Venous air emboli in sheep: reversible increase in lung microvascular permeability

We studied the effects of continuous intravenous air embolization on lung fluid balance in unanesthetized sheep. Following a 2-h base line, we infused 300-micrometers-diam air bubbles into the pulmonary artery at a rate sufficient to increase pulmonary vascular resistance by 60-300% and for periods of 0.25-3 h. Air emboli produced a dose- and duration-dependent increase in lung lymph and protein flow due mainly to an increase in endothelial barrier permeability but also to elevated pressure in the over-perfused microcirculation. When we stopped the air infusion, the vascular resistance fell as the air emboli were reabsorbed. Lung lymph and protein flow increased further and then returned slowly to base line with a half-time proportional to the duration of embolization. Increasing left atrial pressure during air embolization caused an increase in lymph flow and protein clearance and an average decrease of 15 Torr in arterial O2 tension, all changes in excess of that caused by either increased pressure or air emboli alone.