Pulmonary artery pulsatility index (PAPi) is a haemodynamic parameter that is derived from right atrial and pulmonary artery pulse pressures. A number of reports have described the prognostic value of PAPi in patients with advanced heart failure and cardiogenic shock. However, the derivation and physiological interpretation of this parameter have received little attention. This review will examine the physiological interpretation and clinical data for PAPi.