SIMULATION OF CARDIOVASCULAR-RESPONSE TO LOWER-BODY NEGATIVE-PRESSUREFROM 0-MMHG TO -40-MMHG

This paper presents a mathematical model for simulation of the human c ardiovascular response to lower body negative pressure (LBNP) up to -4 0 mmHg both under normal conditions and when arterial baroreflex sensi tivity or leg blood capacity (LBC) is altered. Development of the mode l assumes that the LBNP response could be explained solely on the base s of 1) blood volume redistribution, 2) left ventricular end-diastolic filling, 3) interaction between left ventricle and peripheral circula tion, and 4) modulations of peripheral resistances and heart rate by a rterial and cardiopulmonary baroreflexes. The model reproduced well ex perimental data obtained both under normal conditions and during compl ete autonomic blockade; thus it is validated for simulation of the car diovascular response from 0 to -40 mmHg LBNP. We tested the ability of the model to simulate the changes in LBNP response due to a reduction in LBC. To assess these changes experimentally, six healthy men were subjected to LBNP of -15, -30, and -38 mmHg with and without wearing e lastic compression stockings. Stockings significantly reduced LBC (fro m 3.9 +/- 0.3 to 1.8 +/- 0.4 ml/100 ml tissue at -38 mmHg LBNP; P < 0. 01) and attenuated the change in heart rate (from 23 +/- 4 to 8 +/- 3% at -38 mmHg LBNP; P < 0.05). The model accurately reproduced this res ult. The model is useful for assessing the influence of LBC or other p arameters such as arterial baroreflex sensitivity in diminishing the o rthostatic tolerance of humans after spaceflight, bed rest, or enduran ce training.