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Heat and moisture exchangers (HME) are used as artificial noses for intubated patients to prevent damage resulting from dry and cold inspired gases. HME collect a large fraction of the heat and moisture of the expired air, adding them to the subsequent inspired breath. In a prospective clinical study the air conditioning capacity of a heated humidifier was compared with a hygroscopic HME. METHODS. The water content of the ventilated air of 49 intensive care patients requiring artificial ventilation with tidal volumes between 440 and 1,190 ml (mean 658 +/- 148 ml) was examined. Each patient was ventilated in sequence with an HME (DAR Hygrobac S) and a heated humidifier (Fisher & Paykel MR 630 B). The temperature of the air in the inspiratory limb was maintained at 34 degrees C. The water content of the ventilated air was determined under steady-state conditions directly at the tracheal tube or between tracheal tube and HME using a new, high-resolution humidity meter. The results were compared with the absolute water loss of the exhaled air at the gas outlet of the ventilator as an expression of the water loss from the lower airways. Airway resistance was calculated by a standard formula. The daily running costs for both HME and heated humidifier were estimated. RESULTS AND DISCUSSION. Moisture retention was equivalent in both the HME and the heated humidifier (33.7 +/- 1.85 bzw. 34.1 +/- 2.62 mgH2O/l). These data show that modern HMEs are able to maintain physiological air-conditioning even in long-term ventilated patients. The small increase in airway resistance associated with HMEs (3.1 +/- 2.5 mbar/l.s) has to be noted in difficult weaning procedures. Both labour and costs per day are significantly less with HMEs (8.60 vs. 21.70 DM).

The aeration of operation theatres serves multiple purposes. It is necessary for the regulation of room temperature (waste heat generated by the operation team and technical appliances), air exchange for oxygen supply, removal of narcotic gases and of smells (occupational limit values) as well the elimination of germs and particles emitted during operations by staff and from instruments (avoidance of germ transfer by air). This paper tries on the one hand to estimate the investment and running costs of different indoor air supplies according to the specifications given by DIN 1946, parts 2 and 4) and on the other hand to differentiate the hygienically relevant expenses of turbulent and laminar flow air supply systems. It can be seen that, due to the high heat production generated during operations, no cost can be saved through decreases in air flow volume. Depending on the operation frequency (e.g. 4 operations daily, 200-300 days per year) a hygienically relevant cost of DM 2.8-6.7 per patient is calculated for laminar flow supplies and DM 4.5-6.7 per patient for the turbulent air supply systems ("Stützstrahl"). Thus laminar flow systems are also cost-saving in comparison to turbulent air supply systems in addition to their significant technical and hygienic advantages.