Onkar S. Narula

Significance of the Sinus-Node Recovery Time

The phenomenon of postpacing depression of cardiac pacemakers was utilized to evaluate the sinus-node function in 56 patients by analyzing the sinus-node recovery time (SRT), that is, the interval between the last paced P wave and the following sinus P wave. Corrected SRT (CSRT) is defined as the recovery interval in excess of the sinus cycle (SRT — sinus cycle length). The SRT was measured following sinus-node suppression by (1) isolated premature beats (PABs) and (2) atrial pacing (AP) at rates of 100 to 140/min for periods of 2 to 5 min at each level. Twenty-eight patients had normal heart rates (group A), and 28 patients had sinus bradycardia (SB; group B). Ten of the 28 patients with SB were restudied after receiving atropine (2 mg intravenously). The CSRT with PABs was similar in both group A and group B patients and remained essentially unchanged after atropine despite a decrease in sinus cycle length. The phenomenon of interpolated PABs was demonstrated in seven of the 56 patients. In 27 of the 28 patients with normal heart rates (group A), the CSRT with AP ranged from 110 to 525 msec and was essentially independent of the rate and duration of AP. In the remaining one patient of group A, despite a normal heart rate, the CSRT was prolonged (1810 msec) and directly dependent on the rate and duration of AP. In 12 of the 28 patients with SB, the CSRT was comparable to that in group A (≦525 msec). In the remaining 16 patients with SB (group B), the CSRT ranged from 560 to 3740 msec and was usually directly proportional to the rate and duration of AP. After atropine in most of the patients with a prolonged CSRT, the CSRT remained abnormal whereas in others junctional escape beats appeared first, followed eventually by normal sinus rhythm. In a single patient with SB and an abnormal CSRT, restudy 7½ months later again showed a prolonged CSRT indicating the reproducibility of the measurement. The CSRT with AP provides a potentially useful clinical means of assessing the sinus-node function and thereby aids in the diagnosis of the “sick sinus syndrome.’ It is stressed that AP was found to be more reliable than PABs in eliciting an abnormal response. Furthermore, a normal sinus (atrial) rate does not necessarily provide assurance of a normal sinus-node response to AP, that is, normal sinus-node function.

A new method for measurement of sinoatrial conduction time.

SUMMARY This study describes a new method (NM) for estimation of sinoatrial conduction time (SACT), which utilizes constant atrial pacing (AP) instead of the premature atrial beats (PABs) used in the method reported in 1973 by Strauss et al. The SACTs were obtained by both methods in 20 patients. The SACT by the Strauss method (SM) was calculated as A2A3 minus A1A,. The NM consists of high right AP for a train of eight consecutive beats at rates:510 beats/min faster than the sinus rhythm. The interval between the last paced atrial electrogram (Ap) and the first escape atrial electrogram (A) of sinus origin (Ap-A) was measured along with several post pacing sinus cycles. The SACT by the NM was calculated as follows: SACT = Ap-A minus A,A,. The effect of AP at higher rates was also analyzed. In two patients, the SACT with the SM could not be defined, as all the A2A, intervals were fully compensatory; with the NM the SACT was 217 and 320 msec. In the remaining 18 patients the SACT was obtainable by both methods. With SM, the SACT ranged 105-452 msec (mean 219 ± 102 SD) and with the NM it was 85-492 msec (mean 201 ± 112 SD), and the difference was statistically significant (P = 0.0162). The coefficient of correlation between the two methods was r = 0.97. During AP at faster rates, a rate related increment in Ap-A intervals and also post pacing sinus cycles was noted.This study describes a new and simple method for measurement of SACT in man.

Atrioventricular block: Localization and classification by his bundle recordings

Abstract A new classification for the qualitative and quantitative electrophysiologic localization of atrioventricular (A-V) conduction defects based on His bundle (BH) recordings in 500 patients is proposed. This study demonstrates that the electrocardiographic manifestations of all three degrees of A-V block can result from conduction abnormalities present in either of these three regions, viz, A-V node, BH and the His-Purkinje system. Intraatrial conduction delays may also produce first degree A-V block. It is also suggested that first degree block in any of these four regions of the A-V conduction system may not be apparent in the standard electrocardiogram if the P-R interval is normal. The use of BH recordings in conjunction with standard electrocardiographic leads may be of great value in the diagnosis, therapy and prognosis of disease of the cardiac conduction system in man.

Atrioventricular Conduction Defects in Patients with Sinus Bradycardia Analysis by His Bundle Recordings

A-V conduction was analyzed in 75 patients (mean age, 73 years) with sinus bradycardia (SB) by His bundle (BH) recordings. Forty-five of these patients had normal QRS complexes (≦0.10 sec) and 30 had abnormal QRS complexes (≧0.12 sec). The conduction times through the atrium (P-A), A-V node (A-H), and His-Purkinje system (H-V) were measured during normal sinus rhythm and transient right atrial pacing up to rates of 150/min. The normal range of conduction times through various regions is: P-A = 25 to 45 msec; A-H = 50 to 120 msec, and H-V = 35 to 45 msec. Overall A-V conduction was completely normal only in 25 (33%) patients. Of these 25 patients, 20 had a narrow QRS complex and five had an abnormal QRS complex. The remaining 50 patients showed abnormal A-V conduction in one or more regions (P-A in 10, A-H in five, BH in six, H-V in 10, and mixed in 19). In some patients despite normal ECGs, the H-V time was abnormal and 1:1 A-V conduction was present during atrial pacing up to 150/min. An abnormal H-V time in association with a normal QRS complex is suggestive of delay in the His bundle. Fourteen patients were restudied at intervals of 6 months to 2 years. Three of these 14 patients showed prolongation of conduction times over 1½ to 2 years.Our data demonstrate that SB is often (67%) associated with A-V conduction abnormalities. A-V conduction was abnormal in 56% of patients with normal and 83% with abnormal QRS complexes. The ECG and atrial pacing are of limited value in assessing A-V conduction. The BH recordings in symptomatic patients with sinus bradycardia may be of clinical significance in the selection of site for pacemaker implantation, that is, atrial or ventricular.

Sinus node re-entry: a mechanism for supraventricular tachycardia.

Data in 20 patients with normal sinus rhythm who, following induced premature atrial beats (PABs), manifested re-entry in the region of the sinus node are presented. PABs at coupling intervals ranging between 230-535 msec were followed by sinus node re-entry (SNR). Several criteria were applied to diagnose SNR: the temporal sequence of atrial excitation, the intra-atrial conduction time, the shape and polarity of the P waves, and manifestation of re-entry independent of atrioventricular (A-V) nodal delays. In 18 patients SNR lasting for one to several beats was observed. In the remaining two patients sustained SNR was manifested as supraventricular tachycardia (SVT), with cycle lengths ranging between 320-500 msec, which persisted for several minutes prior to its termination by properly timed atrial stimuli. In one of the latter two patients sustained SNR and SVT were produced only after atropine administration. Sinus node re-entry could be elicited at will in a specific echo zone with a duration ranging between 10-170 msec. In some cases SNR was also observed during and on cessation of regular atrial pacing. The latter observation provides a possible explanation for the unexpected sinus node acceleration seen sometimes on termination of atrial pacing. It is demonstrated that SNR for a single beat may fallaciously simulate sinus node entrance block. Simultaneous occurrence of re-entry at multiple sites, i.e., the sinus node and the A-V node, with resultant fusion P waves is also demonstrated. This study provides another mechanism for the genesis of regular and irregular SVT. The analysis and localization of the re-entry circuit may prove to be of clinical and therapeutic significance in selected patients.

Right bundle branch block with normal, left or right axis deviation: Analysis by his bundle recordings

Abstract His bundle (BH) electrograms were recorded in 123 patients. The electrocardiogram showed only left axis deviation (LAD) in ten, right bundle branch block (RBBB) with a normal axis in thirty, RBBB and LAD with or without old myocardial infarction in sixty-eight and RBBB with right axis deviation (RAD) in fifteen. Thirteen of these patients were restudied at intervals of six months to two years. The conduction times through the atrioventricular (A-V) node (A-H) and His-Purkinje system (H-V) were measured during normal sinus rhythm and atrial pacing. In all but four patients with sinus rhythm in whom second degree A-V block developed only during atrial pacing at rapid rates (range 100 to 180/minute) the block was localized in the A-H interval (A-V node). In the other four with sinus rhythm the second degree block was localized distal to the BH deflection (H-V). Most of the patients with RBBB (normal axis) and first degree A-V block had a normal H-V time with delay localized in the A-V node (A-H), indicating that this combination in the electrocardiogram does not necessarily mean bilateral bundle branch block. In none of the ten patients with only LAD was the H-V time abnormal (normal range=35 to 45 msec). In seven (23 per cent) of the thirty patients with RBBB (normal axis) the H-V time was abnormal. Forty-nine (72 per cent) of the sixty-eight patients with RBBB and LAD with or without myocardial infarction had an abnormal H-V time. Furthermore, patients with similar electrocardiographic patterns (RBBB and LAD) may or may not have an abnormal H-V time. All fifteen patients (100 per cent) with RBBB and RAD had abnormal H-V times. This study demonstrates that (1) the surface electrocardiographic criteria for assessing quantitative damage in the His-Purkinje system are frequently of limited value, and (2) the majority of the patients (72 per cent) with RBBB and LAD, besides RBBB and anterior hemiblock, have additional damage in the remaining fascicles of the His-Purkinje system. This would suggest partial bilateral bundle branch block or trifascicular block.

Wolff-Parkinson-White Syndrome: A Review

Recent developments in the field of electrophysiology and surgical therapy in selected cases of Wolff-Parkinson-White syndrome (W-P-W) support the concept of anomalous A-V pathways. Impulse transmission usually occurs simultaneously through both the normal and anomalous pathways resulting in a fusion QRS complex. Atrial tachycardia is usually due to reentry through the normal and anomalous A-V pathways. However, reentry may occur independently in the A-V node alone exclusive of the anomalous pathway. Anomalous connections, despite varying anatomic locations, may result in similar electrocardiographic manifestations characteristic of W-P-W. His bundle recordings together with electrophysiologic studies may be clinically useful (1) to differentiate various types of anomalous connections, (2) for possible determination of the reentry circuit, (3) to predict the maximum ventricular rate possible during supraventricular tachycardia by evaluating the refractory period of the A-V pathways, or (4) to compare the efficacy of different drugs in a given patient. Surgical interruption of the anomalous pathway in selected cases with W-P-W (type B) is feasible but is most commonly not necessary. The indications for medical and surgical management of symptomatic cases with W-P-W are reviewed.

Second-degree Wenckebach type AV block due to block within the atrium.

Two patients with a history of paroxysmal supraventricular tachycardia were analysed by His bundle and intra-atrial recordings. The electrocardiogram showed sinus bradycardia in I patient and sinus bradycardia with sino-atrial Wenckebach in the other. The conduction times through the AV node and His Purkinje system were normal in both cases. The intra-atrial conduction time was slightly prolonged (55 msec) in one and normal (40 msec) in the other during normal sinus rhythm. In both cases with atrial pacing from high right atrium, the conduction time from the pacing impulse (PI) to atrial activation in the area of the A Vjunction (PI-A) progressively lengthened with increase in atrial pacing rate and finally classical second-degree Wenckebach type of block was manifested at cycle lengths of 460 and 465 msec. The pacing impulse to QRS interval (PI-R) showed a progressive increase before the blocked stimulus. The lengthening of the PI-R interval was due to progressive increase in the intra-atrial (PI-A) conduction time. In the dropped beats, thepacing impulse was notfollowed by anA deflection. This observation indicated Type I block within the atrium. Similarly, during induced premature atrial beats, the PI-A time progressively lengthened as the coupling interval was shortened. These findings were reproducible and were seen despite a fourfold increase in stimulus strength and changes in electrodes and site of stimulation along the lateral right atrial border. This study (I) shows second-degree Wenckebach block within the atrium; (2) supports the existence of sinoatrial Wenckebach; and (3) suggests the atrium as another possible site for re-entry and a cause for supraventricular tachycardia because of the degree of delay and block exhibited within the atrium.

Accommodation of A-V Nodal Conduction and Fatigue Phenomenon in the His-Purkinje System

To understand the mechanisms responsible for disturbances in cardiac rhythm and conduction, it is useful to have a working knowledge of electrophysiologic events underlying normal and abnormal impulse propagation through different segments of the A-V conducting tissue. The ability to study these events in man has been facilitated by the development of the catheter technique for recording His bundle (BH) electrograms. Artificial pacing of the human heart, by permitting variations in site, rate and duration of stimulation, when used in conjunction with BH recordings have contributed significantly to the understanding of disturbances in impulse formation and conduction.

Incomplete Left Bundle-Branch Block A Definite Electrocardiographic Entity

Rate-dependent intermittent incomplete and complete left bundle-branch block (LBBB) is described in a patient without other evidence of heart disease. Continuous electrocardiographic tracings revealed QRS complexes intermediate in configuration between normal conduction and complete LBBB during the transitional phase from normal to abnormal conduction. This provided an opportunity to study the various grades of incomplete LBBB in man. During the incomplete LBBB phase, the morphology of the QRS complexes bore a close resemblance to the QRS form in the Wolff-Parkinson-White syndrome (type B). The differences between these two entities are emphasized.