Blood pressure and heart rate responses to sudden changes of gravity during exercise D. LINNARSSON, C. J. SUNDBERG, B. TEDNER, Y. HARUNA, J. M. KAREMAKER, G. ANTONUTTO, AND P. E. DI PRAMPERO Environmental Physiology Laboratory, Department of Physiology and Pharmacology, Karolinska Institutet, S-l 71 77 Stockholm, Sweden; Department of Health Administration, Tokyo University, Bunkyo-Ku, Tokyo 113, Japan; Department of Physiology, University of Amsterdam, NL-1105AZ Amsterdam, The Netherlands; and Department of Biomedical Science and Technology, University of Udine, I-33100 Udine, Italy Linnarsson, D., C. J. Sundberg, B. Tedner, Y. Haruna, J. M. Karemaker, G. Antonutto, and P. E. di Prampero. Blood pressure and heart rate responses to sudden changes of gravity during exercise. Am. J. Physiol. 270 (Heart Circ. Physiol. 39): H2132-H2142, 1996.-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 (G,+), starting upright and with re- peated 30-s tilts to the supine position. Another group was exposed to sudden G,+ 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 G,+ was suddenly decreased and a similar drop when G,+ was suddenly increased. HR displayed a reverse pattern with larger transients (-22 to -26 min-l) in response to G,+ decreases and more sluggish changes of lower amplitude in the other direction. Central blood volume, as estimated from the inverse of transthoracic impedance (l/TTI), varied in concert with G,+. A model is proposed in which HR responses are described as a function of CDP and l/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-l l mmHg-l (4-7 ms/mmHg). The cardiopulmonary baroreceptor compo- nent 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 impor- tance. carotid baroreceptors; cardiopulmonary baroreceptors; dy- namic modeling; microgravity; parabolic flight; time shifts; vagus nerve MECHANISMS FOR THE DEFENSE Of arterial pressure against gravitational stress have been extensively studied in resting humans (for review, see Ref. 24). From a functional point of view the usual test condition, quiet standing or passive tilting during rest, would seem artificial because most transitions between supine and upright positions involve simultaneous muscular activ- ity Except for studies on active standing that involve a short-lived muscle effort at the change of body posture, to our knowledge no studies have been performed involving sudden gravitational stress superimposed on dynamic exercise. We propose that such an experimen- tal model would provide new information regarding the efficiency of circulatory control (or lack thereof) to maintain blood pressure and blood perfusion under conditions of increased metabolic requirements. In the present study we have exposed exercising, healthy male subjects to sudden gravitational transients up to 1.8 times the normal gravity level during parabolic flight and to rapid tilting between upright and supine in a ground laboratory. A novel model for baroreflex control of heart rate is proposed. MATERIALS AND METHODS Ground-Based Experiments (Series A) Ten healthy male subjects were studied. Age, weight, and height were 20-34 yr, 63-81 kg, and 171-188 cm, respec- tively. The subjects exercised on a tilt board on which an ergometer was mounted with the crank axis at the level of the heart when the subject was supine. The tilt board could be rapidly tilted in -2 s between 80” upright and horizontal (0’) or the reverse. The subjects exercised at a workload of 100 W in the upright position. The 100-W workload was chosen to be roughly physiologically equivalent to the 80- and 90-W work- loads used during the inflight experiments at slightly reduced atmospheric pressures. After a warm-up period of 4 min, each subject was tilted without prior notice to horizontal for 30 s and then back to upright. This procedure was repeated three times at intervals of 2.5 min. Heart rate (HR) was obtained beat-by-beat from a precor- dial electrocardiogram (ECG) recording (Atomenergi AB, Sweden). Blood pressure was measured by a finger-cuff method (34), and readings were compensated for the hydro- static difference between the finger and the heart by measure- ment of the pressure in a water-filled catheter attached to the chest at the level of the fourth intercostal space at one end and to the finger cuff at the other end (cf. Ref. 10). Transthoracic impedance (TTI) was recorded from four tape electrodes as described by Kubicek et .al. (9). This recording was used as an index inversely proportional to changes in central blood volume (18). Data were recorded on a multichannel FM tape recorder together with a continuous recording of the tilt angle, esti- mated from the hydrostatic pressure in a water column parallel to the tilt board. Off-line data reduction included several steps and was performed with an ASYST data handling system at a sam- pling rate of 50 Hz per channel. Beat-by-beat mean arterial pressure (MAP) was computed and stored as a constant level for each period between two consecutive systolic peaks. The on-line analog beat-by-beat HR signal consisted of a series of levels changing for each R peak. The level represented the inverse of the duration of the previous R-R interval. Off-line, H2132 0363-6135/96 $5.00 Copyright o 1996 the American Physiological Society