Atrial pressures in the seated position. Implication for paradoxical air embolism

In order to investigate the circumstances in which air bubbles in the venous circulation could pass to the arterial circulation via a patent foramen ovale, simultaneous right atrial (RAP) and pulmonary capillary wedge pressures (PCWP) were determined in two patient populations undergoing elective neurosurgical procedures in the seated position. In Group 1 (n = 24), quadruple-lumen pulmonary arterial catheters were inserted prior to the induction of anesthesia, and RAP and PCWP were determined while the patients were: 1) awake and supine, 2) awake and seated, 3) anesthetized and supine, 4) anesthetized and seated, and 5) 60 minutes after skin incision. In Group 2 (n = 31), RAP and PCWP were measured prior to, and then during intraoperative episodes of clinical air embolism. In Group 1, placement in the seated position while awake resulted in a significant decrease in PCWP (8.5 mmHg ± 0.9 SE supine vs. 6.6 mmHg ± 0.9 SE seated, P < 0.05), but no change in RAP (6.3 mmHg ± 0.6 SE supine vs. 6.5 mmHg ± 0.9 SE seated). Similar changes were found after induction of anesthesia (PCWP supine = 7.8 mmHg ± 0.8 SE vs. PCWP seated = 5.6 mmHg ± 0.8 SE, P < 0.05, and RAP supine = 6.6 mmHg ± 0.8 SE vs. RAP seated = 4.4 mmHg ± 0.7 SE). Sixty minutes after skin incision, 13 of the 24 patients had PCWP lower than RAP, and mean PCWP was less than RAP (5.6 mmHg ± 0.8 SE vs. 6.2 mmHg ± 0.8 SE). In Group 2, mean pulmonary artery pressure increased markedly with venous air embolism (10.9 mmHg ± 0.7 SE to 18.0 mmHg ± 1.04 SE, P < 0.01). Smaller increases were seen in both RAP (3.7 mmHg ± 0.7 SE vs. 4.7 mmHg ± 0.6 SE, P < 0.01) and PCWP (4.6 mmHg ± 0.6 SE vs. 5.5 mmHg ± 0.7 SE, P < 0.01), but there was no evidence of RAP acutely exceeding PCWP. The authors conclude that use of the seated position inherently predisposes some neurosurgical patients to the risk of paradoxical air embolism, since the normal interatrial pressure gradient frequently becomes reversed in this position.