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| The UK Society for Intravenous Anaesthesia |
| Based in the UK - as a resource for Anaesthesia Worldwide |
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Glasgow Meeting - May 2003 Total Intravenous Anaesthesia in Horses R.E. Clutton, Division of Veterinary Clinical Studies, University of Edinburgh, Edinburgh EH25 9RG While horses represent a high anaesthetic risk in comparison to smaller domestic species, they may still be anaesthetised by any member of the RCVS, whether or not the latter have specialised training in anaesthesia. This partly explains why TIVA is the norm rather than the exception, as the veterinary surgeon/anaesthetist has an aversion to complex techniques, i.e., those involving anaesthetic machines and breathing systems. Another reason is that the majority of routine operations performed on horses, e.g., dental extractions, castration, superficial wound management, are performed at the stable, i.e., under field conditions. This favours techniques which are simple, have minimal equipment and personnel requirements and so are readily deployed in a range of environmental conditions. In its simplest, and possibly most common form, field equine anaesthesia requires only a head collar, lead rope, a subject, an intravenous catheter (optional), needles, syringes, one or two bottles of drug, an anaesthetist/surgeon and an (optional) assistant. Nevertheless, accurate risk assessment and case selection, and restriction to simple, frequently performed and short operations render these minimalist TIVA techniques rather safe. In many situations, surgery may be performed on the standing animal using TIVA techniques referred to generically as standing surgical anaesthesia (SSA). Problems in equine anaesthesia
Standing surgical anaesthesia The objectives of standing surgical anaesthesia (SSA) are to produce conditions for surgery in the standing animal without ataxia. Operations performed under SSA include dental extractions, ocular operations, wound debridement, castration, laparoscopy. SSA is useful in high risk cases, when assistants are unavailable, or when overall conditions for general anaesthesia are unsatisfactory. SSA is safer for the horse, but less safe for the surgeon. Drugs involved in the production of SSA include acepromazine (or other phenothiazines) combined with a 2 agonists and opioids components. The choice of a2 agonist and opioids are governed by factors indicated in table 1 and 2 respectively. Table
1. a2
Components
Table
2; Opioid Components
True surgical anaesthesia is achieved when local anaesthetic techniques - infiltration, conduction block, or more complex neuraxial techniques - are superimposed upon SSA. Pre-emptive analgesia is usually provided by the pre-surgical intravenous (IV) administration of NSAIDs, e.g. flunixin, phenylbutazone, carprofen. Developments to this basic technique include the infusion of detomidine, butorphanol and, or lidocaine. General anaesthesia: single doses with increments. General anaesthesia is required for those operations, e.g. midline laparotomy, which cannot be performed in standing animals, when immobility is required, or when operator safety is of paramount importance. When short procedures are anticipated, most practitioners would attempt to rely on a single injection technique. For many years, drug combinations based on chloral hydrate were commonly used, but were not particularly safe. In the late 1960s, a proprietary etorphine / acepromazine mixture (Immobilon) proved very popular, principally because of the dramatic recovery observed when the opioid component was antagonised with diprenorphine. Poor operating conditions and the lethality of this combination in veterinary surgeons, led to its replacement by single injection techniques based on acepromazine, succinylcholine and thiopental. These in turn gave way to techniques in which a2 agonists were used for pre-anaesthetic medication and induction completed with either thiopental or ketamine. The choice of pre-anaesthetic medication influences the “quality” of anaesthesia. See table 3. Table 3. Field (General) Anaesthesia In Horses; Comparison of a2 - Ketamine Techniques.
With embellishments, these techniques remain currently popular. The author’s current favourite technique is detailed below:
PAM: romifidine 100 mg kg-1 IV Wait 5 minutes I: ketamine 2.5 mg kg-1 + diazepam 50 mg kg-1 IV Upon recumbency Morphine 150 mg kg-1 IV Flunixin 1.1 mg kg-1 IV Top-ups (as required) ketamine (0.25
– 1.0 mg kg-1) + midazolam (12.5 – 50 mg
kg-1) General anaesthesia: Infusions The
principal problem with contemporary techniques is the unpredictable duration of
action, with the need for frequent (embarrassing) incremental dosing (clients
are usually present at field operations). For
this reason, and the need to provide more stable conditions, glyceryl
guaiacolate ether (GGE), or guaifenesin has become increasingly popular in TIVA
techniques. GGE is regarded as a spinally-acting muscle relaxant and is used to provide maintenance conditions as part of several total intravenous anaesthetic techniques. It acts as a muscle relaxant and a vehicle for other drugs. Its mechanism of action is poorly understood; it acts on polysynaptic reflex arcs in the spinal cord, reticular formation and sub-cortical areas of the brain in a mephenesin-like manner. Its prominent feature is an ability to depress internuncial neurone transmission in the spinal cord without impairing breathing. in North America, GGE is considered to have CNS depressant effects beyond muscle relaxation; elsewhere, it is not used as a sole anaesthetic but always given with other CNS depressant drugs. The fact that GGE solutions > 15% lead to haemolysis means that dilute solutions have to be infused. The infusion of a2 agonist / ketamine / GGE solutions to produce conditions for more complex operations lasting up to 60 minutes is becoming increasingly common in veterinary practice, despite the drugs potential for accumulation. General anaesthesia: advanced (non-field) TIVA techniques Propofol
has been assessed alone, and with other drugs for the provision of TIVA in
horses. Infusion rates of
0.18 – 0.33 mg kg-1 minute–1 have been used to
maintain anaesthesia for an hour or so. Haemodynamic
variables are usually satisfactory although mean arterial pH tends to decrease,
while modest hypercapnia and hypoxemia (PaO2 values of 42 to 57 mm
Hg) are encountered. Occasionally
arterial hypotension is severe. Recovery
ranges from good in the majority of horses and acceptable in the rest.
Mean recovery times are usually rapid, being, after 1 hour about 60 +/-
30 minutes with 2 - 3 attempts to stand. Propofol
has been combined with ketamine for TIVA in horses.
After a loading dose propofol 0.5 mg kg–1 propofol was
infused at 0.124 (0.009) mg kg-1 minute–1 along with
ketamine at 40 mg kg–1 minute–1.
Respiratory and cardiovascular variables were better maintained compared
with a sole propofol infusion in which arterial hypotension and marked
respiratory depression were evident. Propofol
has also been infused with the a2
agonist medetomidine in horses. After pre-anaesthetic medication with medetomidine (7 mg
kg–1 ) anaesthesia was induced with propofol (2 mg kg-1
IV) and maintained for 4 hours, using an infusion of medetomidine (3.5 mg
kg–1 hour –1 IV) and propofol at a rate ranging from
0.06 to 0.1 mg kg-1 minute–1. Mean arterial blood
pressure was stable, mean PaO2 & PaCO2 did not change
significantly but hypoxaemia was evident in some animals (minimal PaO2,
47 mm Hg). Recovery was fast and
uneventful It is unlikely that TCI or complex TIVA techniques based on ultra short-acting drugs will ever gain popularity in equine anaesthetic practice outside the institutions. Their expense, complexity and the difficulty in managing side-effects put such techniques at a considerable disadvantage against contemporary options.
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