Human ventricular refractoriness. II. Effects of procainamide

Amiodarone or procainamide for the termination of sustained stable ventricular tachycardia: an historical multicenter comparison

OBJECTIVES

The objective was to compare the effectiveness of intravenous (IV) procainamide and amiodarone for the termination of spontaneous stable sustained ventricular tachycardia (VT).

METHODS

A historical cohort study of consecutive adult patients with stable sustained VT treated with IV amiodarone or procainamide was performed at four urban hospitals. Patients were identified for enrollment by admissions for VT and treatment with the study agents in the emergency department (ED) from 1993 to 2008. The primary measured outcome was VT termination within 20 minutes of onset of study medicine infusion. A secondary effectiveness outcome was the ultimate need for electrical therapy to terminate the VT episode. Major adverse effects were tabulated, and blood pressure responses to medication infusions were compared.

RESULTS

There were 97 infusions of amiodarone or procainamide in 90 patients with VT, but the primary outcome was unknown after 14 infusions due to administration of another antidysrhythmic during the 20-minute observation period. The rates of VT termination were 25% (13/53) and 30% (9/30) for amiodarone and procainamide, respectively. The adjusted odds of termination with procainamide compared to amiodarone was 1.2 (95% confidence interval [CI]=0.4 to 3.9). Ultimately, 35/66 amiodarone patients (53%, 95% CI=40 to 65%) and 13/31 procainamide patients (42%, 95% CI=25 to 61%) required electrical therapy for VT termination. Hypotension led to cessation of medicine infusion or immediate direct current cardioversion (DCCV) in 4/66 (6%, 95% CI=2 to 15%) and 6/31 (19%, 95% CI=7 to 37%) patients who received amiodarone and procainamide, respectively.

CONCLUSIONS

Procainamide was not more effective than amiodarone for the termination of sustained VT, but the ability to detect a significant difference was limited by the study design and potential confounding. As used in practice, both agents were relatively ineffective and associated with clinically important proportions of patients with decreased blood pressure.

Accelerated AV nodal conduction with use of procainamide in atrial fibrillation

BACKGROUND

Atrial fibrillation is a common dysrhythmia seen in the emergency department (ED). Chemical or electrical cardioversion may be performed on patients who have had atrial fibrillation for < 48 h duration and who are at low risk for thromboembolic events. Multiple studies suggest that intravenous procainamide is an appropriate agent in the treatment of acute atrial fibrillation due to its relatively low risk profile and high conversion rate.

OBJECTIVES

A case is presented that demonstrates an adverse reaction to the use of intravenous procainamide for chemical cardioversion of atrial fibrillation in an otherwise hemodynamically stable patient.

CASE REPORT

We report a case of lone paroxysmal atrial fibrillation in a patient with a structurally normal heart who suffered paradoxical accelerated atrioventricular nodal conduction and secondary hypotension in response to procainamide administration.

CONCLUSION

When administering procainamide for chemical cardioversion of atrial fibrillation, a low threshold should be maintained for administration of a complementary rate-controlling agent, and facilities for immediate electrical cardioversion always must be available.

Rate dependent effects of procainamide on the threshold current for pacing in the setting of postrepolarization refractoriness in dogs

Normally, ventricular APD exceeds the VERP. However, under specific circumstances this relation may change and can become inverse. This phenomenon of postrepolarization refractoriness may be caused by a decrease in excitability. The threshold current (TC) for pacing has never been quantified as a possible explanation for these observations. Using a MAP pacing catheter in the right ventricular apex, the rate dependent behavior of TC, VERP, and APD before and after procainamide (dose 20 mg/kg in 10 min + 5 mg/min infusion) was determined in 17 dogs with chronic complete AV block. Initially, TC was determined with 0.1 mA accuracy. Using a pacing current of at least twice TC, VERP and APD showed a similar, rate dependent shortening for PCLs 800, 575, and 350 ms. Procainamide treatment led to an equal, rate independent VERP and APD increase: no post repolarization refractoriness. Subsequently, accuracy for TC determination was increased to 0.01 mA. Comparing PCLs 800 and 250 ms, TC doubled from 0.05 +/- 0.01 to 0.10 +/- 0.09 mA during control and almost tripled from 0.06 +/- 0.02 to 0.17 +/- 0.10 mA (P < 0.05) after procainamide. Using a fixed pacing current of exactly twice TC found at 800 ms PCL during control, VERP exceeded APD after procainamide treatment at 300 and 250 ms PCL: postrepolarization refractoriness. Increasing the pacing current to twice the rate dependent TC, the relation between VERP and APD normalized: no postrepolarization refractoriness. We conclude that after procainamide, rate dependent TC increase is of major importance for the phenomenon of postrepolarization refractoriness.

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)

Promotion of ventricular tachycardia induction by procainamide in dogs with inducible ventricular fibrillation late after myocardial infarction

The influence of procainamide on inducible ventricular tachyarrhythmias was evaluated in 35 dogs with experimental myocardial infarction, and 9 normal dogs. Programmed stimulation was performed from the right ventricular apex via a percutaneously positioned electrode catheter, using up to five extrastimuli before and after intravenous administration of procainamide (15 mg/kg). Procainamide levels in postinfarct dogs were 8.5 +/- 0.7 micrograms/mL (range 5.3-13.6 micrograms/mL). Procainamide exerted its greatest effect in postinfarct dogs with reproducible baseline ventricular fibrillation. Six of nine dogs (P less than 0.05) with ventricular fibrillation had sustained monomorphic ventricular tachycardia (cycle length: 147 +/- 4 msec) induced after procainamide administration. This ventricular tachycardia required significantly more extrastimuli than baseline ventricular fibrillation (3 +/- 0.3 extrastimuli before vs 4 +/- 0.3 extrastimuli after procainamide). Procainamide never converted ventricular fibrillation to ventricular tachycardia in normal dogs. Procainamide had minimal effect on inducible ventricular tachycardia after myocardial infarction. Ventricular tachycardia induction was abolished in only 2 of 17 dogs despite significant prolongation of electrophysiological parameters. Ventricular tachycardia cycle length, and the number of extrastimuli required were unchanged by procainamide in this subgroup.

CONCLUSION

Ventricular tachycardia is insensitive to the antiarrhythmic properties of procainamide in this model. In contrast, procainamide is able to convert postinfarction ventricular fibrillation to ventricular tachycardia, presumably by promoting sustained, organized reentry. This previously undescribed action is an unusual form of proarrhythmic effect, and suggests that this drug should be used cautiously in patients after myocardial infarction.

The effect of procainamide on plasma cholinesterase activity

The in vitro effect of procainamide on plasma cholinesterase (PCHE) activity in the plasma of ten normal ASA physical status I patients was studied using a kinetic method. The mean plasma cholinesterase activity without procainamide (control) was 0.90 +/- 0.09 units.ml-1. The dibucaine numbers of all the samples were in the normal range of 78 to 86, indicating normal genotypes. The mean plasma cholinesterase activity, in the presence of procainamide in concentrations of 5.0, 10.0, 20.0 and 40.0 micrograms.ml-1, was reduced to 0.73 +/- 0.04, 0.61 +/- 0.03, 0.45 +/- 0.02, and 0.36 +/- 0.01 units.ml-1, respectively. At therapeutic concentrations of 4 to 12 micrograms.ml-1, procainamide inhibited cholinesterase activity 15 to 30 per cent. The authors also showed that the concentration of procainamide required to inhibit 50 per cent of plasma cholinesterase activity was 20 micrograms.ml-1 (I50). The authors conclude that procainamide when tested in vitro had a statistically significant depressant effect on plasma cholinesterase activity at all the concentrations studied.

Effects of intravenous N-acetylprocainamide on hemodynamics and left ventricular function in man

N-acetylprocainamide (NAPA) is the active metabolite of procainamide currently undergoing evaluation for its antiarrhythmic properties. Its effects on hemodynamic variables and on ventricular function in man are poorly defined. The effects of intravenously administered NAPA (18 mg/kg over 30 minutes; mean plasma level 40.2 +/- 6.2 micrograms/ml) on hemodynamics and left ventricular ejection fraction (LVEF) assessed by radionuclide ventriculography were therefore determined in 14 patients undergoing diagnostic cardiac catheterization. The peak effects of the drug with respect to most measured variables were seen at 30 minutes and were still apparent at 60 minutes. NAPA increased heart rate (3% at 10 minutes; p less than 0.01), decreased mean pulmonary arterial pressure (14%; p less than 0.05) and capillary wedge pressure (27%; p less than 0.01), decreased mean arterial pressure (12%; p less than 0.01), cardiac index (8%; p less than 0.01), LV dp/dtmax (9%; p less than 0.05), and stroke work index (17%; p less than 0.01). The LVEF increased 5% (p less than 0.05). There was a trend for the systemic vascular resistance to decrease, but this did not reach statistical significance. The data show that NAPA exerts relatively weak peripheral arteriolar and venodilator effects associated with a mild reduction in contractility and cardiac output but an increase in LVEF in patients with preserved ventricular function.

Effects of quinidine versus procainamide on the QT interval

Eighteen patients were given quinidine and procainamide separately to evaluate whether prolongation of the QT interval by type Ia antiarrhythmic agents is a drug-specific phenomenon. Doses were titrated to achieve standard trough therapeutic levels of quinidine (2 to 5 micrograms/ml) and procainamide (4 to 12 micrograms/ml). In 16 of the 18 patients, the increase in corrected QT interval (QTc) was greater with quinidine than with procainamide, averaging 78 +/- 10 ms (+/- standard error of the mean) with quinidine and 39 +/- 7 ms with procainamide (p less than 0.001). The greater degree of QTc prolongation with quinidine than with procainamide was not due to differences in sinus cycle length, QRS duration, serum potassium level or concomitant drug therapy. Differences in relative drug level did not appear to account for the greater effect of quinidine. Thus, at frequently used plasma levels, quinidine prolongs QTc to a greater degree than does procainamide. This effect does not appear to be due to the comparison of "nonequivalent" drug levels.

Changes in ventricular refractoriness after an extrastimulus: effects of prematurity, cycle length and procainamide

This study was performed to determine the ability of extrastimuli to change ventricular refractoriness. We prospectively evaluated the effects of stimulus prematurity and paced cycle length (PCL) in 30 patients and the effect of procainamide in 8 patients on changes in the ventricular effective refractory period (ERP) after a right ventricular extrastimulus (S2). An S2 was introduced at preselected coupling intervals at a PCL (S1-S1) of 600 and 400 ms. At each S1-S2 interval, a second extrastimulus (S3) was introduced in 5-ms decrements and the ERP of S2 measured. The decrease in the ERP after an S2 was directly related to prematurity and most of the shortening occurred over a narrow range of S1-S2 intervals. At a PCL of 600 ms, the ERP of S2 at S1-S2 intervals less than or equal to 400 ms was significantly shorter than the ERP of S1 (maximal shortening 23%). At a PCL of 400 ms, the ERP of S2 at S1-S2 intervals less than or equal to 350 ms was significantly shorter than the ERP of S1 (maximal shortening 25%). The ERP of S2 at the shortest S1-S2 interval was greater with a PCL of 600 ms than with 400 ms (200 +/- 31 versus 180 +/- 26 ms, p less than 0.001). However, the total shortening in ERP (ERPS1 - ERPS2 at shortest S1-S2 interval) was similar at both PCLs (55 +/- 14 versus 59 +/- 13 ms). Procainamide significantly prolonged the ERP of S2 at each S1-S2 interval.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of procainamide on dispersion of ventricular refractoriness

Ventricular arrhythmias after Q-T prolongation by drugs could result from a nonhomogeneous increase in refractoriness (dispersion). Dispersion of effective refractory periods (ERP) was measured before and after infusion of 1 g of procainamide using twice-threshold extrastimuli applied in sinus rhythm and with 500 ms ventricular drive cycle length at 3 right ventricular sites (2 patients) or 2 right and 1 left ventricular site (10 patients). Procainamide prolonged ERP. In drive, average ERP was 247 +/- 5 ms (standard error of the mean) before and 277 +/- 7 ms after procainamide (p less than 0.001). The Q-T interval was prolonged by 50 ms in drive (p less than 0.001), but Q-T prolongation did not reflect the increased ERP (r =-0.05). However, procainamide did not alter measured dispersion (54 +/- 16 to 44 +/- 14 ms in sinus, 48 +/- 14 to 47 +/- 13 ms in drive). Polymorphic ventricular tachycardia (VT) was induced in 6 patients in whom drive itself generally failed to reduce dispersion, and failure to induce tachycardia or shorter runs after procainamide was associated with narrowed dispersion. Polymorphic VT was not induced after procainamide in 2 patients with clinical episodes of torsades de pointes caused by type I agents. The mechanism of torsades de pointes was not explained by dispersion of refractoriness or by polymorphic VT initiated by premature beats after a type I drug.

Electrophysiologic properties and antiarrhythmic mechanisms of intravenous N-acetylprocainamide in patients with ventricular dysrhythmias

To define electrophysiologic properties and antiarrhythmic mechanisms of N-acetylprocainamide (NAPA), we studied 16 patients with symptomatic ventricular dysrhythmias. Electrophysiologic studies were performed before and after intravenous infusion of NAPA at 20 mg/kg over 20 minutes, achieving plasma concentrations of 24 +/- 3.2 to 35.5 +/- 4.5 micrograms/ml. NAPA did not significantly change sinus cycle length or atrioventricular (AV) conduction times (PA, AH, HV, and QRS), but it lengthened the QTc interval (p less than 0.001) during sinus rhythm. Programmed atrial stimulation revealed that NAPA had no discernible effects on AV nodal conduction; however, it exerted depressive effects on the His-Purkinje system in 9 of 16 patients. In 7 of 16 patients who manifested frequent ventricular premature beats (VPBs), NAPA abolished VPBs in only three of them; NAPA induced progressive prolongation of the premature coupling interval before complete abolition of VPBs. In 8 of 16 patients who had inducible repetitive ventricular response (RVR) because of reentry within the His-Purkinje system, NAPA narrowed or abolished the RVR zone in 3 patients and slowed the RVR rate with widening of the RVR zone in the remaining 5 patients. In 2 of 16 patients with slow ventricular tachycardia (VT), NAPA had no antiarrhythmic effects. By contrast, in the other 2 of 16 patients in whom sustained VT could be reproducibly elicited with programmed ventricular stimulation, NAPA slowed the rate of VT and suppressed VT inducibility. We conclude that electrophysiologic properties of NAPA are slightly different from those of procainamide and that NAPA is not uniformly effective for suppressing ventricular dysrhythmias, but its antiarrhythmic mechanisms are similar to those of procainamide.

Termination of ventricular tachycardia: role of tachycardia cycle length

The mode of termination of ventricular tachycardia (VT) and its relation to tachycardia cycle length was evaluated in 139 patients. Tachycardia was terminated by programmed stimulation in 110 patients (79%) and cardioversion was required in 29 patients (21%). Single, double, and triple ventricular extrastimuli terminated the tachycardia in 23 of 85 (27%), 39 of 62 (63%), and 7 of 16 patients (44%), respectively. In all patients requiring 1 extrastimulus, in 35 patients (90%) requiring 2 extrastimuli, and in 6 patients (86%) requiring 3 extrastimuli, the tachycardia cycle length exceeded 300 ms. Rapid ventricular pacing terminated tachycardia in 41 of 54 patients (76%). In 21 (51%) of these patients the tachycardia cycle length exceeded 300 ms. However, rapid ventricular pacing caused acceleration of the arrhythmia in 19 patients (35%). The ability of procainamide to modify the termination of VT was studied in 23 patients. In 7 of these patients (30%) procainamide increased the tachycardia cycle length by 49 +/- 42 ms (p less than 0.01) and did not modify the mode of termination. In 6 patients (26%) procainamide increased cycle length by 142 +/- 108 ms (p less than 0.01), but termination was more difficult. In 10 patients (44%) procainamide increased the cycle length by 138 +/- 110 ms (p less than 0.001) and termination was easier. We conclude that termination of VT by timed extrastimuli requires a tachycardia cycle length longer than 300 ms. Rapid pacing or cardioversion is usually required when the cycle length is less than 300 ms. Although procainamide slows tachycardia, it can unpredictably make termination more difficult in 1 of 4 patients.

Procainamide in the induction and perpetuation of ventricular tachycardia in man

The effects of a single intravenous infusion of 750 mg of procainamide was studied in 12 patients with symptomatic chronic recurrent ventricular tachycardia in whom arrhythmias could reproducibly be initiated and terminated by programmed electrical stimulation of the heart. Sustained ventricular tachycardia was induced in 6 patients and non-sustained tachycardia was induced in the remaining 6 patients during control studies. Following procainamide (plasma level 10.3 +/- 3.7 mcg/ml), ventricular tachycardia could be induced in 10/12 patients, sustained in 4 patients and non-sustained in the remaining 6 patients. In 8/12 patients (66%), induction of ventricular tachycardia was facilitated as demonstrated by: (1) tachycardia zone was widened in 4 patients and was unchanged in another 3 patients; (2) non-sustained ventricular tachycardia was sustained ventricular tachycardia in one patient. the ventricular tachycardia had a faster rate and a different QRS morphology; (3) in 4 patients tachycardia was inducible with a lesser number of extrastimuli and/or by spontaneously occurring ventricular premature depolarization and; (4) increase of the number of induced ventricular responses of non-sustained ventricular tachycardia. In 4/12 patients (33%), procainamide abolished or modified the induction of ventricular tachycardia as demonstrated by: (1) inability to induce ventricular tachycardia in 2 patients; (2) narrowing of the tachycardia zone and conversion from sustained into non-sustained ventricular tachycardia (one patient) and; (3) decrease in the number of induced ventricular responses in one patient. The response to procainamide could not be predicted from rates of spontaneous ventricular tachycardia, induced ventricular tachycardia during control studies, degree of slowing of ventricular tachycardia or from prolongation of the coupling interval after procainamide. These results suggest that instead of abolishing the arrhythmia, procainamide in frequently employed doses in patients with chronic recurrent ventricular tachycardia can facilitate its initiation sometimes at even faster rates. Patients not responsive to the usual doses of procainamide should undergo acute drug trials to determine the optimal dose/drug levels.

Electrophysiologic effects of N-acetylprocainamide in human beings

The electrophysiologic properties of N-acetylprocainamide (NAPA) were studied in 10 patients undergoing cardiac catheterization. Each patient received two successive intravenous infusions: one loading infusion over 15 minutes and one maintenance infusion at a slower rate for 30 minutes. Eight patients received 10.5 mg/kg body weight and two received larger doses (16 and 21 mg/kg, respectively). NAPA plasma concentration was measured at 5 minute intervals from 0 to 25 minutes, and then at 15 and 30 minutes of the second infusion. Mean blood pressure and electrophysiologic data obtained by programmed stimulation were recorded before drug administration and at 15 and 30 minutes of the infusion when the concentration of NAPA was nearly constant in each patient (range 12 to 35 microgram/ml). NAPA decreased blood pressure (p less than 0.005), increased corrected Q-T interval (p less than 0.01) and increased the atrial and ventricular effective refractory periods from 267 +/- 40 to 307 +/- 41 ms (p less than 0.01) and from 278 +/- 37 to 301 +/- 32.8 ms (p less than 0.05), respectively. NAPA did not significantly change sinus cycle length or sinus nodal recovery time, conduction intervals (A-H, H-V, P-R, QRS), atrioventricular nodal functional refractory period or nodal Wenckebach cycle length. The patient receiving the largest dose experienced mild nausea when the plasma concentration was above 35 microgram/ml. These data show that the electrophysiology of NAPA in human beings is different from that reported for procainamide. At the plasma concentrations studied NAPA increases atrial and ventricular refractory periods without increasing cardiac conduction times

Human ventricular refractoriness: effects of increasing current

The ventricular effective refractory period is commonly employed as a measurement of ventricular excitability. Because the current strength used to make this determination varies among laboratories, the relation of refractoriness and current was examined over a range of current strengths from 0.1 to 10 mA. Sixty determinations of refractoriness at variable current strengths were made in 40 patients using the extrastimulus technique with a rectangular pulse of 1 ms duration. These data were obtained by measuring the effective refractory period at threshold current and at 0.25 to 0.50 mA increments from threshold up to 10 mA. In these studies the drive stimulus (S1) and extrastimulus (S2) were kept at the same amplitude. In all patients the ventricular effective refractory period decreased as the current increased. The total decrease ranged from 8 to 100 ms (mean +/- standard deviation 36.9 +/- 17.1). The current strength at which the ventricular effective refractory period became fixed (that is, less than 2 ms change in ventricular effective refractory period with further increase in current strength) varied among the patients, but in all instances equaled or exceeded 1.8 mA, which in all but three patients was greater than three times threshold. The curves relating current strength and refractoriness were shifted to the left at shorter cycle lengths with no change in threshold. These data suggest that (1) current strength-effective refractory period curves more completely characterize ventricular excitability than does a ventricular effective refractory period at single current strength; and (2) studies of drug effects, alterations of autonomic tone, or reentrant arrhythmias, which may affect or are affected by ventricular refractoriness, may be enhanced by more complete measurements of refractoriness afforded by the current strength-effective refractoriness curves.

Strength-interval relation in the human ventricle: effect of procainamide

The effects of procainamide on strength-interval relations were evaluated in 18 patients. At plasma concentrations of 4.3 to 13.6 micrograms/ml procainamide had minimal effects on threshold current in late diastole, but in early diastole it shifted the strength-interval curve to the right. The basic strength-interval relation (that is, decreasing refractory period as current is increased) was not altered. The control refractory period decreased by a mean of 44 ms as the current was increased from threshold to 10 mA, whereas a mean decrease of 42 ms was observed after procainamide. However, the steep portion of the strength-interval curve(absolute refractory period) was shifted to longer coupling intervals by a mean value of 24 ms. These findings suggest that procainamide may primarily affect active membrane properties, but exert little net effect on passive membrane properties late in diastole.

Mechanism of the repetitive ventricular response in man

The electrophysiologic characteristics of the repetitive ventricular response that followed an electrically induced single premature ventricular complex were evaluated to determine its mechanism during atrial pacing or sinus rhythm in 30 patients. Seven patients had preexisting bundle branch block. His bundle or right bundle branch deflections did not precede the repetitive complex in 29 of the 30 patients, which implies that the proximal His-Purkinje system was not involved in the reentry circuit. In 24 of 30 patients the QRS axis ot the repetitive complex was divergent 45 degrees or more from the stimulated complex. In 22 of 30 patients the reprtitive complex had a right bundle branch block configuration. In 14 of 18 patients with two or more repetitive complexes, the QRS pattern changed from beat to beat, which implies that either the reentry pathway or conduction was changing. Thus, the repetitive ventricular response, which can be associated with clinically important ventricular arrhythmias, probably represents intraventricular rather than proximal His-Purkinje system reentry.

Ventricular fibrillation during programmed ventricular stimulation: incidence and clinical implications

Ventricular fibrillation occurred in 10 (3.3 percent) of 300 patients consecutively studied with programmed ventricular stimulation. One hundred twenty-five of these patients were studied with double ventricular extrastimuli including 68 patients with and 57 patients without documented or suspected ventricular tachycardia or fibrillation, or both. Ventricular fibrillation did not develop in response to a single ventricular extrastimulus delivered during sinus rhythm, ventricular pacing or ventricular tachycardia or in response to ventricular pacing at cycle lengths of 300 msec or greater and occurred only in response to double ventricular extrastimuli. All 10 patients who manifested ventricular fibrillation during programmed stimulation were in the group of patients with suspected or documented ventricular tachycardia or fibrillation. Ventricular fibrillation was initiated in seven patients with double ventricular extrastimuli delivered during sinus rhythm or ventricular pacing and in three patients with double ventricular extrastimuli delivered during ventricular tachycardia. Four patients had spontaneous conversion to sinus rhythm and the remainder underwent defibrillation without sequelae. Recurrent ventricular fibrillation occurred clinically in 7 of the 10 patients. This study suggests that ventricular fibrillation occurs uncommonly during programmed ventricular stimulation and only in response to double ventricular extrastimuli in patients in whom spontaneous episodes are likely to occur.

The electrocardiogram in the assessment of the effect of drugs on cardiac arrhythmias

The search for the ideal antiarrhythmic drug continues since none of the available agents offers optimum antiarrhythmic therapy. The continuing search coupled with the interest in the mechanisms of cardiac arrhythmias has led to the development of new techniques for the study of arrhythmias and antiarrhythmic drugs. In this article it is proposed to discuss the electrocardiographic methods used in the assessment of antiarrhythmic drugs. Firstly, to discuss the electrocardiogram in the assessment of the clinical electrophysiological properties of a drug and secondly, the electrocardiogram in the assessment of the value of the drug in the management of cardiac arrhythmias in man.

The repetitive ventricular response in man. A predictor of sudden death

We examined the value of cardiac pacing for assessing ventricular electrical instability and for predicting ventricular tachycardia and sudden death in 50 patients with refractory symptomatic ventricular tachycardia, 12 normal patients, and 48 survivors of a recent myocardial infarction. The repetitive ventricular response (two or more ventricular premature beats produced by a single ventricular pacing stimulus during control of heart rate with atrial pacing) was absent in all 12 normal patients but was present in 44 of the 50 patients (88 per cent) with recurrent ventricular tachycardia (P less than 0.001). Of the 48 survivors of myocardial infarction, 19 had repetitive ventricular responses. During the next 12 months 15 of these patients (79 per cent) had symptomatic ventricular tachycardia or sudden death, or both, as compared with four of 29 patients (14 per cent) who did not have repetitive ventricular responses (P less than 0.001). The repetitive ventricular response identifies patients with life-threatening ventricular instability, but it is still an investigational technic that should be used only with due precautions.