Obstructive Left Heart Lesions

A 2-week-old infant presents with tachypnea, poor perfusion, gallop rhythm, diminished pulses, and hepatomegaly. Arterial blood gas shows metabolic acidosis. Echocardiography reveals critical aortic stenosis.

The infant in the vignette presents with severe congestive heart failure and cardiogenic shock at a typical age for critically obstructive left heart lesions, such as hypoplastic left heart syndrome, critical neonatal coarctation of aorta, and critical congenital aortic stenosis. As the ductus arteriosus closes, the early compensatory right ventricular contribution to systemic blood flow and vital perfusion to the kidneys and other organs are lost. The result is rapidly developing severe metabolic acidosis that contributes to myocardial and other organ dysfunction.

The presentation of affected infants often resembles that of septic shock. A high index of suspicion for congenital left heart obstructive lesions must be maintained for infants who have signs of shock in the first two months of life. These signs include gray color and poor capillary refill, thready or absent peripheral or central pulses, tachypnea and hyperpnea, hypotension, and obtundation.

In the infant who has presumed septic shock, severe metabolic acidosis, and cardiomegaly on chest radiography, echocardiographic evaluation should be undertaken to exclude left heart obstructive lesions. Base deficit pH values greater than 25 on arterial blood gas measurement are common after ductal closure in these infants. Good clinical outcomes have been described even in infants who present with pH values less than 7.0.

Essential to good resuscitation of the infant who has shock from obstructive left heart lesions is reopening of the ductus arteriosus. Presumably, once the ductus closes, the spiral of fatal deterioration develops rapidly. Accordingly, a trial of prostaglandin E₁ (alprostadil) infusion should be administered even to an infant as old as 1 to 2 months of age to open the recently closed ductus arteriosus. The open ductus allows the right ventricle to provide systemic blood flow when the left ventricle cannot.

Inotropic agents such as dobutamine or epinephrine may provide adjunctive and supportive therapy, but they will not be lifesaving if prostaglandin E₁ is not employed to open the ductus.

Serious hypoxia is not a problem for infants who have obstructive left heart lesions. In fact, the arterial Po₂ may be normal on initial blood gas measurement, even in the presence of alarming metabolic acidosis and clinical shock. As the ductus opens with prostaglandin treatment, the arterial Po₂ may decrease, with improving right ventricular flow to the body. Attempts to keep the arterial Po₂ high are detrimental because a high inspired oxygen level is a powerful vasodilator of the pulmonary arteries. Some degree of pulmonary hypertension is essential to promote flow to the systemic circulation via the ductus. Pulmonary artery vasodilatation from supplemental oxygen "steals" the right ventricular output to the pulmonary arteries and away from the body. This "steal phenomenon," not oxygen-induced ductal closure, is the primary risk associated with use of supplemental oxygen when prostaglandin E₁ is being used to keep the ductus open.

Nitric oxide also is a powerful pulmonary arterial vasodilator. Although it may be a lifesaving treatment for newborns who have severe pulmonary hypertension and inadequate pulmonary blood flow, it is contraindicated in the presence of obstructive left heart lesions.

also see VSD

References:
Fedderly RT. Left ventricular outflow obstruction. Pediatr Clin North Am. 1999;46:369-384
Rothman A. Coarctation of the aorta: an update. Curr Probl Pediatr. 1998;28:33-60