Effects of posture on blood flow diversion by hypoxic pulmonary vasoconstriction in dogs

We used differential excretion of sulphur hexafluoride from the left and right lung to measure blood flow diversion by hypoxic pulmonary vasoconstriction (HPV) in the prone and supine positions in dogs (n=9). Gas exchange was assessed using the multiple inert gas elimination technique. Blood flow diversion from the hypoxic (3% oxygen) left lung was mean 70.7 (SD 11.2)% in the supine compared with 57.0 (12.1) % in the prone position (P<0.02). The supine position was associated with increased perfusion to low (V) over dot (A)/(Q) over dot regions (P<0.05). The increased flow diversion with hypoxia in the supine position was associated with more ventilation to high (V) over dot (A)/(Q) over dot regions (P<0.05). We conclude that flow diversion by hypoxic pulmonary vasoconstriction is greater in the supine position. This effect could contribute to the variable response in gas exchange with positioning in patients with ARDS.