EFFICIENCY AND SAFETY OF MECHANICAL VENTILATION WITH A HEAT AND MOISTURE EXCHANGER CHANGED ONCE A WEEK

Bernard Paluch

Nova-VentRx, Inc., Elk Grove Village, Illinois

To the Editor:

I am writing to call into question the method used to measure the humidification performance of a heat and moisture exchanger in the article "Efficiency and Safety of Mechanical Ventilation with a Heat and Moisture Exchanger Changed Only Once a Week" (1). I submit that the wet-bulb psychrometry technique used is not suitable for environments where the airstream temperatures are changing rapidly, as occurs when a heat and moisture exchanger (HME) is used.

When inspiration begins, the HME temperature is at the maximum. It falls rapidly, in milliseconds as inspiration proceeds, because heat is being removed and transferred to the airstream, the temperature of which begins to rise. The temperature differential between the HME and the airstream falls rapidly during inspiration, and consequently, the rate of temperature increase of the airstream also falls. Inspiration is thus characterized by rapidly changing temperatures. Inspiratory gas temperature fluctuations at the ET are usually more severe when an HME is used than when a heated humidifier is used, because an HME can only hold a portion of the heat brought to it by the expired gas. An HME cannot create heat, as a heated humidifier does. The fact that the airstream temperature is always changing leads me to ask why the authors reported specific temperatures that were then used to calculate the specific relative humidities they reported?

Wet-bulb psychrometry, in this environment, is inappropriate because the temperature sensor, being surrounded by a mass of water-soaked cotton, is not exposed to the airstream temperature changes as they occur. The water impedes the transmission of heat from the airstream to the sensor, with the result that the sensor is always lagging behind the actual gas temperature. Primiano and colleagues reported an average temperature fluctuation of 32.5° C at end-expiration to 27.4° C at end-inspiration at the patient-side outlet of a hygroscopic condenser humidifier (2). The temperature measurement device they used had a 10% to 90% full-scale response time of 120 ms. Ricard and colleagues, in the subject paper, do not report the performance specifications of the temperature measurement apparatus.

Wet-bulb psychrometry is appropriate for measurement of stable air temperatures, such as room air. A "sling psychrometer" is a device commonly used to determine ambient saturation temperatures. The instructions for use for such devices require that they be spun until the temperature falls no further, a process which typically takes several seconds. Psychrometry, therefore, is an inappropriate method for determining wet-bulb temperature in an environment of rapidly fluctuating temperatures.

I therefore submit that in the Ricard and colleagues paper, the hygrometric data reported, and the humidification performance attributed to the HME, based upon those data, are as likely to be inaccurate as accurate. The conclusions drawn from those data, are, at minimum, unsupported by accurate data. In my opinion, in the interest of scientific accuracy the authors should retract those portions of their paper based upon that data.

	References

1. 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;1:104-109.

2. Primiano F et al. Conditioning of inspired air by a hygroscopic condenser humidifier. Crit Care Med 12;8:675-678.

From the Authors:

We thank Mr. Bernard Paluch for his interest in our work. We contend in our response that he contests the results of a large number of well conducted clinical studies that stand against his own financial interest. In his letter, Mr. Paluch claims that wet-bulb psychrometrythe method many independent investigators had been using before our study (1) to measure humidity outputs of heat and moisture exchangers (HMEs)is inappropriate. Mr. Paluch's assertion directly questions not only our own personal scientific accuracy, but, more importantly, that of the independent investigators that have published numerous articles on the subject for more than 10 years.

As a whole, dynamic measurements (such as those that can be performed to evaluate HME performance) imply the use of sensors that have specific physical properties that can interact with the measurement. Depending on heat capacity and conductivity, a certain amount of heat must be transferred to the sensor. This process takes time and consumes heat. Therefore, there is inevitably a delay between the actual temperature and the one measured by the sensors. We agree with Mr. Paluch that temperature falls during inspiration and that this fall may be more important with HMEs than with heated humidifiers. Nevertheless, the real and decisive question that Mr. Paluch fails to identify is whether or not psychrometry enables clinicians to measure adequately and in a reproducible manner HMEs humidity outputs at the patient's bedside. The answer is clearly yes. Indeed, even if psychrometry measurements introduce a slight difference with the actual temperature in the airstream (which remains to be proven), they provide consistent results of humidity output that have proved definitely helpful in the clinical settings.

The appropriateness of psychrometry is attested by several arguments:

1. Psychrometry allowed the demonstration that cascade humidifiers provide greater humidity than any type of HME, that purely hydrophobic HMEs provide poor humidity output, and that hydroscopic-hydrophobic ones provide humidity output intermediate between the two former devices. This was shown by independent investigators (2). Moreover, the values for humidity for a given device were found to be exactly the same by different investigators using psychrometry (2), and were also similar to those measured with a different method (Gibeck Humidity Sensor System) (8).

2. The values measured in vivo with psychrometry by independent investigators were the same as those determined on a bench test by the HME manufacturers and given on the advertisement. The only exception was the demonstration of the poorer performance than claimed by the manufacturer of one type of HME (7). This is a strong argument in favor of the determination of objective performance of HMEs by independent clinicians using psychrometry.

3. When dry and cold air passes through the flex tube that connects the HME to the endotracheal tube (ETT), no condensation occurs. Exactly the opposite happens if warm and humid air passes. We showed that there is an excellent correlation between the values for humidity given by the manufacturer and the amount of humidity seen in the flex tube (11). Moreover, we showed that the actual measurement of absolute humidity by psychrometry correlates very well, over a wide range of values (from 22 to 34 mgH₂O/L), with flex tube condensation (6).

In a clinical perspective, a major interest in humidity measurements of HMEs is to detect those that deliver insufficient humidity and put the patients at risk of ETT occlusion (a rare but sometimes fatal complication [12]) and to assess those that perform very well (1). Psychrometry obviously allows these two important goals for the reasons explained above.

In Mr. Paluch's opinion, "in the interest of scientific accuracy," we should retract those portions of our paper (1) based upon psychrometry. Accordingly, he should ask for the retraction of part (or totality) of the following papers (nonexhaustive list): Tsubota K et al, 1991, J Aersol Med; Martin C et al, 1992, Chest; Jackson C et al, 1992, Intensive Care Med; Miyao H et al, 1992, Crit Care Med; Feihl F et al, 1992, Acta Anaesth Italica; Croci M et al, 1993, Intensive Care Med; Martin et al, 1994, Crit Care Med; Martin et al, 1995, Chest; Vanderbroucke-Grauls CM, 1995, J Hosp Infect; Christiansen S, 1998, Anasthesiol Intensivmed Notfallmed Schmerzther; Ricard J-D et al, 1999, Chest; Markowicz P et al, 2000, Crit Care Med.

Our only concern as clinicians and investigators is to give health care providers accurate information they need when using HMEs. We have performed our research (1, 6, 11) in total independence and received no funding whatsoever from any of the brands we have tested. In the paper in question (1) we stated that "we received no financial support from and did not have any commitment to the brand of the device tested in this study," thus ruling out any conflict of interest in this matter. Mr. Paluch is president of Nova-VentRx, a company he has formed and whose webpage can be consulted (http://buyersguide.aarc.org/compdisplay.asp?AcctID=228). Nova-VentRx produces devices Mr. Paluch has invented and for which he owns patents. Some of his devices have been designed to remove the condensation that accumulates in respirator tubings when a heated humidifier is used. It is therefore in Mr. Paluch's financial interest that clinicians use heated humidifiers rather than heat and moisture exchangers in order to sell more of his devices (which is a perfectly laudable goal). It is naturally in Mr. Paluch's financial interest to contest the use of heat and moisture exchangers. He has been doing this for the last 20 years (13).

Our article (1) shows two things clearly: first, the perfect stability of values for absolute humidity provided by the HME and measured over one week of continuous use in non-COPD patients (a finding that could be hardly explained if our values were subjected to random error, as suggested by Mr. Paluch); second, the total clinical safety of the same HME used for one week in non-COPD patients and for two days in COPD patients. Presently, HMEs are more frequently used in France than in North America (at least Canada), which may account in part for lower costs of mechanical ventilation in France (20). If our findings and those from others (1, 7) are taken into account in the USA, considerable savings will undoubtedly ensue, a highly desired goal in many hospitals. But, Mr. Paluch will sell fewer and fewer of the devices he invented. Financial conflicts of interest must be disclosed in order to avoid the threatening of independent clinical research by profit issues (21).

Service de Réanimation Médicale, Hôpital Louis Mourier, Colombes, Assistance Publique-Hôpitaux de Paris, Xavier Bichat Medical School, Paris, France

	References

1. 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: 104-109 [Abstract/Free Full Text].

2. Martin C, Papazian L, Perrin G, Bantz P, Gouin F. Performance evaluation of three vaporizing humidifiers and two heat and moisture exchangers in patients with minute ventilation > 10 L/min. Chest 1992; 102: 1347-1350 [Abstract/Free Full Text].

3. Martin C, Papazian L, Perrin G, Saux P, Gouin F. Preservation of humidity and heat of respiratory gases in patients with a minute ventilation greater than 10 L/min. Crit Care Med 1994; 22: 1871-1876 [Medline].

4. Martin C, Thomachot L, Quinio B, Viviand X, Albanese J. Comparing two heat and moisture exchangers with one vaporizing humidifier in patients with minute ventilation greater than 10 L/min. Chest 1995; 107: 1411-1415 [Abstract/Free Full Text].

5. Sottiaux T, Mignolet G, Damas P, Lamy M. Comparative evaluation of three heat and mositure exchangers during short-term post-operative mechanical ventilation. Chest 1993; 104: 220-224 [Abstract/Free Full Text].

6. Ricard J-D, Markowicz P, Djedaïni K, Mier L, Coste F, Dreyfuss D. Bedside evaluation of efficient airway humidification during mechanical ventilation of the critically ill. Chest 1999; 115: 1646-1652 [Abstract/Free Full Text].

7. Markowicz P, Ricard J-D, Dreyfuss D, Mier L, Brun P, Coste F, 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: 665-671 [Medline].

8. Thomachot L, Vialet R, Viguier JM, Sidier B, Roulier P, Martin C. Efficacy of heat and moisture exchangers after changing every 48 hours rather than 24 hours. Crit Care Med 1998; 26: 477-481 [Medline].

9. Thomachot L, Boisson C, Arnaud S, Michelet P, Cambon S, Martin C. Changing heat and moisture exchangers after 96 hours rather than after 24 hours: a clinical and microbiological evaluation. Crit Care Med 2000; 28: 714-720 [Medline].

10. Davis K Jr,, Evans SL, Campbell RS, Johannigman JA, Luchette FA, Porembka DT, 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: 1412-1418 [Medline].

11. Beydon L, Tong D, Jackson N, Dreyfuss D. Correlation between simple clinical parameters and the in vitro humidification characteristics of filter heat and moisture exchangers. Groupe de Travail sur les Respirateurs. Chest 1997; 112: 739-744 [Abstract/Free Full Text].

12. Martin C, Perrin G, Gevaudan MJ, Saux P, Gouin F. Heat and moisture exchangers and vaporizing humidifiers in the intensive care unit. Chest 1990; 97: 144-149 [Abstract/Free Full Text].

13. Paluch BR. "Artificial nose" claims questioned. Respir Care 1981; 26: 269 .

14. Lekholm A. "Artificial nose" claims questioned (reply). Respir Care 1981; 26: 270 .

15. Paluch BR. More about the Siemens servo humidifier. Respir Care 1981; 26: 680 .

16. Lekholm A. More about the Siemens servo humidifier (reply). Respir Care 1981; 26: 680 .

17. Gedeon A, Mebius C. Hygroscopic condenser humidifiers. Respir Care 1981; 26: 1010 .

18. Paluch BR. Hygroscopic condenser humidifiers (reply). Respir Care 1981; 26: 1011 .

19. Paluch BR. Bacteriologic evaluation of the Servo 150 hygroscopic condenser-humidifier. Crit Care Med 1986; 14: 914 [Medline].

20. Cook D, Ricard J-D, Reeve B, Randall J, Wigg M, Brochard L, Dreyfuss D. Ventilator circuit and secretion management strategies: a Franco-Canadian survey. Crit Care Med 2000; 28: 3547-3554 [Medline].

21. Davidoff F, DeAngelis CD, Drazen JM, Hoey J, Hojgaard L, Horton R, et al . . Sponsorship, authorship, and accountability. N Engl J Med 2001; 345: 825-827 [Free Full Text].