Introduction
In extremely low birthweight infants (ELBW), pulmonary fluid clearance and transcapillary water transport are crucial for pulmonary development.1 The balance between inflammation, water transport, and upper and deep airway humidification is critical and plays a role in surfactant production and continued lung development during the critical period when the preterm infant is dependent on mechanical ventilation.2 A relatively small change (increase or decrease) in the temperature or humidity of the inspired gas may be sufficient to impair the preterm infant’s pulmonary function.3 Recommendations in this field come from adult studies, are uncertain,4 and stipulate that humidification must be maintained between 33 and 44 mg/L of absolute humidity (AH) and 75% to 100% of relative humidity (RH).5 Medical devices to heat and humidify the airways have been optimized for conventional intermittent positive pressure ventilation (IPPV) and can meet these humidification recommendations. High-frequency oscillatory ventilation (HFOV) uses very different settings than IPPV, often with high flow rates and always with small tidal volumes (VT).6
Even though Tarnow-Mordi et al. first raised the problem of the fragility of preterm patients with inadequate humidification in 1989, very few studies have since addressed humidification in ELBW infants.7 In 2009, Chikata et al. and Nagayaet al. reported that humidification during HFOV bench testing may be more problematic than during conventional ventilation.8 9 More recently, Ralphe et al.dedicated a review to humidification during tracheal and noninvasive ventilation for ELBW and insisted on the relation between bronchopulmonary dysplasia and unsuitable humidification.4 We think, however, that conclusions based only on bench-test studies must also be examined in the light of clinical and laboratory data. One animal study has highlighted an interesting link between variation of humidification and lung inflammatory responses.10 All of these authors4 7-10 expressed regret about the lack of studies of humidification during ventilation.
Humidification of the preterm patient’s airways depends on multiple factors such as the ventilator, circuits, heater-humidifier, ventilatory mode and settings, and environmental factors.4 We chose to study two of these: the heater-humidifier and the ventilatory mode. Hence, our aims were, first, to bench test recent heater-humidifiers during IPPV and HFOV, and, second, to compare in-vitro and in-vivo humidification achieved in each mode. Our hypothesis was that the specific parameters of HFOV could result in less adequate humidification in this mode than during IPPV.