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Anesthesia: general - foal

pequis

Synonym(s): General anesthesia in the foal


Introduction

  • Anesthesia is challenging when dealing with foals regardless of their age or health status.
  • The temperament of foals can be difficult.
  • It is unusual for foals to regurgitate while anesthetized and hypoglycemia is common, therefore prevention of sucking is not necessary in foals <3 months of age prior to general anesthesia. Older foals taking solid food can be fasted for 4 h prior to anesthesia but the beneficial effects of this are uncertain.
  • Jugular cannulation is mandatory during anesthesia - strict asepsis should be adhered to when cannulating the jugular vein.
  • The foal should be kept with the dam until anesthesia is induced, or at least until the foal is heavily sedated, to minimize distress.
  • The dam should be sedated prior to removing her foal. Sedation of the mare can be achieved with a combination of an alpha 2 adrenergic agonist with or without acepromazine and/or butorphanol.
  • Foals presenting for emergency cases should have blood taken to evaluate (a minimum of) packed cell volume, white blood cell count, total protein, glucose and electrolytes including potassium and sodium.
  • A thorough physical exam, including cardiac auscultation, should be performed on every foal prior to anesthesia.
  • An accurate bodyweight should be determined.
  • Premedication is not necessary in severely depressed foals prior to induction.
  • Inhalant anesthetics decrease cilia motility in the airways for up to 3 h. In foals with respiratory disease, avoiding inhalant anesthetics is recommended. Instead use intravenous techniques with oxygen insufflation, however controlled ventilation may be necessary in these patients, so intubation and access to a ventilator is advisable. Elective procedures should be delayed until the respiratory disease has resolved.

Goals of general anesthesia of foals

  • Short duration of both sedation and anaesthesia.
  • Controlled unconsciousness, analgesia and muscle relaxation (the triad of anaesthesia).
  • Minimization of physiological perturbations.
  • A smooth recovery with the foal able to nurse as soon as possible.
  • Post-operative analgesia.

Definitions

Neonatal & pediatric periods (metabolically immature)

  • Neonate - typically a foal <2 weeks of age.
  • Pediatric - a foal 2-6 weeks of age.  
  • Neonatal and pediatric foals have alterations in their physiology due to immaturity, including:
    • Body fluid distribution and impaired renal function - foals are poorly tolerant of fluid imbalances.
    • Increased body surface area to weight ratio - prone to hypothermia.
    • Immature hepatic function - delayed drug metabolism, reduced plasma proteins and hypoglycemia.
    • Reduced myocardial contractile capability and persistent ductus arteriosus in foals up to 5 days - can result in hemodynamic perturbations under general anesthesia and flow reversal in the ductus. A machinery murmur may be auscultated in foals up to 5 days of age.
    • Increased blood brain barrier permeability and immature central nervous system - use lower doses of sedative and anesthetic drugs.
  • Sick older foals can respond to anesthetics in a manner similar to a neonate.

MAC & TIVA

Minimum alveolar concentration (MAC)

  • The minimum alveolar concentration (at 1 atmosphere) of an inhalational anesthetic which prevents gross purposeful movement in 50 % of patients in response to noxious stimulation when at steady state.
  • MAC studies are lacking in foals but it is expected that very young foals have a reduced requirement for inhalational anesthetics; older foals may have an increased requirement due to temperament compared with an adult.

Total intravenous anesthesia (TIVA)

  • Using intravenous techniques rather than inhalant anesthetics to maintain general anesthesia.
  • Again, young foals have a reduced requirement whereas older foals may have an increased requirement compared with adult horses.

Premedication

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Induction agents

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Inhalational induction

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Maintenance

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Monitoring

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Valverde A, Giguere S, Sanchez L C, Shih A & Ryan C (2006) Effect of dobutamine, norepinephrine and vasopressin on cardiovascular function in anesthetized neonatal foals with induced hypotension.  Am J Vet Res 67, 1730-1737.
  • Read M R, Read E K, Duke T & Wilson D G (2002) Cardiopulmonary effects and induction and recovery characteristics of isoflurane and sevoflurane in foals. JAVMA 221 (3), 393-398 PubMed.
  • Normal W M, Court M H & Greenblatt D J (1997) Age-related changes in the pharmacokinetic disposition of diazepam in foals. Am J Vet Res 58 (8), 878-880 PubMed.
  • Aida H, Mizuno Y, Hobo S et al (1994) Determination of the minimum alveolar concentration (MAC) and physical response to sevoflurane inhalation in horses. J Vet Med Sci 56 (6), 1161-1165 PubMed.
  • Dunlop C L (1994) Anesthesia and sedation in foals. Vet Clin North Am Equine Pract 10 (1), 67-85 PubMed.
  • Dixon P M (1992) Respiratory mucociliary clearance in the horse in health and disease, and its pharmacoceutical modification. Vet Rec 131 (11), 229-235 PubMed.
  • Carter S W, Robertson S A, Steel C J & Jourdinais D A (1990) Cardiopulmonary effects of xylazine sedation in foals. Equine Vet J 22 (6), 384-388 PubMed
  • Robertson S A, Carter S W, Donovan M & Steele C J (1990) Effects of intravenous xylazine hydrochloride on blood glucose, plasma insulin and rectal temperature in neonatal foals. Equine Vet J 22 (1), 43-47 PubMed.
  • Oijala M & Katila T (1988) Detomidine (Dormosedan) in foals: sedative and analgesic effects. Equine Vet J 20 (5), 327-330 PubMed.
  • Thomas W P, Madigan J E, Backus K Q & Powell W E (1987) Systemic and pulmonary hemodynamics in normal neonatal foals. J Repro Fertil 35, 623-628 PubMed.
  • Rossdale P D (1970) Some parameters of respiratory function in normal and abnormal newborn foals with specific reference to levels of PaO2 during air and oxygen inhalation. Res Vet Sci 11, 270-276.

Other sources of information

  • Doherty T & Valverde A (2006) Management of Sedation and Anesthesia. In: Manual of Equine Anesthesia & Analgesia. Eds: Doherty T & Valverde A. Blackwell Publishing Ltd, UK. pp 219-227.
  • Knottenbelt D C, Holdstock N & Madigan J E (2004) Intensive Care, Therapeutics and Nursing. In: Equine Neonatology: Medicine and Surgery. Saunders, UK. pp 441-454.
  • Mama K R (2000) Anesthetic Management of the Horse: Intravenous Anesthesia. In: Recent Advances in Anesthetic Management of Large Domestic Animals. Ed: Steffey E P.

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