Parameters affecting occurrence of paradoxical air embolism.

The effects of different patterns of ventilation and intravascular volume infusion on the occurrence of paradoxical air embolism (PAE) were evaluated in 15 pigs with a surgically created atrial septal defect (ASD). A balloon atrial septostomy was created transvenously in anesthetized pigs (mean diameter 8.6 mm +/- 1 mm). Monitoring included transesophageal echocardiography (TEE) of the right and left heart, ECG, EEG, direct arterial pressure, right and left atrial pressures (RAP and LAP), pulmonary artery pressure, and pulmonary capillary wedge pressure (PCWP). With the animal in a head up tilt, air was infused into the superior vena cava at a rate of 0.27 ml.kg-1.min-1 for 6 min or until PAE was identified on the TEE. Four situations were studied--intermittent positive pressure ventilation (IPPV), intermittent positive pressure ventilation with 10 cm H2O positive end-expiratory pressure (PEEP), spontaneous ventilation, and IPPV following infusion of 500 ml hetastarch. The incidence of PAE was not different in any of the four situations. Release of PEEP resulted in an increase in the amount of PAE or new PAE in nine of 14 animals. PAE occurred both with and without mean RAP exceeding mean LAP and the incidence of PAE was not significantly different based on the atrial pressure gradient. In situations during which the mean LAP remained greater than mean RAP throughout the venous air infusion and PAE occurred, transient reversal of the right to left atrial pressure gradient during a portion of each cardiac cycle was demonstrated.