Measurement of Inspired Gas Temperature in the Ventilated Neonate
Ventilated neonates routinely have their inspired gases warmed and humidified via a humidifier and a heated inspiratory line as part of the ventilator circuit. The normal physiological temperature of the neonatal airway is largely unknown, as are the optimal temperature and humidity of the inspired gas. The main roles of warming are to achieve adequate levels of humidification and reduce heat loss through the lungs. High levels of humidity are technically difficult to measure; sensors are costly, fragile and when saturated with condensed water give inaccurate measurements. Indirect assessment via calculation from water consumption, gas flow and temperature would provide, at best, a crude long-term average measure of humidity. Temperature probes are slow, durable and inexpensive, and as a result airway temperature, as measured by a probe placed in the inspiratory limb of the ventilator circuit, is routinely used as a proxy measure of humidity. However, these temperature measurements are usually measured at the circuit temperature probe, which is at least 8 cm from the airway opening and even further from the infant’s lungs.