Analgesia for the emergency patient

5 Analgesia for the emergency patient




‘An hour of pain is as long as a day of pleasure’ (English proverb)



Pain in Companion Animals


The advantages that the ability to feel pain confers with respect to survival are well recognized. However, on-going pain results in a number of physiological and psychological responses that are disadvantageous to any animal and especially one that is affected by illness or injury. These are summarized in Box 5.1.



BOX 5.1 Disadvantages associated with on-going pain



• Unnecessary suffering

• Altered mentation: anxiety, agitation; dullness, depression

• Reduced food intake – may slow recovery

• Prolonged recumbency

• Self-mutilation

• Impaired respiration

• Sensitization to noxious stimuli

• Excessive sympathetic stimulation, enhanced catabolism, delayed wound healing, immune suppression

• Inability to monitor patients effectively due to aberrant physiological changes and consequent clinical findings


Pain assessment




Nursing Aspect


Nurses should be considered central to on-going pain assessment as they typically spend more time with and around hospitalized patients. They are therefore often better suited to detect changes in pain status, especially as most of the signs of pain in companion animals are subjective.


The assessment of pain in animals is more difficult and more subjective than in people and veterinary personnel have to rely much more on a combination of sign recognition, patient interaction, experience, and a degree of anthropomorphism. The signs that an animal in pain may exhibit depend on various factors, including:


• The source, severity and duration (acute, chronic) of pain

• Individual patient variability – age, species, breed, tolerance, temperament.

Signs consistent with pain are summarized in Box 5.2 in no particular order and it is important to realize that these signs are derived from both patient observation and interaction. Painful cats are often much less demonstrative than painful dogs, and sick animals in general may be unable to show expected behavioural signs of pain. In addition, physiological parameters are not consistently reliable as indicators of pain.



BOX 5.2 Signs consistent with pain in dogs and cats



• Consistent response on palpation of painful area

• Poor appetite

• Restlessness (especially dogs); dullness, depression and lack of activity (especially cats)

• Looking, licking, biting or scratching at painful area; self-mutilation

• Vocalization: dogs – growling, whining, groaning; cats – purring, growling, hissing

• Reduced grooming behaviour

• Altered gait and reluctance to walk

• Abnormal posture (especially cats who often sit with a hunched sternal posture rather than lying curled in lateral recumbency)

• Reluctance to sleep

• Uncharacteristic behaviour (aggression or affection) towards humans

• Inappropriate urination and/or defecation

• (Unexplained tachycardia, tachypnoea/panting, hypertension, or pupillary dilation)


Clinical Tip



• There are certainly cases where it is not clear whether an animal is in pain. A liberal approach (i.e. if in doubt, treat for pain) is essential and carries minimal risk when based on good knowledge of the analgesics currently available. A low testing dose can be given and the patient monitored for a favourable response.

• The reader is also reminded that cats are not inherently at greater risk of adverse effects from analgesics and pain relief is as important in cats as it is in dogs.


The pain pathway


A basic knowledge of the pain pathway is essential to understand the modes of action of available analgesics and the rationale behind multimodal therapy. The pain pathway essentially consists of the following.


• Peripheral component:
– Tissue receptors that detect painful stimuli (nociceptors)

– Primary afferent nerve fibres that conduct the subsequent signal to the spinal cord (dorsal horn) or cranial nerve nuclei

• Central component:
– Processing of nociceptive signals (modulation) in the spinal cord

– Further processing in higher centres resulting in conscious perception of pain.


Sensitization


Sensitization (or wind-up) refers to increased sensitivity to noxious stimuli following injury to either peripheral tissues (causes inflammatory pain) or the central nervous system (causes neuropathic pain). This increased sensitivity results in:


• Hyperalgesia: an exaggerated responsiveness to a given noxious stimulus

• Allodynia: stimuli that are normally innocuous become painful.

Sensitization can occur in either or both of the peripheral and central components of the pain pathway.



The clinical relevance of sensitization


In practical terms, once sensitization is underway, conscious perception of pain is greater, it is more difficult to control and higher drug doses are required. It is therefore preferable to try to prevent pain rather than merely to treat it once it is established. A significant proportion of emergency patients are already in pain at initial presentation. In order to minimize sensitization, pain management should be instituted as soon as possible and, once the pain is under control, the patient kept on a level pain-free plateau. All animals going on to have operative procedures performed will benefit from adequate preoperative analgesia, i.e. in advance of the significant noxious stimulation resulting from surgical intervention.



Analgesic Agents


Balanced analgesia involves the use of one or more appropriate analgesics (multimodal therapy) in an effective dosing regimen given by the appropriate route(s). Analgesic agents that are commonly available in emergency clinics include:


• Opioids

• Nonsteroidal antiinflammatory agents (NSAIAs)

• Local analgesic/anaesthetic agents

• Ketamine

• Medetomidine.

An intuitive approach to pain management is based on thorough initial evaluation followed by regular reassessment of the patient and adjustment of the treatment protocol as indicated. This is especially relevant to the use of full agonist opioids which may be used at highly variable dose rates and dosing frequencies. As described later in this chapter, nonpharmacological measures are also very important in pain management.



Opioids




Clinical Tip



• In the author’s opinion, all veterinary practices that may see animals in moderate to severe pain should have a full (pure) opioid agonist available, ideally morphine or methadone. These agents are extremely affordable, highly effective, and allow dose titration to effect. Although they need to be stored and administered in accordance with regulations for controlled drugs, this should not be considered an impediment to their use.

Opioids (predominantly) act in the central component of the pain pathway. Most opioids used clinically are selective for the µ-receptor and are full (pure) agonists, partial agonists or agonists–antagonists. They are the mainstay of pain management in the emergency patient and block central sensitization. Opioids are also used in low doses for sedation or anxiolysis. High doses of pure opioids in particular may result in dysphoria (vocalization, anxiety, restlessness) that can be difficult to differentiate from on-going pain. Naloxone (or butorphanol) can be used to reverse the effects of full opioid agonists.


Both respiratory depression and bradycardia are rarely seen as side effects of concern during opioid administration in clinical veterinary patients that are not under anaesthesia. Furthermore, pain itself can cause respiratory compromise and appropriate opioid administration can normalize respiratory status in such cases. Until adequately treated, pain can also confuse assessment of other parameters such as heart rate and blood pressure. Dose rates, routes of administration and dosing frequencies of opioids in current use in emergency clinics are summarized in Table 5.1.



Table 5.1 Clinical information on opioid analgesics

image


Morphine


Morphine is a full µ-agonist and remains the gold standard analgesic for moderate to severe pain. Onset of action is usually 10–15 minutes following intravenous administration. In addition to its analgesic effects, morphine is also extremely useful as an anxiolytic in respiratory distress. Vomiting is more likely than with other opioids and will further exacerbate raised intracranial or intraocular pressure. Morphine should therefore be avoided in patients where this is of concern. Rapid intravenous administration may cause signs associated with histamine release (transient vasodilation and hypotension) and slow intravenous boluses or a constant rate infusion are recommended.



Methadone


Methadone is a full µ-agonist that has the same or greater potency as morphine. Its sedative and dysphoric effects are reportedly less marked than with morphine and it does not typically induce emesis. Methadone does not cause histamine release following intravenous administration. In the author’s experience, panting (due to effects on the thermoregulatory centre) occurs much more commonly following methadone administration in dogs than it does with morphine and this may complicate clinical assessment.



Buprenorphine




Clinical Tip



• Although buprenorphine may be effective for mild to moderate pain, it has a slow onset of action and is therefore not ideal as a first line analgesic for rapid pain relief. Full opioids are preferred in such cases.

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Sep 3, 2016 | Posted by in SMALL ANIMAL | Comments Off on Analgesia for the emergency patient

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