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

A new volatile anaesthetic agent delivery system was tested in 15 horses undergoing scheduled surgical procedures. The delivery system consisted of a laptop computer (with dedicated software), a computer-controlled syringe driver (loaded with liquid isoflurane) connected to the inspiratory arm of a large-animal circle breathing system and a respiratory gas monitor, providing isoflurane end-tidal concentrations (ETmeasured) every 20 s to the computer. Following induction and connection to the breathing system, mechanical ventilation was started. The bodyweight (BW), fresh gas flow, breathing system and ventilator volume, and end-tidal isoflurane target (ETtarget) were entered into the computer. Using Lowe’s equation, the software calculated the prime dose to be delivered by the syringe driver over 2 min. After this, the system delivered each minute the amount of isoflurane as determined by the following equation: Isoflurane (mL) = {2 × λB/G × (200 × BW0.75) × (ETtarget − ETmeasured) + (fresh gas flow − (BW0.75 × 0.07)) × (ETmeasured)}/206. A fresh gas flow of 4 L oxygen min−1 was administered until the inspired fraction of oxygen reached 0.7, and was then decreased. A target of 1.5% end-tidal isoflurane was initially used and subsequently adjusted to the clinical requirements. The system performance was evaluated using the median prediction error (MDPE) and the median absolute performance error (MDAPE), which were −3.6% and 5.29%, respectively. It was concluded that this system was useful to achieve end-tidal target-controlled infusion of isoflurane during equine anaesthesia.