Two basic mechanisms are involved in the development of congestive cardiac failure. In each type, certain physiologic principles, such as the Laplace and Starling relationships (see Chapter 4), describe the derangements that occur with ventricular dilation.

Increased cardiac work

Many neonates and infants experience heart failure from increased cardiac work (e.g. left-to-right shunts and valve regurgitation) despite normal or increased myocardial contractility. This type of heart failure is sometimes referred to as “high-output failure.”

Reduced myocardial contraction

Myocardial contractility can be reduced as in dilated cardiomyopathy. Most adults and some children have failure of this type. Myocardial failure may result from myocarditis, chemotherapy, or familial cardiomyopathies.

In neonates and young infants, severe failure may result from an obstructive lesion including aortic stenosis, coarctation, or severe systemic hypertension. In these infants, myocardial function often improves following relief of obstruction or treatment of hypertension.

Patients with a morphologic right ventricle acting as the systemic pump (e.g. Norwood palliation of hypoplastic left heart syndrome, and the atrial switch repair of complete transposition) frequently develop systolic heart failure. Longstanding pulmonary regurgitation in a patient following tetralogy of Fallot repair may also lead to right ventricular failure, but since these patients have two functioning ventricles, the clinical manifestations are generally less acute.

Unfortunately, the basic cellular abnormalities responsible for decreased myocyte contractility are poorly understood and, usually, no specific therapy is available to repair the cellular problem.

Most therapy, either nonspecific or supportive, is designed to counteract elevation of systemic and pulmonary vascular resistance that accompany neurohumoral abnormalities (including increased sympathetic tone and activation of the renin–angiotensin system) common to both types of failure.

Once the diagnosis of cardiac failure has been made, treatment should be initiated with as many as four types of medication: an inotrope, a diuretic, an agent to reduce afterload, and a beta-blocker for chronic heart failure.

Inotropes include beta-receptor agonists (dopamine and dobutamine), inhibitors of myocardial phosphodiesterases (milrinone and amrinone), and digoxin preparations (which inhibit cell-wall sodium–potassium pumps).

The common end effect of these inotropes is an increase in intracellular calcium ions available to the myocardial contractile proteins.

Inotropes, however, have severe limitations. A child with cardiac failure usually has maximum activation of compensatory mechanisms, including elevated catecholamines, and in chronic heart failure beta-receptors and contractile elements show a blunted response to adrenergic stimulation. Administration of therapeutic inotropes in these children may have little added benefit.

Patients with certain types of heart failure, including ischemic cardiomyopathy, may actually do less well with inotropes and have a better long-term prognosis with beta-receptor blockers rather than beta-stimulants.

Other adverse effects of inotropes include increased heart rate and metabolic work with little increase in myocardial performance. High doses of some inotropic drugs, particularly digoxin or dopamine, may adversely increase systemic vascular resistance.

Intravenous inotropes

These include dopamine (1–15 µg/kg/min) and dobutamine (5–20 µg/kg/min). The inotropic effects of the two are similar, but dopamine may increase renal blood flow more than dobutamine. Dopamine doses in excess of 15 µg/kg/min stimulate alpha-receptors and may adversely increase systemic vascular resistance. Milrinone and amrinone, inotropic by inhibition of the breakdown of phosphorylated “messenger” compounds within the cell, may exert their greatest beneficial effect by vasodilation (see the section Afterload Reduction).

Oral therapy

Digoxin is the preferred and only oral drug for pediatric use, although oral phosphodiesterase inhibitors are under development.

Digoxin may exert its greatest beneficial effect through vagal stimulation and slowing of conduction and heart rate. Although it may be given orally, intramuscularly, or intravenously, digoxin is safest given orally. Digoxin can be initiated at the maintenance dose without a loading dose. This is a safer method of starting outpatient therapy but requires several days to reach full digitalization.

Maintenance digoxin dose

Twenty-four hours after the initial dose of digoxin, maintenance therapy is started. The recommended maintenance dose is 25% of the total digitalizing dose, in divided doses, with half of the maintenance dose given in the morning and the other half in the evening.

These recommendations are merely guidelines, and the dose may be altered according to the patient’s response to therapy or the presence of digitalis toxicity.

Toxicity

During digitalization, monitoring the patient clinically is important. If indicated, an electrocardiographic rhythm strip is used before the administration of each portion of the digitalizing dose to detect digitalis toxicity.