What's new in medicating horses with colic (Proceedings)
Nov 01, 2010
CVC IN SAN DIEGO PROCEEDINGS
There are other agents in each of the above categories, but these drugs can be used as examples in order to develop a treatment plan. For a horse that is actively showing signs of colic, an abbreviated physical examination (heart rate, mucous membrane color, and capillary refill time) should be performed before administering an analgesic. If the horse is extremely painful, an attempt at taking the heart rate should be made, since the α2-agonists will dramatically alter this parameter. Furthermore, heart rate remains one of the most useful predictors of the need for surgery. The first analgesic I administer is xylazine (150-200mg for an adult horse) because it is a moderately potent but short-duration analgesic. This gives the veterinarian an opportunity to determine whether or not the colic is going to recur within the time it takes to complete the remainder of the examination. If flunixin is used as the first line analgesic, it may not be possible to gauge whether or not colic will recur within the time of the initial visit because it is relatively long-acting. However, if the horse remains comfortable on the first dose of xylazine, and the remainder of the examination is normal or the veterinarian feels comfortable with the abnormal findings (such as a mild impaction) flunixin can be administered to control further mild or moderate pain. If the first dose of xylazine does not keep the horse comfortable, it can be repeated at the same dose. In addition, butorphanol can be combined to provide greater pain relief (typically 5-10mg for an adult horse). If xylazine and butorphanol have little or insufficient effect, the next drug I administer is detomidine (5-10mg for an adult horse). If the first dose of detomidine has little effect, I will repeat it. If detomidine fails to control pain, the horse should be referred as rapidly as possible. Large colon volvulus is the most likely diagnosis in a horse with intractable pain, and the horse has approximately 3-4 hours between onset of the volvulus and development of irreversible mucosal injury. Addition of flunixin to the pain treatment plan is of little benefit in these types of cases because the majority of pain results from distension and tension on the mesenteric attachments rather than from inflammation. In addition, all of the drugs mentioned in the table can be repeated as needed except flunixin, which should only be given once every 12 hours.An additional aspect of pain management is treatment of horses that have had colic surgery. These horses may have significant gastrointestinal mucosal injury, so it is advantageous to use low doses of flunixin because NSAIDs have deleterious effects on mucosa. To keep these horses comfortable, we are currently evaluating the use of continuous infusion butorphanol (5-10mg/L at a rate of 2L/hr). Numerous studies in the human literature attest to the benefit of frequent or constant administration of analgesics, because pain is far easier to control if it is treated preemptively. It is also possible that this regime could be useful for pre-operative treatment of severe pain in conjunction with β2 agonists.
There are two major reasons horses with colic may require fluid therapy: hypovolemia and endotoxemia. Hypovolemia results from decreased intake, loss of fluid (typically sweat or reflux), and sequestration of fluid (typically in horses that have intestinal obstruction). Endotoxemia exacerbates hypovolemia by triggering sequestration and leakage of fluid in peripheral capillary beds. The most obvious example of this is a horse with congested gums, in which endotoxin-induced elaboration of prostanoids (particularly PGI2) results in inappropriate vasodilation of capillary beds. The blood that is pooled in these capillary beds becomes de-oxygenated, resulting in progressive deterioration of gum color (from red to purple). Some horses with severe colic will present with predominantly hypovolemia, particularly horses with volvulus of the large colon, in which fluid is sequestered in the large colon, but there has been insufficient duration of disease to trigger endotoxemia. Many horses will present with both hypovolemia and endotoxemia, such as a horse with a small intestinal strangulating obstruction, in which fluid may be sequestered in the lumen of the gut (hypovolemia) and endotoxin has started to leak from degenerating bowel.
The degree of dehydration can be confirmed by running a packed cell volume and total protein. Once the degree of dehydration has been estimated, the percentage is multiplied by the horses body weight to give the fluid deficit (e.g., 8% dehydration x 500kg = 40L). The first half of the fluid deficit should be administered rapidly (up to 100ml/kg/h, or 50L/ hr in an adult horse) followed by the remainder at a slower rate (e.g., 3-5L/ hr) and a re-check of the PCV/ TS. In the field, if fluids are going to be administered, the veterinarian should be prepared to give approximately 15-20L, which corresponds to half the fluid deficit for an adult horse that is 6-8% dehydrated. A lesser volume (such as 2L) is not worth taking the time to set up, and rapid referral is preferable.
In horses with concurrent endotoxemia, the fluid deficit calculations are the same as for hypovolemia. However, consideration should be given to administration of oncotic support because much of the fluid that is administered will continue to pool or leak from capillary beds. The aim of oncotic support is to keep administered fluids within the central circulating blood volume. There are 3 general options: hypertonic saline, polysaccharides, and plasma. Hypertonic saline (4-6ml/kg 7.2% NaCl, or 2-3L for an adult horse) has the same effect as administration of 20L of isotonic fluids on cardiovascular parameters such as central venous pressure. However, the horse should continue to receive its fluid deficit shortly after administration of hypertonic saline because the effects are short lived, and the horse continues to require fluid to normalize total body water. Dextrans (polysaccharide solutions) are clinically available in two molecular weights: dextran-40 (40,000 MW) and dextran-70 (70,000 MW). Dextran-70 is more effective because it exerts greater oncotic pressure, and dextrans are more effective than hypertonic saline because the duration of effect is longer (6-hour half-life for dextran-70). It is administered at a rate of 4ml/kg (2L for an adult horse). For maximal effect, it can be administered with hypertonic saline. The other class of polysaccharide solution is hetastarch, but these solutions are more expensive. The final type of oncotic support is plasma or hyperimmune serum. The latter is available commercially as either J5 E. coli hyperimmune serum, or Re mutant Salmonella hyperimmune serum. Such sera have the advantage of providing oncotic support in addition to antibodies directed at the core polysaccharide of endotoxin. Alternatively, plasma is particularly advantageous in horses that have established endotoxemia with 'vascular-leak syndrome,' in which protein is lost from the circulation into peripheral tissues.