Fresh-gas flow sequence at the start of low-flow anesthesia: clinical application of Maplesons theoretical study

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 (F(adm)) set to 3 MAC, the end fractional expired also expressed as alveolar (F(E')=F(A)) may reach 1 MAC in few minutes, according to the solubility of the inhaled agent. The purpose of this study was to clinically apply.

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; F(adm) 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. F(adm) 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. F(adm) 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. F(adm) 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.

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.

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.