Anesthesia for the Colic Patient

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1 Published in IVIS with the permission of the AAEP Close this window to return to IVIS Anesthesia for the Colic Patient Cynthia M. Trim, BVSc, DVA, Diplomate ACVA, Diplomate ECVA, MRCVS Author s Address: Department of Large Animal Medicine, College of Veterinary Medicine, University of Georgia, Athens, Georgia ctrim@vet.uga.edu. Mortality rate associated with anesthesia and surgery for colic is significantly higher for horses with colic than for non-colicky horses. 1 The statistics associate a significant increase in mortality with old age, long duration of anesthesia, urgent anesthesia, and anesthesia performed outside normal working hours. In a smaller survey, mortality rate was significantly higher in horses that were anesthetized with a pre-induction packed cell volume (PCV) greater than 45%. 2 Increased risk for post-operative ileus after colic surgery was also correlated to high PCV and long duration of surgery in another study. 3 These findings may indicate that failure to maintain normal physiologic function during anesthesia may influence the outcome. This presentation will identify derangements that may be present in horses with colic that alter their responses to anesthesia, present options available to improve control of the patients status, and discuss current controversial issues related to anesthesia of horses with colic. Impact of Colic and Surgery on Anesthesia Hypovolemia: A high PCV before anesthesia may be due in part to splenic contraction from catecholamine release but also infers decreased blood volume. Hypotension and decreased cardiac output are probable occurrences during anesthesia as a result of the adverse impact on cardiovascular function of anesthetic agents administered for induction. The decreases in cardiac output and blood pressure can be attenuated by the administration of lactated or acetated Ringer s solution, 20 ml/kg, or hypertonic 7.5% saline solution, 4 ml/kg, before induction of anesthesia. 4 Electrolyte imbalance: Hypocalcemia may be identified by pre-operative laboratory tests and is frequently present in mares with young foals presented for colic surgery. Hypocalcemia may result in central nervous system (CNS) depression and increases the risk for hypotension and heart block during anesthesia. Initial treatment can include administration of calcium gluconate 23%, 0.5 ml/kg, before induction of anesthesia. Horses with hypokalemia may have muscle weakness that interferes with the ability to stand during recovery from anesthesia. Hypokalemia can also increase susceptibility to dysrhythmias and potentiate post-operative ileus. However, administration of fluids with high potassium content during anesthesia is not advisable as the decreased cardiac output that occurs as a result of anesthetic agent administration increases the likelihood that plasma potassium concentrations will exceed the safe concentration during infusion. Endotoxemia: Endotoxemia has been measured in up to 40% of horses admitted to University Hospitals for treatment of colic. Low levels of endotoxin may initially result in hyperdynamic shock due to increased circulating catecholamines. Physical

2 examination of these horses reveals an appearance of adequate circulatory function because the mucous membranes are bright pink from decreased systemic vascular resistance, the peripheral arterial pulse may feel strong, and the CRT is less than 1 second. Requirement for anesthetic agents may be relatively normal at this point but the cardiovascular function can deteriorate rapidly after induction of anesthesia. Later stages of endotoxemia, or high concentrations of endotoxin, are accompanied by decreases in cardiac contractility, cardiac output, blood pressure, and peripheral perfusion. Clinical signs include pale or grayish mucous membranes, prolonged CRT, and weak arterial pulses. Expansion of blood volume and lower dose rates than usual of anesthetic agents for premedication and induction of anesthesia are important in these horses. Endotoxemia may also develop during anesthesia and surgery as a result of surgical manipulation. 5 The impact on cardiovascular function will depend on the magnitude of endotoxin released; a small influx from an enterotomy or a massive load released by untwisting a colon volvulus. Consequences of endotoxemia include progressive decreases in arterial blood pressure, arterial oxygenation (PaO 2 ), and metabolic acidosis. 6 Increased abdominal pressure: Abdominal distension decreases cardiac output in anesthetized animals and also in horses. 7 Ventilatory compromise created by intestinal distension is further exacerbated by placing horses in dorsal recumbency. In severe cases of abdominal distension, hypoxemia at induction of anesthesia may be prevented by nasal insufflation of oxygen. When the horse is connected to the anesthesia machine, controlled ventilation will maintain higher PaO 2 than spontaneous ventilation. 8 Options to Optimize Patient Status During Anesthesia Cardiovascular support: The most important step towards maintaining satisfactory cardiovascular function is to ensure adequate circulating blood volume by administration of balanced electrolyte solution, lactated or acetated Ringer s solution, preferably before induction of anesthesia. The volume given can be adjusted for the individuals situation. Anesthetic agents decrease cardiac output and, therefore, dose rates should be decreased whenever possible. A common anesthetic protocol for anesthetizing horses with colic is premedication with xylazine a, 1.1 mg/kg, and butorphanol b, 0.02 mg/kg followed by induction of anesthesia with diazepam c, 0.05 mg/kg, and ketamine d, 2.2 mg/kg. Xylazine has significant depressant effects on cardiac output and intestinal motility. The dose rate for premedication will depend on the drugs already administered for sedation and analgesia, but can often be reduced 20-50% from the usual rate. Ketamine is less depressant on the cardiovascular system but can likewise often be reduced. Butorphanol and diazepam can be administered at the rates commonly used in healthy horses. Inhalation agent, isoflurane e, sevoflurane f, or halothane, is administered to maintain anesthesia for surgery. During anesthesia, acceptable values are heart rates beats/min and mean arterial blood pressure (MAP) at or above 70 mm Hg. Catecholamines, dobutamine, dopamine, ephedrine, can be administered to increase cardiac output and MAP. Dobutamine g is a synthetic catecholamine that stimulates β1-receptors to increase cardiac contractility and

3 cardiac output, and subsequently MAP. A convenient dobutamine solution concentration (200 µg/ml) is made by addition of 8 ml 12.5 mg/ml dobutamine to 500 ml N-saline. Heart rate may be unchanged, increased or decreased. Horses with colic may have altered myocardial sensitivity and the administration of dobutamine results in tachycardia at doses necessary to maintain adequate MAP. An adequate MAP is of prime concern, but attempts should be made to decrease the dobutamine dose rate to keep the heart rate less than beats/min. Occasionally, dobutamine induces bradycardia and this results in no change or a decrease in MAP. This situation must be managed by a decrease in the dobutamine administration rate and addition of alternative cardiovascular support, such as ephedrine. Ephedrine h is a plant alkaloid that increases cardiac contractility through release of norepinephrine from neurons and a mild direct agonist effect on α-, β1- and β2-receptors. Cardiac output and blood pressure are increased, heart rate may be slightly increased, some vasoconstriction is induced, and blood volume is mobilized from viscera, especially the spleen. Consequently, ephedrine is specifically used when splenic contraction is needed to facilitate surgical manipulation. Ephedrine is administered IV as a bolus, mg/kg. The onset of action is slow at several minutes and the duration is minutes. Dopamine i is a natural precursor of norepinephrine and part of the cardiovascular effects are mediated through norepinephrine (NE) and part by stimulation at dopamine DA1, α1, β1- and β2-receptors. Effects of dopamine are dose and time dependent. Low dosage, 1-3 µg/kg/min, increases renal and mesenteric blood flow, may increase MAP, and produces a mild increase in cardiac output. Moderate dopamine dosage, 3-6 µg/kg/min, increases cardiac contractility and cardiac output. Vasodilation is produced that slows the rate of rise of MAP and may result in a decrease of MAP in the first 5-7 minutes. Over minutes, plasma NE concentration increases and cardiac contractility and blood pressure continue to increase. High dopamine dosages, >10 µg/kg/min, cause vasocontriction and are indicated for treatment of profound life-threatening hypotension and for advanced atrioventricular heart block. Controlled ventilation: Hypoventilation (increased arterial carbon dioxide concentration, PaCO 2 ) occurs almost invariably in anesthetized horses with colic. Hypercarbia can be confirmed by measurement of blood gases. PaCO 2 in awake horses is around 40 mm Hg and PaCO 2 exceeding 50 mm Hg is hypoventilation. Ventilation can be assessed indirectly using capnography, a technique that measures end-expired or end-tidal CO 2 (ETCO 2 ). In healthy, artificially ventilated anesthetized horses the PaCO 2 is a few mm Hg higher than ETCO 2. Horses with colic or who are breathing spontaneously may have substantial lung collapse, a large ventilation-perfusion inequality, and a greater discrepancy between ETCO 2 and PaCO 2. 9,10 The difference is on average 13 mm Hg and the range of possible values is wide. Controlled ventilation is used to decrease PaCO 2. A respiratory rate of 10 breaths per minute and a tidal volume of 10 ml/kg at an inspiratory to expiratory time ratio of 1:2 should maintain PaCO 2 in an acceptable range. When the abdomen is severely distended, the inspiratory pressure may be high but should not be allowed to exceed 40 mm Hg. The pressure needed to achieve an adequate tidal volume

4 should be less than this after the abdominal incision and decompression. Using artificial ventilation settings to achieve an ETCO 2 of mmhg should maintain an acceptable PaCO 2 in the majority of horses. Control of anesthetic depth: Inhalation agent is delivered at a concentration adequate to prevent movement or increases in heart rate and blood pressure in response to surgical stimulation. Deep anesthesia decreases cardiac output and blood pressure and increases the amount of absorbed anesthetic agent at the end of anesthesia and that contributes to weakness and ataxia in recovery. An anesthetic gas analyzer can measure the inspired and alveolar agent concentration (end-tidal % of isoflurane or sevoflurane) and the vaporizer setting is adjusted to achieve a desired anesthetic depth based on the readings. 9 Controversial anesthetic issues Anesthetic agents: There are a variety of recommendations for anesthetic protocols and one is listed in Table 1. The drug dose rates should be decreased whenever possible and the type and amount of drug administered in the hour before induction will have a significant influence on the patients requirement. Table 1. Anesthetic agent combination for induction of anesthesia Drug name Dose rate Comments Xylazine a 1.1 mg/kg IV Premedication; decrease dose rate 20-50% Butorphanol b 0.02 mg/kg IV Premedication Diazepam 0.05 mg/kg IV Induction Ketamine 2.2 mg/kg IV Induction; decrease dose rate 20-40% Opioids: Balanced (optimal) anesthesia in humans and small animals traditionally includes an opioid for analgesia and contribution to CNS depression. Clinical experience confirms that inclusion of butorphanol in the premedication exerts a beneficial effect in allowing a decrease in the dose of an alpha2- sedative and provides muscle relaxation during induction. The duration of effect of butorphanol is minutes and additional doses or continuous IV infusion must be employed to ensure continued effect. Others have used morphine j, 0.1 mg/kg, for a similar effect. 11 The controversy lies in whether these agents provide substantial analgesia and are a valuable addition to the protocol. One test for analgesia involves determining a reduction in inhalation agent requirement for anesthesia and neither butorphanol or morphine have demonstrated such an effect. 12 On the other hand, continuous intravenous infusion of butorphanol postoperatively in

5 colic horses appears to offer some analgesia based on improved behavior scores, lower plasma cortisol concentrations, and less body weight loss during hospitalization. 13 Lidocaine: A continuous infusion of lidocaine, 0.05 mg/kg/min, with or without a loading dose of 1.5 mg/kg given over 15 minutes, has been recommended for horses with colic. Lidocaine has been demonstrated to have anti-endotoxin properties and can be recommended for this purpose. Further, the sedation provided by lidocaine may allow up to 20% reduction in inhalation agent requirement for anesthesia. 14 The contribution to analgesia during anesthesia is questionable as measurements associated with stress response are not depressed by lidocaine infusion. 14 In summary, anesthetic management should be geared toward maintaining physiologic status as close to normal as possible. Specific anesthetic agents and adjunct drugs need further investigation to determine their individual importance or adverse impact on outcome. References and Footnotes 1. Johnston GM, Eastment JK, Taylor PM. The confidential enquiry into perioperative equine fatalities (CEPEF): mortality results of Phases 1 and 2. Vet Anaesth Analg 2002;29: Pascoe PJ, McDonell WN, Trim CM, et al. Mortality rates and associated factors in equine colic operations A retrospective study of 341 operations. Can Vet J 1983:24: Cohen ND, Lester GD, Sanchez LC, et al. Evaluation of risk factors associated with development of postoperative ileus in horses. J Am Vet Med Assoc 2004;225: Dyson DH, Pascoe PJ. Influence of preinduction methoxamine, lactated Ringer s solution, or hypertonic saline solution or post induction dobutamine infusion on anesthetic-induced hypotension in horses. Am J Vet Res 1990;51: Trim CM, Barton MH, Quandt JE. Plasma endotoxin concentrations in anesthetized horses with colic. Vet Surg 1997;26:163 (abstract). 6. Trim CM, Moore JN, Hardee MM, et al. Effects of an infusion of dopamine on the cardiopulmonary effects of Escherichia coli endotoxin in anaesthetized horses. Res Vet Sci 1991;50: Duke T, Cruz AM, Cruz II, et al. Cardiopulmonary effects associated with headdown position in halothane-anesthetized ponies with or without capnoperitoneum. Vet Anaesth Analg 2002;29: Day TK, Gaynor JS, Muir WW, et al. Blood gas values during intermittent positive pressure ventilation and spontaneous ventilation in 160 anesthetized horses positioned in lateral or dorsal recumbency. Vet Surg 1995;24: Trim CM. Monitoring during anaesthesia: techniques and interpretation. Equine Vet Educ 1998;10: Koenig J, McDonell W, Valverde A. Accuracy of pulse oximetry and capnography in healthy and compromised horses during spontaneous and controlled ventilation. Can J Vet Res 2003;67:

6 11. Mircica E, Clutton RE, Kyles KW, et al. Problems associated with perioperative morphine in horses: a retrospective case analysis. Vet Anaesth Analg 2003;30: Steffey EP, Eisele JH, Baggot JD. Interactions of morphine and isoflurane in horses. Am J Vet Res 2003;64: Sellon DC, Roberts MC, Blikslager AT, et al. Effects of continuous rate intravenous infusion of butorphanol on physiologic and outcome variables in horses after celiotomy. J Vet Intern Med 2004;18: Dzikiti TB, Hellebrekers LJ, van Dijk P. Effects of intravenous lidocaine on isoflurane concentration, physiologic parameters, metabolic parameters and stress-related hormones in horses undergoing surgery. J Vet Med A Physiolo Pathol Clin Med 2003;50: a Sedazine, Fort Dodge Animal Health, Fort Dodge, IA b Torbugesic, Fort Dodge Animal Health, Fort Dodge, IA c Diazepam USP, Abbott Laboratories, North Chicago, IL d Ketaset, Fort Dodge Animal Health, Fort Dodge, IA e IsoFlo, Abbott Laboratories, North Chicago, IL f SevoFlo, Abbott Laboratories, North Chicago, IL g Dobutamine USP, Abbott Laboratories, North Chicago, IL h Ephedrine sulfate USP, 50 mg/ml, Taylor Pharmaceuticals, Decatur, IL i Dopamine HCl Inj. USP, Abbott Laboratories, North Chicago, IL j Morphine sulfate USP, 15 mg/ml, Baxter Healthcare Corporation, Deerfield, IL