Diagnosis of Hypoplastic Left Heart Syndrome Chest x-ray and ECG Echocardiography Diagnosis of hypoplastic left heart syndrome is suspected clinically, particularly in neonates with metabolic acidosis that worsens after receiving oxygen; oxygen lowers pulmonary vascular resistance and thus increases the relative proportion of the right ventricle output that flows to the lungs rather than through the patent ductus arteriosus to the body.