Equis ISSN 2398-2977

Musculoskeletal: anesthesia

Contributor(s): Keith Branson, Craig Johnson

Introduction

  • Surgery to the musculoskeletal system in the horse is frequently carried out on the performance horse which can be expected to be fit and muscular, even if it has been allowed a let-down period prior to surgery.
  • Fit and athletic horses have low energy requirements at rest and during anesthesia, which means that a significant reserve exists in both cardiovascular and respiratory systems, resulting in low respiratory and heart rates.
  • Preoperatively, stabilization of all systems should be carried out, especially where the surgery is for an emergency situation. The patient should be handled calmly but firmly and all possible preparation, eg clipping and scrubbing the surgical site, should be carried out prior to induction, so as to reduce the amount of time the animal spends anesthetized.
  • Drugs used in general anesthesia   Anesthesia: general - overview  depress respiratory and cardiovascular function so both bradycardia (<25 beats/minute) and bradypnoea (<7 breaths/minute) are common and do not respond to surgical stimulation. Reduced cardiac output and respiratory acidosis can result if no attempt is made to correct these problems at an early stage.
  • Monitoring the blood pressure   Cardiovascular: blood pressure monitoring  to detect hypotension can enable the anesthetist to treat quickly and prevent problems escalating.
  • Intravenous fluids and inotropic drugs should always be prepared prior to induction in the athletic patient so treatment for hypotension may be initiated immediately it is detected.
  • During surgery, the development of hypotension may necessitate the use of intravenous anesthetic agents to 'top-up' the anesthetic as an alternative to increasing the inspired concentration of the inhaled agent.
  • Immediately after induction, apnoea is commonly encountered, for up to 45 seconds. In the athletic horse, this can be even more marked and it may be advisable to institute intermittent positive pressure ventilation (IPPV) almost immediately if the surgery time is expected to be more than 60-90 minutes (which most surgeries for the musculoskeletal system are likely to be).
  • Cardiovascular depression due to the inhalation agent can however be enhanced if IPPV is instituted as the horse is given deep, regular breaths, and IPPV reduces venous return and depresses cardiac output.
  • A horse which is exhausted after a competition when it is admitted for urgent musculoskeletal surgery may be dehydrated. In severe cases, it may be necessary to stabilise before induction, but in milder cases, agressive fluid therapy during anesthesia is usually sufficient.
  • A patient which has suffered excessive blood loss prior to surgery should have the circulating volume restored before induction where possible.
    Avoid use of the fenothiazine tranquilizers   Etorphine hydrochloride  in patients which are dehydrated or have suffered significant blood loss until the circulating volume has been restored. This is because the alpha-blocking properties of these drugs can cause severe hypotension in the hypovolemic animal

Elective orthopedic surgery

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Emergency orthopedic surgery

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Further Reading

Publications

Other sources of information

  • Hodgson, D S & Dunlop, C I (1990)General anesthesia for horses with specific problems. The Veterinary Clinics Of North America6(3) 625-643.
  • Hall, L W & Clarke, K W (1983) Veterinary Anesthesia, 8th edn. Bailliere Tindall, London.
  • Taylor, P M (1992)Anesthesia for specific procedures. Lecture notes.


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