Background: The amount of oxygen consumed by the lung itself is difficult to measure because it is included in whole-body gas exchange. It may be increased markedly under pathological conditions such as lung infection or adult respiratory distress syndrome. To estimate normal oxygen consumption of the human lung as a basis for further studies, respiratory gas analysis during total cardiopulmonary bypass may be a simple approach because the pulmonary circulation is separated from systemic blood flow during this period.
Methods: Lung oxygen consumption was determined in 16 patients undergoing cardiac surgery. During total cardiopulmonary bypass their lungs were ventilated with low minute volumes (tidal volume, 150 ml; rate, 6 min-1; inspiratory oxygen fraction, 0.5; positive end-expiratory pressure, 3 mmHg). All expiratory gas was collected and analyzed by indirect calorimetry. As a reference value also, whole-body oxygen consumption of these patients was determined before total cardiopulmonary bypass. In a pilot study of eight additional patients (same ventilatory pattern), the contribution of systemic (bronchial) blood flow to pulmonary gas exchange during cardiopulmonary bypass was assessed. For this purpose, the amount of enflurance diffusing from the systemic blood into the bronchial system was measured.
Results: The human lung consumes about 5-6 ml oxygen per minute at an esophageal temperature of 28 degrees C. Prebypass whole-body oxygen consumption measured at nearly normothermic conditions was 198 +/- 28 ml/min. Mean lung and whole-body respiratory quotients were similar (0.84 and 0.77, respectively). Extrapolating lung oxygen consumption to 36 degrees C suggests that the lung consumes about 11 ml/min or about 5% of total body oxygen consumption. Because the amount of enflurane diffused from the systemic circulation into the bronchial system during cardiopulmonary bypass was less than 0.1%, the contribution of bronchial blood flow to lung gas exchange can be assumed to be negligible.
Conclusions: The lung consumes about 5% of whole-body oxygen uptake.