Intensive Care Unit, Department of Clinical Sciences of Companion Animals, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands
"The inability to communicate in no way negates the possibility that an individual is experiencing pain and is in need of appropriate pain relieving treatment" (International Association for the Study of Pain 1994).
Accurate recognition and effective management of pain are an extremely important part of our patient management and as clinician we have both an ethical reason towards our patients and a medical reason to address pain.
Pain is usually classified as physiologic (protective pain) or pathologic, acute or chronic. The type of pain most commonly recognized in an emergency setting is the pathologic and acute pain. Pathologic pain is a painful stimulus that occurs when tissues have already been damaged. Acute pain is exemplified by traumatic or postoperative pain and is often associated with inflammatory disease. Many diseases that require emergency and critical care management are painful: trauma, organ distension, pleuritis and peritonitis, immobility and especially most of the diagnostic and therapeutic procedures that are often required (IV catheter placement, position of an indwelling urine catheter, position of a chest drain etc.) can be associated with a painful experience.
Pain, when left untreated, besides inducing unnecessary suffering, will also generate a stress response and catabolic state with resultant increased metabolic and energy demands. Other negative effects of pain are delayed wound healing, reduced immune function, weight loss, lack of sleep, decreased mobility, with an increase in conditions associated with prolonged recumbency (e.g., urine and faecal retention, pneumonia, nosocomial infections).
Approach to Pain Management
Careful and compassionate approach of the injured animal, minimal restraint techniques are very important to limit further pain experience during medical procedures and physical examinations.
Drug therapy might be directed to one or more steps in the nociceptive pathway. Multiple drugs with synergic effects and targeting nociception at different steps, can be combined in a multimodal analgesic protocol.
Analgesic drugs available are opioids, nonsteroidal anti-inflammatory drugs (NSAIDs). In addition, local anaesthetics, ketamine, medetomidine, tramadol have analgesic properties, which may be beneficial in a multimodal protocol.
Drugs of choice for moderate to severe pain; commonly used in critically ill patients because of potent analgesic effect, rapid onset of action, large safety, and their reversal is possible.
Opioids have been classified as agonists, partial agonists, or agonist antagonists depending on the dose-response relation of the drug at the different opioid receptors.
μ-receptor agonist: Gold-standard analgesic for moderate to severe pain in dogs and in cats and their analgesic effect is more efficient as there is no ceiling limiting effect to the analgesia provided.1,2
Morphine: It confers sedation and optimal analgesia and they are both dose dependant, reliable in different clinical settings. Used at 0.1–0.4 mg/kg IV, SC, IM or as constant rate infusion (CRI) at 0.1–0.4 mg/kg/h. Rapid morphine intravenous administration has been associated to:
Histamine release, so it should always be administered very slowly
Methadone (0.1–0.4 mg/kg IV, IM, SC): It acts very similar to morphine but it has been associated to less sedation and vomiting.
Fentanyl: It has a rapid onset (3 minutes) of action but a short duration (10 min) of its analgesic effects so it has limited use when used as a bolus (1–3 µg/kg), but it provides great analgesia when used a CRI (3–5 µg/kg/h).
Butorphanol (0.1–0.4 mg/kg IV, SC, IM): Minimal analgesic effect and of short duration (30 min) but adequate sedative effects.
Buprenorphine: Well-recognized analgesic effect for mild to moderate pain. Long onset of action (45 min) but long duration of action (6 hours).
Different side effects have been described with the use mainly of pure µ-opioid (respiratory depression, dysphoria, vomiting, etc.), but these are mainly recognized when a high dose is used in minimally painful animals.
Nonsteroidal anti-inflammatory drugs (NSAIDs) are potent analgesic, antipyretic and anti-inflammatory drugs. NSAIDs have a slow onset of action (up to 45 to 60 min), but they provide analgesia for an extended amount of time.
The most common types of NSAIDs available for use in dogs and cats are:
Carprofen: Licensed in dogs for acute and chronic use and in cats for a single preoperative use of 4 mg/kg.
Meloxicam: Licensed in dogs and cats for acute and chronic pain.
Metacam at a lower dosage (0.05 mg meloxicam/kg body weight) can be used as a follow-up dose in cats for acute pain to be administered once daily (at 24-hour intervals) for up to five days.
NSAIDs block the enzyme cyclooxygenase decreasing the production of prostaglandins. Prostaglandins are necessary to maintain gastrointestinal and kidney perfusion in animals that are in shock, severely dehydrated, in patients with previous history of gastrointestinal ulcerative disease. For these reasons, NSAIDs should never be used in unstable emergency patients as they might place these patients at severe risk of developing:
Acute renal failure
Worsening bleeding because of decreased platelet function
Other Analgesic Drugs
Ketamine is a competitive N-methyl-D-aspartate receptor antagonist. Ketamine causes minimal cardiovascular and respiratory depression. A small bolus of ketamine (0.3–0.5 mg/kg IV), followed by low-dose ketamine CRI (3–5 µg/kg/min) can be used for intra- and postoperative multimodal analgesia and at this dosage the dissociative and dysphoric effects are usually minimal.
Tramadol (2–4 mg/kg IV, PO q 8 h) provides a mild opioid-like analgesic effect. It's useful for the treatment of acute and chronic pain of moderate intensity associated with a variety of conditions.
Medetomidine or dexmedetomidine administered as a low-dose CRI (1–2 µg/kg/h) in stable patients are used to provide sedation, supplement analgesia, and reduce the stress response in agitated animals.
Local anaesthetic: Lidocaine, like other sodium channel-blocking drugs (bupivacaine, Chirocaine) is usually classified as a local anaesthetic. Lidocaine has also been shown to have mild analgesic effects when administered as a low-dose CRI (50 µg/kg/min) in a multimodal protocol.
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