Autonomic tone of patients during an electrophysiological catheterization. The role of autonomic influences on the reproducibility of sinus node function studies

Electrophysiologic effects of mental stress in healthy subjects: a comparison with epinephrine infusion

Mental stress has been associated with serious cardiac arrhythmias, including ventricular tachycardia and ventricular fibrillation. The purpose of this study was to assess cardiac electrophysiologic effects of mental stress and compare them with those of epinephrine infusion. Ten healthy male volunteers participated. Electrophysiologic and hemodynamic variables were measured at baseline, during mental stress produced by Stroop's color word conflict test and during epinephrine infusion at 2 rates (0.025 micromol/kg/min and 0.3 micromol/kg/min). Mental stress produced significant effects on the electrophysiologic properties of the heart with shortening of all measured electrophysiologic variables except atrial, most markedly those of the sinus and the atrioventricular nodes. The effects on the right ventricular myocardium and the His-Purkinje conduction system were less pronounced. During infusion of epinephrine, corresponding effects could only be reproduced at a much higher plasma level. Circulating epinephrine apparently plays a minor role as a mediator of mental stress effects on the heart.

Reproducibility of electrophysiological measurements in cardiac transplant recipients

The clinical usefulness of certain electrophysiological measurements, particularly those of sinus node function, is limited by variation in autonomic tone resulting in poor reproducibility. The denervated transplanted heart is not susceptible to direct autonomic control and, therefore, electrophysiological measurements may be more reproducible in this group. To our knowledge, this hypothesis has not previously been systematically evaluated. Ten adult recipients underwent serial electrophysiological studies between 10-18 days after cardiac transplantation. Five studies were performed at 2-hour intervals during a single day, between 9:00 a.m. and 5:00 p.m. Spontaneous cycle length (SCL) was recorded. Sinus node recovery time (SNRT), sinoatrial conduction time (SACT), and atrioventricular (AV) Wenckebach cycle length were measured using standard techniques. The effective refractory periods of the complete AV conducting system (AVERP), atrium (AERP), and ventricle (VERP) were measured. Corrected maximal SNRT was normal in all subjects. Mean coefficients of variation (Cv) for SCL, corrected maximal SNRT, and SACT were 2.8%, 7.4%, and 3.5%, respectively. AVERP was less than AERP in seven subjects, limiting further analysis. The mean Cv for AV Wenckebach cycle length was 2.1%. The mean coefficients of variation for AERP were 3.6% and 3.7%, and for VERP 3% and 3.3%, at 600- and 400-ms drive cycle lengths, respectively. Previous studies report much greater variation in innervated subjects particularly of indices of sinus node function. Thus, the reproducibility of electrophysiological measurements of sinus and AV node function in the transplanted heart is better than in normal subjects. This may have important implications for the reliability of electrophysiological testing in transplant recipients.

Effect of stimulus intensity on atrial refractoriness and sinus node recovery

INTRODUCTION

Prior studies of sinus node function in man stated that the stimulus intensity of overdrive pacing has no effect on the response of the sinus node to overdrive suppression; however, data documenting these statements were lacking. Previous studies have also suggested that drive train stimulus intensity can alter ventricular refractoriness, but similar studies have not been performed on the human atrium. The purpose of this study was to evaluate the effects of drive train stimulus intensity on atrial effective refractory period and sinus node recovery time.

METHODS AND RESULTS

The effect of drive train stimulus intensity on atrial effective refractory period and sinus node recovery time was studied in 42 patients undergoing clinical electrophysiologic tests. The atrial effective refractory period was shorter at 10 mA (221 +/- 20 msec) and 5 mA (232 +/- 25 msec) than at a drive train stimulus intensity of 1.5 times late diastolic threshold (248 +/- 24 msec, P < 0.05 for pairwise comparison). The sinus node recovery time did not demonstrate a similar effect in the baseline state, following beta-adrenergic blockade, or following combined parasympathetic and beta-adrenergic blockade. However, following isolated parasympathetic blockade with atropine, the corrected sinus node recovery time shortened from 88 +/- 51 msec at 1.5 times late diastolic threshold to 48 +/- 55 msec at 10 mA (P < 0.05). Significant variability was present in sinus node recovery time measurements at baseline and following beta blockade; this variability decreased following parasympathetic blockade.

CONCLUSION

These data suggest that drive train stimulus intensity can affect the electrophysiologic properties of sinus node and atrial tissue. This effect appears to be mediated by local catecholamine and acetylcholine release and provides further evidence that the interaction between pacing stimuli and the cardiac autonomic system may need to be considered in evaluating electrophysiologic effects.

Time-dependent variation in the cardiac conduction system assessed in young healthy individuals at weeks' interval: implications for clinical trials

The time-dependent physiologic variations of the cardiac conduction system were evaluated at repeated invasive studies in 10 healthy individuals. Their mean age was 28 years (range 22 to 34) and they volunteered to undergo two electrophysiologic studies at intervals of 14 to 63 days (mean 25). The coefficients of variation, repeatability and reproducibility, which should be the preferred statistics when assessing the reproducibility of continuous variables, were calculated. The mean sinus cycle length had a high reproducibility, with coefficients of variation between 2% and 6%. The mean and maximal sinus node recovery times, however, varied considerably. The reproducibility was very high for ventricular depolarization and repolarization (QRS, JT, QT), with coefficients of variation between 2% and 6%. The coefficients of variation were below the acceptable 10% value for intraatrial conduction, atrioventricular (AV) node conduction, His-Purkinje conduction as well as the Wenckebach point; for the effective refractory period of the AV node, it was 12%. Repeat invasive electrophysiologic testing is a safe and reproducible method for evaluating and comparing cardioactive drug effects in healthy subjects. The same statistical analyses were applied to previously published studies on continuous electrophysiologic variables, which allowed comparisons among different groups of healthy and sick persons, as well as among different electrophysiologic variables and procedures. Furthermore, the minimal actual treatment differences that can be detected with a reasonable (80%) probability at a predetermined (5%) significance level using a crossover design were estimated for different electrophysiologic variables. These data will assist in the calculation of the necessary sample size for clinical trials and related purposes.

Clinical and electrophysiologic correlates of sinus node dysfunction after orthotopic heart transplantation. Observations in 42 patients

We assessed incidence and course of postoperative SN dysfunction in 42 cardiac transplant recipients. RHY, HR and CSNRT were compared in distinguishing between persistent (9 of 42 patients) and transient (11 of 42) SN dysfunction. Persistent SN dysfunction was distinguished by a significantly lower HR (day 14: 46.2 +/- 12 vs 67 +/- 19 bpm; day 21: 40.3 +/- 21 vs 70.8 +/- 16 bpm; p less than 0.05, respectively) and by RHY: postoperative AS (3 of 42) was found only in patients whose SN dysfunction persisted while patients with SR but prolonged CSNRT more often had transient impairment (6 of 42 vs 2 of 42); CSNRT, in contrast, is unlikely to be useful in distinguishing between transient and persistent SN dysfunction since exceptionally long CSNRTs (11,340 ms. 12,080 ms) occurred in patients with both types. Ischemic times were significantly longer in patients with transient impairment when compared with the group with normal SN function (155.1 +/- 36 vs 109.2 +/- 36 min, p less than 0.05) but did not differ significantly in patients with persistent SN dysfunction (124.8 +/- 32 min). Thus, (1) SN dysfunction after cardiac transplantation is common but most often transient; (2) RHY and HR are adequate follow-up parameters and best predictors of long-term SN function; (3) SN dysfunction after transplantation has important clinical implications given the unreliability of lower pacemakers to take over in 3 patients; (4) transient SN dysfunction is related to duration of ischemia during hypothermic preservation whereas pathogenesis of persistent SN dysfunction remains to be elucidated.

Reproducibility of arrhythmia induction with intracardiac electrophysiologic testing: patients with clinical sustained ventricular tachyarrhythmias

In order to characterize the day to day reproducibility of arrhythmias provoked during electrophysiologic stimulation, 114 patients with documented sustained clinical ventricular tachyarrhythmias were studied. Two baseline electrophysiologic tests were performed in the drug-free state and within 6 to 24 hours of one another. There was a significant increment (p less than or equal to 0.02) in the induction of sustained ventricular tachyarrhythmias as the number of programmed extrastimuli increased from one (10% induction) to four (64% induction). Provoked arrhythmias were observed to be more frequently nonreproducible (as reflected in a major change in rate or duration, or both, of an induced ventricular arrhythmia between baseline tests) as the number of extrastimuli increased from one (7%) to four (27%). Nonreproducibility with three and four extrastimuli was not significantly greater than when two extrastimuli were utilized. Electrophysiology-directed drug trials should be interpreted in light of this observed variability in induced arrhythmias.

Effect of haemodynamic changes during rapid atrial pacing on determination of sinus node recovery time

Arterial blood pressure was continuously monitored during rapid atrial pacing in 31 patients with different types of heart disease to determine sinus node recovery time and corrected sinus node recovery time. Pacing was initiated at 70 beats/min and increased stepwise to 160 beats/min. One to one atrioventricular conduction was maintained throughout the one minute stimulation period. Blood pressure fell initially during at least one stimulation period in 21 of our patients and at pacing rates up to 130 beats/min in 18. Once blood pressure had fallen during overdrive pacing maximal sinus node recovery time and maximal corrected sinus node recovery time could not be prolonged by increasing the pacing rate. Sinus node recovery time and corrected sinus node recovery time during the pacing induced fall in blood pressure was significantly shorter than those during stimulation runs with constant blood pressure. No pacing induced fall in blood pressure and no relation between changes in blood pressure and sinus node recovery time were evident in 10 of the 31 patients. Sinus node recovery time is therefore influenced by alterations in autonomic tone due to pacing induced haemodynamic changes.

The electrophysiologic characteristics of the transplanted human heart

The electrophysiologic characteristics of the denervated human heart were assessed in 14 cardiac transplant recipients. Conduction intervals and refractory periods were measured at pacing cycle lengths of 500 msec and 400 msec. The faster pacing rate caused lengthening of the AH interval (83 +/- 23 msec to 116 +/- 41 msec, p less than 0.01) and shortening of the QT (338 +/- 27 msec to 313 +/- 22 msec, p less than 0.001) and JT (249 +/- 21 msec to 229 +/- 19 msec, p less than 0.001) intervals. There was no change in the SA, HV, or QRS durations. Wenckebach periodicity occurred at a longer cycle length in the retrograde than in the anterograde direction (409 +/- 96 msec vs 318 +/- 46 msec, p less than 0.01) and anterograde conduction was better than retrograde conduction in 13 of the 14 patients (93%). Increasing pacing cycle length resulted in shortening of the atrial effective (203 +/- 28 msec to 190 +/- 25 msec, p less than 0.001), ventricular effective (224 +/- 18 msec to 211 +/- 17 msec, p less than 0.01), and AV nodal functional (367 +/- 38 msec to 357 +/- 36 msec, NS) refractory periods. The AV nodal effective refractory period lengthened (294 +/- 31 msec to 314 +/- 52 msec, p less than 0.05). There was a close correlation between AV Wenckebach cycle length and the functional refractory period of the AV node (r = 0.853, p less than 0.001). These results are qualitatively and quantitatively similar to those reported in the innervated heart. The autonomic nervous system appears to have little influence on the resting electrophysiologic characteristics of the atrioventricular conduction system in the innervated heart.

The electropharmacology of acute drug testing using pacemakers

Acute drug testing in patients is useful to select prophylactic treatment for life-threatening or intractable tachycardias. This is generally done by induction of tachycardias with pacing. Acute studies that depend on temporary insertion of pacing electrodes do not determine efficacy in the same sense as longer term clinical drug trials because of the biased population referred for testing with pacemakers. However, the pharmacologic activity of compounds can be tested in terms of electrical functions such as conductivity and refractoriness not merely of the heart in general, but also of the arrhythmogenic focus. Such data can be directly applied to patients with similar arrhythmias, obviating the confusion often caused by interspecies and disease differences.

Spontaneous termination of circus movement tachycardia using an atrioventricular accessory pathway: incidence, site of block and mechanisms

The incidence, mechanisms and sites of block of spontaneous termination of circus movement tachycardia (CMT) using an atrioventricular accessory pathway (AP) were analyzed in 24 consecutive patients (17 with Wolff-Parkinson-White syndrome and seven with a concealed AP) who were not receiving antiarrhythmic drugs. Spontaneous termination of tachycardia occurred in 10 patients (105 episodes). A reduced "safety margin" of tachycardia was the only factor that was significantly more common in the patients who manifested spontaneous termination (p less than 0.01). The site of spontaneous block was located in the AP in six patients (50 episodes), atrioventricular node (AVN) in six patients (37 episodes) and His-Purkinje system (HPS) in three patients (18 episodes). At least 14 mechanisms leading to block in the tachycardia circuit were identified. Labile conduction during tachycardia occurred at multiple sites (AVN, His bundle, bundle branches, and AP). Analysis of the duration of tachycardia before spontaneous termination showed a characteristic time pattern for block at each site, consistent with the autonomic and electrophysiologic changes that occur after induction of tachycardia. Spontaneous termination of CMT using an AP is a common phenomenon. Many mechanisms are involved, which are often complex and dependent on interplay of the electrophysiologic characteristics of the components of the tachycardia circuit.