During mechanical ventilation

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1 Heat and moisture exchangers in mechanically ventilated intensive care unit patients: A plea for an independent assessment of their performance Guillaume Thiéry, MD; Alexandre Boyer, MD; Etienne Pigné, MD; Amar Salah, MD; Arnaud de Lassence, MD; Didier Dreyfuss, MD; Jean-Damien Ricard, MD, PhD Objective: To determine whether use of a hygroscopic and hydrophobic heat and moisture exchanger (HME) for 7 days without change affects its efficiency in long-term, mechanically ventilated, chronic obstructive pulmonary disease (COPD) patients. Design: Prospective, randomized, controlled clinical study comparing two combined HMEs. Setting: Medical intensive care unit at a university teaching hospital. Patients: Long-term, mechanically ventilated, COPD patients compared with non-copd patients. Interventions: In the first part of the study, COPD patients were studied with the Hygroster HME changed once a week. For the second part, the Hygroster was assessed in non-copd patients and compared with the Hygrobac HME used in COPD and non- COPD patients for 1 wk without change. Devices could be changed if hygrometric measurements indicated insufficient humidity delivery. Measurements and Main Results: Daily measurements were recorded for inspired gas temperature and relative and absolute humidity. Ventilatory variables, clinical indicators of efficient humidification, were also recorded. No tracheal tube occlusion occurred. However, contrary to the manufacturer advertisement, the Hygroster experienced surprisingly low values for absolute humidity in both COPD and non-copd patients. Such events did not occur with the Hygrobac. Absolute humidity with the Hygroster was constantly and significantly lower during the 7-day study period than with the Hygrobac. Absolute humidity measured in COPD patients was identical to that measured in the rest of the study population with both HMEs. Conclusions: Manufacturer specifications and bedside measurements of absolute humidity differed considerably for the Hygroster, which in certain instances did not achieve efficient humidification in both COPD and non-copd patients. This did not occur with the Hygrobac, which performed well throughout the 7-day period in both COPD and non-copd patients. Our results speak for independent and in vivo evaluation of HMEs. (Crit Care Med 2003; 31: ) KEY WORDS: heat and moisture exchanger; acute respiratory failure; mechanical ventilation; absolute humidity; humidification of inspired gases; chronic obstructive pulmonary disease During mechanical ventilation of critically ill patients, heat and moisture must be added to the inspired gases to counterbalance their loss resulting from the bypass of the upper respiratory tract by the endotracheal tube (ETT) (1 3). This process has been achieved efficaciously for many years by heated humidifiers (4). More recently, disposable devices called heat and moisture exchangers (HMEs) have been available to clinicians. During From the Service de Réanimation Médicale, Hôpital Louis Mourier (Assistance Publique-Hôpitaux de Paris), Colombes, France (GT, AB, EP, AS, AdL, DD, JDR); and Faculté Xavier Bichat, Paris, France (DD, JDR). Address requests for reprints to: Didier Dreyfuss, MD, Service de Réanimation Médicale, Hôpital Louis Mourier, 178, rue des Renouillers, Colombes, France. didier.dreyfuss@lmr.ap-hop-paris.fr Copyright 2003 by Lippincott Williams & Wilkins DOI: /01.CCM F5 the last decade, these devices have been increasingly used in intensive care units (ICUs), as an alternative to heated humidifiers, more so in Europe (particularly in France) than in North America (5). This is in part due to the fact that their clinical (6 10) and hygrometric (11 15) performance compare favorably with those of heated humidifiers. In addition, they decrease the workload of the nursing staff (6, 7, 10) and have been shown to reduce the cost of mechanical ventilation (6 8, 10, 16 18). Unlike heated humidifiers, they are not associated with water condensation in the circuit, which may be a source of cross contamination or infection (19), and thus reduce circuit contamination (7). Manufacturers recommend a daily change of the HMEs. However, this recommendation is not substantiated by independent clinical data. Thus, it may be questioned if the duration of use of a given HME can be prolonged. Several clinical studies have provided solid evidence suggesting that some HMEs could be used for longer periods of time than that indicated by the manufacturer (10, 18, 20 24). Extending the duration of use of these devices had no adverse effect for the patients but enabled substantial reduction in the cost of mechanical ventilation. However, a major, unacceptable drawback could arise from this attitude (25). Indeed, one could legitimately fear a progressive decrease over time in the humidity delivered by the HME to the patient, leading to the potential threat of ETT occlusion. It has been shown in a previous study that although absolute humidity (AH) provided by an HME kept for one entire week was constant in the vast majority of the study population, some HMEs experienced a rapid decline in their humidifying performances (22). This rare event was only encountered in COPD patients. As a pre- Crit Care Med 2003 Vol. 31, No

2 caution, it was recommended that this HME (Hygrobac) be used for 48 hrs in COPD patients. Thus, the question that remained unanswered is: would an HME providing even greater humidity outputs than the Hygrobac-Dar be stable for 7 days in COPD patients? This question is worth consideration because COPD patients account for 15% of the patients requiring mechanical ventilation in North America and also undergo ventilation for a longer period of time (26). To try to answer it, we evaluated the humidifying performances over a 7-day period of an HME in mechanically ventilated COPD patients. This HME delivered (according to the manufacturer) greater values for AH. Above all, safety issues were closely examined with daily assessment of the humidifying performances of the HME with both clinical evaluation and hygrometric measurements to detect any insufficient humidity. Unexpectedly, the HME humidity performances measured in our institution were significantly lower than that stated by the manufacturer. Consequently, these results were confirmed in non-copd patients in the second part of the study and compared with the HME we had previously studied (22). MATERIALS AND METHODS Study Design Part One. The first part of the study involved the evaluation of the efficacy of the HME in mechanically ventilated COPD patients. All the COPD patients hospitalized over a 6-month period in the medical ICU of Louis Mourier hospital (a 12-bed ICU in a teaching hospital) who were considered likely to require mechanical ventilation for 48 hrs were included. COPD patients were defined according to the ATS statement for the diagnosis of COPD patients (27). These patients were all acutely ill inpatients. Exclusion criteria were profound hypothermia (body temperature, 33 C), a bronchopleural fistula, poisoning with breath-eliminated drugs such as hydrocarbons, and moribund patients. The HME used was the hygroscopic and hydrophobic Hygroster, (Mallinckrodt Medical, Mirandola, Italy), which, according to the manufacturer, provides greater moisture output than the Hygrobac (33.4 mg H 2 O/L at 500 ml of tidal volume vs mg H 2 O/L, respectively) and higher temperature output (33.9 C vs C, respectively, with a 500-mL tidal volume). The ventilators used were Siemens Servo 900 D (Siemens-Elema, Solna, Sweden), Bird 8400 Sp (Bird Products, Palm Springs, CA), and Evita 4 (Dräger, Lübeck, Germany) respirators. Each HME was initially set for a period of 7 days, after which it was replaced. Patients ventilated for 7 days could provide several 7-day study periods. Part Two. The second part of the study involved evaluation of the humidifying performances of the Hygroster in non-copd patients and comparison of its performances with those of the Hygrobac in both COPD and non-copd patients. After the unexpected results in part one (see below), the humidifying performances of the Hygroster were evaluated in non-copd patients who were likely to require mechanical ventilation for 48 hrs and compared with those obtained with another HME, the Hygrobac (Mallinckrodt Medical) in both COPD and non-copd patients. Patients allocated to the Hygroster HME were provided with HMEs coming from three separate sets of the Hygroster to eliminate technical distortion. To investigate whether poor humidity measurements obtained with an HME were due to the patient s respiratory conditions or to the HME itself, a patient could be switched on to the other HME tested if AH measured with the first one was 25 mg H 2 O/L. Positioning of the HMEs HMEs were placed between the ETT and the Y-piece of the circuit. Particular attention was given to place the HME vertically above the tracheal tube to reduce the risk of partial obstruction of the HME due to refluxed secretions from the tracheal tube (22, 23). Nurses and doctors repeatedly checked the position of the HME. Clinical Evaluation of HME Safety and Efficacy The variables recorded daily to assess HMEs have already been used in our previous studies (7, 20, 22, 23). These include the number of tracheal suctionings (which are usually performed every 4 hrs and whenever breathing sounds are heard) and peak airway pressures that were recorded every 4 hrs and averaged over 24 hrs (they are subsequently referred to as mean peak airway pressure). Tracheal instillation is not performed systematically in our unit, but it is performed in the rare cases of very thick tracheal secretions. Indications for Premature Replacement of HMEs There were three indications for premature replacement of HMEs. First, HME obstruction was identified by an otherwise unexplained rise in peak airway pressure and confirmed by visual inspection of the removed filter and the immediate normalization of airway pressure after replacement of HME. Second, endotracheal tube occlusion was prospectively defined as an unexplained and sudden rise in the peak airway pressure without evidence of filter obstruction and an inability to insert a suction catheter through the previously patent tube. Third, an AH of 25 mg H 2 O/L. Hygrometric Measurements AH, relative humidity (RH), and tracheal temperature were measured within the first hours and then daily from day 1 to day 7. AH is the amount of water vapor contained in air (mg H 2 O/L). Absolute humidity at saturation (AHs) is the maximum amount of water vapor that air can contain at a given temperature. RH is the ratio of AH to AHs expressed as a percentage. These variables were measured by psychrometry, a technique widely used in clinical studies that evaluate HME performances (11 15, 22, 23). A device containing two oneway valves inserted between the ETT and the HME allowed separation of inspired and expired gas flows. In the inspiratory part of this device were inserted two thermal probes: a dry one and a wet one. At the same time, a third thermal probe was inserted in the ETT. These three temperatures were recorded after a 30- min period, allowing optimal thermal equilibrium, and then displayed on a chart recorder (Yokogawa, Tokyo, Japan). Room temperature was constant at C. The psychrometric method compares the temperatures obtained with each probe placed in 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 around the wet probe in the inspiratory part is proportional to the dryness of the gas. The temperature gradient between the two probes increases as the inspired gas humidity decreases. For instance, when the inspired gas is fully saturated with water (100% RH), there is no thermal gradient. RH was calculated by reference to a nomogram, taking into account the difference between temperatures measured by the two probes. AHs (100% RH) was calculated with the following formula (11 15, 22, 23): AHs T T 2 mg H 2 O/L, where T ( C) is the dry probe temperature. AH was obtained with the formula: AH (AHs RH)/100 (in mg H 2 O/L). Data Collection The following variables were recorded prospectively for all the patients: age, sex, indication for and length of ventilatory support, severity of illness according to the Simplified Acute Physiology Score (SAPS II) (28), ventilatory variables (including tidal volume and FIO 2 ), and body temperature at the time of psychrometry measurement. This protocol was approved by the Institutional Review Board for human studies of the 700 Crit Care Med 2003 Vol. 31, No. 3

3 French Intensive Care Society. Informed consent was not requested because all procedures were considered to be routine practice and none was invasive. Statistical Analysis The variables for the HMEs are given as mean SD. Intergroup or intragroup comparisons of quantitative data were performed using analysis of variance. When analysis of variance indicated differences between groups, they were compared using the protected least significant difference. Chi-square statistic was used to compare categorical variables; p.05 was considered significant. RESULTS Part One Study Population. A total of 14 COPD patients were included in the first part of the study (Table 1). Twenty sets (consecutive days of HME measurement) were recorded, of which ten completed the 7-day study period. Five patients, providing five distinct sets of measurements did not complete a 7-day period because of extubation or death before day 7, and five other patients had their HME prematurely replaced (see below). Clinical Evaluation of Humidifying Efficacy. The variables used to evaluate the clinical safety and efficacy of the Hygroster are shown in Table 2. No ETT occlusion occurred. The patients needed no tracheal instillation or HME replacement because of obstruction by secretion during the study period. There were no differences between day 0 and day 7 for the number of tracheal suctionings or the mean peak airway pressure. There were also no differences when these comparisons were made from one day to another (data not shown). Hygrometry Measurements. AH delivered by the Hygroster was stable during the 7-day study period in the ten complete sets (Fig. 1) but markedly lower than that specified by the manufacturer: mg H 2 O/L vs mg H 2 O/L, respectively. In addition, a protocoldefined inadequate AH ( 25 mg H 2 O/L) occurred in five patients. According to the protocol, this measurement led to the immediate replacement of their HME with no adverse effect for the patients. These replacements occurred on days 0, 1, 3, and 4 (two patients). Part Two Hygroster in Non-COPD Patients. The humidifying performance of the Hygroster was measured in six non-copd patients (characteristics of these patients are displayed in Table 1), providing 11 sets of measurements, four of which completed the 7-day study period. Three sets did not complete a 7-day period because of extubation or death before day 7, and four others were prematurely replaced because of an AH of 25 mg H 2 O/L (on day 0, day 1, and day 3). There was no difference between COPD and non-copd patients in AH delivered by the Hygroster (Fig. 2). Table 1. Characteristics of patients Hygroster Hygrobac COPD, n 14 Non-COPD, n 6 COPD, n 6 Non-COPD, n 4 p Value At admission Age, yrs SAPS II Indications for MV Exacerbated COPD Non-COPD ARF During study Duration of MV, days COPD, chronic obstructive pulmonary disease; SAPS II, Simplified Acute Physiology Score (28); MV, mechanical ventilation; ARF, acute respiratory failure, including postoperative respiratory failure, cancer, cardiogenic pulmonary edema, coma, and pneumonia. Table 2. Clinical indicators of adequate humidification, ventilatory variables, and body temperature Hygroster Hygrobac COPD, n 14 Non-COPD, n 6 COPD, n 6 Non-COPD, n 4 p Value Day 0 Peak Paw, cm H 2 O Tracheal aspirations, per day ETT occlusion V T, ml Body temperature, C Day 7 a Peak Paw, cm H 2 O Tracheal aspiration, per day ETT occlusion V T, ml Body temperature, C COPD, chronic obstructive pulmonary disease; Paw, airway pressure; ETT, endotracheal tube; V T, tidal volume. a Or on last day of evaluation in case of cessation of mechanical ventilation before day 7. Crit Care Med 2003 Vol. 31, No

4 Hygrobac in COPD and Non-COPD Patients. Ten (six COPD and four non- COPD) patients (Table 1) were evaluated with the Hygrobac HME over a 7-day period, providing 12 sets of measurements, six of which completed the 7-day study period, and six did not because of mechanical ventilation withdrawal. No premature HME replacement because of an AH of 25 mg H 2 O/L occurred with the Hygrobac HME. COPD patients did not differ from non-copd patients with respect to values for AH (Fig. 3). Comparison Between Hygroster and Hygrobac. Humidifying performances of the Hygroster were compared with those of the Hygrobac, both measured in COPD and non-copd patients. During the 7-day period, the Hygrobac constantly delivered significantly higher values for AH than did the Hygroster (Fig. 4). As stated above, no premature replacement occurred with the Hygrobac because of an inadequate AH ( 25 mg H 2 O/L), whereas nine premature replacements occurred with the Hygroster (five in COPD patients [part 1] and four in non-copd patients [part 2]). Three of these nine prematurely removed Hygrosters were replaced by Hygrobac HMEs, and values of AH measured within the same time frame were significantly greater with the Hygrobac than with the Hygroster ( vs mg H 2 O/L, p.01). Finally, three distinct batches of the Hygroster were tested during this study. Values of AH did not differ between the three batches: batch A (13 measurements), mg H 2 O/L; batch B (ten measurements), mg H 2 O/L; batch C (seven measurements), ; p.12. DISCUSSION The main findings of this study are the following. The Hygroster was found to deliver very low unexpected values for AH during long-term mechanical ventilation of both COPD and non-copd patients. Second, long-term mechanical ventilation with this HME changed only once a week cannot be recommended. Furthermore, the use of this particular HME for short-term mechanical ventilation may even be questioned because of the occurrence of several low humidity outputs measured during the first 24 hrs of use. The meaningful difference between the values for AH measured at the patient s bedside and those provided by the manufacturer stress the importance of independent HME performance assessment. This study confirms our previous findings on the high level of humidity delivered by the Hygrobac throughout a 7-day period. Patients with COPD represent the most important subgroup of mechanically ventilated ICU patients in the United States (26). In addition, their duration of ventilation is longer than that of other patients (26). Thus, adequate humidification is probably more critical in such patients than in those ventilated for shorter periods of time. Although some investigators think that patients with COPD are not appropriate candidates for ventilation with HMEs (16, 29), it has been repeatedly shown that long-term Figure 1. Hygroster heat and moisture exchanger measurements of absolute and relative humidity on days 0, 2, 4, and 7 in 14 chronic obstructive pulmonary disease patients. There was no difference across time for either variables. Data presented as mean SD. Figures at the top of the bars indicate the number of heat and moisture exchangers measured at each time point, and figures at the bottom of the bars indicate the cumulative number of heat and moisture exchangers delivering 25 mg H 2 O/L absolute humidity. Figure 2. Absolute humidity measurements obtained with the Hygroster in chronic obstructive pulmonary disease (COPD) patients were identical to those obtained in non-copd patients throughout the 7-day period. Data presented as mean SD. Figures at the top of the bars indicate the number of heat and moisture exchangers measured at each time point, and figures at the bottom of the bars indicate the cumulative number of heat and moisture exchangers delivering 25 mg H 2 O/L absolute humidity. mechanical ventilation can be safely conducted in such patients with HMEs (8 10, 22, 23). Extending the use of HMEs in this population would undoubtedly reduce cost of mechanical ventilation. Several studies have indeed shown that changing HMEs only every other day was safe and cost-effective in COPD patients (22, 23). Our initial goal was to further evaluate prolonged use of HMEs in COPD patients by using a supposedly more efficient HME. The Hygroster HME was chosen because its manufacturer s advertisement indicated that it delivered more humidity than the Hygrobac HME we had previously studied (22). In this previous study, 33 patients were ventilated with an HME changed only once a week. Overall assessment of this strategy proved to be efficient. However, in three instances, hygrometry measurements indicated that AH delivered by the HME was below the threshold prospectively defined by proto- Figure 3. There was no difference between chronic obstructive pulmonary disease (COPD) patients and non-copd patients in absolute humidity delivered by the Hygrobac. Data presented as mean SD. Figures at the top of the bars indicate the number of heat and moisture exchangers measured at each time point, and figures at the bottom of the bars indicate the cumulative number of heat and moisture exchangers delivering 25 mg H 2 O/L absolute humidity. Figure 4. During the 7-day period, the Hygrobac constantly delivered significantly higher values for absolute humidity than the Hygroster. Data presented as mean SD. *p.05; ***p Crit Care Med 2003 Vol. 31, No. 3

5 col. All three premature HME replacements occurred in COPD patients even though all the other measurements performed in the rest of the COPD population of the study were satisfactory. As a whole, AH measured in COPD patients was slightly but significantly lower than that measured in the rest of the population (22). It thus remained unanswered if these results were due to the HME or to the COPD respiratory condition. This is why we decided to use, in the present study, an HME stated as delivering higher values of AH. As stated above, we noted that values for AH measured in our patients were significantly lower than values provided by the manufacturer for this HME. Importantly, such observations were noted in both COPD and non-copd patients. Thus, it seemed that these events were the consequences of the HME performances rather than that of the respiratory conditions of our patients. Apart from the intrinsic capacity of an HME to deliver a certain level of AH, external conditions (ambient temperature and body temperature) may influence HME performance. In the present study, there was no difference between the body temperature of patients in which an HME delivered 25 mg H 2 O/L of AH and that of the rest of the study population. There was no difference either in the ambient temperature because our unit has a strictly monitored and regulated temperature of C. Two other observations further imply that discrepancies were more likely due to the HME than to environmental or patient-related conditions. First, these low values of AH were measured with the Hygroster in both COPD and non-copd patients. Second, three patients ventilated with a Hygroster in which a very low AH was measured were subsequently ventilated with a Hygrobac that was found to deliver satisfactory values of AH. One may then argue that we accidentally received a faulty batch of Hygroster HMEs. However, to free us from this hypothesis, we randomly chose Hygroster HMEs from three distinct batches of HMEs that all delivered similar AH. Other investigators have studied the same HME, but for only very short periods of time (14, 30), and have measured in some instances low values of AH with the Hygroster. These observations call for two remarks. First, they build up compelling evidence for the need of independent and in vivo evaluation of HMEs. Indeed, most of the manufacturer indications on HME performance are based on in vitro testing (using the International Standards Organization 9360 water loss method and psychrometry), which may be quite different from the clinical setting, and thus yield values that overestimate actual in vivo performance (31, 32). Psychrometry is the method most often used by clinicians investigating HME performance, and consistent results have been obtained with this method (33). It may also be that some HMEs (because of their shape, the surface area of the humidifying media, the exact nature the media) are more sensitive to the conditions of use than others, thus explaining why, with these HMEs, differences are noted between bench measurements and bedside measurements. Second, these discrepancies cast doubt in the mind of clinicians who need to choose an HME for their patients. Indeed, the major risk associated with HME use is ETT occlusion. It has been shown that ETT occlusion occurs after a gradual reduction of the internal diameter of the ETT (34). In this study, the reduction was significantly greater with poor-performing HMEs than with a heated humidifier or the Hygrobac-Dar HME. This is in agreement with the clinical observations of ETT occlusion reported in the literature that occurred with HMEs exhibiting low values for AH (35 38). Furthermore, the reduction of the internal diameter of the ETT with the Hygrobac HME was not different compared with that observed with a Fisher- Paykel heated humidifier (34). It is therefore clear that HMEs performances in terms of heat and humidity outputs may vary considerably from one brand to another (11, 15, 23, 31). Thus, not all available HMEs are appropriate for long-term mechanical ventilation, let alone an extension of their duration of use, and rigorous and independent assessment of such devices must therefore be performed before using them in the ICU. A possible limitation of our study may reside in the relatively small number of patients. However, the object of the study was to call attention on the risk of underhumidification with a given HME. Had we noted only one patient with poor values of AH, it would have been our duty to report it. Although ETT occlusion is currently rare, it may be unfortunately lethal when it occurs. Our data deserve particular attention from clinicians using the Hygroster HME precisely because we observed such an important number of very low values of humidity in a relatively Manufacturer specifications and bedside measurements of absolute humidity differed considerably for the Hygroster, which in certain instances did not achieve efficient humidification in both patients with and patients without chronic obstructive pulmonary disease. small number of patients. Concerning the Hygrobac HME, our results confirm our previous results that showed that mechanical ventilation could be safely conducted with this HME changed only once a week (22). This HME has been extensively studied and results consistently indicate very satisfactory hygrometric performances (11, 15, 36, 37), even during prolonged use (22, 23). As medical care improves and becomes more and more technical (and thus more costly), a higher degree of faultlessness of medical equipment (even the most basic) is required by clinicians and expected by patients and their families. Nevertheless, healthcare resources are not infinite, and administrative supervision is more and more watchful of medical expenditure. This is particularly so in ICUs with mechanical ventilation (39). In this respect, HMEs performances have greatly improved during the last decade, but they participate in a certain extent to the cost of mechanical ventilation. Thus, extending the use of HMEs is a laudable goal as long as it does not put the patient at risk of ETT obstruction. Our results indicate that this cannot be achieved with the Hygroster but can with the Hygrobac. Clinicians must therefore be aware of important disparity in the performance of HMEs underlined by our study. Our results speak for independent and in vivo testing of HME performance that may help clinicians choose the appropriate HME for their patients, without placing them at risk of ETT occlusion. Crit Care Med 2003 Vol. 31, No

6 REFERENCES 1. Chalon J, Patel C, Ali M, et al: Humidity and the anesthetized patient. Anesthesiology 1979; 50: Forbes AR: Temperature, humidity and mucus flow in the intubated trachea. Br J Anaesth 1974; 46: Branson RD: The effects of inadequate humidity. Respir Care Clin North Am 1998; 4: Branson RD, MacIntyre N: Humidification: Current therapy and controversy. Respir Care Clin North Am 1998; 4: Cook D, Ricard JD, Reeve B, et al: Ventilator circuit and secretion management strategies: A Franco-Canadian survey. Crit Care Med 2000; 28: Tenaillon A, Cholley G, Boiteau R, et al: Filtres échangeurs de chaleur et d humidité versus humidificateurs chauffant en ventilation mécanique. Réanim 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: Boots RJ, Howe S, George N, et al: Clinical utility of hygroscopic heat and moisture exchangers in intensive care patients. Crit Care Med 1997; 25: Hurni JM, Feihl F, Lazor R, et al: Safety of combined heat and moisture exchanger filters in long-term mechanical ventilation. Chest 1997; 111: Kollef M, Shapiro S, Boyd V, et al: A randomized clinical trial comparing an extended-use hygroscopic condenser humidifier with heated-water humidification in mechanically ventilated patients. Chest 1998; 113: 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: Jackson C, Webb AR: An evaluation of the heat and moisture exchange performance of four ventilator circuit filters. Intensive Care Med 1992; 18: Sottiaux T, Mignolet G, Damas P, et al: Comparative evaluation of three heat and moisture exchangers during short-term postoperative mechanical ventilation. Chest 1993; 104: 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: Ricard JD, Markowicz P, Djedaïni K, et al: Bedside evaluation of efficient airway humidification during mechanical ventilation of the critcally ill. Chest 1999; 115: Branson RD, Davis KJ, Brown R, et al: Comparison of three humidification techniques during mechanical ventilation: Patient selection, cost, and infection considerations. Respir Care 1996; 41: Kirton O, DeHaven B, Morgan J, et al: A prospective, randomized comparison of an in-line heat and moisture exchange filter and heated wire humidifiers: Rates of ventilatorassociated early-onset (community-acquired) or late-onset (hospital-acquired) pneumonia and incidence of endotracheal tube occlusion. Chest 1998; 112: Thomachot L, Vialet R, Viguier JM, et al: Efficacy of heat and moisture exchangers after changing every 48 hours rather than 24 hours. Crit Care Med 1998; 26: Craven DE, Goularte TA, Make BJ: Contaminated condensate in mechanical ventilator circuits: A risk factor for nosocomial pneumonia. Am Rev Respir Dis 1984; 129: Djedaïni K, Billiard M, Mier L, et al: Changing heat and moisture exchangers every 48 hour rather than every 24 hour does not affect their efficacy and the incidence of nosocomial pneumonia. Am J Respir Crit Care Med 1995; 152: Thomachot L, Boisson C, Arnaud S, et al: Changing heat and moisture exchangers after 96 hours rather than after 24 hours: A clinical and microbiological evaluation. Crit Care Med 2000; 28: Ricard JD, Le Mière E, Markowicz P, et al: Efficiency and safety of mechanical ventilation with a heat and moisture exchanger changed only once a week. Am J Respir Crit Care Med 2000; 161: Markowicz P, Ricard JD, Dreyfuss D, et al: Safety, efficacy and cost effectiveness of mechanical ventilation with humidifying filters changed every 48 hours: A prospective, randomized study. Crit Care Med 2000; 28: Davis K Jr, Evans SL, Campbell RS, et al: Prolonged use of heat and moisture exchangers does not affect device efficiency or frequency rate of nosocomial pneumonia. Crit Care Med 2000; 28: Durbin CGJ: Extended use of disposables: economically sound or asking for trouble [editorial]? Crit Care Med 1998; 26: Esteban A, Anzueto A, Alia I, et al: How is mechanical ventilation employed in the intensive care unit? Am J Respir Crit Care Med 2000; 161: ATS statement: Standards for the diagnosis and care of patients with chronic obstructive pulmonary disease. Am J Respir Crit Care Med 1995; 152:S77 S Le Gall JR, Lemeshow S, Saulnier F: A new Simplified Acute Physiology Score (SAPS II) based on a European/North American multicenter study. JAMA 1993; 270: Branson RD, Davis KJ, Campbell RS, et al: Humidification in the intensive care unit: Prospective study of a new protocol utilizing heated and humidification and a hygroscopic condenser humidifier. Chest 1993; 104: Feihl F, Anglada C, Perret C: The temperature and water output of heat and moisture exchangers during synchronous intermittent mandatory ventilation. Acta Anaesth Italica 1992; 43: Branson RD, Davis KJ: Evaluation of 21 passive humidifiers according to the ISO 9360 standard: Moisture output, dead space, and flow resistance. Respir Care 1996; 41: Ünal N, Kanhai JKK, Buijk SLCE, et al: A novel method of evaluation of three heat and moisture exchangers in six different ventilator settings. Intensive Care Med 1998; 24: Ricard JD, Dreyfuss D: Efficiency and safety of mechanical ventilation with a heat and moisture exchanger changed once a week [letter to the editor]. Am J Respir Crit Care Med 2001; 164: Villafane MC, Cinnella G, Lofaso F, et al: Gradual reduction of endotracheal tube diameter during mechanical ventilation via different humidification devices. Anesthesiology 1996; 85: Cohen IL, Weinberg PF, Fein IA, et al: Endotracheal tube occlusion associated with the use of heat and moisture exchangers in the intensive care unit. Crit Care Med 1988; 16: Martin C, Perrin G, Gevaudan MJ, et al: Heat and moisture exchangers and vaporizing humidifiers in the intensive care unit. Chest 1990; 97: Misset B, Escudier B, Rivara D, et al: Heat and moisture exchanger vs heated humidifier during long-term mechanical ventilation: A prospective randomized study. Chest 1991; 100: Roustan JP, Kienlen J, Aubas P, et al: Comparison of hydrophobic heat and moisture exchangers with heated humidifier during prolonged mechanical ventilation. Intensive Care Med 1992; 18: McGee WT: Cost associated with mechanical ventilation. In: Ventilator Management Strategies for Critical Care. Hill NS, Levy MM (Eds). New York, Marcel Dekker, 2001, pp Crit Care Med 2003 Vol. 31, No. 3