Discussion

Drugs:
Standing chemical restraint involves 5 drug classes; acepromazine; alpha2 agonists, opioids, NSAIDs and local anaesthetics. Acepromazine inexpensively synergises the sedative effects of alpha2 agonist and opioid components. Used alone (33 - 50 microgram/kg bwt i.m. or i.v.) acepromazine is anxiolytic. Xylazine, detomidine and romifidine are the bedrock of standing chemical restraint (SCR) providing sedation with good visceral (and some somatic) analgesia and drug-dependent muscle relaxation - which is important in dental work. They tend to be less effective in frightened or poorly handled horses and are poorly absorbed after i.m. injection.
Xylazine has a short duration but causes profound sedation with visceral analgesia. When repeated doses are necessary, injections made after the return of some mental normality may produce a paradoxical excitation or stimulation and render further doses ineffective. Detomidine as a sole agent produces sedation more predictably when acepromazine 33 microgram/kg bwt is given 30 min beforehand. The muscle relaxant effects of detomidine are advantageous in dental surgery; many are convinced that detomidine is best for keeping the tongue 'inactive'. The principal difference between romifidine and the others is reduced muscle relaxation; when 100 microgram/kg bwt is given over 1 min the horse scarcely moves its feet; and the head is not as low. Several factors affect opioid choice:

Morphine is an effective analgesic for horses in severe pain and a useful sedative. Butorphanol produces transient visceral analgesia in horses with low risk of excitation, even after rapid, i.v. injection in nonpainful cases - or is it sedation? The drug is licensed and is not controlled - which ensures its popularity. Buprenorphine has been used in sedative combinations in horses. NSAIDs reduce post operative inflammation and therefore pain, but do not reduce the requirements for intraoperative sedatives or analgesics required to provide conditions for dental SCR.

SCR for dental surgery:
Complicating factors include: advanced age; repeated procedures (aversiveness); imperfect analgesia and high expensive doses may prove to be unsuccessful. It may prove difficult to eliminate tongue movement and head shaking. Large (high) horses must be predictably relaxed, and in the event that SCR is unsuccessful, the technique must be compatible with a subsequent GA. The author's favoured technique is:
- Romifidine (50 - 100 mg/kg bwt) i.v.
- Wait 3 minutes
- Morphine (0.1 - 0.25 mg/kg bwt) slow i.v.
- Flush cannula
- Flunixin 1.1 mg/kg bwt i.v.
- Wait 5 minutes; proceed or perform local anaesthesia.

Maxillary blocks are straightforward. The maxillary foramen is between the zygomatic bone and the mandible at the level of the posterior one-third of the eye. A 22 gauge spinal needle is inserted just below the zygomatic bone at this level and perpendicular to the face. The needle is advanced until bone is contacted (approximately 5 - 7 cm deep) and then withdrawn. Ten to 20 ml of 2% lidocaine produces anaesthesia in about 15 min which lasts for 100 - 200 min.
Infraorbital nerve blocks are less well tolerated so are less effective. The infraorbital foramen is between the rostral edge of the facial crest and the nasal bone commissure: it is readily located beneath levator labii superioris proprius and levator nasolabialis providing the former is retracted before needle insertion. Injecting lidocaine before each needle advance may prevent the horse from reacting vigorously.
An inferior alveolar block involves locating the mandibular foramen upon a line following the buccal edge of the upper dental arcade extended to the back of the mandible. The foramen is 9 - 11 cm from the caudal mandibular margin and along this line. An inferior alveolar block will effectively desensitise the entire lower dental arcade and surrounding tissue.

Problems:
Keeping the head low for prolonged periods causes facial and nasal oedema; the former impairs respiration and in extreme cases epistaxis may occur. These problems are prevented by tying the horse with the head elevated, i.e., above the level of the heart, thus promoting venous drainage from the head.
The efficacy of chemical restraint depends on the surgery to be performed, its location, the individual horse's temperament, the environment and physical restraint facilities. It is desirable that horses are calm before and during the time drugs are acting. i.e. in quiet surroundings where movement and other distractions are minimised.
Surgical conditions for long operations can be maintained using repeated top-ups of xylazine (0.25 - 1.0 mg/kg bwt) given into a temporarily increased intravenous infusion. Detomidine infusions are also useful: an initial loading dose of 7.5 microgram/kg bwt is followed by an initial infusion rate of 0.6 microgram/kg bwt/min which is halved every 15 min.
When surgery cannot be completed because the horse continues to react, or because it is so ataxic that safe operation is impossible then the operative site must be tidied up and the operation resumed some time later under general anaesthesia.
Most horses may be left once the procedure ends after returning to a foodless box and not offered feed until the animal can effectively masticate. This may take a number of hours. Sedated horses (high in the social hierarchy) should not be
returned to a herd.