EFFECTS OF EMBOLUS SIZE ON HEMODYNAMICS AND GAS-EXCHANGE IN CANINE EMBOLIC PULMONARY-HYPERTENSION

We examined the effects of different-sized glass-bead embolization on pulmonary hemodynamics and gas exchange in 12 intact anesthetized dogs. Pulmonary hemodynamics were evaluated by multipoint pulmonary arterial pressure (Ppa)/cardiac output (Q) plots before and 60 min after sufficient amounts of 100-mu-m (n = 6 dogs) or 1,000-mu-m (n = 6 dogs) glass beads to triple baseline Ppa were given and again 20 min after 5 mg/kg hydralazine in all the animals. Gas exchange was assessed using the multiple inert gas elimination technique in each of these experimental conditions. Embolization increased both the extrapolated pressure intercepts (by 6 mmHg) and the slopes (by 5 mmHg.1(-1).min.m2) of the linear Ppa/Q plots, together with an 80% angiographic pulmonary vascular obstruction. These changes were not significantly different in the two subgroups of dogs. However, arterial PO2 was most decreased after the 100-mu-m beads, and arterial PO2 was most increased after the 1,000-mu-m beads. Both bead sizes deteriorated the distribution of ventilation (VA)/perfusion (Q) ratios, with development of lung units with higher as well as lower than normal VA/Q. Only 100-mu-m beads generated a shunt. Only 1,000-mu-m beads generated a high VA/Q mode and increased inert gas dead space. Hydralazine increased the shunt and decreased the slope of the Ppa/Q plots after 100-mu-m beads and had no effect after 1,000-mu-m beads. We conclude that in embolic pulmonary hypertension, Ppa/Q characteristics are unaffected by embolus size up to 1,000-mu-m. Gas exchange, however, is affected differently, with an increased shunt after the smallest emboli and higher VA/Q after the largest emboli. Pulmonary hypertension after the smallest emboli is partially reversible by hydralazine, but this further increases shunt.