Blood pressure and heart rate responses to sudden changes of gravity during exercise

• Published in: The American journal of physiology Vol. 270; no. 6 Pt 2; p. H2132
• Main Authors: Linnarsson, D, Sundberg, C J, Tedner, B, Haruna, Y, Karemaker, J M, Antonutto, G, Di Prampero, P E
• Format: Journal Article
• Language: English
• Published: United States 01.06.1996
• Subjects: Adult; Aerospace Medicine; Baroreflex - physiology; Blood Pressure; Carotid Arteries - innervation; Carotid Arteries - physiology; Exercise; Gravitation; Head-Down Tilt; Heart Rate; Humans; Male; Models, Cardiovascular; Supine Position
• DOI: 10.1152/ajpheart.1996.270.6.h2132
• ISSN: 0002-9513

Heart rate (HR) and blood pressure responses to sudden changes of gravity during 80- to 100-W leg exercise were studied. One group was exposed to sudden changes between 1.0 and 0 g in the head-to-foot direction (Gz+), starting upright and with repeated 30-s tilts to the supine position. Another group was exposed to sudden Gz+ changes between 1.8 and 0 g in an aircraft performing parabolic flight. Arterial blood pressure at the level of the carotid (carotid distending pressure, CDP) showed a large transient increase by 27-47 mmHg when Gz+ was suddenly decreased and a similar drop when Gz+ was suddenly increased. HR displayed a reverse pattern with larger transients (-22 to -26 min-1) in response to Gz+ decreases and more sluggish changes of lower amplitude in the other direction. Central blood volume, as estimated from the inverse of transthoracic impedance (1/TTI), varied in concert with Gz+. 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 including a low-pass filter function with time constants of 0.34-0.35 s for decreasing HR and time constants of 2.9-4.6 s for increasing HR. The sensitivity of the carotid component was around -0.8 to -1.0 min-1 . mmHg-1 (4-7 ms/mmHg). The cardiopulmonary baroreceptor component was an additive input but was of modest relative importance during the initial HR responses. For steady-state HR responses, however, our model suggests that inputs from carotid and cardiopulmonary receptors are of equal importance.