The Effects of Right Atrial and Ventricular Pacing

assess changes in auscultatory findings produced by pacing-induced variations in cardiac rate, rhythm, and conduction. As the heart rate was increased in stepwise intervals to the maximum possible extent by right atrial pacing (RAP) in 14 patients, the interval between the Q wave and the click (Q-C) decreased in all cases (21 ± 2 msec/10 beats/min; P < 0.001). In two patients, RAP at rates of and above 118 and 159 per minute, respectively, resulted in disappearance of the click. During right ventricular pacing (RVP) without evidence of atrioventricular (A-V) dissociation on the surface ECG in ten patients, the click was inaudible at all pacing rates in three instances. In all seven of the cases in which the click was audible at the lowest rate of RVP with VA conduction, Q-C was greater and C-S2 less than that in sinus rhythm. As the rate of RVP was increased, Q-C was noted to decrease (26 ± 4 msec/10 beats/min) and C-S2 to increase (19 ± 7 msec/10 beats/min) in all patients. In three patients in whom RVP induced atrioventricular dissociation, the click was seen only in beats closely preceded by a P wave. In ten of 11 patients the click occurred earlier in systole with a postextrasystolic or post tachycardia beat as compared to its appearance after a sinus beat when a shorter preceding diastolic filling period was present (P < 0.001). In the eleventh patient a loud systolic murmur was present during postextrasystolic cycles. It is concluded that pacing-induced rhythm disturbances can result in disappearance of a midsystolic click or can alter its timing and cause it to mimic sonic phenomena seen in other disease states. The possibility of similar changes taking place as a result of spontaneously occurring disturbances of rate, rhythm, and conduction should be recognized in order that the possible diagnosis of mitral valve prolapse not be overlooked.