Das Wärme-Feuchte-Profil des PhysioFlex Untersuchungen am Modell*

Zusammenfassung

Ziel: Das Wärme-Feuchte-Profil des Narkosegeräts PhysioFlex wurde am Modell beim Betrieb im geschlossenen System bestimmt.

Methodik: An vier Meßstellen (Atemkalk, Geräteausgang, Geräteeingang, inspiratorisch vor dem Y-Stück) wurden der Temperatur- und Feuchteverlauf bei standardisierten Umgebungsbedingungen innerhalb der ersten 2 h nach Inbetriebnahme ermittelt.

Ergebnisse: Nach 10 min herrschte an allen Meßorten Wasserdampfsättigung. Durch das konstruktionsbedingte Umwälzen des Systemvolumens mit 70 l/min kommt es zu einem schnellen Temperatur- und Feuchteausgleich im System. Die höchsten Werte (bis 32° C, bis 35 mg H₂O/l Atemgas) werden über dem Atemkalk gemessen. Inspiratorisch werden im Mittel bereits nach 10 min mit 20 mg/l Feuchtewerte erreicht, die die minimalen Anforderungen an eine Narkosegasklimatisierung erfüllen. Nach 2 h werden mit annähernd 30 mg/l Feuchtewerte erreicht, die bislang an konventionellen Narkosegeräten nicht beschrieben werden konnten.

Diskussion: Die im Vergleich zu konventionellen Narkosegeräten niedrigen Temperaturen über dem Atemkalk könnten den Einsatz thermolabiler Inhalationsanästhetika im PhysioFlex eher erlauben als in herkömmlichen Narkosegeräten unter „Minimal Flow”-Bedingungen. Der hohe Temperatur- und Feuchtegradient zwischen Atemkalk und Geräteausgang deutet auf einen Ansatzpunkt für eine weitere Optimierung hin.

Abstract

Closed-system anaesthesia provides the best prerequisites for optimal warming and humidification of anaesthetic gases. The PhysioFlex anaesthesia machine fascilitates quantitative closed-system anaesthesia. Furthermore, its design may improve the climatisation of the anaesthetic gases by revolving the system volume at 70 l/min, using a small soda-lime canister to allow optimal usage of the heat and moisture generated by CO₂ absorption and by integrating all system components in thermally isolating housing. To determine the capacity of the PhysioFlex to climatise anaesthetic gases, we evaluated the heat and humidity profile at four characteristic places in the anaesthetic circuit under standardised conditions in a model.

Materials and methods: In an air-conditioned room at 19–20° C ambient temperature, the PhysioFlex was operated with a fresh gas flow of less than 500 ml/min, similar to quantitative closed-system anaesthesia in adults. With a respiratory rate of 10/min and a tidal volume of 600 ml, a humidifier was ventilated, that delivered humidity-saturated gas at 33–34° C; 200 ml/min CO₂ were added to the system at the humidifier to mimic the heat, moisture, and CO₂ input of a patient into the anaesthetic circuit. A total of six series were performed, each starting with a cold and dry anaesthetic circuit. For 2 h the time-courses of temperature and humidity of the anaesthetic gases were measured at four distinct places: (1) in the soda-lime canister (M₁); (2) at the outlet of the anaesthesia machine (M₂); (3) at the inlet of the anaesthesia machine (M₃); and (4) in the inspiratory limb close to the Y-piece (M₄). Capacitive humidity sensors (VAISALA Type HMM 30 D without a protective cap) and very small thermocouples were used to measure relative humidity (rH) and temperature. The data were recorded at 5 min intervals. Due to the continuous gas stream in the system, the response time of the sensors, which is in the range of a few seconds, did not affect the accuracy of the measurement. With the temperature-dependent humidity content of 100% rH obtained from equation 1, absolute humidity was calculated.

Results: The time courses of temperature and humidity at the different measuring points are depicted in Figs. 2 and 3, respectively. The steepest increase in temperature and humidity was observed at M₁. Within 10 min 100% rH was achieved at all measuring points. Initially, there was a considerable temperature gradient between M₁ and M₂; this became gradually smaller, indicating system components with high heat capacities. There was only a small gradient between M₂ and M₄, indicating that there was only a small heat loss compared to the heat input. The recommended minimal climatisation of the anaesthetic gases of 20 mg H₂O/l [20] was obtained within 10 min at M₄. During the whole measuring period heat and humidity increased in the system, reaching a maximum at M₄ after 120 min with average values of more than 28° C and 27 mg H₂O/l, respectively.

Conclusion: With the PhysioFlex anaesthesia machine employing closed-system conditions, minimal climatisation of anaesthetic gases was reached within 10 min. After a period of 120 min, the anaesthetic gases were nearly climatized to the extent recommended for long-term respirator therapy. To date, no comparable temperature and humidity level has been reported with conventional anaesthesia machines. The time course of the gradient between M₁ and M₂ may give an opportunity for further optimising the system in reducing heat loss after the soda-lime canister, the active heat and moisture source in the circuit. At about 32° C, the temperature in the soda-lime canister is 10–15° C less than in conventional anaesthesia machines. Thus, the use of thermally instable volatile anaesthetics in the PhysioFlex under closed-system conditions may be less critical than in conventional anaesthesia machines under minimal-flow conditions.