The Science Journal of the American Association for Respiratory Care

2003 OPEN FORUM Abstracts

BEDSIDE VISUAL COMPARISON OF VENTILATOR CIRCUIT HUMIDITY

Jodette A Brewer, RCP, RRT, Ruta Steinell, RCP, RRT, David Willms, MD Sharp Memorial Hospital, San Diego, CA.

Background:
Currently, patients on ventilators have several general options for humidification. A passive heat-moisture exchanger (HME) consists of an in-line filter which collects expired humidity and returns it to the patient during mechanical inspiration. Heated humidifiers, or active humidification, consist of an evaporative water bath placed in-line with the ventilator inspiratory circuit. A third, hybrid device (Humid-Heat, Gibeck, Upplands Väsby, Sweden) consists of an HME-like filter coupled with a heating jacket and constant slow water delivery, and has been termed an "active-passive HME". Experience suggests that simple inspection of the inspiratory circuit for condensation roughly correlates with delivery of adequate humidity to the airways. A visual scale for assessment of condensation in the flex-tube connecting the ventilator circuit to the artificial airway was reported to correspond well to direct hygroscopic measures of humidity (Ricard et al Chest 1999). The scale assigns values from 1 ("dry") to 6 ("dripping wet") to indicate the amount of moisture observed in the flex-tube. We implemented the Ricard scale in our units and utilized it to assess humidity delivered from three varieties of humidification in use in our hospital.

Method:
This study consisted of investigator observation of the flex-tube just proximal to the airway at 4 (+/-1) and 24 (+/-1) hours after beginning humidification. Selection of the humidification device for a particular patient was at the treating clinician's discretion. The three types of devices used were: a) Passive HME (Humid-Vent 2S, Gibeck, Sweden), b) Active-passive HME (Humid-Heat), and c) Active humidification with heated-wire circuit (Fisher Paykel FP 730, Auckland, New Zealand). The same 6-point visual evaluation scale was used per Ricard's study. The scale reflects numbers 1-6 as follows: 1. dry, 2. moisture only, 3. moisture + few water droplets, 4. moisture + several water droplets, 5. moisture + numerous water droplets, 6. dripping wet. Humidity scores were analyzed using MINITAB software (Minitab Inc, State College, PA.).

RESULTS:
95 determinations were made at 4 h and 78 at 24 h. Mean visual scores for the devices are shown in the table [Mean ± SD, (n)]:

Time a) HME b) Active-passive HME c) Active, heated wire
4 h 3.8±1.14 (22) 4.2±1.38 (35) 3.7±1.29 (38)
24 h 3.9±1.3 (15) 3.3±1.3 (34) 3.7±1.2 (29)


A one-way ANOVA of humidity scores revealed no significant difference between the devices at 4 hours (F (2,92)=1.11, p=0.33) or at 24 hours (F (2,75)=1.12, p=0.33). Airway temperatures for the active humidification systems and the Humid-Heat were recorded. Humid-Heat averaged a temperature of 36.5 deg C, and the FP heater averaged 34.4 deg C. No endotracheal tube occlusions were reported during the evaluation interval. Cost of initial setup for disposable items was: a) HME with a non-heated wire circuit, $4.20, b) Active-passive HME with a non-heated wire circuit, water and the water transfer set tubing, $12.22, c) Active heated humidifier, heated wire circuit, water chamber, water and transfer set tubing, $19.10.

CONCLUSION:
All three devices appeared to provide adequate humidity, based on prior correlation of the visual score with acceptable absolute humidity. The lack of significant differences in the visual humidity scores among the three devices may be due to small sample size or variables not controlled. Variables not controlled in this study include: heliox in-line (two patients), fans (three patients), and the different levels of airway temperature achieved with the various systems. Further research comparing humidification techniques using this visual scale may need to be done using control, randomization and larger sample sizes.

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