In a patient whose trachea is intubated, heat and moisture must be added to the inspired gases to compensate for losses resulting from the bypass of the upper respiratory tract by the endotracheal tube (1). In addition to CO2 absorbers, heated humidifiers have been used to provide humidification and warming of the inspiratory gases during general anesthesia. Currently disposable devices called heat and moisture exchangers (HMEs) are also available to anesthesiologists. The manufacturers of these devices generally do not specify heat-conserving properties but specify moisture output (absolute humidity, AH) based on in vitro testing using the International Standards Organization (ISO) 9360 water loss method and psychrometry. The ISO standard specifies the lung model, ventilator settings, and the accuracy and tolerance of the measurement method (2). In vitro testing has the advantages of allowing standardized measurements in a well-controlled setup and allowing for the assessment of each individual physical variable by keeping all the others constant. However, the disadvantage of in vitro testing is that the properties of living tissues, anatomical details, and variable interactions cannot be reproduced. Therefore, in vitro measurements might not reproduce in vivo behavior (3). Studies that quantify the effect of different HMEs on humidity during current routine anesthesia practice are not available. The goal of this study was to evaluate the performance of three different HMEs with similar ISO specifications in patients whose lungs were mechanically ventilated and using an anesthesia machine equipped with a circle system and a CO2 absorber. Performance was assessed by determining the effect of the HME on temperature, convective heat loss, evaporative heat loss, total heat loss, relative humidity, and absolute humidity of inspiratory gases.
After obtaining informed consent and IRB approval 8 patients, ASA physical status I–II, aged between 36 and 55 yr, scheduled for nonthoracic surgical procedures under general anesthesia, were studied. General anesthesia was induced, and the trachea was intubated. Anesthesia was maintained with 70% N2O in oxygen and isoflurane 0.5%–1%. The anesthesia machine was a Narkomed 2B (North American Draeger, Telford, PA) equipped with the standard 2 L double canister absorber system filled with soda lime (Sodasorb, Portex Inc, Keene, NH). The fresh gas flow was 3 L/min. The ventilator rate was set at 8 breaths/min with an inspiratory:expiratory ratio of 1:2. Tidal volume was adjusted to attain end-tidal CO2 of 35–40 mm Hg, and it was maintained at this level. A heating mattress and blanket were used to maintain normothermia of the patient. Temperature and relative humidity of the gases were measured with a humidity sensor system (Gibeck Respiration, Sweden). The humidity sensor system was placed between the Y-piece and the endotracheal tube or, when used, between the HME and the endotracheal tube. The system uses a capacity sensor method with a sampling rate of 21 times per second. The accuracy is ±4% for relative humidity and ±1°C for temperature. The response times are <1.4 s for a 90% relative humidity response and <150 ms for a 90% temperature response. These response times are fast enough for the system to settle at correct values at the end of inspiration and expiration if breathing frequency is normal. The humidity and the temperature sensors were connected to a computer interface that transformed the signals into a computer readable signal of the ASCII type. The signals were transformed into graphs and values by an IBM-compatible computer and a specially designed computer program. The value of each variable at the end of the inspiration and expiration phase was automatically collected for analysis. AH was calculated using the following formula:
where T = temperature in °C and RH = relative humidity (%).
Convective heat loss (Wcv) was calculated as follows (4):EQUATION
where V = minute ventilation, ρ = volumetric mass of the ventilatory gas (N2O = 1.997 g/L, O2 = 1.43 g/L), Cp = specific heat of the inspired and expired gases (N2O = 0.2098 cal/g per 1°C, O2 = 0.2198 cal/g per 1°C), Tex = temperature of expired gas, and Tinsp = temperature of inspired gas.
Evaporative heat loss (Wev) was calculated as follows (4):EQUATION
λ = latent heat of water evaporation (585 cal/g H2O), AHexp = absolute humidity of expired gas calculated from Tex with the hypothesis that expired gases were fully saturated in water vapor (relative humidity: 100%), and AHinsp = absolute humidity of inspired gas.
Total heat loss was calculated as the sum of Wcv and Wev.
The HMEs studied were Humi-Vent 1 (H), Humi-Vent 2S (H2), both produced by Gibeck (Indianapolis, IN), and Edith 1000 (E) produced by Datex-Ohmeda (Helsinki, Finland). The manufacturers’ specifications are shown in Table 1. At the time of study, these were the HMEs in use at our hospital. One hour after general anesthesia had been induced, the 3 HMEs were studied in a randomized crossover fashion in the same patient separated by a 15-min period. Baseline measurements were taken immediately before inserting an HME. Measurements for analysis of HME performance were taken 15 min after insertion.
Results are expressed as mean ± sd or median (interquartile range) and analyzed by one-way repeated-measures analysis of variance, or Friedman repeated-measures analysis of variance on ranks. The Tukey test or Dunnett’s method was used for pairwise multiple comparison. P < 0.05 was considered significant.
Figure 1 shows box plots of the Tinsp, Wcv, Wev, total heat loss, RH, and AH for the baseline measurements and for each of the HMEs. There were no significant differences between baseline values for any of the measurements. However, there were significant differences in the performance of the three HMEs. The Tinsp increased significantly for all three HMEs, but the temperature was significantly greater with the use of H than with the use of E. Wcv and total heat loss were not affected by E, but decreased significantly with both H and H2 when compared with baseline. Wev was significantly less than baseline values for all three HMEs, but H and H2 outperformed E significantly. RH was significantly increased for all three HMEs when compared with baseline values. AH was significantly more than baseline values for all three HMEs, but H and H2 outperformed E significantly. All HMEs achieved a steady state in measured variables within 15 min.
In this randomized crossover study we determined T, Wcv, Wev, total heat loss, RH, and AH of inspiratory gases. The effect of three different HMEs on these variables was determined in patients whose lungs were ventilated with a fresh gas flow of 3 L/min 70% N2O in oxygen and using an anesthesia machine with a 2 L CO2 absorber. The main finding of this study is that manufacturer’s specifications do not reliably predict performance during routine anesthesia practice. The measured moisture output of E (AH, 21.5 (3.7) mg H2O/L) was considerably less than the value specified by the manufacturer (30 mg H2O/L). The measured AH of H was 27.1 (3.1) mg H2O/L versus 30.5 mg H2O/L specified. Only H2 performed according to manufactures specifications (28 mg H2O/L). This discrepancy between manufacturers specifications and in vivo measurements is in agreement with the findings of Thiery et al. (5), who measured much lower AH than the values specified by the manufacturer for the Hygroster HME but not for the Hygrobac HME.
The minimum humidity level needed to preserve normal pulmonary function and prevent tracheobronchial damage is unknown, but AH values >20 mg H2O/L are recommended (6). In our study, the AH of the inspiratory gas without the use of an HME was low (10 mg H2O/L). Henriksson et al. (7) studied the effect of fresh gas flow on inherent humidifying properties of a circle system with a low volume (1 L) CO2 absorber. Without the use of an HME, he found significantly lower AH values at a fresh gas flow of 5 L/min (approximately 18 mg H2O/L) than at fresh gas flows of 1 and 2 L/min (approximately 22 mg H2O/L). With the use of an HME (H) there was no difference in AH at different flow rates, and values similar to those for H in our study were found. In a recent study (8) it was reported that at low gas flows the benefit of an HME might be small. At a fresh gas flow of 1 L/min the use of a Humid Vent 2 increased AH from a baseline value of 26.6 ± 3.2 mg H2O/L to 32.7 ± 3.1 mg H2O/L. Tinsp increased from approximately 28°C to 33°C without an effect on body temperature.
As expected, we found that most of the total airway heat loss occurred by evaporation. E, H, and H2 all reduced Wev significantly. E however did not have a significant effect on airway Wcv or on total airway heat loss. Metabolic heat production during anesthesia is approximately 930 cal/min (9). Without the use of an HME, we found a heat loss of 120 cal/min, which is approximately 13% of the metabolic heat production during anesthesia. The use of H and H2 reduced heat loss to 40–50 cal/min, which is a conservation of approximately 7%–8% of the estimated total heat production during anesthesia. These values are in the same range as those calculated by Bickler and Sessler (10). They studied the performance of 5 different HMEs not by measuring humidity but by measuring reduction in water loss and changes in airway temperature in patients whose lungs were ventilated with a fresh gas flow of 5 L/min and an anesthesia machine with a circle system (10). They also found a considerable range in the performance of the different HMEs. Evaporative water loss reduction ranged from 60%–90%, and airway temperature increase ranged from 2°C–8°C for the different types.
In conclusion, we found that there is considerable variability in the performance of HMEs that have similar manufacturer’s specifications. These specifications, based on the ISO 9360 standard cannot be used to predict clinical performance.
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