Pain Management for Gastrointestinal Conditions

ArticleLast Updated September 20158 min readPeer ReviewedWeb-Exclusive

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Gastrointestinal (GI) diseases are common presenting complaints in veterinary patients. Both primary and secondary GI diseases can be painful, including pancreatitis with its associated inflammation and autodigestion,¹ ileus secondary to viral illnesses (eg, parvovirus) or other causes, cramping from colitis, and distention from obstruction. The clinical manifestations of abdominal pain in dogs and cats can be subtle and easily overlooked, but if there is any concern as to whether a patient is experiencing pain, initiating treatment and monitoring the patient’s response represent the most humane course of action.² Analgesia, therefore, is an essential part of case management.

Analgesics are considered standard therapy in patients with pancreatitis and gastric dilatation-volvulus (GDV); however, pain management may not be readily implemented in patients with obstruction, ileus, and inflammation. Abdominal pain in and of itself can cause a functional ileus. Patient comfort is thus paramount in maintaining a good quality of life while diagnostics are performed and more targeted therapies pursued.³ Effective management of visceral pain can be more challenging than managing other types of pain (eg, musculoskeletal) because of the lack of overt clinical signs, difficulty with GI absorption (if a patient is not eating or has a malabsorptive syndrome), and adverse effects of the medications on GI function.

There are a variety of medication choices to treat abdominal pain, but decisions may be limited by inventory availability, routes of administration, and comorbid conditions.

Opioids

Opioids as a class provide very effective analgesia in the treatment of abdominal pain. They can, however, decrease GI motility and cause constipation, although this primarily occurs with chronic use.⁴ If a patient is obstipated, long-term use of opioids is not recommended; however, a short-term course of opioids after manual evacuation should not worsen the constipation.

Opioids can reportedly increase plasma lipase and amylase up to 24 hours after being administered, so serial laboratory work should be interpreted accordingly.⁵ Although there is no evidence that opioids can induce pancreatitis, laboratory results for serial amylase and lipase may not decrease as quickly as expected.

Morphine & Its Derivatives

Morphine and its derivatives (eg, oxymorphone, hydromorphone) are mu-receptor agonists that provide effective GI analgesia. However, these drugs can cause vomiting, especially when administered as intermittent boluses, although this adverse effect is more noticeable with administration of morphine than with its derivatives.⁶ Morphine and its derivatives can also cause weakness and respiratory depression when administered at high dosages. In addition, these agents can initially cause panting, which in turn can increase the patient’s body temperature.

Morphine and its derivatives have a high first pass effect with low oral bioavailability; therefore, they are mostly administered IV.⁵

Fentanyl

Fentanyl, a mu-receptor agonist, can be useful in dogs with severe abdominal pain, including as postsurgical pain relief and in those with severe pancreatitis. Its short duration of action requires continued exposure to the drug (eg, via constant-rate infusion [CRI], sustained-release transdermal patch or solution) for effective pain management. The sustained-release patch and solution are useful in patients that cannot be managed with CRI or oral administration because of client finances or patient temperament, respectively. The patches take about 12 to 24 hours to reach effect and typically last about 72 hours; however, duration of action can be variable, and therapeutic levels do not occur in some patients.³

Buprenorphine & Butorphanol

Buprenorphine, a partial mu-opioid agonist, and butorphanol, a mixed mu-opioid agonist–antagonist, are also common choices for pain management. Buprenorphine may be preferable because of its longer duration of action. Because of the short duration of action of butorphanol, CRI administration may be more effective. However, butorphanol has documented antiemetic properties, and in cases of overdose, is easier to reverse than buprenorphine.³ Both agents can be administered by IV, SC, or buccal routes, thereby enabling their use as part of outpatient care.⁷

Cats tolerate the taste of buprenorphine better than tramadol, and the liquid formulation is easier to administer than pills.⁸ Buprenorphine, however, is not highly bioavailable when swallowed, but it does absorb through the buccal mucosa quite quickly. Partial mu-opioid agonists are less likely to cause spasms in the sphincter of Oddi (spasms that theoretically could increase pain associated with pancreatitis), the concerns of which may be exaggerated, as demonstrated by recent studies in humans.⁹

Tramadol

Tramadol, an opiate-like weak mu-opioid agonist with monoamine reuptake inhibition, can be used for mild visceral pain, but its oral administration and bitter taste limits its administration to patients that are eating and/or are not vomiting.

Lidocaine & Ketamine

Lidocaine administered IV can be useful in alleviating generalized pain or as a local perioperative anesthetic. Adding lidocaine to the pain management plan for a patient may help decrease ileus (although very high IV doses can induce ileus). In addition, lidocaine is a free radical scavenger.¹⁰

Ketamine can decrease central sensitization and also enhance opioid receptor sensitivity.⁸ Low-dose ketamine has also been identified as a safe additive to opioid analgesic protocols in a review of clinical trials; such protocols may be useful in surgical patients requiring large perioperative opioid doses, including major abdominal surgeries.¹¹

Nonsteroidal Anti-inflammatory Drugs

NSAIDs are not recommended for GI pain management, as they generally lack efficacy for this purpose³ and are associated with high risk of adverse effects in patients with GI disorders. The most concerning adverse events are ischemic kidney injury when given to dehydrated patients, GI ulcers (especially in hypovolemic patients or when used in combination with steroids), and hepatotoxicity.¹²

Common Drugs Used for GI Pain Management in Dogs & Cats

Drug (Used Alone)	Dose & Route (Dogs)	Dose & Route (Cats)	Comments
Morphine	0.5–2 mg/kg IM, SC, or IV (slowly); redose every 2 hours prn 0.1–0.2 mg/kg/hr CRI	0.05 – 0.4 mg/kg IM, SC	Efficacy and adverse effects unreliable in cats Can cause vomiting Avoid with obstipated patients
Hydromorphone	0.1-0.2 mg/kg IV, IM or SC every 2–4 hours 0.01-0.05 mg/kg/hr CRI	0.05-0.1 mg/kg IV, IM or SC every 2-6 hours 0.01-0.05 mg/kg/hr CRI	Approximately 5x more potent than morphine Efficacy and adverse effects unreliable in cats Generally less sedating than morphine
Oxymorphone	0.05-0.2 mg/kg IV, IM, or SC every 1-6 hours as necessary	0.05-0.2 mg/kg IV, IM, or SC every 1-6 hours as necessary	10x more potent than morphine Efficacy and adverse effects unreliable in cats Less likely to cause nausea and vomiting compared to morphine
Fentanyl	2-5 µg/kg/hr CRI ¹⁰ For information on sustained release transdermal patch or solution, see specific product information	0.1-0.4 µg/kg/hr CRI ¹⁰ For information on sustained release transdermal patch or solution, see specific product information	Cat generally dosed at the low ends of dosage range Transdermal application variable in cats
Buprenorphine	5-30 µg/kg (0.005-0.03 mg/kg) IV, IM or SC every 6–12 hours 2-4 mcg/kg/hour CRI 120 µg/kg buccally	10-30 µg/kg (0.01-0.03 mg/kg) IM, IV, buccal every 6-8 hours	Buccal administration may provide better analgesia in cats than parenteral morphine or oxymorphone For additional formulations, see individual package inserts from manufacturer
Butorphanol	0.1-0.5 mg/kg IV, IM, SC every 2-4 hours 0.1-0.4 mg/kg/hr CRI	0.1-0.5 mg/kg IV, IM, SC every 6 hours 0.1-0.2 mg/kg/hr CRI	Antiemetic properties Duration of analgesia may be less than 1 hour
Tramadol	2-10 mg/kg PO every 8 hours	1-2 mg/kg PO every 12 hours	Reserved for patients eating and not vomiting
Lidocaine 2%	1-2 mg/kg IV as single bolus, given slowly 1.5-6 mg/kg/hr CRI	Not regularly used	Cats especially sensitive to CNS effects Reduce dosage by 50% if liver dysfunction present
Ketamine	When in conjunction with an opioid: 0.5 mg/kg IV loading bolus followed by 10 µg/kg/min CRI during surgery and 2 µg/kg/min for 24 hours following surgery	When in conjunction with an opioid: 0.5 mg/kg IV loading bolus followed by 10 µg/kg/min CRI during surgery and 2 µg/kg/min for 24 hours following surgery	Avoid in cats with Hypertrophic Cardiomyopathy (confirmed or suspected)

Drugs (Used in Combination)	Dose & Route (Dogs)	Dose & Route (Cats)
	Morphine (3.3 µg/kg/min), Lidocaine (50 µg/kg/min), and Ketamine (10 µg/kg/min) CRI	Morphine (0.2 mg/kg/hr), lidocaine (40 µg/kg/min), and ketamine (2 mg/kg/hr)

Routes of Administration

Pain management for patients with GI disease can be accomplished via various administration routes of the opioid class of drugs (morphine, morphine derivatives, fentanyl, buprenorphine, butorphanol, tramadol), lidocaine, or ketamine. Administration of NSAIDs is not recommended.

Intermittent bolus: IV, IM, SC
By mouth: PO (tablet or liquid formulation), buccal
CRI: For complete pain management of GI conditions, combination CRIs are very useful in patients with severe abdominal pain (eg, postoperative pain, severe pancreatitis). Opioids (fentanyl, morphine) with other analgesics that target alternative aspects in the pain pathway (lidocaine, ketamine) provide better multimodal pain control and at lower doses than when used alone, yielding a more constant rate of pain control.
Epidural: In very severe cases of abdominal pain, epidural injections or catheters can be quite effective. Patients must be monitored for respiratory depression, as these drugs can migrate cranially, causing paralysis of the diaphragm.

Closing Remarks

A variety of medications can be used to manage GI pain in dogs and cats. Effective clinical management requires close patient monitoring, recognizing that animals may only reveal subtle clinical signs of pain. In some cases, a single drug may be effective while other patients may require multimodal analgesia. When one medication is not working, consideration should be given to trying an alternative drug, as efficacy can vary among patients.

Dr. Durocher is affiliated with Red Bank Veterinary Hospital (Hillsborough and Cherry Hill locations), a group of specialty referral and 24-hour emergency hospitals located in New Jersey. She was previously affiliated with Carolina Veterinary Specialists, Winston-Salem, North Carolina. Dr. Durocher received her DVM from University of Georgia and completed a rotating small animal internship at Ontario Veterinary College, Guelph, Ontario, followed by a small animal internal medicine residency at The Ohio State University.

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