Impact of CPAP on Forehead Near-infrared Spectroscopy Measurements in Patients With Acute Respiratory Failure: Truth or Illusion.

Background: Critically ill patients with acute respiratory failure admitted to an intensive care unit are at high risk for cerebral hypoxia. We investigated the impact of continuous positive airway pressure (CPAP) therapy on regional cerebral tissue oxygenation (rSO(2)). Materials and Methods: In total, 40 extubated surgical intensive care unit patients requiring classic oxygen therapy (COT) for acute respiratory failure were examined. Near-infrared spectroscopy (INVOS 5100C, Covidien) was used for 30 minutes to detect bilateral rSO(2) during COT via facemask (6 L/min) and CPAP therapy (40% fraction of inspired oxygen, 8 cm H2O CPAP) using a randomized crossover study design. Patients served as their own control. Continuous hemodynamic routine monitoring and blood gas analysis were performed. The effect of CPAP therapy on rSO(2) and influence of assessed covariables were investigated using a mixed linear model. Results: Median rSO(2) increased from 57.9% (95% confidence interval [CI], 54.2-61.5) during COT to 62.8% (95% CI, 59.2-66.5) during CPAP therapy (P<0.0001). The estimated difference from the mixed model between COT and CPAP is -5.0 (95% CI, -6.3 to -3.7). Median arterial partial pressure of carbon dioxide decreased from 47.8 +/- 5.1 mm Hg during COT to 43.1 +/- 5 mm Hg during CPAP (P<0.001), whereas arterial partial pressure of oxygen remained unchanged (P=0.329). In total, 23% of patients had SO2 levels <50%, with a higher prevalence under COT. Conclusions: Our results reveal that CPAP therapy compared with COT may influence rSO(2) in patients with acute respiratory failure. However, the cause of the rSO(2) increase following CPAP application remains to be elucidated, and the accuracy of cerebral oximetry during CPAP therapy in patients with acute respiratory failure remains questionable.