Analgesia for dogs during the perioperative period
Introduction
As vets, we must provide our patients with adequate analgesic support, not just because of the ethical obligation, but because it contributes to a better recovery.1-3 We also need to remember that pain during the perioperative period is a cause of concern for owners.4 Unless there is an underlying chronic disease, analgesics for dogs during the perioperative period should be planned to control acute pain. In general, pain of less than 3 months’ duration is considered acute.1,3
As dogs cannot express any pain they might be feeling, the vet and everyone else involved in the patient’s care must assess and grade its existence and severity in order to establish the correct treatment. Accordingly, pain assessment scales can be used to make objective measurements of the severity of pain and provide the most appropriate analgesic plan.2
Characteristics of perioperative pain
Pain is a complex multidimensional sensation with various origins. It has been defined as an unpleasant sensory and emotional experience associated with actual or potential tissue damage.3 The response to pain is unique to each individual and comprises two components:
- a sensory componenet (nociception), which involves the neuronal processing of the painful stimulus,
- and another affective one (pain perception), which involves the unpleasant sensory and emotional experience associated with actual or potential tissue damage.1
Nociceptive pain occurs when peripheral nerve receptors are activated by a painful stimulus (in this case, surgical incision or trauma to tissues).1 Inflammatory pain develops following the activation of the immune system in response to tissue damage during surgery or to an infection.2 Inflammatory pain is the cause of acute postoperative pain and may persist until the tissue heals.3 In these patients, the sensation of pain emerges rapidly, and its intensity and duration depend on the severity and duration of the tissue damage.
A key to the correct assessment of perioperative pain is the active involvement of veterinary hospital staff in identifying the signs shown by patients that allow them to anticipate and recognise the presence of pain. It is important to assess the dog in its environment, without any external interactions, paying attention to its posture, movements and facial expression, and to evaluate how it interacts with its caregiver, its response to palpation of the wound, etc.1 The use of numerical scoring scales (CMIs) helps objectify the individual interpretation of the existence and severity of pain.1
Analgesic plans for dogs during the perioperative period
Correct analgesia reduces the dose of anaesthetic agents and, therefore, their possible side effects, while optimising patient recovery.4
MULTIMODAL ANALGESIA
Treatment with a single drug is unlikely to achieve adequate pain control in the perioperative period.4 Multimodal analgesia combines drugs with different mechanisms of action which when used simultaneously achieve better control of the painful process. It is currently the analgesic modality of choice in the surgical patient.4 An example of multimodal analgesia is the administration of an anti-inflammatory that reduces nociceptive activity alongside a local anaesthetic that blocks transmission of the painful impulse, together with an opioid or α-2 agonist that reduces receptor response in the central nervous system.4
PREVENTIVE ANALGESIA
An important concept in analgesics for dogs during the perioperative period is preventive analgesia, that is, the administration of analgesic drugs before the painful stimulus occurs (shortly before or during anaesthetic induction). This has been shown to reduce surgical and postsurgical analgesic requirements.
So, analgesics for dogs during the perioperative period should systematically include the administration of an opioid and a nonsteroidal anti-inflammatory drug (NSAID). Furthermore, depending on each individual case, the possible benefits of incorporating a local anaesthetic, α-2 agonists, additional analgesics, such as ketamine or gabapentin, or the use of nonpharmacological techniques for pain control should also be considered.1
OPIOIDS
In terms of opioids:
- μ-Agonists (the μ-opioid receptor or mu-opioid receptor is an opioid receptor located primarily in the brain and spinal cord), such as morphine or fentanyl, are preferred for the control of moderate-to-severe pain.
- Partial agonists, for example, buprenorphine, are recommended for dogs with moderate pain.
- κ-Agonists/μ-agonists, such as butorphanol, are reserved for mild pain.
The side effects associated with opioids include bradycardia, respiratory depression, tachypnoea, emesis, nausea, sedation, dysphoria and impaired thermoregulation.2,4
NSAIDs
NSAIDs play an important role in the management of mild-to-moderate inflammatory pain. Their most frequent side effects include digestive disorders, bleeding, nephrotoxicity, hepatoxicity and even death. It is advisable to avoid their use or use them with caution in hypovolaemic patients, those with heart, liver or renal failure, and those with gastric ulcers or coagulation disorders.2
LOCAL ANAESTHESIA
Local anaesthetics make an excellent addition to analgesic plans for dogs thanks to their efficacy and few side effects. Any side effects are generally due to accidental intravenous administration or overdose, and include ataxia, nystagmus, tremors, seizures and respiratory arrest. They can be administered topically, systemically, intrathecally or epidurally or by infiltration.2
α-2 AGONISTS
α-2 Agonists have a sedative and analgesic action and work in synergy with opioids, so are potentially useful in analgesic plans for dogs. However, their effects on the cardiorespiratory system (bradycardia, hypertension/hypotension and a reduced cardiac output, respiratory rate and tidal volume) should be considered when deciding if they are suitable for a given patient. In any event, their preoperative coadministration with opioids improves analgesia and sedation, while allowing for lower doses of both analgesics. Furthermore, they provide sedation and analgesia during the postoperative period, especially in dysphoric patients.2,4
OTHER ANALGESICS
Treatment refractory patients may need to receive additional analgesic agents. This includes anaesthetics such as low doses of ketamine or anticonvulsants, e.g., gabapentin or pregabalin.2 Gabapentin is primarily used for the treatment of neuropathic pain.
These drugs are unlikely to have a significant analgesic effect on the management of acute pain, but they can be very useful in surgical patients with prior experience of neuropathic pain, such as patients undergoing surgery for intervertebral disc disease.4,5 Another drug whose actual analgesic effect is not completely understood but, given its potential as an antiemetic, may be recommended in anaesthetic protocols is maropitant.4
Conclusions
Fortunately, the epoch when perioperative analgesia was disregarded, or even when a certain degree of pain in the incision area was considered to contribute to improved recovery (because the patient did not touch the wound), is already part of the dark history of veterinary medicine. The importance of good analgesia during the perioperative period is now widely recognised and accepted.
