Unresolved Questions in ASD Pathophysiology: Shunt Dynamics, Exercise Physiology, and Pulmonary Vascular Disease Risk

Authors, Journal, Affiliations, Type, DOI

• Author: Ashish H. Shah
• Affiliation: Section of Cardiology, Department of Internal Medicine, Max Rady College of Medicine, University of Manitoba, Winnipeg, MB, Canada
• Journal: Annals of Pediatric Cardiology, Volume 18, Issue 6, November–December 2025 (published 16 March 2026)
• Type: Brief Communication (narrative review / opinion)
• DOI: https://doi.org/10.4103/apc.apc_164_25

Overview

This brief communication identifies three underappreciated knowledge gaps in ASD physiology that may limit current risk stratification. First, the Qp:Qs ratio is a relative index that obscures widely varying absolute pulmonary blood flow burdens — two patients with identical Qp:Qs 1.8:1 but cardiac outputs of 4 vs 6 L/min have pulmonary flows of 7.2 vs 10.8 L/min. Second, exercise-induced changes in shunt volume are poorly characterized, with conflicting data from three small studies and no prospective invasive or 4D flow MRI dataset. Third, predictors of post-closure PAH are incompletely defined, with delayed closure, pre-closure elevated PVR, BMPR2 variants, and pulmonary microvascular disease as candidate risk factors lacking prospective validation.

Keywords

Atrial septal defect, pathophysiology, unresolved questions

Key Takeaways

Limitations of Qp:Qs and the Need for Absolute Shunt Volume Metrics

• Qp:Qs ≥1.5:1 has long guided ASD closure decisions; it is influenced by defect size and ventricular compliance
• Qp:Qs is a relative index — it does not capture absolute pulmonary blood volume traversing the pulmonary vasculature
• Example: two patients both with Qp:Qs 1.8:1 — one with resting cardiac output 4 L/min (pulmonary flow 7.2 L/min) vs another with 6 L/min (pulmonary flow 10.8 L/min) — have very different right heart and vascular burdens
• A tall, athletic individual with hyperdynamic circulation may have substantially higher absolute pulmonary flow at the same Qp:Qs as a less fit patient
• Qp:Qs assumes stable systemic and pulmonary vascular resistances and does not account for physiological fluctuations during exercise or neurohumoral modulation

Exercise-Induced Augmentation of Pulmonary Flow — Underappreciated?

• Left atrial pressure rises more than right atrial pressure during exercise; left atrium is described as stiffer in ASD patients
• The trans-atrial pressure gradient is dynamically modulated by exercise-induced changes in RV and LV compliance, PVR-compliance, and systemic arterial pressure
• Age, cardiovascular risk factors, and atrioventricular compliance all modulate exercise-related shunt behavior
• The assumption that Qp:Qs remains stable at rest and exercise is largely unvalidated
• Three published studies with contradictory findings:
  • Bay et al. (Acta Med Scand 1971): stress testing → increased absolute shunt volume
  • Nielsen and Fabricius (Acta Med Scand 1968): exercise → decreased shunt volume
  • Stephensen et al. (Eur Heart J Cardiovasc Imaging 2017): dobutamine/atropine stress → no change in shunt volume
• Caveats: small sample sizes; some used surrogate stressors (pharmacological) rather than true exercise; individual variability within each study contradicted average trends
• A declining Qp:Qs during exercise does NOT necessarily mean decreased absolute pulmonary flow — if systemic cardiac output rises faster than the ratio falls, absolute pulmonary flow may still increase
• Clinical question raised: Should unrepaired ASD patients — especially those engaged in endurance sports — limit physical activity to avoid episodic or chronic right heart volume overload? Current guidelines provide no clear directive
• Advanced tools (real-time 4D flow MRI; invasive exercise hemodynamics) could characterise dynamic shunt behavior but are not in routine clinical use

Predictors of PAH in ASD — Who Is at Risk?

• Eisenmenger syndrome (shunt reversal from elevated PVR) is well described but rare in secundum ASD
• Late-onset PAH after ASD closure is increasingly recognized, occurring years or decades post-intervention
• Risk factors for post-closure PAH:
  • Delayed closure (typically >40 yr) — irreversible pulmonary vascular changes may have occurred
  • Pre-closure elevated PVR even if below Eisenmenger threshold
  • Genetic susceptibility — BMPR2 variants (data limited in ASD cohorts specifically)
  • Unrecognized pulmonary microvascular disease, especially in women or patients with autoimmune conditions
• Belief that ASD closure reduces pulmonary pressures in all patients is overly simplistic
• Pre-closure hemodynamic assessment, including vasoreactivity testing and PVR quantification, remains essential particularly in older adults or symptomatic patients
• Some patients develop PAH despite early closure — implicating nonhemodynamic factors: endothelial dysfunction, inflammation, in situ thrombosis

Knowledge Gaps and Future Research Directions

• What is the true range of absolute shunt volumes in patients with identical Qp:Qs?
• How does exercise-induced shunt flow vary across ASD morphologies and ventricular compliance states?
• Can serial exercise hemodynamics identify patients at risk of early right heart dysfunction or PAH?
• Are there biomarkers (NT-proBNP, endothelin-1) or molecular signatures predictive of adverse remodeling despite closure?
• What is the role of genetic testing (BMPR2) or endothelial function assessment in PAH risk stratification in ASD?

Limitations of the Document

• Brief communication with no primary patient data
• All three cited exercise studies have very small sample sizes and methodological limitations (surrogate stressors, variable ages, no invasive flow measurement)
• BMPR2 data in ASD-specific cohorts are described as "limited" — no quantification provided
• No systematic review methodology; references selective
• Conclusions and clinical questions are speculative and hypothesis-generating rather than evidence-based

Key Concepts Mentioned

• concepts/Atrial-Septal-Defect — primary topic; limitations of Qp:Qs; exercise shunt dynamics; PAH predictors
• concepts/Intracardiac-Shunts — absolute vs relative shunt quantification
• concepts/Pulmonary-Hypertension — ASD-associated PAH; post-closure PAH risk factors

Key Entities Mentioned

• No dedicated entity pages required for this source

Wiki Pages Updated

• wiki/sources/asd-japc-2025 — created
• wiki/wikiindex.md — updated
• wiki/sourceindex.md — updated
• concepts/Atrial-Septal-Defect — new section on Qp:Qs limitations, exercise physiology, PAH predictors; new open questions
• concepts/Intracardiac-Shunts — updated with absolute vs relative shunt volume distinction