Closed‐Circuit Liquid Injection Isoflurane Anesthesia in the Horse

Abstract

Six horses were administered isoflurane anesthesia by liquid injection into a closed breathing circuit according to the square root of time model. The unit dose (UD) was calculated using Lowe's formula to provide an end-tidal concentration of 1.3%, or the minimum alveolar concentration of isoflurane. The mean UD was 4.2 +/- 0.2 mL. The mean end-tidal isoflurane concentration (ETiso) for each interval after injection, and the peak and minimum concentrations for each injection interval, did not change beginning with the second injection, indicating that the square root of time model accurately predicted isoflurane uptake in the horse. Mean ETiso measured for the interval after the first injection was 0.68 +/- 0.06%, which was significantly (p < .05) lower than the mean concentration after all subsequent injections (1.1 +/- 0.1%). Mean peak end-tidal concentration was 1.1 +/- 0.25% after the first injection and 1.7 +/- 0.26% for all other injections. Mean minimum end-tidal concentration was 0.77 +/- 0.13% for all injection periods. This model proved to be an acceptable technique for administration of isoflurane anesthesia to horses.

Closed system anaesthesia in dogs using liquid sevoflurane injection; evaluation of the square-root-of-time model and the influence of CO2 absorbent

“…This suggests that the intervals between injections proposed by Lowe were over‐long. The results contrast with the situation in horses in which the square‐root‐of‐time model evaluated by liquid injection of isoflurane produced constant end‐tidal concentrations for 100 minutes (Olson et al. 1993).…”

“…Some of these authors found the square‐root‐of‐time model accurately predicted anaesthetic uptake while others judged it to be an inadequate or potentially harmful means of achieving closed system anaesthesia, finding alveolar anaesthetic concentrations to differ markedly from predicted values. The square‐root‐of‐time model has been evaluated in dogs with halothane and isoflurane (Moens 1988) and in horses with isoflurane (Olson et al. 1993) and is considered useful for performing closed circuit anaesthesia.…”

Clinical evaluation of an end-tidal target-controlled infusion closed-loop system for isoflurane administration in horses undergoing surgical procedures

“…As is shown by the high correlation coefficient between the amount of Iso used to produce 1 hour of anaesthesia and horse weight, the ETCLo-TCI system produces quantitative volatile agent anaesthesia. Although quantitative anaesthesia is often associated, as shown in the literature, with the use of a closed-circuit breathing system [6], it has not previously been demonstrated for an ETCLo-TCI system. One advantage of the tested delivery system is that it is able to adapt the anaesthetic administered to the FGF set by the anaesthetist, because the FGF is used in Equation (2) to calculate the amount of Iso that has to be delivered One advantage of the tested delivery system is that it is able to adapt the anaesthetic administered to the FGF set by the anaesthetist as per Equation (2).…”

“…In order to avoid a potentially serious cardiovascular consequence, as previously reported [6], the loading dose was administered over 2 minutes. This can explain why there was no significant difference in the time taken by both systems to reach the FE'Iso target.…”

“…Injecting a volatile agent, in its liquid form, This conflict is resolved when the volatile anaesthetic agent is administered by injecting it directly into the respiratory circle system [3]. However, though satisfactory results were obtained when quantitative anaesthesia was attempted by manual injection of a volatile liquid anaesthetic into a closed breathing circuit according to the square root of the time, there was a failure to predict the volatile anaesthetic uptake and cardiovascular depression [4][5][6][7][8].…”