I read with great interest the article by Fields et al. exploring practice variability among paediatric interventional cardiologists in diagnosing pulmonary arteriovenous malformations in single-ventricle physiology.^{Reference Fields, Boon and Aldoss1} The authors deserve commendation for highlighting the variability in diagnostic approaches and the absence of consensus in assessing this important clinical problem. However, several pathophysiologic and diagnostic nuances warrant further consideration.

First, a subset of Fontan patients demonstrating intrapulmonary shunting in the absence of discrete vascular malformations likely have hepatopulmonary syndrome secondary to Fontan-associated liver disease rather than true pulmonary arteriovenous malformations.^{Reference Shah, Qureshi and Krasuski2} Even in early, pre-cirrhotic stages, Fontan-associated liver disease may induce intrapulmonary vasodilatation through inflammatory and angiogenic mediators, resulting in a high cardiac output state.^{Reference Bews, Mittal, Tam and Shah3} Distinguishing between hepatopulmonary syndrome and micro pulmonary arteriovenous malformations remains extremely challenging, as neither imaging nor bubble contrast timing can reliably separate the two unless a large, discrete malformation is present. Our recent work demonstrated elevated bile acid levels in patients with Fontan circulation, which were closely associated with Fontan pathophysiology.^{Reference Shah, Surendran and Hassan-Tash4} Bile acids are known to trigger pulmonary inflammation and may contribute to the development of hepatopulmonary syndrome,^{Reference Horvatits, Drolz and Rutter5} an association that warrants further investigation and clarification.

Second, while the hepatic factor deficiency hypothesis remains widely accepted, it is plausible that excess rather than deficiency contributes to pulmonary arteriovenous malformation formation. Venous blood that bypasses hepatic filtration may contain pro-angiogenic or vasodilatory mediators, such as vascular endothelial growth factor, angiopoietin-2, and transforming growth factor-β, that are normally degraded by hepatocytes. Consequently, pulmonary arteriovenous malformation formation may reflect not only the absence of a protective “hepatic factor” but also the unfiltered passage of vasoactive substances that drive aberrant pulmonary vascular remodelling.^{Reference Fields, Boon and Aldoss1} This alternate interpretation deserves further biochemical and mechanistic validation.

Third, future consensus statements should incorporate parallel evaluation for both pulmonary arteriovenous malformations and hepatopulmonary syndrome in all Fontan patients presenting with hypoxaemia or positive bubble contrast studies. Quantitative perfusion scanning and arterial blood gas analysis (room air) can help identify diffuse intrapulmonary shunting. Moreover, visualisation of bubbles passing through all pulmonary veins is more consistent with hepatopulmonary syndrome than with discrete pulmonary arteriovenous malformations, though variations may occur.^{Reference Shah, Shaikh, Malik, Blouw and Kass6} Relying solely on angiography or echocardiography risks under-recognising hepatogenic vasodilatation. Moreover, assessment under room air rather than high-flow oxygen is essential, as hyperoxia artificially elevates pulmonary venous oxygen content and may overestimate true oxygenation. Consistent procedural standards, such as FiO₂ setting, saline-air (+/− blood) ratios, and timing of injection, must be established to ensure reproducibility across centres.

Finally, I echo the authors’ call for uniform diagnostic practices but stress that standardisation must go beyond procedural technique to encompass a broader understanding of Fontan-related pulmonary vascular pathophysiology. The intertwined mechanisms of hepatic congestion, endothelial dysfunction, and angiogenic dysregulation are central to both pulmonary arteriovenous malformations and hepatopulmonary syndrome development. Clarifying these biochemical pathways may identify therapeutic targets capable of modulating intrapulmonary shunting and improving systemic oxygen delivery in this uniquely vulnerable population.

I once again commend Fields et al. for their important contribution to harmonising practice in this evolving domain. Their work provides the foundation for a new era of multicentre collaboration that integrates haemodynamic, hepatic, and biochemical insights into the management of Fontan-associated pulmonary vascular disease.