SUMMARYThe 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 sinusnode 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. Twentyeight patients had normal heart rates (group A), and 28 patients had sinus bradyeardia (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 7,i 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 sinusnode response to AP, that is, normal sinus-node function.

Despite the widespread use of pervenous right ventricular pacing, there are relatively few reports concerning the occurrence of a right bundle-branch block pattern during apparent right ventricular pacing (Siddons and Sowton, 1967;Mower, Aranaga, and Tabatznik, 1967). This communication presents our observations of these patterns during pervenous ventricular pacing and analyses their clinical and electrophysiological implications. (Gordon, 1965) before and during the withdrawal of the catheter, suggested that it had been located in the coronary sinus (Fig. 2) block with a 0 13 sec. QRS complex with a left bundlebranch block pattern. A temporary bipolar pacing catheter was inserted into what was considered to be the outflow tract of the right ventricle, and ventricular pacing was achieved at a threshold of about 4-5 mA. Several days later when the threshold had risen to 22 mA and the chest x-ray suggested that the catheter was in the coronary sinus, the significance of the right bundlebranch block pattern recorded in the electrocardiogram (Fig. 3) became evident. The catheter was withdrawn and replaced by another which was positioned in the outflow tract of the right ventricle where a left bundlebranch block pattern was recorded during pacing (Fig. 3).Case 3. Cardiac catheterization demonstrated no abnormality in a 16-year-old boy who had been referred for the investigation of a cardiac murmur and incomplete right bundle-branch block in the electrocardiogram. At the end of the study, a bipolar pacing catheter was manipulated, in turn, to the outflow tract, apex, and inflow tract of the right ventricle, and surface electrocardiograms were obtained during pacing at these sites (Fig. 4, 5, and 6). The pacing rate, which was faster than the control rate, and voltage used for pacing were identical for all the three sites. A right bundle-branch block pattern was recorded during pacing of the apex of the right ventricle and retrograde ventriculo-atrial conduction with reciprocal (echo) beating was also observed (Fig. 7). Electrograms before pacing at all the sites demonstrated intraventricular morphology, and the threshold for ventricular pacing was under 1-5 mA at all sites.Case 4. An 86-year-old man was admitted with congestive heart failure and the electrocardiogram demonstrated atrial fibrillation and complete heart block with a 0-13 sec. QRS complex exhibiting a right bundlebranch block pattern. When he became compensated, a permanent pervenous pacemaker was implanted with the tip of the catheter wedged in the apex of the right ventricle where the threshold for pacing was 0 5 mA, and the electrogram displayed an intraventricular QRS complex. After operation the electrocardiogram showed 285

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