Heart failure is defined as a pathophysiologic state in which the heart loses its ability to provide an output sufficient for the metabolic demands of the peripheral tissues, by whatever means. Myocardial failure is a more restricted definition: the cause is specifically because of inadequate contractile performance of the myocardium.
Heart failure is a common cause of death within 1 month of birth. The etiology involves left to right shunting, either through a patent ductus or through a VSD. Thus, the condition is heart failure as defined above, not myocardial failure.