Factors enhancing cardiac output in resting subjects during cold exposure in air environment

Six young males, in semi-nude conditions, resting in dorsal reclining position, were exposed successively to a thermoneutral environment (30 min), a cold environment (1 degree C) (Cold) or thermoneutrality (Control) (120 min), and during 60 min recovery in thermoneutral conditions. Cardiac output has been measured using a Dual Beam Doppler. During cold stress a significant increase of heart rate (66.4 +/- 6.4 to 91.0 +/- 14.9 beats.min-1), systolic blood pressure (119.5 +/- 7.8 to 218.7 +/- 18.7 mmHg), diastolic blood pressure (68.1 +/- 11.7 to 114.3 +/- 28.3 mmHg) and cardiac output (5.42 +/- 0.96 to 8.08 +/- 1.28 l.min-1) were observed. On the contrary initial systolic aorta flow acceleration is significantly lowered (1130 +/- 120 to 840 +/- 170 cm.s-2). Systolic ejection volume remained unchanged throughout the whole experiment. Increase in cardiac output during cold air (1 degree C) exposure is thus only imputed to the higher heart rate partly due to hypersecretion of catecholamines. The diminution of the blood flow acceleration could be related to a lesser arterial compliance and/or to the cold induced hemoconcentration. Enhanced heart's mechanical work due to sympathetic stimulation, seems thus to be absorbed by the increase in the peripheral resistance.