Pain Recognition and Management in Critical Care Patients
Pain management is crucial in critical care patients. Pain has multiple negative effects that can delay or prevent healing, and veterinary technicians play a central role in pain management. Understanding pain, its consequences, and how it can be addressed increases veterinary technicians’ ability to work with veterinarians to ensure that patients are comfortable both during hospitalization and when they go home.
Physiology of Pain
Nociceptors are unmyelinated peripheral neurons sensitive to noxious stimuli. They are found throughout the body and are triggered by mechanical, chemical, or thermal stimuli.1
When a stimulus activates a nociceptor, a nerve impulse is produced. This impulse is transmitted to the dorsal horn of the spinal cord, where it causes the release of neurotransmitters (e.g., aspartate, glutamate γ-aminobutyric acid [GABA]) and neuropeptides (substance P). Some of these are excitatory and continue to send the signal up the spinal cord to the brain, while others are inhibitory and inhibit the signal from traveling further. Inflammation causes an increase in excitatory neurotransmitters while reducing inhibitory neurotransmitters.2
There are three types of nerve fibers: A-delta, A-beta, and C fibers. A-delta fibers are medium to small, thinly myelinated, and conduct at a speed of 6 to 30 m/sec. A-beta fibers are larger, myelinated, and conduct at a speed of 30 to 70 m/sec. C fibers are small, unmyelinated, and conduct at a speed of 0.5 to 2 m/sec.1 A fibers are sensitive to thermal or mechanical stimulation, causing a sharp, localized pain, while C fibers can be sensitive to thermal, mechanical, or chemical stimuli, causing a dull, aching pain that is more diffuse.1,3
Pathophysiology of Pain
Pain, whether from injury or surgery, can have many detrimental effects if left untreated. The release of catecholamines in response to a painful stimulus results in tachycardia, hypertension, and an increase in oxygen consumption by the myocardium. If prolonged, this can lead to left ventricular dysfunction, ischemia, and possible infarction. An increase in the release of cortisol and glucagon can cause insulin resistance and hyperglycemia. Stress caused by pain increases the activity of clotting factors, leading to an increased risk for developing a coagulopathy. Stress also suppresses the immune system, leading to an increased risk for infection.4
If thoracic pain is present, the patient may be unwilling to breathe normally, leading to atelectasis. Decreased gastrointestinal motility and the development of ileus and urine retention after abdominal surgery (secondary to pain and an unwillingness to posture to urinate) can cause decreased bladder function. The patient may also be anxious, agitated, and unable to rest, prolonging the recovery period.4
Recognition of Pain
Recognition of pain has always been difficult in veterinary patients. Some patients are stoic and do not demonstrate obvious signs of pain, while others are very sensitive and will react to the slightest touch. The patient may be hunched, splint (stiffen its muscles) with abdominal palpation, vocalize (growl, hiss), refuse to lie down, stretch (FIGURE 1), be restless or agitated, be tachycardic or hypertensive, refuse to rise, or walk stiffly. If measures taken to relieve pain seem unsuccessful, it is helpful to ask the owner about the patient’s normal behavior and attitude.
While pain management is a team effort, the veterinary technician plays a large role in recognition and treatment of pain. The patient should be assessed at the beginning of the shift so that any changes can be noted throughout the day.
Pain Versus Anxiety
It is important to be able to distinguish between pain and anxiety because both can cause tachycardia, hypertension, and changes in posture. Removing the patient from its kennel or taking it out for a short walk may alleviate some anxiety. If an outside area is available, it is often advantageous to assess the patient outdoors to see if it is more relaxed out of the hospital environment.
Patient comfort while in the kennel is equally important to reducing anxiety; however, individual preferences vary. A soft, thick bed can keep older patients more comfortable, while other patients prefer a thin blanket and refuse to lie down if the bed is too thick or soft. Cats often prefer a bed, although occasionally they prefer to lie in the litterbox. Cats that will not use a standard bed may be happier with a blanket placed in a second litterbox or no bed at all.
Some canine breeds are normally vocal (e.g., beagles, Northern breeds). Veterinary technicians should be able to differentiate between normal vocalization and vocalization secondary to pain or discomfort.
Gentle palpation of the abdomen can reveal abdominal pain. Some patients become tense immediately on being touched, and it is helpful to place your hands on either side of the abdomen, wait for the patient to relax, and then slowly palpate the abdomen. If the patient tenses, leave your hands where they are until the animal relaxes again; then continue palpating. If the patient does not relax or is vocal, pain is likely present, and the veterinarian should be notified.
If the patient is tachycardic when auscultated, it is advantageous to leave the stethoscope in place for a minute or two to give the patient a chance to relax and the heart rate to normalize. Pain is only one cause of tachycardia and other causes (e.g., hypovolemia, anxiety, excitement) should be ruled out.
Painful patients are reluctant to move and often refuse to lie down or stand up because they anticipate pain associated with changing position (FIGURE 2). If a patient does not appear painful but refuses to lie down, it may be anticipating pain. It is often helpful to physically lay the patient down and sit with it until it has relaxed.
Pain scales can be used to ensure pain assessment remains consistent. Colorado State University has developed pain scales for dogs and cats that assess behavior as well as response to environment and palpation (see CLINIC RESOURCES page).
Cats are often difficult to assess. They may be fractious by nature, and so hissing or growling may not indicate pain. Tachycardia and increased respiratory rate may be signs of anxiety rather than pain, and hiding, while a possible sign of pain, can also be caused by simple fear. To decrease stimuli that may contribute to fear and anxiety, covering the cage with a blanket may be beneficial. If possible, keeping feline patients in quieter surroundings (e.g., away from noisy dogs) may also help.
A painful cat may be quiet while refusing to move or may become aggressive and growl or hiss while rolling in its kennel.5 It is worthwhile to try observing the cat from a distance in case your presence causes anxiety. When palpating for pain, it is recommended to palpate the area several times and watch for a repeatable pain response. If you are unsure whether the patient is painful, it is always best to assume that it is and administer analgesics.
If the veterinary technician believes a patient is in pain, he or she should approach the clinician to discuss additional pain management. It is in the patient’s best interest to obtain as much information as possible before going to the clinician with concerns (BOX 1). When talking to the clinician, technicians should be ready to discuss observed signs of pain, what has been done to address other possible causes of discomfort (e.g., walking the patient, checking the bedding, ensuring the intravenous catheter is patent), and what analgesics the patient is receiving, including the dose, frequency, and time of last administration.
Of equal importance is communication between veterinary technicians. Rounds at the end of the shift should include information on the patient’s pain status as well as any concerns and opinions, including whether the patient is more anxious than painful or how it is responding to new analgesics. The same information given to the clinician should be given to the technician taking over care.
It is vital that veterinary technicians be knowledgeable about the different types of pain medication available and the mechanism of action for each drug. TABLE 1 lists the most common pain medications used in veterinary medicine, the level of pain relief they provide, and their standard dosages.
|DRUG||PAIN LEVEL||CANINE DOSAGE||FELINE DOSAGE||COMMENTS|
|Tramadol||Mild to moderate||2–4 mg/kg PO
|2–4 mg/kg PO
|Morphine||Severe||0.5–2.0 mg/kg IV
|0.2–0.5 mg/kg IV
|Butorphanol||Mild to moderate||0.1–0.4 mg/kg IV/IM q1–4h||0.1–0.4 mg/kg IV/IM q2–6h||
|Hydromorphone||Severe||0.05–0.2 mg/kg IV
|Contraindicated in cats because of potential for hyperthermia10||
|Oxymorphone||Severe||0.05–0.4 mg/kg IV
|0.2–0.5 mg/kg IV
|Buprenorphine||Moderate to severe||0.005–0.02 mg/kg IV/IM q6–8h||0.005–0.02 mg/kg IV/IM/PO
|Fentanyl||Moderate to severe||2–5 mcg/kg/h CRI||2–5 mcg/kg/h
|0.0005–0.0008 mcg/kg/h CRI||
|N-METHYL-D-ASPARTATE (NMDA) AGONIST|
|Lidocaine||Adjunct||25–75 mcg/kg/min CRI||10–40 mcg/kg/min CRI, but use with caution in this species||
|2.5–5.0 mg/kg PO q12h17||
|Carprofen||Mild alone; moderate with opioid||2.2 mg/kg SC or PO BID
4.4 mg/kg SC or PO q24h
|Contraindicated in cats||
Several opioids are used in veterinary medicine. Opioids are classified based on the receptor they bind to as well as the effect of binding on receptor activity.6 Some opioids bind to only one receptor, while others bind to multiple receptors. There are three primary opioid receptors6:
- Mu—located in multiple areas, including the brain (thalamus and cortex) and spinal cord
- Delta—located in the brain
- Kappa—located in the brain and spinal cord
When an opioid binds to a receptor, it may cause increased receptor activity with a maximum effect (agonist), increased receptor activity that plateaus at an effect lower than maximum (partial agonist), or decreased receptor activity (antagonist).6
The potency of each opioid is compared with that of morphine (TABLE 1).6 The greater the potency, the smaller the dosage required for the drug to be effective.
Nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit cyclooxygenase (COX), an enzyme involved in the production of prostaglandins, some of which are mediators of the inflammatory response. There are two forms of COX: COX-1 and COX-2. COX-1 produces prostaglandin E2 (PGE2), which plays a role in several functions involving the gastrointestinal system, such as increasing mucus and bicarbonate secretion, decreasing gastric acid secretion, and increasing the rate of turnover of gastric mucosa cells. COX-1 is also indirectly involved in coagulation processes.
In addition to PGE2, COX-2 produces prostacycline (PGI2), which causes vasodilation, inhibits platelet aggregation, and is involved with inflammation. PGE2 and PGI2 also decrease blood flow to the kidneys.
Carprofen is a COX-2 specific NSAID, meaning it inhibits COX-2 without blocking the activity of COX-1. As a result, it has fewer gastrointestinal side effects, is less likely to cause bleeding secondary to platelet inhibition, and is less likely to cause renal disease.19
Veterinary technicians are usually the ones calculating drug dosages and constant-rate infusions. It is important for technicians to be comfortable with these calculations and to be able to perform them accurately. Often, multiple calculations are required for a single drug, as shown in the CASE EXAMPLE, below.
If the patient is on a constant-rate infusion (such as with fentanyl), a dose range allows the veterinary technician to make adjustments based on the patient’s pain level. This gives technicians the freedom to address pain quickly.
Assessing Response to Medication
Veterinary technicians play an important role in pain management. Often, because technicians monitor patients through their hospital stay, they are more able to recognize changes in patients’ behavior and can quickly tell if a patient is showing signs of pain or appears more comfortable.
Changes in patient attitude, such as willingness to lie down or a decrease in anxiety, indicate whether pain management is adequate (FIGURE 3). Normalized heart rate, respiratory rate, and blood pressure are also signs that pain is well controlled.
Other Methods of Minimizing Pain
It is also helpful to combine treatments so the patient is disturbed as little as possible. Bloodwork can be scheduled during normal treatment times, as can subcutaneous or intramuscular injections. If the patient is recumbent, taking it outside on a gurney can brighten its spirits. Depending on the cause of the pain, warm or cold compresses can be helpful.
Considerations in Special Patient Populations
Patients with Thoracostomy Tubes
Thoracostomy tubes are known to cause severe pain that prevents the patient from fully expanding its lungs, which can lead to atelectasis. These patients are also reluctant to lie down and unable to rest. While systemic analgesia (primarily opioids) does address this pain, these patients benefit greatly from additional analgesia, including intercostal blocks or intrapleural analgesia.
Intercostal blocks are performed by injecting bupivacaine (0.5%) caudal to the head of the ribs surrounding the insertion site of the thoracostomy tube. The bupivacaine dose should not exceed 5 mg/kg. If the patient and the volume of the dose are both small, the dose can be diluted with 0.9% saline. The total amount can be divided between several injection sites and can be given every 6 to 12 hours.20
Intrapleural analgesia may be more effective because the medication is injected directly into the pleural space. At the author’s hospital, bupivacaine at a dose of 1 mg/kg is injected into the thoracostomy tube, followed by 0.9% saline to ensure the drug reaches the pleural space. This is followed by injection of 1 mg/kg of lidocaine into the thoracostomy tube, and the tube is again flushed with 0.9% saline. This procedure is performed every 6 hours until the thoracostomy tube is removed.
Neonatal, Pregnant, and Lactating Patients
Drug absorption is disrupted in pregnant animals because of a decrease in gastrointestinal motility, increases in cutaneous blood flow, low-normal serum albumin levels, rapid renal secretion, and increases in total body water.21 Drugs that are lipophilic will cross the placental barrier. Ionized, polar, or protein-bound drugs are less likely to do this.21
Drugs that are highly lipid soluble or non-ionized are excreted in milk and should be avoided in nursing animals. Current estimates state that nursing neonates will receive 1% to 2% of the drug dose.21
Pediatric patients may not be affected by some medications (e.g., ketamine) due to their underdeveloped N-methyl-D-aspartate (NMDA) system.21
NSAIDs should be avoided in pregnant patients because they block the production of prostaglandins. This can cause fetal abnormalities (orofacial cleft, ductus arteriosus, and underdeveloped kidneys). Fetuses and neonates may also eliminate some drugs more slowly due to their increased body water, increased tissue perfusion, lower plasma protein level, and immature hepatic system.21
Veterinary technicians must act as patient advocates. Because technicians have the most contact with patients, they are in an ideal position to monitor and address pain. Understanding the physiology and pathophysiology of pain, as well as being aware of the options available, can only serve to elevate patient care.
- Bourne S, Machado AG, Nagel SJ. Basic anatomy and physiology of pain pathways. Neurosurg Clin N Am 2014;25(4):629-638.
- Reardon DP, Anger KE, Szumita PM. Pathophysiology, assessment, and management of pain in critically ill adults. Am J Health Syst Pharm 2015;72(18):1531-1543.
- 3. Steeds CE. The anatomy and physiology of pain. Surgery (Oxford) 2013;31(2):49-53.
- Dunwoody CJ, Krenzischek DA, Pasero C, et al. Assessment, physiological monitoring, and consequences of inadequately treated acute pain. J Perianesth Nurs 2008;23(1 suppl):S15-S27.
- Taylor PM, Robertson SA. Pain management in cats—past, present and future. Part 1. The cat is unique. J Feline Med Surg 2004;6(5):313-320.
- KuKanich B, Clark TP. The history and pharmacology of fentanyl: relevance to a novel, long-acting transdermal fentanyl solution newly approved for use in dogs. J Vet Pharmacol Ther 2012;35(suppl 2):3-19.
- Gaynor SG, Muir WW. Handbook of Veterinary Pain Management. 3rd ed. St. Louis, MO: Elsevier; 2015.
- Epstein ME. Opioids. In: Gaynor JS, Muir W, eds. Handbook of Veterinary Pain Management. 3rd ed. St. Louis, MO: Elsevier; 2015:161-195.
- Mathews KA, Dyson DH. Analgesia and chemical restraint for the emergent patient. Vet Clin North Am Small Anim Pract 2005;35(2):481-515, viii.
- Hansen B. Analgesia for the critically ill dog or cat: an update. Vet Clin North Am Small Anim Pract 2008;38(6):1353-1363, vii.
- Ko JC, Freeman LJ, Barletta M, et al. Efficacy of oral transmucosal and intravenous administration of buprenorphine before surgery for postoperative analgesia in dogs undergoing ovariohysterectomy. JAAHA 2011;238(3):318-328.
- Robertson SA, Taylor PM. Pain management in cats—past, present and future. Part 2. Treatment of pain—clinical pharmacology. J Feline Med Surg 2004;6(5):321-333.
- Hansen BD. Analgesia and sedation in the critically ill. J Vet Emerg Crit Care 2005;15(4):285-294.
- Berry SH. Analgesia in the perioperative period. Vet Clin North Am Small Anim Pract 2015;45(5):1013-1027.
- Terkawi AS, Sharma S, Durieux ME, et al. Perioperative lidocaine infusion reduces the incidence of post-mastectomy chronic pain: a double-blind, placebo-controlled randomized trial. Pain Physician 2015;18(2):E139-E146.
- Dyson DH. Analgesia and chemical restraint for the emergent veterinary patient. Vet Clin North Am Small Anim Pract 2008;38(6):1329-1352, vii.
- Lamont LA. Adjunctive analgesic therapy in veterinary medicine. Vet Clin North Am Small Anim Pract 2008;38(6):1187-1203, v.
- Quandt J. Analgesia, anesthesia, and chemical restraint in the emergent small animal patient. Vet Clin North Am Small Anim Pract 2013;43(4):941-953.
- KuKanich B, Bidgood T, Knesl O. Clinical pharmacology of nonsteroidal anti-inflammatory drugs in dogs. Vet Anaesth Analg 2012;39(1):69-90.
- Pavlidou K, Papazoglou L, Savvas I, Kazakos G. Analgesia for small animal thoracic surgery. Compend Contin Educ Pract Vet 2009;31(9):432-436.
- Mathews KA. Analgesia for the pregnant, lactating and neonatal to pediatric cat and dog. J Vet Emerg Crit Care 2005;15(4):273-284.