Temperature loss from gases in the ETT exposed to ambient

Introduction

It is essential to heat and humidify gases delivered to mechanically ventilated patients in order to maintain airway function. Humidifiers will control the gas temperature along the breathing circuit up to the temperature probe at the Y-piece. Beyond this point the gases will cool as they pass through an unheated length of circuit. In order to deliver optimally conditioned gases (ie core temperature and 100% RH) to the patient this temperature loss, and concomitant humidity loss, must be compensated for.

Method

Gas temperatures in the breathing circuits of 5 adult ICU patients undergoing mechanical ventilation, were monitored.Vt=0.75 ml (SD=0.04) and RR=11.4 bpm (1.5). The humidity and temperature of the inspired gases were controlled to 30°C, 34°C, 37°C, 40°C at 100%RH in sequence, by a heated humidifier with heated wire circuit (Fisher & Paykel MR730). The ambient temperature was 25°C with still air. The unheated length of breathing circuit on each patient was 10.8 cm (0.2) of endotracheal tube (ETT) protruding from the teeth, and 2 cm of suction port attached directly to the Y-piece. A K type thermocouple (response time 0-90% = 0.1 s) mounted on a suction catheter was inserted through the port to measure temperature in the centre of the circuit at two sites: the middle of the suction port at the exit of the Y-piece, and in the ETT just prior to the teeth.

Results

Temperature and humidity loss were recorded over the portion of ETT protruding from the teeth during inspiration and expiration (see figure for a typical breath). The losses during inspiration were:

Conclusions

Gases undergo a significant temperature and humidity loss as they pass through even the short length of ETT outside the patient threshold. To compensate for this, the humidifier must be set at least 1.4°C above the desired gas temperature. These losses will be greater when flexible extensions are used (eg a 10 cm extension will incur a loss of 2.9°C and 6 mgH₂O/l).

Table

Figure 1