978-66 Prolonged Bed Rest Impairs Response of the Small Arteries to Cold Pressor Test

Although most of the effects of prolonged bed rest on cardiovascular system is considered to be mediated by autonomic nervous system, there has been no evidence of change in α adrenergic activity. Cold pressor test has been known to increase vascular resistance through α adrenergic stimulation. To assess if change in α adrenergic activity occurs after prolonged bed rest, blood flow velocity of the proper palmar digital artery of the index finger (resistance vessel) was measured during 90 seconds of cold pressor test by pulsed Doppler ultrasound using a 7.5MHz linear transducer in 10 healthy volunteers before and after 20 days of strict horizontal bed rest. Lower body negative pressure (LBNP) test was performed to measure LBNP tolerance time as an indicator of orthostatic tolerance.Flow velocity decreased during the first 45 seconds of cold pressor test both before and after bed rest, and decreases in peak flow velocity were more prominent (p<0.05) before bed rest (-68±25%) than after bed rest (-48±20%). After 20 days bed rest, LBNP tolerance time shortened (p<0.02) from 19.5±5.6min to 10.8±5.0min in all but one subject who demonstrated prominent decrease in peak flow velocity during the first 45 seconds of cold pressor test. During the latter 45 seconds of cold pressor test, recovery of peak flow velocity was found in 7 subjects before bed rest, but in only 2 subjects after 20 days bed rest (p<0,05).In conclusion, 20 days bed rest impairs response of the small digital arteries to cold pressor test, which suggests that α adrenergic response diminishes after prolonged bed res