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.
Cerebral tissue oxygen saturation values in volunteers and patients in the lateral decubitus and beach chair positions: a prospective observational study
