High-frequency ventilation in lung edema: effects on gas exchange and perfusion

In six open-chest dogs, unilobar pulmonary edema was induced by injection of oleic acid into a lower lobe pulmonary arterial branch. Two hours later, intermittent positive-pressure ventilation (IPPV) and high-frequency oscillatory ventilation (HFOV) at matched mean alveolar pressures (MAP) were compared at 30-min intervals. The edematous lobar venous admixture (Qva/QT) increased by 0.14 (P less than 0.1) and its relative perfusion (QL/QT) decreased by 0.03. (P less than 0.1) during HFOV compared with IPPV. In another six dogs the MAP was increased by 1.5 cmH2O during HFOV; now the edematous lobar Qva/QT decreased by 0.15 (P less than 0.03) and QL/QT increased by 0.03 (P less than 0.03) during HFOV compared with IPPV. Greater MAP during HFOV also improved the edematous lobar ventilation (lobar venous PCO2) during HFOV compared with IPPV. Conceivably, the increase in MAP inflated collapsed, flooded regions, and thereby increased their O2 transfer, perfusion, and ventilation. Whole-lung Qva/QT was similar during IPPV and HFOV in both groups because of the opposing effects of lobar Qva/QT and QL/QT. We conclude that HFOV supports cardiovascular-respiratory function in open-chest unilobar pulmonary edema compared with IPPV at similar MAP; gas exchange and perfusion of edematous units may be sensitive to small changes in MAP, and we detected no intrinsic effects of HFOV on these variables. We speculate that similar beneficial effects in diffuse pulmonary edema may be effected by small variations in MAP during HFOV without the barotrauma side effects sometimes observed when MAP is increased during IPPV.