VALIDATION OF LOWER BODY NEGATIVE PRESSURE AS AN EXPERIMENTAL

23 Lower body negative pressure (LBNP), a model of hemorrhage (Hem), shifts blood to 24 the legs and elicits central hypovolemia. This study compared responses to LBNP and 25 actual Hem in sedated baboons. Arterial pressure, pulse pressure (PP), central venous 26 pressure (CVP), heart rate, stroke volume (SV), and +dP/dt were measured. Hem steps 27 were 6.25%, 12.5%, 18.75%, and 25% of total estimated blood volume. Shed blood 28 was returned, and four weeks after Hem, the same animals were subjected to four 29 LBNP levels which elicited equivalent changes in PP and CVP observed during Hem. 30 Blood gases, hematocrit (Hct), hemoglobin (Hb), plasma renin activity (PRA), 31 vasopressin (AVP), epinephrine (EPI), and norepinephrine (NE) were measured at 32 baseline and maximum Hem or LBNP. LBNP levels matched with 6.25%, 12.5%, 33 18.75% and 25% hemorrhage were -22±6, -41±7, -54±10, and -71±7 mmHg, 34 respectively (mean ± SD). Hemodynamic responses to Hem and LBNP were similar. 35 SV decreased linearly such that 25% Hem and matching LBNP caused a 50% reduction 36 in SV. Hem caused a decrease in Hct, Hb, and central venous oxygen saturation 37 (ScvO2). In contrast, LBNP increased Hct, and Hb, while ScvO2 remained unchanged. 38 Hem caused greater elevations in AVP and NE than LBNP, while PRA, EPI, and other 39 hematologic indices did not differ between studies. These results indicate that while 40 LBNP does not elicit the same effect on blood cell loss as Hem, LBNP mimics the 41 integrative cardiovascular response to Hem, and validates the use of LBNP as an 42 experimental model of central hypovolemia associated with Hem. 43