BLOOD-PRESSURE AND HEART-RATE RESPONSES TO SUDDEN CHANGES OF GRAVITY DURING EXERCISE

Heart rate (HR) and blood pressure responses to sudden changes of grav ity during 80- to 100-W leg exercise were studied. One group was expos ed to sudden changes between 1.0 and 0 g in the head-to-foot direction (G(z+)), starting upright and with repeated 30-s tilts to the supine position. Another group was exposed to sudden G(z+) changes between 1. 8 and 0 g in an aircraft performing parabolic flight. Arterial blood p ressure at the level of the carotid (carotid distending pressure, CDP) showed a large transient increase by 27-47 mmHg when G(z+) was sudden ly decreased and a similar drop when G(z+) was suddenly increased. HR displayed a reverse pattern with larger transients (-22 to -26 min(-1) ) in response to G(z+) decreases and more sluggish changes of lower am plitude in the other direction. Central blood volume, as estimated fro m the inverse of transthoracic impedance (1/TTI), varied in concert wi th G(z+). A model is proposed in which HR responses are described as a function of CDP and 1/TTI after a time delay of 2.3-3.0 s and includi ng a low-pass filter function with time-constants of 0.34-0.35 s for d ecreasing HR and time constants of 2.9-4.6 s for increasing HR. The se nsitivity of the carotid component was around -0.8 to -1.0 min(-1). mm Hg(-1) (4-7 ms/mmHg). The cardiopulmonary baroreceptor component was a n additive input but was of modest relative importance during the init ial HR responses. For steady-state HR responses, however, our model su ggests that inputs from carotid and cardiopulmonary receptors are of e qual importance.