Adjunctive agents used to support blood pressure

Anesthetizing a patient with cardiac disease requires a plan for the use of supportive measures to maintain adequate tissue perfusion. As in the case of left sided cardiac dysfunction patients, volume administration frequently is not an option to support blood pressure. In these cases, should a positive inotropic or pressor agent be indicated, the volume of the adjunctive agent required should be deducted from the volume of crystalloid administered to maintain a balanced hourly rate.

Cardiomyopathies

Dilated Cardiomyopathy

Dilated cardiomyopathy is most commonly seen in certain large breed dogs. This condition is characterized by elongation, weakening of the heart muscle, so that chamber size is increased, but contractility is compromised. Stroke volume is reduced and resulting in decreased blood pressure. Compensatory mechanisms for hypotension ensue. Chamber enlargement may alter the shape and function of the mitral valve, leading to valvular insufficiency. In addition, the chamber may be so enlarged as to affect cell communication and organized depolarization. Atrial fibrillation, premature ventricular contractions and ventricular tachycardia may develop. Whether heart failure is due to valvular insufficiency or cardiomyopathy, compensatory mechanisms in response to poor cardiac output are similar. The sympathetic nervous system, the renin-angiotensin-aldosterone system are activated. Secretion of antidiuretic hormone increases. Results are increased heart rate and vascular tone, and sodium and water retention.

Treatment of heart failure from either cause is also similar, as it is aimed at minimizing this compensatory mechanism which becomes harmful when they are chronic. Medications to decrease heart rate and improve contractility, decrease blood volume by diuresis, and decrease vascular resistance against which the heart must pump.

Digitalis and digitoxin are positive inotropes which increase the concentration of calcium within the myocardial cells thus increasing the force of contractions and decreasing heart rate. A relatively new addition to management is pimobendan which elicits calcium sensitization of the myofilaments. Anti-arrhythmics such as procainamide, quinidine or mexiletine may be used if indicated. Calcium channel blockers and beta blockers may be used to help control supraventricular arrhythmias.

Diuretics such as furosemide are used for decreasing circulating blood volume. ACE inhibitors cause relaxation of the blood vessels, thereby decreasing systemic vascular resistance. Anesthetic protocols for patients with dilated cardiomyopathy should aim to avoid drugs that would increase heart rate or decrease contractility. Fluids must be delivered with caution to avoid overload. Ideally central venous pressure would be monitored to determine how well the heart is able to handle the circulating volume and fluids may be regulated accordingly.

Hypotension may be corrected with the use of CRI opioid/benzodiazepines to reduce vaporizer setting thus avoiding the dose dependent peripheral vasodilation attributed to inhalant anesthetics. Positive inotropes or beta agonists such as dobutamine may be added if further support of adequate blood pressure is required. These drugs focus on improving contractility without increasing afterload. Dobutamine should be diluted and administered as a constant rate infusion, beginning at 2 micrograms per kilogram per minute, and increased as needed. The use of dobutamine is not benign as increased rate of administration may cause an increase in heart rate or arrhythmias may be noted as it acts as a positive inotrope as well at higher doses as a vasocontrictor. Dopamine as a CRI may also be used to support blood pressure and is calculated to be delivered at a starting rate of 2 mcg/kg/minute.

Ventricular arrhythmias may be exacerbated with the use of beta agonists; however, so close monitoring of the EKG is necessary.

Suggested protocols may include +/- glycopyrrolate at 0.011 mg/kg given subcutaneously, dependent on patient's heart rate

Hydromorphone or oxymorphone at 0.1 mg/kg or 0.05mg/kg given subcutaneously

Pre-medicants will be given 20-30 minutes prior to induction time

Should patients be of a brachycephalic breed where respiratory difficulties may be present, butorphanol at 0.2 mg/kg will be used in lieu of hydromorphone and the patient will be observed for any signs of distress.

pre-oxygenation will occur prior to induction unless the patient becomes distressed by mask placement.

Induction agents:
diazepam /midazolam at 0.2 mg/kg given IV followed by
etomidate at 1-2 mg/kg given IV to effect to facilitate placement of the endotracheal tube.

Anesthetic maintenance-
Inhalant gas anesthesia maintenance will be sevoflurane with oxygen.
Fentanyl at 0.8 mcg/kg/minute and midazolam at 8 mcg/kg/minute combined may be given IV via syringe pump as an anesthetic adjunct for a MAC sparing effect. This will allow the vaporizer setting to be reduced to between 0.25-1% if sevoflurane is the inhalant used. Contraindicated: Ketamine, any alpha 2 agonist, and precaution should be taken with propofol administration.