Suspected vagal reflex and hyperkalaemia inducing asystole in an anaesthetised horse.
Authors: Ryan Aoife, Gurney Matthew, Steinbacher Roswitha
Journal: Equine veterinary journal
Summary
# Editorial Summary: Vagal Reflex and Hyperkalaemia-Induced Asystole in an Anaesthetised Horse A 10-year-old mustang gelding undergoing surgical repair of nasal, frontal and lacrimal bone fractures experienced intraoperative asystole 137 minutes into general anaesthesia, prompting investigation into the underlying mechanisms. Perianaesthetic monitoring revealed progressive hyperkalaemia (peak potassium 6.4 mmol/L; reference range 3.5–5.1 mmol/L) alongside bradycardia, though characteristic electrocardiographic changes associated with severe hyperkalaemia—flattened P waves and prolonged QRS complexes—were notably absent. Whilst elevated potassium levels were documented, the authors conclude these were unlikely to be the primary cause of asystole based on their review of relevant literature; instead, they propose that a trigeminocardiac reflex (potentially a previously undescribed maxillomandibulocardiac reflex in equines) triggered by surgical manipulation during maxillary repair was the dominant causative factor, with hyperkalaemia acting as a secondary contributor. This case highlights an important gap in equine anaesthetic knowledge and suggests that farriers, vets and anaesthetists should remain vigilant for vagal reflexes during oro-maxillary procedures, particularly when bradycardia develops without typical hyperkalaemic ECG signatures. Regional anaesthetic techniques and careful surgical technique around sensitive facial structures warrant careful consideration during equine oro-maxillofacial surgery to minimise reflex-mediated cardiovascular complications.
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Practical Takeaways
- •When performing maxillary regional anaesthetic blocks in horses, be alert to the possibility of vagal reflexes triggering cardiac arrhythmias, particularly asystole, even when ECG changes typical of hyperkalaemia are absent
- •Perianaesthetic hyperkalaemia may be a contributing rather than primary cause of intraoperative asystole in horses; consider reflex mechanisms as differential diagnoses
- •Close cardiac monitoring during facial surgery under general anaesthesia is essential, as vagal stimulation from surgical manipulation of trigeminal nerve territory may precipitate life-threatening arrhythmias
Key Findings
- •A 10-year-old mustang gelding developed progressive hyperkalaemia (6.4 mmol/L) during general anaesthesia for facial bone fracture repair
- •Asystole occurred 137 minutes after induction despite hyperkalaemia levels unlikely to be the primary cause based on literature review
- •A trigeminocardiac reflex (potentially a maxillomandibulocardiac reflex) is hypothesised as the primary contributory factor to asystole
- •Classic ECG changes of severe hyperkalaemia (flattened P waves, tall T waves, prolonged QRS) were not documented prior to asystole