CHAPTER 86 Multimodal Pain Management of the Horse with Acute Abdomen
Proper management of the horse with an acute abdomen, commonly referred to as colic or abdominal pain, requires the practitioner to be able to control pain but not mask signs of a strangulating lesion that would indicate the need for surgical intervention. In many instances, this means providing the least amount of analgesia necessary to prevent the horse from harming itself or its handlers while still allowing the veterinarian to differentiate a surgical from a nonsurgical condition. The pain associated with an acute abdomen arises from intestinal distension, ischemia, spasm, or inflammation; therefore, the appropriate choice of analgesic is associated with relieving the distension or spasm or preventing transmission of pain to or modulating it in the central nervous system.
Some of the analgesics used to treat an acute abdomen include the nonsteroidal anti-inflammatory drugs (NSAIDs) flunixin meglumine, phenylbutazone, ketoprofen, and dipyrone; the opioid agonists and agonist-antagonists morphine, methadone, and butorphanol; the local anesthetic lidocaine; and the α2-adrenergic agonists xylazine, romifidine, and detomidine. Buscopan, a spasmolytic, is a combination of the NSAID dipyrone and the anticholinergic drug hyoscine and is approved for use in the United States for treatment of spasmodic colic. Of these medications, the most commonly used drugs for management of acute abdominal pain include xylazine, butorphanol, the combination of dipyrone and hyoscine, and flunixin meglumine.
Flunixin meglumine is the most potent commonly administered NSAID for management of acute abdominal pain. By preventing conversion of arachidonic acid to prostaglandins via inhibition of the enzyme cyclooxygenase, this drug has benefits beyond providing analgesia. Low doses (0.25 to 0.5 mg/kg, every 6 to 12 hours) confer an antiendotoxin effect by inhibiting the generation of eicosanoids that arises from damage to the vasculature by gram-negative bacteria. By so doing, NSAIDs decrease fever and inflammation and maintain intestinal blood flow. The recommended dose for pain management is 1.1 mg/kg,administered intravenously (IV) and intramuscularly (IM), although IM administration has been associated with the risk of necrotizing myositis at the injection site. An oral formulation is also available, with 85.5% bioavailability, although the presence of feed in the stomach decreases absorption of the drug. Flunixin meglumine is dependent on renal excretion, so caution should be used in horses with renal compromise. Additionally, dehydrated horses administered NSAIDs are at risk of renal papillary necrosis.
Clearance of these drugs from the plasma is longer in foals less than 1 month of age and in horses older than 9 years. Therefore, after administration of the initial dose in such horses, the interval should be increased to prevent adverse effects such as gastrointestinal tract ulceration or renal compromise.
The α2-adrenergic agonists induce analgesia and sedation by acting on cell membrane receptors in the central nervous system to decrease neurotransmitter release. The effect of these drugs on the central nervous system is to induce analgesia and sedation, whereas peripheral receptor stimulation results in vasoconstriction and inhibited insulin release. All the α2-adrenergic agonists induce ataxia, but romifidine induces the least analgesia compared with xylazine and detomidine.
These drugs are effective analgesics for horses with colic pain. The analgesic effect is evident within minutes of administration, but the effect might not last as long as the sedation. The α2– adrenergic agonists are associated with decreased rates of gastrointestinal motility; duodenal motility may be decreased for 30 minutes or less after administration of xylazine and for at least an hour after administration of detomidine. However, the benefits of these drugs in the management of a horse with acute abdomen outweigh the concern over decreased gastrointestinal motility.
Typically, a critically ill horse with surgical disease will have recurrence of pain (i.e., breakthrough pain) shortly after xylazine administration with signs such as flank biting, thrashing, and rolling. Detomidine administration may temporarily mask pain associated with a strangulating lesion; therefore, it may be inappropriate for use during diagnosis. Detomidine is a good choice for pain management when surgery is not an option or when the decision to undergo surgery has been made and preoperative procedures such as physical examination, catheter placement, and antimicrobial administration must be performed. An additional advantage of α2– adrenergic agonists is the availability of antagonist drugs if reversal of the sedation is needed. Yohimbine (0.5 mg/kg, IM or IV), tolazoline (2 to 4 mg/kg, IV), and atipamezole (0.1 to 0.4 mg/kg, IV) can all reverse the sedative properties of the α2