What PAPi measures: ratio of pulmonary artery pulse pressure to right atrial pressure (RV pulsatility vs congestion).
Rule-of-thumb interpretation:

• Normal / healthy RV: > 1.5–2.0
• RV dysfunction: < 1.0
• Severe RV failure: < 0.5

Clinical hint: Low PAPi with markedly elevated PA pressures (e.g., ARDS, PE) may improve as afterload falls (transient). Low PAPi despite modest PA pressures suggests intrinsic RV pump failure (more persistent).