CHAPTER 88 Medical Management of Horses After Colic Surgery
The prognosis for survival in horses undergoing colic surgery has improved dramatically during the past 10 to 15 years, with short-term survival rates of horses recovered from general anesthesia being as high as 90%. Although much of the improvement in survival rate can be attributed to early referral and surgical intervention, improvements in general anesthesia and postoperative medical management have also been important. With development of the specialty of equine emergency and critical care, clinicians are beginning to focus more on monitoring and treatment of the critically ill postoperative colic patient. Providing adequate cardiovascular and metabolic support in addition to analgesia is vital for healing of the damaged gastrointestinal tract and surgical site. Diagnostic tests and treatment should be goal directed to optimize treatment success and minimize expense. Meticulous medical records should be maintained for postoperative colic patients. Horses should be monitored closely with physical examination findings and various monitoring tools.
Horses that have undergone colic surgery should be observed for appropriateness of attitude and appetite. Although some horses are dull immediately after abdominal surgery, horses should have a good appetite when offered feed and should become bright on refeeding. Postoperative colic patients that do not become bright when hand-walked outside or are not interested in grazing warrant thorough examination, and, in my experience, this clinical appearance is not associated with a good prognosis. Common reasons for persistent dullness and inappetence in the postoperative period include (1) gastric dilatation, necessitating passage of a nasogastric tube and checking for reflux; (2) intestinal obstruction; (3) enterocolitis caused by Salmonella spp. infection, tentative diagnosis of which can be made on the basis of fever, leukopenia, and sonographic evidence of liquid accumulation in the colon; (4) nonviable or leaking intestine; (5) metabolic alterations such as hypertriglyceridemia; and (6) oral, pharyngeal, or esophageal lesions, especially as a consequence of repeated nasogastric tube passage. Routinely leaving a nasogastric tube in place is not recommended because of trauma and the possibility of inducing reflux of luminal fluid.
The postoperative horse should also be observed for abdominal distension, colic, and fecal production. Any horse with postoperative pain, particularly in association with abdominal distension and an absence of feces, should undergo thorough examination. Some causes include failure to remove an enterolith at surgery, reimpaction of the colon with sand, reimpaction of the small colon, redisplacement, intestinal ischemia, gastric dilation, severe postoperative ileus, and complications related to an anastomosis.
Water consumption and urination should also be monitored closely. Horses receiving intravenous (IV) fluids generally do not drink large volumes of water. The normal rate of urine production in an adult horse is 0.6 to 1.25 mL/kg per hour (e.g., 300 to 625 mL per hour or 7.2 to 15 L daily for a 500-kg horse). Fluid “ins and outs” are not usually measured in adult horses, and urine production is subjectively assessed by observing actual urination and the volume of urine in the stall. A horse should produce a good stream of urine every few hours, and the stall should have several wet areas but not be soaking wet.
Horses should be examined every 2 to 6 hours after colic surgery, depending on the severity of illness and the time since surgery. Observations should be recorded on a form similar to the one in the example provided (Figure 88-1).
Packed cell volume (PCV) and total protein (TP) concentration should be monitored every 6 to 24 hours in postoperative colic patients, depending on the severity of the disease and the time since surgery. Although PCV and TP are not the gold standard criteria for assessing hydration status, they can be used as a rough guideline for adjusting the rate of fluid administration as well as the need for other fluids, such as colloids in a horse with hypoproteinemia.
Serum creatinine concentration can be measured to assess organ and tissue perfusion. If a horse that has undergone colic surgery is persistently azotemic (Table 88-1), either the IV fluid rate is inadequate (prerenal azotemia) or the horse is in renal failure. Renal failure can be diagnosed on the basis of isosthenuria, urinary casts, and decreased urinary fractional excretion of sodium.
|Serum or plasma creatinine concentration||<1.8 mg/dL|
|Serum triglyceride concentrations||<50 mg/dL|
|Blood lactate concentration||<2 mmol/L|
|PcvO2||45 ± 4.7 mm Hg|
|Systolic arterial pressure (direct)||126-168 mm Hg|
|Diastolic arterial pressure (direct)||85-116 mm Hg|
|Mean arterial pressure (direct)||110-133 mm Hg|
|Systolic arterial pressure (indirect)||98-125 mm Hg|
|Diastolic arterial pressure (indirect)||54-82 mm Hg|
|Mean arterial pressure (indirect)||70-96 mm Hg|
|Central venous pressure||7-12 cmH2O|
|Colloid oncotic (osmotic) pressure||19-31 mm Hg|
Direct, Directly measured; indirect, indirectly measured; PcvO2, central venous partial pressure of oxygen; ScvO2, central venous oxygen saturation.
Blood and urine glucose concentration should be monitored, particularly in horses receiving dextrose-containing fluids. An insulin constant-rate infusion should be considered in some horses with persistent hyperglycemia and glucosuria. Blood glucose concentration should be maintained from 80 and 120 mg/dL. The renal threshold for glucose is 160 to 180 mg/dL. Serum triglyceride concentration should be measured in horses at risk for developing hyperlipemia-hyperlipidosis (see Table 88-1).
Lactate is produced as the end product of anaerobic glycolysis and is a marker of peripheral tissue perfusion and oxygen delivery. Although there are several metabolic causes of hyperlactatemia, tissue hypoxia resulting from hypovolemia, hypoxemia, hypotension, or a hypermetabolic state is the most common cause. Central venous oxygen tension (PcvO2) and saturation (ScvO2) reflect tissue oxygen delivery and can be measured by collecting blood from the vena cava through a central venous pressure catheter. Low PcvO2 and ScvO2 are indications of inadequate tissue oxygenation, particularly with concurrent hyperlactatemia.
Coagulopathies are also relatively common in critically ill postoperative colic patients. A coagulation profile includes a platelet count and measurements of fibrinogen, fibrin degradation products, activated partial thromboplastin time, prothrombin time, and antithrombin III activity and should be performed on any horses suspected of having coagulopathy, as indicated by clinical signs or findings of hypofibrinogenemia, or thrombocytopenia.
Arterial Blood Pressure
Arterial pressure, particularly mean arterial pressure (MAP), can be used to estimate organ and tissue perfusion. Horses may have low MAP (hypotension) as a result of hypovolemia or activity of the systemic inflammatory response syndrome (SIRS), which can be a response to endotoxemia. Arterial blood pressure can be measured directly or indirectly. Direct measurement of arterial pressure is performed with an arterial catheter, pressure transducer, and continuous recorder. Indirect arterial pressure is most commonly measured by means of the oscillometric method, with an occlusion cuff placed over the coccygeal (tail), dorsal metatarsal, median, or palmar digital (limb) artery (Figure 88-2; see Table 88-1). The internal inflatable bladder length should be 80% of the tail or limb circumference, and the bladder width should be 20% to 25% and 40% to 50% of the tail and limb circumferences, respectively. For measured pressures to be considered reliable, the heart rate recorded by the oscillometric method should be the same as that obtained on physical examination. The accuracy of indirect blood pressure measurements can be improved by taking serial measurements, with the horse minimally restrained in a quiet environment and the head maintained in a resting position.