Effects of CPAP-application via nasal mask and mouth-piece on gas exchange and respiratory mechanics after cardiac surgery

Abstract

Objective: To compare the effects of continuous positive airway pressure (CPAP) applied via nasal mask (NM-CPAP) vs. mouthpiece (MP-CPAP) on arterial oxygenation, respiratory mechanics and breathing pattern. Design: Clinical randomized trial. Setting: Intensive care unit of the University Hospital Ulm. Subjects: 17 postoperative patients after aortocoronary-bypass surgery. Interventions: All Patients had trials of spontaneous breathing with both types of CPAP application (NM-CPAP and MP-CPAP). Control measurements during breathing at ambient pressure were performed before or after each CPAP period. The order of measurement (control vs. CPAP and NM-CPAP vs. MP-CPAP) were randomly assigned. Measurements and main results: At each point of measurement we continuously recorded airway and oesophageal pressure as well as flow curves for off-line breath-to-breath calculation of tidal and minute volumes (VT and MV), respiratory rate (f), work of breathing (WOB), and pressure-time product (PTP). Furthermore, we measured arterial oxygen and carbon dioxide partial pressures (PaO2 and PaCO2), arterial oxygen saturation (SaO2), heart rate (HR), and mean systemic arterial pressure (MAP). WOB did not differ significantly with NM- and MP-CPAP at both points of measurement, in contrast, PTP was significantly lower with NM-CPAP 24 but not 6 h after extubation. When compared to control, PTP also was significantly lower with NM-CPAP at the first but not at the second point of measurement. In contrast, when comparing control vs. MP-CPAP, PTP significantly increased 24 h after extubation. MV was significantly higher during MP-CPAP at both points of measurement when compared to NM-CPAP. In contrast, only 24 h after extubation VT measured during MP-CPAP significantly differed from NM-CPAP. When compared to control, MV was significantly lower during NM-CPAP at the first point of measurement and VT significantly higher at the second one. f was almost identical comparing NM- and MP-CPAP. When compared to control, however, a statistically significant decrease of this parameter was observed 24 h after extubation. When compared to control, PaO2 significantly increased during CPAP regardless of the application mode. In contrast, PaCO2 was unchanged throughout the study period. Haemodynamics did not reveal any significant change regardless of the study conditions. Conclusions: Both ways of CPAP-application have similar beneficial effects on arterial oxygenation. NM-CPAP, however, did not affect the breathing pattern as much as MP-CPAP, and was less exhaustive. These observations are likely to be partially responsible for the better acceptance of NM-CPAP therapy.