Venous return in lower limb during heat stress.

We used ultrasound to study venous return during heat stress. We measured venous cross-sectional area (CSA) and blood flow velocity (BFV) of nine femoral veins and nine saphenous veins. During heat stress, saphenous CSA increased from 4.7 +/- 2.6 mm2 (mean +/- SD) to 9.1 +/- 2.3 mm2 (P < 0.01), whereas femoral CSA was 22.7 +/- 9.5 mm2 at rest and 22.0 +/- 9.6 mm2 during heat stress (NS). Meanwhile, BFV increased from 0.06 +/- 0.02 to 0.30 +/- 0.10 m/s (P < 0.01) in the saphenous vein and from 0.14 +/- 0.08 to 0.38 +/- 0.23 m/s (P < 0.005) in the femoral vein. Maximal venous outflow (MVO) was the product of CSA and BFV. During heat stress, MVO showed an eightfold increase in the saphenous veins (from 22.7 +/- 18.2 to 180.7 +/- 86.7 ml/min) and a 2.5-fold increase in the femoral veins (from 143.4 +/- 52.9 to 354.0 +/- 126.9 ml/min). The results showed that one-half of the cutaneous blood flow increase during heat stress returned through the deep collecting veins in the lower limb. Thereafter, although there was no venodilation of deep veins compared with superficial veins, the deep veins remain the main pathway for the venous return during heat stress.