K. Züchner

Airway humidification in mechanically ventilated neonates and infants: A comparative study of a heat and moisture exchanger vs. a heated humidifier using a new fast-response capacitive humidity sensor

OBJECTIVE To study the efficiency of a heated humidifier and a heat and moisture exchanger in mechanically ventilated neonates and infants. DESIGN Prospective, controlled, clinical study. SETTING University pediatric intensive care unit. PATIENTS Forty neonates and infants who needed mechanical ventilation were enrolled in the study. INTERVENTIONS None. MEASUREMENTS AND MAIN RESULTS A heat and moisture exchanger and active airway humidification were alternately used in the same patients to exclude interindividual differences in airway humidification. Airway humidity was measured by a new fast-response capacitive humidity sensor which measures airway humidity with an acquisition rate of 20 Hz throughout the respiratory cycle. The humidity sensor was placed at the endotracheal tube adapter. Measurements were done at the beginning and at the end of three consecutive sessions of passive, active, and again passive airway humidification, each session lasting 6 hrs. There was no significant difference between mean inspiratory airway humidity with the heated humidifier (33.8 +/- 2.9 mg/L) and with the heat and moisture exchanger (34.0 +/- 2.6 mg/L). Moreover, the mode of airway humidification did not significantly influence body temperature or PCO2. No serious side effects such as endotracheal tube occlusion were observed. CONCLUSIONS Passive airway humidification by a heat and moisture exchanger is effective in mechanically ventilated neonates and infants over a 6-hr period. However, the performance and safety of a heat and moisture exchanger in prolonged mechanical ventilation remain to be proven.

Determination of airway humidification in high-frequency oscillatory ventilation using an artificial neonatal lung model : Comparison of a heated humidifier and a heat and moisture exchanger

Objective: Thus far only few data are available on airway humidification during high-frequency oscillatory ventilation (HFOV). Therefore, we studied the performance and efficiency of a heated humidifier (HH) and a heat and moisture exchanger (HME) in HFOV using an artificial lung model. Methods: Experiments were performed with a pediatric high-frequency oscillatory ventilator. The artificial lung contained a sponge saturated with water to simulate evaporation and was placed in an incubator heated to 37 °C to prevent condensation. The airway humidity was measured using a capacitive humidity sensor. The water loss of the lung model was determined gravimetrically. Results: The water loss of the lung model varied between 2.14 and 3.1 g/h during active humidification; it was 2.85 g/h with passive humidification and 7.56 g/h without humidification. The humidity at the tube connector varied between 34.2 and 42.5 mg/l, depending on the temperature of the HH and the ventilator setting during active humidification, and between 37 and 39.9 mg/l with passive humidification. Conclusion: In general, HH and HME are suitable devices for airway humidification in HFOV. The performance of the ventilator was not significantly influenced by the mode of humidification. However, the adequacy of humidification and safety of the HME remains to be demonstrated in clinical practice.

Heat and moisture exchangers for humidifying the inspired air of intubated patients in intensive care medicine. An investigation of humidification properties of heat and moisture exchangers under clinical conditions

Zusammenfassung. Die Befeuchtungsleistung unterschiedlicher Wärme- und Feuchtigkeitstauscher (DAR Hygrobac, Gibeck HumidVent 2P, Pall BB 22-15 T und BB 100) zur Klimatisierung der Atemgase wurde im beatmeten Lungenmodell unter standardisierten Bedingungen sowie bei 25 intubierten Patienten untersucht. Mit Hilfe eines neuartigen, hochauflösenden Feuchtesensors konnte nachgewiesen werden, daß die Klimatisierungsleistung von Wärme- und Feuchtigkeitstauschern (HME, Heat and Moisture Exchanger) unter Nichtrückatmungsbedingungen nicht nur durch Messung der inspiratorischen Feuchte direkt am Tubus bestimmt werden kann, sondern einfacher und genauer durch Messung des exspiratorischen Wassergehalts in der Abluft des Respirators. Diese Meßmethodik ist nicht-invasiv und erlaubt jederzeit die Bestimmung der Befeuchtungsleistung des verwendeten HME in der spezifischen Beatmungssituation. Anders als bei der schwierigen Interpretation der tubusnah ermittelten Feuchteparameter ist durch die Bestimmung des Wassergehalts der Exspirationsluft am Respiratorauslaß eine eindeutige Beurteilung der Befeuchtungsleistung des HME durch Vergleich mit den physiologischen Wärme- und Feuchtigkeitsverhältnissen in den Atemwegen möglich. Durch Beschichtung des Innenmaterials mit hygroskopischen Substanzen kann die Wasserbindungskapazität von HME soweit erhöht werden, daß– gemessen an physiologischen Gegebenheiten – adäquate Befeuchtungsleistungen erreicht werden können. HME stellen damit prinzipiell eine gute Alternative zu aktiven Befeuchtungssystemen dar.

Klimatisierung von Narkosegasen bei Einsatz unterschiedlicher Patientenschlauchsysteme

ZusamenfassungFragestellung. Die Klimatisierung der Narkosegase wird in erheblichem Maß vom Frischgasfluss beeinflusst und ist bei Durchführung von Niedrigflussnarkosen besser als bei hohem Flow. Feuchte und Wärme der Gase werden darüber hinaus aber auch von den Wärmeverlusten und entsprechender Wasserkondensation in den Atemschläuchen beeinflusst, die ihrerseits vom technischen Design und den Materialeigenschaften der Atemschläuche abhängen. Mit dieser prospektiven Studie sollte untersucht werden, 1. inwiefern bei Durchführung von Niedrigflussnarkosen die Klimatisierung der Narkosegase im klinischen Routinebterieb durch das technische Design der Schlauchsysteme beeinflusst wird. Des weiteren sollte 2. untersucht werden, ob bei Einsatz des Schlauchsystems, mit dem optimale Klimatisierungseffekte erreicht werden, diese sich auch bei Erhöhung der Frischgasflows noch realisieren lassen. Methodik. Drei unterschiedliche Schlauchsysteme, das konventionelle Doppelschlauchsystem, ein koaxiales Schlauchsystem und ein Doppelschlauchsystem mit beheizten Schläuchen wurden bei der Durchführung von Minimal-flow-Anästhesien mit einem Frischgasfluss von 0,5 l/min an einem Dräger Cicero EM Narkosegerät eingesetzt. Die Atemgastemperatur und die absolute Feuchte im Inspirationsschenkel des jeweiligen Schlauchsystems wurden sowohl Atemsystem- als auch tubuskonnektornah gemessen. Die besten Klimatisierungseffekte wurden bei Einsatz der beheizten Schläuche erreicht. Mit diesem Schlauchsystem wurden deshalb weitere Messungen der inspiratorischen Atemgasfeuchte und -wärme mit Frischgasflows von 1,0, 2,0 und 4,4 l/min durchgeführt. Die Messungen in allen Gruppen erfolgten während der Durchführung von Inhalationsnarkosen von zumindest 45 min Dauer an jeweils acht Arbeitstagen. Ergebnisse. Bei der Durchführung von Minimal-flow-Narkosen wurden gleichermaßen während des Ablaufs der Arbeitstage mit allen Schlauchsystemen tubuskonnektornah Feuchtewerte zwischen 17 und 30 mgH2O/l erreicht. Nur während der ersten Narkosen zu Beginn der Arbeitstage wurde eine kurze zeitliche Verzögerung von 15 bis 30 min bis zum Erreichen einer Feuchte von zumindest 17 mgH2O/l beobachtet. Bei Einsatz der beheizten Schlauchsysteme wurden gehäuft auch Feuchtewerte über 30 mgH2O/l gemessen. Die tubuskonnektornah gemessenen Atemgastemperaturen waren bei Einsatz des konventionellen und des koaxialen Schlauchsystems während der Durchführung von Minmal-flow-Narkosen mit Werten zwischen 23 und 30°C in einem akzeptablen Bereich. Bei Einsatz der beheizten Schläuche aber waren die inspiratorischen Atemgastemperaturen mit Werten zwischen 28 bis 32 °C deutlich höher und nahezu optimal. Während der Durchführung von Minimal-flow-Narkosen waren somit bei Einsatz beheizter Schlauchsysteme die beste Klimatisierung der Atemgase zu erreichen. Mit diesem Atemsystem wurden deshalb weitere Messreihen mit zunehmenden Frischgasflows von 1,0, 2,0 und 4,4 l/min durchgeführt. Während bei einem Frischgasfluss von 1,0 l/min die Atemgasklimatisierung weiterhin optimal ist, fällt bei weiterer Steigerung des Frischgasflows auf 2,0 l/min die Feuchte drastisch auf Werte unter 17 mgH2O/l, bei einem Flow von 4,4 l/min sogar auf Werte um nur 10 mgH2O/l ab. Dagegen erwiesen sich die Atemgastemperaturen bei Einsatz beheizter Schläuche als weitestgehend flowunabhängig und lagen immer, auch bei einem Flow von 4,4 l/min, um 28–32°C. Schlussfolgerungen. Mit konventionellen und koaxialen Schlauchsystemen wird bei der Durchführung von Minimal-flow-Anästhesien eine zwar nicht optimale, aber ausreichende Atemgasklimatisierung erreicht. Der Einsatz koaxialer Systeme scheint im Vergleich zum konventionellen Doppelschlauchsystem nur bei langdauernden Narkosen zu einer weiteren Verbesserung der Klimatisierung zu führen. Während des klinischen Routinebetriebs sind beide Systeme bezüglich der Klimatisierungseffekte als gleichwertig anzusehen. Bei Einsatz beheizter Schläuche ist unter den Bedingungen der Minimal-flow-Anästhesie die Klimatisierung der Atemgase deutlich besser als bei Einsatz konventioneller oder koaxialer Schlauchsysteme. Es werden nicht nur ausreichende, sondern gar optimale Feuchte- und Wärmewerte erreicht. Diese optimale Klimatisierung wird aber nur bei Durchführung von Niedrigflussverfahren realisiert, dann also, wenn der Frischgasflow nicht größer als 1 l/min ist. Bei höheren Frischgasflows nimmt die Feuchte der Atemgase drastisch ab, während die Atemgastemperaturen hoch bleiben. Es ist anzunehmen, dass die Beatmung mit warmem und trockenem Atemgas zu verstärkter Austrocknung des Atemwegsepithels der unteren Atemwege führt. Beheizte Schläuche sollten deshalb nur bei Durchführung von Niedrigflussnarkosen eingesetzt werden. Während die Feuchte der Atemgase im wesentlichen von der Frischgasflussrate bestimmt wird, hängt die Atemgastemperatur im wesentlichen vom Wärmeverlust am Inspirationsschenkel des Patientenschlauchsystems ab.AbstractBackground. During general anaesthesia gas climate significantly is improved by performance of low flow techniques. Gas climatisation, however, markedly also will be influenced by the temperature loss at, and corresponding water condensation within the hoses, factors which are related to the technical design and material of the patient hose system. The objective of this prospective study was to investigate 1. how anaesthetic gas climatisation during minimal flow anaesthesia is influenced by the technical design of different breathing hose systems in clinical practice. 2. to investigate, whether a sufficient gas climatisation also can be gained with higher fresh gas flows if that hose system is used, proven beforehand to optimally warming and humidifying the anaesthetic gases. Methods. Three different systems, a conventional two-limb hosing consisting of smooth silicone hoses, a coaxial hosing, and a hosing consisting of actively heated breathing hoses, attached to a Dräger Cicero EM anaesthesia machine, were used during minimal flow anaesthesia with a fresh gas flow of 0.5 l/min. Gas temperature and absolute humidity were measured at the tapered connection between the inspiratory limb and the breathing system as well as at its connection to the endotracheal tube. The best gas climatisation was observed if heated breathing hoses were used. Thus, using this hosing, additionally gas temperature and humidity in the inspiratory limb were taken at fresh gas flow rates of 1.0, 2.0 and 4.4 l/min respectively. Measurements were performed in all groups at all general anaesthesias lasting at least 45 minutes during the lists of eight different days each. Results. In minimal flow anaesthesia, with all hose systems likewise, generally an absolute humidity between 17 to 30 mgH2O/l is reached at the endotracheal tubes connector during the course of the list. Only in the first cases of the day there was a short delay of 15 to 30 minutes before reaching a humidity of at least 17 mgH2O/l. Only with heated hoses, however, humidity frequently even exceeded 30 mgH2O/l. If conventional or coaxial hosings were used, during minimal flow anaesthesia gas temperatures in an acceptable range between 23 to 30 °C were measured at the tube connector. With heated hoses, however, warming of the gases was excellent with gas temperatures betwen 28 to 32 °C. In minimal flow anaesthesia climatisation of the anaesthetic gases proved to be best if heated hoses were used. Thus, using heated hose systems another three trials with increasing fresh gas flow rates of 1.0, 2.0 and 4.4 l/min respectively were performed. Whereas climatisation of the anaesthetic gases still was found to be optimal with a fresh gas flow of 1.0 l/min, the humidity dropped drastically to values lower than 17 mgH2O/l at 2.0 l/min and even down to 10 mgH2O/l at a flow rate of 4.4 l/min. Gas temperatures, however, turned out to be independent of the flow and remained at 28–32°C, even at a flow as high as 4.4 l/min. Conclusions. Using conventional hose systems and coaxial hosings acceptable, but not optimal climatisation of the anaesthetic gases can be gained if minimal flow anaesthesia is performed. The use of a coaxial hose system seems to lead to improved climatisation in long lasting procedures only. In routine clinical practice, however, conventional and coaxial hose systems are similar in respect to the climatisation of breathing gases. Heated breathing hoses performed markedly better in terms of climatisation of the breathing gas than the coaxial and the conventional hose system. With this hosing not only sufficient but optimal moisture and temperature values are realized. Optimal climatisation, however, only can be gained if low flow anesthetic techniques with fresh gas flows equal or less than 1 l/min are performed. With higher fresh gas flow rates the humidity decreases markedly while high gas temperatures are maintained. It seems justified to assume, that ventilation with warm but dry gases may result in increasingly drying out of the respiratory epithelium of the lower air ways. Heated hoses only should be used if low flow anaesthetic techniques are performed. While moisture content of the breathing gases mainly is influenced by the fresh gas flow rate, temperature mainly is depending on the convectional loss of heat at the inspiratory limb of the hosing.

A Novel, Simplified Ex Vivo Method for Measuring Water Exchange Performance of Heat and Moisture Exchangers for Tracheostomy Application

BACKGROUND: Breathing through a tracheostomy results in insufficient warming and humidification of inspired air. This loss of air-conditioning can be partially compensated for with the application of a heat and moisture exchanger (HME) over the tracheostomy. In vitro (International Organization for Standardization [ISO] standard 9360–2:2001) and in vivo measurements of the effects of an HME are complex and technically challenging. The aim of this study was to develop a simple method to measure the ex vivo HME performance comparable with previous in vitro and in vivo results. METHODS: HMEs were weighed at the end of inspiration and at the end of expiration at different breathing volumes. Four HMEs (Atos Medical, Hörby, Sweden) with known in vivo humidity and in vitro water loss values were tested. The associations between weight change, volume, and absolute humidity were determined using both linear and non-linear mixed effects models. RESULTS: The rating between the 4 HMEs by weighing correlated with previous intra-tracheal measurements (R2 = 0.98), and the ISO standard (R2 = 0.77). CONCLUSIONS: Assessment of the weight change between end of inhalation and end of exhalation is a valid and simple method of measuring the water exchange performance of an HME.

Infection prevention during anaesthesia ventilation by the use of breathing system filters (BSF): Joint recommendation by German Society of Hospital Hygiene (DGKH) and German Society for Anaesthesiology and Intensive Care (DGAI).

An interdisciplinary working group from the German Society of Hospital Hygiene (DGKH) and the German Society for Anaesthesiology and Intensive Care (DGAI) worked out the following recommendations for infection prevention during anaesthesia by using breathing system filters (BSF). The BSF shall be changed after each patient. The filter retention efficiency for airborne particles is recommended to be >99% (II). The retention performance of BSF for liquids is recommended to be at pressures of at least 60 hPa (=60 mbar) or 20 hPa above the selected maximum ventilation pressure in the anaesthetic system. The anaesthesia breathing system may be used for a period of up to 7 days provided that the functional requirements of the system remain unchanged and the manufacturer states this in the instructions for use. The breathing system and the manual ventilation bag are changed immediately after the respective anaesthesia if the following situation has occurred or it is suspected to have occurred: Notifiable infectious disease involving the risk of transmission via the breathing system and the manual bag, e.g. tuberculosis, acute viral hepatitis, measles, influenza virus, infection and/or colonisation with a multi-resistant pathogen or upper or lower respiratory tract infections. In case of visible contamination e.g. by blood or in case of defect, it is required that the BSF and also the anaesthesia breathing system is changed and the breathing gas conducting parts of the anaesthesia ventilator are hygienically reprocessed. Observing of the appropriate hand disinfection is very important. All surfaces of the anaesthesia equipment exposed to hand contact must be disinfected after each case.

Evaluation der neuen Kehlkopfmasken Ambu AuraOnce™ und Intersurgical i-gel™

BACKGROUND Supraglottic airway devices (SGAD) have become more important in airway management over the past years and an objective comparison of the available devices is in order. METHODS In a prospective study the four SGADs LMA-Classic(cLMA), LMA-ProSeal (PLMA), Ambu AuraOnce and Intersurgical i-gel were compared in groups of 40 patients in ambulatory surgery, with respect to the feasibility of positioning, leak tightness, patient comfort and airway morbidity. The seal test of the airway devices was carried out with a specially constructed pneumotachograph. RESULTS Adequate placement on the first attempt was achieved in 92.5% with the cLMA, 85% with the PLMA, 92.5% with the AuraOnce and 82.5% with the i-gel (p>0.05). There were no clinically relevant differences in mean insertion times: cLMA 13.8 s (+/-3.4 s), PLMA 13 s (+/-3.2 s), AuraOnce 11.2 s (+/-2.7 s; p<0.05) and 13.9 s (+/-3.6 s) with the i-gel. A tight seal at a constant oropharyngeal pressure of 15 cmH(2)O was achieved in 85% of the cases (34 cases) with the cLMA, 90% (36 cases) with the PLMA, 97.5% (39 cases) with the AuraOnce and 72.5% (29 cases) with the i-gel (p<0.05). A tight seal at a constant oropharyngeal pressure of 20 cmH(2)O was seen in 62.5% with the cLMA, 60% with the PLMA, 67.5% with the AuraOnce and in 50% with the i-gel of the cases (p>0.05). Airway morbidity was not observed in any group. Significantly more patients complained of a sore throat after using the cLMA (p<0.05). CONCLUSION The tested SGADs were comparable with regard to ease of insertion, insertion times and airway morbidity. Considering leak tightness and patient comfort the PLMA and the AuraOnce fared better with regard to tightness of seal and patient comfort.

Atemgasklimatisierung mit leistungsfähigen HME (Heat and Moisture Exchanger) – eine effektive und kostengünstige Alternative zu aktiven Befeuchtern bei beatmeten Patienten

ZusammenfassungIn einer prospektiven klinischen Studie an 49 intubierten und kontrolliert beatmeten Patienten wurde gezeigt, daß die inspiratorischen Befeuchtungsleistungen des HME DAR Hygrobac S und des aktiven Befeuchters Fisher & Paykel MR 630 B äquivalent sind (33,7± 1,85 vs. 34,1±2,62 mgH2O/l). Bei bestimmungsgemäßem Einsatz (tubusnahe Plazierung bzw. inspiratorische Atemgastemperatur von 34 °C) und Tidalvolumina zwischen 440 und 1190 ml (Mittel 658±148 ml) gewährleisteten beide Systeme eine physiologische Klimatisierung der Atemluft. Die geringgradige Erhöhung der inspiratorischen Resistance durch die HME (3,1±2,5 mbar/l/s) muß ggf. bei der schwierigen Respiratorentwöhnung mit berücksichtigt werden. Die Verwendung von HME ist erheblich kostengünstiger und weniger personalintensiv als der Einsatz von aktiven Befeuchtungssystemen. Leistungsstarke HME stellen damit eine gute Alternative zu aktiven Befeuchtungssystemen in der Intensivbeatmung dar.AbstractHeat 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 °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).

[Evaluation of the new supraglottic airway devices Ambu AuraOnce and Intersurgical i-gel. Positioning, sealing, patient comfort and airway morbidity].

BACKGROUND Supraglottic airway devices (SGAD) have become more important in airway management over the past years and an objective comparison of the available devices is in order. METHODS In a prospective study the four SGADs LMA-Classic(cLMA), LMA-ProSeal (PLMA), Ambu AuraOnce and Intersurgical i-gel were compared in groups of 40 patients in ambulatory surgery, with respect to the feasibility of positioning, leak tightness, patient comfort and airway morbidity. The seal test of the airway devices was carried out with a specially constructed pneumotachograph. RESULTS Adequate placement on the first attempt was achieved in 92.5% with the cLMA, 85% with the PLMA, 92.5% with the AuraOnce and 82.5% with the i-gel (p>0.05). There were no clinically relevant differences in mean insertion times: cLMA 13.8 s (+/-3.4 s), PLMA 13 s (+/-3.2 s), AuraOnce 11.2 s (+/-2.7 s; p<0.05) and 13.9 s (+/-3.6 s) with the i-gel. A tight seal at a constant oropharyngeal pressure of 15 cmH(2)O was achieved in 85% of the cases (34 cases) with the cLMA, 90% (36 cases) with the PLMA, 97.5% (39 cases) with the AuraOnce and 72.5% (29 cases) with the i-gel (p<0.05). A tight seal at a constant oropharyngeal pressure of 20 cmH(2)O was seen in 62.5% with the cLMA, 60% with the PLMA, 67.5% with the AuraOnce and in 50% with the i-gel of the cases (p>0.05). Airway morbidity was not observed in any group. Significantly more patients complained of a sore throat after using the cLMA (p<0.05). CONCLUSION The tested SGADs were comparable with regard to ease of insertion, insertion times and airway morbidity. Considering leak tightness and patient comfort the PLMA and the AuraOnce fared better with regard to tightness of seal and patient comfort.

[Is reduction of intraoperative heat loss and management of hypothermic patients with anesthetic gas climate control advisable? Heat and humidity exchangers vs. active humidifiers ina functional lung model].

ZusammenfassungPulmonal 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).AbstractHeated 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.