Anesthesia: general - foal | horses | Vetlexicon Equis from Vetlexicon

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.