Background: Low inspired oxygen acutely increases pulmonary vascular resistance and decreases pulmonary-systemic blood flow ratio. We present a simple method to lower inspired oxygen fraction (FIO2 < 0.21) without supplemental nitrogen, during mechanical ventilation by an anesthesia machine. Methods: After institutional approval, seven healthy adult volunteers and three infants (0-12 month old) scheduled for congenital heart surgery were enrolled in this study. All the infants were diagnosed with congestive heart failure because of high pulmonary blood flow and were thought to benefit from low FIO2. The volunteers performed spontaneous ventilation (fresh air flow rate = 10 l center dot min(-1), tidal volume = 600 ml, frequency = 10 br center dot min(-1)). The infants were mechanically ventilated with air (fresh air flow rate = 6 l center dot min(-1), tidal volume = 10 ml center dot kg(-1), 15 < frequency < 30 br center dot min(-1) to adjust PaCO2 between 5.8 kPa and 6.5 kPa (45-50 mmHg), after induction of general anesthesia and tracheal intubation. The fresh gas flow rates were determined by the following formula. Fresh gas flow rate = (FIO2 - FEO2) EVE/(0.21 + FIO2 - FEO2 - target FIO2). We recorded FIO2 every 5 min for 30 min. When arterial oxygen saturation decreased > 15%, fresh gas flow rates were increased to adjust FIO2 to 0.21. Results: In all of the seven volunteers and three infants target FIO2 was achieved in < 10 min. FIO2 was kept at 0.18 +/- 0.01 (SD) by calculated fresh air flow rates. In one infant, SpO(2) decreased > 15% 20 min after lowering FIO2, we had to discontinue this study, and increase fresh gas flow to ventilate the infant with FIO2 0.21. In the other two infants, FIO2 was maintained throughout the study. Conclusions: This simple and convenient method to decrease FIO2, has a utility in clinical situations, in which pulmonary vascular resistance is to be increased to improve systemic oxygen delivery in patients with high pulmonary blood flow during cardiac surgery.
