Water's to-and-fro-type systems generate warm, moist gases but are little used these days. Bain-type circuits result in both counter-current heating of the inspired gases and rebreathing of exhaled gas for some humidification, but they are probably only about 10% to 20% efficient on intermittent positive pressure ventilation and even less when used for spontaneous breathing (because of the high fresh gas flows required). Circle circuits warm up after a period of time and do generate water but, again, are relatively little help.
Rarely used these days, as the nebulisation process results in an inhaled aerosol of 100% relative humidity at lower than room temperature, which increases heat loss rather than reducing it. Water uptake may be excessive and there is a real potential for infection from the water bath. Used only to liquefy secretions, but these days active humidification is far preferable.
Heat and moisture exchangers (HMEs)
Initally made for tracheostomied patients from copper mesh ("Swedish nose"). Most are now cheap and disposable and made from modified hygroscopic paper filters encased in a plastic case.
HMEs are usually 50% to 80% efficient at best (depending on inspired humidity); consequently, heat and water loss and dehydration of respiratory secretions still occur, particularly if the inspired gases are completely dry. Efficiency is reduced further by large tidal volumes or by failure to place the HME right on the endotracheal tube, permitting rain-out. Full function is not immediate, typically taking 5 to 20 minutes to achieve steady state.
Nonetheless, they are almost as effective as the human nose and provide enough humidification to maintain ciliary action and mucus flow and to reduce heat and moisture losses during anaesthesia to insignificant levels. They are simple to use, cheap, act as a macroscopic particle trap and avoid many of the problems associated with active humidifiers. Although the importance of breathing system contamination has been disputed, cases of hepatitis C transmission have supported the increasing use of filters as a routine. They can be placed anywhere in the breathing system, but most commmonly placed between the patient and the system itself require changing between cases. All filters increase deadspace and resistance to spontaneous ventilation.