Spatial pattern of ventilation-perfusion mismatch following acute pulmonary thromboembolism in pigs

Abstract

We studied the spatial distribution of the abnormal ventilation-perfusion (V̇a/Q̇) units in a porcine model of acute pulmonary thromboembolism (APTE), using the fluorescent microsphere (FMS) technique. Four piglets (∼22 kg) were anesthetized and ventilated with room air in the prone position. Each received ∼20 g of preformed blood clots at time t = 0 min via a large-bore central venous catheter, until the mean pulmonary arterial pressure reached 2.5 times baseline. The distributions of regional V̇a and blood flow (Q̇) at five time points ( t = −30, −5, 30, 60, 120 min) were mapped by FMS of 10 distinct colors, i.e., aerosolization of 1-μm FMS for labeling V̇a and intravenous injection of 15-μm FMS for labeling Q̇. Our results showed that, at t = 30 min following APTE, mean V̇a/Q̇ (V̇a/Q̇ = 2.48 ± 1.12) and V̇a/Q̇ heterogeneity (log SD V̇a/Q̇ = 1.76 ± 0.23) were significantly increased. There were also significant increases in physiological dead space (11.2 ± 12.7% at 60 min), but the shunt fraction (V̇a/Q̇ = 0) remained minimal. Cluster analyses showed that the low V̇a/Q̇ units were mainly seen in the least embolized regions, whereas the high V̇a/Q̇ units and dead space were found in the peripheral subpleural regions distal to the clots. At 60 and 120 min, there were modest recoveries in the hemodynamics and gas exchange toward baseline. Redistribution pattern was mostly seen in regional Q̇, whereas V̇a remained relatively unchanged. We concluded that the hypoxemia seen after APTE could be explained by the mechanical diversion of Q̇ to the less embolized regions because of the vascular obstruction by clots elsewhere. These low V̇a/Q̇ units created by high flow, rather than low V̇a, accounted for most of the resultant hypoxemia.