Post-Intracardiac Repair ITU: "Free PR" and "Free MR" - What They Mean and How to Manage Them

What Does "Free PR" / "Free MR" Mean?

• During diastole, the entire PA diastolic pressure column bounces back into the RV - this is an acute, enormous volume load on the right ventricle
• The RV was already hypertrophied (TOF) and non-compliant; now it has to handle massively increased end-diastolic volume
• Acutely this causes RV dilation and dysfunction - the RV "stiffens" and cannot handle the extra volume well
• Right-sided filling pressures rise (CVP rises), tricuspid annulus dilates causing secondary TR, and forward flow into the pulmonary circulation is impaired
• In children with a compliant pericardium and chest, this may be tolerated initially, but the stunned myocardium + CPB insult + hypothermia makes the early post-op period very dangerous

• Repair of AV canal defects (complete or partial) where the mitral (or "left AV valve") repair leaves significant regurgitation
• Anomalous left coronary artery (ALCAPA) repair with residual LV dysfunction
• Mitral valvuloplasty that has failed or left a residual cleft

• Each systole, a large fraction of LV stroke volume regurgitates back into the left atrium instead of going forward through the aorta
• LA pressure rises acutely - pulmonary venous hypertension and flash pulmonary edema can follow
• LA enlarges, pulmonary pressures rise, and RV is now also under pressure stress from the downstream elevation
• The LV is volume-loaded and dilated; with CPB-induced myocardial stunning, this combination is especially dangerous
• A key insight: the LV is ejecting against two orifices (aorta + regurgitant mitral) - the aortic output is reduced even as total LV work is high. Decreasing afterload dramatically helps reduce regurgitant fraction

Why These Two Lesions Are Particularly Troublesome in the ITU

1. Myocardial stunning is universal for 6-24 hours
2. Catecholamine receptor downregulation from CPB reduces native inotropic reserve
3. Hypocalcemia is common from citrated blood products
4. Metabolic acidosis from ischemia-reperfusion impairs contractility
5. Capillary leak from CPB inflammatory response causes volume shifts
6. Junctional ectopic tachycardia (JET) is a common early arrhythmia (2-14% after TOF/VSD repair) - it increases myocardial oxygen demand and causes AV dyssynchrony, which dramatically worsens both RV and LV function when the valve is already severely regurgitant

Role of Each Drug

1. Milrinone (Milicor / Primacor) - PDE-3 Inhibitor / "Inodilator"

• For free PR + RV dysfunction: Milrinone reduces pulmonary vascular resistance (PVR) - this is the single most important pharmacological lever you have for the RV. By lowering PVR, you lower the RV afterload and allow better forward flow. It also provides inotropy to the stunned RV. The PRIMACORP trial established milrinone's benefit in preventing low cardiac output syndrome (LCOS) post-congenital heart surgery
• For free MR + LV dysfunction: Milrinone is an afterload reducer for the LV - lower SVR means the LV "prefers" to push blood forward through the aorta rather than backwards through the incompetent valve. This directly reduces the regurgitant fraction. It also improves diastolic relaxation (lusitropic effect), which helps the over-loaded LV
• Pediatric pharmacokinetics: Children have larger volume of distribution and faster clearance than adults; the loading dose may need to be 50-100 mcg/kg (can be given on CPB), followed by infusion at 0.5-1.0 mcg/kg/min in older infants and children (Miller's Anesthesia, 10e)
• Caution: Milrinone is a vasodilator - if the child is already vasoplegic from CPB (as many are), it can worsen hypotension. Often needs a vasopressor backstop (vasopressin or norepinephrine) to maintain coronary perfusion pressure
• 2024 Cochrane network meta-analysis (Burkhardt et al., PMID 39588800, 13 studies, 937 children): milrinone likely largely reduces LCOS in pediatric cardiac surgery with moderate-certainty evidence

2. Dobutamine - Beta-1 / Beta-2 Agonist / Inodilator

• Dobutamine increases contractility and has mild vasodilating properties (making it an "inodilator" too, though weaker than milrinone for vasodilation)
• In children after cardiac surgery, dobutamine increases cardiac output primarily through increased heart rate - this is important: in young children with developmental downregulation of alpha-receptors and high baseline catecholamines, the predominant response is chronotropic rather than inotropic (Miller's Anesthesia, 10e, page 10810)
• For free PR: There is a nuanced problem here. The RV with free PR is tachycardia-sensitive - at faster rates, diastolic time is shorter, which actually reduces regurgitant volume (less time for blood to flow back). But in an RV that is already stiff and failing, excessive tachycardia increases oxygen demand in an already compromised ventricle. So dobutamine's chronotropy is a double-edged sword after TOF repair with free PR
• For free MR: Dobutamine improves LV contractility and its mild afterload reduction helps reduce regurgitant fraction, similar to milrinone but less potent for this purpose
• Risk of arrhythmia (JET, SVT) limits dose - structural similarities to isoproterenol make arrhythmogenic potential real
• Usual dose: 2-20 mcg/kg/min IV

3. Levosimendan - Calcium Sensitizer

• The key advantage over catecholamines and milrinone: It does not increase intracellular calcium or cAMP, so it does not increase oxygen demand in an already oxygen-starved stunned myocardium. This is theoretically protective
• For free PR + RV dysfunction: Levosimendan has significant pulmonary vasodilatory properties and improves RV contractility. The 2022 RCT on levosimendan for TOF repair LCOS prevention (PMID 35763357) specifically studied this scenario
• For free MR + LV dysfunction: Levosimendan improves LV systolic function, reduces filling pressures, and the vasodilation reduces regurgitant fraction - similar benefits to milrinone
• 2022 systematic review and meta-analysis (Lapere, Rega, Rex, PMID 35792892, 9 RCTs, 539 children): Levosimendan reduced LCOS incidence (RR 0.80, p=0.01) and increased cardiac index (MD +0.17 L/min/m²) compared to other inotropes or placebo in pediatric cardiac surgery. Effect did not translate to improved mortality or ICU LOS
• 2024 Cochrane network meta-analysis (PMID 39588800): levosimendan resulted in a large reduction in mortality vs placebo (RR 0.57), with high-certainty evidence for LCOS reduction (RR 0.45). It ranked highest among inotropes for LCOS prevention
• Practical note: Levosimendan has a long half-life (~1 hour for parent compound, but active metabolite OR-1896 lasts 70-80 hours). A single loading infusion of 6-24 mcg/kg over 10-24 hours may be given. Effects persist for days
• Dose in children: Typically 0.1-0.2 mcg/kg/min IV (some protocols with loading dose 6-12 mcg/kg over 10 min, though loading is often omitted to avoid hypotension)
• Levosimendan is superior to dobutamine as an inodilator for post-cardiac surgery pulmonary hypertension in children (Ebade et al., 2013)

4. Beta-Blockers (Esmolol, Propranolol) - What Is Their Role?

• Esmolol (ultra-short-acting IV beta-1 selective blocker) has been used routinely perioperatively after TOF repair at some centers. The 2022 single-center retrospective study by Affolter et al. (PMID 36200768) showed that routine esmolol post-TOF repair was associated with good cardiac output, minimal vasoactive support, and only 3/26 patients developing JET - with no 30-day mortality. They proposed esmolol as part of a routine multi-disciplinary protocol
• However: Classic teaching says beta-blockers are not reliably effective for JET because JET arises from enhanced automaticity (not re-entry), which is not abolished by beta-blockade alone. Amiodarone + hypothermia (surface cooling to 35-36°C) + atrial overdrive pacing remain the mainstay of JET management

• Low cardiac output state
• Hypotension
• Need for multiple inotropes
• Signs of reduced perfusion

Practical ITU Framework for a 5-7 Year Old Post-Intracardiac Repair with Free PR or Free MR

Key Points Summary

1. "Free PR" = no effective pulmonary valve - classic after trans-annular patch for TOF. Causes acute RV volume overload and potentially catastrophic RV failure in the stunned post-CPB heart
2. "Free MR" = no effective mitral valve coaptation - classic after AV canal repair or failed valvuloplasty. Causes acute LV volume overload, LA hypertension, and pulmonary edema with reduced forward cardiac output
3. Milrinone (Milicor) is the backbone - reduces PVR, reduces SVR (decreasing regurgitant fraction for MR), and provides inotropy. Use in both conditions
4. Dobutamine is an inotrope + mild vasodilator; useful but carries arrhythmia risk and its cardiac output benefit in young children is largely chronotropic
5. Levosimendan is the calcium sensitizer with the best evidence base in pediatric cardiac surgery (2024 Cochrane: highest-ranked inotrope for LCOS prevention). Uniquely does not increase myocardial oxygen demand. Preferred in severe or refractory LCOS
6. Beta-blockers (esmolol) are NOT first-line inotropes - they are used specifically for rate control in JET (where amiodarone + cooling is the main treatment) and at specific centers as a routine post-TOF protocol to reduce adrenergic drive on the stunned myocardium. Contraindicated in low cardiac output states
7. Free PR is better tolerated long-term than acutely - the RV adapts over years but needs intensive support immediately post-bypass

• Miller's Anesthesia (10e), pp. 10809-10812 - Pediatric post-CPB ventricular dysfunction management
• Burkhardt BE et al. (Cochrane 2024), PMID 39588800 - Network meta-analysis of inotropes for pediatric CHD surgery (levosimendan ranked best)
• Lapere M et al. (Eur J Anaesthesiol, 2022), PMID 35792892 - Levosimendan systematic review in pediatric cardiac anaesthesiology
• Affolter JT et al. (Pediatr Crit Care Med, 2022), PMID 36200768 - Routine esmolol after infant TOF repair
• Harriet Lane Handbook (23e) - Inotrope dosing table
• Current Surgical Therapy (14e) - Milrinone and dobutamine mechanisms