Bubble CPAP elicits decreases in exhaled nitric oxide in rabbits

Endogenous nitric oxide (NO) has a key role in pulmonary function, and the application of positive end-expiratory pressure (PEEP) can increase exhaled NO (FE(NO)) in anesthetized animals and isolated lungs. The influence of continuous positive airway pressure (CPAP), which is similar to PEEP, on the FE(NO) level has not been investigated in humans or in animals. The present study was undertaken to determine whether and how the application of CPAP in spontaneously breathing rabbits influences levels of FE(NO). We performed a randomized crossover study to measure FE(NO) levels in 12 ketamine-anesthetized rabbits that were intubated via tracheostomy for bubble CPAP (B-CPAP) or ventilator-derived CPAP (V-CPAP), which are two of the most popular CPAP modes and which have different pressure sources. The baseline FE(NO) level was 23.8 +/- 2.6 ppb, which increased to 27.1 +/- 2.9 ppb (P < 0.001) during V-CPAP and decreased to 18.6 +/- 2.2 ppb (P < 0.001) during B-CPAP. We used one high-frequency oscillatory ventilator to repeat the experiment, in which the conventional ventilation function of the ventilator was used in the baseline and V-CPAP periods, and the high-frequency function was used to replace B-CPAP. Changes in FE(NO) were similar to our previous findings. This study demonstrated that the application of CPAP did influence levels of FE(NO). We speculate that the decrease in FE(NO) during B-CPAP may have been related to the bubble-associated high-frequency oscillation.