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.