Physiological Relationship Between AV Interval and Heart Rate in Healthy Subjects: Applications to Dual Chamber Pacing

The purpose of this paper is to specify the mathematical relationship between spontaneous AV interval (AVI) and heart rate (HR), the amplitude and rate of variation of AVI, and the physiological factors likely to affect these characteristics. Ten patients with healthy hearts were studied. Two catheter electrodes were positioned in the right atrium and at the tip of the right ventricle respectively, allowing the detection of endocardial signals. The AV and AA intervals for each heart cycle were digitized to an accuracy of +/- 1 msec. Measurements were made at rest, then during a stress test on an exercise bicycle, and finally during the recovery phase. The results show that adaptation is very precise and takes place instantly. Any variation in heart rate causes an immediate, inversely proportional variation in AVI. Adaptation follows a linear pattern, generally with relatively low amplitude and an average AVI reduction of 27.5 +/- 11.2 msec for an average HR increase of 78.7 +/- 22.5 bpm, i.e., a decrease of 4 +/- 2.1 msec for an HR variation of 10 bpm. The amplitude and variation rate of AVI seem to be independent of the age and base value of the PR interval. These observations may be useful for designing new VDD or DDD pacemakers that automatically adapt the AV interval to the instantaneous heart rate. The hemodynamic benefits of this adaptation were also demonstrated.