Mechanism of change in pulmonary shunt flow with hemorrhage

Abstract

In the presence of regional atelectasis, hemorrhage increases the pulmonary shunt fraction (.ovrhdot.Qs/.ovrhdot.Qt). The increase in .ovrhdot.Qs/.ovrhdot.Qt which occurs may be due to inhibition of hypoxic vasoconstriction or to hydrostatic effects associated with a collapsed lung and decrease in cardiac output. The effects of hemorrhage on flow to a hypoxic lung (.ovrhdot.Qh) produced in 2 ways in anesthetized dogs were compared. In 6 dogs, atelectasis of 1 lung was produced by bronchial occlusion and in a 2nd group of 6 dogs, 1 lung was ventilated with 100% N2. The other lung in each group was ventilated with 100% O2. Hemorrhage (20 ml/kg) had no effect on .ovrhdot.Qh/.ovrhdot.Qt in dogs with a N2 ventilated lung suggesting that hemorrhage did not inhibit hypoxic vasoconstriction. Hemorrhage caused a marked increase in .ovrhdot.Qh/.ovrhdot.Qt in dogs with a collapsed lung because the flow to the inflated ventilated lung fell, while flow to the collapsed lung did not change. The increase in (.ovrhdot.Qs/.ovrhdot.Qt) in the dogs with atelectasis can be explained solely on the basis of mechanical effects produced by the differences in lung heights and alveolar pressures between ventilated and atelectatic lung.