1996 OPEN FORUM Abstracts
Devices and Techniques Used to Increase Airway Humidity In Intubated Patients
Robert. S. Campbell, RRT Monday, November 4, 1996
Providing adequate humidification of inspired gases to patients with bypassed upper airways is a standard of care. There are numerous reports in the literature illustrating the negative effects of inadequate humidification. The "adequate" level of humidity has been the topic of much debate among practitioners and manufacturers of humidification devices. There is no simple answer to the question of adequate humidification delivery and this will vary between patient populations and care settings. For example, patients requiring an artificial airway for a one hour operation will have humidification requirements much different from mechanically ventilated ICU patients or long term ventilator dependent patients. Likewise, patients with a disease process that affects sputum production and consistency will have needs far different from those with normal sputum production and consistency.
Currently available humidification devices may be divided into two categories: active and passive. Active humidifiers increase the water content of and may also heat the inspired gas. The most commonly used active humidifier is a heated humidifier which incorporates a heated water bath through which inspired gases travel prior to delivery to the patient. Configuration of the humidifier chamber affects the gas-liquid interface which will determine the water content of the inspired gas. Configurations used to increase to water content over a basic passover humidifier chamber include the use of baffles and/or wicks within the chamber. Typical placement of the humidifier chamber is near the mechanical ventilator which requires the gas to be heated well above the desired delivery temperature to compensate gas cooling within the inspiratory limb of the breathing circuit. Condensation occurs as gas temperature drops within the breathing circuit. Many breathing circuit designs have attempt to prevent, minimize, remove, or eliminate condensation accumulation and it's associated problems. Circuit designs have incorporated heated wires within the inspiratory and/or expiratory limbs, insulation of the breathing circuit, use of various water trap designs, or coaxial circuit designs to name a few. It is also important to recognize that the exhaled gas from the patient may need to be "deconditioned" to protect pressure and flow sensors located on the expiratory side of some mechanical ventilators. Previously, heated humidifiers were thought to contribute to the development of noscomial pneumonia. Recent studies have shown that the humidifier, breathing circuit, and condensation within the breathing circuit actually plays a small role in the development of noscomial pneumonia. Passive humidifiers are designed to return a portion of the heat and humidity within the patient's exhaled gas on the subsequent breath. Passive humidifiers are fondly referred to as "artificial noses" of which there are four types depending on the material used to store expired heat and humidity. The most basic passive humidifier is the Heat and Moisture Exchanger (HME) which typically utilize a layered aluminum insert with or without an additional fibrous element to retain moisture and heat during exhalation. The HME is the least efficient of all passive humidifiers. A Heat and Moisture Exchanging Filter (HMEF) has a hydrophobic fibrous insert that repels (stores) moisture and heat on the patient side of the insert, but also acts as an effective bacterial filter. The most commonly utilized passive humidifier is the Hygroscopic Condensor Humidifier (HCH). The HCH consists of a polypropylene or paper insert which is laced with either Lithium Chloride or Calcium Chloride. The HCH is more efficient than HME's due to the chemical attraction of the insert to water. The addition of a thin bacteria filter media between the insert and source gas makes this an HCHF. HCHF's may be useful in the operating room because most anesthesia ventilators allow rebreathing. Any benefit from use of an HCHF over a HCH in respiratory care applications is yet to be determined.
In conclusion, choice of each particular humidification technique, device, and/or brand is currently made largely based on cost and personal bias rather than by effectiveness and appropriateness of use in each particular application. Because of the variety of patient populations within each health care institution, it is unlikely that any one humidification technique can be safely used and remain cost efficient.