Objectives:
To discuss how hypoxaemia might be harmful and why the horse is particularly predisposed to developing it. To review the strategies that are used to manage hypoxaemia in anaesthetised horses, to describe how successful these strategies are and the adverse events associated with them.
Databases used:
Google Scholar and PubMed using the search terms – horse; pony; exercise; anaesthesia; hypoxaemia; oxygen; mortality; morbidity; ventilation perfusion mismatch.
Conclusions:
Although there is no evidence that hypoxaemia is associated with increased morbidity and mortality in anaesthetised horses, most anaesthetists would agree that it is important to recognise and prevent or treat it. The favourable anatomical and physiological adaptations of the horse for exercise, adversely affect gas exchange once the animal is recumbent. Hypoxaemia is recognised more frequently than in other domestic species during general anaesthesia, although its incidence in healthy horses remains unreported. The management of hypoxaemia in anaesthetised horses is challenging and often unsuccessful. Positive pressure ventilation strategies to address alveolar atelectasis in humans have been modified for implementation in the recumbent anaesthetised horse, but are often accompanied by unpredictable and unacceptable cardiopulmonary adverse effects, and some strategies are difficult or impossible to achieve in adult horses. Furthermore, the anticipated beneficial effects of these techniques are inconsistent. Increasing the inspired fraction of oxygen during anaesthesia is often unsuccessful since much of the impairment in gas exchange is a direct result of shunt. Alternative approaches to the problem involve the manipulation of pulmonary blood away from atelectatic regions of lung to better ventilated areas. However, further work is essential, with particular focus upon survival associated with general anaesthesia in the horse, before any technique can be accepted into widespread clinical use
