Could a common heart drug fix a rare surgery complication in kids?

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

Inclusion Criteria: * ≥7 days and ≤18 years old at time of original cardiac surgery * Congenital structural cardiac anomaly necessitating surgical correction. Examples include but are not limited to: ASD, VSD, single ventricles, HLHS, Tetralogy of Fallot, transposition of great vessels, AV canal, heart transplant * developed high output postoperative chylothorax (≥10mL/kg/day), or * persistent chylous drainage at any volume for ≥7 days after open heart surgery for a congenital cardiac defect * ≥70% lymphocytes, or * pleural triglyeride (TG) ≥ half of serum TG, or chylomicron positive * for patients with lower than normal serum lymphocyte count (agedependent), ≥60% lymphocytes in pleural fluids * Must have measurable output (chylothorax output in mL) * Any level of respiratory support (room air, supplemental oxygen, CPAP/BIPAP, ventilatory support) * Any level of inpatient support (ICUs, step-down units, floor) * Study participants can be on concomitant treatment for postoperative chylothorax started prior to study initiation * Study participants can continue on on-going treatment for their primary cardiac other medical conditions * Study participants can initiate new treatments for their primary cardiac or other medical conditions during trial period * Adequate renal function * Not on dialysis * No hemodynamically unstable bradycardia * No systolic hypotension not corrected by pressor support * Not in 2nd or 3rd degree heart block * No history of asthma * A parent or a legal guardian must sign a written informed consent and HIPAA Form * Patients will be required to also be enrolled in a related study (AAAQ6902) which collects chylothorax fluid for cell isolation and genetic analysis Exclusion Criteria: * Pregnancy * Renal failure at time of enrollment * Hypotension despite pressor support * Unstable bradycardia without capacity for pacing * History of asthma or chronic bronchodilator therapy * Uncontrolled hypoglycemia or hyperglycemia as per investigators' judgment * Study participant will be removed from study if they failed 2 consecutive attempts to initiate propranolol (\>10% drop in BP/HR from age-adjusted normative range) * Study participant experiences ≥ Grade 3 AE (SAE)