1995 OPEN FORUM Abstracts
TEMPERATURE LOSS IN DEADSPACE EXPOSED TO AMBIENT
B Peterson BE Hon, N Rankin MD, D Galler MD. Intensive Care Unit, Middlemore Hospital, Auckland, New Zealand.
Introduction: Delivery of heated and humidified gases to mechanically ventilated patients is essential to maintain airway integrity. Because heated humidifiers typically use a temperature probe at the Y-piece, we sought to determine the temperature drop from the site of measurement to the teeth in mechanically ventilated patients. This will suggest the appropriate humidifier settings to deliver core temperature saturated gases to the patient. Method: Gas temperatures were monitored in the breathing circuit of general ICU patients during mechanical ventilation. A Fisher & Paykel MR730 humidifier with heater wire circuit was controlled at various temperatures between 32 and 39ºC in a 25ºC ambient. Each patient had approximately 6cm of endotracheal tube (ETT) protruding from the teeth, and a suction port with 10cm of flexible extension placed between the ETT and the Y-piece. K type thermocouples with a 0.1 second response time measured temperature in the centre of the tubing at three sites: 1) the Y-piece; 2) in the suction port between the flexible extension and ETT; 3) in the ETT just prior to the teeth (inserted through the wall of the ETT).
Results: Temperature drops were recorded over both the flexible extension and the portion of ETT protruding from the teeth during inspiration and expiration. During inspiration temperature drops of up to 4ºC from the temperature setting on the humidifier to the teeth were recorded. (See figure for a typical breath).
Conclusions: The magnitude of the temperature drop is dependent on ambient air movement, ambient temperature, tubing geometry, tubing length, and flow rate. Gases undergo a significant temperature drop in the unheated tubing between the Y-piece and the patient. To compensate for this, the humidifier must be set approximately 2ºC above the desired gas temperature.
Humidifier Y-Piece Temp.Temp. at teeth Temp. Drop
Measurements given as time averages for ten breaths, ±2SD.
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