Effect of 50% and maximal inspired oxygen concentrations on respiratory variables in isoflurane-anesthetized horses

Abstract

BackgroundThe purpose of this study was to compare the effects of 0.5 fraction of inspired oxygen (FiO2) and >0.95 FiO2 on pulmonary gas exchange, shunt fraction and oxygen delivery (DO2) in dorsally recumbent horses during inhalant anesthesia. The use of 0.5 FiO2 has the potential to reduce absorption atelectasis (compared to maximal FiO2) and augment alveolar oxygen (O2) tensions (compared to ambient air) thereby improving gas exchange and DO2. Our hypothesis was that 0.5 FiO2 would reduce ventilation-perfusion mismatching and increase the fraction of pulmonary blood flow that is oxygenated, thus improving arterial oxygen content and DO2.ResultsArterial partial pressures of O2 were significantly higher than preanesthetic levels at all times during anesthesia in the >0.95 FiO2 group. Arterial partial pressures of O2 did not change from preanesthetic levels in the 0.5 FiO2 group but were significantly lower than in the >0.95 FiO2 group from 15 to 90 min of anesthesia. Alveolar to arterial O2 tension difference was increased significantly in both groups during anesthesia compared to preanesthetic values. The alveolar to arterial O2 tension difference was significantly higher at all times in the >0.95 FiO2 group compared to the 0.5 FiO2 group. Oxygen delivery did not change from preanesthetic values in either group during anesthesia but was significantly lower than preanesthetic values 10 min after anesthesia in the 0.5 FiO2 group. Shunt fraction increased in both groups during anesthesia attaining statistical significance at varying times. Shunt fraction was significantly increased in both groups 10 min after anesthesia but was not different between groups. Alveolar dead space ventilation increased after 3 hr of anesthesia in both groups.ConclusionsReducing FiO2 did not change alveolar dead space ventilation or shunt fraction in dorsally recumbent, mechanically ventilated horses during 3 hr of isoflurane anesthesia. Reducing FiO2 in dorsally recumbent isoflurane anesthetized horses does not improve oxygenation or oxygen delivery.

Impact of low inspired oxygen fraction on oxygenation in clinical horses under general anesthesia

“…In the present study, the delivered volume could not be appropriately reported but it was not allowed to decrease below 10 mL/kg. This is in agreement with previous studies where VT close to 15 mL/kg were maintained with PIP between 22 (Moens et al 1998) and 30 cmH 2 O (Hubbell et al 2011). In humans under general anesthesia, VT as low as 6 mL/kg did not induce more lung atelectasis than traditional VT (Cai et al 2007), but high pressures and large volumes induced more lung injury (Hong et al 2010).…”

“…Similar findings were reported in a retrospective study comparing 0.8 and 0.6 FIO 2 (Schauvliege et al 2015). Another trial exposing 5 mechanically ventilated horses to 0.5 FIO 2 reported no effect of FIO 2 in mechanically ventilated horses placed in dorsal recumbency (Hubbell et al 2011). In horses anesthetized with isoflurane (Crumley et al 2013) or intravenous anesthetics (Karrasch et al 2015) and breathing spontaneously in dorsal recumbency, the use of 0.5 FIO 2 did not improve the pulmonary gas exchange compared to maximal oxygen concentrations.…”

Effect of low inspired oxygen fraction on respiratory indices in mechanically ventilated horses anaesthetised with isoflurane and medetomidine constant rate infusion

“…Horses anaesthetised with 70 isoflurane in low FiO 2 (0.6) had significantly lower PaO 2 and lower P(A-a)O 2 , but similar 71 PaO 2 :FiO 2 ratios and similar numbers of hypoxaemic animals, when compared to horses 72 anaesthetised with isoflurane in a higher FiO 2 (0.78) (Schauvliege et al, 2015). In two additional 73 studies using oxygen/air mixtures, there was no benefit in using a FiO 2 of 0.5, with no improvement 74 in oxygen delivery and significant hypoxaemia (Hubbell et al, 2011;Crumley et al, 2013). 75…”

Evaluation of intranasal oxygen supplementation in mules anesthetized with the combination of ketamine, butorphanol, and guaifenesin

“…Shunt may be due to areas of atelectasis, i.e., blood flow in non-ventilated alveoli. Atelectasis can occur in anesthetized horses due to high fraction of inspired oxygen or simply to animal recumbency (Hubbel and Muir, 2015;Hubbell et al, 2011). When the animal is breathing 100% O 2 , there will only be oxygen in the alveoli, and as it is easily diffused to the blood, the alveoli tends to collapse, causing absorption atelectasis.…”

“…This was the probable cause of the decrease in PaO 2 in the TG along of anesthesia. Hubbell et al (2011), suggested reducing FiO 2 to 0.5 in order to reduce the occurrence of shunt caused by the absorption atelectasis, however, PaO 2 decreased without a significant improvement in shunt fraction or alveolar dead space ventilation.…”