Nasal CPAP therapy: effects of different CPAP levels on pressure transmission into the trachea and pulmonary oxygen transfer

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Abstract
Background: Nasal continuous positive airway pressure (nCPAP) is considered useful for prophylaxis and treatment of respiratory complications following major thoracic surgery. It is unknown, however, which CPAP levels are required to avoid alveolar derecruitment and to consistently improve pulmonary oxygen transfer in patients following thoracotomy. We therefore studied the effects of different nCPAP levels on pressure transmission into the trachea as well as on pulmonary oxygen transfer.Methods:   In 10 consecutive patients after cardiac or thoracic vascular surgery, following extubation in the ICU, nCPAP was generated by means of a high‐flow gas source and applied randomly at levels of 5 or 10 cm H2O. Airway pressure was recorded continuously in the nasal mask and the trachea. The PaO2/FiO2ratio was calculated from the tracheal oxygen concentration, and PaO2 was determined while breathing at an ambient and elevated airway pressure. Haemodynamic variables (heart rate, arterial blood pressure, central venous pressure) were also recorded.Results:   Mean pressures in the nasal mask were 5.4±0.1 and 9.7±0.3 cm H2O. Corresponding tracheal pressures were 2.8±1.0 vs. 7.2±1.1 cm H2O (P=0.007). With higher mask pressure, the fraction of pressure transferred from the nasal mask into the trachea was larger (0.75±0.03 vs. 0.52±0.05; P=0.04), and tracheal pressures remained positive during the entire respiratory cycle in all patients. In contrast, with 5.4 cm H20, negative pressure changes during inspiration occurred in five out of 10 patients. The PaO2/FiO2 ratio increased from 183±53 (ambient pressure) to 199±74 (nCPAP 5.4 cm H2O; P=0.25) and to 333±54 (nCPAP 9.7 cm H2O; P=0.003). Nasal CPAP did not alter hemodynamics.Conclusion: Nasal CPAP is an effective non‐invasive means of increasing tracheal and thus intrathoracic pressure without adverse hemodynamic effects. Only mask pressures of 9–10 cm H2O were sufficient to consistently improve pulmonary oxygen transfer in patients following thoracotomy.