Influence on horse´s pulmonary function using a modified "Open-Lung-Concept"-Ventilation with different oxygen-concentration during general anaesthesia

Abstract

The aim of this study was the evaluation of the influence of three different concentrations of inspired oxygen on the pulmonary function of horses in dorsal recumbency during general anaesthesia, and the recovery period. Thirty-six warmblood horses weighing at least 400 kg were included in the study. All horses were presented to the clinic for colic symptoms and underwent an emergency surgical procedure in dorsal recumbency. The horses were randomly allocated to one of three groups. Group A(35%) was ventilated with an inspired oxygen fraction (FiO(2)) of 35%, group B-55% with 55% and group C-90% with 90% using the open-lung-concept. Premedication (0.04-0.08 mg/kg romifidine i.v.), induction (0.05 mg/kg diazepam i.v. and 2.2 mg/kg ketamine i.v.), maintenance (inhalation: isoflurane) and hemodynamic pharmacological support (dobutomine and intravenous fluids) were identical in all horses. Arterial partial oxygen pressure (PaO2), arterial partial carbon dioxide pressure (PaCO2), arterial pH, PaO2/FiO(2) ratio, heart rate, arterial blood pressure, respiratory rate, end-expiratory rate of carbon dioxide (EtCO2), alveolar dead space, as well as inspiratory and end-expiratory concentration of isoflurane were analysed. Intro- and postoperatively arterial blood gases and acid-base status were measured. The quality and duration of the anaesthetic recovery period was scored. Included were the number of attempts to rise, time to first movement, time to sternal recumbency, time to standing position and the quality of recovery. The PaO2 arterial oxygenation was significantly higher in horses of group C-90% than in horses of group A(35%) or B-55%. However, there was no significant difference between the oxygenation of horses in group A(35%) or B-55%. In the intro- and postoperative period, the PaO2/FiO(2) ratio of group C-90% was higher than those of group A(35%) and B-55%, with group B-55% having even lower values than group A(35%). At least one measurement during the anaesthetic period revealed arterial hypoxemia (PaO2 <60 mmHg) in 33% of the horses in group A(35%), 50% of horses in group B-55% and only in 8% of horses in group C-90%. Arterial hypoxemio was observed particularly at the beginning of anaesthesia and mainly in horses with large intestinal distension. In the postanaesthetic period the PaO2 was highest in horses of group C-90%, followed by those of group A(35%). Stats There was no significant difference in timing of events during the recovery period, except for an earlier regain of the swallowing reflex of horses in group B-55% compared to horses in group A(35%). The number of attempts to rise, the over-all manner as well as the balance and coordination during the recovery period was equal in all groups. In conclusion, using lower concentrations of oxygen for modified open-lung-concept-ventilation during anaesthesia is possible, even in the critically ill equine patient. There was no improvement of the ventilation-perfusion mismatch when using lower oxygen concentrations. In horses with large intestinal distension, decreased concentrations of FiO(2) are accompanied with a high risk of hypoxemia. The oxygen concentration did not influence the duration or quality of the recovery period.