Print version ISSN 0034-7094
Rev. Bras. Anestesiol. vol.55 no.1 Campinas Jan./Feb. 2005
Anesthesia breathing systems with CO2 absorption, circle valve circuit: comparison of thermal behavior of coaxial system and conventional system with different fresh gas flows*
Sistemas respiratorios con absorción de CO2, circulares, valvulares: comparación del comportamiento térmico entre el sistema coaxial y convencional con diferentes flujos de gas fresco
Marcelo Luís Abramides Torres, TSA, M.D.I; Eduardo Tsuyoshi Yamaguchi, TSA, M.D.II; Ubirajara Sabbag Fonseca, M.D.III
IProfessor Doutor da Disciplina de
Anestesiologia da FMUSP
IIAnestesiologista do Hospital das Clínicas da FMUSP
IIIEx-ME3 (2003) da Disciplina de Anestesiologia da FMUSP
BACKGROUND AND OBJECTIVES: The adequate
maintenance of inhaled gases temperature during anesthetic procedures is critical
to prevent perioperative respiratory complications. This study aimed at comparing
the ability to warm up inhaled gases of coaxial breathing system and conventional
system, by varying fresh gas flows (FGF).
METHODS: Breathing systems were tested in a lung simulator ventilated with 600 mL tidal volume and respiratory frequency of 10 bpm. The model simulated human CO2 production by delivering 250 mL.min-1 of CO2 flow. Then, exhaled gas from the model was directed to a pre-warmed humidifier to simulate human exhaled gas. Both systems were classified as circle, valve circuits with CO2 absorption. In the coaxial system (model A), the inspiratory branch was enveloped by the expiratory branch, whereas the conventional one (model B) presented separated respiratory branches. Inhaled gas temperature was measured at the following moments: 0, 5, 10, 20, 30, 40, 50, 60 and 90 minutes, with low (0.5 and 1 L.min-1) and high (3 and 6 L.min-1) FGF.
RESULTS: Model A presented significant thermal variation between beginning and end of experiment (22.47 ± 1.77 ºC and 24.27 ± 3.52 ºC respectively, p < 0.05). Both models A and B produced similar temperatures at the end of the study (24.27 ± 3.52 ºC and 23.61 ± 1.93 ºC respectively). There was no difference between final temperatures of both models and environmental temperature (21.25 ± 1.20 ºC and 21.81 ± 1,87 ºC respectively). Low FGF has produced similar temperatures to those observed at the end of the study with higher flows in both models (A: 25.53 ± 4.78 ºC and 23.02 ± 0.80 ºC; B: 24.50 ± 0.85 ºC and 22.72 ± 2.36 ºC, respectively).
CONCLUSIONS: The coaxial system presented significant thermal variation between beginning and end of experiment, while this was not observed in the conventional one. No difference was observed in final temperatures when comparing both systems, regardless of the FGF.
Key words: EQUIPMENTS: coaxial circuit, heat permuter; GASES: temperature
JUSTIFICATIVA Y OBJETIVOS: El mantenimiento
de la temperatura del gas inhalado por el paciente durante el procedimiento
anestésico es de fundamental importancia para evitar complicaciones respiratorias
durante el peri-operatorio. El objetivo de este estudio es comparar, a través
de modelo experimental, la capacidad de calentamiento de los gases inhalados
con la utilización de sistemas respiratorios con absorción de CO2,
circulares, valvulares coaxial y convencional, variándose el flujo de gas
MÉTODO: Fue realizado en estudio experimental en laboratorio, testándose dos sistemas respiratorios en un simulador de pulmón, que fue ventilado con volumen corriente de 600 mL y frecuencia de 10 incursiones por minuto. El modelo simulaba la producción de CO2, a través de la administración de flujo de 250 mL .min-1 de CO2, y el gas exhalado del pulmón de prueba pasaba por un humidificador calentado para simular el gas expirado. Los dos sistemas fueron clasificados como circulares, valvulares, con absorción de CO2. En el sistema A (coaxial), la rama inspiratoria pasaba por el interior de la rama expiratoria, mientras que el sistema B fue el convencional. Las medidas de temperatura del gas inhalado fueron realizadas en los momentos 0, 5, 10, 20, 30, 40, 50, 60 y 90 minutos, siendo empleados FGF bajos (0,5 e 1 L.min-1) y altos (3 y 6 L.min-1).
RESULTADOS: El sistema A presentó variación térmica significativa entre el inicio y el final de los ensayos (22,47 ± 1,77 ºC y 24,27 ± 3,52 ºC p < 0,05 respectivamente). Los sistemas A y B produjeron temperaturas semejantes al final del estudio (24,27 ± 3,52 ºC y 23,61 ± 1,93 ºC respectivamente), y no hubo diferencia entre las temperaturas finales de los sistemas y la temperatura ambiental (21,25 ± 1,20 ºC y 21,81 ± 1,87 ºC respectivamente). La utilización de bajos FGF produjo temperaturas semejantes a las temperaturas observadas al final del estudio con flujos más elevados en los dos sistemas (A: 25,53 ± 4,78 ºC y 23,02 ± 0,80 ºC; B: 24,50 ± 0,85 ºC y 22,72 ± 2,36 ºC, respectivamente).
CONCLUSIONES: El sistema coaxial presentó variación térmica significativa entre el inicio y el final del experimento, lo que no fue observado en el sistema convencional. Mientras, no hubo diferencia de las temperaturas finales cuando comparados los dos sistemas entre sí, independientemente del FGF empleado.
Maintenance of inhaled gas temperature and humidity during anesthetic procedures is critical to prevent perioperative respiratory complications 1-9.
Aiming at solving this problem, thus decreasing mechanical ventilation-induced complications during general anesthesia, several methods have been proposed to warm and humidify inhaled gases, among them: adding warmed humidifiers 10, fresh gas flow (FGF) admission directly to soda lime reservoir 11, FGF decrease 12-14, placement of humidifiers inside soda lime reservoir 15, use of heat and humidity exchangers (artificial nose) 16-18 and coaxial system in the anesthesia machine 11.
The coaxial system is made up of a corrugated tube (through which inspired gas flows) enveloped by another tube (filled with expired gas), both ending in a common pathway connected to anesthetized patient's tracheal tube. This way, expiratory branch gas warms inspiratory branch gas, thus optimizing its acclimatization.
This study aimed at comparing the ability to warm up inhaled gases of coaxial breathing system and the conventional system, by varying fresh gas flows (FGF) on an experiment model.
After the Ethics Committee for Research Projects Analysis, Hospital das Clínicas, Faculdade de Medicina, Universidade de São Paulo approval, this experimental lab study was performed with two breathing systems tested in a lung simulator mechanically ventilated with 600 mL tidal volume and respiratory rate of 10 breaths/minute. An electronic ventilator with 1000 mL bellows was used and a reservoir bag was placed at the ventilator's excess gases outlet to prevent room air aspiration to the ventilator and the respiratory system. The model simulated CO2 production through 250 mL.min-1 carbon dioxide flow directly administered to the simulator bellows. In addition, exhaled gas from the test lung would go through a warmed humidifier to simulate humidity and temperature of patients' exhaled gases. Lab system assembly is shown in figure 1.
Both systems were classified as circle, valve circuits with CO2 absorption. In model A, the inspiratory branch was enveloped by the expiratory branch, whereas the conventional one (model B) presented separated respiratory branches (Figure 2).
Inhaled gas temperatures were measured at moments 0, 5, 10, 20, 30, 40, 50, 60 and 90 minutes, with low (0.5 and 1 L.min-1) and high (3 and 6 L.min-1) FGF. A sensor was placed in the inspiratory branch to measure inhaled gas temperature. Three essays were used for each system and FGF, in a total of 24 essays. Ventilator's bellows, reservoir's bag, corrugated tubes, soda lime, valves and connections were replaced after every essay to prevent the influence of water vapor built in the system.
Statistical analysis consisted of ANOVA test with repeated measures for two factors (comparison of final systems temperature and initial and environmental temperatures) and Friedman's test (comparison of systems' initial and final temperatures with different flows), considering significant p < 0.05.
Model A presented significant thermal variation between beginning and end of experiment as opposed to Model B. Both models A and B produced similar temperatures at the end of the study (Table I).
Both models presented significant differences between final and environmental temperatures, however with no difference between systems (Table II).
Inhaled gas temperature variation in moments 0, 5, 10, 20, 30, 40, 50, 60 and 90 minutes, according to FGF used in models A and B, is shown in figure 3.
Final temperature of both models were similar, even when grouped in low and high FGF (Table III).
Maintaining inhaled gases temperature in conditions close to patient's respiratory system is still an anesthesiologist's challenge during general anesthesia. In our study, the coaxial system has provided higher temperatures at 90 minutes as compared to initial temperatures of the same model. However, when coaxial system was compared to conventional system, both had the same behavior at the end of the study.
Similarly, FGF variation has not influenced test lung inhaled gas temperatures since both low (0.5 and 1 L.min-1) and high (3 and 6 L.min-1) FGF have led to similar temperatures at the end of the essays. These data are in disagreement with other studies which indicate significant temperature increase with CO2 absorption systems and low FGF 12,19. It might be that this difference in results is a consequence of the number of essays of our study (3 essays with each flow and system, in a total of 24 essays).
It is interesting to note that when low FGF (0.5 and 1 L.min-1) was used with Model A (coaxial), mean initial temperature was higher as compared to higher FGF (3 and 6 L.min-1). Although data are statistically similar, low FGF could lead to a rapid system warming.
We decided to group different FGF into low (0.5 and 1 L.min-1) and high (3 and 6 L.min-1). It is possible that significant differences could have been found if only 3 and 6 L.min-1 FGF were used.
The coaxial system has produced higher differences between inhaled gases at 90 minutes, and environmental temperature. Torres et al. 14 have shown that thermal insulation in the corrugated tubes of the respiratory system of the anesthesia machine increases absolute inhaled gas temperature and humidity, and inhaled gas temperature has major correlation with environmental temperature in conventional respiratory system with the same FGF used in our study. Torres et al. have used three aluminum foils with the brilliant face towards the interior of corrugated tubes. In our study, the coaxial system itself has worked as thermal insulation since expiratory branch enveloped the inspiratory branch.
In conclusion, coaxial system has presented significant thermal variation between beginning and end of experiment, what has not been observed with the conventional system. However, there were no final temperature differences when both systems were compared, regardless of FGF.
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Dr. Marcelo Luís Abramides Torres
Address: Av. Dr. Enéas de Carvalho Aguiar, 255 8º Andar
PAMB Divisão de Anestesia
ZIP: 05403-900 City: São Paulo, Brazil
Submitted for publication June 8, 2004
Accepted for publication October 26, 2004
* Received from Laboratório de Biofísica da Disciplina de Anestesiologia da Faculdade de Medicina da Universidade de São Paulo (FMUSP)