Postoperative pain control: spotting signs and choosing relief

Obvious signs of pain, such as the lame orthopaedic patient, the eye blepharospasm or the horse kicking at its side with abdominal discomfort, are easier to assess. It is patients with low-grade pain – or discomfort in body systems that are not easy to monitor – that often prove to be more problematic to the clinician, and often require the assessment of more subtle indicators of pain.

Respiratory rate and heart rate have traditionally been used to quantify the pain a horse is experiencing, and are very useful in most instances postoperatively. Clinical examination, with auscultation of the heart and lungs (Figure 1), at repeated times throughout the day, allows early identification of increases that may be consistent with rising levels of discomfort. These examinations are also very useful in monitoring the efficacy of pain management regimes, allowing rapid intervention both in doses and drug choices.

In some individuals, tachypnoea and tachycardia may be directly associated with disease processes, and must be accounted for in such cases. Additionally, research has shown they may be poorly correlated to subjective assessments of pain in the horse¹, and perhaps should not be relied on as a sole means of pain assessment.

Reliable pain scales for the equine patient are embryonic compared to those used in human and small animal practice and are not routinely used. Until such systems are developed, the use of heart and respiratory rate are still invaluable in the assessment of pain in the horse.

In some patients, tissue damage and swelling can alert the clinician to focus a clinical examination to a particular area, where responses to palpation may elicit a reaction deemed as painful. Therefore, palpation around incisions or wounds is encouraged in the postoperative patient to monitor the degree of localised pain. In a similar manner, providing a post-oral procedure patient with chewable feed and monitoring the efficiency of eating is a good indicator of comfort.

Postoperative lameness assessment of a horse will often indicate the presence of orthopaedic pain, but it must be remembered other painful conditions (for example, caudal abdominal pain) can also alter a horse’s ability to ambulate normally.

Subtle changes in demeanour, posture and behaviour can be useful indicators of discomfort in the horse, with signs such as restlessness, weight shifting, tail swishing, vocalisation, dullness, watching a painful body part and a “tucked-up” appearance. All of these should be routinely monitored in the postoperative patient where development of pain is anticipated. Depending on the body part involved and the cause, other signs including sweating, muscle fasciculations and bruxism may also be noted and should be considered as a sign of discomfort.

Long-term and chronic pain may lead to more subtle signs such as weight loss and behavioural changes. In these cases, and when other causes have been eliminated, the responses to analgesic trials appear very useful in determining if it is pain that is the cause.

Systemic pain relief strategies

A number of analgesic drugs can be administered to the horse, and sometimes in combination, to provide additive and synergistic analgesic effects. These include NSAIDs, opioids, local anaesthetics, α2-adrenoceptor agonists (α2-AAs) and ketamine. The use of pre-emptive preoperative analgesia is encouraged to optimise the effectiveness of the chosen regime and reduce the development of central sensitisation that may reduce the effect of the postoperative analgesia provided.

The amount of analgesia afforded to the patient can be variable and is often dependent on the individual, the clinical condition and the surgical procedure. Close peri and postoperative monitoring of the horse, as discussed, should ensure modifications of the analgesia regime are made as soon as possible.

NSAIDs have long been a core of equine analgesia protocols, with phenylbutazone, suxibuzone, flunixin meglumine, meloxicam, carprofen, vedaprofen and ketoprofen being widely available for systemic medication. Topical diclofenac cream has also shown to give some benefit in the equine patient².

NSAIDs work by cyclo-oxygenase (COX) enzyme inhibition, which is involved in prostaglandin production during the inflammatory response. These drugs can have adverse side effects, with damage to the gastrointestinal mucosa being of particular concern in the horse³ as a result of COX-1 enzyme inhibition. It has been suggested newer generations of NSAIDs, such as firocoxib, may cause fewer gastrointestinal problems as they work by more selective inhibition of the COX-2 enzyme.

Despite the inherent side effects, NSAIDs are an efficient and cost-effective method of pain relief in most clinical scenarios, and their judicious use should minimise the inherent risks in most cases. Injectable and oral forms provide ease of administration, and their anti-inflammatory and antiendotoxic properties provide invaluable additional effects in some postoperative patients.

Opioids deliver an analgesic effect by binding to a range of receptors in the brain, spinal cord and within inflamed tissues. Butorphanol is regularly used in the horse, often in combination with α2-AA drugs for sedation. As an analgesic, its short duration of effect (30 to 60 minutes) and very high dose requirements often negates its use as a one-off analgesic⁴. However, constant rate infusions (CRIs) of butorphanol in postexploratory laparotomy horses have been shown to provide useful adjunctive analgesia⁵. Intravenous buprenorphine has reportedly provided long-lasting analgesia with minimal side effects in the horse, with sublingual administration also being effective⁶.

Morphine is used for its analgesic properties by many institutions, in both general anaesthesia protocols and in standing pre-sedated patients. Excitement of horses following intravenous injection has been reported when used without presedation⁷. Morphine has also been implicated in the development of postanaesthetic colic⁸ in one study, but many other institutions have used it during^9,10 and after general anaesthesia without complication.

These side effects have widely been reported and, along with its legislative restrictions, have perhaps been a cause of its limited use in the equine veterinary world. However, it anecdotally appears useful in a number of cases, including fracture repair and dental extractions that may otherwise be refractory to other methods of pain relief. Fentanyl patches have also successfully been used in equine patients, especially foals⁶.

Alpha2-AAs are often used in the horse to provide sedation and in general anaesthesia regimes, and include detomidine, romifidine and xylazine. Their analgesic mode of action has not been fully determined, but their concomitant sedative effect often precludes their beneficial use as analgesics in the horse. They can, however, be useful in cases of abdominal discomfort, affording rapid cessation of clinical signs of pain, and provide time for other drugs to reach maximum efficacy or for general anaesthesia to be induced. Using α2-AAs via an epidural route allows much lower doses to be used, minimising the sedative effects, and provide a much more suitable method of analgesia.

Lidocaine infusions are often used in horses following gastrointestinal surgery, as they are considered useful in reducing postoperative ileus¹¹, and it has been suggested they may provide analgesic effects in these patients.

Ketamine infusions have also been used in equine pain management, and are thought to act via an antihyperalgesic effect¹². Oral gabapentin has been suggested in the treatment of neuropathic pain in the horse¹³, and may be indicated in horses showing suspected chronic pain and headshaking.

Local analgesia techniques

Postoperative epidural analgesia can be administered through a long-stay catheter usually placed between the first two coccygeal vertebrae. This method provides pain relief to body systems and structures caudally in the horse.

Aseptic techniques for placement and maintenance are paramount in preventing complications. Local anaesthetics, α2-AAs, and opioids and their combinations can all be used in the epidural space to provide longer-term regional analgesia.

Local anaesthetic use may result in profound ataxia and, potentially, a recumbent horse, with the effect appearing to be volume-dependent so careful administration is essential. Opioids, such as preservative-free morphine, have been shown to provide long-lasting analgesia, with known side effects being limited to perineal pruritus and wheals¹⁴.

Xylazine and detomidine used in this way can provide up to three hours of analgesia, but as previously discussed, may cause some systemic sedation¹⁵.

Other methods of pain relief

Adequate limb coaptation in the orthopaedic patient (Figure 2), especially fractures, is often essential to prevent further injury, and the extra support often helps to achieve pain-free weight bearing. This not only provides comfort to the individual, but prevention of contralateral overload will help avoid the risk of laminitis in the adult and soft tissue injury in the juvenile. Equally, provision of support to the contralateral foot at the time of surgery is often advocated if non-weight bearing lameness is anticipated postoperatively. Bandaging support to limbs can also help reduce postoperative swelling, which may be a source of postoperative discomfort.

In postoperative colic cases with ileus, removal of excess reflux by passage of a nasogastric tube at regular intervals, not only prevents catastrophic gastric rupture, but also prevents uncomfortable distension.

Bladder and urinary surgery cases may also benefit from an indwelling urinary catheter for the first few days following surgery to increase comfort.

Summary

Some degree of postoperative pain should be assumed in all cases and dealt with as quickly as possible. Adequate monitoring of the patient should ensure modifications and cessation of analgesia protocols are made rapidly, and the horse is kept as comfortable as possible with minimal side effects and complications.