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Print version ISSN 0034-7094
Rev. Bras. Anestesiol. vol.52 no.2 Campinas Mar./Apr. 2002
Fresh-gas flow sequence at the start of low-flow anesthesia: clinical application of Maplesons theoretical study*
Secuencia de flujo de gas fresco para inicio de la anestesia con bajo flujo: aplicación clínica del estudio teórico de Mapleson
Marisa Miziara Jreige Borges, M.D.I; Renato Ângelo Saraiva, TSA, M.D.II
IAnestesiologista do Hospital SARAH
- Brasília, DF
IICoordenador de Anestesiologia da Rede SARAH de Hospitais
BACKGROUND AND OBJECTIVES:
In a theoretical study, Mapleson using a multicompartmental pharmacokinetic
model in a standard 40-year old and 70 kg man, has shown that with a fresh gas
flow (FGF) initially equal to total pulmonary minute ventilation and then decreased
to 1 L.min-1, and with fractional anesthetic administration (Fadm)
set to 3 MAC, the end fractional expired also expressed as alveolar (FE=FA)
may reach 1 MAC in few minutes, according to the solubility of the inhaled agent.
The purpose of this study was to clinically apply.
METHODS: Twenty-eight patients of both genders, aged 18 to 55 years, scheduled to undergo general anesthesia, were randomly divided in four groups of seven patients each according to the anesthetic drug to be used (halothane, isoflurane, sevoflurane and desflurane). Anesthesia was induced with intravenous propofol, fentanyl and vecuronium, and maintained with the inhalational agent diluted in oxygen under mechanical ventilation. Gas parameters were set, according to the agent as follows: Halothane group: initial FGF of 5 L.min-1 up to the 4th minute, followed by 2.5 L.min-1 up to the 10th minute and 1.5 L.min-1 up to the 20th minute; Fadm was 3 MAC during the first 20 minutes of anesthesia. Isoflurane group: initial FGF of 5 L.min-1 for 1.5 minute, followed by 1.5 L.min-1 up to the 7th minute and 1 L.min-1 up to the 20th minute. Fadm was 3 MAC up to the 7th minute and 2.5 MAC up to the 20th minute. Sevoflurane group: initial FGF of 5 L.min-1 for 1 minute and 1 L.min-1 up to the 20th minute. Fadm was 3 MAC for 1 minute, 2.5 MAC up to 7 minutes and 1.8 MAC up to the 20th minute. Desflurane group: initial FGF of 3,5 L.min-1 for 1 minute and 1 L.min-1 up to the 20th minute. Fadm was 3 MAC for 1 minute followed by 1.5 MAC up to the 10th minute and 1.2 MAC up to the 20th minute. In addition to routine monitoring of physiological (cardiovascular and respiratory) variables, FI and FE (FA) of the inhaled agents were measured.
RESULTS: Halothane group: FA reached 1.15 MAC in 2 minutes and varied from 1.21 to 1.47 MAC until the 20th minute. Isoflurane group: FA reached 1.03 MAC in 1 minute and varied from 1.11 to 1.21 MAC until the 20th minute. Sevoflurane group: FA reached 1.53 MAC in 1 minute and varied from 1.10 to 1.34 MAC until the 20th minute. Desflurane group: FA reached 0,94 MAC in 1 minute and varied from 1.07 to 1.14 MAC until the 20th minute.
CONCLUSIONS: Results obtained confirm the clinical feasibility of Maplesons theoretical model. This way, a fast FA increase of the inhaled agent was achieved, which reached 1 MAC in 1 to 2 minutes and was maintained within this value with minor variations and low anesthetic consumption.
Key words: ANESTHETICS, Volatile: desflurane, halothane, isoflurane, sevoflurane; ANESTHETIC TECHNIQUES, Inhalational: low-flow
JUSTIFICATIVA Y OBJETIVOS: En
estudio teórico, Mapleson utilizando un modelo farmacocinético multicompartimental,
con un hombre padrón de 40 años y 70 kg, demostró que, con flujo
de gas fresco (FGF) inicial igual a la ventilación pulmonar total, siendo
después reducido hasta 1 L.min-1 y concentración (Fraccional)
administrada del anestésico (Fadm) igual a 3 CAM, la
fraccional expirada final, también expresa como alveolar (FE=FA),
puede llegar a 1 CAM en pocos minutos, de acuerdo con la solubilidad del agente
inhalado. El objetivo del presente trabajo fue realizar la aplicación clínica
de este modelo teórico.
MÉTODO: Después de la aprobación por la comisión de Ética, fueron estudiados 28 pacientes de ambos sexos, con edad entre 18 y 55 años, sometidos a anestesia general, divididos aleatoriamente en 4 grupos de 7 pacientes de acuerdo con anestésico utilizado (halotano, isoflurano, sevoflurano y desflurano). La inducción fue venosa con propofol, fentanil y vecuronio. Y manutención con el agente inhalatorio diluido en oxígeno, bajo ventilación pulmonar mecánica. Los parámetros fueron los siguientes, de acuerdo con el agente utilizado: Grupo del halotano: FGF inicial de 5 L.min-1 hasta 4 minutos, seguido por 2,5 L.min-1 hasta 10 minutos y 1,5 L.min-1 hasta 20 minutos, Fadm igual a 3 CAM durante los 20 minutos iniciales de la anestesia. Grupo del isoflurano: el FGF inicial fue de 5 L.min-1 por 1,5 minuto, seguido por 1,5 L.min-1 hasta 7 minutos y 1 L.min-1 hasta 20 minutos. La Fadm fue de 3 CAM hasta 7 minutos y 2,5 CAM hasta el vigésimo minuto. Grupo del sevoflurano: el FGF inicial fue de 5 L.min-1 por 1 minuto y 1 L.min-1 hasta el vigésimo minuto y la F Fadm de 3 CAM por 1 minuto, después 2,5 CAM hasta 7 minutos y 1,8 CAM hasta 20 minutos. Grupo del desflurano: el FGF inicial fue de 3,5 L.min-1 por 1 minuto y 1 L.min-1 hasta completar los 20 minutos y la Fadm de 3 CAM por 1 minuto, seguido de 1,5 CAM hasta 10 minutos y 1,2 CAM hasta 20 minutos. Además de la monitorización rutinera de las variables fisiológicas (cardiovasculares e respiratorias) fueron medidas FI, y FE (FA) de los agentes utilizados.
RESULTADOS: Grupo del halotano: la FA llegó a 1,15 CAM en 2 minutos y varió de 1,21 a 1,47 CAM hasta 20 minutos. Grupo del isoflurano: la FA fue de 1,03 CAM en 1 minuto, variando de 1,11 a 1,21 CAM hasta 20 minutos. Grupo del sevoflurano: la FA de 1,53 CAM fue alcanzada en 1 minuto, variando de 1,10 a 1,34 CAM durante los 19 minutos restantes. Grupo del desflurano: la FA fue de 0.94 CAM en 1 minuto, variando de 1,07 hasta 1,14 CAM hasta el vigésimo minuto.
CONCLUSIONES: Los resultados obtenidos comprueban la aplicabilidad clínica del modelo teórico de Mapleson. De esta manera, se consiguió un rápido aumento de la FA del agente inhalatorio que llegó a 1 CAM, en 1 a 2 minutos, manteniéndose en este valor con pequeñas oscilaciones y bajo consumo de anestésico.
Low gas flow inhalational anesthesia was introduced by John Snow in the end of 19th Century aiming at decreasing consumption and preventing anesthetic pollution, being able to significantly decrease chloroform and ether odor in operating rooms 1. He created an experimental close circuit device in which patients would breath oxygen while exhaled CO2 was absorbed by potassium hydroxide. Snow himself performed the first tests. Ralph Watters, during the 20s, used a to-and-fro system with low flows 2 to reduce costs and increase cyclopropanes safety margin, which is an excellent through high-cost gaseous anesthetic, flammable and explosive even in low concentrations 3. This technique was later reintroduced by Harry Lowe with very low flows, called basal flows, in a quantitative anesthesia method 4.
Drug administration is based on dosing. In inhalational anesthesia, doses are measured by fractions or multiples of Minimum Alveolar Concentration (MAC) where 1 MAC might be the initial target for inducing anesthesia. In the quantitative method, the dose expressed in vapor volume is calculated from MAC values, agents solubility, alveolar ventilation and cardiac output. This volume is then converted into volume of anesthetic liquid to be administered.
Although very efficient, this method is seldom used for involving several calculations. This is why anesthesiologists in clinical practice need a simple method to be safely used with the available equipment and with no risk of calculation mistakes. This way, simply adjusting flow and administered concentration in the vaporizer, according to time, it would be possible to use a fresh gas flow low enough to meet the concerns of consumption and surgery room environmental pollution.
Mapleson 5 has shown in a 20-minute anesthesia simulation that, with an initial fresh gas flow equal to total minute ventilation and with an anesthetic concentration or fractional (Fadm) of 3 MAC, end expired fractional (FE) is close to 1 MAC in 1 minute with desflurane and sevoflurane, 1.5 minute with isoflurane, 2.5 minutes with enflurane and 4 minutes with halothane.
The purpose of this study was to clinically apply Maplesons theoretical study, which proposes a fast end expired fractional (alveolar) increase of the inhalational agent to reach 1 MAC, and to maintain it constant there after, with minimum anesthetic consumption.
After the Ethical Committee approval, 28 patients of both genders, aged 18 to 55 years, physical status ASA I and II, scheduled for surgical procedures under general anesthesia with mechanical ventilation were included in this study.
Patients were randomly distributed in four groups of seven patients, according to the anesthetic agent used: halothane, isoflurane, sevoflurane and desflurane.
Patients were premedicated with oral midazolam (15 mg). Monitoring consisted of cardioscope, non-invasive BP, pulse oximetry, temperature and capnometry. Anesthetics inspired and end expired (alveolar) fractionals were measured by a gas analyzer built in an Ohmeda monitor. After venoclysis, patients and ventilation system were pre-oxygenated for 3 minutes with 100% O2. Anesthesia was then induced with propofol (2.5 to 3 mg.kg-1), fentanyl (1 µg.kg-1) and vecuronium (0.1 mg.kg-1), and maintained with a single inhalational agent diluted in oxygen.
Anesthetics fresh gas flow and agents fractional sequences according to Maplesons model, are shown in table I. The study was started with open vaporizers for the administration of concentration equivalent to 3 MAC of the anesthetic agents. A chart was used to convert anesthetics concentrations into MAC, according to age 6. It was adjusted for an altitude of 1000 meters and an atmospheric pressure of 700 mmHg (Figure 1). In the halothane group, with an FGF of 5 L.min-1 and Fadm equivalent to 3 MAC up to 4 minutes, inspired fractional was increased in the first minute and was maintained slightly below 3 MAC. After the 4th minute, flow was decreased to 2.5 L.min-1 and after the 10th minute to 1.5 L.min-1 until the 20th minute. Fadm was maintained in 3 MAC. The isoflurane group received an initial fresh gas flow of 5 L.min-1 for 1.5 minutes which was decreased to 1.5 L.min-1 until the 7th minute and to 1 L.min-1 until the 20th minute. Fadm was 3 MAC for 7 minutes and 2.5 MAC until the 20th minute. The sevoflurane group received an initial fresh gas flow of 5 L.min-1 and Fadm was of 3 MAC in the first minute. Then, the flow was reduced to 1 L.min-1 until the 20th minute and Fadm was decreased to 2.5 MAC until the 7th minute and to 1.8 MAC until the end. Initial fresh gas flow for the desflurane group was 3.5 L.min-1 and Fadm was of 3 MAC in the first minute. Then, the flow was reduced to 1 L.min-1 until the 20th minute and Fadm was decreased to 1.5 MAC until the 10th minute and to 1.2 MAC until the end.
Patients were maintained in mechanical ventilation with respiratory rate (RR) of 10 cycles per minute (c.p.m.) and tidal volume (Vt) of 10 ml.kg-1 (100 ml.kg-1.min-1).
Data were collected during the first 20 minutes of anesthesia and liquid anesthetic volume consumption was measured for all anesthetic agents, except for desflurane due to technical problems (extremely volatile anesthetic, vaporizer and bottle with valves). For desflurane, the volume was calculated by the formula:
V = Concentration x flow x time
D x 2.24 x (273 + t)
MW x 273
(Concentration in %/100; flow in ml/min; time in minutes; V = volume; D= density; MW = molecular weight; and t = temperature in ºC) 4.
Statistical analysis was performed with analysis of variance using F test and considering significant a = 0.05 (5%).
There were no statistical differences among groups in weight and age (Table II). Anesthetic consumption was obtained by volume measurements in 20 minutes (Table III) and can be considered low compared to the literature reports 7.
Regarding hemodynamic variables, mean blood pressure (Figure 2) varied from 98 to 63 mmHg. There was one case of a 22.56% pressure decrease after 9 minutes in the halothane group (BP 69/35 mmHg) which required 10 mg ephedrine, followed by an improvement (BP 83/41 mmHg). Heart rate (Figure 3) varied from 98 to 68 bpm. In 1 sevoflurane group patient there was a 25.39% decrease after 17 minutes but without clinical relevance. Regarding ventilatory variables, end expired CO2 fractional (Figure 4) varied from 35 to 30 mmHg, also without clinical relevance.
In the halothane group, FA/MAC ratio equal to 1, that is, 1 MAC, was reached in 2 minutes, varying from 1.21 to 1.47 MAC during the first 20 minutes, which may be considered normal for a clinically stable anesthesia, considering that Anesthetic Dose 95 (AD95) is approximately 1.3 MAC 8 (Figure 5).
In the isoflurane group, FA/MAC ratio equal to 1 was reached in 1 minute and varied from 1.11 to 1.21 MAC during the 20 minutes (Figure 6).
In the sevoflurane group, FA/MAC ratio was 1.53 in one minute and FA varied from 1.10 to 1.34 MAC for 20 minutes (Figure 7).
In the desflurane group, FA/MAC ratio was equal to 0.94 in 1 minute and FA varied from 1.07 to 1.14 MAC (Figure 8) for 20 minutes.
Using Maplesons theoretical study, fresh gas flow and anesthetic concentrations (fractional) were evaluated in an attempt to improve their use during the first minutes of anesthesia in order to obtain a fast induction with a minimum volatile anesthetics consumption 5. This rapid induction requires high fresh gas flow (FGF) and high anesthetics fractional (Fadm) to obtain a rapid kinetic balance between this fractional and the inspired fractional (FI) and then, through efficient alveolar ventilation, to bring alveolar fractional (FA) closer to FI.
This theoretical model is able to simulate volatile anesthetics uptake (absorption) and distribution through data obtained from physiological variables: ventilation, dead space, cardiac output and organs blood perfusion, based on a standard 40 years old man weighing 70 kg 9. Values of anesthesia machine components volume and of agents solubility, expressed by blood/gas and tissue/blood partition coefficient of each organ, were obtained from the literature 10,11.
In the model, the respiratory system has three compartments, as follows: the anesthesia machine (including mechanical dead space), anatomic dead space and pulmonary alveolus air. Circulation and tissues have multiple compartments and are laid in series, according to blood perfusion: brain, viscera, muscles and fat tissue. They are connected to arterial blood perfusion which brings the anesthetic drug from the alveolus to the organs, and to venous system, which takes the anesthetic drug back to the alveolus.
Simulation is performed by imagining the anesthetic drug being taken as a vapor from the vaporizer to the respiratory system by the fresh gas flow, and from this to the pulmonary alveolus through alveolar ventilation, being then removed by the blood and distributed to the organs according to blood perfusion and solubility. During this trip, the anesthetic drug suffers the effect of dilution in the anesthesia machine and the alveolus. Alveolar ventilation is the major factor determining the kinetic balance between alveolar and inspired fractional.
The alveolar anesthetic uptake by the blood to distribute it throughout the body is accounted for in this model. Since brain perfusion is very high, the brain-alveolar balance is promptly reached. Hence, alveolar anesthetic concentration, that is, end expired fractional, expresses brain fractional.
When starting inhalational anesthetics administration, there is a great difference between administered fractional, which immediately leaves the vaporizer, and the inspired fractional, at patients mouth level after going through the inhalational system of the anesthesia machine, as well as between it and alveolar fractional which of course, is still for below MAC of the agent being used. These differences are explained by the lack of anesthetics in the anesthesia machine inhalation system and in the alveolar air at this point in time.
FA equal or close to MAC is the first goal to be met during inhalational anesthesia. For such, a very high anesthetic offering is needed for the inhalation system through fresh gas flow and from it to the pulmonary alveolus through alveolar ventilation.
Considering this pharmacokinetic concept, Maplesons theoretical model accounts for obtaining 1 MAC during the first minutes, depending on agents solubility. This is why fresh gas flow is high, equal to pulmonary minute ventilation, and Fadm is equal to 3 MAC. The idea was also to keep alveolar ventilation within normal standards to maintain patients in normocapnia or mild hypocapnia and take to the alveolus the volume of anesthetics needed for MAC to be promptly reached.
Using FGF and Fadm values in the moments scheduled by the method and described on the results, FA/MAC ratios were obtained and their peaks varied from 0.94 to 1.5 (mean of 1.2). These results were very close to Maplesons study, in which the target was an FA/MAC ratio of 1 to be obtained during the first minutes and then maintained constant.
Halothane, a more soluble agent, reached MAC in 2 minutes. The other anesthetics (isoflurane, sevoflurane and desflurane) reached MAC in 1 minute. Since they are less soluble agents, the balance between alveolar partial pressure and inspired partial pressure is faster.
The difference between our results and those of Maplesons theoretical study was certainly due to the fact that our sample was not standardized (40 years old and 70 kg men). Patients were ordinary people found in our daily practice. Even so, it must be stressed that differences between results were minor and without statistical significance.
Drug consumption during the first minutes of anesthesia was high due to the high fresh gas flow. When the flow was decreased, obviously consumption also decreased. Regardless of this higher volume used in the beginning of anesthesia, mean anesthetic volume consumption during 20 minutes was low.
It is important to stress that, to effectively and safely use this method, it is advisable to have available an anesthetic gas monitor to continuously measure end expired fractionals of such agents, in addition to a complete anesthesia machine with standard monitoring, such as pulse oximetry and capnography. Currently, this equipment should be part of the anesthesiologist routine whenever possible.
Our results have confirmed the clinical feasibility of Maplesons theoretical model. A fast increase in inhalational agent end expired fractional (alveolar) was obtained, which reached 1 MAC already during the first minute, being then maintained constant. Anesthetic consumption may be considered among the lowest ever described.
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Submitted for publication May 23, 2001
Accepted for publication October 09, 2001