POSITIVE END-EXPIRATORY PRESSURE INCREASES THE RIGHT-TO-LEFT SHUNT IN MECHANICALLY VENTILATED PATIENTS WITH PATENT FORAMEN OVALE

Objective: To determine the effect of the presence of a patent foramen ovale on the right-to-left shunt in patients with respiratory failure who receive positive end-expiratory pressure (PEEP). Design: Convenience sample with randomized application of PEEP. Setting: General intensive care unit of a university teaching hospital. Patients: A total of 46 mechanically ventilated patients with respiratory failure requiring an inspired oxygen concentration of at least 50% and a PEEP of at least 5 cm of H2O. Intervention: Randomized application of PEEP (0 and 10 cm of H2O). Measurements: A patent foramen ovale was detected by saline contrast transesophageal echocardiography. The alveolar-to-arterial oxygen difference and the right-to-left shunt were calculated from arterial and venous blood gas sampling. Results: In patients without a patent foramen ovale (n =39), the alveolar-to-arterial oxygen difference and the shunt fraction decreased (-50 mm Hg [95% CI, -21 to -67] and -0.05 [CI, -0.03 to -0.07], respectively) after adding PEEP (10 cm of H2O). In patients with a patent foramen ovale (n = 7), minimal changes were noted in the alveolar-to-arterial oxygen difference (4 mm Hg, P > 0.2), but the shunt fraction increased (0.05, CI, 0 to 0.09). Adding PEEP (10 cm of H2O) increased the shunt fraction in 6 of 7 (86%) patients with a patent foramen ovale, whereas the shunt increased in only 7 of 39 (18%) patients without a patent foramen ovale (P < 0.007). Conclusions: A patent foramen ovale was found in 7 of 46 patients (15%; CI, 6% to 29%) with acute respiratory failure. This condition is a common cause of lack of improvement in oxygenation with the addition of PEEP in the mechanically ventilated patient. In patients with a patent foramen ovale, the right-to-left shunt is usually increased by using PEEP.