Electrophysiology of normal sinus node with and without autonomic blockade

Age-dependent contribution of intrinsic mechanisms to sinoatrial node function in humans

Average beat interval (BI) and beat interval variability (BIV) are primarily determined by mutual entrainment between the autonomic-nervous system (ANS) and intrinsic mechanisms that govern sinoatrial node (SAN) cell function. While basal heart rate is not affected by age in humans, age-dependent reductions in intrinsic heart rate have been documented even in so-called healthy individuals. The relative contributions of the ANS and intrinsic mechanisms to age-dependent deterioration of SAN function in humans are not clear. We recorded ECG on patients (n = 16 < 21 years and n = 23 41-78 years) in the basal state and after ANS blockade (propranolol and atropine) in the presence of propofol and dexmedetomidine anesthesia. Average BI and BIV were analyzed. A set of BIV features were tested to designated the "signatures" of the ANS and intrinsic mechanisms and also the anesthesia "signature". In young patients, the intrinsic mechanisms and ANS mainly contributed to long- and short-term BIV, respectively. In adults, both ANS and intrinsic mechanisms contributed to short-term BIV, while the latter also contributed to long-term BIV. Furthermore, anesthesia affected ANS function in young patients and both mechanisms in adult. The work also showed that intrinsic mechanism features can be calculated from BIs, without intervention.

Comprehensive evaluation of electrophysiological and 3D structural features of human atrial myocardium with insights on atrial fibrillation maintenance mechanisms

Atrial fibrillation (AF) occurrence and maintenance is associated with progressive remodeling of electrophysiological (repolarization and conduction) and 3D structural (fibrosis, fiber orientations, and wall thickness) features of the human atria. Significant diversity in AF etiology leads to heterogeneous arrhythmogenic electrophysiological and structural substrates within the 3D structure of the human atria. Since current clinical methods have yet to fully resolve the patient-specific arrhythmogenic substrates, mechanism-based AF treatments remain underdeveloped. Here, we review current knowledge from in-vivo, ex-vivo, and in-vitro human heart studies, and discuss how these studies may provide new insights on the synergy of atrial electrophysiological and 3D structural features in AF maintenance. In-vitro studies on surgically acquired human atrial samples provide a great opportunity to study a wide spectrum of AF pathology, including functional changes in single-cell action potentials, ion channels, and gene/protein expression. However, limited size of the samples prevents evaluation of heterogeneous AF substrates and reentrant mechanisms. In contrast, coronary-perfused ex-vivo human hearts can be studied with state-of-the-art functional and structural technologies, such as high-resolution near-infrared optical mapping and contrast-enhanced MRI. These imaging modalities can resolve atrial arrhythmogenic substrates and their role in reentrant mechanisms maintaining AF and validate clinical approaches. Nonetheless, longitudinal studies are not feasible in explanted human hearts. As no approach is perfect, we suggest that combining the strengths of direct human atrial studies with high fidelity approaches available in the laboratory and in realistic patient-specific computer models would elucidate deeper knowledge of AF mechanisms. We propose that a comprehensive translational pipeline from ex-vivo human heart studies to longitudinal clinically relevant AF animal studies and finally to clinical trials is necessary to identify patient-specific arrhythmogenic substrates and develop novel AF treatments.

Cardiac Pacemaker Activity and Aging

A progressive decline in maximum heart rate (mHR) is a fundamental aspect of aging in humans and other mammals. This decrease in mHR is independent of gender, fitness, and lifestyle, affecting in equal measure women and men, athletes and couch potatoes, spinach eaters and fast food enthusiasts. Importantly, the decline in mHR is the major determinant of the age-dependent decline in aerobic capacity that ultimately limits functional independence for many older individuals. The gradual reduction in mHR with age reflects a slowing of the intrinsic pacemaker activity of the sinoatrial node of the heart, which results from electrical remodeling of individual pacemaker cells along with structural remodeling and a blunted β-adrenergic response. In this review, we summarize current evidence about the tissue, cellular, and molecular mechanisms that underlie the reduction in pacemaker activity with age and highlight key areas for future work.

Biology of the Sinus Node and its Disease

The sinoatrial node (SAN) is the normal pacemaker of the heart and SAN dysfunction (SND) is common, but until recently the pathophysiology was incompletely understood. It was usually attributed to idiopathic age-related fibrosis and cell atrophy or ischaemia. It is now evident that changes in the electrophysiology of the SAN, known as electrical remodelling, is an important process that has been demonstrated in SND associated with heart failure, ageing, diabetes, atrial fibrillation and endurance exercise. Furthermore, familial SND has been identified and mutations have been characterised in key pacemaker genes of the SAN. This review summarises the current evidence regarding SAN function and the pathophysiology of SND.

QT/RR curvatures in healthy subjects: sex differences and covariates

Data of a large clinical study were used to investigate how much are the QT/RR patterns in healthy subjects curved and whether these curvatures differ between women and men. Daytime drug-free 12-lead Holter recordings were repeated 4 times in each of 176 female healthy subjects and 176 male healthy subjects aged 32.7 ± 9.1 yr. In each of the subjects, up to 1,440 carefully verified QT interval measurements were obtained with QT/RR hysteresis-corrected RR intervals. Individual subject data were used to fit the following regression equation: QT = χ + (δ/γ)(1 - RR(γ)) + ε, where QT and RR are QT and RR measurements (in s), χ is regression intercept, δ is the QT/RR slope, γ is the QT/RR curvature and provides the lowest regression residual, and ε represents normally distributed zero-centered errors. The bootstrap technique showed the intrasubject reproducibility of QT/RR slopes and curvatures. In women and men, QT/RR curvatures were 0.544 ± 0.661 and 0.797 ± 0.706, respectively (P = 0.0006). The corresponding QT/RR slopes were 0.158 ± 0.030 and 0.139 ± 0.023, respectively (P < 0.0001). QT/RR curvatures were related to QT/RR slopes but not to individually corrected mean QTc intervals or individual QT/RR hysteresis profiles. The individual heart rate correction formula derived from the curvilinear regression provided a significantly lower intrasubject variability of QTc interval than individual optimisation of linear or log-linear QT/RR heart rate corrections. The QT/RR curvature can be reliable measured and expressed numerically. The corresponding heart rate correction formula provides more compact data than the previously proposed approaches. There are substantial sex differences in QT/RR patterns. Women have a QT/RR pattern that is not only steeper than men but also more curved.

Sex differences in cardiac autonomic regulation and in repolarisation electrocardiography

The review summarises the present knowledge on the sex differences in cardiac autonomic regulations and in related aspects of electrocardiography with particular attention to myocardial repolarisation. Although some of the sex differences are far from fully established, multitude of observations show consistent differences between women and men. Despite more pronounced parasympathetic cardiac regulation, women have higher resting heart rate and lower baroreflex sensitivity. Of the electrocardiographic phenomena, women have longer QT interval duration, repolarisation sequence more synchronised with the inverse of the depolarisation sequence, and likely increased regional heterogeneity of myocardial repolarisation. Studies investigating the relationship of these sex disparities to hormonal differences led frequently to conflicting results. Although sex hormones seem to play a key role by influencing both autonomic tone and electrophysiological properties at the cellular level, neither the truly relevant hormones nor their detailed actions are known. Physiologic usefulness of the described sex differences is also unknown. The review suggests that new studies are needed to advance the understanding of the physiologic mechanisms responsible for these inequalities between women and men and provides key methodological suggestions that need to be followed in future research.

Consecutive administration of atropine and isoproterenol for the evaluation of asymptomatic sinus bradycardia

AIMS

Sinus node function is commonly evaluated by the atropine test. The isoproterenol test is less used. The aim of this study was to evaluate chronotropic reserve in patients with asymptomatic sinus bradycardia using the combined administration of atropine and isoproterenol.

METHODS AND RESULTS

A total of 100 patients were studied, 18-70 years old, with permanent, asymptomatic, sinus bradycardia and no detectable cardiac disease. The standard administration protocols for atropine and isoproterenol were used and successive heart rate recorded. Patients were stratified into three groups: Group A (control), showing normal response to atropine and isoproterenol; Group B, demonstrating abnormal response to atropine; Group C, with abnormal response to atropine and isoproterenol. No statistically significant difference was observed between Groups A and B (P = 0.11), whereas Group C differed statistically from both Groups A (P < 0.000001) and B (P = 0.000003) to a significant extent. By the end of the 3-year follow-up period, 47% of the Group C patients had undergone permanent pacemaker implantation (DDDR)--Kaplan-Maier survival curves predict only 35% survival without pacing--whereas none did so in Groups A and B.

CONCLUSIONS

In patients with deficient chronotropic response to atropine administration, isoproterenol tests could differentiate those with inadequate chronotropic reserves, possibly requiring preventive pacemaker implantations.

Long-term transvenous temporary pacing with active fixation bipolar lead in the management of severe autonomic dysfunction in Miller-Fisher syndrome: A case report

Management of severe autonomic dysfunction in patients with Guillain-Barre syndrome (GBS) or its variant Miller-Fisher syndrome (MFS) include placement of permanent pacemaker. We report a case of MFS with severe bradycardia and asystole treated initially with an external permanent (temporary-permanent) pacemaker with a transvenous, active fixation right ventricular lead placement as a "bridge" to permanent pacing.

Clinical characteristics of hypervagotonic sinus node dysfunction

BACKGROUND

Sinus node dysfunction (SND) is caused not only by intrinsic sinus node disease, but also by the extrinsic factors. Among the extrinsic factors, autonomic imbalance is most common. Symptomatic SND usually requires permanent pacemaker therapy. However, the clinical characteristics and patient response to medical therapy for hypervagotonic SND have not been properly clarified.

MATERIALS AND METHODS

Thirty two patients (14 men, 18 women, 51 +/- 14 years) with hypervagotonic SND were included in this study, but those patients who had taken calcium antagonists, beta-blockers or other antiarrhythmic drugs were excluded. Hypervagotonic SND was diagnosed if the abnormal electrophysiologic properties of the sinus node were normalized after the administration of atropine (0.04 mg/kg).

RESULTS

The presenting arrhythmias were 16 cases of sinus bradycardia (50.0%), 12 of sinus pause (37.5%), 3 of sinoatrial block (9.4%) and 1 of tachy-bradycardia (3.1%). Nine (28.1%) patients had hypertension, 7 (21.9%) smoked, 2 (6.3%) had diabetes mellitus, and 1 (3.1%) had hypercholesterolemia. Among the patients, 3 had no remarkable symptoms, 13 had dizziness, 7 had syncope, 3 had weakness and 6 had shortness of breath. Twenty five (78.1%) patients were treated with theophylline, 1 patient with tachy-bradycardia syndrome was treated with digoxin and propafenone, and 6 (18.8%) were treated with no medication. During the 43 +/- 28 month follow-up, 25 patients remained asymptomatic, but 6 who took no medication developed mild dizziness. One patient needed permanent pacemaker implantation owing to recurrent syncope despite of theophylline treatment.

CONCLUSION

These results show that hypervagotonic SND has a benign course and most of the patients can be managed safely without implanting a pacemaker. (Ed note: I like the abstract. It is short and direct, as it should be.)

Altered sinus nodal and atrioventricular nodal function in freely moving mice overexpressing the A1 adenosine receptor

To investigate whether altered function of adenosine receptors could contribute to sinus node or atrioventricular (AV) nodal dysfunction in conscious mammals, we studied transgenic (TG) mice with cardiac-specific overexpression of the A1 adenosine receptor (A1AR). A Holter ECG was recorded in seven freely moving littermate pairs of mice during normal activity, exercise (5 min of swimming), and 1 h after exercise. TG mice had lower maximal heart rates (HR) than wild-type (WT) mice (normal activity: 437 +/- 18 vs. 522 +/- 24 beats/min, P < 0.05; exercise: 650 +/- 13 vs. 765 +/- 28 beats/min, P < 0.05; 1 h after exercise: 588 +/- 18 vs. 720 +/- 12 beats/min, P < 0.05; all values are means +/- SE). Mean HR was lower during exercise (589 +/- 16 vs. 698 +/- 34 beats/min, P < 0.05) and after exercise (495 +/- 16 vs. 592 +/- 27 beats/min, P < 0.05). Minimal HR was not different between genotypes. HR variability (SD of RR intervals) was reduced by 30% (P < 0.05) in TG compared with WT mice. Pertussis toxin (n = 4 pairs, 150 microg/kg ip) reversed bradycardia after 48 h. TG mice showed first-degree AV nodal block (PQ interval: 42 +/- 2 vs. 37 +/- 2 ms, P < 0.05), which was diminished but not abolished by pertussis toxin. Isolated Langendorff-perfused TG hearts developed spontaneous atrial arrhythmias (3 of 6 TG mice vs. 0 of 9 WT mice, P < 0.05). In conclusion, A1AR regulate sinus nodal and AV nodal function in the mammalian heart in vivo. Enhanced expression of A1AR causes sinus nodal and AV nodal dysfunction and supraventricular arrhythmias.

Sick sinus syndrome

Sinus-node dysfunction is common in the elderly and, in most cases, does not cause any symptoms. Despite the high number of laboratory investigations, most diagnoses of sinus-node dysfunction are made by 12-lead electrocardiography, which shows severe sinus bradycardia, sinus arrest, or sinoatrial block. Continuous electrocardiographic monitoring, exercise testing, and electrophysiologic investigations (including pharmacologic interventions to cause complete autonomic blockade) are sometimes useful in detecting transient or latent sinus-node abnormalities. The term sick sinus syndrome should be reserved for patients with symptomatic sinus-node dysfunction. Sick sinus syndrome has a protean presentation with variable degrees of clinical severity. Symptoms are often intermittent, changeable, and unpredictable. Because these symptoms can be observed in several other diseases, none are specific to sick sinus syndrome. Owing to the nonspecific nature of its symptoms, sick sinus syndrome can be diagnosed only when clear electrocardiographic signs corroborate symptoms. In the absence of a demonstrable link between signs and symptoms, a diagnosis can be presumed only when signs of severe sinus dysfunction are present and when every other possible cause of symptoms has been excluded carefully. Sinus-node dysfunction frequently is associated with diseases of the autonomic nervous system, and autonomic reflexes play a major role in the genesis of syncope. Survival does not seem to be affected by sick sinus syndrome. Atrioventricular block, chronic atrial fibrillation, and systemic embolism are major pathologic conditions that affect the outcome of the syndrome. Treatment should be aimed at controlling morbidity and relieving symptoms. Cardiac pacing is the most powerful therapy; physiologic pacing (atrial or dual-chamber) has been shown definitively to be superior to ventricular pacing.

The natural course of untreated sick sinus syndrome and identification of the variables predictive of unfavorable outcome

We performed a prospective study in 35 untreated patients aged > or = 45 years, who had a mean sinus rate at rest of < or = 50 beats/min and/or intermittent sinoatrial block, and symptoms attributable to sinus node dysfunction. The patients were followed up for up to 4 years (mean 17 +/- 15 months). During follow-up, 20 patients (57%) had cardiovascular events that required treatment: 8 had syncope (23%); 6 had overt heart failure (17%); 4 patients had chronic atrial fibrillation (11%); and 2 patients had poorly tolerated episodes of paroxysmal tachyarrhythmias (6%). Actuarial rates of occurrence of all events were 35%, 49%, and 63%, respectively, after 1, 2, and 4 years. At univariate analysis, age > or = 65 years, end-systolic left ventricular diameter > or = 30 mm, end-diastolic left ventricular diameter > or = 52 mm, and ejection fraction < 55% were predictors of cardiovascular events. At multivariate analysis, age, end-diastolic diameter, and ejection fraction remained independent predictors of events. Actuarial rates of occurrence of syncope were 16%, 31%, and 31%, respectively, after 1, 2, and 4 years. Both univariate and multivariate predictors of syncope were history of syncope and corrected sinus node recovery > or = 800 ms. A favorable outcome was observed in the remaining 43% of patients. Thus, clinical cardiovascular events occur in most untreated sick sinus syndrome patients during long-term follow-up, even though a favorable course can be expected in 43% of patients. The outcome can be partly predicted on initial evaluation. In the patients with a favorable outcome, treatment can safely be delayed.

Effects of pharmacological autonomic blockade on dual atrioventricular nodal pathways physiology in patients with slow-fast atrioventricular nodal reentrant tachycardia

The purpose of this study was to investigate the atrioventricular AV nodal physiology and the inducibility of AV nodal reentrant tachycardia (AVNRT) under pharmacological autonomic blockade (AB). Seventeen consecutive patients (6 men and 11 women, mean age 39 +/- 17 years) with clinical recurrent slow-fast AVNRT received electrophysiological study before and after pharmacological AB with atropine (0.04 mg/kg) and propranolol (0.2 mg/kg). In baseline, all 17 patients could be induced with AVNRT, 5 were isoproterenol-dependent. After pharmacological AB, 12 (71%) of 17 patients still demonstrated AV nodal duality. AVNRT became noninducible in 7 of 12 nonisoproterenol dependent patients and remained noninducible in all 5 isoproterenol dependent patients. The sinus cycle length (801 +/- 105 ms vs 630 +/- 80 ms, P < 0.005) and AV blocking cycle length (365 +/- 64 ms vs 338 +/- 61 ms, P < 0.005) became shorter after AB. The antegrade effective refractory period and functional refractory period of the fast pathway (369 +/- 67 ms vs 305 +/- 73 ms, P < 0.005; 408 +/- 56 ms vs 350 +/- 62 ms, P < 0.005) and the slow pathway (271 +/- 30 ms vs 258 +/- 27 ms, P < 0.01; 344 +/- 60 ms vs 295 +/- 50 ms, P < 0.005) likewise became significantly shortened. However, the ventriculoatrial blocking cycle length (349 +/- 94 ms vs 326 +/- 89 ms, NS) and effective refractory period of retrograde fast pathway (228 +/- 38 ms vs 240 +/- 80 ms, NS) remained unchanged after autonomic blockade. Pharmacological AB unveiling the intrinsic AV nodal physiology could result in the masking of AV nodal duality and the decreased inducibility of clinical AVNRT.

Serial measures of sinoatrial and atrioventricular nodal function in ambulatory patients

We hypothesized that the outpatient assessment of SA and AV nodal (SAN, AVN) function could be a useful tool to determine the effectiveness of drugs and other treatments. We sought to examine the reproducibility, safety and ease of acquiring serial measurements of these parameters. Ten patients with permanent pacemakers underwent low current chest wall stimulation while their device was programmed to unipolar atrial triggered mode. Measurements at multiple conditioning drive train frequencies were obtained for: sinus nodal recovery time (SNRT); corrected sinus nodal recovery time (CSNRT); SA conduction time (SACT); AVN block cycle length (AVNBCL); and AVN effective refractory period (AVNERP). AVN function curves were also constructed. All studies were repeated after 2 weeks. Measures of sinus nodal and AVN function did not show significant differences between the two studies. The following co-efficients of correlation were obtained: SNRT800, r = 0.79; CSNRT800, r = 0.71; SNRT600, r = 0.71; CSNRT600, r = 0.44; SACT, r = 0.75; AVNBCL, r = 0.98; AVNERP800, r = 0.55; and AVNERP600, r = 0.99. AVN function curves did not significantly differ between week 1 versus week 2 at conditioning drive trains of either 800 ms or 600 ms. These data suggest that serial noninvasive electrophysiological measures of AVN and SAN function are reproducible over 2 weeks. Using data in this study, estimates of the sample size necessary for the evaluation of the effects of investigational drugs on the SAN and AVN in future studies are possible.

Electrophysiologic properties of the adult human sinus node

Electrophysiologic properties of the sinoatrial (SA) node have been studied extensively in various species. Published studies on human SA nodal properties were carried out on fetal SA node preparations; however, the electrophysiologic properties of adult human cardiac tissues have never been examined. In this study we report, for the first time, recordings of transmembrane potentials in the adult human SA node. Our data demonstrate that the electrical behavior of adult human SA node pacemaker cells resemble those of SA nodal tissue of different animal species. We conclude that the latter ideally mimic the former. Furthermore, human SA node preparations could contribute to a better understanding of some sinus arrhythmias.

Sinus node recovery time assessment revisited: role of pharmacologic blockade of the autonomic nervous system

Sinus node recovery time assessment is used to diagnose clinically significant sinus node dysfunction (SND) when Holter has failed to prove a relationship between sinus bradyarrhythmias and symptoms, but consensus has not been reached as to the value of including assessment after pharmacologic blockade of the autonomic nervous system. This issue was addressed in the present study performed on 52 patients with syncope or presyncope/dizziness (n = 48), sinus bradyarrhythmias (n = 45), or both (n = 41). Group 1 consisted of 13 patients with a proven relationship between symptoms and sinus bradyarrhythmias. Group 2 consisted of 39 patients with suspected SND. The protocol included three pacing periods at two pacing rates and was performed at baseline (n = 52), after single doses of atropine and propranolol (0.02 mg/kg and 0.1 mg/kg, respectively) (n = 41), and again after a second dose (n = 29). The sensitivity of prolonged recovery times was 77% in group 1. Among group 2 patients, 56% had prolonged recovery times at baseline (79% when including the results after the first dose of drugs). The second dose did not contribute diagnostic information, but it caused significant adverse reactions in 7 of 29 patients (P < 0.001). These 7 patients were all older than 60 years. Assessment of sinus node recovery time after pharmacologic blockade of the autonomic nervous system thus increases the sensitivity of the method in patients with suspected SND and normal baseline results. However, only 50% of the initially suggested doses of atropine and propranolol is sufficient and eliminates the risk for significant adverse reactions.

Cardiac asystole post-exercise: a report of two cases

Two patients are described with reproducible cardiac asystole post-exercise. No structural heart disease was demonstrable. At autonomic function testing no abnormal responses were noted. Also, head-up tilt tests were normal. However, electrophysiologic testing and heart rate variability during 24-h Holter monitoring were indicative of a high vagal tone in both patients. The findings suggest that post-exertional asystole may not be due solely to a vasovagal mechanism; excessive rebound vagotonia per se may also play a role.

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.

An abnormal neural reflex plays a role in causing syncope in sinus bradycardia

OBJECTIVES

This study investigates the role of an abnormal neural reflex in causing syncope in patients with sinus bradycardia.

BACKGROUND

Syncope is commonly considered an indication of severity in sinus bradycardia. However, the occurrence of syncope is unpredictable, and the prognosis appears to be similar in patients with and without syncope.

METHODS

Head-up tilt testing (60 degrees for 60 min), carotid sinus massage in the supine and standing positions, 24-h Holter ambulatory electrocardiographic (ECG) recording and electrophysiologic study before and after pharmacologic autonomic blockade were performed in 25 patients with sinus bradycardia and syncope (group I, sinus rate < 50 beats/min, age 71 +/- 12 years) and 25 patients with sinus bradycardia and no neurologic symptoms (group II, sinus rate < 50 beats/min, age 67 +/- 16 years).

RESULTS

Clinical characteristics and ambulatory ECG monitoring data were similar in the two study groups. A positive response (induction of syncope or presyncope with hypotension and/or bradycardia) was obtained by head-up tilt testing in 15 group I (60%) and in 3 group II (12%) patients (p < 0.001) and by carotid sinus massage in 11 group I (44%) and 6 group II (24%) patients (p = NS). Results of at least one test (head-up tilt testing or carotid sinus massage, or both) were positive in 19 group I (76%) and 9 group II (36%) patients (p < 0.01). Basal and intrinsic corrected sinus node recovery time did not differ significantly between the two groups. An abnormal intrinsic heart rate was present in 66% of group I and 26% of group II patients (p < 0.01). The different percentage of positive findings on head-up tilt testing and carotid sinus massage in the two groups was independent of the presence of intrinsic sinus node dysfunction.

CONCLUSIONS

These results indicate that an abnormal neural reflex plays a role in causing syncope in patients with sinus bradycardia. This reflex seems to be unrelated to the severity of sinus node dysfunction, even if the latter could enhance the cardioinhibitory response.

Sinus node dysfunction during long-term lithium treatment

BACKGROUND AND OBJECTIVE

Lithium has occasionally been reported to cause symptomatic sinus node bradyarrhythmias. The prevalence and mechanism of these arrhythmias during long-term treatment are unknown. The aims of this study were (a) to evaluate the systemic effects of lithium treatment on cardiac conduction in individuals who were free from cardiovascular disorders; (b) to assess the prevalence of lithium treatment in a group of patients with pacemakers; and (c) to evaluate the interaction between the parasympathetic limb of the autonomous nervous system and the sinus node cells during long-term lithium treatment.

PATIENTS AND METHODS

45 patients who had been treated with lithium for > 12 months were investigated in a long-term electrocardiography study. Only patients without cardiovascular disease, or concomitant chronotropic medication, or metabolic disorders known to cause rhythm disturbances were included. An age-stratified population was used as a reference group. 21 patients also underwent analysis of carotid sinus pressure and sinus cycle length before and after atropine to clarify whether neural mechanisms were involved. The prevalence of lithium treated patients was determined in 650 patients with pacemakers.

RESULTS

(a) Signs of moderate sinus node dysfunction (sinus arrest > 1.5 s, minimum heart rate < 50 beats/min) were found in 56% and 78% respectively in the lithium-treated group compared with 30% and 30% respectively in the reference group (p < 0.01). Severe sinus node dysfunction was equally common in both groups. (b) The prevalence of chronic lithium treatment in the pacemaker population was 0.46%. (c) Sinus cycle variations were abnormal in the basal state in three (14%) patients and in 11 (52%) patients after atropine despite signs of intact and normal parasympathetic innervation.

CONCLUSIONS

Depressed sinus node function was significantly more common in a lithium-treated population than in an age-stratified reference group. Clinically significant dysfunction, however, was uncommon. The effect of lithium on the sinus node seemed to be intrinsic and was not caused by increased parasympathetic tone.

The electrophysiology of cardiac allograft rejection: independent effects of rejection and perioperative ischemia on the sinus node recovery phenomenon after cardiac transplantation

We characterized the effect of cardiac allograft rejection on the sinus node (SN) recovery response from overdrive suppression. A total of 54 corresponding data sets (SN recovery time [SNRT]/endomyocardial biopsy [EMB]) was available in 24 transplant recipients with normal SNRT. Data were pooled in the rejection vs the no-rejection group (n = 16 vs n = 38, respectively). During cardiac rejection (defined as a 7-day period starting 3 days prior to and lasting until 3 days after the EMB) the SNRT curves were moderately, but significantly shifted towards higher values (F = 13.4, p = .0003). All changes occurred within accepted normal limits for the SNRT. Multivariate analysis indicated independent effects of donor heart ischemic time (p = .0005) on SNRT in addition to that of rejection. After accounting for that influence of ischemic time respective F values regarding the influence of rejection on the SNRT excursions were 10.8 (ischemic time < 100 min, p = .0014) and 4.36 (ischemic time > or = 100 min, p = .039). This study shows that cardiac allograft rejection significantly delays the SN recovery response from overdrive suppression. These changes, however, are subtle and, hence, are an unlikely explanation for the often grossly abnormal postoperative SN function.

Electrophysiological effect of the maze procedure on canine sinoatrial node function

The maze procedure is an operation that has had great initial success in curing atrial fibrillation. This procedure includes several right atrial incisions that may interrupt the integrity of the sinoatrial node or its arterial supply. To assess the effect of the maze procedure on sinus node function (SNF), the following studies were performed: sinus node recovery times (SNRT), corrected SNRT (CSNRT), CSNRT under autonomic blockade maximal heart rate and intrinsic heart rates. Thirty-four dogs underwent a right thoracotomy with cardiopulmonary bypass (CPB). The dogs were divided into three groups. Group 1 (n = 9), the sham group, underwent CPB without any incisions. Group 2 (n = 8) underwent CPB and one of the right atrial incisions. Group 3 (n = 18) underwent CPB and all three of the right atrial incisions. SNF was determined before and after the procedure. Groups 1 and 2 had no significant difference in measured SNF acutely after the procedure. In Group 3 the mean SNRT increased from 552 msec to 1,984 msec (P = 0.005). Sinus node dysfunction was corroborated by all studies. In the chronic studies, a trend toward recovery of SNF was observed. The maze procedure results in significant acute sinus node dysfunction. This dysfunction may resolve spontaneously over the ensuing months. Modifications of the maze procedure that avoid the sinus node or its blood supply area may reduce procedure related sinus node dysfunction.

Comparative class 1 electrophysiologic and anticholinergic effects of disopyramide and its main metabolite (mono-N-dealkylated disopyramide) in healthy humans

During steady-state treatment with disopyramide, its main and active metabolite, mono-N-dealkylated disopyramide, was reported to reach concentrations that were equal to or higher than the parent drug in 25% of 70 evaluated patients. This metabolite has been found to have a more pronounced anticholinergic action than the parent drug on in vitro evaluation, but neither its anticholinergic nor its direct electrophysiologic effects on the human heart have been properly assessed. We therefore compared the acute electrophysiologic and anticholinergic effects (the standard being atropine, 0.04 mg/kg) of disopyramide and its main metabolite, given 2 mg/kg body weight intravenously to 10 healthy individuals in a double-blind, randomized, crossover design. The anticholinergic effect of these substances on sinus and atrioventricular node function was unexpectedly found to be of similar magnitude and more pronounced than previously thought (at least one-third the effects of the atropine dose). The class 1 electrophysiologic effects were as follows: intra-atrial and His-Purkinje conduction (the PA and the HV interval, respectively) was prolonged 33% (95% CI: 18-47%) and 27% (21-32%) by disopyramide, and 15% (10-19%) and 13% (10-17%), respectively, by the metabolite. Disopyramide also prolonged the QRS, JT, and QT intervals by 15% (9-21%), 10% (8-13%), and 10% (7-12%), respectively. The metabolite caused a 9% (7-12%) prolongation of the QRS interval (significantly less than disopyramide), but shortened repolarization (as reflected by the JT interval) by -7% (-2 to -11%; p < 0.01), which is similar to the acute effects of lidocaine 2 mg/kg body weight.(ABSTRACT TRUNCATED AT 250 WORDS)

Parasympathetic overactivity and its evaluation in patients with sinus nodal dysfunction

In 17 controls, and 17 patients with sinus nodal dysfunction, an electrophysiologic study was made of sinus nodal function and atrioventricular nodal conduction in the basal state. The study was then repeated in all patients after atropine. The heart rate, mean sinus cycle length, variations of sinus cycle length, sinus node recovery times, sinuatrial conduction time, AH interval, and atrioventricular nodal Wenckebach threshold were significantly different in patients from those of controls. All these parameters changed significantly in patients after atropine, and were comparable to those of controls except for the atrioventricular nodal Wenckebach threshold. Atropine failed to increase the heart rate beyond 90 beats per minute in 10 of 17 patients (sensitivity of 59%) or by at least 30% above the resting heart rate only in 4 of them (sensitivity of 24%). The variations of sinus cycle length, and their standardized value, could detect sinus nodal dysfunction with sensitivities of 59 and 47%, respectively. From our results, we conclude that there is parasympathetic overactivity in patients with sinus nodal dysfunction. Because of their very low sensitivities, the atropine test and variations of sinus cycle length were not useful in identifying sinus nodal dysfunction noninvasively. The normal response of the heart rate to atropine does not exclude sinus nodal dysfunction, but atropine may help to differentiate abnormalities intrinsic and extrinsic to the sinus node during the electrophysiologic study.

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.

Role of sinus node artery disease in sick sinus syndrome in inferior wall acute myocardial infarction

This study was undertaken to evaluate a possible role of sinus node (SN) artery disease in the pathogenesis of sick sinus syndrome (SSS) in patients with an inferior wall acute myocardial infarction (AMI). Coronary angiography and electrophysiologic studies of the SN, both in the basal state and after pharmacologic autonomic blockade, were performed in 23 study patients (mean age 60 years) with SSS and a previous inferior wall AMI and in another 23 control patients (mean age 57 years) with normal sinus rate and a previous inferior AMI. Stenosis of the SN artery (or that proximal to its origin) greater than 50% was present in 13 study patients (56%) and in 8 control patients (34%) (p less than 0.05). In the study group, the intrinsic heart rate was abnormal in 5 of the 6 patients (83%) with severe SN artery stenosis (greater than or equal to 75% narrowing), in 3 of the 7 (43%) with moderate stenosis (50 to 75% narrowing) and in 3 of the 10 (30%) with insignificant stenosis (less than 50% narrowing). In the study group, the correlation between the SN measures (heart rate, corrected SN recovery time and sinoatrial conduction time) and the severity of SN artery stenosis was good after autonomic blockade (r between 0.59 and 0.64) and poor in the basal state. These data provide evidence for a role of SN artery disease in the pathogenesis of SSS in patients with an inferior wall AMI.

Electrophysiologic evaluation of sinus node dysfunction in postoperative children and young adults utilizing combined autonomic blockade

Sinus node dysfunction is a recognized problem following surgery for congenital heart disease. Seven postoperative patients with sinus node dysfunction (5 Mustard, 1 tetralogy of Fallot, 1 Fontan) underwent electrophysiology study of sinus node function during combined autonomic blockade (CAB) utilizing propranolol 0.2 mg/kg i.v. and atropine 0.04 mg/kg i.v. to evaluate intrinsic sinus node function isolated from autonomic control. During CAB, intrinsic heart rate, intrinsic corrected sinus node recovery time, and intrinsic sinoatrial recovery time were measured. These results were compared with age-matched normal intrinsic data from our lab [normal (n = 7, mean age 9 years) IHR 128 +/- 24, intrinsic corrected sinus node recovery time 135 +/- 40 ms, intrinsic sinoatrial conduction time 86 +/- 19 ms]. Among postoperative Mustard patients (n = 5, mean age 13 years, mean years postoperative 11) 2 of 5 had clearly abnormal intrinsic sinus node function with nonsinus rhythm during CAB; 3 of 5 had sinus rhythm during CAB with normal or mildly abnormal intrinsic sinus node function. The postoperative case of tetralogy of Fallot (age 20 years, postoperative 14 years) had mildly abnormal intrinsic sinus node electrophysiology study. The postoperative case of Fontan (age 16 years, postoperative 1.5 years) had sinus rhythm at rest but left atrial rhythm during CAB. Different aspects of sinus node dysfunction may be expressed during resting electrophysiology study vs. electrophysiology study utilizing CAB. The pathophysiology of sinus node dysfunction among postoperative pediatric patients is not homogeneous with regard to the contribution of intrinsic sinus node dysfunction. In those patients with normal or mildly abnormal intrinsic sinus node function, an important pathophysiologic influence of the autonomic nervous system is implicated.

Intrinsic heart rate in children and young adults: an index of sinus node function isolated from autonomic control

Standard evaluation of children with sinus node (SN) dysfunction cannot distinguish abnormal autonomic tone from intrinsic SN disease. This distinction has potentially important therapeutic and prognostic implications. Intrinsic heart rate (IHR)--the peak heart rate (HR) measured during pharmacologic combined autonomic blockade--reflects intrinsic SN function. The purpose of this study was to evaluate the use of IHR--and its relationship with resting heart rate (RHR)--in distinguishing autonomic influence from intrinsic SN disease among children with SN dysfunction. IHR was determined in the electrophysiology laboratory using intravenous propranolol, 0.2 mg/kg, followed by intravenous atropine, 0.04 mg/kg; the peak HR recorded was the IHR. IHR was measured in two groups. Seven control subjects, defined as patients with normal noninvasive SN testing, had IHR mean of 128 +/- 24 beats/min; this was greater than RHR mean of 89 +/- 16 beats/min (p less than 0.01). Eight patients with abnormal noninvasive SN testing had IHR mean of 103 +/- 6 beats/min; this was greater than RHR mean of 71 +/- 9 beats/min (p less than 0.01). We therefore reached the following conclusions. (1) Among this particular group of patients with abnormal noninvasive SN testing, IHR was consistently in the normal range for age and greater than RHR, suggesting that excess vagal tone can play a significant role in the expression of SN dysfunction. (2) Among normal individuals, IHR is age-related, decreasing with advancing age. IHR greater than RHR suggests that vagal tone predominates in the normal resting state as the net increase in HR during combined autonomic blockade is due to blockade of vagally mediated chronotropic inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiological evaluation of the sinus node in patients with atrioventricular and/or intraventricular conduction defects

The authors evaluated retrospectively sinoatrial conduction time (SACT), sinus node recovery time (SNRT), and corrected sinus node recovery time (CSNRT) in 272 patients with atrioventricular and/or intraventricular conduction defects without evidence of overt sinus node dysfunction. The study was designed to determine the prevalence of electrophysiologic sinus node abnormalities in patients with overt atrioventricular and/or intraventricular conduction defects. One or more sinus node electrophysiological abnormalities were observed in 133 cases (48.9%). There was a significant prevalence of electrophysiologic sinus node abnormalities only in patients older than 71 years of age. The data suggest that the involvement of the specialized conduction system is much more diffuse than one might expect simply observing the single recorded ECG defect and that the prevalence of associated defects of the conduction system increases with increasing age of patients.

Asystole with syncope secondary to hyperventilation in three young athletes

We describe three athletes who had syncope after (case 1) or during (cases 2, 3) hyperventilation. During the episode, ECG showed prolonged sinus arrest. Clinical data and noninvasive investigations were normal and the phenomenon was not reproducible. Electrophysiological study after autonomic blockade allowed a prolonged intrinsic heart rate in case 1, and abnormal corrected sinus node recovery time in cases 1 and 2. During follow-up, symptomatic sinus arrest provoked by deep inspiration occurred in case 3. These cases document prolonged asystole of unknown etiology, secondary to hyperventilation, and probably caused by different vagally-mediated mechanisms.

Reliability of transesophageal pacing in the assessment of sinus node function in patients with sick sinus syndrome

The purpose of this study was to find out whether transesophageal pacing could be utilized for assessment of sinus node function in patients with sick sinus syndrome (SSS). In 17 patients with SSS (study group) we compared the results of sinus node tests obtained both in the basal state and after pharmacological autonomic blockade by endocavitary stimulation and, 24 hours later, by transesophageal pacing. In another group of 17 patients with SSS (control group), we compared the results obtained by two endocavitary studies. In "study group", sinus cycle length (SCL) and corrected sinus node recovery time (CSRT) did not show significant differences between the two studies both in the basal state and after autonomic blockade, whereas sinoatrial conduction time (SACT) was more prolonged during esophageal pacing (P less than 0.01). In "control group", sinus node measures did not show significant differences between the two studies. In the "study group," the following coefficients of correlation were obtained in the basal state; SCL, r = 0.65, CSRT, r = 0.57, SACT, r = 0.52 and after autonomic blockade: SCL, r = 0.95, CSRT, r = 0.62 and SACT, r = 0.53. In the basal state, the correlation for SCL and CSRT between the two studies was lower in the "study group" than in the "control group" (P less than 0.05), whereas after autonomic blockade the correlation for sinus node measures did not show significant differences between the two groups of patients. These data suggest that transesophageal study influences the autonomic tone regulating the sinus node; however, it is not responsible for important variations in sinus node measures.(ABSTRACT TRUNCATED AT 250 WORDS)

Direct and autonomically mediated effects of oral flecainide

This study was undertaken to determine if oral flecainide exerts autonomically mediated actions in addition to its direct depressant effect. Electrophysiologic studies were performed twice in each of 15 patients (mean age 59 years) with normal resting and intrinsic heart rate and normal A-H interval. In the first study, the variables of sinus node and atrioventricular node were evaluated both in the basal state and after autonomic blockade (propranolol 0.2 and atropine 0.04 mg/kg). Oral flecainide was administered for 4 to 5 days (200 to 250 mg daily) and the study was then repeated using the same methods. From comparison of data obtained in the 2 studies in the basal state, the overall effect of flecainide was evaluated and by comparing those obtained after autonomic blockade, the direct action of the drug was assessed. The overall effect of flecainide on sinus node was slight; sinus cycle length, corrected sinus node recovery time and sinoatrial conduction time did not change significantly after the drug. In contrast, after autonomic blockade the variables of sinosal automaticity were increased significantly (p less than 0.01). Flecainide significantly prolonged the atrioventricular node variables both in the basal state and after autonomic blockade (p less than 0.01), but the degree of increase was more marked after autonomic blockade (p less than 0.05). These data show dual effects of oral flecainide: a direct depressant action and an autonomically mediated opposing action, likely of vagolytic type.

Assessment by autonomic blockade of age-related changes of the sinus node function and autonomic regulation in sick sinus syndrome

Age-related changes of the sinus node (SN) function and the autonomic influence on the SN function were evaluated in 65 patients with sick sinus syndrome (range 14 to 84 years). Heart rate (HR), corrected SN recovery time and sinoatrial conduction time were measured before (basic) and after (intrinsic) autonomic blockade (propranolol 0.2 mg/kg plus atropine 0.04 mg/kg intravenously). Percent of autonomic chronotropies of the SN function were calculated by the following formulas: (1)--(intrinsic HR--basic HR/intrinsic HR) X 100; (2) (intrinsic corrected SN recovery time--basic corrected SN recovery time/intrinsic corrected SN recovery time) X 100; (3) (intrinsic sinoatrial conduction time--basic sinoatrial conduction time/intrinsic sinoatrial conduction time) X 100. Basic HR, basic corrected SN recovery time and basic sinoatrial conduction time did not vary with age. Intrinsic HR decreased with age, but this correlation was weak (r = -0.54, p less than 0.001). Intrinsic corrected SN recovery time and intrinsic sinoatrial conduction time tended to increase with age (r = 0.26, p less than 0.05; r = 0.29, p less than 0.05, respectively). Percent chronotropies of HR, corrected SN recovery time and sinoatrial conduction time were negative values in younger patients and positive values in elderly patients; they correlated positively with age (r = 0.59, p less than 0.001; r = 0.60, p less than 0.001; r = 0.43, p less than 0.001, respectively). Thus, the basic SN function did not change with age, while the intrinsic SN function deteriorated with age.(ABSTRACT TRUNCATED AT 250 WORDS)

Bedside evaluation of sinus bradycardia: usefulness of atropine test in discriminating organic from autonomic involvement of sinus automaticity

In 55 patients with persistent sinus bradycardia who underwent an electrophysiologic study of sinus node, both in the basal state and after autonomic blockade (propranolol, 0.2 mg/kg, and atropine, 0.04 mg/kg), an atropine test (0.02 mg/kg) was performed the following day. The 49 patients in whom sinus rate could be evaluated after atropine were subdivided into two groups--group I, 24 patients (age: 54 +/- 13 years) with normal intrinsic sinus automaticity (normal intrinsic heart rate and intrinsic corrected sinus node recovery time) and group II, 25 patients (age: 62 +/- 9 years) with abnormal intrinsic sinus automaticity. In group I, atropine increased sinus rate from 53.7 +/- 4 to 87.9 +/- 17 bpm (delta %: 65.5 +/- 33) and in group II from 51.6 +/- 5 to 73.9 +/- 14 bpm (delta %: 43.1 +/- 26). The discriminant threshold of sinus rate after atropine and its percent increase, obtained by discriminant analysis, was 80 bpm and +52%, respectively, with a misleading classification of 32% and 36%, respectively. The overall predictive accuracy of sinus rate after atropine was higher than the percent change in sinus rate (73% and 65%, respectively). These data evidence that the atropine test is not very helpful in discriminating between an organic and an autonomic involvement of sinus automaticity in patients with sinus bradycardia.

Transesophageal versus intracardiac atrial stimulation in assessing electrophysiologic parameters of the sinus and AV nodes and of the atrial myocardium

Electrophysiological parameters of the sinus and AV nodes and of the atrial myocardium were assessed with both transesophageal atrial stimulation (TAS) and intracardiac atrial stimulation (ICS) in the same patient during the same study. The study group was comprised of nine men and seven women, aged 45 to 79 years, referred for the evaluation of syncope of possible arrhythmogenic origin. Twelve patients were included for analysis. Autonomic inhibition (AI) was obtained in five patients. The most striking result was the significantly longer AERP with TAS (mean 286 +/- 9 ms) than with ICS (mean 244 +/- 12 ms; p than 0.02). After AI, the AERP was even more prolonged with TAS (mean 332 +/- 20 ms) than with ICS (mean 237 +/- 8 ms; p less than 0.01). Intraatrial and AV nodal conduction times assessed at multiple paced cycle lengths were significantly shorter with TAS than with ICS. There was no difference between TAS and ICS with regard to AVERP, Wenckebach periodicity and H-V intervals. Although a tendency towards shorter sinus node recovery time (SNRT) and sinoatrial conduction time (SACT) was observed with TAS, the difference was not statistically significant. Possible mechanisms of the differences are discussed. It seemed clear that the site of origin of an atrial impulse can have definite effects upon excitability and conduction properties of atrial and AV nodal fibers. Enhanced sympathetic activity during TAS was also suggested. The electrophysiological properties inherent in the TAS technique warrant further elucidation.

Effect of pindolol and propranolol on sinus node recovery time and atrioventricular conduction intervals

In a randomized, observer-blind study, the effect of incremental doses of pindolol 0.001, 0.002, 0.003, and 0.004 mg/kg IV and propranolol 0.01, 0.02, 0.03, and 0.04 mg/kg IV on SA nodal recovery time (SNRT) and atrioventricular conduction interval (AH) was assessed in 20 patients (15 men and 5 women age range thirty to seventy-two, mean age fifty-three). AH and His bundle-to-ventricle (HV) intervals and SNRT were measured at spontaneous heart rate and at incremental atrial pacing rates (80, 100, 120, 140 bpm). Both drugs caused significant beta blockade as estimated by the percentage suppression of heart rate increment induced by 3 mcg isoproterenol administered intravenously (pindolol 67.6 +/- 5.3%, P less than 0.007; propranolol 38.6 +/- 10.6%, P less than 0.001). Propranolol significantly prolonged SNRT (P less than 0.05) and AH interval (P less than 0.05). Pindolol did not significantly affect either SNRT (P = 0.25) or AH intervals (P = 0.78). Significant effects on HV interval were not seen. Thus, in the doses tested that resulted in significant beta blockade, propranolol prolonged SA nodal recovery times and depressed AV nodal conduction while pindolol did not affect these variables.

Intrinsic electrophysiologic properties of reentrant supraventricular tachycardia involving bypass tracts

This study evaluates the effects of autonomic blockade (propranolol, 0.2 mg/kg, and atropine, 0.04 mg/kg) in 20 patients with paroxysmal supraventricular tachycardia (SVT). In 8 patients the SVT circuit involved a concealed atrioventricular bypass for retrograde conduction (group I) and in 12 a concealed atrio-His pathway (group II). Autonomic blockade did not significantly change atrial and ventricular refractory periods, whereas it prolonged atrioventricular nodal refractoriness without varying AH interval. The ventriculoatrial interval did not change in any patient. The H2A2 interval was unchanged in all but 2 group II patients. In both groups, the effective refractory period of the concealed bypass was prolonged by autonomic blockade. In the basal state, SVT was induced in all patients; after autonomic blockade, SVT was induced in 7 patients in group I (87%) and in 7 in group II (58%) (p less than 0.05). Cycle length of SVT was prolonged after autonomic blockade in 11 of these 14 patients. The variations were observed only in the anterograde conduction (Ae-H interval), whereas retrograde conduction (H-Ae interval) was unchanged in all patients. These data indicate that the autonomic system appears to facilitate induction of SVT in patients with concealed atrio-His bypass as well as shorten the cycle length of SVT in both groups of patients.

Differential effects of autonomic blockade on sinus and atrioventricular nodal function in normals and in intrinsic sinus node dysfunction

The effect of pharmacologic total autonomic blockade on sinus and atrioventricular nodes was studied in 10 normals and 21 patients with sick sinus syndrome with abnormal intrinsic corrected sinus node recovery time. In normals the intrinsic heart rate (113.3 +/- 11.6 beats/min) was higher than the resting heart rate (87.3 +/- 12 beats/min; P less than 0.001). The AH interval at an identical paced rate decreased from 119 +/- 36 msec to 93 +/- 17.6 msec after autonomic blockade (P less than 0.05). Mean atrial paced cycle length at AH Wenckebach block was not different during control and after drugs (319 +/- 46 msec vs. 311.5 +/- 39 msec; P = NS). Although sinus cycle length shortened in all cases after autonomic blockade, paced cycle length at AH Wenckebach increased (4) or remained unchanged (3) in 7 cases. Maximum normal "intrinsic" paced cycle length at AH Wenckebach was 390 msec (mean +/- 2 SD). In sick sinus syndrome, resting heart rate (66.3 +/- 18.8 beats/min) and intrinsic heart rate (74.6 +/- 16.4 beats/min) were similar (P = NS); AH at identical paced rate: control 136.6 +/- 54 msec, after drugs 130.5 +/- 35 msec (P = NS); cycle length at AH Wenckebach: control 380.5 +/- 73 msec, after autonomic blockade 383 +/- 49 msec (P = NS). Two of 3 cases with abnormal atrioventricular nodal response to atrial pacing during control normalized after autonomic blockade; 9/21 (42.8%) cases developed AH Wenckebach at cycle length greater than 390 msec after autonomic blockade. The data suggest that the autonomic nervous system has differential effects on sinus and atrioventricular nodes. Patients with sick sinus syndrome frequently have abnormalities of "intrinsic" atrioventricular nodal conduction unmasked by autonomic blockade.

Effects of autonomic blockade on dual atrioventricular nodal pathways pattern

Fifteen patients (age: 57.6 +/- 14 years) showing dual A-V nodal pathways pattern during basal electrophysiological testing were studied following pharmacological autonomic blockade (iv propranolol 0.2 mg/Kg and iv atropine 0.04 mg/Kg). After induction of the autonomic blockade, the dual A-V nodal pathways pattern was not present in four patients due to disappearance of the slow pathway; the pattern remained in 11 (73%). The longest A2-H2 interval, the effective and functional refractory periods of the fast pathway did not change significantly following autonomic blockade. Even the electrophysiological measures of the slow pathway, in the 11 patients in whom they were comparable, did not change significantly after autonomic blockade. These data suggest that: the dual A-V nodal pathways pattern is mainly related to the intrinsic structure of the A-V node; the autonomic nervous system only affects in a variable way the refractoriness and the conduction velocity in the two pathways.

Reproducibility of electrophysiologic parameters of extrinsic sinus node function in patients with and without sick sinus syndrome

In a group of 50 patients we investigated the daily reproducibility of the resting heart rate (RHR), the maximal corrected sinus node recovery time (CSNRTM), the pacing rate at which the CSNRTM occurred (OP), and the sinoatrial conduction time (SACT) during basal state. The study population was divided into two groups according to the presence or absence (as evidenced during the initial electrophysiologic study) of sinus node disease: group I included 35 patients with normal sinus node function, and group II included 15 patients with sick sinus syndrome. The electrophysiologic study was repeated approximately at the same hour and under similar conditions after an interval of at least two days (mean: 3.2 days for group I and 4.7 days for group II). The results showed good reproducibility with the exception of RHR in group I which slightly but significantly decreased in the second electrophysiologic study. The daily variations of the sinus node parameters appeared to be of similar levels in the two groups except for the CSNRTM; this parameter showed wide variations in single values in both groups, more marked in group II than in group I. Furthermore, if the CSNRTM and/or SACT were normal, it was likely that they would remain normal whatever the electrophysiologic status of the patient. Inversely, the change of status from abnormal to normal CSNRTM or SACT was not uncommon in patients with electrophysiologic signs of sinus node disease.