Background: The amount of oxygen consumed by the lung
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| The amount of oxygen consumed by the lung |
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, the 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 enflurane
diffusing from the systemic blood into the bronchial system was measured.
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Results: The human lung consumes about 5-6 ml
oxygen per minute at an esophageal temperature of 28 degrees C. Pre bypass
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.
