Abstract 276: The Effects of Peep and Blood Pressure on Cerebral Tissue Oxygen Saturation During Hemorrhage in Swine

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.

Modern technology–derived normative values for cerebral tissue oxygen saturation in adults

Modern near-infrared spectroscopy technology is increasingly adopted to measure cerebral tissue oxygen saturation. However, the normal range of cerebral tissue oxygen saturation in adults with such technology is unknown. We sought to measure cerebral tissue oxygen saturation in healthy volunteers using the novel O3 Regional Oximetry® device (Masimo Corporation, Irvine, CA, USA) and assess its relationship with key physical and haemodynamic characteristics. For ≥5 minutes, we continuously recorded cerebral tissue oxygen saturation, pulse oximetry, cardiac index and mean arterial pressure. We assessed for differences in cerebral tissue oxygen saturation between hemispheres, sex, skin type, comorbidity or smoking status, and for associations between cerebral tissue oxygen saturation and age, height, weight, SpO2and haemodynamic parameters. We recorded >32,000 observations in 98 volunteers aged 22 to 60 years, including 41 (42%) males. One-fifth had one or more co morbidities ( n=22, 22.5%), one-tenth were either current or former-smokers ( n=13, 13%), and most had a Fitzpatrick skin type of 3 or lower ( n=84, 86%). The mean combined average cerebral tissue oxygen saturation was 67.6% (95% confidence interval 66.8%–68.6%). We found statistically significant differences in cerebral tissue oxygen saturation according to hemisphere and an association between cerebral tissue oxygen saturation and mean arterial pressure and cardiac index. The combined average cerebral tissue oxygen saturation in 98 healthy volunteers was 67.6% with a narrow confidence interval and no combined average cerebral tissue oxygen saturation was below 56%. We also observed statistically significant yet quantitatively small cerebral tissue oxygen saturation differences between hemispheres, and an association between cerebral tissue oxygen saturation and mean arterial pressure and cardiac index.