Effect of end-expiratory pressure on total oxygen dynamics

The role of altered end-expiratory pressure on total oxygen dynamics was studied prospectively in 18 patients with injuries and sepsis. Eight patients received high tidal volumes (12 to 18 ml/kg), continuous positive airway pressure, and intermittent mandatory ventilation (CPAP/IMV); 10 patients received low tidal volumes (8 to 10 ml/kg) with zero end-expiratory pressure and assist control mode of ventilation (ZEEP/A-CM). CPAP/IMV patients had better oxygen tension, reduced physiologic shunting in the lung (24% versus 18%), and an improved arterial tension: inspired oxygen concentration ratio. CPAP/IMV patients also had significantly higher central filling pressures that were associated with significant reductions in cardiac output (8.2 L/min versus 6.4 L/min). The reduced cardiac output appeared to be a result of a reduction in left ventricular stroke work index. Consequently, the total oxygen delivery was reduced for all 3 days following insult and for the cumulative data for all 3 days (266 versus 306 ml/min) in the CPAP/IMV patients. Oxygen consumption was also reduced in the CPAP/IMV patients; this reduction was not significant for each of the first 3 days but was significant when the data for the 3 days were added to the analysis (306 versus 272 ml/min). Future prospective randomized studies are needed to determine the most effective use of ventilatory support on total oxygen dynamics including oxygen delivery and oxygen consumption.