Anesthetic management of foals during common surgical procedures.

The neonatal period is generally classified as the first four weeks of life and is a period of very rapid physiologic change and adaptation, particularly in the cardiovascular, respiratory, immune, and neurologic systems. Foals younger than 1 month have an increased risk of anesthetic-related death (seven times higher than that of an adult horse) and complications. At 1 to 4 months of age the risk level decrease but there is still immature anatomy. Common procedures that require anesthesia include angular limb deformities, abdominal surgery, endoscopy, umbilical hernia, ribs fractures, umbilical infections, etc. General anesthetic considerations will be mentioned.

Respiratory system of the neonatal foal

Pulmonary changes occur during the first few hours but the most important changes to the adult circulatory system take 48–72 hours. Expansion of the lungs results in increased pulmonary blood flow, closure of foramen ovale, and closure of the ductus arteriosus.

Foals have an increased respiratory rate in comparison with adult horses and the chest wall compliance is greater too. PaO2 values are low (75 mmHg) by 4 h after birth. However, adult values are not achieved for about 7 days. PaCO2 values are 50 mmHg in the first hour of life and decrease thereafter. Hypoxia (oxygen saturation <95%), hypercapnia, and acidosis may be common in these patients under anesthesia. The administration of oxygen is always recommended.

Cardiovascular system

Foals have a higher average heart rate and a lower MAP compared with adult horses. Contractility of the heart ventricular compliance are limited. Adequate heart rate is important to maintain cardiac output and blood pressure when a patient is under general anesthesia. The sympathetic system and baroreceptors are also immature.

Thermoregulation

Neonatal foals are prone to heat loss under general anesthesia because of a lack of involuntary muscle activity (e.g., shivering) caused by the anesthesia. The attempting to thermo-regulate also contribute to the development of hypoglycemia and increase in oxygen consumption.

Fasting

It is unclear whether this is beneficial. In general, the neonatal foal should be allowed to suck up to the time of anesthesia induction. Older foals, on solid food, may have food withheld from 4–6 hours.

Sedation

The mare and foal if possible, should be kept together as long as the foal is conscious to avoid fear. Mares could also be sedated if necessary. Foals are very needle-shy and it is easy to accidentally hit the carotid artery while attempting jugular puncture. Intradermal or subcutaneous local anesthesia makes jugular catheterization easier, and IM sedation in older foals can be useful. To prevent hypothermia and hypoglycemia, avoid keeping the neonatal foal sedated for long periods. Diazepam/midazolam 0.05–0.1 mg/kg, IV, can be used. Alpha2 agonists should be used with caution (low doses to effect e.g., xylazine 0.2 – 0.5 mg/kg) in the very young. If possible, avoid the use of alpha 2 agonists in foals less than a week of age.

Induction

Ketamine (2–3 mg/kg, IV) with diazepam/midazolam (0.1 - 0.2 mg/kg IV) given slowly to effect ± a low-dose α2 agonist (depending on physical status and age).

Propofol (4 mg/kg, IV) given slowly to effect with or without prior sedation with a benzodiazepine.

Maintenance Agents

For short procedures (30 minutes), inhalant anesthetics isoflurane and sevoflurane are the mainstay. This requires tracheal intubation and the use of an anesthesia machine.

Analgesia

As part of a balanced anesthesia the administration of infusions or boluses of ketamine, lidocaine, α2 agonists, and/or opioids as adjuncts to inhalational anesthesia, it is always appropriate. Butorphanol is the most used opioid analgesic in horses. Foals, unlike adult horses, do not exhibit behavior effects when administered butorphanol, and neonatal foals become sedate. For analgesia, foals seem to need higher doses of butorphanol than do adults. In foals aged from 1 to 8 weeks, a butorphanol dose of 0.1mg/kg, IV provided 150minutes of analgesia in a thermal model of nociception. A dose of 0.05mg/kg, IV was not associated with analgesia. The pure mu agonists, morphine and methadone, may be used at adult doses; thus, it would seem reasonable to assume that hydromorphone can also be used in foals at adult doses. However, PK/ PD data on these drugs are limited in foals. Morphine (0.05–0.1mg/kg, IV, IM). Methadone (0.05–0.1mg/kg, IV, IM). Hydromorphone (0.02–0.04mg/kg, IV, IM). NSAIDs dosing interval may be longer in foals than in adults. Flunixin in neonatal foals, dosages of 1.1mg/kg, IV q 24–36hours seem to be safe. For meloxicam a dose of 0.6mg/kg, IV or orally q12–24hours have been recommended.

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