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

Abstract

OBJECTIVE\nTo determine the effect of fraction of inspired oxygen (FiO2) on intrapulmonary shunt fraction as measured by F-shunt in ponies during isoflurane anaesthesia.\n\n\nSTUDY DESIGN\nProspective, randomized clinical study.\n\n\nANIMALS\nA group of 23 adult Shetland ponies undergoing a total of 32 anaesthetic procedures.\n\n\nMETHODS\nPonies 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\xa0= 0.95) or a mixture of oxygen and medical air (group TL; FiO2\xa0= 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).\n\n\nRESULTS\nPaO2 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\xa0= 0.016) and T90 (TH: 11.7% ± 3.5 versus TL: 18.6% ± 9.5; p\xa0= 0.036).\n\n\nCONCLUSIONS AND CLINICAL RELEVANCE\nOur findings do not support a beneficial effect of using a reduced FiO2 to improve oxygenation in anaesthetized and mechanically ventilated Shetland ponies.