Systolic time interval. Heart rate relationship during atrial or ventricular pacing.

In order to evaluate the relative importance of heart rate (HR) and A-V synchrony during pacing, 16 patients with VVI pacemakers were compared with 8 patients with AAI pacemakers using systolic time intervals. The patients were examined at rest at increasing HR with increments of 10 beats/min until a maximum of 150 beats/min was reached or for AAI pacing until 2 degrees A-V block occurred. The slopes of the linear regression equations for PEP versus HR and PEP/LVET versus HR were used. To compensate for differences in baseline values, delta PEP% versus delta HR% and delta PEP/LVET% versus delta HR% were also calculated and used to evaluate the response to the two different pacing modes. PEP seemed in this situation to be less dependent on HR than expected from earlier data concerning spontaneous differences in heart rate. This is probably due to a lower catecholamine level during pacing. During AAI pacing the slopes were slightly more normal than during VVI pacing, but the differences did only reach statistical significance using PEP/LVET versus HR (p less than 0.05). At heart rates above 80 blood pressure had a tendency to be higher during AAI pacing, a trend which was only significant for the diastolic blood pressure at HR from 130-150. These results support the view, that the ability to increase heart rate is more crucial for left ventricular function than A-V synchrony in these patients.