Initial effect of smoke inhalation injury on oxygen consumption (response to positive pressure ventilation)

Our purpose was to determine the effect of a smoke inhalation injury on initial oxygen demands measured as oxygen consumption. In addition, we wanted to determine the effect of positive pressure ventilation (PPV) on this process.

Adult sheep were insuffated with cotton toweling smoke to a carboxyhemoglobin level of 45% +/- 3% and then monitored unanesthetized for 24 hours. Oxygen delivery was maintained at a constant state.

A significant increase in oxygen consumption (VO2), indicating increased metabolic demands, occurred during the first 2 hours after smoke with peak increase of 75% +/- 10% above baseline. A second increase occurred peaking at 18 hours with a 40% +/- 11% increase. Both increases were due to increased O2 extraction from hemoglobin rather than increased cardiac output. Use of PPV during the first 2 hours had no effect on VO2 but did correct impaired lung function manifested by an increased shunt fraction. Use of PPV during the later increased VO2 totally reversed the process but had less effect on improving lung function.

We concluded that the initial increase in oxygen demands is likely due to an acute release of inflammatory mediators from the airway injury. The PPV response is to reexpand airways and alveoli, but it has no effect on the metabolic response. The late increase is likely due to increased work of breathing, which is removed by PPV. However, lung dysfunction from established airway edema at this stage is less reversible with PPV.