The effects of cycle length on cardiac refractory periods in man

Effect of intravenous amiodarone on electrophysiologic variables and on the modes of termination of atrioventricular reciprocating tachycardia in Wolff-Parkinson-White syndrome

Atrioventricular reciprocating tachycardia (AVRT) associated with the Wolff-Parkinson-White (WPW) syndrome, sometimes terminates spontaneously, generally sustains and eventually becomes drug resistant. Amiodarone is a potent antiarrhythmic drug that is sometimes effective in patients with AVRT which is resistant to conventional antiarrhythmic drugs. However, systematic studies concerning the effects of amiodarone on AVRT have not been reported. This study evaluated the effects of intravenous amiodarone on electrophysiologic variables, and on the sites and the modes of termination of AVRT. Fifteen WPW patients were studied. Nine had overt, and 6 had concealed WPW syndrome. Measurements of electrophysiologic variables and the induction of AVRT were performed by atrial and/or ventricular programmed stimulations. Amiodarone was then administered at a dose of 5 mg/kg over 5 min. The effective refractory periods (ERP) of the atrial, atrioventricular node, ventricular and accessory pathway were increased significantly by amiodarone. The conduction times of all the components were significantly lengthened by amiodarone, except for the His-ventricular (HV) interval in concealed WPW patients. AVRT was induced in all patients, and was terminated by amiodarone in 12 of 13 patients with sustained AVRT. After amiodarone, AVRT was induced in 9 patients. Spontaneous termination was observed 11 times in 3 of the 9 patients in whom AVRT was still induced. In these cases, the modes and sites of termination were the same as during the baseline state. The ERPs and conduction times of all components of AVRT, except the HV interval, were significantly lengthened by amiodarone. Amiodarone is efficacious for terminating AVRT wherever weak links exist. However, sites of spontaneous termination are not significantly affected by amiodarone.

Human atrial repolarization: effects of sinus rate, pacing and drugs on the surface electrocardiogram

OBJECTIVES

We studied the effects of rate and some cardioactive drugs on the atrial surface electrocardiogram (ECG).

BACKGROUND

In atrioventricular block, atrial surface ECG is unmasked. The effect of rate alone permits detection of the effect of other exogenous stimulations such as drugs in the presence of rate alterations.

METHODS

High fidelity, high gain ECG leads I, II and III were recorded from 51 patients with heart block. Durations of P and Ta waves and the total PTa interval were measured from nonconducted atrial events.

RESULTS

No relationship was found between sinus cycle length and PTa, P or Ta in 31 patients. In 20 patients, progressively decreasing the atrial pacing cycle length from 853 ms to 381 ms resulted in a linear reduction of the PTa interval from 444 to 291 ms (rho = 0.76, slope = 0.24). This was largely due to shortening of Ta. A linear rate correction formula was derived: corrected PTa = PTa - 0.24 (PP - 1000). Atropine (0.02 mg/kg) shortened the PP interval (p < 0.001) and the PTa interval (p < 0.01). Propranolol (0.1 mg/kg) prolonged the PP interval (p < 0.001) but did not alter the PTa interval. Neither disopyramide (2.0 mg/kg) nor flecainide acetate (2.0 mg/kg) altered the PP interval, but both prolonged the PTa interval (p < 0.001). This was largely due to P wave lengthening after flecainide (p < 0.001) and to Ta prolongation after disopyramide (p < 0.001).

CONCLUSIONS

In heart block, PTa, P and Ta waves can be measured reliably. The effects of pacing and some antiarrhythmic drugs on the atrial myocardium are similar to those known at the ventricular level.

Lack of benefit of very short basic drive train cycle length or repetition of extrastimulus coupling intervals for induction of ventricular tachycardia

INTRODUCTION

There are considerable variations of uncertain importance in basic drive train cycle lengths and degree of repetition of extrastimuli used in programmed ventricular stimulation protocols in different laboratories. We compare prospectively three different stimulation protocols to examine the influence of a short basic drive train cycle length and repetition of extrastimuli on induction of ventricular tachycardia.

METHODS AND RESULTS

Thirty consecutive patients who had documented ventricular tachycardia or fibrillation based on underlying coronary artery disease underwent programmed ventricular stimulation with each of the three study protocols. Protocol A used a basic drive train cycle length of 400 msec with each extrastimulus coupling interval delivered only once. Protocol B used the same basic drive train cycle length, but with each extrastimulus coupling interval repeated three times before decrementing. Protocol C used 300 msec as the cycle length of basic drive trains without repetition of extrastimuli. Sixty-three percent, 67%, and 63% of the study patients had ventricular tachycardia inducible with protocols A, B, and C, respectively (P = NS). Ventricular fibrillation was induced in 23% of the 30 patients in all three protocols. There were no significant differences in the mean cycle lengths of induced ventricular tachycardia, the number of extrastimuli used, and the coupling interval of the last extrastimulus inducing ventricular tachycardia among the three protocols.

CONCLUSION

This study showed no clinical benefit for repetition of extrastimuli that have failed to induce a ventricular tachyarrhythmia during programmed ventricular stimulation. A short basic cycle length of 300 msec was not superior to 400 msec for induction of ventricular tachyarrhythmias. We recommend the use of basic cycle length 400 msec with delivery of each extrastimulus interval only once as the initial protocol for programmed ventricular stimulation.

Changes in cardiac repolarization following short periods of ventricular pacing

INTRODUCTION

"Cardiac memory" (primary T wave change) is thought to occur after 15 minutes to several hours of right ventricular (RV) pacing. The two components of the temporal change in repolarization are memory and accumulation. The purpose of this study was to examine quantitatively the effect of short periods of ventricular pacing on the human cardiac action potential, using monophasic action potential (MAP) recordings.

METHODS AND RESULTS

Thirty-one patients (ages 43+/-14 years) with structurally normal hearts undergoing a clinically indicated electrophysiologic procedure were enrolled. Catheters were placed in the right atrium (RA) and RV, and a MAP catheter was positioned at the RV septum. APD90 was calculated from digitized MAP recordings. MAP morphology comparisons were performed using the root mean square (RMS) of the difference between complexes. All pacing was at 500-msec cycle length. There were four pacing protocols: (1) RA pacing was performed for approximately 15 minutes to evaluate temporal stability of the MAP recordings (5 pts); (2) to evaluate the memory phenomenon, four successive 1-minute episodes of RV pacing were interspersed with 2 minutes of RA pacing (5 pts); (3) the accumulation phenomenon was evaluated by assessing the effects of 1, 5, 10, and 15 minutes of RV pacing on the MAP during RA pacing (16 pts); and (4) 20 minutes of RV pacing was followed by 10 minutes of RA pacing to correlate visually apparent T wave changes with changes in MAP recordings (5 pts). In the control patients, no changes in APD90 or RMS analysis were noted during 14.9+/-1.4 minutes of RA pacing. In the second protocol, RMS of the difference between the baseline MAP complexes and the signal average of the first 50 beats following each of four 1-minute RV pacing trains demonstrated progressively greater differences in morphology after successive episodes of RV pacing. In protocol 3, RMS analysis identified a progressively greater difference between the baseline MAP recording and the average of the first 50 beats after 1, 5, 10, and 15 minutes of RV pacing. In protocol 4, visually apparent changes in T waves occurred in parallel with the RMS of the difference between the baseline MAP recordings and the average of the first 50 beats after 20 minutes of RV pacing. Similar changes also were demonstrated by APD90 analysis.

CONCLUSION

This study is the first to demonstrate that episodes of abnormal ventricular activation as short as 1 minute in duration may exert lingering effects on the repolarization process once normal ventricular activation resumes.

Developmental changes of atrioventricular nodal recovery properties

Atrioventricular (AV) nodal recovery properties can be studied by a periodic premature stimulation protocol performed at a slow basic rate. Developmental aspects of these properties have not been determined. The purpose of this study was to determine the developmental changes of AV nodal recovery properties. Forty-three children and young adults (male:female ratio 25:18) without AV nodal disease (aged 3.3 to 21.9 years) were studied by delivering premature atrial extrastimuli coupled to basic driven atrial beats. The individual recovery curve was fitted to the equation: A2H2 = A0H0 + exp(alpha -H1A2/tau) for H1A2 > or =theta, where A0H0 is the minimum AH interval, H1A2 is any recovery interval that exceeds the nodal effective refractory period, A2H2 is the corresponding nodal conduction time at any given H1A2, alpha is a constant, tau is the recovery time constant, and theta is the nodal effective refractory period. We found that: (1) A0H0 and alpha constant did not change significantly with age; (2) both tau (r = 0.324; p <0.05) and theta (r = 0.401; p <0.05) had a positive correlation with age; and (3) the maximum change in A2H2 with a 10-ms decrement in H1A2 was 32 ms and did not change significantly with age. Our results suggest that AV nodal recovery properties are age-dependent and both the recovery time constant and effective refractory period lengthen with age.

Procainamide induced change of the width of the zone of entrainment and its relation to the inducibility of reentrant ventricular tachycardia

Procainamide depresses conduction velocity and prolongs refractoriness in myocardium responsible for reentrant VT, but the mechanism by which the induction of VT is suppressed after procainamide administration remains to be determined. In the present study, the relationship between electrophysiological parameters and the noninducibility of VT was assessed during procainamide therapy with a special reference to the change of an excitable gap. Clinically documented monomorphic sustained VT was induced in 30 patients and, utilizing the phenomenon of transient entrainment, the zone of entrainment was measured as the difference between the cycle length of VT and the longest paced cycle length interrupting VT (block cycle length) which was determined as the paced cycle length decreased in steps of 10 ms, and used as an index of the excitable gap. The effective refractory period was measured at the pacing site and the paced QRS duration was used as an index of the global conduction time in the ventricle. The cycle length of VT, the block cycle length, and the width of the zone of entrainment were determined and compared between the responders and nonresponders. In 15 patients, these parameters were determined at the intermediate dose and related to subsequent noninducibility at the final dose. At the final doses of procainamide, VT was suppressed in 8 (26.7%) of 30 patients. However, the cycle length of VT, the block cycle length, and the width of the zone of entrainment were unable to predict the drug efficacy, i.e., noninducibility. The change in the effective refractory period at the pacing site or the width of the paced QRS duration was not different between the responders and nonresponders. Among the variables, only the width of the zone of entrainment showed a significant narrowing in the responders at the intermediate dose of procainamide, and it was smaller than that of the nonresponders. The significant narrowing of the width of the zone of entrainment was associated with the subsequent noninducibility of VT at the final dose. The present study showed that the baseline cycle length of VT, the block cycle length, the drug induced change of the effective refractory period, or the paced QRS duration was not a predictor of the noninducibility after procainamide administration. However, a significant narrowing of the width of the zone of entrainment at the intermediate dose was associated with the noninducibility of VT at the final dose.

Atrial electrophysiological features in patients with Wolff-Parkinson-White and atrial fibrillation: absence of rate adaptation of intraatrial conduction time parameters

Clinical electrophysiology has not yet clearly defined atrial features that can predict spontaneous occurrence of atrial fibrillation (AF). The aim of this work was to identify atrial electrophysiological features that can distinguish Wolff-Parkinson-White patients with spontaneous AF from those without this arrhythmia. Sixty-nine patients with Wolff-Parkinson-White were divided into three groups: group I (16 patients) with spontaneous AF; group II (35 patients) with reciprocating tachycardia but not AF; and group III (18 patients) asymptomatic without documented arrhythmias. Atrial effective refractory periods (ERPs) and intraatrial conduction times in response to premature extrastimuli were analyzed. The latter were evaluated as the A1A2 interval minus the correspondent S1S2 interval (A1A2-S1S2), S1A2 and the interval A1A2 following the shortest S1S2 producing atrial activation (FRP'). All the parameters have been evaluated in two atrial sites and at two atrial pacing cycle lengths (600 and 400 ms). For all the parameters, the difference ("gradient") was calculated between the values of the same parameter measured at the atrial pacing cycle length of 600 ms and that found at the atrial pacing cycle length of 400 ms in the same recording site in each patient was calculated. Atrial ERP did not differ significantly in the three groups. Intraatrial conduction parameters, evaluated in the high right atrium (HRA), were longer when measured at an atrial pacing of 400 ms and showed a lack of rate adaptation in patients with spontaneous AF. In group I patients in particular, FRP' became longer with the increase of atrial rate, while in groups 2 and 3, it usually shortened. The mean gradient of HRA FRP' was -15.0 +/- 19 ms in group I as compared to 5.7 +/- 13 ms in group II and 6.4 +/- 13 ms in group III (P < 0.001); sensitivity, specificity, and negative predictive value of a negative gradient in the identification of patients with spontaneous AF, were, respectively, 83%, 75%, and 93%. Patients from groups 2 and 3 did not differ in any of the analyzed parameters. Patients with Wolff-Parkinson-White and spontaneous AF showed prolonged intraatrial conduction times and a different behavior in response to modification of heart rate.

Electrophysiologic properties of the atrioventricular node in pediatric patients

OBJECTIVES

The purpose of this study was to characterize anterograde and retrograde properties of the atrioventricular (AV) node in children and to determine the presence of ventriculoatrial (VA) conduction and dual AV node pathways.

BACKGROUND

Although AV node reentry is common in adults, it accounts for 13% of pediatric supraventricular tachycardia (SVT). The age-related changes in the AV node with development are poorly understood. The incidence of dual AV node pathways and VA conduction in the pediatric population is unknown.

METHODS

Electrophysiologic studies were performed in 79 patients with normal hearts and no evidence of AV node arrhythmias. Patients were classified into two groups by age: group I = 49 patients (0.39 to 12.8 years old, mean [+/- SD] age 8.5 +/- 3.6); group II = 30 patients (13.4 to 20.0 years old, mean age 15.6 +/- 1.8).

RESULTS

There was a significant difference (p < 0.05) in the cycle length (CL) at which anterograde AV block occurred between group I (305 +/- 63 ms) and group II (350 +/- 91 ms). Sixty-one percent of children had VA conduction with no age-related differences. There was no significant difference in the mean CL of retrograde VA block (360 ms). The incidence of dual AV node pathways in group I was 15% and 44% in group II (p < 0.05).

CONCLUSIONS

These findings suggest that AV node electrophysiology undergoes maturational changes. The increase in AV node reentrant tachycardia in adults may relate to changes in the relative refractoriness and conduction of the AV node or to differences in autonomic input into the AV node that allow dual pathway physiology to progress to SVT.

Influence of duration of rapid ventricular pacing on ventricular refractoriness in the presence of propafenone and lidocaine

Propafenone and lidocaine have a rate dependent negative dromotropic effect on intraventricular conduction. We investigated the use dependent actions of propafenone and lidocaine on intraventricular conduction in isolated guinea pig hearts perfused by the method of Langendorff. Of primary interest was how the number of stimuli of the conditioning train (S1) might influence the ventricular effective refractory period (VERP) when refractoriness is assessed at a high pacing rate. Propafenone (0.3 microM) and lidocaine (50 microM) caused a comparable prolongation of the intraventricular conduction time during sinus rhythm. During ventricular pacing in the presence of propafenone an abrupt decrease of the pacing cycle length (220 to 120 ms) resulted in an initial peak of rate dependent prolongation of the QRS interval that subsequently decreased to a stable steady-state level. Lidocaine also induced a rate dependent increase of the intraventricular conduction time up to a steady-state level. The time constant, characterizing the changes of the intraventricular conduction time after shortening the ventricular pacing cycle length from 220 to 120 ms was significantly (P < 0.01) longer in the presence of propafenone (tau = 31 +/- 4 beats; mean +/- SEM; n = 11) than for lidocaine (tau = 3 +/- 1; n = 10). Both drugs caused the greatest increase of the VERP when the number of conditioning stimuli (S1, interstimulus interval = 120 ms) was in the range of their respective time constant. However, when the number of conditioning stimuli was further increased, VERP progressively diminished. These effects may be explained by a shortening of the action potential during high rates that results in a decreased binding of propafenone to Na+ channels and by the direct shortening of repolarization period by lidocaine (Class IB drug).

Effects of AWD 23-111, a new antiarrhythmic substance, on cardiac conduction and refractoriness

In isolated spontaneously beating guinea pig hearts, the effects of AWD 23-111 (N-(dicyclohexylcarbamoylmethyl)-N-(3-diethylamino-propyl)-4-nit robenzamid -hydrochloride), a new synthetic class III antiarrhythmic agent with sodium antagonistic properties, were investigated on cardiac electrophysiological parameters, that is, conduction and refractoriness. Concentration-dependent prolongation of the atrioventricular, intraventricular, and His bundle conduction times and of sinus node cycle length were present. At 0.3 microM the repolarization period was prolonged significantly. No reverse use-dependent effect on the repolarization period was observed. During rapid pacing (pacing cycle length = 120 ms for the ventricle and 180 ms for the atrium) the rate-dependent intraventricular (QRS) or atrioventricular conduction time (AVCT) prolongation follows an exponential function of the beat number and is characterized by a drug-specific time constant. The time constant for the intraventricular conduction time prolongation in the presence of 0.1 microM AWD 23-111 was very long at 150 +/- 29 beats (mean +/- SEM; n = 6), indicating a slow binding kinetic to the sodium channel. At 0.1 microM AWD 23-111, a significant increase in the ventricular effective refractory period was reached when the interstimulus interval (S1-S1) was 120 ms and the number of conditioning stimuli (S1) was higher than the time constant. The time constant for the rate-dependent AVCT prolongation in the presence of 0.3 microM AWD 23-111 was 34 +/- 6 beats (n = 6). The effective refractory period of the atrioventricular conduction significantly increased with the number of conditioning stimuli (S1), until the number was comparable with the time constant. In conclusion, AWD 23-111 exerts a wide variety of actions on the cardiac conduction system. Its combined effects on the potassium and sodium channels seem to be responsible for the marked rate-dependent effect on ventricular refractoriness and for the lack of a reverse use-dependency on JT prolongation.

Unusual resetting patterns in response to single atrial extrastimuli during AV junctional reentrant tachycardia

INTRODUCTION

Two unusual resetting patterns were observed in two patients with slow-fast AV junctional reentrant tachycardia (AVJRT) submitted to an electrophysiologic study.

METHODS AND RESULTS

After AVJRT induction, resetting was evaluated by introducing single extrastimuli at progressively shorter coupling intervals from the high right atrium (HRA) and the proximal coronary sinus (CS). An alteration in the return cycle length duration allowed demonstration of resetting. In the first patient, during and AVJRT with a large excitable gap, properly timed extrastimuli delivered both from the HRA and CS simultaneously reset the tachycardia and advanced the H electrogram of the preceding tachycardia beat. In the second patient, both HRA and CS stimulation apparently failed to reset AVJRT (return cycle length unchanged), but, at critical coupling intervals, the cycle length duration of the tachycardia beat following the return cycle was consistently shortened.

CONCLUSION

During slow-fast AVJRT, single atrial stimulation from sites remote to the reentrant circuit may result in unusual resetting patterns. Further studies are required to evidence the full spectrum of resetting in AVJRT.

Atrioventricular node properties in patients with accessory pathways

A study during the era of surgical ablation suggested that atrioventricular (AV) nodal conduction is faster in patients with accessory pathways than in controls. In the present study, AV nodal characteristics were studied in 30 patients who underwent radiofrequency ablation of an accessory pathway and compared to 23 control patients. Sinus cycle length, AH and HV intervals, AV block cycle length, ventriculoatrial (VA) block cycle length, AV nodal effective refractory period, and VA effective refractory periods were measured in control and postablation accessory pathway patients before and after autonomic blockade with 0.04 mg/kg of atropine and 0.2 mg/kg of propranolol. The mean sinus cycle length in the control and accessory pathway groups did not differ significantly at baseline (798 +/- 211 and 766 +/- 156 msec, respectively) or after autonomic blockade (654 +/- 98 and 649 +/- 108 msec, respectively). The mean AH interval in the accessory pathway group (77 +/- 15 msec) was significantly shorter than in the control group (91 +/- 22 msec; p < 0.05) at baseline; however, there was no difference after autonomic blockade. No other significant differences were observed between the accessory pathway and control groups. These results demonstrate that AV nodal properties of patients with accessory pathways are not significantly different from controls and suggest that previously reported differences may have been due to selection bias.

How to measure AV nodal refractoriness in the presence of verapamil, amiodarone, digoxin, and diltiazem

On the AV node the negative dromotropic action of verapamil, amiodarone, digoxin, and diltiazem is known to be rate dependent. The effective refractory period of the AV node (AV-ERP) at a short cycle length is related to the AV conduction at that cycle length. We investigated how the number of stimuli during the conditioning train (S1) (during measurement of refractoriness at a high pacing rate [cycle length = 180 ms]) might influence the AV-ERP in isolated guinea pig hearts in a Langendorff preparation. Verapamil (10 nM), amiodarone (10 microM), digoxin (0.6 nM), and diltiazem (30 nM) caused a comparable prolongation of the AV conduction time (AVCT). All four drugs caused a significant prolongation of the AV-ERP when evaluated by a standard stimulation protocol with a conditioning train of 10 stimuli (10 S1) at a pacing cycle length of 180 ms followed by the test stimulus (S2). When the number of stimuli during the conditioning train (S1) was increased (> 10), until the prolongation of AVCT reached steady state, the AV-ERP in the presence of verapamil (132 +/- 4 vs 141 +/- 3 ms; P < 0.05, mean +/- S.E.M.) and diltiazem (143 +/- 3 vs 151 +/- 3 ms; P < 0.05) was prolonged significantly further. These results indicate that the effect of drugs on AV-ERP should be measured with a modified stimulation protocol, whereby the number of conditioning stimuli is comparable to the time constant characterizing the prolongation of AVCT at fast pacing rates.

Rate-dependent effects of detajmium and propafenone on ventricular conduction and refractoriness in isolated guinea pig hearts

Detajmium (4--3'-diethylamino-2'-hydroxypropyl--ajmalin) is an Na(+)-channel-blocking drug with an extremely long recovery from use-dependent sodium channel block. The aim of the present study was to investigate the rate-dependent effects of detajmium on the intraventricular conduction of isolated, spontaneously beating, guinea pig hearts in comparison with the effects of propafenone. Detajmium (0.3 microM) and propafenone (0.3 microM) caused comparable prolongations of the intraventricular conduction time during sinus rhythm. The time to steady state of the rate-dependent QRS prolongation during rapid ventricular pacing follows an exponential function of the beat number after an abrupt change of frequency and is characterized by a drug-specific time constant. This time constant was significantly longer for detajmium (tau = 265 +/- 165 beats; mean +/- SEM; n = 6) than for propafenone (tau = 31 +/- 4 beats; n = 11; p < 0.01). In the presence of propafenone, QRS duration peaked initially before decreasing to a steady state. Detajmium, in contrast, progressively broadened the QRS complex. Both substances caused the greatest increase in the ventricular effective refractory period (V-ERP) when the number of conditioning stimuli (interstimulus interval, 120 ms) was in the range of the time constant. However, when the number of conditioning stimuli was further increased, the V-ERP for propafenone diminished progressively. In conclusion, propafenone displayed, in comparison with detajmium, only a transient rate-dependent effect on intraventricular conduction and V-ERP.

Premature atrial beat eliciting atrial fibrillation after coronary artery bypass grafting

Of patients undergoing coronary artery bypass grafting 30% develop atrial fibrillation (AF) or flutter. To determine if AF is initiated from the right or left atrium, atrial electrograms were continuously recorded in patients undergoing this procedure. In addition, to study whether the prematurity index of premature atrial contractions (PACs) eliciting AF differs from PACs not provoking AF, the distribution of prematurity indices was evaluated from R-R interval analysis. The right and left atrial recording electrodes were first activated by the ectopic beat provoking AF in six and eight patients, respectively. The prematurity index of the PAC eliciting AF was located in the middle (in half of the patients) or to the left of the median distribution of prematurity indices. The variability in activation of the atrial electrodes suggests that the PAC provoking AF can have its origin in the right, the septal, or the left region of the atrium. The initiation of AF depends on the prematurity index of the PAC.

Atrial ectopic activity and atrial fibrillation/flutter after coronary artery bypass surgery. A case-base study controlling for confounding from age, beta-blocker treatment, and time distance from operation

To evaluate the impact of ectopic atrial activity and cardiac autonomic function for development of atrial fibrillation or flutter we studied 128 consecutive patients undergoing elective coronary artery bypass grafting. The patients were Holter monitored during a 96-h postoperative period. Autonomic balance was estimated by heart rate variability measurement. Thirty-eight (30%) patients developed sustained atrial fibrillation or flutter. Holter recordings were analyzed in a case-base design matching for age, beta-blocker treatment, and time elapsed after the operation. The number of supraventricular ectopic beats/h was increased (P < 0.01) in the case group during each of the last 7 h before onset of atrial fibrillation or flutter. Episodes of supraventricular tachycardia 62% vs. 26%; cases vs. controls (P = 0.007), episodes of non-sustained atrial fibrillation 29% vs. 0% (P = 0.0009), and episodes of non-sustained atrial flutter 15% vs. 0% (P = 0.05) were significant predictors of atrial fibrillation or flutter. Mean heart rate/h was increased in each of the 24 h preceding the arrhythmia. However, indexes of autonomic heart function did not differ significantly between groups. Patients developing atrial fibrillation or flutter after coronary artery bypass surgery present 'warning' atrial arrhythmias: numerous supraventricular ectopic beats, paroxysmal supraventricular tachycardia, and episodes of nonsustained atrial fibrillation and flutter.

Spatial and temporal inhomogeneity of adenosine's effect on atrial refractoriness in humans: using atrial fibrillation to probe atrial refractoriness

INTRODUCTION

Adenosine is an antiarrhythmic agent widely used for the diagnosis and treatment of supraventricular tachyarrhythmias. Bolus administration of adenosine has been associated with transient atrial fibrillation, but the underlying mechanisms have not yet been delineated, due in part to the technical limitations resulting from adenosine's short half-life. In this study, the rapid, repetitive activation of atrial tissue during atrial fibrillation was exploited to yield a nearly continuous estimate of atrial refractoriness, enabling a description of adenosine's action on atrial refractoriness in humans.

METHODS AND RESULTS

Simultaneous right and left atrial, narrow-spaced, bipolar electrograms were recorded in 15 patients before, during, and after bolus administration of 12 mg of adenosine during atrial fibrillation. By estimating the local cycle length of excitation from the atrial activation frequency, a refractory period index (RPI) of local tissue was obtained. Adenosine dramatically increased the activation frequency from 6.4 +/- 0.7 to 12.2 +/- 12.2 Hz in the right atrium and from 6.1 +/- 0.6 to 8.7 +/- 1.2 Hz in the left atrium, corresponding to a decrease in the RPI from 156 to 82 msec (P < 0.0001) and from 163 to 116 msec (P < 0.0001), respectively. The magnitude of adenosine's effect was greater (47% vs 29% shortening) (P < 0.001) and the duration of adenosine's effect longer (28 vs 19 sec) (P < 0.05) in the right compared to the left atrium.

CONCLUSION

Adenosine, at doses routinely used clinically, produces a transient, but spatially and temporally inhomogeneous, shortening of atrial refractoriness, which may explain the increased propensity toward atrial fibrillation associated with its use.

Overdrive prolongation of refractoriness and fatigue in the early stages of human bundle branch disease

OBJECTIVES

The aim of this study was to assess the response of refractoriness in normal and diseased human bundle branches to changes in cycle length, as well as during a long period of continuous overdrive pacing.

BACKGROUND

The anterograde refractory period of the bundle branches in patients with functional bundle branch block shortens as the rate is increased. The rate-dependent response of refractoriness in diseased bundle branches is quite different. However, this difference has not been precisely delineated, and its physiologic meaning is uncertain.

METHODS

Refractoriness of the bundle branches was measured by the extrastimulus technique in 16 patients with tachycardia-dependent bundle branch block and 10 patients with functional bundle branch block, both after basic trains of 8 atrial-paced impulses at different cycle lengths and during a 10-min period of continuous overdrive pacing.

RESULTS

The baseline refractory period in the bundle branches of patients with functional bundle branch block measured 430 +/- 32 ms (mean +/- SD) and shortened to 368 +/- 30 ms at the shortest cycle length. The maximal effect was reached within the 1st min of overdrive pacing. The baseline refractory period of the bundle branches was significantly longer in patients with tachycardia-dependent bundle branch block (611 +/- 184 ms) and demonstrated a cumulative overdrive prolongation in 15 (83%) of 18 studies with typical manifestations of fatigue. In two other studies, this occurred only after ajmaline administration.

CONCLUSIONS

A rate- and time-dependent prolongation of refractoriness frequently occurs in diseased human bundle branches. When absent, this response may be induced under the effects of sodium channel blockers. This would suggest that an abnormality in the recovery from inactivation of the sodium channel might underlie the early stages of bundle branch disease.

Dynamic behavior of the atrioventricular node: a functional model of interaction between recovery, facilitation, and fatigue

The wide variety of delays that the atrioventricular node can generate in response to an increased rate are explained by dynamic interactions between the three intrinsic properties of recovery, facilitation, and fatigue. The functional model presented suggests that any deviation of nodal conduction time from its minimum basal value represents, at any given time, the net sum of the effects produced by these properties. When a constant fast atrial rate is suddenly initiated, the node first "sees" a shortening in recovery time and responds by an increase in conduction time. This increase further shortens the recovery time of the ensuing beat, which is accordingly further delayed, and so on until a steady state is reached or a block occurs. However, these events do not occur alone. The second beat at the fast rate is conducted with a shorter conduction time than expected from the recovery time alone, and is therefore facilitated. These facilitatory effects develop within one short cycle and dissipate within one long cycle. They affect increasingly the conduction time of beats occurring with shorter cycle lengths. While steady-state effects of recovery and facilitation occur within seconds, nodal conduction time continues to increase slowly over several minutes when a rapid rate is maintained. This effect is attributed to fatigue, which develops and dissipates with a slow, symmetric time course. The dynamics of these properties can now be directly studied with selective stimulation protocols, and have many implications for the understanding of nodal behavior in the context of supraventricular tachyarrhythmias.

Comparison of number of extrastimuli versus change in basic cycle length for induction of ventricular tachycardia by programmed ventricular stimulation

OBJECTIVES

The purpose of this study was to examine the effects of varying basic cycle lengths in a programmed stimulation protocol if up to seven extrastimuli were available at each basic cycle length.

BACKGROUND

There is no uniformly accepted protocol for induction of ventricular tachycardia. Most protocols limit the number of extrastimuli to two or three but use several basic cycle lengths.

METHODS

Twenty-eight patients with coronary artery disease and documented spontaneous sustained ventricular tachycardia or ventricular fibrillation were studied. In the absence of antiarrhythmic drugs, each patient underwent three inductions of ventricular tachycardia/ventricular fibrillation using sinus rhythm or right ventricular pacing at 600 or 400 ms as the basic cycle length. Up to seven extrastimuli were allowed at each basic cycle length.

RESULTS

The maximal yield of clinical tachycardia (96%) was identical for each basic cycle length and was achieved using a maximum of seven, five and four extrastimuli for sinus rhythm and 600 and 400 ms, respectively. A basic cycle length of 400 ms required fewer extrastimuli (2.4 +/- 0.7) to induce ventricular tachycardia/ventricular fibrillation than did 600 ms (2.7 +/- 1.1, p = 0.014) or sinus rhythm (3.4 +/- 1.2, p < 0.001). There was no significant difference in the cycle lengths of the induced ventricular tachycardia, incidence of induced ventricular fibrillation or requirement for direct current countershock.

CONCLUSIONS

The use of an adequate number of extrastimuli obviates the need for multiple basic cycle lengths for induction of ventricular tachycardia and does not increase induction of unwanted ventricular fibrillation. If only one basic cycle length is used, the ease of inducibility can be quantified in terms of the number of extrastimuli required. Fewer extrastimuli were required for induction of ventricular tachycardia if a basic cycle length of 400 ms was used. These data favor the use of ventricular pacing at a basic cycle length of 400 ms with up to at least four extrastimuli as the standard stimulation protocol for induction of ventricular tachycardia.

Radiofrequency versus pharmacologic modification of the atrioventricular node

Although transcatheter radiofrequency modification of the atrioventricular (AV) node has been proposed as curative treatment in AV nodal reentry tachycardias, its role for the control of the ventricular rate in atrial tachyarrhythmias remains unclear. The aim of this study was to analyze the acute effect of radiofrequency current on AV nodal conduction and refractoriness, and to compare it with the effects of two antiarrhythmic drugs such as amiodarone (class III) and flecainide (class I). Twenty-one dogs were studied: (1) radiofrequency group (5 W for less than 45 seconds; 2 to 12 discharges; seven dogs); (2) amiodarone group (5 mg/kg intravenously; seven dogs); and (3) flecainide group (2 mg/kg intravenously; seven dogs). The following parameters were measured under basal conditions and after each procedure: AH interval, AV nodal functional refractory period, Wenckebach cycle length, minimum R-R interval during atrial fibrillation, and fitting of AV nodal function curve to a hyperbolic equation using its linear transformation. The AV nodal effective refractory period could not be calculated in any dog in the basal study because it was shorter than the atrial functional refractory period.(ABSTRACT TRUNCATED AT 250 WORDS)

Atrioventricular nodal conduction and refractoriness

The AV node is ideally suited to act as a weigh station between atrium and ventricle. It is believed that the latter function is achieved by virtue of nodal conduction being dependent on slow channel conduction probably carried by calcium currents. The AV nodal structure allows for encouragement of decremental conduction but also allows for two or more pathways or collection of cells which may trigger reentrant arrhythmias.

A unified functional/anatomic substrate for circus movement atrial flutter: activation and refractory patterns in the canine right atrial enlargement model

OBJECTIVES

This study was designed to test the concept of a functional/anatomic interaction in a canine model of reentry based on right atrial enlargement and to elucidate the electrophysiologic basis for functional conduction block.

BACKGROUND

The monotonic feature of atrial flutter suggests a uniform substrate for the arrhythmia. Atrial flutter in the sterile pericarditis model is due to single-loop circus movement around a functional or a functional/anatomic obstacle near the atrioventricular (AV) ring. Sustained circus movement requires a critical interaction of a functional arc of block, a natural obstacle, the AV ring and a zone of slow conduction. The location of the inferior vena cava predisposes the lower right atrium to single-loop reentry.

METHODS

In 11 dogs with right atrial enlargement, 127 bipolar epicardial electrograms were obtained during atrial flutter. For correlation of activation and refractory maps, the effective refractory period under each electrode was determined using the extrastimulus technique.

RESULTS

Atrial flutter was due to single-loop reentry around functional arcs of block near the AV ring (n = 2) or around functional/anatomic obstacles (n = 8) involving the inferior vena cava. A slow zone was located between the arc and the AV ring and between the inferior vena cava and AV ring, respectively. During initiation, the arc joined the AV ring, forcing activation to proceed around the free end of the arc before breaking through the arc near the AV ring. Arrhythmia termination required the arc of block to rejoin the AV ring. Inducibility of sustained atrial flutter was associated with a marked spatial dispersion of refractoriness. The configuration of the functional arc of block was critically dependent on the spatial pattern of refractoriness.

CONCLUSIONS

Atrial flutter requires a similar functional or functional/anatomic substrate independent of the underlying etiology. The spatial distribution of refractoriness in enlarged canine atria provides an adequate substrate for the development of functional conduction block.

Changes in ventricular effective refractory periods after two extrastimuli and ventricular electrical instability

Using programmed stimulation with one and three extrastimuli delivered in the right ventricular apex, we compared the effective refractory period (ERP) during sinus rhythm (ERP-SR) and during the third extrastimulus (ERP-S3) in patients without ventricular tachycardias (control group, n = 87) and in patients with documented ventricular tachycardia (VT group, n = 76). The protocol was not completed to determine ERP-S3 in one patient in the control group and in 15 patients in the VT group. We observed a significantly greater change (i.e., shortening) in ERP after two extrastimuli in the VT group compared with patients without VT (delta ERP = 45 +/- 20 msec in the control group and 70 +/- 16 msec in the VT group, P < 0.001). This electrophysiological phenomenon, along with conduction delay, may play an important role in VT induction.

Physiological substrate for antidromic reciprocating tachycardia. Prerequisite characteristics of the accessory pathway and atrioventricular conduction system

BACKGROUND

Although the anatomic "substrate" for the occurrence of antidromic reciprocating tachycardia (ART) has been previously examined, the underlying physiological substrate for this unusual arrhythmia in patients with the Wolff-Parkinson-White syndrome has not been thoroughly characterized.

METHODS AND RESULTS

The electrophysiological properties of the accessory pathway and normal ventriculoatrial conduction system in 30 patients with ART and a single accessory pathway were compared with those observed in a control group of 36 patients without this arrhythmia to elucidate the critical physiological substrate essential for the development and maintenance of ART. Inducible ART had a mean cycle length of 286 +/- 31 msec. The average retrograde ventriculoatrial conduction system effective refractory period in ART patients was significantly less than that in the control group (244 +/- 32 versus 291 +/- 46 msec, p = 0.0002). All of the ART patients showed retrograde conduction over the normal conduction system at cycle lengths of less than or equal to 360 msec; 23 had 1:1 conduction to less than or equal to 300 msec, and 16 showed 1:1 propagation at cycle lengths of less than or equal to 260 msec. The shortest cycle length accompanied by 1:1 retrograde propagation over the normal conduction system in patients with ART was also significantly less than that observed in the control group (274 +/- 39 versus 347 +/- 73 msec, p less than 0.001). The accessory pathway anterograde ERP in ART patients with 1:1 retrograde conduction over the normal ventriculoatrial conduction system at cycle lengths of less than or equal to 360 was significantly less than that seen in comparable control patients (247 +/- 23 versus 284 +/- 56 msec, p = 0.001), and the accessory pathway location was significantly further from the atrioventricular node in 21 patients with ART undergoing surgery than that in 22 operated control patients (3.8 +/- 0.8 versus 2.9 +/- 0.8 mapping units, p = 0.0025) who also had retrograde ventriculoatrial conduction to cycle lengths of less than or equal to 360 msec. No significant differences in anterograde atrioventricular conduction system properties, retrograde accessory pathway refractoriness, or shortest ventricular pacing cycle lengths maintaining 1:1 conduction via the accessory pathway were observed between groups.

CONCLUSIONS

This quantitative characterization of the properties of conduction and refractoriness of both the accessory pathway and ventriculoatrial conduction system and the relation between these characteristics and the accessory pathway location in ART patients provides additional insight into the prerequisites for the initiation and maintenance of this rhythm disturbance.

Effect of pacing drive cycle length on induction of sustained monomorphic ventricular tachycardia

To assess the effect of pacing drive cycle length on induction of sustained monomorphic ventricular tachycardia, 40 patients were prospectively studied in the drug-free state. Ventricular extrastimuli were sequentially delivered at the same coupling interval at each of three drive cycle lengths (600, 500, and 400 ms) before the coupling interval was shortened and the process repeated. This protocol was continued until sustained monomorphic ventricular tachycardia was induced at all three drive cycle lengths or until pacing was completed through three extrastimuli. Of the 27 patients in whom sustained monomorphic ventricular tachycardia was induced, tachycardia was induced at all 3 drive cycle lengths in 12 (44%), only 2 drive cycle lengths in 11 (41%), and only 1 drive cycle length in 4 (15%) patients. Although the yield of inducible ventricular tachycardia increased with each additional extrastimulus, this yield increased even further with the use of multiple drive cycle lengths. In 10 of 11 patients, failure to induce ventricular tachycardia at any drive cycle length was not due to failure to achieve at that drive cycle length the critical extrastimulus coupling interval required to induce tachycardia at other drive cycle lengths. Induction of sustained monomorphic ventricular tachycardia is often drive cycle length specific, and failure to induce tachycardia cannot be explained by failure to achieve critically short coupling intervals. Pacing at multiple drive cycle lengths increases the yield of induced ventricular tachycardia.

Three-dimensional computer model of the entire human heart for simulation of reentry and tachycardia: gap phenomenon and Wolff-Parkinson-White syndrome

A computer model of the entire human heart has been developed for simulation of the excitation and repolarization process. Spatial distribution of refractory periods and conduction velocities in the different cardiac tissues, the anisotropy of conduction in the ventricles, and the cycle length dependence of refractory periods and conduction velocities are taken into account. The algorithm calculating the activation process is based on a modified version of Huygen's principle for constructing wavefronts. This study presents simulations concerning the gap phenomenon of the conduction system and the initiation of tachycardias in a heart with Wolff-Parkinson-White syndrome. Results are compared for different basic cycle lengths and for normal and prolonged refractory periods in the His-Purkinje system. The gap phenomenon was found to be present only when using the prolonged refractory periods in the His-Purkinje-system at a cycle length of 700 ms. Induction of tachycardia by a single extrastimulus in the high right atrium in a heart with a bidirectionally conducting accessory pathway is possible by properly timed extrastimuli. The coupling interval of the stimulus for initiating a reentrant tachycardia depends on the cycle length, the conduction velocities and the set of refractory periods used. The same parameters determine whether or not a gap phenomenon in atrioventricular conduction occurs. The model may be useful for investigating similar questions concerning the reentry phenomena of tachycardia.

Variability in the measurement of human ventricular refractoriness

The degree of variability in ventricular refractoriness and factors potentially affecting this variability were evaluated in 80 patients undergoing an electrophysiological study. Each of seven variables (stimulation current, coupling interval of the basic drive train to spontaneous rhythm, pause between determinations, bipolar pacing configuration, bipolar vs unipolar pacing, atrioventricular synchrony, and autonomic tone) was evaluated in a group of ten patients to determine its effects on the reproducibility of refractoriness. Measurements were repeated ten times in every patient under each of two conditions. Five variables had significant effects on the reproducibility of measurements. Pacing at 10 mA was associated with less variability in the determination of ventricular refractoriness than pacing at twice threshold (within-subject variance component 4.5 vs 10.1 msec; P less than 0.001). The mean difference between the longest and shortest determinations of refractory periods (range) was 6.2 msec at 10 mA and 8.6 msec at twice threshold. The use of a conditioning period of pacing and continuous trains (eight beats with a 3-sec pause) rather than a variable pause between serial trials reduced the mean within-subject variance component from 16.5 to 3.3 (P less than 0.001) and the mean range of refractory period determinations from 10.8 to 4.8. The use of the distal rather than the proximal pole as the cathode decreased the mean within-subject variance component from 9.4 to 3.3 (P less than 0.001) and the range of determinations from 6.4 to 5.8 msec. Unipolar pacing was associated with less variability than bipolar pacing (mean within-subject variance component 4.6 vs 6.4; P less than 0.05, mean range 5.0 vs 7.6 msec).(ABSTRACT TRUNCATED AT 250 WORDS)

Monophasic action potential duration during programmed electrical stimulation

To examine changes in monophasic action potential duration (APD) with a pacing protocol similar to that used during electrophysiological testing, action potentials were recorded in vivo from the left ventricular apical endocardium of 12 normal mongrel dogs. The atrioventricular node was ablated and the dogs paced from the anterior right ventricle at a baseline cycle length of 1000 ms between interventions. Mean steady-state APD (APDss) was 266 +/- 7 ms at a pacing cycle length (PCL) of 1000 ms. Two pacing protocols were used. The first consisted of a sudden acceleration in pacing from a cycle length of 1000 ms to one between 300 and 600 ms. The second consisted of an 8-beat train at a cycle length of 400 ms followed by a premature beat at a coupling interval of 280 ms followed by a pause. The inter-train pause varied between 1 second and 32 seconds. With a sudden acceleration in pacing rate, steady-state values for APD at the faster PCLs were significantly smaller than APDss at 1000 ms with a change to cycle lengths of 600 ms (247 +/- 29 ms), 500 ms (229 +/- 21 ms), 400 ms (220 +/- 17 ms), and 300 ms (203 +/- 31 ms; P less than 0.01 for all comparisons). The time constant of the change in APD was shorter at a PCL of 300 ms (14.9 +/- 0.8 s) than 600 ms (20.3 +/- 4.7 s; P less than 0.05). With drive train pacing and incorporating an inter-train pause, the percent drop in steady-state APD compared to APD for the first train ranged from 10.1% with a 1-second inter-train pause to 2.1% with a 32-second pause. The difference in APD between the first drive train and drive trains after at least 3 minutes of pacing when APD had stabilized was not significant for an inter-train pause exceeding 8 seconds.

IN CONCLUSION

(1) with a sudden acceleration in pacing rate, endocardial APD in vivo decreases exponentially. The faster the new rate, the shorter the new steady-state APD and the shorter the time constant. (2) When pacing using an 8-beat drive train and an inter-train pause, there is a decremental shortening in APD for pause lengths shorter than 16 seconds. Thus, while performing programmed stimulation using a pause, a conditioning period of at least 2 minutes should be used prior to diastole scanning to allow APD to achieve a steady state.

Prinzmetal angina. Multifocal ischemia, recurrent AV block, and bradycardia with patent coronary arteries responsive to verapamil

Coronary vasospasm may result in recurrent angina pectoris and cause acute myocardial infarction. The extent to which the "sudden death syndrome" occurs is unknown. The case described herein is unique in that the clinical features, including hypotension, AV block, and ventricular arrhythmias, were similar to those seen in myocardial infarction with a poor prognosis, yet infarction was not documented. In subsequent, long-term follow-up evaluation, chest pain has been recurrent, but despite close observation, no further major cardiac complications have been documented. Long-term use of verapamil has contributed to better control of clinical symptomatology.

Arrhythmia analysis by successive RR plotting

A successive RR interval plot was developed to analyze arrhythmia. The plot consisted of a set of points with the x-value of (N)th RR interval and the y-value of (N + 1)th RR interval. This method was applied in the arrhythmia analysis of Holter electrocardiograms obtained from 35 patients. In the analysis of ventricular premature contractions (VPCs) this method was useful not only in detecting VPCs but also in demonstrating coupling interval-dependent characteristics of VPCs. In the analysis of atrial fibrillation the successive RR plot enabled the authors to estimate the functional refractory period of the atrioventricular conduction. In conclusion, despite its simplicity, the successive RR plot was found to be powerful in analyzing arrhythmia. Specifically, the potential to analyze integrally the coupling interval-dependent properties of various types of arrhythmia makes it attractive as a clinical tool.

Bradycardia-mediated tachyarrhythmias in congenital heart disease and responses to chronic pacing at physiologic rates

The coexistence of bradycardia and a tachyarrhythmia may preclude effective pharmacologic treatment of 1 arrhythmia without paradoxic aggravation of the other. This study evaluated the potential relation between the 2 types of arrhythmias and the effect of conventional modes and rates of pacing for bradycardia on the frequency of the associated tachyarrhythmias. Twenty-one young patients, aged 2 to 19 (mean 11) years with congenital heart disease and a tachyarrhythmia occurring in the setting of chronic bradycardia were studied. The effects of pacing were evaluated by comparison of the number of episodes of clinical tachycardia during the 12-month intervals before and after pacemaker implantation. During these intervals, antiarrhythmic drug therapy was not altered. Patients were analyzed as independent groups, based on the type of tachyarrhythmia: supraventricular (n = 5), atrial flutter (n = 9) and ventricular (n = 7). The modes of chronic pacing were AAI (n = 4), DDD (n = 6) and VVI (n = 11). The prevention of bradycardia by pacing was associated with a significant decrease in the frequency of supraventricular (p = 0.008) and ventricular (p = 0.02) tachyarrhythmias. However, the frequency of atrial flutter was not altered. Prevention of tachycardia was more frequently associated with the AAI and DDD modes of pacing compared to VVI (p = 0.08). Pacing represents an effective therapy for certain tachyarrhythmias associated with chronic bradycardia, although critical modes may be required.

The diagnostic sensitivity of electrophysiologic testing in patients with syncope caused by transient bradycardia

Although electrophysiologic testing accurately delineates abnormalities in patients with fixed cardiac-conduction defects, its sensitivity in identifying transient rhythm disturbances is unknown. We prospectively studied 21 patients who had electrocardiographically documented intermittent atrioventricular block (n = 13) or sinus pauses (n = 8) causing syncope, but whose cardiac rhythm had reverted to normal by the time of referral. There were 14 men and 7 women, with a mean age (+/- SD) of 63 +/- 13 years. Fourteen patients had organic heart disease, and 8 were taking cardioactive medications. Electrophysiologic testing was performed before the implantation of a permanent pacemaker. Only three of the eight patients with documented sinus pauses had abnormalities during their tests that suggested the correct diagnosis (sensitivity, 37.5 percent), including a prolonged sinus-node recovery time in one and carotid-sinus hypersensitivity in two. Three of the eight patients had abnormalities detected that were unrelated to syncope, including atrial flutter, dual atrioventricular nodal pathways, and sustained monomorphic ventricular tachycardia. Of the 13 patients with documented atrioventricular block, only 2 had abnormalities suggesting the correct diagnosis (sensitivity, 15.4 percent). Additional observations unrelated to syncope among these 13 patients included abnormal sinus-node function, atrial flutter, and atrial fibrillation causing hypotension. These preliminary observations suggest that a negative electrophysiologic test in a patient with a normal cardiac rhythm who has experienced syncope does not exclude a transient bradyarrhythmia as a cause of the syncope. Furthermore, electrophysiologic testing may sometimes reveal unrelated rhythm disturbances that may mistakenly be designated as the cause of the syncope.

Origin, domain, and dynamics of rate-induced variations of functional refractory period in rabbit atrioventricular node

Different aspects of the intrinsic regulation of rate-induced variations of functional refractory period of atrioventricular node (FRPN) were studied in isolated rabbit heart preparations. First, the hypothesis that these variations originate from the net interaction between facilitation and fatigue was tested. For a constant fast rate, selective effects of faciliation and of steady-stage fatigue were independently shown to shorten and prolong, respectively, FRPN while their combined effects were shown to result in intermediate changes corresponding to the sum of their individual effects. Second, selective and combined effects on FRPN were shown to start for rates corresponding to the upper half of the 1:1 nodal conduction range and to reach their maximums at the fastest rate tested. Third, the time-courses of fatigue-induced prolongations in nodal conduction time and FRPN were shown to be closely linked. Facilitation effects on conduction time and FRPN were confirmed, as previously shown for in situ dog hearts, to be linked, but time-independent. Fourth, FRPN was shown not to correspond to particular limits in its subintervals, but to be, nevertheless, related to nodal refractoriness. Fifth, it was demonstrated that, in conditions of combined facilitation and transient fatigue such as those prevailing in current endocavitary investigations of nodal function, FRPN could be shortened, left unchanged or prolonged by a constant fast rate depending on its duration. In conclusion, the present study demonstrates the dual origin of rate-induced FRPN variations, their rate and time dependence, their relation to changes in nodal refractoriness, and their involvements in various nodal responses.

Atrial refractory periods after atrial premature beats in patients with paroxysmal atrial fibrillation

To study the effects of an atrial premature beat on atrial refractory periods, we investigated 11 patients (group A) who were the control group, 12 patients suffering from paroxysmal atrial fibrillation (group B), and 10 patients (group C) without arrhythmias but with cardiopathy or cardiomyopathy. At every eighth complex of a constant atrial electrostimulated rhythm a fixed premature extrastimulus was introduced, and effective and functional refractory periods (ERP and FRP) were measured in three different sites of the right atrium, before and after introduction of this extrastimulus. Average ERP and FRP shortened respectively in group A, from 220.28 +/- 25.68 msec and 281.17 +/- 28.15 msec before extrastimulation, to 190.58 +/- 22.74 msec and 245.88 +/- 19.86 msec after; in group B, from 219.44 +/- 27.38 msec and 284 +/- 30.06 msec to 191.66 +/- 28.72 msec and 253.23 +/- 34.01 msec; and in group C from 229.03 +/- 29.65 msec and 289.67 +/- 51.62 msec to 194.19 +/- 24.6 msec and 237.74 +/- 39.59 msec. The average dispersions of ERP and FRP in group A were, respectively: 41.81 +/- 21.36 msec and 36.36 +/- 18.04 msec before extrastimulation, 28.18 +/- 18.14 msec and 35.45 +/- 15.72 msec after. In group B: 26.66 +/- 19.46 msec and 41.66 +/- 16.96 msec versus 45.83 +/- 23.91 msec and 45 +/- 34.77 msec and in group C: 27 +/- 11.59 msec and 45 +/- 29.15 msec versus 29 +/- 18.52 and 27 +/- 18.88.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiologic manifestations of the excitable gap of orthodromic atrioventricular reciprocating tachycardia demonstrated by single extrastimulation

To assess the electrophysiologic characteristics of the excitable gap, 12 patients with orthodromic atrioventricular (AV) reciprocating tachycardia were studied. During tachycardia, 8 patients used a left-sided and 4 patients a right-sided anomalous bypass tract for retrograde conduction. QRS complex-synchronized single extrastimuli were delivered from high right atrium, right ventricular apex and coronary sinus, respectively, scanning the whole cycle length of tachycardia. An excitable gap was determined to be present if tachycardia resetting or tachycardia termination occurred. The duration of the excitable gap varied among different pacing sites and occupied 0 to 48% (mean 17 +/- 16) of basic tachycardia cycle length (240 to 480 ms, mean 327 +/- 70). Three patterns of tachycardia resetting were observed: the sum of coupling interval and return cycle being (1) less than a fully compensatory pause in 12 of 12 patients, (2) more than a fully compensatory pause in 5 of 12 patients and (3) equal to a fully compensatory pause in 2 of 12 patients, depending on extent of AV nodal conduction delay exhibited in return cycle. Tachycardia termination was possible when extrastimuli were delivered from right ventricular apex and coronary sinus but not from high right atrium, and only when basic tachycardia cycle length was greater than or equal to 290 ms in 7 of 12 patients. Tachycardia termination was accounted for by development of orthodromic conduction block in AV node in 7 of 7 patients and in bypass tract in 2 of 7 patients. Therefore, site of extra-stimulation and basic tachycardia cycle length affect electrophysiologic manifestations of excitable gap. Further, functional properties of the AV node influence patterns of tachycardia resetting and are primarily responsible for tachycardia termination during programmed single extrastimulation.

Effect of disopyramide on initiation of atrial fibrillation and relation to effective refractory period

Electrophysiologic studies were performed before and after intravenous administration of disopyramide (2 mg/kg) to 40 patients with either documented or suspected atrial fibrillation (AF). In control studies, sustained AF (greater than 1 minute), nonsustained AF (1 to 60 seconds) and no AF were found in 14, 18 and 8 patients, respectively. After disopyramide, the ability to initiate AF was totally lost in 18 patients (group A), while 22 patients (group B) showed sustained AF (11 patients) or nonsustained AF (11 patients). The effective refractory period of the atrium was 232 +/- 41 ms in the control study and 266 +/- 49 ms after disopyramide. Atrial functional refractory periods before and after disopyramide were 282 +/- 43 and 317 +/- 48 ms, respectively. The differences and ratios of the refractory periods after and before disopyramide were higher in group A than in group B (p less than 0.001). The prolongation of atrial refractoriness after disopyramide was important to suppress the initiation of AF in group A. In some group B patients, on the other hand, the initiation of AF was promoted after disopyramide. Disopyramide may alter the atrial electrophysiologic substrate required for AF initiation.

Electrophysiological study of young patients with exercise related paroxysms of palpitation: role of atropine and isoprenaline for initiation of supraventricular tachycardia

Electrophysiological studies were performed in eight patients (four men and four women, mean (SD) age 24 (5) years with paroxysmal attacks of palpitation during or immediately after exercise. Five patients were competitive athletes at college. In two patients spontaneous supraventricular tachycardia during exercise was recorded by ambulatory electrocardiographic monitoring and in another it was induced by treadmill exercise testing. Two had dual atrioventricular nodal pathways, three had manifest atrioventricular accessory pathways, and three had concealed atrioventricular pathways. Programmed stimulation induced sustained supraventricular tachycardia in six patients--in two after intravenous injection of atropine sulphate (1 mg) and in four during infusion of isoprenaline (0.01 microgram/kg/min). In one patient, non-sustained atrioventricular nodal reentrant tachycardia was induced during isoprenaline infusion. In the remaining patient, who had dual atrioventricular nodal pathways, tachycardia was not inducible. AH block prevented maintenance of reentry in five patients. In five patients shortening of the effective refractory period of the atrioventricular node with atropine (one patient) and isoprenaline (four patients) caused sustained supraventricular tachycardia. The present study indicates that treatment with atropine and isoprenaline may be an important factor in the initiation of supraventricular tachycardia in patients with exercise related paroxysms of palpitation.

Bundle branch block during orthodromic reciprocating tachycardia onset in infants

Transesophageal electrophysiologic studies were performed in 58 infants (age less than or equal to 1 year, median 10 days) with electrocardiographically documented orthodromic reciprocating tachycardia (ORT). The aim was to evaluate the occurrence, type and electrophysiologic effects of bundle branch block (BBB) during ORT onset. Of the 58 infants, 25 (43%) had BBB with pacing-induced tachycardia onset. BBB was initiated by single or double premature atrial extrastimuli and by burst atrial pacing; 4 infants also demonstrated BBB with spontaneous ORT onset during transesophageal study. Two of 25 infants had BBB only after intravenous procainamide. Comparison of the 25 infants exhibiting BBB at ORT onset with the 33 infants not demonstrating BBB revealed that age was not statistically different in the 2 groups, but that severity of illness (based on a 1 to 3 scale) was greater (p less than 0.05) and normal QRS ORT cycle length was shorter (p less than 0.02) in the infants with BBB. Of the 25 infants with BBB at ORT onset, 17 had left BBB, 3 had right BBB and 5 had both left and right BBB. Ventriculoatrial interval or cycle length increases during ORT with BBB in 16 of 25 (64%) infants suggested left free wall-accessory atrioventricular connections.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of abrupt cycle length changes on the refractoriness of accessory pathway, His-Purkinje system, atrial and ventricular myocardium in Wolff-Parkinson-White syndrome

We compared the response of the accessory pathway (AP), the atrial myocardium, the His-Purkinje system (HPS) and the ventricular myocardium during steady state (constant cycle length) and following an abrupt alteration in cycle length in 23 patients with Wolff-Parkinson-White syndrome. The durations of the anterograde and retrograde refractory periods were measured during constant drive cycle lengths of 600 and 400 ms (Method I) and during an abrupt change in cycle length of either short-to-long (400 to 600 ms) (Method II) or long-to-short (600 to 400 ms) (Method III) just before the extra stimulus. The mean durations of the anterograde effective refractory periods of the APs were 295 +/- 43, 243 +/- 39 and 273 +/- 37 ms at 600, 400 and 400 to 600 ms cycle lengths, respectively. For the atrial effective refractory periods at the three cycles, they were 238 +/- 18, 217 +/- 11 and 241 +/- 17 ms, respectively. During ventricular stimulation, the mean durations of the retrograde effective refractory periods of the APs were 263 +/- 25, 245 +/- 19 and 253 +/- 21 ms at cycle lengths of 600, 400 and 400 to 600 ms, respectively. For the relative refractory periods of the HPS, they were 335 +/- 29, 239 +/- 23 and 367 +/- 38 ms, respectively and, for the effective refractory periods of the ventricular myocardium, they were 227 +/- 17, 206 +/- 15 and 215 +/- 18 ms, respectively. The retrograde effective refractory period of the HPS could be measured in only five patients at the three cycles (600, 400 and 400 to 600 ms) and the mean values were 265 +/- 57, 225 +/- 14 and 305 +/- 27 ms, respectively. With Method III, AP and ventricular myocardium responded in a cumulative manner while HPS demonstrated paradoxical effect. Compared to Method I, changes with Methods II and III were statistically significant for all variables measured. During all three cycles, the retrograde effective refractory period of the HPS exceeded the effective refractory period of the AP; and the HPS demonstrated progressive conduction delay while the AP responded to no or minimal delays when the V1V2 intervals were similar. An abrupt cycle length change of the short-to-long type facilitated the induction of orthodromic tachycardia during ventricular pacing.(ABSTRACT TRUNCATED AT 400 WORDS)

The maximum effect of an increase in rate on human ventricular refractoriness

The purpose of this study was to determine the maximum shortening of ventricular refractoriness that occurs following an increase in rate and to quantitate the duration of ventricular pacing required to obtain this maximum shortening of refractoriness. The subjects of the study consisted of 41 patients who underwent a clinically indicated electrophysiologic study. Ventricular refractory periods were measured with an extrastimulus (S2) at basic cycle lengths of 600 and 400 ms by Method A (8 beat basic drive trains and 4 second intertrain paue and Method B (drive train duration of 3 minutes, then an S2 after every eighth basic drive beat, with no pause after the S2). In 23 subjects, the mean ventricular effective refractory period determined by Method B was 12 +/- 7 ms (+/- standard deviation) shorter than when determined by Method A at a basic drive cycle length of 600 ms (p less than 0.0001) and 33 +/- 9 ms shorter at a basic drive cycle length of 400 ms (p less than 0.001). In these 23 subjects, the drive train duration required for maximum shortening of ventricular refractoriness was estimated by counting the number of drive train beats preceding ventricular capture by an S2 inserted after every fourth basic drive beat at a coupling interval fixed at 5 ms longer than the ventricular effective refractory period determined in that subject by Method B. The mean number of basic drive beats preceding capture by S2 was 114 +/- 84 beats at a basic drive cycle length of 600 ms and 233 +/- 85 beats at a BDCL of 400 ms. In six subjects the ventricular effective refractory period was measured by Methods A and B before and after autonomic blockade with propranolol and atropine, and the amount of shortening in the ventricular effective refractory period with Method B was not affected by autonomic blockade. In conclusion, the basic drive train has a cumulative effect on ventricular refractoriness in humans, and a drive train duration substantially longer than 50 beats often is required to obtain the maximum shortening of ventricular effective refractory period after an increase in rate. Therefore, ventricular effective refractory periods determined conventionally using 8 beat drive trains and a 4 second intertrain pause often may be overestimates of the actual ventricular effective refractory period. The shortening of ventricular refractoriness with long drive train durations is probably related to a prolonged duration of pacing required to obtain a steady-state action potential duration after an increase in rate.

Effect of pacing cycle length and autonomic blockade on sinus node refractoriness

Sinus node (SN) refractoriness can be measured indirectly by observing the return responses after the introduction of progressively earlier atrial premature beats. The SN effective refractory period (ERP) is defined as the longest premature interval resulting in an interpolated atrial return response. In the present study, SNERP was analyzed in 71 subjects--51 control persons and 20 patients with evidence of SN dysfunction. SNERP could be measured in 40 of 51 control subjects and was shown to prolong at shorter basic pacing cycle lengths. At a basic cycle length of 600 ms, SNERP was 330 +/- 40 ms, whereas at 500 ms it was 350 +/- 50 ms (p less than 0.05). At a basic cycle length of 600 ms, SNERP was measured in 31 control subjects and 7 patients with SN dysfunction. The values of 330 +/- 40 and 520 +/- 20 ms, respectively, in these 2 groups suggested that this method can be used to differentiate patients with SN dysfunction (p less than 0.001). In 12 control subjects, SNERP was measured before and after partial autonomic blockade with propranolol and atropine. SNERP shortened from 360 +/- 40 to 320 +/- 40 ms (p less than 0.05). It shortened with atropine and prolonged with propranolol. Thus, SNERP prolongs with a shorter basic pacing cycle length and is affected by autonomic manipulation, in a fashion analogous to the atrioventricular node.

Atrioventricular nodal conduction and refractoriness following abrupt changes in cycle length

The properties of the atrioventricular (AV) nodal conduction and effective refractory period in man are generally evaluated at a constant basic cycle length (CL) and, in most cases, they demonstrate an inverse relationship to the drive cycle. The response of AV node to abrupt change in CL is less defined. We therefore studied the effects of abrupt changes in CL on AV nodal conduction time and refractoriness in 18 patients. AV nodal conduction time, and effective and functional refractory periods were measured during: (1) a constant long CL, (2) a constant short CL, and (3) after an abrupt increase in CL just prior to the introduction of extrastimuli. In 10 of the 18 patients a constant long CL of 600 ms, a constant short CL of 400 ms and a sudden short-to-long change in CL (400 to 600 ms) were tested. AV nodal conduction times (A2H2) were measured at the shortest and longest comparable A1A2 intervals. The mean value of the shortest A2H2 intervals for constant CL of 600 ms was 144 +/- 18 ms; for a constant CL of 400 ms it was 162 +/- 17 ms; after a sudden short-to-long change in CL (400 to 600 ms) it was 142 +/- 14 ms. The mean value of the longest A2H2 intervals at a constant CL of 600 ms was 185 +/- 18 ms; at a constant CL of 400 ms it was 236 +/- 26 ms (p less than 0.01) and after a short-to-long change in CL (400 to 600 ms) 199 +/- 21 ms. AV nodal effective refractory periods measured at the same three CLs had mean values of 279 +/- 13 ms; 300 +/- 15 ms and 294 +/- 13 ms, respectively. Similar results were obtained when other CLs such as 700 to 900, 500 to 900, and 400 to 700 ms were tested. The data suggest that after abrupt short-to-long changes in CL, AV nodal function curves shift from long constant CL toward short constant CL as the coupling intervals decrease, indicating a cumulative pattern. Although the return to baseline conduction time after the fast basic rate is known to be slow, the limitation of this effect to the very early premature beat in the human has not been reported previously.

Failure of the exercise test to predict the anterograde refractory period of the accessory pathway in Wolff Parkinson White syndrome

Modifications of the delta wave on the surface ECG during an exercise stress test were compared to electrophysiological variations in accessory pathway (AP) refractoriness and in AV node conduction, during intravenous isoproterenol infusion in ten patients with WPW syndrome. In one patient, the delta wave persisted unchanged at the end of exercise and, with isoproterenol, there was a greater reduction in the AP anterograde effective refractory period (AERP) than in AV node conduction time. In three patients, the delta wave became less and less apparent but without completely disappearing; in these patients, the slight reduction of the AERP in the accessory pathway with isoproterenol was comparable to the reduction in AV node conduction time, explaining the progressive fusion between the two activation fronts. In the six other patients, the delta wave completely disappeared during exercise: in two cases, suddenly from one cycle to the next with strong concordance between the measured (isoproterenol) and the estimated (exercise test) AERP in the AP; in four cases, the disappearance was progressive with a significantly greater reduction in the AV node conduction time than in the measured AERP of AP which was nonetheless very short, 190 to 225 ms, during isoproterenol infusion. These findings confirm the limitations of the exercise test to predict the AERP of the AP. In addition, they demonstrate that modifications in the delta wave during exercise result from a balance between the relative effects of sympathetic stimulation on refractoriness of AP and normal AV conduction.

Reappraisal of criteria for assessing drug efficacy in patients with ventricular tachyarrhythmias: complete versus partial suppression of inducible arrhythmias

To test whether increased difficulty in inducing ventricular tachycardia during antiarrhythmic therapy can be considered a sufficient criterion for predicting long-term efficacy of such therapy in patients with ventricular tachyarrhythmias, 95 patients were studied with a graded stimulation protocol (single and double premature stimuli during sinus rhythm and ventricular drives of 120, 140, 160 and 180 beats/min). After a control study, the effects of oral antiarrhythmic drugs on the ability to induce ventricular tachycardia were assessed. The median number of drug trials was four per patient. After antiarrhythmic therapy, four subgroups of patients were identified. In 36 patients, there was no change in inducibility (group 1), whereas in 18 patients ventricular tachycardia was rendered more difficult to induce; that is, a sustained ventricular tachycardia was inducible at a basic drive at least 40 beats/min faster than during the control study (group 2). In 34 patients, ventricular tachycardia induction was suppressed (group 3) and in 7 patients with nonsustained ventricular tachycardia, only 3 to 5 repetitive ventricular responses were induced after treatment (group 4). During follow-up of 15.5 +/- 11.5 months, 10 patients of group 1 had a recurrence of ventricular tachycardia and 6 died suddenly, whereas in group 2 only 1 patient died suddenly and in group 3, 2 patients had a recurrence of ventricular tachycardia (group 1 versus 2 and 3, p less than 0.001, Mantel-Cox and Breslow; group 2 versus 3, no difference). Thus, increased difficulty in inducing ventricular tachycardia is a sufficient criterion for predicting long-term efficacy of an antiarrhythmic drug regimen.

Effect of right ventricular pacing on ventricular rhythm during atrial fibrillation

In 13 patients with atrial fibrillation, the effect of right ventricular pacing at various rates on spontaneous RR intervals was studied. Five hundred consecutive RR intervals were recorded and measured before and during varying right ventricular pacing rates. As anticipated, all RR intervals longer than the right ventricular pacing intervals were abolished. However, RR intervals shorter than the right ventricular pacing intervals were also eliminated. It is difficult to explain the elimination of RR intervals shorter than the pacing intervals with the accepted concepts concerning the mechanisms governing the rate and rhythm of the ventricular response to atrial fibrillation. An alternative explanation may be that during atrial fibrillation the atrioventricular node behaves as a nonprotected pacemaker that is electrotonically modulated by the chaotic atrial electrical activity. The result is a random ventricular rhythm. With right ventricular pacing, the automatic focus is depolarized by the retrogradely concealed conducted ventricular impulses, the short RR intervals are not generated as a consequence and the rhythm becomes pacemaker dependent.

The effects of the novel anti-anginal compound RS 43285 on myocardial conduction in the anaesthetized dog

1. A pentobarbitone-anaesthetized canine model of myocardial conduction was developed to evaluate drug effects on intra-atrial (I-A), intra-ventricular (I-V) and atrioventricular (A-V) conduction parameters, both at rest and during electrical pacing of the right atrium or ventricle. Drug effects on the ability of the sino-atrial (SA) node to re-establish sinus rhythm on switching off electrical pacing were also considered. The effects of the novel anti-anginal compound RS 43285-193 ((+/-)-N-(2,6-dimethyl-phenyl)-4[2-hydroxy-3-(2-methoxyphenoxy)propyl] -1-piperazine acetamide dihydrochloride) were compared to those of the standard anti-anginal compounds nicardipine, nifedipine and verapamil. 2. In the dose range 15-7000 micrograms kg-1, RS 43285 had no significant effects on I-A, I-V or A-V conduction either at rest or during electrical pacing and did not affect the re-establishment of sinus rhythm. 3. Nicardipine had no effects on conduction parameters at resting heart rate. There were no effects on I-A or I-V conduction on electrical pacing but A-V conduction was increased at 200-500 micrograms kg-1 (with a 2:1 A-V conduction block in two out of six dogs); this was accompanied by a prolongation of the interval to reversion of sinus rhythm. 4. Nifedipine had no significant effects on I-A or I-V conduction but significantly prolonged A-V conduction at 1000 micrograms kg-1 and this dose also increased the interval to SA node recovery. 5. Verapamil did not effect I-A or I-V conduction. However, A-V conduction was affected with a significant prolongation occurring at resting heart rate at 100-400 Atg kg-' and a 2:1 A-V block in one dog at rest. During right atrial pacing verapamil significantly increased A-V conduction at 50- 400 fig kg-'. All dogs exhibited a 2:1 A-V conduction block at the highest frequency at 400 jig kg-'.