The loss of moisture and heat that occurs when the

Transcription

1 Bedside Evaluation of Efficient Airway Humidification During Mechanical Ventilation of the Critically Ill* Jean-Damien Ricard, MD; Philippe Markowicz, MD; Kamel Djedaini, MD; Laurence Mier, MD; François Coste, MD; and Didier Dreyfuss, MD Study objective: To determine the correlation between simple rating of condensation seen in the flex-tube connecting the heating and humidifying device used with the endotracheal tube and hygrometric parameters (absolute and relative humidity and tracheal temperature) measured by psychrometry. Design: Prospective randomized clinical trial. Setting: Medical ICU of Louis Mourier Hospital, Colombes, France, a university-affiliated teaching hospital. Patients: Forty-five consecutive mechanically ventilated critically ill patients. Interventions: Patients undergoing mechanical ventilation were randomly assigned to receive humidification with one of the four heat and moisture exchangers (HMEs) tested or with a conventional heated humidifier. Measurements: The hygrometric performances of four HMEs (BB2215, BB50, and BB100 from Pall Biomedical, Saint-Germaine-en-Laye, France; and Hygrobac-Dar from Mallinckrodt, Mirandola, Italy) and a heated humidifier (Fisher & Paykel; Auckland, New Zealand) were studied after 3 h and also after 48 h of use for the Hygrobac-Dar and correlated to a clinical visual inspection rating the amount of condensation in the flex-tube of the endotracheal tube. Results: A total of 95 measurements in 45 patients were performed. The best hygrometric parameters were obtained with the heated humidifier (p < 0.001). The Hygrobac-Dar yielded significantly higher values for both humidities and tracheal temperature than the other three HMEs (p < 0.001). The performance of Hygrobac-Dar was unchanged after 48 h of use. There was a significant correlation between the condensation seen in the flex-tube and the hygrometric parameters measured by psychrometry (absolute humidity, rho 0.7; relative humidity, rho 0.7; tracheal temperature, rho 0.5, p < ). Conclusion: In mechanically ventilated ICU patients, visual evaluation of the condensation in the flex-tube provides an estimation of the heating and humidifying efficacy of the heating and humidifying device used, thus allowing the clinician bedside monitoring of airway humidification. (CHEST 1999; 115: ) Key words: acute respiratory failure; airway humidification; endotracheal tube; heat and moisture exchangers; mechanical ventilation; psychrometry Abbreviations: AH absolute humidity; HD Hygrobac-Dar; HH heated humidifier; HME heat and moisture exchanger; RH relative humidity; SAPS Simplified Acute Physiology Score The loss of moisture and heat that occurs when the upper respiratory tract is bypassed by the insertion of an endotracheal tube can lead to severe *From the Service de Réanimation Médicale, Hôpital Louis Mourier (Assistance Publique-Hŏpitaux de Paris), Colombes, and Unité de Recherches Inserm U82, Faculté de Médecine Xavier Bichat, Université Paris VII, Paris, France. The authors did not receive any financial support from and do not have any commitment with any of the brands of the humidifying devices tested in this study. Manuscript received July 8, 1998; revision accepted December 1, Correspondence to: Didier Dreyfuss, MD, Service de Réanimation Médicale, Hôpital Louis Mourier, 178, rue des Renouillers, Colombes, France; didier.dreyfuss@lmr.ap-hop-paris.fr airway damage during mechanical ventilation. 1,2 Heated humidifiers (HHs), and more recently, heat and moisture exchangers (HME), have been used to prevent this. HMEs are now widely used in ICUs. Many types of HME are available, but not all of them are suitable for use during long-term ventilation. Indeed, some have been responsible for endotracheal tube occlusions in 12 h, 3 and cases of death from endotracheal tube occlusion have been reported. 4 Endotracheal tube occlusion seemed to be due to insufficient airway humidification with HMEs exhibiting hydrophobic properties only. 3 7 In contrast, the capacity of HMEs with both hydropho Clinical Investigations in Critical Care

2 bic and hygroscopic properties to heat and humidify inspired gases has been validated in several studies These HMEs are also safe and effective when used for 48 h. Indeed, changing these HMEs every 48 h did not affect their capacity to humidify inspired gases or the incidence of nosocomial pneumonia. 11 The clinician wishing to use an HME must identify the ones that perform poorly without having to conduct hygrometric measurements. This may make him/her reluctant to use HMEs for all ICU patients. Recommendation for using or not using an HME lack evidence-based guidelines. 12 Furthermore, indications for when to use an HME during mechanical ventilation (including the type of patients appropriate for HME use and the duration of ventilation with an HME) are still a matter of debate. Indeed, Branson and colleagues tend to limit their use of HMEs to surgical patients and for a maximum of 5 days of mechanical ventilation, while others, ourselves included, use HMEs for all patients, surgical and medical, including patients with COPD, whatever the duration of mechanical ventilation. 8 11,16 However, as pointed out in a recent editorial, 17 a simple method for evaluating HMEs is needed to allow comparison of the various brands of HMEs available and selection of the appropriate one for mechanical ventilation. The lack of evidence-based guidelines is undoubtedly due to the few data available from independent laboratories on the performance of HMEs. Indeed, the characteristics of the HMEs indicated by the manufacturer have not necessarily been established by an independent laboratory. Objective measurements are time consuming and require equipment that is not routinely available. This study was therefore carried out to provide an objective assessment of the performance of HMEs and to determine whether a clinician can assess this performance at the bedside by observing simple visual parameters. We recently reported that visual inspection of the amount of moisture in the flex-tube correlated adequately with the performance of the HME given by manufacturers. 18 However, we did not check our visual scale against objective hygrometric measurements performed at the bedside. This study compares the amounts of moisture seen in the flex-tube with the hygrometric parameters measured psychometrically, thus enabling the clinician to evaluate the efficacy of a given HME without hygrometric measurements. Patients Materials and Methods All consecutive mechanically ventilated patients were eligible for inclusion in the study. Patients were randomly allocated to ventilation using one of five humidification devices (see below) for a 3-h period, after which they were ventilated with the device normally used in our unit, the Hygrobac-Dar (HD) (Mallinckrodt Medical S.p.A.; Mirandola, Italy). Indications for mechanical ventilation according to Zwillich and coworkers 19 were recorded along with the Simplified Acute Physiology Score (SAPS). 20 Materials Four HMEs and one HH were tested. Two HMEs were purely hydrophobic (Pall Ultipor Filter BB2215 and BB50; Pall Biomedical; Saint-Germaine-en-Laye, France); the two others had both hygroscopic and hydrophobic properties (Pall Ultipor Filter BB100; Pall Biomedical; and HD). The conventional HH was the Fisher & Paykel MR 450; Auckland, New Zealand (FP450). The ventilators used were either a Servo 900D ventilator (Siemens- Elema; Solna, Sweden) or a Bird 8400 (Bird Products Corp; Palm Springs, CA). Hygrometric Measurements Hygrometric measurements (absolute humidity [AH], relative humidity [RH]) and tracheal temperature measurement were performed after 3hofmechanical ventilation for all devices and also after 48 h for the HD, which is routinely used for this time in our unit. 11 The number of times devices delivered AH below 24 mg H 2 O/L was recorded. AH and RH were obtained using the psychrometric method A device to separate inspiratory and expiratory gas flow by the means of two unidirectional valves was inserted between the endotracheal tube and the HME (or between the endotracheal tube and the Y-piece when the HH was used). Two thermal probes a dry one and a wet one were placed in the inspiratory part of the device. The temperatures recorded by the two probes were measured and displayed on a chart recorder (Yokogawa; Tokyo, Japan). Tracheal temperature was measured with another thermal probe inserted in the endotracheal tube and also displayed on the chart recorder. Mean temperatures were recorded from both probes after a 30-min period allowing for optimal thermal equilibrium. The psychrometric method is based on comparing the temperatures obtained with the two probes placed on the inspiratory part of the separating device. The dry probe is placed upstream and measures the actual gas temperature. The downstream probe is coated with sterile cotton wetted with sterile water. Evaporation in the inspiratory part is proportional to the dryness of the gas. The temperature gradient between the two probes varies inversely as the humidity of the inspired gas. There is no thermal gradient when the inspired gas is fully saturated with water (100% RH). RH was calculated by reference to a psoriometric diagram taking into account the temperature difference between the two probes. 25 AH at saturation point (100% of RH) (AHs) was calculated with the following formula: AHs T T 2 (mg H 2 O/L), where T( C) is the dry probe temperature. AH was obtained with the following formula: AH (AHs RH)/100 (mg H 2 O/L). Room temperature was kept constant between 23.5 and 25 C throughout the trials. Visual Evaluation The amount of moisture in the flex-tube connecting the HME (or the Y-piece when the HH was used) to the endotracheal tube was rated at the same time by an independent observer unaware CHEST / 115 / 6/ JUNE,

3 Table 1 Characteristics of Patients According to the Device Used* BB2215 (n 20) BB50 (n 20) BB100 (n 20) HD (n 10) FP450 (n 15) p Value Age, yr SAPS Minute ventilation, L/mn Tidal volume, ml Indication for mechanical ventilation 0.6 Chronic airway obstruction Other pulmonary diseases Postoperative respiratory failure Neurologic emergency Miscellaneous *Values are mean SD or No. of patients. of the hygrometric results as follows: dry, moisture only, moisture few water droplets, moisture several water droplets, moisture numerous water droplets, and dripping wet. These items were assigned values from 1 (dry) to 6 (dripping wet). The first three items of the scale were counted as dry and the last three as wet in some analyses to make comparisons between devices easier. This protocol was approved by the institutional review board for human studies of the French Intensive Care Society. Informed consent was not requested since all procedures were considered routine practice, and none was invasive. Statistical Analysis All results are means SD. Hygrometrics parameters were compared by analysis of variance. When analysis of variance indicated differences among groups, the groups were compared using protected least significant difference. Kruskal-Wallis analysis was used to compare the clinical rating scores of each humidifying device. Contingency table analysis was used to compare the number of times AH was 24 mg H 2 O/L. The correlation between the clinical rating of humidifying efficiency and the measured values was performed using Spearman s rank correlation coefficient. A p value 0.05 was considered significant. Statistical analysis was done with a commercial statistical package (Statview; Abacus Concepts Inc; Berkeley, CA). Results A total of 87 clinical measurements and 95 psychrometric measurements were performed on 45 patients. There were 20 measurements for each HME and 15 measurements for the HH. There were no differences between the patients allocated to each of the devices in terms of age, SAPS, 20 minute ventilation, tidal volume, or indications for mechanical ventilation 19 (Table 1). Hygrometric Measurements There were significant differences between the hygrometric parameters (AH, RH, and tracheal temperature) of the various HMEs and the HH (Table 2). The best results were obtained with the FP450 HH, which had significantly better AH and tracheal temperature than all the HMEs. The HD was the HME that gave the best values (Table 2). These performances were unchanged after 48 h of use. The poorest performances were obtained with the purely hydrophobic HMEs (BB2215 and BB50). The BB100 showed values that were intermediate between those of the poorest performing devices and those of HD. Table 3 shows the number of times each of the devices was found to deliver 24 mg H 2 O/L AH. This occurred significantly more frequently with the BB2215 and BB50 HMEs than with all the other devices. Table 2 Hygrometric and Clinical Assessment of the Performances of the Devices* BB2215 (n 20) BB50 (n 20) BB100 (n 20) HD H3 (n 10) HD H48 (n 10) FP450 (n 15) Hygrometric assessment AH, mg H 2 O/L RH, % Tracheal temperature, C Clinical assessment *Results are expressed as mean SD. Measurements are performed after 3hofuseforalldevices, and also after 48 h for HD HME only. HD H3 denotes evaluation of HD HME after 3hofuseandHDH48denotes evaluation of HD HME after 48 h of use. p (difference with HD). p 0.05 (difference with BB100). p 0.01 (difference with HD). p 0.05 (difference with HD) Clinical Investigations in Critical Care

4 Table 3 Number of Times Each Device Delivered < 24 mg H 2 O/L AH and Results of Visual Inspection No. of Times AH 24 mg H 2 O/L No. of Times Dry No. of Times Wet With AH 24 mg H 2 O/L BB /20* 15/20 4/20 BB50 19/20* 16/20 3/20 BB100 4/20 3/20 4/20 HD H3 0/10 2/10 0/10 HD H48 0/10 0/10 0/10 FP450 0/15 0/7 0/7 *Different from all the others with p Different from all the others with p Visual Evaluation of Humidifying Performance Table 2 shows the results of visual ratings of humidifying efficacy. Statistical analysis revealed significant differences in these parameters, with the purely hydrophobic HMEs being the least effective. Table 3 shows the number of times each device was rated dry by visual inspection. The number of times the two purely hydrophobic devices were dry was significantly higher than in the four other conditions where this event was rare. Miscategorization of HMEs (ie, flex-tube rated wet when in fact AH is 24 mg H 2 O/L) is shown in Table 3. Accuracy of the scale was 88%. Correlations Between Hygrometric Measurements and Visual Evaluation There was a significant correlation between the visual rating of the condensation in the flex-tube and the AH measured by psychrometry (rho 0.7, p ) (Fig 1). The correlation was also significant with the other two parameters (RH and tracheal temperature), though less powerful (rho 0.7, and rho 0.5, respectively, both with p ). Discussion This study shows that simple visual inspection of the amount of moisture in the flex-tube connecting the HME to the endotracheal tube (or the Y-piece to the endotracheal tube when using a HH) provides an accurate estimation of the humidifying performances Figure 1. Correlation between the visual scale rating the condensation in the flex-tube and the actual AH measured by psychrometry after 3hofuseforallthedevices and after 48 h for the HD; 1 dry; 2 moisture only; 3 moisture few water droplets; 4 moisture several water droplets; 5 moisture numerous water droplets; 6 dripping wet. CHEST / 115 / 6/ JUNE,

5 of the heating and humidifying device used (ie, HME or HH). These findings may be important since recommendations for the use of HMEs in the ICU lack evidence-based guidelines partly because of the few data available on assessment of HME performance at the bedside. 12 Characteristics of HMEs given by the manufacturer may not have been established by an independent laboratory and the need for accurate bedside evaluation of the efficiency of HMEs has been emphasized recently. 17 Thus, our results indicate that such evaluation is possible and may be useful for managing and monitoring HMEs in the ICU. The gas inspired during mechanical ventilation must be adequately heated and humidified. HMEs are increasingly employed because they are easy to use and cost-effective. 8,9,11 Though the clinical importance of sufficient humidification has been often debated, there is no simple, visual method for evaluating the efficacy of HMEs during long-term mechanical ventilation. Indirect parameters such as the occurrence of nosocomial pneumonia, number of endotracheal tube occlusions, number of tracheal suctionings, and instillations required 8,9,11 have been reported, but they do not always reflect accurately the hygrometric performance of a given HME. 16 Indeed, in this previous report, 16 the clinical evaluation of three different HMEs found no differences in humidifying properties, whereas hygrometric measurements revealed great differences between them. The only direct parameters suitable for evaluation are the hygrometric measurements. They may be expressed as an amount of water loss 26 or expressed as humidity outputs obtained by psychrometric method. This method is the one most often used, but it requires a flow divider, three thermal probes, and 30 to 60 min to ensure thermal equilibrium. 4 6 These measurements are not available in all ICUs and are not feasible in routine practice. This is why a suitable visual evaluation method is worthwhile. We found that the humidity in the flex-tube was correlated with values provided by the manufacturers for AH. 18 We have now evaluated each heating and humidifying device using a visual rating and precise hygrometric measurements to determine whether there was a correlation between the two means of evaluation on the same patient. A group of four commonly used HMEs was tested to validate our visual rating of the condensation in the flex-tube. As the room temperature was constant in our unit, the condensation in the flex-tube reflected the humidity delivered by the HME, rather than thermal variations. The hygrometric parameters measured were consistent with those measured in other studies for each HME tested, and confirm the superiority of the HH FP450 and of the HD The humidifying properties of this hygroscopic and hygrophobic filter have been studied and it consistently gives better hygrometric parameters than many other HMEs. These data were also obtained after use of the HME for 24 h or 48 h. 16 We selected HMEs having very different efficacies to emphasize the differences in the visual assessment. It would have been difficult to detect hygrometric differences with a visual scale if all the HMEs had produced similar humidities. Thus, the visual rating provides an accurate, satisfactory evaluation of both the effective HMEs (HD and BB100), and the less effective ones (BB2215 and BB50). The AH delivered by the HME to the patient via the inspired gas is a decisive determinant of the humidifying performance of an HME. The lower the AH, the higher total amount of water lost by the patient during mechanical ventilation. The ideal gas is 37 C with 100% RH (44 mg H 2 O/L water vapor) because it does not remove water from the patient. 27 This is clinically important since endotracheal tube occlusions occurred when the AH delivered was insufficient. 3 6 These occlusions occurred after 12 hofuse 3 and some were lethal. 4 Endotracheal tube occlusions occur after gradual reduction of the tube s diameter, and depend partly on the type of humidification device used, purely hydrophobic HMEs having a significantly greater reduction of their tracheal tube diameter. 7 The precise AH an HME should deliver to avoid endotracheal tube occlusion is not clear, but some recommendations suggest that 24 mg H 2 O/L is the minimum an HME should deliver when used for long-term ventilation. 28 However, International Standards Organization recommends that a minimum of 30 mg H 2 O/L should be provided by devices during mechanical ventilation. Interestingly, a number of studies evaluating different HMEs at the bedside report, however, much lower humidity output measured with commonly used HMEs. Indeed, measured values of absolute humidity are well below 30 mg H 2 O/L: 20.6 mg H 2 O/L for the Pall BB2215, mg H 2 O/L for the Intersurgical Filtatherm (Intersurgical; Wokingham, England), mg H 2 O/L for the Clear-Thermal (Intersurgical), mg H 2 O/L for the Pall BB50, 23 and 28.8 mg H 2 O/L for the Hygrobac filter. 22 It also appears that no endotracheal occlusion has been reported with AH 24 mg H 2 O/L. This is the reason why 24 mg H 2 O/L was set as the lowest acceptable inspired gas AH. In this setting, the scale described herein enables satisfactory rating of the poorly performing HMEs, since most of those delivering 24 mg H 2 O/L AH were rated dry. Similarly, most of those delivering 24 mg H 2 O/L AH were rated humid. Our findings are consistent with 1650 Clinical Investigations in Critical Care

6 those of Miyao and colleagues 27 who reported that the condensation in the breathing circuit is a good way of monitoring the humidity delivered by a humidifying device. They also speculated that the condensation helps to prevent the secretions inside the endotracheal tube from drying out. Both HMEs and HHs present several drawbacks. Placing an HME in the respiratory circuit generates gasflow resistance. This increase in the circuit s resistance has been studied previously; it reaches approximately 2.5 cm H 2 O/Ls and is related to the water load of the HME. 29,30 The clinical impact of this resistance has not been studied extensively. Conti and coworkers 31 found that in COPD patients receiving volume-controlled mechanical ventilation, the presence of an HME in the circuit did not modify intrinsic positive end-expiratory pressure. Moreover, the additional resistance occasioned by the HME is not higher than the one caused by an HH. 32 Additional dead space may also be caused by an HME, and increased Paco 2 and minute ventilation during weaning from mechanical ventilation in comparison with HHs. 33 However, a possible increase in total weaning time caused by HMEs has not been evaluated (to our knowledge) and the problem of increased dead space is easily overcome by increasing pressure support level. 34 It has been shown clearly in the literature either by in vitro or in vivo studies that increase in tidal volume or minute ventilation affected the performances of some HMEs. However, only the purely hydrophobic HMEs (Pall BB2215 and BB50) suffered from this drop in performance. Indeed, Martin and coworkers 22 have shown that the efficacy of the HD was not affected by minute ventilation 10 L/min. In our study, humidifying devices were used with minute ventilation 10 L/min. In fact, minute ventilation ranged from 7 to 15 L/min. Therefore, our scale can assess the performances of a humidifying device whatever the minute ventilation used. The use of purely hydrophobic HMEs cannot be recommended during mechanical ventilation with minute ventilation 10 L/min for which clinicians may use either hydrophobic and hygroscopic HMEs or HHs. 22,23 Excessive humidification (which can be caused only by a HH) can lead to epithelial and mucociliary damage. 35 However, insufficient humidification still remains the major problem that faces a clinician wishing to choose a humidifying device. Evaluation of the efficacy of this device whether it be an HH or an HME can be achieved with this simple inspection. This inspection must be performed several times during the use of the HME. A flex-tube that is always dry indicates a HME delivering insufficient humidity, and therefore at risk of endotracheal tube occlusion. In such instances, we believe it is wise either to change the HME used for a high-performing HME or to switch the patient to an HH. In our unit, we use only high-performing HMEs and such instances have never occurred whatever the type of patient. 9,11,16 However, a flex-tube that is constantly covered in water droplets indicates that the HME is appropriate for long-term mechanical ventilation without fear of endotracheal tube occlusion. Our observations indicate that clinicians can evaluate the performance of an HME at the bedside. This may result in considerable cost containment since mechanical ventilation with HMEs is much less expensive than with HH, 8,9,11 and could lead to the widespread use of HMEs with further substantial savings as long as contraindications to the use of HMEs be carefully respected; these include profound hypothermia, bronchopleural fistulas, and breath-eliminated drug poisoning. It is thus possible to evaluate the efficacy of HMEs visually by rating the amount of moisture in the flex-tube connecting the HME to the endotracheal tube. References 1 Chalon J, Patel C, Ali M, et al. Humidity and the anesthetized patient. Anesthesiology 1979; 50: Forbes A. Temperature, humidity and mucus flow in the intubated trachea. Br J Anaesth 1974; 46: Cohen I, Weinberg P, Fein I, et al. Endotracheal tube occlusions associated with the use of heat and moisture exchangers in the intensive care unit. Crit Care Med 1988; 16: Martin C, Perrin G, Gevaudan M, et al. Heat and moisture exchangers and vaporizing humidifiers in the intensive unit. Chest 1990; 97: Misset B, Escudier B, Rivara D, et al. Heat and moisture exchangers vs heated humidifier during mechanical ventilation. Chest 1991; 100: Roustan J, Kielen J, Aubas P, et al. Comparison of hydrophobic heat and moisture exchangers with heated humidifiers during prolonged mechanical ventilation. Intensive Care Med 1992; 18: Villafane M, Cinella G, Lofaso F, et al. Gradual reduction of endotracheal tube diameter during mechanical ventilation via different humidification devices. Anesthesiology 1996; 85: Tenaillon A, Cholley G, Boiteau R, et al. Filtre échangeurs de chaleur et d humidité versus humidificateurs chauffant en ventilation mécanique. Réan Soins Intens Méd Urg 1989; 5: Dreyfuss D, Djedaini K, Gros I, et al. Mechanical ventilation with heated humidifiers or heat and moisture exchangers: effects on patient colonization and incidence of nosocomial pneumonia. Am J Respir Crit Care Med 1995; 151: Hurni J, Feihl F, Lazor R, et al. Safety of combined heat and moisture exchangers in long-term mechanical ventilation. Chest 1997; 111: Djedaïni K, Billiard M, Mier L, et al. Changing heat and moisture exchangers every 48 hours rather 24 hours does not affect their efficacy and the incidence of nosocomial pneumonia. Am J Respir Crit Care Med 1995; 152: CHEST / 115 / 6/ JUNE,

7 12 Cook D, Sibbald W, Vincent J, et al. Evidence based critical care medicine; what is it and what can it do for us? evidence based medicine in critical care group. Crit Care Med 1996; 24: Branson R, Davis K Jr, Campbell R, et al. Humidification in the intensive care unit: prospective study of a new protocol utilizing heated humidification and a hygroscopic condenser humidifier. Chest 1993; 104: Branson R, Davis K Jr, Brown R, et al. Comparison of three humidification techniques during mechanical ventilation: patient selection, cost, and infection considerations. Respir Care 1996; 41: Branson R, Davis K Jr. Evaluation of 21 passive humidifiers according to the ISO 9360 standard: moisture output, dead space, and flow resistance. Respir Care 1996; 41: Djedaïni K, Markowicz P, Mier L, et al. Comparisons between three types of heat and moisture exchangers (HME) changed every 48 hours during mechanical ventilation [abstract]. Am J Respir Crit Care Med 1997; 155:A Branson RD. Is a nose just a nose [editorial]? Chest 1997; 112: Beydon L, Tong D, Jackson N, et al. Correlation between some simple clinical parameters and the in vitro humidification characteristics of filter heat and moisture exchangers. Chest 1997; 112: Zwillich C, Pierson D, Creagh C, et al. Complications of assisted ventilation: a prospective study of 354 consecutives episodes. Am J Med 1974; 57: Legall JR, Loirat P, Alperovitch A, et al. A simplified acute physiologic score for ICU patients. Crit Care Med 1984; 12: Jackson C, Webb A. An evaluation of the heat and moisture performance of four ventilator circuit filters. Intensive Care Med 1992; 18: Martin C, Papazian L, Perrin G, et al. Performance evaluation of three vaporizing humidifiers and two heat and moisture exchangers in patients with minute ventilation 10 L/min. Chest 1992; 102: Martin C, Thomachot L, Quinio B, et al. Comparing two heat and moisture exchangers with one vaporizing humidifier in patients with minute ventilation greater than 10 L/min. Chest 1995; 107: Sottiaux T, Mignolet G, Damas P, et al. Comparative evaluation of three heat and moisture exchangers during shortterm postoperative mechanical ventilation. Chest 1993; 104: Condon E. Handbook of physics. New York, NY: Churchill Livingstone, Bickler P, Sessler D. Efficiency of airway heat and moisture exchangers in anesthetized humans. Anesth Analg 1990; 71: Miyao H, Hirokawa T, Miyasaka K, et al. Relative humidity, not absolute humidity, is of great importance when using a humidifier with a heating wire. Crit Care Med 1992; 20: Emergency Care Research Institute. Heat and moisture exchangers. Health Devices 1983; 12: Ploysongsang Y, Branson R, Rashkin MC, et al. Pressure flow characteristics of commonly used heat and moisture exchangers. Am Rev Respir Dis 1988; 138: Chiaranda M, Verona L, Pinamonti O, et al. Use of heat and moisture exchangers filters in mechanically ventilated ICU patients: influence on airway flow resistance. Intensive Care Med 1993; 19: Conti G, De Blasi RA, Rocco M, et al. Effects of HME on dynamic hyperinflation of mechanically ventilated COPD patients. Intensive Care Med 1990; 16: Oh TE, Lin ES, Bhatt S. Resistance of humidifiers, and inspiratory work imposed by a ventilator-humidifier circuit. Br J Anaesth 1991; 66: Le Bourdellès G, Mier L, Fiquet B, et al. Comparison of the effects of heat and moisture exchangers and heated humidifiers on ventilation and gas exchange during weaning trials from mechanical ventilation. Chest 1996; 110: Iotti GA, Olivei MC, Palo A, et al. Unfavorable mechanical effects of heat and moisture exchangers in ventilated patients. Intensive Care Med 1997; 23: Williams R. The effects of excessive humidity. Respir Care Clin North Am 1998; 4: Clinical Investigations in Critical Care