Chapter 6 Pain Management in the Surgical Patient
Like anesthesia, the perioperative assessment of pain and the provision of analgesia have become an integral part of companion animal medicine. Regardless of the type of medical or surgical procedure performed, pain management must be considered a part of the perianesthetic drug plan.
DEFINITIONS
• Pain: Pain is defined as an unpleasant sensory and emotional experience associated with actual or potential tissue damage. 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.
• Nociceptor: Specialized sensory nerve endings that respond to the mediators of tissue damage. These mediators are mechanical, thermal, and chemical.
• Nociception: The sensing and ascending neural transmission of a noxious stimulus. Nociception that is centrally processed can be interpreted as pain.
• Somatic pain: Pain caused by activation of nociceptors within the skin and musculoskeletal tissue. This is usually described in people as sharp and localized when associated with superficial tissue and dull or aching when associated with deeper tissues.
• Visceral pain: Pain that is poorly localized and described by people as a pressure-like or squeezing sensation. It is associated with activation of nociceptors within a body cavity.
• Neuropathic pain: Pain related to nervous system damage. This is usually described in people as a burning sensation.
• Acute versus chronic pain: Acute pain is the normal response to an inciting stimulus, such as the pain resulting from surgical stimulation, trauma, or thermal injury. Perioperative analgesic plans are formulated to minimize this type of pain. Chronic pain persists longer than that associated with healing of an acute condition or that associated with a malingering condition. Perianesthetic analgesia can temporarily alleviate this type of pain.
PAIN PATHWAYS
• Transduction: Tissue injury releases chemical mediators (prostaglandins, histamine, and cytokines) that can activate nociceptors. Continued nociceptive stimulation sensitizes the nociceptors (peripheral nociceptive sensitivity), resulting in hyperalgesia (increased response to a painful stimulus) and allodynia (pain from a normally non-painful stimulus) localized to the area of insult.
• Transmission: Nociceptive signals are transmitted to the spinal cord by primary afferent A-delta or C fibers. The thinly myelinated A-delta fibers conduct rapidly and are responsible for rapid reflex responses such as limb withdrawal from the inciting stimulus. The non-myelinated C fibers conduct more slowly. Cell bodies of these fibers are located in the spinal cord dorsal horn. Axons from these neurons synapse in the superficial layers of the spinal cord dorsal horn. Activated dorsal horn neurons send their impulses toward the brain via ascending spinal tracts. Interneurons located within the dorsal horn can modify the nociceptive signals via the release of endogenous opioids. Continued C-fiber dorsal horn stimulation increases sensitivity of these neurons to nociceptive stimulation (central sensitization), resulting in allodynia in anatomic areas adjoining the area of initial insult. This central sensitization is referred to as “wind-up.”
• Perception: Ascending nociceptive transmission results in perception or awareness of pain in some part of the body. Cortical and limbic systems of the brain are involved. This evokes the autonomic and affective behavioral aspect of pain.
• Modulation: Descending inhibitory signals from the brain act in the dorsal horn to modify nociceptive signals via opioids, norepinephrine, serotonin, and γ-aminobutyric acid (GABA).
PAIN ASSESSMENT
• Pain is an individual experience and can be difficult to quantitate given the diversity of species, breeds, and individual temperament.
• Pain-related behaviors such as vocalizing, guarding, lameness, and trembling are easily recognized. However some animals, particularly cats, will not develop these “obvious” signs (Table 6-1). Furthermore, vocalization can be a manifestation of emergence from anesthesia, breed or individual temperament, or opioid induced dysphoria.
• As yet, no one blood chemical value specifically relates to pain. Plasma glucose, cortisol, catecholamine, and endorphin concentrations are not specific for pain.
• Changes in physiologic variables such as heart rate, blood pressure, and respiratory rate are nonspecific for pain and can change in response to anesthetic recovery.
• In addition to recognizing the commonly perceived signs of pain, anticipating pain related to a type of procedure facilitates preemptive treatment (Table 6-2). In general, the greater the degree of tissue trauma related to a surgical procedure, the greater the pain. The degree of tissue trauma also relates to the surgeon’s experience and skill as well as the duration of the procedure.
Table 6-1 SIGNS OF PAIN
Behavioral Signs
Withdrawal, avoidance, fear, and fear biting
Refusal to lie down, sleep, or groom
Withdrawal to the back of the cage or enclosure
PAIN SCORING SYSTEM
• Use a pain scoring system when assessing pain. A standardized scoring system minimizes person-to-person inconsistency in evaluation. A pain scoring system minimizes observer bias based on breed characteristics.
• Scoring systems range from extensive to simple and take into account combinations of physiologic, behavioral, and neurochemical responses. A simplified scoring system that relies on observation of behavior can be performed quickly (Table 6-3).
• Regardless of the pain score, use common sense. If a given procedure would cause you pain, then administer perioperative analgesic therapy. If you think the animal is experiencing pain, administer analgesia.
Table 6-3 SIMPLIFIED PAIN SCORING SYSTEM
| Reaction to Gently Probing of Wound While Animal Is Awake | |
| No response | 0 |
| Looks at wounded area, may slightly withdraw affected area | 1 |
| Turns toward wounded area, withdrawal of affected area, some vocalization | 2 |
| Dramatic response such as violent withdrawal, loud vocalization | 3 |
| Comfort | |
| Asleep, calm and resting quietly | 0 |
| Refusal to lie down or assume normal posture but willing to approach when coaxed | 1 |
| Severe vocalizing, thrashing, extreme guarding of affected area, withdrawn and motionless, unresponsive to coaxing | 2 |
| Physiologic Responses | |
| Normally expected postoperative changes in heart rate and respiratory rate | 0 |
| Tachycardia, tachypnea not explainable by other physiologic causes (dehydration, stress, hypoxemia, etc.), dilated pupils | 2 |
| Respiratory Pattern | |
| Normal | 0 |
| Guarded, mild abdominal | 1 |
| Marked abdominal | 2 |
| Observation and scoring must be influenced by observation of behavior prior to induction of painful stimulus | |
| Minimum score (no pain) | = 0 |
| Maximum score (maximum pain) | = 9 |
| Any score other than zero warrants consideration of analgesic therapy, with higher scores suggesting more potent and multimodal therapy | |
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