Cardiovascular response to acute hypovolemia in relation to age. Implications for orthostasis and hemorrhage

Venous compliance in the legs of aging man has been found to be reduced wit h decreased blood pooling (capacitance response) in dependent regions, and this might lead to misinterpretations of age-related changes in barorecepto r function during orthostasis. The hemodynamic response to hypovolemic circ ulatory stress was studied with the aid of lower-body negative pressure (LB NP) of 60 cmH(2)O in 33 healthy men [18 young (mean age 22 yr) and 15 old ( mean age 65 yr)]. Volumetric technique was used in the study of capacitance responses in the calf and arm as well as transcapillary fluid absorption i n the arm. LBNP led to smaller increase in heart rate (P < 0.001) and perip heral resistance (P < 0.01) and reduced transcapillary fluid absorption in the arm (P < 0.05) in old subjects. However, blood pooling in the calf was reduced in old subjects (1.66 +/- 0.10 vs. 2.17 +/- 0.13 ml/100 mi tissue; P < 0.01). Accordingly, during similar blood pooling in the calf (LBNP 80 c mH(2)O in old subjects), no changes in cardiovascular reflex responses with age were found. The capacitance response in the arm (mobilization of perip heral blood to the central circulation) was still reduced, however (0.67 +/ - 0.10 vs. 1.37 +/- 0.11 ml/100 mi tissue; P < 0.01). Thus the reduced card iovascular reflex response found in the elderly during orthostatic stress s eems to be caused by a reduced capacitance response in the legs with age an d a concomitant smaller central hypovolemic stimulus rather than a reduced efficiency of the reflex response. With similar hypovolemic circulatory str ess, no changes in cardiovascular reflex responses are seen with age. The c apacitance response in the arm (mobilization of peripheral blood toward the central circulation) is reduced, however, by similar to 50% in the elderly . This might seriously impede the possibility of survival of an acute blood loss.