Introduction:
Positive end-expiratory pressure (PEEP) is used to increase oxygen delivery by preventing end-expiratory alveolar collapse. However, the associated increased intrathoracic pressure can lead to an increase in right atrial pressure, and a decrease in venous return and cardiac output. Near infrared spectroscopy (NIRS) can be used as a non-invasive tool to continuously monitor cerebral tissue oxygen saturation. In this pilot study, we examined the effects of PEEP on cerebral oxygen saturation during a controlled hemorrhage.
Methods:
Four female, domestic swine (~30 kg), were bled to 3 target levels of mean arterial pressure (MAP; 55, 45, and 35 mm Hg). At each MAP target, 3 levels of PEEP were applied using a mechanical ventilator (5, 10, and 15 cm H
2
O) for ~10 minutes each. Following the reinfusion of shed blood and a recovery period, these interventions were repeated. Measurements included invasive aortic pressure and cerebral oxygen saturation using a commercially available tissue oximeter. A total of 61 epochs were entered into the following regression model: cerebral oxygen saturation = MAP + PEEP + animal number. Each epoch contained data from the last ~2 minutes of each MAP target and PEEP level.
Results:
The regression model yielded a coefficient of 0.30 for MAP (
P
< 0.001) and -0.08 for PEEP (
P
= 0.09) and overall, explained 94% of the variance in cerebral oxygenation (adjusted
R
2
= 0.94,
P
< 0.001). While MAP was a stronger predictor in the model, higher PEEP levels appear to result in lower levels of cerebral oxygenation (see figure).
Conclusions:
Cerebral tissue oxygen saturation declines with lower mean arterial pressures and increased levels of PEEP. NIRS to measure cerebral oxygen saturation may be a useful clinical tool to ensure adequate cerebral oxygenation in patients with hypotension related to hemorrhage, particularly in those patients that require greater than physiologic PEEP to maintain central oxygenation.