Today's Veterinary Practice

SEP-OCT 2018

Today's Veterinary Practice provides comprehensive information to keep every small animal practitioner up to date on companion animal medicine and surgery as well as practice building and management.

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PEER REVIEWED 24 SEPTEMBER/OCTOBER 2018 Etomidate Etomidate is a short-acting hypnotic anesthetic that produces its anesthetic action through binding and positive modulation of GABA A receptors in the CNS. Cardiovascular function is largely preserved, with minimal changes to heart rate, blood pressure, or cardiac output. Because of its hyperosmolarity, etomidate is considered a vascular irritant and therefore can be administered intravenously only. It is highly desirable for anesthetic induction of cats with cardiovascular disease but is not useful as a sedative. CARDIOVASCULAR DRUGS THAT AFFECT ANESTHESIA Cats with cardiomyopathy that need to be sedated or immobilized are often receiving cardiac drugs that can affect the sedation or anesthetic period. To develop a safe sedative protocol, a working knowledge of the effects of these drugs is necessary. When dynamic left ventricular outflow tract obstruction is present in the absence of congestive heart failure, beta-blockers such as atenolol are used. Atenolol unloads the work of the hypertrophic heart through its negative chronotropic (decreased heart rate), dromotropic (decreased conduction), and inotropic (decreased contractility) as well as its antiarrhythmic and ischemic effects. These effects will be compounded by sedative or anesthetic drugs that have myocardial depressant effects (propofol, alfaxalone, alpha-2 agonists, and inhalant anesthetics) and bradycardic effects (opioids). Withdrawal of beta-blockers before inducing sedation or anesthesia is generally not indicated. For bradycardic or hypotensive cats, heart rate and blood pressure support during deep sedation or anesthesia may be necessary. For cats with echocardiographic evidence of systolic dysfunction, pimobendan, a phosphodiesterase 3 inhibitor, is used because of its inodilator (positive inotropic and vasodilatory) effects. These effects are generally desirable, and pimobendan should not be withdrawn before inducing anesthesia. For cats that have a current thrombus, a history of thromboembolic disease, spontaneous echogenic contrast, and left atrial enlargement, antithrombotic drugs are used. These include both antiplatelet (clopidogrel, aspirin) and anticoagulant (enoxaparin, dalteparin, apixaban, rivaroxaban, and factor IIa inhibitors) drugs. Hemodynamic effects (i.e., hypotension resulting from hemorrhage) might be expected if antithrombotic drugs are not withdrawn before surgical procedures are performed or if inadequate hemostasis occurs during surgery. RISK REDUCTION Isoflurane and sevoflurane are halogenated volatile inhalant anesthetic drugs used to maintain anesthesia in small animals. These drugs produce dose-dependent cardiovascular depression through depression of inward calcium currents through the sarcolemma and decreased sarcoplasmic reticulum function. The result is reduced myocardial contractility (negative inotropy) and vasodilation, which are more profound in hypertrophic myocardium. 38,39 Given the dose- dependent depression of myocardial contractility and vasodilation, it is never advised to use chamber or mask induction or immobilization with a volatile inhalant anesthetic in difficult-to-handle cats that have or are suspected to have cardiovascular disease. This technique induces a profound stress response and requires excessive anesthetic depth to facilitate handling or anesthetic induction. Other ways to reduce risk for death associated with administration of sedatives or anesthetics, even if only for brief periods of sedation, is to monitor pulse and pulse oximetry, which is strongly recommended. 4 For cats with cardiovascular disease, additional monitoring (e.g., performing electrocardiography and measuring blood pressure and temperature) is also advised. References 1. American Veterinary Medical Association. U.S. Pet Ownership and Demographics Sourcebook, 2012 edition. Schaumberg,IL: American Veterinary Medical Association, 2012. 2. Hoyumpa Vogt A, Rodan I, Brown M, et al. AAFP AAHA feline life stage guidelines. J Fel Med Surg 2010;12:43–54. For bradycardic or hypotensive cats, heart rate and blood pressure support during deep sedation or anesthesia may be necessary.

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