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Equine veterinary journal2021; 54(5); 927-933; doi: 10.1111/evj.13535

Suspected vagal reflex and hyperkalaemia inducing asystole in an anaesthetised horse.

Abstract: A 10-year-old 466 kg mustang gelding presented to an equine referral hospital for surgical repair of nasal, frontal and lacrimal bone fractures from an unknown trauma. Surgical repair was performed under general anaesthesia, including a right-sided maxillary regional anaesthetic block with mepivacaine hydrochloride. Progressive hyperkalaemia was documented perianaesthetically (T-3 mins; 134 mins after induction; potassium 6.4 mmol/L (ref 3.5-5.1 mmol/L). Perianaesthetic bradycardia was attributed to alpha -2 agonist infusion administration, and other characteristic ECG changes (flattened P waves, narrow T waves of increased amplitude, prolonged QRS complex) were not documented. Asystole occurred 137 min after induction of anaesthesia; however, a review of the available literature suggests the degree of hyperkalaemia documented was unlikely to be the primary cause of asystole but may have been a contributing factor. It is hypothesised that a trigeminocardiac reflex was the primary contributory factor to asystole in the described case, and may represent a maxillomandibulocardiac reflex that has not been previously described in the horse.
© 2021 The Authors. Equine Veterinary Journal published by John Wiley & Sons Ltd on behalf of EVJ Ltd.
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Publication Date: 2021-11-19 PubMed ID: 34738246PubMed Central: PMC9543672DOI: 10.1111/evj.13535Google Scholar: Lookup
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Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

This research article investigates an unusual case of a horse experiencing heart stoppage (asystole) during and after anesthesia for surgery. The researchers propose that an uncommon type of reflex action involving the nerves of the face and heart, coupled with an abnormally high potassium level in the blood, may have been the cause.

Case Description

  • The researchers present a case of a 10-year-old mustang horse that was admitted to a veterinary hospital for surgical repair of fractures in the nasal, frontal, and lacrimal bones. The manner in which the horse acquired these fractures is not known.
  • The surgical procedure was carried out while the horse was under general anesthesia, which included a specific regional anaesthetic block (using a drug called mepivacaine hydrochloride) for the maxillary area on the right side of the horse’s face.

Potassium Levels and Bradycardia

  • Throughout the anesthesia phase, the horse showed a gradual increase in blood potassium levels well above the normal range, a condition known as hyperkalemia.
  • The horse also developed an abnormally slow heart rate (bradycardia), which the researchers initially attributed to the administration of an alpha-2 agonist infusion – a type of medication often used during anesthesia to sedate and decrease stress in animals.
  • However, the heart’s electrical activity (as seen on an ECG) did not show the typical alterations one would expect in hyperkalemia, such as specific changes in the P waves, T waves, and QRS complex.

Cardiac Arrest (Asystole)

  • About 137 minutes after the induction of anesthesia, the horse’s heart stopped (asystole).
  • Although hyperkalemia can have serious effects on the heart, the researchers pointed out that, based on their review of the relevant literature, the level of hyperkalemia the horse experienced was unlikely to be the primary trigger for asystole. Instead, it may have contributed to the occurrence of the asystole in some way.

Possible Involvement of Trigeminal Reflex

  • The central hypothesis from the researchers is the involvement of a reflex action connecting the nerves of the face (the trigeminal nerve area) with the heart, known as the trigeminocardiac reflex. They propose this as the key reason for the asystole in the horse.
  • The authors suggest this could potentially represent a specific type of reflex, which they denote as a “maxillomandibulocardiac reflex”, something not previously reported in horses.

Cite This Article

APA
Ryan A, Gurney M, Steinbacher R. (2021). Suspected vagal reflex and hyperkalaemia inducing asystole in an anaesthetised horse. Equine Vet J, 54(5), 927-933. https://doi.org/10.1111/evj.13535

Publication

Equine veterinary journal
ISSN: 2042-3306
NlmUniqueID: 0173320
Country: United States
Language: English
Volume: 54
Issue: 5
Pages: 927-933

Researcher Affiliations

Ryan, Aoife
  • University of Veterinary Medicine, Vienna, Austria.
  • Anderson Moores Veterinary Specialists, Winchester, UK.
Gurney, Matthew
  • Anderson Moores Veterinary Specialists, Winchester, UK.
Steinbacher, Roswitha
  • University of Veterinary Medicine, Vienna, Austria.

MeSH Terms

  • Animals
  • Arrhythmias, Cardiac / veterinary
  • Blood Pressure
  • Heart Arrest / chemically induced
  • Heart Arrest / veterinary
  • Horse Diseases / chemically induced
  • Horses
  • Hyperkalemia / complications
  • Hyperkalemia / veterinary
  • Male
  • Reflex

Conflict of Interest Statement

No competing interests have been declared.

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Citations

This article has been cited 2 times.
  1. Rollet M, Flyps J, Vernemmen I, van Loon G, Schauvliege S. Case Report: Inhaled salbutamol in the successful treatment of life-threatening acute hyperkalaemia in an anaesthetised horse. Front Vet Sci 2025;12:1663681.
    doi: 10.3389/fvets.2025.1663681pubmed: 41624282google scholar: lookup
  2. Natterson-Horowitz B, Wright K, Van Steenkiste G, Decloedt A, Gagnon AL, Cai X, Mazmanian A. Arrhythmias across the tree of life: comparative insights for human electrophysiology. Front Cardiovasc Med 2025;12:1652591.
    doi: 10.3389/fcvm.2025.1652591pubmed: 41567384google scholar: lookup
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