Anesthetic management of small animals with preexisting cardiac conditions (Proceedings)
To safely anesthetize small animal patients with preexisting cardiac condition, it is important that specific hemodynamic goals are set for the particular heart condition. It is easy to state that for patients with cardiac problem, "heart-friendly" drugs need to be used. A specific anesthetic drug will be appropriate for a certain heart condition and not for another. Understanding the underlying problem(s) in a cardiac disease will help in planning the appropriate anesthetic regime. In this presentation, the anesthetic management of the more common cardiac diseases in dogs and cats will be discussed.
Mitral Regurgitation (MR) in dogs
MR in dogs develops as a result of endocardiosis. The clinical consequences of mitral valve endocardiosis are seen in elderly small-breed dogs. The anesthetic risk associated with MR depends on its severity. Mild MR is not associated with hemodynamic instability. Patients with mild MR have low anesthetic risk. Dogs with untreated congestive heart failure will have the highest risk. Dogs being treated with an ACE inhibitor and diuretics for congestive heart failure can be anesthetized but extreme care should be taken. Anesthesia should not be performed in dogs with untreated congestive heart failure.With MR, part of the left ventricular stroke volume is ejected through the incompetent mitral valve into the left atrium. It creates a volume load on the left atrium and left ventricle. The pressures in the left atrium and left ventricle increase. As a result of increased pressures, the LA and LV dilate and develop hypertrophy. Later in the disease process, the increased pressures will be reflected back upon the pulmonary venous circulation and ultimately results in pulmonary edema.
When anesthetizing dogs with MR, the following hemodynamic goals should be considered: (1) reduce preload slightly to reduce regurgitant flow, (2) avoid acute increases in afterload, (3) maintain contractility, (4) avoid bradycardia, (5) maintain sinus rhythm, and (6) do not increase myocardial oxygen requirement by avoiding severe tachycardia and hypotension.
To achieve the hemodynamic goals for MR, there are important steps that need to be done. The patient should not be overloaded with fluids. In case of hypotension during anesthesia, fluid boluses should be avoided. Fluid should be given at a lower rate of 3.0-5.0 ml/kg/hour. Those patients with compensated CHF should receive fluid with lower sodium load like 2.5% dextrose and half-strength LRS. Sinus bradycardia should be corrected because it results in longer systolic time and more regurgitant flow. If possible, normal heart rate should be maintained. Slight tachycardia is acceptable in MR. Atropine (0.02 mg/kg IV) or glycopyrrolate (0.01 mg/kg IV) should be given to correct sinus bradycardia. Myocardial contractility should be maintained and supported using beta-1 agonist like dobutamine or dopamine. Dobutamine and dopamine can be given at 5.0-10.0 μg/kg/min. Slight vasodilation is acceptable. Drugs that cause peripheral vasoconstriction, e.g., alpha2-agonists, should not be used in dogs with MR. The patient should be handled gently to minimize excitement and stress. Increased catecholamine release may cause vasoconstriction.