Bubble CPAP: not all bubbling is good bubbling.

Since its introduction over 30 years ago,1 bubble CPAP has become an important part of the management of neonatal respiratory distress syndrome. Similar to “ventilatorderived” or “machine-derived” CPAP devices, bubble CPAP devices apply pressure to the neonatal respiratory system via nasal prongs placed into the infant’s nostrils, forming a tight seal to minimize leak. On the other hand, bubble CPAP devices are much simpler than ventilatorderived CPAP, often consisting only of humidified bias flow being delivered into the inspiratory limb, and the CPAP level set based on the distance the expiratory limb of the circuit is submerged into a water seal chamber. The simplicity of this system makes it attractive to many centers, especially resource-limited neonatal units in developing countries.2 Bubble CPAP also differs from ventilator-derived CPAP mechanistically. The mean pressure applied to the infant’s airway in bubble CPAP is not constant, as it is in ventilator-derived systems, but, rather, resonant, with the airway pressure actually fluctuating approximately 4 cm H2O around the mean.3 This resonance results from the bubbling that occurs when the bias flow reaches the water seal chamber. For example, in a bubble CPAP system in which the expiratory limb of the circuit is submerged 5 cm H2O, the pressure applied to the airway may actually be “oscillating” between 3 and 7 cm H2O. These pressure swings are caused by pressure amplitudes created by the bubbling in the water seal chamber that reflects back through the expiratory limb of the circuit. Several studies have suggested this “noise” contributes to alveolar recruitment and maintenance of airway patency, and is thus likely responsible for some of the beneficial effects of bubble CPAP on gas exchange, lung volumes, and outcomes that have been reported in infants with neonatal respiratory distress syndrome.4-6 Further, this effect seems to be most pronounced early after birth, when compliance is low and the oscillating pressure amplitudes are minimally dampened as they are transmitted through the respiratory system.3 In this month’s issue of RESPIRATORY CARE, Youngquist and colleagues report an elegant set of experiments evaluating the effect of a different kind of “noise” on the pressure delivered to the airway in infants receiving bubble CPAP.7 For most infants receiving positive-pressure ventilation, whether from traditional mechanical ventilators or bubble CPAP devices, condensate forms in the expiratory limb of the circuit as warm humidified gas is exposed to room temperature tubing. As the authors note, if there is a low point in the expiratory tubing, a considerable amount of condensate will form and “bounce/bubble” around in the tubing as more gas flows past. In traditional mechanical ventilators, this “bubbling” can lead to excessive activation of the flow trigger and lead to hyperventilation.8