Hepatopulmonary Syndrome (HPS) & Portopulmonary Hypertension

Hepatopulmonary syndrome (HPS) and portopulmonary hypertension (PoPH) are two distinct but related pulmonary vascular disorders that complicate chronic liver disease and portal hypertension. Both arise from the haemodynamic and molecular consequences of portal hypertension but differ fundamentally in pathophysiology, clinical presentation, and management. Recognising and distinguishing these conditions is critical because they affect transplant candidacy, require different therapeutic approaches, and each carries specific MELD exception provisions in Australia.

Epidemiology in Australia

HPS is detected in approximately 5–30% of patients evaluated for liver transplantation, depending on the diagnostic criteria applied. In Australia, where an estimated 500–700 liver transplants are performed annually across major centres (Austin Health Melbourne, Royal Prince Alfred Sydney, Princess Alexandra Brisbane, Sir Charles Gairdner Perth), HPS represents a significant proportion of transplant indications with MELD exception scoring. PoPH is less common, affecting approximately 2–6% of patients with portal hypertension, but is associated with substantially higher peri-transplant mortality if not identified and treated pre-operatively.

The Australian burden of cirrhosis is increasing, driven by metabolic dysfunction-associated steatotic liver disease (MASLD, formerly NAFLD), alcohol-related liver disease, and chronic viral hepatitis. Australian Institute of Health and Welfare (AIHW) data indicate that liver disease mortality has risen over the past two decades. Aboriginal and Torres Strait Islander peoples experience cirrhosis at 2–4 times the rate of non-Indigenous Australians, underscoring the importance of screening for HPS and PoPH in this population.

Definition & Triad

HPS is defined by the simultaneous presence of three features:

1. Liver disease and/or portal hypertension — cirrhosis of any aetiology, non-cirrhotic portal hypertension, or acute/acute-on-chronic liver failure.
2. Intrapulmonary vascular dilatations (IPVDs) — confirmed by contrast-enhanced (bubble) echocardiography or macroaggregated albumin (MAA) lung perfusion scanning.
3. Impaired oxygenation — alveolar–arterial (A–a) oxygen gradient ≥15 mmHg (≥20 mmHg in patients aged >64 years) on arterial blood gas breathing room air.

Pathophysiology

Portal hypertension triggers release of vasoactive mediators — including nitric oxide (NO), carbon monoxide, and endothelin-1 — that cause diffuse dilatation of the pulmonary capillary bed. Normal capillary diameter is 8–15 µm; in HPS, dilatation to 15–500 µm occurs. This impairs the diffusion–perfusion relationship: oxygen molecules cannot traverse the widened alveolar–capillary membrane fast enough to fully oxygenate red blood cells during their shortened transit time. A component of true intrapulmonary right-to-left shunting through arteriovenous communications also contributes in severe cases.

Clinical Presentation & Diagnostic Criteria

Patients typically present with progressive exertional dyspnoea, often out of proportion to the degree of liver dysfunction. Platypnea (worsening dyspnoea on upright posture) and orthodeoxia (≥5% fall in PaO₂ or ≥4% fall in SpO₂ from supine to upright) are characteristic but present in only 25–30% of HPS cases. Physical examination may reveal digital clubbing, cyanosis, and spider naevi (though the latter reflects liver disease rather than HPS specifically).

Diagnosis

Diagnosis requires demonstration of both intrapulmonary vascular dilatation and abnormal oxygenation in the context of portal hypertension.

Contrast-Enhanced Echocardiography (Bubble Study)

This is the preferred first-line screening investigation. Agitated saline (creating microbubbles >10 µm) is injected intravenously during echocardiography. In normal physiology, microbubbles are trapped in the pulmonary capillary bed and do not reach the left atrium. In HPS, intrapulmonary dilatations allow bubbles to pass through; their appearance in the left atrium after 3–6 cardiac cycles is diagnostic of IPVDs. Immediate appearance (within 1–3 cardiac cycles) indicates an intracardiac shunt (e.g., patent foramen ovale) and should be excluded.

Arterial Blood Gas (ABG)

ABG on room air confirms abnormal oxygenation. The A–a gradient should be calculated using the alveolar gas equation. In patients with cirrhosis, a baseline mild A–a gradient elevation is common due to hyperdynamic circulation; HPS is diagnosed when the gradient meets or exceeds 15 mmHg (or ≥20 mmHg if age >64 years). Seated and supine ABGs should be compared to assess for orthodeoxia.

Excluding Primary Lung Disease

Before diagnosing HPS, intrinsic pulmonary pathology must be excluded. This includes COPD, interstitial lung disease, pulmonary fibrosis, pleural effusions, and pneumonia. Chest X-ray, CT thorax (if indicated), pulmonary function tests (spirometry and diffusing capacity), and high-resolution CT may be required. Coexisting primary lung disease and HPS can co-occur; the key is to establish that IPVDs are contributing to hypoxaemia beyond what the primary lung condition explains.

Management

Supplemental Oxygen

Supplemental oxygen is the primary supportive therapy for HPS. It does not reverse the underlying vascular dilatation but improves oxygenation, reduces symptoms, and may mitigate secondary polycythaemia. Oxygen should be titrated to maintain SpO₂ ≥92% at rest and with exertion. Home oxygen therapy (HOT) can be arranged through state-based programmes (e.g., Victoria's Respiratory Support Services, NSW Oxygen Home Service) where available.

MELD Exception Points

In Australia, patients with HPS who are listed for liver transplantation may receive MELD exception points. The standard allocation assigns a MELD score of 22 (equivalent to approximately 15–20% three-month mortality) when all three diagnostic criteria are met and PaO₂ <60 mmHg on room air. This exception must be approved by the relevant state transplant allocation authority and reviewed periodically. The Transplant Society of Australia and New Zealand (TSANZ) coordinates exception criteria across centres.

Liver Transplantation

Liver transplantation is the only definitive treatment for HPS. Post-transplant, intrapulmonary vascular dilatations resolve over weeks to months, and oxygenation normalises in the majority of patients. However, HPS is associated with increased post-transplant morbidity (prolonged ICU stay, hypoxaemia-related complications) and mortality compared to transplant for other indications. Pre-transplant PaO₂ <50 mmHg is associated with worse outcomes, though transplantation remains indicated as the only curative option. Post-transplant outcomes in Australian centres are consistent with international data, with 1-year survival approximately 80–85% for HPS patients.

Definition

Portopulmonary hypertension is defined as pulmonary arterial hypertension (Group 1 PH, per WHO classification) occurring in the setting of portal hypertension, with or without underlying cirrhosis. Haemodynamic criteria, confirmed by right heart catheterisation (RHC), require:

• Mean pulmonary arterial pressure (mPAP) ≥25 mmHg at rest
• Pulmonary vascular resistance (PVR) >3 Wood units (240 dyn·s·cm⁻⁵)
• Pulmonary capillary wedge pressure (PCWP) ≤15 mmHg

The updated 2022 ESC/ERS haemodynamic definitions now use mPAP >20 mmHg with PVR ≥3 Wood units as the threshold for pre-capillary PH, though the 25 mmHg threshold remains standard for clinical decision-making in transplantation contexts.

Pathophysiology

The pathogenesis of PoPH involves pulmonary arterial vasoconstriction, endothelial dysfunction, and medial hypertrophy of small pulmonary arteries — histologically identical to idiopathic PAH. Portal hypertension leads to gut-derived vasoactive substances (serotonin, endothelin-1, interleukin-1, thromboxane A₂) reaching the pulmonary circulation via portosystemic shunts, bypassing hepatic first-pass metabolism. These mediators promote pulmonary arterial smooth muscle proliferation, intimal fibrosis, and in situ thrombosis. A hyperdynamic circulatory state (high cardiac output) may mask early PoPH; disease manifests when PVR rises sufficiently to overcome the elevated cardiac output.

Clinical Presentation & Diagnostic Criteria

Symptoms are often non-specific and attributed to underlying liver disease, leading to delayed diagnosis. Progressive exertional dyspnoea, fatigue, syncope (in severe cases), and chest pain are typical. Physical findings include a loud P2, right ventricular heave, tricuspid regurgitation murmur, elevated jugular venous pressure, peripheral oedema (which may be attributed to hypoalbuminaemia or cirrhosis), and hepatomegaly.

All patients being evaluated for liver transplantation should undergo transthoracic echocardiography to estimate right ventricular systolic pressure (RVSP) via tricuspid regurgitation velocity. If RVSP is elevated (>40 mmHg) or right ventricular dysfunction is suspected, right heart catheterisation is mandatory to confirm PoPH and quantify haemodynamics.

Severity Grading

Investigations

PAH-Directed Therapy

PAH-directed pharmacotherapy is used to reduce mPAP and PVR to levels compatible with safe liver transplantation. Therapy selection is guided by severity, haemodynamic response, hepatic tolerability, and drug interactions. All agents below are used off-label or under specialist guidance for PoPH in Australia.

PDE5 Inhibitors

Endothelin Receptor Antagonists (ERAs)

Prostacyclin Analogues

Therapy Response & Transplant Criteria

The goal of PAH-directed therapy is to reduce mPAP to <35 mmHg with low PVR (ideally <3–5 Wood units) and preserved cardiac output — this is considered the acceptable risk threshold for liver transplantation. Patients are typically re-assessed with repeat RHC after 3–6 months of optimised therapy.

MELD Exception in Portopulmonary Hypertension

Selected patients with PoPH may receive MELD exception points if they have demonstrated a response to PAH-directed therapy (mPAP reduced but still elevated, or disease severity warrants accelerated listing). This is assessed on a case-by-case basis by the relevant state transplant authority, with input from the treating hepatologist, respiratory/pulmonary hypertension specialist, and transplant surgeon. Exception status requires documentation of RHC-documented haemodynamic parameters and evidence of PAH therapy optimisation.

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