Reduktion intraoperativer Wärmeverluste und Behandlung hypothermer Patienten durch atemgasklimatisierende Maßnahmen? Wärme- und Feuchtigkeitstauscher vs. aktive Befeuchter im beatmeten Lungenmodell

Zusammenfassung

Pulmonal bedingte Wärmeverluste bei Beatmung mit trockenen und kalten Atemgasen sind in der perioperativen Phase ebenso wie in der Intensivmedizin durch atemgasklimatisierende Maßnahmen weitgehend vermeidbar. Im Vergleich zu Energieverlusten durch Radiation, Konvektion und Evaporation von Wärme und Wasser von der Körperoberfläche und aus geöffneten Körperhöhlen ist die Wärmetransportkapazität der Atemluft jedoch gering. Die Kompensation hoher perioperativer Wärmeverluste sowie die Wiedererwärmung hypothermer Patienten ist auch durch Beatmung mit überkörperwarmen Inspirationsgasen nicht möglich. Aktive Befeuchtungssysteme (heated humidifier, HH) bieten somit aus energetischer Sicht keine wesentlichen Vorteile gegenüber leistungsfähigen Wärme- und Feuchtigkeitstauschern (heat and moisture exchanger, HME).

Abstract

Heated humidifiers (HH) as well as heat and moisture exchangers (HME) are commonly used in intubated patients as air-conditioning devices to raise the moisture content of the air, thus preventing mucosal damage and heat loss resulting from ventilation with dry inspired gases. In contrary to HME, HH are able to add heat and moisture to the inspired air in surplus, which is often stressed as an advantage in warming hypothermic patients or reducing major heat losses, e.g., during long operations. The impact of air conditioning on the energy balance of man was calculated comparing HME and HH.

Methods. The efficiency of a HME (Medisize Hygrovent) and a HH (Fisher & Paykel MR 730) was evaluated in a mechanically ventilated lung model simulating the physiological heat and humidity conditions of the upper airways. The gas flow from the central supply was dry; the model temperature varied between 32 and 40 °C. By using a HH in the inspiratory limb, a circle system was simulated with water-saturated inspired air at room temperature. The water content of the ventilated air was determined at the tracheal tube connection using a fast, high-resolution humidity meter and was compared with the moisture return of the HME. The energy balance was calculated according to thermodynamic laws.

Results. Both HME and HH were able to create physiological heat and humidity conditions in the airways. With the normothermic patient model, the moisture return of the HME was equal to that of the HH set at 34 °C. Increasing the heating temperature resulted only in reduced water loss from the lung; heat and water input in the normothermic model was not possible. This was only effective with almost negligible amounts under hypothermic patient model conditions.

Discussion. The water content in the inspired and expired air is the most important parameter for estimating pulmonary heat loss in mechanically ventilated patients. In adults (minute volume ∼7 1/min) the main fraction of pulmonary heat loss results from water evaporation from the airways (∼6 kcal/h), whereas the heat loss due to convection is negligible (∼1.2 kcal/h). In intubated patients ventilated with dry air, the heat loss increases to ∼8 kcal/h due to greater water evaporation from the airways. Both HME and HH are able to reduce the pulmonary heat loss to 1–2 kcal/h. In normothermic as well as hypothermic patients, HH do not offer significant advantages in heat balance compared to effective HME. In conclusion, air conditioning in intubated patients is neither a powerful too for maintaining body temperature during long-lasting anaesthesia nor a sufficient method of warming hypothermic patients in intensive care units.