Inhaled gases humidification and heating during artificial ventilation with low flow and minimal fresh gases flow

Gorayb, Susane Bruder Silveira; Braz, José Reinaldo Cerqueira; Martins, Regina Helena Garcia; Módolo, Norma Sueli Pinheiro; Nakamura, Giane

doi:10.1590/S0034-70942004000100004

Abstracts

BACKGROUND AND OBJECTIVES: Inhaled gas humidification and heating are necessary in patients under tracheal intubation or tracheostomy to prevent damage to respiratory system resulting from the contact of cold and dry gas with the airways. This study aimed at evaluating the effect of respiratory circle systems with carbon dioxide absorbers from Dräger's Cicero anesthesia machine (Germany) as to inhaled gases heating and humidification ability using low fresh gases flow (1 L.min-1) or minimum flow (0.5 L.min-1). METHODS: Participated in this study, 24 patients, physical status ASA I, aged 18-65 years, submitted to general anesthesia using Dräger's Cicero workstation (Germany) for abdominal surgery, who were randomly distributed in two groups: low flow group (LF) received 0.5 L.min-1 oxygen and 0.5 L.min-1 nitrous oxide, and minimum flow group (MF) received 0.5 L.min-1 oxygen only. Evaluated attributes were temperature, relative and absolute humidity of the operating room and of respiratory circuit gas. RESULTS: There were no significant differences in inhaled gas temperature, relative and absolute humidity between groups, but they have increased along time in both groups, with influence of operating room temperature on inhaled gas temperature for both groups. Near optimal levels of humidity and temperature were reached as from 90 minutes in both groups. CONCLUSIONS: There have been no significant differences in inhaled gas humidity and temperature with fresh gases low flow or minimum flow.

ANESTHETIC TECHNIQUES, General; ANESTHETIC TECHNIQUES, General; EQUIPMENTS; EQUIPMENTS

JUSTIFICATIVA E OBJETIVOS: Em pacientes sob intubação traqueal ou traqueostomia, a umidificação e o aquecimento do gás inalado são necessários para a prevenção de lesões no sistema respiratório, conseqüentes ao contato do gás frio e seco com as vias aéreas. O objetivo da pesquisa foi avaliar o efeito do sistema respiratório circular com absorvedor de dióxido de carbono do aparelho de anestesia Cícero da Dräger, quanto à capacidade de aquecimento e umidificação dos gases inalados, utilizando-se fluxo baixo (1 L.min-1) ou mínimo (0,5 L.min-1) de gases frescos. MÉTODO: O estudo aleatório foi realizado em 24 pacientes, estado físico ASA I, com idades entre 18 e 65 anos, submetidos à anestesia geral, utilizando-se a Estação de Trabalho Cícero da Dräger (Alemanha), para realização de cirurgias abdominais, os quais foram distribuídos aleatoriamente em dois grupos: grupo de Baixo Fluxo (BF), no qual foi administrado 0,5 L.min-1 de oxigênio e 0,5 L.min-1 de óxido nitroso e fluxo mínimo (FM), administrando-se somente oxigênio a 0,5 L.min-1. Os atributos estudados foram temperatura, umidade relativa e absoluta da sala de operação e do gás no sistema inspiratório. RESULTADOS: Os valores da temperatura, umidade relativa e umidade absoluta no sistema inspiratório na saída do aparelho de anestesia e junto ao tubo traqueal não apresentaram diferença significante entre os grupos, mas aumentaram ao longo do tempo nos dois grupos (BF e FM), havendo influência da temperatura da sala de operação sobre a temperatura do gás inalado, nos dois grupos estudados. Níveis de umidade e temperatura próximos dos ideais foram alcançados, nos dois grupos, a partir de 90 minutos. CONCLUSÕES: Não há diferença significante da umidade e temperatura do gás inalado utilizando-se baixo fluxo e fluxo mínimo de gases frescos.

EQUIPAMENTOS; EQUIPAMENTOS; TÉCNICAS ANESTÉSICAS, Geral; TÉCNICAS ANESTÉSICAS, Geral

JUSTIFICATIVA Y OBJETIVOS: En pacientes bajo intubación traqueal o traqueostomia, la humidificación y calentamiento del gas inhalado son necesarios para la prevención de lesiones en el sistema respiratorio, consecuentes al contacto del gas frío y seco con las vías aéreas. El objetivo de la pesquisa fue evaluar el efecto del sistema respiratorio circular con absorbedor de dióxido de carbono del aparato de anestesia Cícero de Dräger, cuanto a la capacidad de calentamiento y humidificación de los gases inhalados, utilizándose flujo bajo (1 L.min-1) o mínimo (0,5 L.min-1) de gases frescos. MÉTODO: El estudio aleatorio fue realizado en 24 pacientes estado físico ASA I, con edades entre 18 y 65 años, sometidos a anestesia general, utilizándose la Estación de Trabajo Cícero de Dräger (Alemania), para realización de cirugías abdominales, los cuales fueron distribuidos aleatoriamente en dos grupos: grupo de Bajo Flujo (BF), en el cual fue administrado 0,5 L.min-1 de oxígeno y 0,5 L.min-1 de óxido nitroso y flujo mínimo (FM), administrándose solamente oxígeno a 0,5 L.min-1. Los atributos estudiados fueron: temperatura, humedad relativa y absoluta de la sala de operación y del gas en el sistema inspiratorio. RESULTADOS: Los valores de la temperatura, humedad relativa y humedad absoluta en el sistema inspiratorio en la salida del aparato de anestesia y junto al tubo traqueal no presentaron diferencia significante entre los grupos, pero aumentaron a lo largo del tiempo en los dos grupos (BF y FM), habiendo influencia de la temperatura de la sala de operación sobre la temperatura del gas inhalado, en los dos grupos estudiados. Niveles de humedad y temperatura próximos de los ideales fueron alcanzados, en los dos grupos, a partir de 90 minutos. CONCLUSIONES: No hay diferencia significante de la humedad y temperatura del gas inhalado utilizándose bajo flujo y flujo mínimo de gases frescos.

SCIENTIFIC ARTICLE

Inhaled gases humidification and heating during artificial ventilation with low flow and minimal fresh gases flow^* * Received from Hospital das Clínicas (HC) do CET/SBA do Departamento de Anestesiologia da Faculdade de Medicina de Botucatu (FMB), UNESP, no Programa de Pós-Graduação em Cirurgia

Humidificación y calentamiento del gas inhalado durante ventilación artificial con bajo flujo y flujo mínimo de gases frescos

Susane Bruder Silveira Gorayb, M.D.^I; José Reinaldo Cerqueira Braz, TSA, M.D.^II; Regina Helena Garcia Martins, M.D.^III; Norma Sueli Pinheiro Módolo, TSA, M.D.^IV; Giane Nakamura, M.D.^V

^IPós-Graduanda (Mestrado) do Programa de Pós-Graduação em Cirurgia da FMB-UNESP. Enfermeira-Chefe da Divisão Médica e Apoio Diagnóstico do HC da FMB-UNESP

^IIProfessor Titular do Departamento de Anestesiologia da FMB-UNESP

^IIIProfessora Doutora do Departamento de Oftalmologia, Otorrinolaringologia e Cirurgia de Cabeça e Pescoço da FMB-UNESP

^IVProfessora Adjunta Livre-Docente do Departamento de Anestesiologia da FMB-UNESP

^VPós-Graduanda (Doutorado) do Programa de Pós-Graduação em Anestesiologia da FMB-UNESP

^{Correspondence} Correspondence to Prof. Dr. José Reinaldo Cerqueira Braz Deptº de Anestesiologia da FMB-UNESP 18618-970 Botucatu, SP

SUMMARY

BACKGROUND AND OBJECTIVES: Inhaled gas humidification and heating are necessary in patients under tracheal intubation or tracheostomy to prevent damage to respiratory system resulting from the contact of cold and dry gas with the airways. This study aimed at evaluating the effect of respiratory circle systems with carbon dioxide absorbers from Dräger's Cicero anesthesia machine (Germany) as to inhaled gases heating and humidification ability using low fresh gases flow (1 L.min^-1) or minimum flow (0.5 L.min^-1).

METHODS: Participated in this study, 24 patients, physical status ASA I, aged 18-65 years, submitted to general anesthesia using Dräger's Cicero workstation (Germany) for abdominal surgery, who were randomly distributed in two groups: low flow group (LF) received 0.5 L.min^-1 oxygen and 0.5 L.min^-1 nitrous oxide, and minimum flow group (MF) received 0.5 L.min^-1oxygen only. Evaluated attributes were temperature, relative and absolute humidity of the operating room and of respiratory circuit gas.

RESULTS: There were no significant differences in inhaled gas temperature, relative and absolute humidity between groups, but they have increased along time in both groups, with influence of operating room temperature on inhaled gas temperature for both groups. Near optimal levels of humidity and temperature were reached as from 90 minutes in both groups.

CONCLUSIONS: There have been no significant differences in inhaled gas humidity and temperature with fresh gases low flow or minimum flow.

Key Words: ANESTHETIC TECHNIQUES, General: inhalational, low flow; EQUIPMENTS: anesthesia machine, CO₂ absorber

RESUMEN

JUSTIFICATIVA Y OBJETIVOS: En pacientes bajo intubación traqueal o traqueostomia, la humidificación y calentamiento del gas inhalado son necesarios para la prevención de lesiones en el sistema respiratorio, consecuentes al contacto del gas frío y seco con las vías aéreas. El objetivo de la pesquisa fue evaluar el efecto del sistema respiratorio circular con absorbedor de dióxido de carbono del aparato de anestesia Cícero de Dräger, cuanto a la capacidad de calentamiento y humidificación de los gases inhalados, utilizándose flujo bajo (1 L.min^-1) o mínimo (0,5 L.min^-1) de gases frescos.

MÉTODO: El estudio aleatorio fue realizado en 24 pacientes estado físico ASA I, con edades entre 18 y 65 años, sometidos a anestesia general, utilizándose la Estación de Trabajo Cícero de Dräger (Alemania), para realización de cirugías abdominales, los cuales fueron distribuidos aleatoriamente en dos grupos: grupo de Bajo Flujo (BF), en el cual fue administrado 0,5 L.min^-1 de oxígeno y 0,5 L.min^-1 de óxido nitroso y flujo mínimo (FM), administrándose solamente oxígeno a 0,5 L.min^-1. Los atributos estudiados fueron: temperatura, humedad relativa y absoluta de la sala de operación y del gas en el sistema inspiratorio.

RESULTADOS: Los valores de la temperatura, humedad relativa y humedad absoluta en el sistema inspiratorio en la salida del aparato de anestesia y junto al tubo traqueal no presentaron diferencia significante entre los grupos, pero aumentaron a lo largo del tiempo en los dos grupos (BF y FM), habiendo influencia de la temperatura de la sala de operación sobre la temperatura del gas inhalado, en los dos grupos estudiados. Niveles de humedad y temperatura próximos de los ideales fueron alcanzados, en los dos grupos, a partir de 90 minutos.

CONCLUSIONES: No hay diferencia significante de la humedad y temperatura del gas inhalado utilizándose bajo flujo y flujo mínimo de gases frescos.

INTRODUCTION

Patients under general anesthesia breathe gases with low humidity content and at low temperatures, sometimes for prolonged periods of time. When tracheal tube is used, nasal, pharyngeal, laryngeal and part of tracheal functions are abolished and gases humidification and heating have to be performed in lower airways. If inhaled gases humidification and heating are inadequate, there will be functional airway mucosa damage with changes in ciliary movements and increased mucous viscosity promoting secretions hardening and incrustations which interfere with tracheal ability to heat and humidify inhaled gases and predisposing to cork formation which obstructs airways, especially smaller ones, and tracheal tubes ^1,2 causing atelectasis and pulmonary complications ³.

So, during artificial ventilation under tracheal intubation, inspired gases heating and humidification are critical to ensure airways integrity, preserve muco-ciliary function and improve gaseous changes ⁴.

Humidity may be defined in several ways: maximum humidity, which is the maximum amount of water vapor existing in the gaseous phase of a certain atmosphere at a certain temperature; absolute humidity, which is water vapor mass present in a certain gas volume, being usually expressed in mg of water per liter of gas; and relative humidity, represented by the ratio between water vapor mass contained in a certain gas volume at a certain temperature and maximum water vapor mass that this gas volume could contain at the same temperature. To help humidity calculations, there are tables proposed to calculate absolute humidity as a function of temperature ⁵.

There is still no consensus on optimal inhaled gases heating and humidification levels, but it is necessary to determine inspired gases heating and humidification limits ^6,7. In theory, gas temperature should be close to central temperature, which is 37 ºC, and relative humidity should not go beyond 100%, to prevent respiratory tree mucosa dehydration and promote maximum ciliary transport velocity ³.

In a Brazilian experimental study with dogs submitted to tracheal intubation and mechanical ventilation for 3 hours, the effects of absolute inspired gases heating and humidification were observed according to histological tracheobronchial tree mucosa epithelial changes at light microscopy. Authors have concluded that absolute inspired gases heating and humidification should not be higher than 36 ºC and 36 mg of water per liter of inspired gas, respectively, and should not be lower than 27 ºC or present absolute humidity below 23 mg of water.L^-1⁸.

The interest in low fresh gases flow during general inhalational anesthesia has increased in recent years for its major advantages, such as low inhalational anesthetic consumption, more effective inspiratory gases humidification and heating and major environmental pollution decrease ^9-11. However, low fresh gases flow system has disadvantages, such as the need for anesthesiologists' better understanding, attention and care, the impossibility of rapid changes in inspired inhalational anesthetic concentration, higher risk for hypercarbia by faster exhaustion of the carbon dioxide absorption system, possible build up of undesirable gases, such as CO₂, acetone, methane, in addition to toxic metabolites of anesthetic agents, argon and nitrogen ^12,13, requiring periodic "washing" of the system with high fresh gases flow for some minutes.

There are several and different opinions in the literature about proposed gases flow to define baseline flow, minimum flow and low flow. Baker (1994) ¹² has suggested a modification on gas flows based on Simionescu's classification (1986) ¹⁴ (Table I).

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Brazilian Journal of Anesthesiology, 2004; 54: 1: 20 - 36

Inhaled gases humidification and heating during artificial ventilation with low flow and minimal fresh gases flow

Susane Bruder Silveira Gorayb; José Reinaldo Cerqueira Braz; Regina Helena Garcia Martins;

Norma Sueli Pinheiro Módolo; Giane Nakamura

Decreased fresh gases flow leads to better use of heat and humidity generated in the absorber's reservoir, by CO₂ neutralization reaction of the mixture exhaled by soda lime, which is exothermal and leads to water formation ^15,16. Several authors have used low fresh gases flow with good results, but with variable efficacy in terms of reusing heat and humidity ^2,16-19. Differences found may be explained by changes in respiratory systems assembly, by different methods used to measure humidity present in inspired gases and by different flows used in the low fresh gases flow technique ^2,18.Among anesthesia machines to administer inhalational anesthetics to patients under general inhalational anesthesia, Dräger's Cicero (Germany) has special respiratory system characteristics. It has been projected for fresh gases flow to cross soda lime three times before being administered to patients, in addition to the presence of a plate to heat expired gases to prevent water condensation in the system. These characteristics may be highly efficient to improve inhaled gases heating and humidification levels.

This study aimed at evaluating the effect of the respiratory circle system with CO₂ absorber of the Cicero's anesthesia machine, using low fresh gases flow (1 L.min^-1) or minimum flow (0.5 L.min^-1), as to inhaled gases heating and humidification ability.

METHODS

After the local Research Ethics Committee approval and their informed consent, participated in this study 24 adult patients of both genders, aged 18 years or above and less than 65 years, physical status ASA I, submitted to abdominal surgery lasting 2 hours or more, under general anesthesia and mechanical ventilation.

Patients were randomly distributed in two groups of 12 patients, differentiated by the flow administered during ventilation, as follows:

Group LF: low fresh gases flow group receiving oxygen (0.5 L.min^-1) and nitrous oxide (0.5 L.min^-1);

Group MF: minimum fresh gases flow receiving oxygen only (0.5 L.min^-1).

After 15 mg oral midazolam 60 minutes before, anesthesia was induced with sufentanil (0.7 µg.kg^-1), propofol (2 mg.kg^-1) and cisatracurium (150 µg.kg^-1). After tracheal intubation, anesthesia was maintained with isoflurane in maximum concentration of 0.8%, that is, minimum alveolar concentration, with fresh gases flow according to the studied group in a closed system with CO₂ absorber, using Dräger's Cicero anesthesia machine and continuous sufentanil (0.01 - 0.005 µg.kg^-1.min^-1) and cisatracurium (2 µg.kg^-1.min^-1) infusion through two-channel infusion pump. Nitrous oxide, oxygen and CO₂ inspired and expired fractions, tidal volume and respiratory rate were continuously monitored by gases and halogenate collector located between the anesthetic system and the tracheal tube. For both groups, tidal volume was 8 ml.kg^-1 and respiratory rate was adjusted to maintain P_ETCO₂ between 30 and 35 mmHg.

Monitoring also consisted of ECG (D_IIand V5 lead), pulse oximetry (SpO₂), capnometry (P_ETCO₂), non-invasive blood pressure, neuromuscular block by TOF stimulation, always looking for the lack of response or minimum response to stimulations, and central temperature by a sensor placed in the lower esophagus.

Temperature and relative room air humidity readings were obtained from Gulton do Brasil Higrotermo 95 electronic digital thermo-hygrometer, through a sensor placed close to patients during studied moments. Inspiratory system gases temperature and relative humidity readings were obtained from the same thermo-hygrometer with the sensor initially connected by a T piece to the respiratory system outlet close to the workstation and then between the Y piece of the respiratory system and the tracheal tube during studied moments. The relative humidity sensor of this thermo-hygrometer is a Parametrics^® capacitative polymer film. Sensor's dielectric constant changes with changes in relative humidity expressed on a digital display, together with temperature. The following formula was used to calculate absolute humidity ⁵:

Where:

DA = absolute inspired air humidity (mgH₂O.L^-1);

DS = relative air humidity in saturation conditions (mgH₂O.L^-1), obtained from specific table ⁵ and using inspired gas temperature (ºC);

F = relative inspired gas humidity (%).

Cicero anesthesia machine respiratory system receives additional heating from a heated plate located close to the expiratory part of the system, lateral to the ventilator plunger. So, gases expired by patients initially go through the expiratory valve which, during this phase is opened. Then, they cross the heated plate which remains heated and humidified without water condensation because there is no cold surface within the respiratory system and cross soda lime for the first time toward the fresh gas reservoir (balloon). Next, expired gas and additional fresh gases flow from the canister are "pulled" by plunger's movement to fill the ventilator, after having passed through soda lime (expired gas for the second time and fresh gases flow for the first time). With the closing of expiratory intermittent positive pressure ventilation valves and the opening of the inspiratory valve, ventilator's plunger movement sends the gaseous mixture to patient, after crossing soda lime one more time (expired gas for the third time). Soda lime pills have been replaced before each experiment. Soda lime canister capacity is 1.5 L. Two corrugated 1.20 cm length silicone tubes were used in the respiratory system and were also replaced before each experiment. No filters or heat and humidity exchangers were used in the respiratory system.

To determine plate and soda lime canister temperature, Mallinckrodt's Thermistor 400, mod. 6150 two-channel thermometer was used together with Mallinckrodt's skin thermometer. Temperature of the plate located close to the internal respiratory system was installed in one channel to which a temperature sensor was coupled. The second channel recorded soda lime temperature through a sensor fixed on the internal canister wall, in its lower part, where the temperature was the highest.

Humidity and temperature attributes were evaluated at moments: M0 (control) - immediately after ventilatory system installation, according to the studied group, and obtained 10 minutes after ventilatory system installation with 2 L.min^-1 oxygen necessary to totally filling respiratory system with gases and anesthetics, and M30, M60, M90 and M120, respectively 30, 60, 90 and 120 minutes after ventilation system installation, according to the group.

Statistical Analysis

Student's t test was used for demographics and Fisher's Exact test was used for gender distribution. ANOVA, followed by Tukey's multiple comparisons test were used for variables with normal distribution and variance homogeneity. Mann-Whitney test was used for non-parametric distribution to compare values of both groups, and Friedman's test was used to compare values within the same group. Dispersion diagram was also used for correlation analysis between operating room temperature and inhaled gas temperature. Significance levels below 0.005 were considered significant.

Attributes within normal assumptions are expressed in mean and standard deviation. Non-parametric attributes are expressed in median and 1^st and 3^rd quartiles; 25% of observed values are within the 1^st quartile, while 75% of observed values are in the 3^rd quartile.

RESULTS

Both groups were homogeneous in demographics data (Table II). Gender distribution was also homogeneous between groups with predominance of females (Table II).

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Brazilian Journal of Anesthesiology, 2004; 54: 1: 20 - 36

Inhaled gases humidification and heating during artificial ventilation with low flow and minimal fresh gases flow

Susane Bruder Silveira Gorayb; José Reinaldo Cerqueira Braz; Regina Helena Garcia Martins;

Norma Sueli Pinheiro Módolo; Giane Nakamura

Operating room temperature and absolute and relative humidity were not statistically different between groups and among studied moments (p > 0.05) (Table III).

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Brazilian Journal of Anesthesiology, 2004; 54: 1: 20 - 36

Inhaled gases humidification and heating during artificial ventilation with low flow and minimal fresh gases flow

Susane Bruder Silveira Gorayb; José Reinaldo Cerqueira Braz; Regina Helena Garcia Martins;

Norma Sueli Pinheiro Módolo; Giane Nakamura

Esophageal temperature was not statistically different between groups and among moments (p > 0.05) (Table IV). Soda lime canister temperature, however, was significantly higher in moment M90 for group MF as compared to group LF (p < 0.05), with significant increase along time for both groups (p < 0.05) (Table IV). Temperature of the plate close to the expiratory system was not statistically different between groups (p > 0.05), but has increased along time for both groups (p < 0.05).

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Brazilian Journal of Anesthesiology, 2004; 54: 1: 20 - 36

Inhaled gases humidification and heating during artificial ventilation with low flow and minimal fresh gases flow

Susane Bruder Silveira Gorayb; José Reinaldo Cerqueira Braz; Regina Helena Garcia Martins;

Norma Sueli Pinheiro Módolo; Giane Nakamura

Gas temperature in the inspiratory branch at the anesthesia machine outlet and close to the tracheal tube was not significantly different between groups (p > 0.05), but has significantly increased along time for both groups (p < 0.05) (Figures 1 and 2 respectively).

Brazilian Journal of Anesthesiology, 2004; 54: 1: 20 - 36

Inhaled gases humidification and heating during artificial ventilation with low flow and minimal fresh gases flow

Susane Bruder Silveira Gorayb; José Reinaldo Cerqueira Braz; Regina Helena Garcia Martins;

Norma Sueli Pinheiro Módolo; Giane Nakamura

Brazilian Journal of Anesthesiology, 2004; 54: 1: 20 - 36

Inhaled gases humidification and heating during artificial ventilation with low flow and minimal fresh gases flow

Susane Bruder Silveira Gorayb; José Reinaldo Cerqueira Braz; Regina Helena Garcia Martins;

Norma Sueli Pinheiro Módolo; Giane Nakamura

Dispersion diagram for correlation analysis between operating room temperature and inhaled gas temperature in groups LF and MF is shown in figures 3 and 4 respectively. There has been significant correlation between both temperatures for both groups (p < 0.001).

Brazilian Journal of Anesthesiology, 2004; 54: 1: 20 - 36

Inhaled gases humidification and heating during artificial ventilation with low flow and minimal fresh gases flow

Susane Bruder Silveira Gorayb; José Reinaldo Cerqueira Braz; Regina Helena Garcia Martins;

Norma Sueli Pinheiro Módolo; Giane Nakamura

Brazilian Journal of Anesthesiology, 2004; 54: 1: 20 - 36

Inhaled gases humidification and heating during artificial ventilation with low flow and minimal fresh gases flow

Susane Bruder Silveira Gorayb; José Reinaldo Cerqueira Braz; Regina Helena Garcia Martins;

Norma Sueli Pinheiro Módolo; Giane Nakamura

Relative humidity in the inspiratory branch of the anesthesia machine outlet and close to the tracheal tube was not significantly different between groups (p > 0.05), but has increased along time for both groups (p < 0.05) (Figures 5 and 6 respectively).

Brazilian Journal of Anesthesiology, 2004; 54: 1: 20 - 36

Inhaled gases humidification and heating during artificial ventilation with low flow and minimal fresh gases flow

Susane Bruder Silveira Gorayb; José Reinaldo Cerqueira Braz; Regina Helena Garcia Martins;

Norma Sueli Pinheiro Módolo; Giane Nakamura

Brazilian Journal of Anesthesiology, 2004; 54: 1: 20 - 36

Inhaled gases humidification and heating during artificial ventilation with low flow and minimal fresh gases flow

Susane Bruder Silveira Gorayb; José Reinaldo Cerqueira Braz; Regina Helena Garcia Martins;

Norma Sueli Pinheiro Módolo; Giane Nakamura

Absolute humidity of the inspiratory branch of the anesthesia machine outlet and close to the tracheal tube was not significantly different between groups (p > 0.05), but has increased along time for both groups (p < 0.05) (Figures 7 and 8 respectively).

Brazilian Journal of Anesthesiology, 2004; 54: 1: 20 - 36

Inhaled gases humidification and heating during artificial ventilation with low flow and minimal fresh gases flow

Susane Bruder Silveira Gorayb; José Reinaldo Cerqueira Braz; Regina Helena Garcia Martins;

Norma Sueli Pinheiro Módolo; Giane Nakamura

Brazilian Journal of Anesthesiology, 2004; 54: 1: 20 - 36

Inhaled gases humidification and heating during artificial ventilation with low flow and minimal fresh gases flow

Susane Bruder Silveira Gorayb; José Reinaldo Cerqueira Braz; Regina Helena Garcia Martins;

Norma Sueli Pinheiro Módolo; Giane Nakamura

DISCUSSION

Inhaled gases temperature at the anesthesia machine was adequate for both groups since the beginning of the experiment and has increased during the experiment (Figure 1). This result was already expected, since Dräger's Cicero device has features to improve inhaled gas heating and humidification. According to the manufacturer, its heating plate coupled to the respiratory system aims especially at preventing expired air water condensation when it cools within the respiratory system, which could interfere with the adequate operation of valves and ventilator. But this heating plate also helps maintaining gas temperature in the respiratory system at adequate levels. In our study, plate temperature has not changed between groups, but has gradually increased along moments (Table IV).

Other equally or more important features to maintain inhaled gas temperature were also incorporated to Cicero, such as reutilization of heat and humidity from soda lime neutralization reaction, because the expired gas passes 3 times through soda lime canister before getting to patients.

It has been shown that the lower the fresh gases flow, the higher the temperature in the absorber¹⁸. So, the authors used 0.5 L.min^-1 or 1 L.min^-1 flows and have observed that soda lime temperature has reached a mean temperature of 39 ºC after 30 to 40 minutes. The difference in temperature observed in that study, as well as in our study, may be credited to different devices and methods used to measure temperature. Torres et al. (1997) ¹⁸ have measured canister temperature at the external surface with an infrared thermometer. This method allows temperature screening in a wider surface, being the reading captured where the temperature is the highest. Our study has used simple electric thermometer with the sensor fixed to the lower part external of soda lime canister surface, where the temperature has always been the highest.

In an experimental study with dogs under low fresh gases flow (1 L.min^-1) and two canisters with 1800 g soda lime each, it has been observed that external temperature at the highest temperature site, went from 23 ºC to a maximum temperature of 29 ºC after three hours, using electric thermometer to measure temperature ²⁰. In the circle system used, fresh gases inlet was close to the inspiratory branch and there has been excess gas escape in the system before all exhaled gas went through soda lime, which certainly has contributed to decrease the intensity of soda lime neutralization reaction. Some authors ^22-24 have obtained better inhaled gas heating results with changes in anesthesia systems assembly and decreased gas flows. A better use of heat and humidity was also observed when fresh gas flow inlet was located before soda lime canister ¹⁸.

The conclusion was that fresh gases from hospital gases canisters (at room temperature close to 20 ºC), using a valve system without CO₂ absorber, or those with CO₂ absorber where soda lime gases are mixed to additional fresh gases flow, reach the inspiratory branch at a temperature close to room temperature, if no heating system or thermal insulation is used ^15,19.

A different way to make inspired gases heat and humidity more efficient is the use of small soda lime canisters. Our study has used a large 1.5 liter canister. An experimental study with two types of canisters has observed that there is more heat and humidity release with the small soda lime canister and, as a consequence, better use of inspired gases heat and humidity ²⁴. The disadvantage is the need for frequent soda lime replacement because it is easily saturated.

Gases temperature at the anesthesia machine outlet was similar to that of soda lime at 60 minutes of study. These results have shown that gas flow has incorporated heat produced by soda lime chemical reaction with CO₂.

Other authors' ¹⁸ results, however, were different from ours. Using low fresh gases flows (1 L.min^-1 and 0.5 L.min^-1) in respiratory system with CO₂ absorber, they have observed much higher soda lime canister temperatures, around 40 ºC, but with inhaled gas temperatures of approximately 19 ºC to 22 ºC. According to the authors, this might have been a consequence of heat loss to the environment, the temperature of which has been maintained in approximately 20.5 ºC, or even to other parts of the respiratory system, indicating poor respiratory system thermal insulation.

In our study, inhaled gas temperature close to the tracheal tube was not significantly different between groups, but there has been major inhaled gas temperature decrease as compared to gas temperature at anesthesia machine outlet, with a loss of approximately 4 ºC in group LF and 5 ºC in the group MF (Figure 2).

It should be highlighted that most respiratory systems corrugated tubes are made of silicone, poly vinyl chloride (PVC) or other poorly thermal insulated materials.

There has been a significant correlation in both groups between inhaled gas temperature and room temperature (Figure 3 and 4). These results are in line with other authors ^{15,18,20-22,25}. Authors ¹⁸ have also observed that this correlation is independent of additional gas flow passing or not through soda lime before being incorporated to the inspiratory branch. In this study, authors have concluded that heat generated in soda lime has not been totally incorporated to inspired gas, but has been partially transferred to the environment, which is normally kept at a lower temperature. This heat transfer was only absent when the authors have thermally insulated the corrugated tubes with aluminum foil and have obtained higher inhaled gas temperatures.

It has to be highlighted in our study that higher inhaled gas temperatures were only observed 30 to 60 minutes after the beginning of the study, when mean inhaled gas temperature has reached 27.4 ºC in group LF and 27.3 ºC in group MF. A similar result was also observed by Kleeman (1994) ², with adequate inhaled gas temperature obtained only when the author used 0.6 L.min^-1 fresh gases flow after 90 minutes.

Differently from temperature, relative gas humidity at anesthesia machine outlet and close to the tracheal tube was similar for both groups (Figures 5 and 6 respectively).

Most studies preconize high relative inspired gas humidity values (close to 100%) to prevent respiratory epithelium and muco-ciliary activity changes ^26,27. Our research has obtained values close to this only 60 minutes after beginning of experiment.

Commercially available anesthetic gases are intentionally dry to prevent respiratory system valves obstruction. These gases, however, decrease the amount of humidity available to patients. Anesthesia machine humidity source becomes dependent on the water incorporated to soda lime granules, on exothermal soda lime neutralization reaction producing water vapor, on wet and heated gases exhaled by patients in the respiratory system, on the previous use of the ventilator system which returns water from condensation built up during its use, on the use of low and minimum gases flow during anesthesia and on the use of heat and humidity exchangers ^3,18,19,28.

Aldrete et al. (1981) ²⁸ have used valve respiratory system with CO₂ absorber and fresh gas flows of 5; 2; 0.5 and 0.3 L.min^-1 in patients submitted to abdominal surgeries and have shown that relative humidity in the inspiratory branch has gradually increased with fresh gases flow decrease, reaching 98% with 0.3 L.min^-1 flow.

Conversely, absolute humidity at anesthesia machine outlet has always remained above 20 mg H₂O.L^-1, for both groups, except for the control moment, reaching values of approximately 34 to 35 mg H₂O.L^-1 120 minutes after (Figure 7). However, absolute inhaled gas humidity close to the tracheal tube has shown lower values but still above 20 mg H₂O.L^-1, except for the control moment, and has reached even higher values as from 60 minutes, reaching a mean of 27 to 28 mg H₂O.L^-1 at the end of the study (Figure 8).

So, in our study and differently from what has been observed with relative humidity, there have been major differences between absolute humidity at anesthesia machine outlet and that of the inhaled gas, certainly by the major influence of room temperature over inhaled gas temperature. Kleemann (1990) ²⁹, in an experimental study with swine under anesthesia, has ventilated animals for 10 hours using the Dräger AV1 (Germany) device with 0.5 and 6 L.min^-1 oxygen (40%) and nitrous oxide (60%). At the end of the experiment, the author has removed the tracheobronchic trees for biopsy under electronic screening microscopy. In the control group, the tracheobronchic tree was removed 20 minutes after anesthetic induction. In the group receiving high fresh gases flow (6 L.min^-1), mucus droplets have shown important dehydration-related changes and cilia have formed clusters with several ciliary rarefaction areas, with epithelial cells exposure, particularly more frequent in tracheal bifurcation and major bronchi areas. In the minimum 0.5 L.min^-1 flow group, ciliary epithelium was relatively unchanged with no significant ciliary and mucous changes as compared to the control group.

The same author has carried out a clinical trial ²⁹ to determine inhaled gas temperature and humidity during anesthesia, using Dräger's AV1 device (Germany) and low 0.6 L.min^-1 fresh gases flow in group I, and using Dräger's Sulla 800 V device (Germany) with 1.5, 3 and 6 L.min^-1 fresh gases flow in groups II, III and IV, respectively. Results have shown that medium (1.5 L.min^-1) and high (3 and 6 L.min^-1) flows determine inadequate inhaled gas levels, with maximum temperature of 24.5 ºC and maximum absolute humidity of 16 mg H₂O.L^-1, while minimum gas flow (0.6 L.min^-1) determines adequate inhaled gas temperature (31 ºC) and acceptable absolute humidity (21.3 mg H₂O.L^-1) levels 1.5 to 2 hours later and is a feasible alternative to heat and humidity exchangers in prolonged surgeries and in patients with respiratory complications.

Henriksson et al. (1997) ¹⁷ have evaluated for 60 minutes humidity and temperature of inhaled gases in circle system with low volume absorber (1 L), with and without heat and humidity exchangers, with three different fresh gases flows (1, 2 or 5 L.min^-1), using the Servo 900 ventilator (Siemens - Elemo, Sweden). Their conclusion was that in groups with fresh gases flows equal to or lower than 2 L.min^-1, absolute inspired gas humidity of 21 to 23 mg H₂O.L^-1 was obtained without statistical differences between groups, at a temperature of 26 to 27 ºC and 84% to 87% relative humidity, which the authors have considered enough to prevent respiratory tract dehydration. When heat and humidity exchangers, were used, significantly increased absolute humidity (between 28 to 30 mm H₂O.L^-1) and inhaled gas temperature (mean of 32 to 34 ºC), regardless of the fresh gas used and already in the first minutes of the study.

Something to be considered for the attaining of relatively low humidity levels in the first moments of our study, is the fact that higher fresh gases flows (2 L.min^-1) were used in the beginning of the procedure to rapidly fill the whole system and reach desired inhalational anesthetic concentration in a shorter time.

Bisinotto et al. (1999) ²⁰ have studied in dogs the effects of high fresh gases flows (5 L.min^-1) with respiratory system without CO₂ absorber, and of low fresh gases flow (1 L.min^-1) in circle system with CO₂ absorber, associated or not to heat and humidity exchangers, during 3 hours, and have evaluated inhaled gases temperature and humidity and the effects on the tracheobronchic tree, analyzed by screening electronic microscopy. Authors have observed that the respiratory system without CO₂ absorber and with high fresh gases flow has shown significantly lower relative (between 37% and 39%) and absolute (between 8 and 9 mg H₂O.L^-1) humidity values as compared to the group with low flow and CO₂ absorber, where relative humidity has remained in approximately 75% to 79% and absolute humidity in approximately 15 to 17 mg H₂O.L^-1. Authors have not observed significant differences between groups in inhaled gas temperature, even with the introduction of the heat and humidity exchanger. Regardless of gases flows used, groups with the exchanger had higher relative humidity (around 88% to 94%) and absolute humidity (22 to 24 mg H₂O.L^-1) values. These results were reflected in tracheobronchial tree mucosa exams, in which major mucous and ciliary changes were seen in the group with high flow and without exchanger. In the exchanger groups, regardless of the flows, there were minor changes in the muco-ciliary system, but still with areas indicating some degree of dehydration. Authors have concluded that heat and humidity exchanger attenuates, but does not prevent muco-ciliary changes in the tracheobronchial tree.

Studies have recommended absolute inhaled gas humidity between 20 and 30 mg H₂O.L^-1 to decrease the risk for respiratory tract dehydration ^{2,3,8,17,20,30}.

Minimum absolute humidity limit accepted during prolonged ventilation is 30 mg H₂O.L^-1³¹, but, in our opinion, it is difficult to determine the lowest humidification limit during anesthesia¹ because it depends on anesthesia duration, previous pulmonary conditions and the respiratory system used. Humidification studies were primarily performed with animals ^8,32-35. In men, investigations are limited by the small number of patients involved and the lack of information about patients involved in each study ^2,17. New information about upper airways is scarce ¹, especially when there are already previous respiratory changes and little have we advanced in measurement techniques.

There have not been, in our study, significant esophageal temperature changes during surgery, with patients' central temperature remaining around 36 ºC. We believe that patients' central temperature maintenance was due to operating room temperature maintenance around 24 ºC, to respiratory system assembly, in which gases went three times through soda lime before reaching patients, thus supplying patients with heated and humidified gases, and to the use of low and minimum fresh gases flows.

So, inhaled gases heating and humidification and operating room temperature maintenance have been described as important measures to prevent heat loss and, as a consequence, body temperature decrease ^21,36,37.

In conclusion; in men and under our experimental conditions, there have been no significant differences in humidity and temperature of inhaled low (1 L.min^-1) or minimum (0.5 L.min^-1) fresh gases flow; humidity and temperature levels were initially adequate to prevent tracheobronchial tree mucosa dehydration and, as from 90 minutes, they were close to optimal levels, according to the literature.

REFERENCES

Submitted for publication February 24, 2003

Accepted for publication May 22, 2003

01. Williams R, Rankin N, Smith T et al - Relationship between the humidity and temperature of inspired gas the function of the airway mucosa. Crit Care Med, 1996;24:1920-1929.
02. Kleemann PP - Humidity of anaesthetic gases with respect to low flow anaesthesia. Anaesth Intensive Care, 1994;22:396-408.
03. Chalon J, Ali M, Ramanathan S et al - The humidification of anaesthetic gases. Its importance and control. Can Anaesth Soc J, 1979;26:361-366.
04. Branson RD, Chatburn RL - Humidification of inspired gases during mechanical ventilation. Respir Care, 1993;38:461-468.
05. Tubelis A, Nascimento FJL - Umidade do Ar, em: Meteorologia Descritiva: Fundamentos e Aplicações Brasileiras. São Paulo, Distribuidora Brasil, 1980;94-127.
06. King M, Tomkiewiaz RP, Boyd WA - Mucociliary clearance and epithelial potential difference in dogs mechanically ventilated with air humidified by heated hot water and heat and moisture exchange devices. J Aerosol Med, 1995;8:66-84.
07. Brock-Utne JG - Humidification in pediatric anaesthesia. Paediatr Anaesth, 2000;10:117-119.
08. Martins RHG, Braz JRC, Defaveri J et al - Estudo da umidificação e do aquecimento dos gases durante a ventilação mecânica no cão. Rev Bras Otorrinolaringol, 1996;62:206-218.
09. Möllhoff T, Burgard G, Prien T - Low-flow and minimal-flow anaesthesia using the laryngeal mask airway. Eur J Anaesthesiol, 1996;13:456-462.
10. Baxter AD - Low and minimal flow inhalational anaesthesia. Can J Anaesth, 1977;44:643-653.
11. Couto JMS, Aldrete JA - Fluxos de gases empregados em anestesia. Rev Bras Anestesiol, 1988;38:445-450.
12. Baker AB - Low-flow and closed circuits. Anaesth Intensive Care, 1994;22:341-342.
13. Dorsch JA, Dorsch SE - Anesthesia Ventilators, em: Understanding Anesthesia Equipment. 4^th Ed, Baltimore, Williams & Wilkins, 1999;309-353.
14. Simionescu R - Safety of low flow anaesthesia. Circular, 1986;3:7-9.
15. Dorsch JA, Dorsch SE - The Circle System, em: Understanding Anesthesia Equipment. 4^th Ed, Baltimore, Williams & Wilkins, 1999;229-269.
16. Torres MLA, Mathias RS - Aparelho de Anestesia: Componentes e Normas Técnicas, em: Ortenzi AV, Tardelli MA - Anestesiologia SAESP, 5ª ª Ed, São Paulo, Atheneu, 2001; 99-136.
17. Henriksson BA, Sundling J, Hellman A - The effect of a heat and moisture exchanger on humidity in a low-flow anaesthesia system. Anaesthesia, 1997;52:144-149.
18. Torres MLA, Carvalho JCA, Bello CN et al - Sistemas respiratórios valvulares com absorção de CO₂: capacidade de aquecimento e umidificação dos gases inalados em três tipos de montagens utilizadas em aparelhos de anestesia no Brasil. Rev Bras Anestesiol, 1997;47:89-100.
19. Bisinotto FMB, Braz JRC, Martins RHE - Umidificação dos gases inalados. Rev Bras Anestesiol, 1999;49:349-359.
20. Bisinotto FMB, Braz JRC, Martins RHG et al - Trachebronchial consequences of the use of heat and moisture exchanges in dogs. Can J Anesth, 1999;46:897-903.
21. Tausk HC, Miller R, Roberts RB - Maintenance of body temperature by heated humidification. Anesth Analg, 1976;55:719-723.
22. Chalon J, Patel C, Ali M et al - Humidity and the anesthetized patient. Anesthesiology, 1978;50:195-198.
23. Berry FA, Ball CG, Blankenbaker WL - Humidification of anesthetic systems for prolonged procedures. Anesth Analg, 1975;54:50-54.
24. Bengtson NM, Johnson A - Failure of a heat and moisture exchanger as a cause of disconnection during anaesthesia. Acta Anaesthesiol Scand, 1989;33:522-523.
25. Bengtson JP, Sonander H, Stenqvist O - Preservation of humidity and heat of respiratory during anesthesia. A laboratory investigation. Acta Anaesthesiol Scand, 1987;31:127-131.
26. Mercke U, Hakansson CH, Toremalm NG - The influence of temperature on mucociliary activity. Temperature range 20 ºC - 40 ºC. Acta Otolaryngol, 1974;78:444-450.
27. Mercke U - The influence of temperature on mucociliary activity: temperature range 40 ºC - 50 ºC. Acta Otolaryngol, 1974;78: 253-258.
28. Aldrete JA, Cubillos P, Sherrill D - Humidity and temperature changes during low flow and closed system anaesthesia. Acta Anaesthesiol Scand, 1981;25:312-314.
29. Kleemann PP - The climatisation of anesthetic gases under conditions of high flow to low flow. Acta Anaesthesiol Belg, 1990;41:189-200.
30. Mebius C - A comparative evaluation of disposable humidifiers. Acta Anaesthesiol Scand, 1983;27:403-409.
31. Shelly MP, Lloyd GM, Park GR - A review of the mechanisms and methods of humidification of inspired gases. Intensive Care Med, 1988;14:1-9.
32. Nouguchi H, Takumi Y, Aochi O - A study of humidification in tracheostomized dogs. Br J Anaesth, 1973;45:844-848.
33. Chalon J, Loew DAY, Malebranche J - Effect of dry anesthetic gases on the tracheobronquial ciliated epithelium. Anesthesiology, 1972;37:338-343.
34. Tsuda T, Nouguchi H, Takumi Y et al - Optimum humidification of air administered to a tracheostomy in dogs. Scanning electron microscopy and surfactant studies. Br J Anaesth, 1977;49: 965-977.
35. Eckebon B, Lindholm CE - Heat end moisture exchangers and body temperature: a preoperative study. Acta Anaesthesiol Scand, 1990;34:538-542.
36. Stone DR, Downs JB, Paul WL et al - Adult body temperature and heated humidification of anesthetic gases during general anesthesia. Anesth Analg, 1981;60:736-741.
37. Tonelli D, Toldo A - Regulação da tempertura e anestesia. Rev Bras Anestesiol, 1994;44:195-204.

Correspondence to

Prof. Dr. José Reinaldo Cerqueira Braz

Deptº de Anestesiologia da FMB-UNESP

18618-970 Botucatu, SP

*

Received from Hospital das Clínicas (HC) do CET/SBA do Departamento de Anestesiologia da Faculdade de Medicina de Botucatu (FMB), UNESP, no Programa de Pós-Graduação em Cirurgia

Publication Dates

Publication in this collection
06 May 2004
Date of issue
Feb 2004

History

Accepted
22 May 2003
Received
24 Feb 2003

This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

[1] 01. Williams R, Rankin N, Smith T et al - Relationship between the humidity and temperature of inspired gas the function of the airway mucosa. Crit Care Med, 1996;24:1920-1929.

[2] 02. Kleemann PP - Humidity of anaesthetic gases with respect to low flow anaesthesia. Anaesth Intensive Care, 1994;22:396-408.

[3] 03. Chalon J, Ali M, Ramanathan S et al - The humidification of anaesthetic gases. Its importance and control. Can Anaesth Soc J, 1979;26:361-366.

[4] 04. Branson RD, Chatburn RL - Humidification of inspired gases during mechanical ventilation. Respir Care, 1993;38:461-468.

[5] 05. Tubelis A, Nascimento FJL - Umidade do Ar, em: Meteorologia Descritiva: Fundamentos e Aplicações Brasileiras. São Paulo, Distribuidora Brasil, 1980;94-127.

[6] 06. King M, Tomkiewiaz RP, Boyd WA - Mucociliary clearance and epithelial potential difference in dogs mechanically ventilated with air humidified by heated hot water and heat and moisture exchange devices. J Aerosol Med, 1995;8:66-84.

[7] 07. Brock-Utne JG - Humidification in pediatric anaesthesia. Paediatr Anaesth, 2000;10:117-119.

[8] 08. Martins RHG, Braz JRC, Defaveri J et al - Estudo da umidificação e do aquecimento dos gases durante a ventilação mecânica no cão. Rev Bras Otorrinolaringol, 1996;62:206-218.

[9] 09. Möllhoff T, Burgard G, Prien T - Low-flow and minimal-flow anaesthesia using the laryngeal mask airway. Eur J Anaesthesiol, 1996;13:456-462.

[10] 10. Baxter AD - Low and minimal flow inhalational anaesthesia. Can J Anaesth, 1977;44:643-653.

[11] 11. Couto JMS, Aldrete JA - Fluxos de gases empregados em anestesia. Rev Bras Anestesiol, 1988;38:445-450.

[12] 12. Baker AB - Low-flow and closed circuits. Anaesth Intensive Care, 1994;22:341-342.

[13] 13. Dorsch JA, Dorsch SE - Anesthesia Ventilators, em: Understanding Anesthesia Equipment. 4^th Ed, Baltimore, Williams & Wilkins, 1999;309-353.

[14] 14. Simionescu R - Safety of low flow anaesthesia. Circular, 1986;3:7-9.

[15] 15. Dorsch JA, Dorsch SE - The Circle System, em: Understanding Anesthesia Equipment. 4^th Ed, Baltimore, Williams & Wilkins, 1999;229-269.

[16] 16. Torres MLA, Mathias RS - Aparelho de Anestesia: Componentes e Normas Técnicas, em: Ortenzi AV, Tardelli MA - Anestesiologia SAESP, 5ª ª Ed, São Paulo, Atheneu, 2001; 99-136.

[17] 17. Henriksson BA, Sundling J, Hellman A - The effect of a heat and moisture exchanger on humidity in a low-flow anaesthesia system. Anaesthesia, 1997;52:144-149.

[18] 18. Torres MLA, Carvalho JCA, Bello CN et al - Sistemas respiratórios valvulares com absorção de CO₂: capacidade de aquecimento e umidificação dos gases inalados em três tipos de montagens utilizadas em aparelhos de anestesia no Brasil. Rev Bras Anestesiol, 1997;47:89-100.

[19] 19. Bisinotto FMB, Braz JRC, Martins RHE - Umidificação dos gases inalados. Rev Bras Anestesiol, 1999;49:349-359.

[20] 20. Bisinotto FMB, Braz JRC, Martins RHG et al - Trachebronchial consequences of the use of heat and moisture exchanges in dogs. Can J Anesth, 1999;46:897-903.

[21] 21. Tausk HC, Miller R, Roberts RB - Maintenance of body temperature by heated humidification. Anesth Analg, 1976;55:719-723.

[22] 22. Chalon J, Patel C, Ali M et al - Humidity and the anesthetized patient. Anesthesiology, 1978;50:195-198.

[23] 23. Berry FA, Ball CG, Blankenbaker WL - Humidification of anesthetic systems for prolonged procedures. Anesth Analg, 1975;54:50-54.

[24] 24. Bengtson NM, Johnson A - Failure of a heat and moisture exchanger as a cause of disconnection during anaesthesia. Acta Anaesthesiol Scand, 1989;33:522-523.

[25] 25. Bengtson JP, Sonander H, Stenqvist O - Preservation of humidity and heat of respiratory during anesthesia. A laboratory investigation. Acta Anaesthesiol Scand, 1987;31:127-131.

[26] 26. Mercke U, Hakansson CH, Toremalm NG - The influence of temperature on mucociliary activity. Temperature range 20 ºC - 40 ºC. Acta Otolaryngol, 1974;78:444-450.

[27] 27. Mercke U - The influence of temperature on mucociliary activity: temperature range 40 ºC - 50 ºC. Acta Otolaryngol, 1974;78: 253-258.

[28] 28. Aldrete JA, Cubillos P, Sherrill D - Humidity and temperature changes during low flow and closed system anaesthesia. Acta Anaesthesiol Scand, 1981;25:312-314.

[29] 29. Kleemann PP - The climatisation of anesthetic gases under conditions of high flow to low flow. Acta Anaesthesiol Belg, 1990;41:189-200.

[30] 30. Mebius C - A comparative evaluation of disposable humidifiers. Acta Anaesthesiol Scand, 1983;27:403-409.

[31] 31. Shelly MP, Lloyd GM, Park GR - A review of the mechanisms and methods of humidification of inspired gases. Intensive Care Med, 1988;14:1-9.

[32] 32. Nouguchi H, Takumi Y, Aochi O - A study of humidification in tracheostomized dogs. Br J Anaesth, 1973;45:844-848.

[33] 33. Chalon J, Loew DAY, Malebranche J - Effect of dry anesthetic gases on the tracheobronquial ciliated epithelium. Anesthesiology, 1972;37:338-343.

[34] 34. Tsuda T, Nouguchi H, Takumi Y et al - Optimum humidification of air administered to a tracheostomy in dogs. Scanning electron microscopy and surfactant studies. Br J Anaesth, 1977;49: 965-977.

[35] 35. Eckebon B, Lindholm CE - Heat end moisture exchangers and body temperature: a preoperative study. Acta Anaesthesiol Scand, 1990;34:538-542.

[36] 36. Stone DR, Downs JB, Paul WL et al - Adult body temperature and heated humidification of anesthetic gases during general anesthesia. Anesth Analg, 1981;60:736-741.

[37] 37. Tonelli D, Toldo A - Regulação da tempertura e anestesia. Rev Bras Anestesiol, 1994;44:195-204.