Post operative complications in horses: causes and treatment

Comparative anaesthetic morbidity and mortality rate suggests that horses are 20 times more likely to suffer an anaesthesiaassociated fatality than dogs and cats (>1 in 100 vs. 1 in >2000) and 5–8,000 times more likely to die from anaesthesia than humans (1 in 100 vs. 1 in 500–800,000). Fatality rates are even higher if the horse presents for anaesthesia as an emergency (1 in 6) or for colic (1 in 3). The most common causes for mortality are post anaesthetic fractures and myopathies or neuropathies requiring euthanasia within 7 days of the anaesthetic event and cardiac collapse or arrest. Although mortality rates are reported to be more favourable (lower mortality) when equine anaesthesia is performed at a private specialty surgery practice or academic institution, rates as high as 1 in 1000 are still reported.
Complications and death caused by or associated with equine anaesthesia can be categorised into these major categories: species dependent, drug induced, equipment and facility linked, surgery related or human error. Most if not all drugs used to produce anaesthesia in horses have the potential to produce unwanted side effects including death.
Bolus injections of injectable anaesthetic drugs can induce marked hypoventilation and apnoea in horses early during the course of anaesthesia. Drug induced decreases in respiratory rate or volume (tidal or minute volume) produce hypoxaemia (low PaO2 ) and hypercarbia (high PCO2 ) leading to tissue acidosis (lactic acidosis, respiratory acidosis). Hypoventilation combined with decreases in pulmonary blood flow (low cardiac output) worsen ventilation– perfusion mismatching compromising arterial PO2 values and tissue oxygenation. Ventilation–perfusion mismatching is exaggerated by both compression and absorption atelectasis in laterally recumbent and especially in large, supine anaesthetised horses. […]