Efficiency and Safety of Mechanical Ventilation with a Heat and Moisture Exchanger Changed Only Once a Week

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1 Efficiency and Safety of Mechanical Ventilation with a Heat and Moisture Exchanger Changed Only Once a Week JEAN-DAMIEN RICARD, ERIC LE MIÈRE, PHILIPPE MARKOWICZ, SERGE LASRY, GEORGES SAUMON, KAMEL DJEDAÏNI, FRANÇOIS COSTE, and DIDIER DREYFUSS Service de Réanimation Médicale et de Bactériologie, Hôpital Louis Mourier (Assistance Publique-Hôpitaux de Paris), Colombes, France; and Institut National de la Santé et de la Recherche Médicale, Unité 82, Faculté de Médecine Xavier Bichat, Paris, France The cost of mechanical ventilation (MV) is high. Efforts to reduce this cost, as long as they are not detrimental for the patients, are needed. MV with heat and moisture exchangers (HME) changed every 48 h is safe, efficient, and cost-effective. Preliminary reports suggest that the life span of these filters may be prolonged. We determined prospectively whether a hygroscopic and hydrophobic HME (Hygrobac-Dar; Mallinckrodt) provided safe and efficient heating and humidification of the inspired gases when changed only once a week. Patients who were considered to require mechanical ventilation for more than 48 h were included in the study. HMEs were initially set for 7 d. Efficient airway heating and humidification were assessed by clinical parameters (number of tracheal suctionings and instillations required, peak airway pressures) and hygrometric measurements performed by psychrometry. Resistance was measured from 0 to 7. Bacterial colonization of circuits and HMEs was studied. A total of 377 days of mechanical ventilation with 60 HMEs was studied. Clinical parameters and hygrometric measurements did not change between 0 and 7. Mean absolute humidity was mg H 2 O/L on 0 and mg H 2 O/L on 7 (p 0.7). Endotracheal tube occlusion never occurred. Three HMEs were replaced prematurely because of insufficient absolute humidity. This rare event occurred only in patients with COPD and after the third day of use. In addition, the absolute humidity delivered by the HMEs was significantly lower in patients with COPD than in the rest of the population. Resistance did not change from 0 to 7 ( versus cm H 2 O/L/s; p 0.4). Bacterial samples of both circuits and ventilator sides of HMEs were sterile in most cases. We conclude that mechanical ventilation can be safely conducted in non- COPD patients using an HME changed only once a week, leading to substantial cost savings (about $110,000 per year if these findings were applied to the university-affiliated hospitals in Paris). Ricard J-D, Le Mière E, Markowicz P, Lasry S, Saumon G, Djedaïni K, Coste F, Dreyfuss D. 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: Reducing costs is a major issue when treating critically ill patients. Mechanical ventilation of these patients requires adequate airway heating and humidification to counterbalance the bypassing of the upper respiratory tract. Without such conditioning, the dry inspired gases may severely damage the respiratory epithelium (1, 2). Heated humidifiers have been used for many years. While the hot water system used provides adequate heat and humidity to the inspired gases, there are undesired side effects, such as excessive condensation in (Received in original form in February 16, 1999 and in revised form June 10, 1999) Disclosure of potential conflict of interest: We received no financial support from and do not have any commitment to the brand of the device tested in this study. Presented in part at the ATS meeting, April 24 29, Correspondence and requests for reprints should be addressed to Didier Dreyfuss, Service de Réanimation Médicale, Hôpital Louis Mourier, 178, rue des Renouillers, Colombes, France. didier.dreyfuss@lmr.ap-hop-paris.fr Am J Respir Crit Care Med Vol 161. pp , 2000 Internet address: the circuit, circuit contamination, high cost, and increased work load (3 6). Disposable devices called heat and moisture exchangers (HMEs) have been developed. Their clinical efficacy is comparable to that of the heated humidifier (6 8), despite their slightly lower humidity outputs. The types of HMEs include (1) purely hydrophobic HMEs, which possess high antimicrobial properties, but perform poorly in terms of humidity output and have been responsible for endotracheal tube occlusions (9 12), (2) hygroscopic HMEs, which have better humidifying qualities than the hydrophobic HMEs, but do not possess antimicrobial filtration properties, and (3) hydrophobic and hygroscopic HMEs, which have both satisfactory humidity outputs and antimicrobial properties. The manufacturers recommend that these disposable HMEs be changed every 24 h, although this is not supported by objective data. It has been previously shown that a hydrophobic and hygroscopic HME (Hygrobac-Dar; Mallinckrodt Medical, Mirandola, Italy) could be changed only every 48 h without any adverse effects for the patients (13). The rates of nosocomial pneumonia were identical whether the HME were replaced every 24 h or

2 Ricard, Le Mière, Markowicz, et al.: Heat and Moisture Exchangers Changed Weekly 105 every 48 h. Finally, extending the use period to 48 h provided measurable savings per year (13). But clinical evaluation of the humidifying and heating performances of HMEs with parameters such as the number of tracheal suctionings and instillations required daily, and the peak airway pressure, is insufficient because it does not always detect less effective HMEs. A preliminary study showed that the clinical parameter of adequate airway humidification of a moderately performing HME was not different from that of efficient HMEs, whereas hygrometric measurements revealed much lower values for both absolute humidity and relative humidity than those exhibited by the efficient HMEs (14). These data clearly indicate the need for an objective in vivo evaluation of the heating and humidifying performances of HMEs before extending the duration of their use. The remarkable stability of the values for absolute humidity and relative humidity obtained with the Hygrobac-Dar after 48 h of use indicates that this hygroscopic and hydrophobic HME may be appropriate for even longer periods of mechanical ventilation without being changed. Although the economic impact of further extending the use of HMEs may be important in an era of diminished resources, studies addressing this issue must certainly not, for obvious ethical reasons, place the patients at risk. Extending the use of HMEs on clinical grounds only is insufficient. We therefore assessed the safety and efficacy of mechanical ventilation with a hygroscopic and hydrophobic HME changed only once a week, using both hygrometric measurements and bacterial colonization surveillance. METHODS Study Design All of the patients hospitalized over a 9-mo period in the Service de Réanimation Médicale of the Louis-Mourier University Hospital (a 12-bed intensive care unit) who were considered likely to require continuous mechanical ventilation for more than 48 h were included. Exclusion criteria included profound hypothermia (temperature 33 C), a bronchopleural fistula, or poisoning with breath-eliminated drugs (hydrocarbons). The HME used was the hygroscopic and hydrophobic Hygrobac-Dar (Mallinckrodt Medical). The ventilators used were Siemens Servo 900 D (Siemens-Elema, Solna, Sweden), Bird 8400 Sp (Bird Products, Palm Springs, CA), and César respirators (CFPO, Paris, France). Each HME was initially installed for a period of 7 d after which it was replaced. Premature replacement could occur under certain circumstances (see below). Patients ventilated for more than 7 d therefore provided several 7-d study periods. Positioning of the HMEs HMEs were placed between the endotracheal tube and the Y-piece of the circuit. Particular care and attention were given to positioning each HME in relation to the endotracheal tube. The HME had to be placed vertically above the tracheal tube (by means of a flex tube) in order to reduce the risk of partial obstruction of the HME due to refluxed secretions from the tracheal tube. Nurses and doctors repeatedly checked the position of each HME. Clinical Evaluation of HME Safety and Efficacy The same simple parameters as those used in our previous studies (7, 13) were recorded. The number of tracheal suctionings and tracheal instillations was recorded daily (in our unit, tracheal aspirations are performed every 4 h and whenever breathing sounds are heard, and instillations are performed by the nurse only when the secretions are considered thick and difficult to suction). Peak airway pressures were recorded every 6 h and averaged (they are subsequently referred to as mean peak airway pressures). These measurements were done after patient suctioning. Tracheal tube occlusion was prospectively defined as an unexplained rise in the peak airway pressure without evidence of filter obstruction and an inability to insert a suction catheter through the previously patent tube, leading to its replacement. Episodes of HME obstruction were identified by unexplained rises in airway pressure and confirmed by visual inspection of the removed filter and the immediate normalization of airway pressures after replacement of the HME. Hygrometric Measurements Absolute humidity, relative humidity, and tracheal temperature were measured within the first 48 h and then daily from 3 until 7. Absolute humidity 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. Relative humidity is the ratio of absolute humidity to absolute humidity at saturation, and is expressed as a percentage. These parameters may be measured by psychrometry, a technique widely used in clinical studies that evaluate HME performance (14 18). Briefly, inspiratory and expiratory gas flows were separated by a device containing two one-way valves inserted between the endotracheal tube and the HME. 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 to allow for optimal thermal equilibrium. The psychrometric method compares the temperatures obtained with the two probes 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. There is no thermal gradient when the inspired gas is fully saturated with water (100% relative humidity). Relative humidity (RH) was calculated by reference to a nomogram, taking into account the difference between temperatures measured by the two probes. Absolute humidity at saturation (100% 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. Absolute humidity (AH) was obtained from the formula AH (AHs RH)/100 (in mg H 2 O/L). Room temperature was constant at C. HMEs were replaced before the seventh day of use when the absolute humidity was 26 mg H 2 O/L or when there was partial obstruction of the HME by tracheal secretion. Resistance of the HMEs to Air Flow HME resistance to air flow was measured daily in one of every three patients, from 0 until 7, according to the International Standards Organization ISO 9360 (19). Briefly, resistance was calculated from the pressure drop from either side of the HME when a constant 60-L/min flow was applied through the HME (Flowmeter; Fischer & Porter, Warminster, PA). Pressure was measured with pressure sensors (SCX DV, AST, Vanves, France) and the signal was analyzed and displayed on a two-track recorder (Windograf; Gould, Cleveland, OH). TABLE 1 NUMBER OF HMEs STUDIED ACCORDING TO THE NUMBER OF DAYS ON MECHANICAL VENTILATION AND THE REASON FOR VENTILATION Reason for Ventilation Number of s on Mechanical Ventilation Chronic airway obstruction (n 10)* Other pulmonary disease (n 13) Other (n 10) Total HMEs studied * Numbers in parentheses represent the number of patients studied. Postoperative mechanical ventilation, neurologic emergency, miscellaneous.

3 106 AMERICAN JOURNAL OF RESPIRATORY AND CRITICAL CARE MEDICINE VOL TABLE 2 VENTILATORY PARAMETERS AND PATIENT BODY TEMPERATURE: COMPARISONS BETWEEN PATIENTS WITH AND WITHOUT COPD COPD (n 10) Others (n 23) p Value Tidal volume, ml NS Respiratory rate, breaths per minute NS FI O2, % NS Time spent under pressure support ventilation (percentage of total time under mechanical ventilation) NS Temperature, C NS Colonization of HMEs and Circuits The bacteria-filtering properties of the HME were assessed by opening them aspetically after their seventh day of use (day of replacement), and placing the ventilator side of the HME on CLED agar. Circuit bacterial colonization was assessed by plating a 1-cm 2 swabbing of the Y-piece on CLED agar and incubating the plates for 48 h. Colonies were counted and identified. Bacterial counts are expressed as colony-forming units (CFU) per HME or per square centimeter of the Y-piece. Data Collection and Definitions The parameters recorded for each patient included age, sex, indication for ventilatory support according to the classification of Zwillich and colleagues (20), simplified acute physiology score on admission (21), mean tidal volume, respiratory rate, FI O2 (fraction of inspired oxygen), body temperature over time, and the percentage of time spent on inspiratory pressure ventilation (defined as the ratio of time spent on inspiratory pressure ventilation to total duration of mechanical ventilation). This protocol was approved of by the Institutional Review Board for human studies of the French Intensive Care Society. Informed consent was not requested since all procedures were considered to be routine practice, and none was invasive. Statistical Analysis The parameters for the HMEs are given as means SD, or as the number of patients affected. Continuous data for the groups were assessed by analysis of variance (ANOVA). When ANOVA indicated differences between groups, they were compared using the protected least significant difference. Hygrometric values over time were compared according to the indication for mechanical ventilation, using a t test with Bonferroni s correction. Categorical data were analyzed by contingency table analysis. A p value 0.05 was considered significant. RESULTS Study Population Thirty-three patients were included in the study. The sex ratio (M/F) was 18/15. The mean age was yr (range, yr). The marker of acute illness used was the Simplified Acute Physiologic Score 2 (21), with a mean of (range, 21 87). Reasons for mechanical ventilation were the following: there were 10 patients with chronic airway obstruction, 13 patients with other pulmonary diseases, and 10 patients with nonrespiratory diseases (4 postoperative mechanical ventilation, 1 neurologic emergency, and 5 miscellaneous). Of the 60 HMEs studied, 40 were studied for 7 d. The 33 patients enrolled provided a total of 377 d of mechanical ventilation. Of these, 12 patients (6 chronic airway obstruction, 4 other pulmonary diseases, 1 neurologic emergency, 1 miscellaneous) provided several 7-d study periods: 7 periods in 1 patient, 4 periods in 2 patients, 3 periods in 2 patients, and 2 periods in 7 patients; 5 patients provided one 7-d period. The remaining patients (16) provided 20 periods of less than 7 d because of mechanical ventilation cessation or premature HME replacement (see below). Table 1 shows the number of HMEs studied according to the number of days of mechanical ventilation and the reasons for it. Only two modes of mechanical ventilation were used: assist-control ventilation and inspiratory pressure support ventilation. The conditions of mechanical ventilation and body temperature were identical in patients with COPD and in the rest of the patient population (Table 2). Clinical Evaluation of Humidifying Efficacy The parameters used to evaluate the safety and efficacy of the HMEs are shown in Table 3. No endotracheal tube occlusion occurred. There were no differences between s 0 and 7 in terms of the number of tracheal suctionings, tracheal instillations, or the mean peak airway pressures. There were also no differences when these comparisons were performed from one day to another (data not shown). Only 6 of the 60 HMEs used were changed prematurely (before 7) because of partial obstruction of the HME by tracheal secretions, thus amounting to HME changed per day of mechanical ventilation. This rare event was not correlated with the duration of HME use (data not shown), was not related to the performance of the HMEs, but was encountered in productive patients and had no adverse effect on the patient. Hygrometric Measurements The hygrometric parameters of the Hygrobac-Dar, measured within the first 48 h of mechanical ventilation (referred to as 0 2) and then daily ( 3 to 7), were remarkably constant throughout the 7-d study period (Table 4). The values for absolute humidity, relative humidity, and thracheal temperature measured on 7 were identical to those measured on 0 2 (Table 4). During the whole study period, only three HMEs were replaced prematurely because of insufficient absolute humidity. This represented 3 HMEs/377 d, or 0.7 time per 100 d of mechanical ventilation. All three HMEs replaced concerned patients with chronic obstructive pulmonary disease (COPD). One HME delivered less than 26 mg H 2 O/L of absolute humidity on 5, and the other two HMEs were replaced, de- TABLE 3 CLINICAL ASSESSMENT OF HUMIDIFYING EFFICIENCY 0 7 p Value Tidal volume, ml Respiratory rate, breaths per minute Mean peak airway pressure,* cm H 2 O No. of tracheal suctionings, per day No. of tracheal instillations, per day * Peak airway pressures were recorded every 6 h and averaged.

4 Ricard, Le Mière, Markowicz, et al.: Heat and Moisture Exchangers Changed Weekly 107 TABLE 4 HYGROMETRIC PARAMETERS p Value Absolute humidity, mg H 2 O/L Relative humidity, % Tracheal temperature, C spite study protocol, on 3 and 4. Although their absolute humidity values were not less than 26 mg H 2 O/L, they were constantly declining and were close to the set threshold (Table 5). Importantly, the gradual decrease in absolute humidity before HME replacement observed in these three instances led to no adverse effect on the patients. As this rare event occurred solely in patients with COPD, we analyzed the results of this category of patients separately from those of the remaining population. Table 6 shows the values for absolute humidity in patients with COPD (including those in whom the HME was replaced prematurely) and the values for absolute humidity measured in the other patients. The HMEs used by the patients with COPD had significantly lower values of absolute humidity than did those of the other patients after 72 h of use. This difference did not reach significance on 6 and 7 because of the smaller number of HMEs left for analysis. Resistance of HMEs to Air Flow The resistance of the HME to air flow was measured daily in 10 patients; the results of these measurements are shown in Table 7, and indicate that the resistance did not change from 0 to 7. Bacteriological Study Thirty-three Y-pieces and ventilator sides of HMEs were sampled. HMEs (ventilator side) were sterile in 20 cases, whereas 13 grew negligible amounts of microorganisms (fewer than 10 CFU/side of coagulase-negative staphylococci in 11 cases, 1 CFU/side of Candida albicans or of a Bacillus sp. in one instance each). The results were comparable for the Y-piece: no growth in 30 instances, and fewer than 10 CFU/cm 2 of coagulase-negative staphylococci in 2 cases and an Enterococcus sp. in one case. DISCUSSION This prospective clinical study clearly shows that a hygroscopic and hydrophobic HME (Hygrobac-Dar) can be used safely for seven continuous days of mechanical ventilation in all ICU patients except patients with COPD. These important findings are based on both clinical and hygrometric assessment, and confirm that the duration of use of this particular TABLE 5 ABSOLUTE HUMIDITY* DELIVERED BY THE THREE REPLACED HMEs Patient * ln mg H 2 O/L. The HME was replaced after this measurement. HME can be extended (13). These findings may have important medical and economical consequences. We believe that this study is the first to evaluate an HME for such a long period with daily hygrometric measurements. Indeed, more than 350 d of mechanical ventilation were monitored. Clinical parameters, such as the number of tracheal suctionings and instillations required per day, and peak airway pressures, may help to assess adequate airway humidification, but daily survey of these parameters is not sensitive enough to detect a gradual fall in the humidity delivered by a humidifying device. We have previously measured wide differences in the absolute humidity delivered by different HMEs, whereas the clinical parameters used to assess airway humidification indicated no such differences (14). We therefore consider it important to perform hygrometric measurements when investigating HMEs, and most importantly, when exploring the extended duration of use of an HME. Hygrometric parameters were measured once a day after 2 in the present study, to detect over time any fall in humidity performance that could have been detrimental to the patient. Absolute humidity remained remarkably stable over time in the non-copd patients. This constant humidity output reflects the integrity of the epithelial and mucociliary function. The HMEs act as passive heating and humidifying devices: they store heat and moisture from the expired gas and return them to the patient via the inspired gas. As the capacity to heat and humidify fresh gas coming into the respiratory tract depends on the integrity of the epithelial and mucociliary function, a constant absolute humidity output over 7 d of mechanical ventilation indicates indirectly that the heating and humidifying functions of the respiratory tract have been preserved during this time. However, this was not true in all patients. Three HMEs were replaced prematurely. In one case, the absolute humidity was below the safety threshold (26 mg H 2 O/L), and it was constantly decreasing in the other two cases, being significantly below the mean absolute humidity in the other patients on the same day. The protocol was violated in these two patients because the particularly low absolute humidity could have continued to decrease and place the patient at risk of endotra- PATIENT 6 ABSOLUTE HUMIDITY VALUES FOR PATIENTS WITH COPD AND FOR ALL OTHER PATIENTS Patients with COPD Other Patients p Value (25)* (30) (26) (33) (24) (34) (20) (28) (13) (20) (15) (25) 0.7 * Numbers in parentheses indicate the number of HMEs.

5 108 AMERICAN JOURNAL OF RESPIRATORY AND CRITICAL CARE MEDICINE VOL TABLE 7 HME RESISTANCE TO AIR FLOW* * Resistance units, cm H 2 O/L/s; ANOVA for repeated measurements revealed a p value of 0.4 for the changes in resistance from 0 to 7. cheal tube occlusion. This rare decrease in HME objective performance occurred only in patients with COPD. The values for absolute humidity were significantly lower in the COPD patient group than in the rest of the study population. This difference remained even when the three HMEs replaced prematurely were excluded from the analysis. These surprisingly low values for absolute humidity in patients with COPD had not been encountered in our previous study, in which 371 measurements were performed, one-third on patients with COPD (14). The HMEs were changed every 2 d in this previous study. Thus, it is possible that this is the longest period that some patients with COPD can be on an HME before its replacement; however, this remains to be confirmed. The reasons for this decline in absolute humidity are not clear. As mentioned above, an HME acts passively and cannot restitute during the inspiratory phase more humidity than it has received during the previous expiratory phase (22). The anatomical and functional alterations in the bronchial epithelium of patients with COPD may have contributed to the gradual decrease in absolute humidity in these three particular patients. Thus, a decrease in HME performance after 48 h of use may result in a marked decrease in humidity in those patients with an altered respiratory epithelium. However, all patients with COPD did not behave in the same way. Indeed, one patient with COPD was ventilated for 47 d and two for 21 d with their HME changed every week without any problem. Insufficient humidification may lead to respiratory tract desiccation and damage, and to life-threatening endotracheal tube occlusion (9 12, 23). Tracheal tube occlusion never occurred during the 377 d of mechanical ventilation we have studied. Furthermore, HMEs have been changed every week in our unit (except for patients with COPD) since this study was completed (1 yr ago), and no tracheal tube occlusion has occurred in 246 ventilated patients, 176 of whom were ventilated for more than 3 d outside the research setting. Placing an HME in the breathing circuit generates resistance to gas flow. Thus a fear of a major increase in this resistance over time is a legitimate concern. This is why we measured the resistance of 10 HMEs every day from 0 to 7. Resistance did not change significantly over the 7-d use of the HME. The additional resistance due to an HME is no greater than that of a heated humidifier (24). Additional dead space may be a potential drawback to the use of HMEs. The use of HMEs leads to greater Pa CO2 and minute ventilation than with heated humidifiers, during weaning from mechanical ventilation (25). Pelosi and coworkers showed that the use of HMEs increased the work of breathing, although this increase can be easily overcome by increasing pressure support (26). The clinical impact of these observations on the course, duration, and outcome of weaning remains unknown. In our daily practice, patients are kept on HMEs during weaning from mechanical ventilation. Several studies have shown that the bacteria-filtering properties of the HME used in this study (Hygrobac-Dar) remain stable after 48 h of use (13, 14). Our present findings are consistent with these results. The bacterial colonization of the Y-piece and the ventilator side of the HME was low after 7 d of use. In one of these previous studies addressing the issue of nosocomial pneumonia (13), we showed that changing the Hygrobac-Dar every 48 h did not affect the rate of nosocomial pneumonia. There is now considerable evidence that HMEs may be used for longer than the 24 h recommended by the manufacturers (13, 14, 27, 28). However, and most importantly, this is not true of all HMEs. Our previous study of three different hydrophobic and hygroscopic HMEs found that one of them was clearly not suited for use beyond 24 h (14). These results were based on daily measurements of humidity outputs. Such measurements are essential since the clinical parameters used to evaluate the humidification performances did not detect the lower performances of this particular HME, and obviously not all available HMEs are appropriate for extended use (14). We therefore believe that hygrometric performance must be assessed in any clinical study investigating the extended use of HMEs. Tracheal tube occlusion is the most dangerous risk of insufficient humidification. Although it appears to be rare, fatal cases have been reported (11). Reducing the cost of mechanical ventilation is a laudable goal but it must certainly not be done at the expense of a patient s life. This study demonstrates that a particular HME (Hygrobac- Dar) can be safely used in non-copd patients for 1 wk. Given the fact that most patients are ventilated for shorter periods (29), this implies that this HME can be used with total security in patients for more than 2 d, without change. This is the first objective demonstration of this possibility. One study (27) compared the use of a hygroscopic HME (without bacteria-filtering properties) over 7 d with a classic heated humidifier. The authors concluded that this hygroscopic HME may be used during the first 7 d of mechanical ventilation, reducing staff labor and the cost of mechanical ventilation. But the study had some drawbacks: first, the HMEs were systematically replaced by heated humidifiers at the end of the first week of mechanical ventilation, which precluded assessment of longer periods of use; second, several HMEs were replaced by a heated humidifier before the seventh day of use because of abundant secretions. Tracheal tube occlusion might have occurred if the HME had been continued in these patients, and hygrometric measurements (which were not performed) might have detected insufficient humidification in these patients; third, prematurely replaced HMEs were excluded from the analysis; last and most important, the mean duration of mechanical ventilation was d, thus only 21 of 147 HMEs were used for at least 7 d. Our study included 40 HMEs used for more than 7 d. It is important to note that the economic savings obtained by prolonging the use of HMEs may be considerably reduced if they are replaced prematurely owing to partial obstruction by secretions. For instance, in the study by Kollef and coworkers (27), the number of prematurely replaced HMEs was per day of mechanical ventilation (27 HMEs/750 d of

6 Ricard, Le Mière, Markowicz, et al.: Heat and Moisture Exchangers Changed Weekly 109 mechanical ventilation) whereas this number is (6 HMEs/ 377 d of mechanical ventilation) in our study. This difference is probably due to the attention paid by the nurses and doctors in our unit to keeping the HMEs above the endotracheal tube: they are kept vertically above the endotracheal tube by means of a flex-tube in order to limit refluxes of secretion. The balance between cost savings and quality improvement in clinical research in the ICU is a major issue. In the present study, most of the patients with COPD were ventilated without any problem when their HME was changed only once a week. If hygrometric measurements had not been performed, the conclusion of this study (based on the results of the clinical parameters only) would probably have been that mechanical ventilation with an HME changed only once a week was safe and efficient in all patients, COPD included. However, because measurements were performed, three patients with COPD in whom HMEs delivered an absolute humidity significantly lower than the values measured in all of the other patients were detected. This event occurred only 0.7 time per 100 d of mechanical ventilation. However, we prefer to stay on the safe side, and recommend a 48-h change of HMEs in patients with COPD. We decided to adopt a conservative approach in order that patient safety issues not be sacrificed for economic reasons. In addition, less frequent HME changes reduce the number of septic procedures, thus improving quality of care. In conclusion, our study shows that mechanical ventilation with an HME changed every week is safe, efficient, and costeffective for non-copd patients. Patients with COPD should have their HMEs changed every 48 h (13). Expanding the life span of this HME may result in considerable cost savings more than $110,000 if these findings are extended to the 800 intensive care beds (medical and surgical ICUs) of the 20 acute-care teaching hospitals of the Administration Générale de l Assistance Publique de Paris (to which the Hôpital Louis Mourier belongs). Nevertheless, focusing only on cost reduction, on the assumption that all available HMEs are safe and suitable for prolonged use, may lead to unacceptable adverse effects (30). References 1. Chalon, J., C. Patel, M. Ali, S. Ramanathan, L. Capan, C. Tang, and H. Turndorf Humidity and the anesthetized patient. Anesthesiology 50: Forbes, A. R Temperature, humidity and mucus flow in the intubated trachea. Br. J. Anaesth. 46: Craven, D., M. Connolly, D. Lichtenberg, P. Primeau, and W. McCabe Contamination of mechanical ventilators with tubing changes every 24 or 48 hours. N. Engl. J. Med. 306: Craven, D., T. Goularte, and B. Make Contaminated condensate in mechanical ventilator circuits: a risk for nosocomial pneumonia. Am. Rev. Respir. Dis. 129: Dreyfuss, D., K. Djedaïni, P. Weber, P. Brun, J. Lanore, J. Rahmani, Y. Boussougant, and F. Coste Prospective study of nosocomial pneumonia and of patient and circuit colonization during mechanical ventilation with circuit changes every 48 hours versus no change. Am. Rev. Respir. Dis. 143: Tenaillon, A., G. Cholley, R. Boiteau, D. Perrin-Gachadoat, and M. Burdin Filtre échangeurs de chaleur et d humidité versus humidificateurs chauffant en ventilation mécanique. Réan Soins Intens. Méd. Urg. 5: Dreyfuss, D., K. Djedaini, I. Gros, L. Mier, G. Le Bourdellés, Y. Cohen, P. Estagnasié, F. Coste, and Y. Boussougant 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. 151: Hurni, J. M., F. Feihl, R. Lazor, P. Leuenberger, and C. 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