Under normal physiological conditions, inhaled air is heated and humidified when passing through the nose and upper airways, reaching the alveoli at body temperature (37°C) with 100% relative humidity (RH) and approximately 44 mgH2O · L−1 of absolute humidity (AH). During expiration, heat and moisture are conserved by the upper airways and nose to minimize these losses from the lower airways. During tracheal intubation or tracheotomy and mechanical ventilation, this countercurrent mechanism is partly bypassed. In this situation, ventilation with dry and cold compressed gases leads to considerable loss of water and heat from the respiratory tract, unless appropriate means of humidification and heating are used.1,2 Failure to achieve and maintain efficient humidification and heating predispose to airway damage, such as destruction of cilia and mucus glands, decrease in ciliary transport, and respiratory dysfunction.2–5
The Primus anesthesia workstation (Dräger, Lübeck, Germany) has a built-in hotplate to heat exhaled gases in the breathing circuit.6 One would expect that the inhaled gases are warmed by the hotplate in this machine. On the other hand, in this anesthesia workstation, the cold and dry fresh gas flows (FGF) are mixed with the exhaled gases after the exhaled gas has passed through the soda lime canister. The temperature and humidity of the inhaled gases coming from this anesthesia machine have not yet been investigated.
A heat and moisture exchanger (HME) is designed to achieve temperature and humidification of the inhaled gases similar to normal upper airways.7,8 Adding a HME to the breathing circuit of this machine would increase the inhaled gas temperature and humidity.
The aim of this investigation was to compare the temperature and humidity of the inhaled gas from a low-flow breathing system of the Primus anesthesia workstation with or without a HME.
The study was approved by the Medical Ethics Committee (Ref: 409/2006) from the Botucatu Medical School, UNESP, Brazil, and registered with ClinicalTrials.gov (NCT00997295), and written informed consent was obtained from all patients. Thirty adult (18 to 64 years old), afebrile (T < 38°C), nonobese (body mass index <30 kg · m−2), and ASA I and II women scheduled for elective abdominal gynecologic surgeries lasting 120 minutes or more were enrolled in this study.
Patients received midazolam (7.5 mg by mouth) 60 minutes before their admission to the operating room (OR). In the OR, patients were randomly allocated by sealed envelope assignment to the study group, in which an HME was placed in the breathing circuit, or to the control group, in which an HME was not used.
Fluid deficits were replaced with lactated Ringer's solution at 8 to 10 mL · kg−1 · h−1. In all patients, fluids were kept at OR temperature. Standard clinical monitoring was performed: electrocardiogram (DII and V5 leads), peripheral oxygen saturation (SpO2), noninvasive arterial blood pressure (systolic and diastolic), and monitoring of neuromuscular blockade. All patients received active skin-surface warming with a specific blanket on the lower limbs from a warming device (WarmTouch, model 5200, Mallinckrodt Medical, Hazelwood, MO) at a temperature ranging from 42°C to 46°C after anesthesia induction until the end of the surgery.
Anesthesia was induced with IV sufentanil 0.5 μg · kg−1 and propofol 2.0 mg · kg−1. Rocuronium bromide 0.6 mg · kg−1 was given to facilitate orotracheal intubation. The lungs were mechanically ventilated using volume-controlled mode of the Primus anesthesia workstation (Dräger Medical, Lübeck, Germany) with a tidal volume of 8 mL · kg−1. An HME (Pall BB 100; Pall Corporation, East Hills, NY) was placed between the Y-piece of the breathing circuit and the tracheal tube in the HME group. FGF was supplied to the circle system at 1.0 L · min−1 (0.5 L · min−1 O2 in 0.5 L · min−1 N2O) with an inspiratory concentration of isoflurane at 1 minimum alveolar concentration. Anesthesia was also maintained with sufentanil and rocuronium bromide. Respiratory rate was adjusted to maintain an end-tidal CO2 (PETCO2) concentration close to 35 mm Hg. Intraoperative distal esophageal (core) temperatures were measured after tracheal intubation using a thermocouple sensor (Mon-a-therm 90,044; Mallinckrodt Medical, Juares, Mexico).
The anesthetic system was equipped with clean and dry silicone corrugate tubes (1.5 m; Dräger Medical, Lübeck, Germany) and soda lime canister (1.5 L) and fresh soda lime (Dragersorb 800 plus, Dräger, Germany) before each anesthesia. The breathing circuit, including the CO2 absorber, had an internal volume of 4.5 L.
The temperature and RH of the gas were measured using an electronic digital thermo-hygrometer (Hygrotermo 95; Gulton, São Paulo, Brazil) connected by a T-piece between the Y-piece of the breathing circuit and the tracheal tube in the no-HME group and between the HME and the tracheal tube in the HME group; and also in the inspiratory limb outlet close to the anesthesia workstation in both groups (Fig. 1).
The RH sensor operates on a capacitative principle with a polymer film (Panametrics®). Its accuracy is ±2% for RH and ±0.5°C for temperature. The response times are 1.4 seconds for 90% RH response and <150 ms for 90% temperature response. The temperature and humidity of the gases in the inspiratory limb fluctuated with the phases of the respiratory cycles. We recorded the minimal values averaged over 4 respiratory cycles after 15, 30, 60, 90, and 120 minutes of the connection of the patient to the respiratory circuit after tracheal intubation. Room temperature was measured by a thermocouple placed near the patients. The temperature probes were attached to electronic 2-channel thermometers (Thermistor 400, model 6150; Mallinckrodt Medical, St. Louis, MO). AH was calculated using the formula AH = RH × MH/100, where AH is absolute humidity (mgH2O · L−1), RH is relative humidity (percentage), and MH is maximum air humidity in saturation conditions (mgH2O · L−1), according to a specific chart.9
Sample size of the groups was calculated on the basis of literature data10,11 and assuming a difference of approximately 2.6 mgH2O · L−1 between AH mean values of the groups. A minimum of 15 patients in each group was necessary to detect this difference using a 2-tailed test with the probability of a type I error (α) of 0.05 and a type II error (β) of 0.1 (power of 90%). Statistical analysis was performed using Statistic Package for Social Sciences for Windows Software (version 6.0; SPSS, Inc., Chicago, IL). Normal distribution of the data was confirmed using the Lilliefors test. Anthropometric variables were compared between the groups by 2-sided Student test with equal variances. Continuous parametric data were compared between groups and in the same group by analysis of variance for repeated measurements followed by Tukey test to investigate differences at different times. The covariance structure was the symmetric compound. We used Akaike's information and Schwarz's Bayesian criteria to choose this model. The observed covariance matrices of the times are equal across groups (Box's M test, P > 0.05). The assumption of sphericity was observed in this analysis (Mauchly's test, P > 0.05). The Time × Group interaction was studied. Data were expressed as mean ± SD and 95% confidence interval (95% CI) for the mean between group differences. Person's coefficient was used for correlation analysis between room temperature and inhaled temperature in the no-HME group. For all analyses, P < 0.05 was considered statistically significant.
There were no statistically significant differences between groups regarding patients' characteristics (Table 1) or hemodynamic variables (Table 2).
OR temperature and gas temperature in the inspiratory limb of the workstation were not statistically significantly different between groups, with 95% CI [−1.17°C to 1.41°C] and [−1.91°C to 1.87°C], respectively (Table 3). The core temperature was maintained in the HME group, but not in the no-HME group (Table 4). The temperature of the inhaled gas was higher with the HME 120 minutes after patients were connected to the respiratory circuit (P < 0.001) with 95% CI [3.8°C to 6.4°C] (Table 4). There was significant and positive correlation between OR and inhaled gas temperatures in the no-HME group (r2 = 0.37; P = 0.001).
The RH and AH in the inspiratory limb of the workstation increased approximately 46% 120 minutes after patient connection to the respiratory circuit (P < 0.001) but did not differ between groups. The AH of the inhaled gas was higher (approximately 50%) when the HME was used (P < 0.001) (Table 5).
There were 2 main findings in this study: (1) the Primus anesthesia workstation partially humidifies the inspired gas when a low FGF is used; (2) insertion of an HME increases the humidity in inhaled gas, bringing it close to physiological values.
During spontaneous breathing, inhaled gas in the subglotic space normally has a temperature of 31.2°C to 33.6°C, RH of 95%–100%, and AH of 33 mgH2O · L−1.12–14
When breathing from a circle absorber breathing circuit, there are 2 sources of heat and water vapor: rebreathing of exhaled gas, and water vapor and heat released from the CO2 absorbent. In an exothermic reaction, 2 mol of water and 14 kcal of heat are liberated from each mole of CO2 absorbed.15
In the breathing system in the Primus anesthesia workstation, the exhaled gases move through the hotplate and cross soda lime once before mixing with cold and dry FGF. The mixed gases are then pulled by the plunger to fill the ventilator. The opening of the inspiratory valve ventilator's plunger sends the gaseous mixture to the inspiratory limb of the respiratory circuit6 (Fig. 1). The inhaled gas will be warmer than breathing systems without a hotplate. However, inhaled gas temperature was lower (around 4°C) than the gas temperature in the inspiratory limb of the workstation, when HME was not used. The use of corrugated tubes with thermal insulated materials could improve the temperature and humidity of the inhaled gases from the Dräger Primus machine. The temperature and humidity of the inhaled gas from the inspiratory circuit at the Primus workstation were similar to the other conventional nonheated circuits with low FGF, such as the Nikkei (K. Takaoka, São Paulo, Brazil)2 and Aestiva/5 (Datex-Ohmeda, Helsinki, Finland).16 Anesthetic machines such as the Dräger Cato and Dräger Cicero (Dräger, Lübeck, Germany) also have a built-in hotplate to heat exhaled gas.16,17 In these machines, different from the Dräger Primus, the hot exhaled gases mixed with the cold and dry FGF, and passed through the CO2 absorber twice during a breath. Thus, the temperature and humidity of the inhaled gases were higher in former studies of these machines16,17 in relation to our findings.
There is no minimum requirement for humidification performance in the draft standard for HMEs.18 The draft standard for heated humidifiers were set to a minimum AH of 33 mgH2O · L−1 for safe and effective performance of humidifiers in patients whose upper airways have been bypassed by an endotracheal tube.19 The mean values of AH of inhaled gases (30 mg · H2O.L−1) recorded in our clinical setting with the HME were closer to those required by the International Standards Organization (ISO, 2007).19
The Primus hotplate, active skin-surface warming, and an HME resulted in the maintenance of a normal core temperature in the HME group. The HME increased the inhaled gas temperature by about 5°C. However, <10% of metabolic heat production is lost through respiration, and the use of a HME alone will not prevent a decrease in core temperature.10 Active skin-surface warming minimizes intraoperative hypothermia20,21; however, heat applied to the skin surface requires considerable time to compensate for the redistributive loss of core heat to the cool and vasodilated peripheral tissues that follows the induction of anesthesia.22
A limitation of the study is that it was not blinded, which could raise some bias. Another limitation is that humidity values were recorded until 120 minutes after the connection of the respiratory circuit to the patient. This time frame was based on a previous study that showed no significant alterations of the temperature and humidity values of the inhaled gas beyond 120 minutes of ventilation from 2 low-flow anesthesia systems with or without HME.16
In conclusion, the Primus anesthesia workstation partially humidifies inspired gas when a low FGF is used. Insertion of an HME increases the humidity in inhaled gas, bringing it close to physiological values.
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Name: Jair de Castro, Jr., MD, PhD.
Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.
Attestation: Jair de Castro, Jr., has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.
Name: Fernanda Bolfi, MD.
Contribution: This author helped conduct the study.
Attestation: Fernanda Bolfi has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.
Name: Lidia R. de Carvalho, PhD.
Contribution: This author helped analyze the data.
Attestation: Lidia R. de Carvalho has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.
Name: Jose R. C. Braz, MD, PhD.
Contribution: This author helped design the study, analyze the data, and write the manuscript.
Attestation: Jose R. C. Braz has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.© 2011 International Anesthesia Research Society