The Science Journal of the American Association for Respiratory Care

2006 OPEN FORUM Abstracts

The Effects On Temperature And Moisture Content Of Inspired Gas When Adding Non-Heated Tubing To A Heated Wire Ventilator Circuit:

John Newhart RRT, Timothy A. Morris MD, UCSD Medical Center San Diego CA.

Background:
Gas delivered to intubated patients by mechanical ventilation should be conditioned as close as possible to BTPS so as to maintain moist and warm airways. The AARC guidelines for intubated mechanically ventilated patients are 30mg/L H2O at 30 oC minimum. Gas exiting the humidifier can be more effectively kept warm via heated wire circuits. These circuits contain heating wires inside the tubing to maintain gas temperature and therefore moisture, as it flows towards the patient airway. The entire system is microprocessor controlled. We sought to quantify temperature and moisture loss resulting from added fittings and sections of non-heated tubing. It is common practice to add these components to circuits to allow various therapies to be administered to the patient. These include metered dose inhalers, nebulizers and sample lines for such gasses as nitric oxide. Any section of the inspiratory limb that is not heated will cause a drop in gas temperature and resulting loss of moisture content of the delivered gas.

Methods:
We used a Puritan Bennett 840 ventilator with a Hudson Concha IV humidifier utilizing a dual heated wire circuit. A 6" section of 22mm corrugated tubing was added to the inspiratory limb as the non-heated segment. The ventilator was connected to a single rubber test lung partially insulated inside a plastic box. Thermocouple type digital thermometers were placed inside the circuit at three locations. TP1 (test point 1) was located at the distal end of the heated wire in the inspiratory limb, TP2 at the inspiratory side of the patient "Y" and TP3 at the end of the patient connector. A digital hygrometer was also inserted at these locations to accurately measure relative humidity (RH) at these same locations. From RH and temperature, water content of the gas can be determined. The ventilator and humidifier were turned on with airway temp set to 37 oC, humidity at the "neutral" position and the system was allowed to stabilize.

Results:
RH was 100% at all locations. The temperature readings were as follows. TP1 38.8 oC, TP2 32.5 oC, TP3 29.8 oC. Based on a published "temperature/RH/water content table" (Sykes, McNicol, and Campbell,  Respiratory Failure, 2nd ed., Oxford England 1976), H2O content was as follows TP1 49 mg/L, TP2 35 mg/L, TP3 30mg/L.

Conclusion:
The humidity deficit at the patient connection based at 100% RH and 29.8 oC is 13mg/L (compared to BTPS). The Temperature deficit is 7.2 oC.

Discussion:
Significant heat and moisture loss can occur when adding non-heated tubing or adaptors to the end of the inspiratory limb of a heated wire ventilator circuit. In this study the water content was at the minimum based on AARC clinical guidelines (30mg/L) and the temperature was approximately at the minimum standard of these same guidelines (30 oC). Increasing the set temperature of the humidifier would likely increase these to a more desirable level.

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