Discussion

Introduction:
The most important way to avoid cardiovascular trouble is to be prepared. Preoperative assessment will help identify underlying disease. Adequate preoperative preparation will allow you to optimise or stabilise cardiovascular function before anaesthesia. Appropriate and detailed monitoring during the anaesthetic period will allow problems to be identified quickly and treated appropriately before further trouble develops. However despite this preparation, cardiovascular problems will occasionally occur suddenly and without warning. Having trained personnel and drugs available to deal with unforeseen situations will help to ensure a favourable outcome.

Preoperative assessment:
Ensure that you familiarise yourself with the history of the case and spend enough time examining the horse so that important information is not overlooked. 'Lone' atrial fibrillation (AF) is a common cause of poor performance in equine athletes and is easily missed. Whilst this does not usually pose a problem during anaesthesia, it is important to recognise AF and discuss its effects with the client before embarking on performance enhancing surgery. It is important to establish whether this arrhythmia is present before surgery, as the alpha2 agonist drugs commonly used during anaesthesia increase vagal tone, and can predispose to the development of atrial fibrillation in the post anaesthetic period.
The horse has a much greater cardiac reserve than humans and other domesticated species and in compensated disease, no relationship has been found between racing performance and the presence of a cardiac murmur (Young et al. 2008). Therefore in most cases a cardiac murmur will not significantly compromise cardiac function during anaesthesia. One possible exception to this is severe aortic regurgitation (AR). As it becomes more common to treat geriatric horses the chance of being asked to anaesthetise a horse with significant AR will increase. Horses with AR have a low diastolic blood pressure due to the run off of blood back into the left ventricle during diastole. Anything that decreases heart rate will increase the time available for regurgitation and decrease the diastolic pressure. Horses with a dilated left ventricle as a result of AR will also have increased resting myocardial oxygen consumption and will be more prone to arrhythmias.
Hypotension remains the most significant problem that needs to be identified in the preoperative period. Hypotension is common in horses presented for colic surgery and when profound and untreated, can lead to cardiovascular collapse following anaesthetic induction. Animals presenting with hypotension will have an elevated heart rate, although this can also be due to other factors including pain. Palpation of the arterial pulse will provide further information. The pulse pressure merely reflects the difference between the systolic and diastolic pressures. If the systolic pressure is 120 and the diastolic pressure is 80 the pressure difference will be 40. This pulse pressure will feel exactly the same as that from a horse where the systolic pressure is 60 and the diastolic pressure is 20, yet this horse is more severely compromised. The difference between these 2 horses can be determined during palpation of the arterial pulse by assessing the amount of pressure taken to occlude the artery. The pulses in the latter horse would be easily occluded and the need for preoperative fluids and cardiovascular support appreciated.

Bradycardia due to elevated potassium levels can also be missed in the preoperative period, especially in foals where the normal resting heart rate is higher than that of the adult horse. Hyperkalaemia, when present in the preoperative period will tend to increase following induction to anaesthesia if the horse is breathing spontaneously. Respiratory acidosis in spontaneous breathing horses will cause potassium to move out of the cells which may increase blood potassium to a critical level causing cardiac arrest. Blood samples should be taken preoperatively if hyperkalaemia is suspected.

Preoperative preparation
Preoperative preparation allows cardiac reserve to be purchased before anaesthetic drugs are given which may reduce cardiac performance. In horses with preoperative hypotension and tachycardia, 2 l of hypertonic saline 7.5% (4 ml/kg bwt) can restore palpable pulses and ensure that the horse at least survives the initial induction period to allow surgery and further cardiovascular stabilisation.
Medical treatment of hyperkalaemia should be undertaken before induction to anaesthesia and horses with known hyperkalaemic periodic paralysis should also be treated (Robertson
2010).

Anaesthetic monitoring
During anaesthesia careful monitoring will allow most problems to be identified before an emergency situation develops. Whilst an ECG is a very useful monitor during anaesthesia it is important to remember that it merely records the electrical activity of the heart and does not indicate mechanical activity. Arrhythmias are relatively uncommon in horses during anaesthesia compared to other species however electromechanical dissociation is common. In these cases the ECG will continue to be unremarkable many minutes after cardiac output has ceased.
Measurement of arterial pressure is important when inhalant anaesthetics are used. Hypotension is common and drugs to support cardiovascular function should be available however the many causes of hypotension should be appreciated. Increased analgesia may allow the plane of anaesthesia to be reduced; if hypercapnia is causing vasodilatation and hypotension, positive pressure ventilation will increase arterial pressure despite reductions in preload. Where lack of preload is considered to be a causative factor, a modest increase in arterial pressure can be achieved by using relatively small volumes of colloids.
Whilst hypotension is a common problem in horses and results in the loss of autoregulation of blood flow to the tissues, hypertension should also be avoided as this will dramatically reduce cardiac output and muscle blood flow (Raisis 2005).

References
Raisis, A.L. (2005) Skeletal muscle blood flow in anaesthetized horses. Part II: effects of anaesthetics and vasoactive agents. Vet. Anaesth. Analg. 32, 331-337.
Robertson, S.A. (2010) Cardiovascular emergencies associated with anaesthesia. In: Cardiology of the Horse, 2nd edn., Eds: C.M. Marr and M. Bowen, Elsevier Ltd, Edinburgh. pp 253-266.
Young, L.E., Rogers, K. and Wood, J.L. (2008) Heart murmurs and valvular regurgitation in thoroughbred racehorses: epidemiology and associations with athletic performance. J. Vet. Intern. Med. 22, 418-426.

15.10
Practical analgesia
R. Eddie Clutton
Department of Veterinary Clinical Studies, Easter Bush Veterinary Centre, Easter Bush, Roslin, Midlothian EH25 9RG, UK.


Pain management is necessary for ethical, legal, medical and in horses especially - practical reasons. Acute pain behaviours in horses are ostentatious and may be violent; effective analgesia may be required for diagnosis. In contrast, chronic pain behaviours may be misleadingly undramatic. Equine pain can be managed in 5 broad ways: 1) pharmacologically; 2) surgical/ manipulative (wound inspection/dressing); 3) neuro-physiological,
4) euthanasia; 5) complementary (evidence-free). The relative importance of each component depends on circumstances. These may vary from animals injured 'in the field' to those presenting with severe colic requiring transport to a referral centre for laparotomy. The effective management of acute laminitis pain remains elusive.

1) Pharmacological analgesia
The most important drug classes are: 1) opioids; 2) local anaesthetics; 3) NSAIDS; 4) alpha2 agonists; 5) ketamine; 6) spasmolytics; 7) general anaesthetics; 8) SAIDs; 9) benzodiazepines; 10) anticonvulsants; 11) anti-depressants.
Pure mu-agonists are effective analgesics in horses and synergise the sedative effects of other drugs. Concerns over their side effects have been critically over-stated in the past (Clutton
2010). The choice of opioid(s) used depends on these factors: analgesic efficacy; likelihood of side effects; sedative properties; autonomic nervous effects; onset time; duration of action; duration of action and concurrent alpha2 agonist administration; CDR requirement; VMD requirement; personal preference; and cost.
Local anaesthetics are true analgesics insofar that certain routes of administration eliminate all pain sensation. There are 7 ways to administer local anaesthesia: topically; by infiltration; conduction blockade; intra-articular; intravenous regional analgesia and by neuraxial anaesthesia (extra [epi-] dural and spinal, sub- arachnoid anaesthesia). The drugs used depend on efficacy, onset time, duration and toxicity. Those most commonly used in horses are: lidocaine; mepivacaine; bupivacaine; proxymetacine.
NSAIDs are important for providing musculoskeletal analgesia, but some exert useful anti-endotoxic effects. Used in conjunction with other analgesics as part of PMPT, it is often difficult to identify the superiority of one NSAID over another.
Alpha-2 agonists are useful because they sedate, whilst providing visceral analgesia. The principle choices (xylazine, detomidine and romifidine) are chosen on the basis of their analgesic efficacy, onset time, duration, cardiovascular effects and the sedation muscle relaxation and ataxia produced.
Ketamine produces profound analgesia and chemical restraint at 'normal' doses, but the use of 'stun' doses at 1/20th may also be beneficial. Ketamine infused at very low doses may prevent central sensitisation.
Smooth muscle relaxants, e.g. anti-muscarinic drugs, reduce pain arising from gastro-intestinal spasm.
General anaesthetics only control pain for the duration of anaesthesia, and some may promote pain appreciation post operatively.
The versatility of many of these drug classes in providing pain relief has expanded over recent years as new methods and routes of administration have been devised, e.g. both opioids and alpha2 agonists can be used within synovial structures or the extradural space. Lidocaine can be infused intravenously as well as applied peri-neurially. Ketamine dose manipulations have similarly improved its versatility. The use of transdermal 'patches' provides one opportunity for the long-term administration of drugs, e.g. fentanyl. Another option achieving the same goal is the use of constant rate infusions. The use of battery driven syringe drivers facilitates this in the ambulatory animal.
Developments in therapeutic strategies have also provided greater options for more effective pain management: 1) pre- emptive analgesia; 2) polymodal pain therapy; 3) partial intravenous anaesthesia; 4) prolonged post operative pain therapy.

2) Surgical/manipulative measures
Wound dressing, the effective casting of fractures, dental and foreign body extractions and abscess lancing are obvious ways surgery relieves (rather than creates) pain. Hot and cold compresses may have some useful effect. Good nursing practice, i.e. grooming, wound management, feeding, watering and exercise, may improve the animal's comfort

3) Neurophysiological methods
TENS (transcutaneous nerve stimulation) which stimulates nerves at
100 Hz does not appear to have been tested in horses whereas TSE (transcutaneous spinal electroanaesthesia) has. Using stimulation frequencies of 1800 - 2500 Hz acupuncture, this was alleged to be effective in treating lumbar injuries in Australian racehorses.

4) Euthanasia
The most effective means of treating pain in horses

5) Complementary methods
Unproven and faddish methods of treating pain are important on
2 counts: at worst, they may aggravate the pain, at best, they will delay the time when the animal receives effective treatment or euthanasia.

Practical analgesia (surgical) equine colic
To facilitate diagnosis (paracentesis, nasogastric intubation and rectal examination) xylazine (1.1 mg/kg bwt i.v.) and pethidine (2 mg/kg bwt i.m.) would be suitable as both are short-acting sedative and analgesics. For transfer to a referral centre, detomidine (10 microgram/kg bwt i.v.) flunixin (1.1 mg/kg bwt i.v.) and Buscopan would provide a long duration of action. NB the receiving centre would require pretransfer information arising from physical examination, e.g. heart and respiratory rate and the doses of transfer drugs given and time. For cases going immediately to surgery, preanaesthetic medication with an alpha2 agonist and opioid, followed by ketamine (2.5 mg/kg bwt i.v.) would provide
conditions for endotracheal intubation, allowing anaesthesia, but not necessarily analgesia to be maintained with an inhalational anaesthetic. The depressant effects of inhalation agents could be reduced by infusing lidocaine and, or morphine. The infusion alpha2 agonists, i.e. medetomidine and ketamine is not uncommon practice these days. Nonsteroidal anti-inflammatory drugs should be given before surgery begins. Many would give an additional alpha2 agonist dose to smooth the initial parts of recovery.