Effect of different inspired fractions of oxygen on F-shunt and arterial partial pressure of oxygen in isoflurane-anaesthetized and mechanically ventilated Shetland ponies

Vet Anaesth Analg. 2021 Nov;48(6):930-934. doi: 10.1016/j.vaa.2021.05.005. Epub 2021 Aug 13.

Abstract

Objective: To determine the effect of fraction of inspired oxygen (FiO2) on intrapulmonary shunt fraction as measured by F-shunt in ponies during isoflurane anaesthesia.

Study design: Prospective, randomized clinical study.

Animals: A group of 23 adult Shetland ponies undergoing a total of 32 anaesthetic procedures.

Methods: Ponies were premedicated intravenously (IV) with detomidine (0.01 mg kg-1) and either morphine (0.1 mg kg-1) or butorphanol (0.02 mg kg-1). Anaesthesia was induced with ketamine (2.2 mg kg-1) and midazolam (0.07 mg kg-1) administered IV. Ponies were randomly allocated to maintenance of anaesthesia with isoflurane in oxygen (group TH; FiO2 = 0.95) or a mixture of oxygen and medical air (group TL; FiO2 = 0.65); all ponies were given a constant rate of infusion of detomidine. Animals were mechanically ventilated to maintain PaCO2 between 40 and 50 mmHg. Arterial blood gas analysis was performed every 30 minutes. The F-shunt equation was calculated for each time point T0, T30, T60 and T90. Data were analysed using linear mixed model analysis and presented as mean ± standard deviation (p < 0.05).

Results: PaO2 was greater in group TH than in group TL (TH: 406 ± 90, 438 ± 83, 441 ± 69 and 464 ± 53 mmHg versus TL: 202 ± 90, 186 ± 84, 172 ± 85 and 191 ± 98 mmHg at T0, T30, T60 and T90, respectively; p < 0.0001). In TH, F-shunt was < TL. Significant differences were found at T60 (TH: 13.2% ± 4.3 versus TL: 19.4% ± 8.3; p = 0.016) and T90 (TH: 11.7% ± 3.5 versus TL: 18.6% ± 9.5; p = 0.036).

Conclusions and clinical relevance: Our findings do not support a beneficial effect of using a reduced FiO2 to improve oxygenation in anaesthetized and mechanically ventilated Shetland ponies.

Keywords: absorption; atelectasis; equine; inspired oxygen fraction; pony.

Publication types

  • Randomized Controlled Trial, Veterinary

MeSH terms

  • Animals
  • Horses*
  • Isoflurane*
  • Oxygen
  • Partial Pressure
  • Prospective Studies
  • Respiration, Artificial / veterinary

Substances

  • Isoflurane
  • Oxygen