Base heart rate during sleep in hypertensive and normotensive subjects

Many studies have reported that the resting heart rate (HR) is intimately related to the prognosis of cardiovascular disease. However, HR in the waking state is influenced by psychological and physical activity and has low reproducibility. To measure the base HR (minimum and stable HR during sleep) with ease, we developed a wristwatch-type HR recorder. We then investigated the pathophysiological significance of the base HR.

The subjects were 158 patients (82 men and 76 women) with essential hypertension (HT) on no medication and 204 normotensive (NT) subjects (105 men and 99 women, matched for age). On the basis of pulse waves from the wristwatch-type photoelectric plethysmograph, using a blue sensor with a wavelength of 450 nm, HR was recorded easily minute by minute during sleep time. In addition, 40 (23 men and 17 women) of the HT patients were hospitalized. Their cardiac index (CI) and stroke volume index (SVI) were measured by means of the cuvette method during waking time and night-time deep sleep when their HR had reached a minimum level.

Mean base HR (HRo) in NT subjects was 49+/-4 beats per minute (bpm) and tended to rise with increasing age (r = 0.51, P < 0.01). In HT patients, HR0 was significantly higher: 60+/-7 bpm (P < 0.01). CI of HT patients, measured during night-time deep sleep, maintained a stable value of 2.45+/-0.36 l/min per m2. SVI in patients whose HR0 was above 60 was significantly lower than that in patients whose HR0 was below 60 (37+/-4.5: 45+/-4.8 ml/m2, P < 0.01). Corresponding differences in mean HR and SVI during waking or night-time, became smaller.

These findings suggest that there is a close relation between base HR and cardiac function caused by such conditions as age and hypertension. This relation became indistinct during waking time. Moreover, the wristwatch-type HR recorder appears to be a useful tool for measuring base HR.