Assessing Operability in Left-to-Right Shunt Lesions

Assessing the operability of a left-to-right shunt (such as an ASD, VSD, or PDA) fundamentally hinges on quantifying the degree of pulmonary vascular disease (PVD). The core objective is to determine whether the pulmonary vascular resistance (PVR) is dynamically reversible or permanently fixed. Closing a defect in the presence of irreversible pulmonary vascular disease removes the “pop-off” valve, which can precipitate acute right ventricular failure and accelerate mortality.

1. Non-Invasive Assessment (Echocardiography)

While hemodynamics dictate the final decision, echocardiography establishes the initial trajectory by assessing the direction of flow and right heart remodeling.

• Favorable for closure: Clear left-to-right shunting, significant right heart volume overload (RV/RA dilation), and an estimated pulmonary artery systolic pressure (PASP) < 50% of systemic pressure.
• Warning signs: Bidirectional or right-to-left shunting (Eisenmenger physiology), severe right ventricular hypertrophy (indicating pressure overload rather than just volume overload), and loss of the continuous murmur (in PDA) or pansystolic murmur (in VSD).

2. Right Heart Catheterization (RHC)

RHC is the gold standard and mandatory for any patient with non-invasive evidence of elevated pulmonary pressures or suspected PVD. The decision relies heavily on the shunt fraction (Q_p:Q_s) and the PVR.

Blood gases must be meticulously sampled from the SVC, IVC, RA, RV, and PA to accurately calculate the step-up and apply the Fick principle.

Operability Thresholds (based on ESC/AHA Guidelines)

Classification	PVR (Wood Units)	Qp​:Qs​ Ratio	Management Implication
Clearly Operable	< 3 WU	> 1.5	Proceed with surgical or percutaneous closure.
Grey Zone	3 – 5 WU	> 1.5	Individualized decision; closure generally considered safe if hemodynamics are favorable.
Likely Inoperable	≥ 5 WU	< 1.5	High risk. Defect acts as a necessary pop-off. Closure generally contraindicated upfront.

3. Vasoreactivity Testing

For patients falling into the grey zone or those presenting with PVR ≥ 5 WU, baseline hemodynamics are insufficient. Vasoreactivity testing is critical to differentiate between reactive vasoconstriction and permanent vascular remodeling.

Administer a potent, short-acting pulmonary vasodilator (typically inhaled nitric oxide, often combined with 100% O₂, or IV epoprostenol).

Favorable response criteria:

• A significant drop in PVR (typically by ≥ 20%).
• PVR falling below the 5 WU (ideally < 3 WU) threshold.
• A concurrent increase in the Q_p:Q_s ratio (indicating the pulmonary bed can accommodate more flow when relaxed).
• No significant drop in systemic cardiac output.

4. The “Treat-and-Repair” Strategy

If the PVR remains elevated (≥ 5 WU) despite acute vasoreactivity testing, the lesion is traditionally considered inoperable. However, a “treat-and-repair” strategy is increasingly utilized.

This involves initiating dual advanced PAH therapies (e.g., endothelin receptor antagonists and PDE-5 inhibitors) for 6 to 12 months. RHC is then repeated. If the PVR drops into the operable or grey zone with a robust left-to-right shunt, the patient may become a candidate for closure.

When intervening on high-risk or borderline cases, a fenestrated device (for ASDs) or leaving a small residual shunt (for VSDs) is often employed. This allows partial closure to reduce volume load while preserving a safety valve for right-to-left shunting during pulmonary hypertensive crises.

Limits to the Strategy: Patients who present with classic Eisenmenger syndrome—characterized by marked right-to-left shunting, desaturation at rest, and severe, irreversible pulmonary vascular disease—are strictly contraindicated for defect closure. For these patients, the defect acts as a necessary “pop-off” valve, and closure leads to worsened survival.

Reference

Humbert M, Kovacs G, Hoeper MM, Badagliacca R, Berger RMF, Brida M, Carlsen J, Coats AJS, Escribano-Subias P, Ferrari P, Ferreira DS, Ghofrani HA, Giannakoulas G, Kiely DG, Mayer E, Meszaros G, Nagavci B, Olsson KM, Pepke-Zaba J, Quint JK, Rådegran G, Simonneau G, Sitbon O, Tonia T, Toshner M, Vachiery JL, Vonk Noordegraaf A, Delcroix M, Rosenkranz S; ESC/ERS Scientific Document Group. 2022 ESC/ERS Guidelines for the diagnosis and treatment of pulmonary hypertension. Eur Respir J. 2023 Jan 6;61(1):2200879. doi: 10.1183/13993003.00879-2022. PMID: 36028254.