Gaseous homeostasis during low‐flow anaesthesia

Abstract

The object of this clinical study was to investigate the circle system gas homeostasis during low-flow anaesthesia using a technique designed to keep a constant inspired oxygen fraction of 0.30. Denitrogenation was adequately accomplished with mask preoxygenation, 10 l/min, for 1 min and an initial fresh gas flow of 5 l/min for 6 min after intubation. There was no need to wash out accumulated nitrogen at intervals, since the already low nitrogen concentration in the system tended to decrease after 1 h. The fresh gas flow of nitrous oxide to oxygen ratio and the inspiratory to end-expiratory oxygen concentration difference both reflected the uptake of nitrous oxide. The calculated rates of uptake of nitrous oxide, a subject of controversy, were in accordance with those found by Severinghaus and Barton & Nunn.

Brief review: Theory and practice of minimal fresh gas flow anesthesia

“…29 After a period of high-flow denitrogenation, \ 1 L of nitrogen remaining in the body will be slowly exhaled and mix and accumulate in the gas space of a closed breathing system. 30 Assuming a totally closed system, a patient's FRC of about 2 L, and a breathing system volume of 3.6 L, the theoretical maximal nitrogen concentration within the circle breathing system would be a magnitude of up to 18% (1 L:5.6 L). After a period of 6-8 min of denitrogenation, both Morita et al 31 and Bengtson et al 30 found that the average nitrogen concentration in the closed circuit increased from about 6% to a plateau of about 16%.…”

“…30 Assuming a totally closed system, a patient's FRC of about 2 L, and a breathing system volume of 3.6 L, the theoretical maximal nitrogen concentration within the circle breathing system would be a magnitude of up to 18% (1 L:5.6 L). After a period of 6-8 min of denitrogenation, both Morita et al 31 and Bengtson et al 30 found that the average nitrogen concentration in the closed circuit increased from about 6% to a plateau of about 16%. Dilution of gas in the circle system by nitrogen can be minimized by an effective denitrogenation of the gaseous compartment during induction.…”

Should nitrous oxide be discontinued before desflurane after anaesthesia with desflurane/N2O?

“…A total of 1.3 MAC was used for maintenance of anaesthesia. The fresh gas flow was 4.5 1/min during the first 6 min and then reduced to less than 1 l/min according to the Gothenburg low-flow concept (10). The inspired oxygen fraction was kept at 0.30 and E'C02 was kept between 4.5 and 5 ~01%.…”

“…End-tidal concentrations of desflurane, nitrous oxide, and carbon dioxide, and total MAC immediately before extubation, median values (25-75 percentiles). Group I : DesfluranelN20 anaesthesia with N 2 0 (E' 6065%)for10 rnin prior to emergence. Group 2: Desfluranel N 2 0 anaesthesia with desflurane (E' 4.6%)for 10 min prior to emergence.…”

Predictable nitrous oxide uptake enables simple oxygen uptake monitoring during low flow anaesthesia

“…Any gas leak from the closed circuit would result in overestimated oxygen uptake rates. A low flow technique with a constant inspired oxygen concentration, a small surplus of fresh gas supply as compared to uptake, and a clinically insignificant nitrogen accumulation, has been developed in this department [4]. If the uptake of nitrous oxide and the waste eas composition can be approximated for an anaesthetised patient, oxygen uptake rates could be calculated in a situation of known inspired oxygen and nitrous oxide concentrations and accurate flowmeters [5].…”