Sedation and Anesthesia

Sedation and anesthesia may be required in neonatal foals to allow diagnostic and therapeutic procedures to be carried out in a safe fashion. There are several differences in foals compared to adult horses that require consideration [1].

I. Neonatal Characteristics

1. The newborn foal (especially during the first 72 hrs of life) and the premature foal are in circulatory transition from in utero right ventricular dominance to left ventricular dominance and can revert to fetal circulation following birth (See Respiratory Distress section III E).

1. The ductus arteriosus and the foramen ovale may re-open under conditions of hypoxia, hypotension and bradycardia. Therefore caution needs to be used with the use of agents affecting circulatory status to prevent hypoxia and hypotension.
2. Neonates rely on heart rate to maintain cardiac output compared to adults [7].
3. Foals may have altered responses to hypoxemia and hypercapnia leading to alveolar hypoventilation, ventilation perfusion mismatch and R-L shunts.

2. Immature drug metabolism may also cause different outcomes compared to adult horses using the same agents. (See Drug Formulary-Equine Neonate).
3. Susceptibility to hypothermia requires efforts to prevent heat loss and maintain body temperature during some procedures with sedation and anesthesia.
4. Low energy stores will cause hypoglycemia and thus foals should have dextrose containing solutions infused during anesthesia.
5. Because of limitations in stroke volume in neonates cardiac output depends more on heart rate.

1. Drugs that lower the heart rate (xylazine and detomidine) and those that decrease preload (acepromazine) may produce diminished cardiac output and tissue perfusion.

II. Sedation

1. α2-agonists

1. Xylazine is not recommended as a preanesthetic agent in ill foals - Significant effects on decreasing heart rate and can compromise circulatory status.

1. In sick neonatal foals doses of 0.44 mg/kg IV (about 22 mg in a 50 kg foal) produced blanching of mucous membranes, marked depression and prolonged recovery and 2 sick foals died that received 0.55 mg/kg IV (about 28 mg in a 50 kg foal) [5].
   Editor's Comment - Use diazepam in sick foals for sedation.
2. Healthy foals 10 days to 28 days of age received a dose of 1.1 mg/kg IV which produced sedation and ataxia and recumbency in 4 of 6 foals with no adverse effects.3
3. Xylazine at lower doses 0.25 to 0.5 mg/kg IV (12-25 mg in 50 kg foal) combined with butorphanol 0.02-0.04 mg/kg (1-2 mg in a 50 kg foal) produces good sedation and analgesia [4]
4. Xylazine (0.5-1.0 mg/kg) mixed with pentazocine 0.03-0.08mg/kg (Talwin®).

2. Detomidine widely used with safety in healthy foals at 10-20 μg/kg (0.5 to 1 mg in a 50 kg foal) [6]

1. Combination with butorphanol 20-40 μg/kg (2-4 mg in 50 kg foal) produces good analgesia and sedation.

3. Medetomidine has been used by infusion in experimental settings in ponies. It is used as a component of TIVA or PIVA in adult horses although it has not been evaluated in neonatal foals yet. It is very potent and should be carefully dosed (0.001-0.01 mg/kg).
4. Romifidine is not used commonly in foals. The dose is 0.04-0.12 mg/kg. It has a shorter duration compared to detomidine.

2. Benzodiazepines

1. Diazepam (Valium) 0.1-0.2 mg/kg (5-10 mg in a 50 kg foal) is a safe and effective tranquilizer in neonatal foals but has no analgesia. Good for sick foals because it has minimal cardiovascular effects. Catheter placement is consequently easier in foals sedated with diazepam as jugular distensibility is less affected.
   Editor's Comment - I use this when I get a sick foal that is flopping around, can’t stand and won’t lie quietly.
2. Midazolam (0.05-0.1 mg/kg iv) is a shorter acting agent. Doses can be repeated frequently or used as a CRI if needed.

3. Phenothiazine Tranquillizers

1. Acepromazine has its major drawback as it causes dose dependent cardiovascular effects and vasodilatation that produces hypothermia.
   Editor's Comment - We don’t use this in neonatal foals

III. Anesthesia

1. Premedication

1. Premedication of neonatal foals with a sedative or tranquilizer is not usually performed when using inhalation anesthetics.
2. A report of 2 cases of cardiac dysrhythmias with xylazine premedication and halothane anesthesia.
3. If needed, use diazepam 0.1-0.2 mg/kg
4. In foals >1 month and that are less tractable can use alpha-2 agent and butorphanol followed by mask or tube induction [6].

2. Induction agents (See references for more details and consult [10])
   Editor's Comment - With inhalational anesthesia, withhold milk from neonates 30-60 minutes prior.
3. Inhalational agents

1. Isoflurane produce a smooth induction and rapid recovery [8].
2. Sevoflurane has become the most popular and the safest inhalational anesthetic in young foals. MAC is slightly higher for sevoflurane than for isoflurane (2.31% vs. 1.31%)
3. Halothane is not recommended for equine neonates.
4. Inhalational methods

1. Face mask -can be used to deliver oxygen and inhalant
2. Naso-tracheal intubation with cuffed silicone tubes (Bivona) 55 cm or longer with internal diameters of 8-14 mm.
3. Large clear plastic canine mask or 1 liter plastic bottles if nothing else is available.
4. Single carbon dioxide absorber canisters suitable for foals up to 60 kg and double canisters for foals up to 150 kg [4]

5. Keep mare next to foal during induction to prevent stress. May wish to tranquilize mare in preparation for separation
6. Small animal anesthesia machines will work on most neonatal foals.
7. Keep body temperature stable with warm water blankets, warm IV fluids, etc. [4]

4. Induction with intravenous anesthetics (following sedation)

1. Ketamine (2-3 mg/kg)

1. Ketamine (2-3 mg/kg) + diazepam (0.05-0.2 mg/kg)
2. Ketamine (2-3 mg/kg) + midazolam (0.050.1 mg/kg)
3. Propofol (2 mg/kg) can be used in foals alone or in combination with ketamine or a benzodiazepine for induction [9,10]

5. Maintenance of anesthesia [8,10]

1. Inhalational anesthesia

1. Isoflurane (1-3%)
2. Sevoflurane (1-5%)

2. TIVA (Total Intravenous Anesthesia)

1. Ketamine + benzodiazepine
2. Ketamine + α2 -agonists + guaiphenesin (older, healthy foals only)
3. Propofol
4. Propofol + Ketamine

3. PIVA (Partial Intravenous Anesthesia)
   This technique is widely used nowadays because of the safety and lower mortality rate. It will result in a significant decrease of the inhalational anesthetic use (25-90% reduction in MAC).

1. Ketamine
2. α2–agonists (older, healthy foals only)
3. Ketamine + α2–agonists (older healthy foals only)
4. Lidocaine
5. Lidocaine + Ketamine
6. Lidocaine + Ketamine + α2–agonists (older healthy foals only)

IV. Supportive Therapy

1. Intravenous fluids and electrolytes (See Fluid and Electrolyte Balance) should be administered based on laboratory and blood gas values. Monitor for glucose levels during prolonged anesthesia and give isotonic fluids with 1-2.5% dextrose depending on the blood glucose.
2. Heart rate problems

1. Bradycardia (normal is 70-90 beats/min) or AV blocks.

1. Treat with atropine 0.02-0.04 mg/kg or glycopyrrolate 0.001 mg/kg [4].
2. Administration of intravenous calcium-gluconate may be beneficial in foals with decreased myocardial contractility.

2. Ventricular premature contractions may be treated with a bolus of Mg-sulfate and lidocaine 0.5-1 mg/kg slowly iv.

3. Hypotension/Shock (See Shock (SIRS))

1. Suggested to keep mean arterial pressure (MAP) above 60 mmHg (optimal is between 70-90 mmHg).
2. Treat first by increasing fluids and decreasing plane of anesthesia.
3. Inotropes: Dobutamine (2-5 ug/kg/min) produces dose dependent increase in heart rate, arterial blood pressure, and renal and splanchnic blood flow. Mix 50 mg dobutamine in 500 ml of saline to make 100 ug/ml solutions.
4. Vasopressors: Norepinephrine 0.1-1.5 μg/kg/min and Vasopressin 0.25-1.5 mU/kg/min
5. Dopamine is not recommended as first line inotrope or vasopressor anymore.