Blood pressure and heart rate responses to sudden changes of gravity during exercise.
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.
Linnarsson
D
D
Department of Physiology and Pharmacology, Karolinska Institutet, Stockholm, Sweden.
Sundberg
C J
CJ
Tedner
B
B
Haruna
Y
Y
Karemaker
J M
JM
Antonutto
G
G
Di Prampero
P E
PE
D000328:Adult; D000337:Aerospace Medicine; D017704:Baroreflex / Q000502:physiology; D001794:Blood Pressure; D002339:Carotid Arteries / Q000294:innervation / Q000502:physiology; D015444:Exercise; D006112:Gravitation; D018475:Head-Down Tilt; D006339:Heart Rate; D006801:Humans; D008297:Male; D008955:Models, Cardiovascular; D016683:Supine Position