Impact of highly concentrated contaminants on the quality of oxygen 93 % produced by pressure swing adsorption.

A zeolite based pressure swing adsorption (PSA) module designed to produce medicinal oxygen with 90 - 96 % oxygen content was exposed to high input concentrations and high total amounts of CO (17.7 %, 44 mol), CO2 (16.5 %, 23 mol), NO2 (0.98 %, 2 mol), NO (6.2 %, 6 mol) and SO2 (4.2 %, 6 mol). In addition the system was operated with up to 35 % argon in the feed gas. An empirical model was developed to describe the dependence of the oxygen concentration in the product on the oxygen concentration in the input. If the oxygen concentration in the feed gas was reduced below 18 % by dilution, the oxygen concentration in the product fell under the 90 % threshold. Additional effects were observed with NO, NO2 and SO2 which are apparently due to chemical reactions on the adsorbent. These effects consisted of a further decrease in the oxygen concentration measured in the product and could not be reversed by excessive regeneration of the module with air. Under the experimental conditions used, only CO was detected in the product. Appropriate CO monitoring of the input gas is considered a possible remedy for PSA modules in order to ascertain the pharmaceutical quality of the oxygen produced.

[Prevention of infections under anesthetic breathing with breathing filters: concerted recommendations of the Deutsche Gesellschaft für Krankenhaushygiene e.V. (DGKH) and the Deutsche Gesellschaft für Anästhesiologie und Intensivmedizin e.V. (DGAI)]. / Infektionsprävention bei der Narkosebeatmung durch Einsatz von Atemsystemfiltern: Gemeinsame Empfehlung der Deutschen Gesellschaft für Krankenhaushygiene e.V. (DGKH) und der Deutschen Gesellschaft für Anästhesiologie und Intensivmedizin e.V. (DGAI).

An interdisciplinary working group from the German Society of Hospital Hygiene (DGKH) and the German Society for Anesthesiology and Intensive Care (DGAI) worked out the following recommendations for infection prevention during anesthesia 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 anesthetic system.The anesthesia 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 anesthesia 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 colonization 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 anesthesia breathing system is changed and the breathing gas conducting parts of the anesthesia ventilator are hygienically reprocessed.Observing of the appropriate hand disinfection is very important. All surfaces of the anesthesia equipment exposed to hand contact must be disinfected after each case.

[Evaluation of the new supraglottic airway devices Ambu AuraOnce and Intersurgical i-gel. Positioning, sealing, patient comfort and airway morbidity]. / Evaluation der neuen Kehlkopfmasken Ambu AuraOnce und Intersurgical i-gel: Platzierbarkeit, Dichtigkeit, Patientenkomfort und Atemwegsmorbidität.

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.

Influence of intrinsic noise generated by a thermotesting device on thermal sensory detection and thermal pain detection thresholds.

Various factors can influence thermal perception threshold measurements and contribute significantly to unwanted variability of the tests. To minimize this variability, testing should be performed under strictly controlled conditions. Identifying the factors that increase the variability and eliminating their influence should increase reliability and reproducibility. Currently available thermotesting devices use a water-cooling system that generates a continuous noise of approximately 60 dB. In order to analyze whether this noise could influence the thermal threshold measurements we compared the thresholds obtained with a silent thermotesting device to those obtained with a commercially available device. The subjects were tested with one randomly chosen device on 1 day and with the other device 7 days later. At each session, heat, heat pain, cold, and cold pain thresholds were determined with three measurements. Bland-Altman analysis was used to assess agreement in measurements obtained with different devices and it was shown that the intersubject variability of the thresholds obtained with the two devices was comparable for all four thresholds tested. In contrast, the intrasubject variability of the thresholds for heat, heat pain, and cold pain detection was significantly lower with the silent device. Our results show that thermal sensory thresholds measured with the two devices are comparable. However, our data suggest that, for studies with repeated measurements on the same subjects, a silent thermotesting device may allow detection of smaller differences in the treatment effects and/or may permit the use of a smaller number of tested subjects. Muscle Nerve 40: 257-263, 2009.

Population pharmacokinetics of sevoflurane in conjunction with the AnaConDa: toward target-controlled infusion of volatiles into the breathing system.

BACKGROUND: The Anesthetic Conserving Device (AnaConDa) uncouples delivery of a volatile anesthetic (VA) from fresh gas flow (FGF) using a continuous infusion of liquid volatile into a modified heat-moisture exchanger capable of adsorbing VA during expiration and releasing adsorbed VA during inspiration. It combines the simplicity and responsiveness of high FGF with low agent expenditures. We performed in vitro characterization of the device before developing a population pharmacokinetic model for sevoflurane administration with the AnaConDa, and retrospectively testing its performance (internal validation). MATERIALS AND METHODS: Eighteen females and 20 males, aged 31-87, BMI 20-38, were included. The end-tidal concentrations were varied and recorded together with the VA infusion rates into the device, ventilation and demographic data. The concentration-time course of sevoflurane was described using linear differential equations, and the most suitable structural model and typical parameter values were identified. The individual pharmacokinetic parameters were obtained and tested for covariate relationships. Prediction errors were calculated. RESULTS: In vitro studies assessed the contribution of the device to the pharmacokinetic model. In vivo, the sevoflurane concentration-time courses on the patient side of the AnaConDa were adequately described with a two-compartment model. The population median absolute prediction error was 27% (interquartile range 13-45%). CONCLUSION: The predictive performance of the two-compartment model was similar to that of models accepted for TCI administration of intravenous anesthetics, supporting open-loop administration of sevoflurane with the AnaConDa. Further studies will focus on prospective testing and external validation of the model implemented in a target-controlled infusion device.

[Measurement of water vapour pressure in the airways of mechanically ventilated patient using different types of humidifiers]. / Wassergehaltsmessungen im Tracheobronchialsystem beatmeter Patienten bei Verwendung unterschiedlicher Befeuchtersysteme*

UNLABELLED: AIM OF THE INVESTIGATION: To evaluate the performance of different types of humidifying systems (heat and moisture exchanger, HME, and heated humidifier, HH) in the tracheo bronchial airway system of intubated mechanically ventilated patients. METHODS: A heated and fast responding capacitive sensor with a time constant of 0,23 s was used to measure the water vapor pressure at different locations in the tracheo bronchial airway system of 12 patients after therapeutic bronchoscopy. The sensor was immersed in an airstream of app. 1 ml/s continuously sampled with a bronchoscope in which the working pipe and the handle have been equipped with a heating system to prevent condensation. The sampling positions were 3 cm distal of the bifurcation in the right main bronchus, 2 cm proximal of the bifurcation and at the tube connector respectively. RESULTS: Without any humidifying system there was a dramatic reduction of the climatisation index respectively increase of the pulmonary water loss index, most prominently visible at the tube connector. There were significant differences between different types of HME (Pall BB 100 vs. Medisize Hygrovent S) but no significant differences between the Hygrovent S and the HH Fisher & Paykel 630 set at 34 degrees C. In consideration of applied tidalvolumens between 550 and 950 ml, isothermic saturation has not been reached in close vicinity to the bifurcation. CONCLUSIONS: HME may differ substantially from one type to the other and should not be used as climatisation system without careful consideration of their performance data. Effective HME are equivalent to HH at 34 degrees C and may therefore also be used for long term ventilation.

[Rapid measurement of water vapor partial pressure at the saturation point with a new hybrid humidity sensor]. / Schnelle Messung des Wasserdampfpartialdrucks im Sättigungsbereich mit einem neuartigen Hybrid-Feuchtesensor.

A new hybrid humidity sensor comprising a conventional capacitance humidity sensor and a planar heating element is described. Owing to its short response time of only a few milliseconds, its great measuring accuracy, large measuring range, and simplicity in handling, the sensor is suitable for measuring water vapour partial pressure in gases with rapidly changing flow and direction, e.g. in ventilated patients in the fields of anaesthesia and intensive care medicine. The small dimensions permit measurements at almost any location within the ventilation system.

[Climatization of anesthetic gases using different breathing hose systems]. / Klimatisierung von Narkosegasen bei Einsatz unterschiedlicher Patientenschlauchsysteme.

BACKGROUND: 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 tube's 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 degrees C were measured at the tube connector. With heated hoses, however, warming of the gases was excellent with gas temperatures between 28 to 32 degrees 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 degrees 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. (ABSTRACT TRUNCATED)

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 degrees 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.

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.

Prevention of patient bacterial contamination of anaesthesia-circle-systems: a clinical study of the contamination risk and performance of different heat and moisture exchangers with electret filter (HMEF).

The microbiological contamination of 250 breathing system tubes after use in anaesthesia circle systems with reduced fresh gas flow was investigated. The lungs of 50 patients were ventilated without any filtering device between the endotracheal tube and the Y-piece. A total of 51, 49 and 100 patients, respectively, were given different types of heat and moisture exchanger with electret filters (HMEF). With no filtering device the tubing system was contaminated by microorganisms originating from the patient's tracheal secretion in 13% of the patients. In contrast, no bacterial migration into the tubing system was detected when any of the investigated HMEF-devices were used. We therefore conclude that heat and moisture exchangers with electret filters prevent contamination of the anaesthesia breathing system with microorganisms from the patients airways.

[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]. / Reduction intraoperativer Wärmeverlusteund behandlung hypothermer patienten durch atemgas-klimatisierende Massnahmen? Wärme- und Feuchtigkeitstauscher vs. aktive Befeuchter im beatmeten Lungenmodell.

UNLABELLED: 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 degrees 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 degrees 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 approximately 71/min) the main fraction of pulmonary heat loss results from water evaporation from the airways (approximately 6 kcal/h), whereas the heat loss due to convection is negligible (approximately 1.2 kcal/h). In intubated patients ventilated with dry air, the heat loss increases to approximately 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.

[Air conditioning with a high-performance HME (heat and moisture exchanger)--an effective and economical alternative to active humidifiers in ventilated patients. A prospective and randomized clinical study]. / Atemgasklimatisierung mit leistungsfähigen HME (Heat and Moisture Exchanger)--eine effektive und kostengünstige Alternative zu aktiven Befeuchtern bei beatmeten Patienten. Eine prospektive und randomisierte klinische Studie.

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).

[Efficiency of a mobile oxygen concentrator for mechanical ventilation in anesthesia. Studies with a metabolic lung model and early clinical results]. / Leistungsfähigkeit eines mobilen Sauerstoff-konzentrators für die Narkosebeatmung. Untersuchungen am metabolischen Lungenmodell und erste klinische Erfahrungen.

Oxygen (O2) for clinical application is generally provided from either a central gas supply via a hospital pipeline system or is delivered to the working place in cylinders as compressed gas. An alternative source is the one-site generation of O2 from air using O2 concentrators based on molecular sieve technology. Whereas O2 concentrators for anaesthesia in remote areas or underdeveloped countries are wide-spread, in Germany their use is common in neither hospitals nor anaesthesiological practice. The maximum O2 content produced by concentrators is 96% with about 4% argon (Ar) and minimal amounts of nitrogen and other noble gases. The total O2 production is systematically limited, and therefore, the delivered concentration decreases with higher flows. There is also a potential possibility of Ar accumulation in rebreathing anaesthesia systems with reduced fresh gas flow. We investigated the efficiency and potential disadvantages of using O2 concentrators in anaesthesia and the influence of Ar on the accuracy of anaesthetic gas monitors. METHODS. The efficiency of the concentrator was characterised as O2 concentration depending on delivered gas flow. The degree of Ar accumulation in rebreathing anaesthesia systems was obtained with an O2-consuming and CO2-producing metabolic lung model consisting of a water-cooled burning chamber with an adjustable gas jet. The expiratory CO2 content was set to approximately 7%, representing an O2 consumption of 350 ml/min while ventilating the model with 500 ml tidal volume and 10 breaths/min. The inspiratory O2 concentration was adjusted to 35% or 70%; the fresh gas flow was set to 0.5 or 1 l/min. The accuracy of different types of anaesthesia monitors for O2, CO2, volatile anaesthetics, and nitrous oxide in the presence of Ar was checked in comparison with data obtained with a mass spectrometer. To evaluate the usefulness of O2 concentrators for anaesthetic practice, the function of a respirator-concentrator unit was investigated in clinical routine for 8 weeks. RESULTS. The efficiency of the concentrator is flow-rate dependent: O2 concentrations higher than 90% are only achieved with flow rates below 5 l/min and decrease to values lower than 50% at 12 l/min or more. Ar accumulation occurred in rebreathing circuits but exceeded values higher than 10% only under minimal-flow conditions (fresh gas flow 0.5 l/min). Ar did not influence the accuracy of common anaesthetic gas monitors. In clinical practice, the performance of anaesthesia using O2 from an O2 concentrator generated no additional problems. CONCLUSIONS. For the future, the use of O2 concentrators for anaesthesia seems to be a practicable alternative to compressed O2 from cylinders. The main application could be in small operating units or anaesthesia practices. The method is safe and without additional risk of hypoxia, even in rebreathing systems and closed circuits, when the O2 concentration in the inspired gas is measured.

[Heat and moisture exchangers for conditioning of inspired air of intubated patients in intensive care. The humidification properties of passive air exchangers under clinical conditions]. / Wärme- und Feuchtigkeits-tauscher zur Klimatisierung der Inspirationsluft intubierter Patienten in der Intensivmedizin. Untersuchungen zur Befeuchtungsleistung von passiven Atemluftbefeuchtern unter klinischen Gesichtspunkten.

Heat and moisture exchangers (HME) are used as artificial noses for intubated patients to prevent tracheo-bronchial or pulmonary damage resulting from dry and cold inspired gases. HME are mounted directly on the tracheal tube, where they collect a large fraction of the heat and moisture of the expired air, adding this to the subsequent inspired breath. The effective performance depends on the water-retention capacity of the HME: the amount of water added to the inspired gas cannot exceed the stored water uptake of the previous breath. This study evaluates the efficiency of four different HME under laboratory and clinical conditions using a new moisture-measuring device. METHODS. In a first step, the absolute efficiency of four different HME (DAR Hygrobac, Gibeck Humid-Vent 2P, Pall BB 22-15 T, and Pall BB 100) was evaluated using a lung model simulating physiological heat and humidity conditions of the upper airways. The model was ventilated with tidal volumes of 500, 1,000, and 1,500 ml and different flow rates. The water content of the ventilated air was determined between tracheal tube and HME using a new high-resolution humidity meter and compared with the absolute water loss of the exhaled air at the gas outlet of a Siemens Servo C ventilator measured with a dew-point hygrometer. Secondly, the moisturizing efficiency was evaluated under clinical conditions in an intensive care unit with 25 intubated patients. Maintaining the ventilatory conditions for each patient, the HME were randomly changed. The humidity data were determined as described above and compared with the laboratory findings. RESULTS AND DISCUSSION. The water content at the respirator outlet is inversely equivalent to the humidity of the inspired gases and represents the water loss from the respiratory tract if the patient is ventilated with dry gases. Moisture retention and heating capacity decreased with higher volumes and higher flow rates. These data are simple to obtain without affecting the patient and can easily be interpreted. It was demonstrated that, compared to physiological conditions, the DAR Hygrobac and Gibeck Humid Vent 2P-HME coated with hygroscopic salts-were able to maintain sufficient inspiratory humidity and heat. The Pall-HME, solely a condensation humidifier, did not meet the physiological requirements.

[Is the lithium chloride-coated heat and moisture exchanger a danger for patients?]. / Sind Lithiumchlorid-beschichtete "Heat and Moisture Exchanger" ("Künstliche Nasen") eine Gefährdung für Patienten?

Such hygroscopic compounds as LiCl, CaCl2, and MgCl2 are used to improve water retention capacity and, as a consequence, the effectiveness of heat and moisture exchangers (HME). Resorption of these substances via the bronchopulmonary tract and a resulting systemic action cannot be excluded, especially if additional active moisturizing devices are used. The narrow therapeutic range of lithium is known, as are its unwanted side effects, such as nausea, vomiting, somnolence and even cardiac arrhythmia. These are symptoms that also frequently occur during anaesthesia and intensive care, so that differentiation against effects of lithium is nearly impossible. We investigated whether, in theory and in practice, LiCl-coated HME could result in effective Li plasma concentrations. We measured (1) total LiCl content of HMEs, (2) release of this content, simulating the worst-case situation with a breathing model, and (3) lithium plasma concentrations of adult patients being ventilated during anaesthesia with a rebreathing circuit and LiCl-coated HME, but with no additional active moisturizing system incorporated. RESULTS. The results show striking differences with LiCl content ranging from 3 to 251 mg varying not only between different types of HME but also within the same lots. After 20 min of ventilation more than 90% of the LiCl coating was rinsed into the test lung of the breathing model. In practical use, we observed an increase in lithium plasma concentration in 3 of 20 investigated patients. The plasma values of maximum 49.5 micrograms/l (= 0.007 mmol/l) do not amount to potentially toxic concentrations. Nevertheless, clinically relevant concentrations might occur in patients with small distribution volumes, e.g. newborns or infants with frequent exposition within short intervals such as in intensive care units. The differences in lithium content also indicate qualitative differences in water retention capacity. Because of the potential side effects of lithium, we prefer qualitatively equivalent HMEs, e.g., with MgCl2 or CaCl2 as hygroscopic substance.