Problems and anti-tachyarrhythmic effects of chronic atrial pacing

Sixty-six patients in whom atrial pacemaker (AAI) were implanted were followed for one year to 5 years for the occurrence of pacing failure, sensing problems, and later AV block. Pacing failure occurred in only one patient and sensing problems occurred in 15 patients but 10 of them improved after a change of sensing. Temporal change of AV conductivity was not recognized in the majority of patients. Eighteen patients developed transient decrease in AV conductivity. Two patients developed persistent decrease in AV conductivity and ended in clinical AV block for which the pacemaker was implanted. Out of 66 patients, 22 had a history of paroxysmal atrial flutter or fibrillation (AFF) prior to AAI implantation. They were divided into two groups. Group I consisted of 20 patients in whom paroxysmal AFF disappeared after AAI implantation. Group II consisted of 22 patients in whom paroxysmal AFF persisted after AAI implantation. Electrophysiological studies prior to the AAI implantation showed that sinus rate at control was significantly slower (36.3 +/- 10.1 beats per min in Group I, 57.1 +/- 10.8 beats per min in Group II), atrial fragmented activity zone was significantly narrower (62.7 +/- 32.9 msec in Group I, 88.1 +/- 19.7 msec in Group II), and the occurrence of PAC was less at an atrial pacing rate of 70 beats per min (8% in Group I, 67% in Group II) in Group I compared to Group II.(ABSTRACT TRUNCATED AT 250 WORDS)     

Long-term clinical performance of AAI pacing in patients with sick sinus syndrome: a comparison with dual-chamber pacing.

AIMS: In this clinical study, we compared two groups of age-matched patients, AAI and DDD, to evaluate the clinical benefits of AAI pacing in patients with sick sinus syndrome (SSS) and normal atrioventricular (AV) conduction. METHODS AND RESULTS: Ninety-five patients with SSS implanted with AAI pacemakers were compared with 101 SSS patients implanted with DDD pacemakers. Mortality, chronic atrial fibrillation, lead survival rates, and reoperation rates were compared by Kaplan-Meier analysis. Eight AAI devices were switched to DDD due to high-degree (grade 2-3) AV block. The incidence of high-degree AV block was 1.104%/year, with a freedom rate of 88.6% at 10 years. There were no significant differences between the two groups in survival rates (87.8% in AAI vs. 93.4% in DDD at 10 years), freedom from atrial fibrillation (93.6% vs. 90.6%), or freedom from reoperation (71.3% vs. 76.3%). On the other hand, lead failure was twice as frequent in the DDD group than in the AAI group (relative risk=2.045, P=0.0382). CONCLUSION: AAI pacing, a simple system using a single lead and single-chamber pacemaker, can achieve a clinical outcome similar to that of the DDD mode in patients with SSS and normal AV conduction.     

Development of sinus node disease in patients with AV block: implications for single lead VDD pacing

OBJECTIVE: To investigate the incidence of sinus node disease after pacemaker implantation for exclusive atrioventricular (AV) block. DESIGN: 441 patients were followed after VDD (n = 219) or DDD pacemaker (n = 222) implantation for AV block over a mean period of 37 months. Sinus node disease and atrial arrhythmias had been excluded by Holter monitoring and treadmill exercise preoperatively in 286 patients (group A). In 155 patients with complete AV block, a sinus rate above 70 beats/min was required for inclusion in the study (group B). Holter monitoring and treadmill exercise were performed two weeks, three months, and every six months after implantation. Sinus bradycardia below 40 beats/min, sinoatrial block, sinus arrest, or subnormal increase of heart rate during treadmill exercise were defined as sinus node dysfunction. RESULTS: Cumulative incidence of sinus node disease was 0.65% per year without differences between groups. Clinical indicators of sinus node dysfunction were sinus bradycardia below 40 beats/min in six patients (1.4%), intermittent sinoatrial block in two (0.5%), and chronotropic incompetence in five patients (1.1%). Only one of these patients (0.2%) was symptomatic. Cumulative incidence of atrial fibrillation was 2.0% per year, independent of the method used for the assessment of sinus node function and of the implanted device. CONCLUSIONS: In patients undergoing pacemaker implantation for isolated AV block, sinus node syndrome rarely occurs during follow up. Thus single lead VDD pacing can safely be performed in these patients.     

AAI pacing mode: When is it indicated and how should it be achieved?

AAI pacing offers better hemodynamic characterstics than dual-chamber pacing and is the optimal mode for patents with sick sinus syndrome without AV conduction disorders. AAI pacing may be achieved by single-chamber atrial, by programming a dual-chamber pacemaker to the AAI mode, or by programming a dual-chamber pacemaker to DDD mode with a long AV delay. The annual incidence of AV block development in patients with sick sinus syndrome is low, probably 1-5%, but there is no method of detecting patients immune to future development of AV block. Chronotropic is often present in patients with sick sinus but the value of additional rate response is not yet established. Our recommendations for the choice of the method of pacing are discussed.     

Stability of AV conduction in sick sinus node syndrome patients with implanted atrial pacemakers

Single-chamber atrial pacing is effective in the management of sinus node dysfunction, subject to the uncertainty of long-term atrioventricular conduction. Despite the accepted observation that many patients with sinus node dysfunction also have atrioventricular conduction disease, data do not exist on the development of atrioventricular block in those patients with permanent single-chamber atrial pacing. Of 70 patients who received single-chamber atrial pacing from 1967 to 1982 (mean duration of pacing was 33 months), only two patients of 58 (3.4%) of those with sinus node dysfunction developed atrioventricular (AV) block—after 14 months in one patient and after 23 months of successful atrial pacing in the other. None of the 12 patients paced for tachyarrhythmia management developed AV block. Of the 70 patients, 37 had assessment of AV conduction by incremental atrial pacing at the time of implant and 20 patients underwent atrial pacing on the basis of surface ECG and clinical judgment. Electrophysiologic studies were conducted only in those patients being paced for control of supraventricular arrhythmias. Only 5 of the 70 patients required conversion to ventricular pacing for technical difficulties; three of these conversions occurred in the early 1970's before the advent of atrial tined or J leads; one was for irreparable lead fracture and only one occurred in a patient with a newer design atrial lead. In conclusion, progression to AV block in patients with permanent atrial pacing is uncommon; formal electrophysiologic studies are necessary mainly in patients with supraventricular arrhythmias; and in the majority of patients, AV conduction can be assessed at the time of implant. Continued improvement in atrial leads should make atrial pacing even more successful.     

心房按需起搏的临床应用

AAI起搏是治疗房室传导正常的病态窦房结综合征(SSS)的理想方法,它可保持正常的房室收缩顺序及防止VVI起搏时心房压力上升,对预防心房纤颤(Af)有一定作用。本组17例AAI起搏主要用于房室传导正常的SSS,其中4例合并间断发作的Af,1例合并阵发性室上性心动过速(PSVT)。在随诊观察1～15个月之间,Af未再发作,PSVT也得到控制。本组17例全都采用螺旋形主动电极。我们体会其优点为电极可固定在心房之任何部位,操作简单,不易移位,采用可程控多参数的SSI型起搏器,便于定期进行心房调搏,观察房室传导功能的变化及处理可能发生的感知障碍及输出阻滞。     

Atrial adaptive rate pacing in sick sinus syndrome: effects on exercise capacity and arrhythmias.

OBJECTIVE--To test the hypotheses that adaptive rate atrial (AAIR) pacing: significantly increases maximal exercise capacity, and results in significant suppression of supraventricular and ventricular arrhythmia compared with fixed rate atrial (AAI) pacing. DESIGN--Prospective, randomised, single blind, crossover study with maximal treadmill exercise testing and 24 hour ambulatory electrocardiographic monitoring in AAIR and AAI modes. SETTING--Regional pacing centre. PATIENTS--30 consecutive patients (mean SD age 65 (12) years) with sick sinus syndrome who required permanent pacing, without evidence of conduction disturbance on 12 lead electrocardiograms or 24 hour ambulatory electrocardiographic monitoring and without other cardiovascular or systemic disease. INTERVENTIONS--Activity sensing or minute ventilation driven systems (AAI/AAIR) were implanted alternately. RESULTS--The mean (SD) peak heart rate in AAI mode was 122(28)v 130(22) in AAIR mode (p < 0.02) for the whole group and 104(17) v 120(5) (p < 0.003) for the patients with chronotropic incompetence. Exercise time was 12.3 (4.1) minutes in AAI and 12.3 (3.8) minutes in AAIR mode (NS) in the chronotropically incompetent patients. There were no significant differences in the Borg scores at peak exercise in AAI v AAIR mode in either group. The frequency per hour of atrial and ventricular arrhythmias showed no significant differences between the two modes in either the group as a whole or in the subgroups with chronotropic incompetence. CONCLUSION--AAIR pacing confers little benefit in sick sinus syndrome compared with AAI pacing.     

Comparison of atrial and VVI pacing modes in symptomatic sinus node dysfunction without associated tachyarrhythmias

The natural course of patients with symptomatic sinus node dysfunction who did not have associated tachyarrhythmias before pacemaker implantation was compared after VVI and atrial pacemaker implantation. Between April 1981 and June 1989, forty-seven such patients (mean age 52 + 13 years) received VVI pacemakers and forty patients (mean age 54 + 13 years) received AAI or DDD pacemakers. Baseline clinical characteristics and severity of sinus node dysfunction were comparable in the two groups. Over a follow up of 10 to 96 months (mean 49.2 + 26 months), 11 (23.4%) VVI patients were in functional class II or more compared to 2 (5%) atrially paced patients (p less than 0.01). Other complication rates were also higher in the VVI group as compared to AAI group viz. atrial fibrillation (21.2% vs 2.5% p less than 0.01) and stroke (10.6% vs 2.5%) though the number of deaths (14.9% vs 10%) was not significantly different in the two groups. Two patients in atrial paced group and one patient in VVI group developed first degree heart block. There was no incidence of second or third degree heart block. Transient loss of atrial sensing occurred in 3 patients and atrial lead displacement in 2 cases, but overall incidence of lead related problems was low and comparable in both groups. Thus atrial pacing is superior to ventricular pacing in sinus node dysfunction and risk of developing high grade atrioventricular block on follow up is low.     

Electrophysiological studies in four patients with atrial flutter with 1:1 atrioventricular conduction

Electrophysiological studies of atrioventricular conduction during rapid atrial overdrive pacing and during programmed premature atrial stimulation are reported in four patients with an unusually rapid 1:1 ventricular response to atrial flutter (ventricular rates 240 to 310 per minute). Second-degree AV block developed during atrial overdrive pacing at rates well below those during which 1:1 AV conduction was sustained during spontaneous atrial flutter. Although none of the four patients showed evidence of pre-excitation on the standard 12-lead electrocardiogram, evidence suggesting a partial AV nodal bypass was demonstrated at electrophysiological study in one case. It is postulated that the profile of the atrial wavefront presented to the normal AV node by atrial flutter differs from that during high right atrial pacing and may account for the lower ventricular rates achieved during high right atrial overdrive pacing than during spontaneous atrial flutter in the remaining three cases.     

Observations on variability of atrioventricular nodal conduction in man and the dual-pathway response

A study was made of nodal conduction times of atrial stimuliwith fixed coupling intervals, in 23 patients divided into twogroups according to their atrial stimulus test response: GroupI (continuous AV node function curve; 17 cases) and Gruop II(dual AV node pathway; six cases). The stimulation protocolinvolved the delivery of 75 stimuli with a fixed coupling interval20 ms greater than the effective refractory period (ERP) ofthe AV node (Group I) or fast pathway (Group II). The atrialcoupling intervals (A1A2) and node conduction times (A2H2) weremeasured. An evaluation was made of the dispersion of intervalswith range (R) and of the distribution of A2H2 times (² test). In both Groups, R (A2H2) was greater than R(A1A2) (P<0.05):R(A2H2) in Group II was greater than R(A2H2) in Group I (P <0.001). In Group I the distribution of A2H2 was non-normal infour cases and bimodal in five; in Group II the distributionwas non-normal and bimodal in all cases. It is concluded that: (1) the AV node generates a dispersionin its conduction times in the vicinity of its ERP, althoughthe nodal conduction curve is continuous: and (2) the so-calleddual pathway may constitute an exaggeration of AV node responseinhomogeneity.     

Electrophysiological studies in thyrotoxicosis with and without associated sick sinus syndrome

Electrophysiological studies in 13 patients with thyrotoxicosis (5 men and 8 women, aged 17 to 76 years) are reported. Five patients presented with features of sick sinus syndrome (SSS) (Group A) while the remaining 8 patients (Group B) had no detectable cardiovascular abnormality. Sinus node function (corrected sinus node recovery and sinoatrial conduction time) was abnormal in all Group A but normal in Group B patients. Intra-atrial, artioventricular (AV) nodal, and infranodal conduction time and effective refractory period of atrium were normal in all patients in both groups. Effective refractory period of AV node was decreased in 6 patients (3 in each group). All Group A patients received radioiodine with complete clinical remission of sick sinus state in 4 subjects. Repeat electrophysiological studies in two of these patients, 6 and 12 months after treatment, showed complete normalization of sinus node function. This is the first reported electrophysiological study documenting the occurrence of SSS in thyrotoxicosis reversed by effective antithyroid treatment. We suggest that attempts should be made to identify underlying thyrotoxicosis in all patients with SSS, especially in the older age group. Appropriate medical treatment may prevent unnecessary implantation of permanent pacemakers in such patients.     

VVI与AAI起搏对病态窦房结综合征合并阵发性房性快速心律失常的影响

目的比较ＶＶＩ与ＡＡＩ起搏对病态窦房结综合征（病窦综合征）合并阵发性房性快速心律失常的远期影响。方法分别对４１和３８例病窦综合征合并阵发性房性心律失常的患者行ＶＶＩ和ＡＡＩ起搏,术后进行了临床、心电图、２４小时动态心电图的定期随访,随访时间分别为３９±２．６和２４±１．８个月。结果ＶＶＩ组２１例（５１．２％）发展成持续性心房颤动,２０例阵发性房性心律失常发作次数及持续时间较术前显著增加（Ｐ＜０．０５）,６例（１４．１％）出现血栓栓塞,无１例死亡,无电极移位等并发症。ＡＡＩ组,所有患者生活质量明显改善,１９例（５０％）阵发性心房颤动、心房扑动发作的频度及持续时间较术前显著减少（Ｐ＜０．０５）,无１例发展为持续性心房颤动、血栓栓塞和死亡,１例（３％）出现电极移位,１例（３％）出现一度房室阻滞。结论ＡＡＩ起搏是一种安全、可靠的起搏方式,对病窦综合征合并的阵发性房性心律失常有明显的抑制作用,显著优于ＶＶＩ。     

Atrial rate-responsive pacing in sinus node disease

Patients with sinus node disease (SND) who are unable to achieve an adequate increase in heart rate during exercise are candidates for atrial rate-responsive pacing (AAI-R). We have implanted 40 AAI-R systems in SND patients with an average follow-up of 12.5 +/- 8 (range 3-30) months. All the patients received an activity-sensing pulse generator (Activitrax, Medtronic or Sensolog, Siemens-Pacesetter) with a single atrial lead. Only patients with an intraoperative AV nodal block cycle-length above 100 beats min-1 were included. During follow-up, one patient was observed to have transient asymptomatic 2:1 AV-block during sleep. No patient developed persistent AV-block or chronic atrial fibrillation. Twelve patients with persistent chronotropic incompetence were assigned for a randomized double-blind crossover study, comparing exercise treadmill capacity in AAI-R with conventional atrial inhibited pacing (AAI). During AAI-R pacing the maximum heart rate during exercise was 120 +/- 1 beats min-1 compared with 97 +/- 21 beats min-1 during AAI pacing (P less than 0.01). The average exercise time increased from 11.2 +/- 2 min during AAI-pacing to 13.4 +/- 3 min during AAI-R pacing (P less than 0.01). AAI pacing should be considered for patients with SND and chronotropic incompetence.     

Carotid sinus syndrome diagnosed by transcutaneous cardiac pacing]

The diagnostic usefulness and safety of non-invasive heart stimulation has been tested on 20 persons with full symptomatic cardiodepressive type of carotid sinus syndrome. The estimation of the cardiodepressive syndrome has been conducted on the grounds of electrophysiological tests of the heart with the method of endocavitary stimulation introduced by Stryjer and adopted by us for non-invasive heart stimulation. Massage of the carotid sinus was performed during sinus rhythm, during transesophageal atrial stimulation and during transcutaneous ventricle stimulation. In 17 persons massage of the carotid sinus caused sinus node inhibition. In this group carotid sinus massage during atrial stimulation revealed a group of 10 persons with only sinus node inhibition (type A) and a group of 7 persons with the coexistence of sinus node inhibition and an AV III block (type C). Only in 3 persons massage of the carotid sinus merely caused an AV III block (type B). Massage of the carotid sinus during transcutaneous ventricle stimulation showed retrograde AV conduction suppression in type B and C of the carotid cardiodepressive syndrome. To recapitulate the obtained results, it can be assumed that electrophysiological estimation of the carotid sinus syndrome is also possible and safe with the non-invasive method. Such procedure can also facilitate the selection of patients for permanent stimulation of AAI type for the type A syndrome or DVV for types B and C.     

Determinants of the ventricular rate during atrial fibrillation

Determinants of the ventricular cycle length during atrial fibrillation were examined in 52 patients. Thirty-three patients had structural heart disease and none had an accessory atrioventricular (AV) connection. The AV node effective and functional refractory periods, the shortest atrial pacing cycle length associated with 1:1 conduction, the AV node conduction time and indexes of concealed conduction in the AV node were measured in the baseline state (36 patients) and after modification of sympathetic tone by infusion of isoproterenol or propranolol (8 patients each). Atrial fibrillation was then induced with rapid atrial pacing, and the mean, shortest and longest ventricular cycle lengths were measured. Variables that correlated most strongly with the mean RR interval during atrial fibrillation were the AV node effective refractory period (r = 0.93; p less than 0.001), AV node functional refractory period (r = 0.87; p less than 0.001) and shortest atrial pacing cycle length associated with 1:1 conduction (r = 0.91; p less than 0.001). The AH interval during sinus rhythm (r = 0.74; p less than 0.001) and during atrial pacing at the shortest cycle length with 1:1 conduction (r = 0.52; p less than 0.001) had weaker correlations. Measures of concealed conduction did not improve the prediction of the mean or longest ventricular cycle length during atrial fibrillation. In conclusion, the refractory periods and conductivity of the AV node are the best indicators of the potential of the node to transmit atrial impulses to the ventricles during atrial fibrillation. The degree of concealed conduction in the AV node is a less important determinant of the mean ventricular rate during atrial fibrillation.     

Atrial pacing for sick sinus syndrome

Atrial pacing is the most physiological way to pace patients with sinus node disease, as it provides both AV synchrony and a normal ventricular activation pattern. Long-term studies comparing atrial and ventricular pacing imply that atrial pacing results in fewer cardiac complications and, possibly, reduced mortality. Ventricular pacing should thus, if possible, be avoided in patients with sinus node disease. The potential risk of impending high-grade AV block during atrial pacing is low, with an annual incidence around 1% if patients are selected appropriately. Approximately 40-50% of patients with sinus node disease show signs of chronotropic incompetence during physical exercise, and are thus candidates for atrial rate responsive pacing. A preoperative evaluation of candidates for atrial pacing should include long-term Holter/telemetry, exercise test, carotid sinus stimulation, and an electrophysiological study excluding significant AV conduction disturbances.     

Pre-implant determinants of adequate long-term function of single lead VDD pacemakers.

AIMS: An inherent limitation of single lead VDD pacing is the inability to stimulate the atria. Reprogramming and upgrading the pacemaker system may be required when sinus node dysfunction, atrial undersensing, or atrial fibrillation develop. We evaluated whether routine clinical information is sufficient to select patients to benefit in long-term from VDD pacing. METHODS AND RESULTS: We collected data on 12-lead and monitored electrocardiograms and routine clinical information at implantation of a VDD pacing system in 350 consecutive patients with grade II or III atrioventricular conduction block. The age at implantation was 74.5 +/- 8.0 years, and the follow-up lasted 1.5 +/- 1.5 years. The cumulative maintenance of VDD pacing mode was 91.%. Loss of VDD mode was due to permanent atrial fibrillation in 16 (4.6%), sinus node dysfunction in six (1.7%). atrial undersensing in 11 (3.1%). Chronic atrial fibrillation developed in 23% of patients who had heart enlargement in chest x-ray and a history of paroxysmal atrial fibrillation or flutter. A criterion of normal sinus rate at implantation sufficiently predicted adequate sinus node function. Poor atrial sensing was not predicted by pre-implant characteristics. CONCLUSIONS: According to our data, adequate sinus-driven atrial rate and no history of paroxysmal atrial fibrillation and cardiac enlargement predict maintenance of the VDD pacing mode in elderly patients treated for heart block. Routine information available at implantation is sufficient to guide acceptance of single lead VDD pacing therapy.     

Clinical electrophysiologic effects of lorcainide in patients and its effect on the conduction of accessory pathway in Wolff-Parkinson-White syndrome

The electrophysiologic effects of the intravenous administration of a new antiarrhythmic drug, lorcainide, were evaluated by programmed electrical stimulation of the heart in 20 patients with and without Wolff-Parkinson-White (WPW) syndromes. Lorcainide shortened the sinus cycle length from 721.0 +/- 125.9 to 649.5 +/- 100.1 ms (P less than 0.001), but did not influence sinus node function and AV node conduction and refractoriness, slightly increased atrial effective period (ERP) (P less than 0.02) and did not change ventricular ERP (P less than 0.2), obviously lengthened atrial conduction time, H, H-V interval and the width of V wave. Lorcainide caused complete antegrade block of the accessory pathway (AP) in six of 9 WPW patients and resulted in exclusive conduction over the AV nodal. His conduction in two patients with atrial flutter. It also prolonged the retrograde conduction time and refractoriness of AP, and prevented initiation of orthodromic atrioventricular tachycardia (O-AVRT) in six of 12 patients by blocking of the retrograde conduction of the AP, increased the cycle length of tachycardia from 321.7 +/- 43.6 to 361.7 +/- 54.9 ms (P less than 0.005) by marked prolongation of retrograde AP conduction time in 6 patients in whom O-AVRT could still be induced. It is concluded that intravenous lorcainide does not affect sinus node and AV node function, slightly influences atrial and ventricular refractoriness, obviously suppresses atrial, His bundle and intraventricular conduction, and is an effective antiarrhythmic drug for patients with WPW by blocking both the antegrade and retrograde conduction of the AP.     

Selective radiofrequency ablation of the slow pathway for the treatment of atrioventricular nodal reentrant tachycardia. Evidence for involvement of perinodal myocardium within the reentrant circuit.

BACKGROUND. The circuit of atrioventricular (AV) nodal reentrant tachycardia may include perinodal atrial myocardium. Furthermore, in patients with dual AV nodal pathways, the atrial insertion of the slow pathway is likely to be located near the ostium of the coronary sinus, caudal to the expected location of the AV node. The present study was designed to evaluate the safety and efficacy of selective catheter ablation of the slow pathway using radiofrequency energy applied along the tricuspid annulus near the coronary sinus ostium as definitive therapy for AV nodal reentrant tachycardia. METHODS AND RESULTS. Among 34 consecutive patients who were prospectively enrolled in the study, the slow pathway was selectively ablated in 30, and the fast pathway was ablated in four. Antegrade conduction over the fast pathway remained intact in all 30 patients after successful selective slow pathway ablation. There was no statistically significant change in the atrio-His interval (68.5 +/- 21.8 msec before and 69.6 +/- 23.9 msec after ablation) or AV Wenckebach rate (167 +/- 27 beats per minute before and 178 +/- 50 beats per minute after ablation) after selective ablation of the slow pathway. However, the antegrade effective refractory period of the fast pathway decreased from 348 +/- 94 msec before ablation to 309 +/- 79 msec after selective slow pathway ablation (p = 0.005). Retrograde conduction remained intact in 26 of 30 patients after selective ablation of the slow pathway. The retrograde refractory period of the ventriculo-atrial conduction system was 285 +/- 55 msec before and 280 +/- 52 msec after slow pathway ablation in patients with intact retrograde conduction (p = NS). There were three complications in two patients, including an episode of pulmonary edema and the development of spontaneous AV Wenckebach block during sleep in one patient after slow pathway ablation and the late development of complete AV block in another patient after fast pathway ablation. Over a mean follow-up period of 322 +/- 73 days, AV nodal reentrant tachycardia recurred in three patients, all of whom were successfully treated in a second ablation session. CONCLUSIONS. Radiofrequency ablation of the slow AV pathway is highly effective and is associated with a low rate of complications.     

Electrophysiologic manifestations in so-called "sick sinus syndrome"

Electrophysiologic disorders in 17 patients with sick sinus syndrome (SSS) were assessed by recording of intracardiac electrograms, atrial overdrive pacing and extrastimulus technique. Significant suppression of the sinoatrial node (SAN) by overdrive pacing (maximum corrected SAN recovery time of longer than 560 msec) was noted in 14 or 16 patients studied. In nine patients, scanning with atrial extrastimuli, sinus rest was defined in all. In one patient there was a longer interpolation zone. Calculated sinoatrial conduction time (SACT) in individual patients varied considerably. The mean SACT was over 110 msec in 5 of 9 patients (56%). Sinus echo was demonstrated in 3; one manifested SAN- re-entrant tachycardia with rates of 72 to 77 beats/min. AV nodal echo was demonstrated in 3, two of them manifested AV nodal re-entrant tachycardia. Intracardiac electrograms revealed prolonged AV conduction time in 2 of 15 patients and prolonged His-Purkinje system conduction time in 2 of 17 patients studied. Two patients disclosed what we thought to be manifestation of intraatrial conduction disturbance. Both had considerable time interval between pacing impulse and atrial response. In one of them Mobitz type 1 and 2:1 intraatrial blocks were observed on atrial pacing and a possible internodal tract depolarization was also recorded. It is concluded that the electrophysiologic manifestations of patients with SSS cover a wide spectrum. The machanism of tachycardia can be due to either SAN or AV nodal re-entry.