The pulmonary artery pulsatility index (PAPi) is calculated by subtracting diastolic pulmonary artery pressure from systolic pulmonary artery pressure, then dividing by right atrial pressure. The formula is: PAPi = (sPAP − dPAP) / RAP. A higher number generally indicates better right heart function, while a lower number suggests the right ventricle is struggling.
Breaking Down the Formula
Each component of the PAPi equation captures a different piece of right heart function:
- Systolic pulmonary artery pressure (sPAP): The peak pressure in the pulmonary artery when the right ventricle contracts and pushes blood toward the lungs.
- Diastolic pulmonary artery pressure (dPAP): The lowest pressure in the pulmonary artery between heartbeats, when the right ventricle relaxes.
- Right atrial pressure (RAP): The pressure in the right atrium, which reflects how much blood is backing up before entering the right ventricle. This is sometimes referred to as central venous pressure (CVP).
The top half of the equation (sPAP minus dPAP) is the pulmonary artery pulse pressure, which reflects how much blood the right ventricle ejects with each beat. Dividing that by right atrial pressure creates a ratio that balances the right ventricle’s pumping ability against its filling pressure. When the right ventricle weakens, pulse pressure drops and right atrial pressure climbs, so PAPi falls on both ends of the fraction.
A Worked Example
Say a patient’s right heart catheterization shows a systolic pulmonary artery pressure of 45 mmHg, a diastolic pulmonary artery pressure of 20 mmHg, and a right atrial pressure of 10 mmHg. Plugging those in: (45 − 20) / 10 = 2.5. That PAPi of 2.5 suggests the right ventricle is generating a reasonable pulse pressure relative to the backup pressure in the right atrium.
Now consider a sicker patient with pressures of 38, 25, and 15 mmHg. The calculation becomes (38 − 25) / 15 = 0.87. The narrower pulse pressure and higher right atrial pressure both push PAPi down, signaling that the right ventricle is failing to keep up.
How the Pressures Are Measured
PAPi is traditionally measured during right heart catheterization. A thin, flexible catheter is threaded through a vein into the right side of the heart and then into the pulmonary artery. As it advances, it records pressure waveforms at each location: right atrium first, then right ventricle, then pulmonary artery. All pressures are recorded at end-expiration to minimize the effect of breathing on the readings.
Before any pressures are taken, the transducer system is zeroed to atmospheric pressure and leveled at the midpoint of the chest (roughly the fourth intercostal space). This step ensures the numbers reflect actual intracardiac pressures rather than artifacts from the patient’s position.
Some newer research has explored estimating PAPi noninvasively using echocardiography, which uses ultrasound-derived pressure estimates instead of catheter measurements. This approach is less precise but avoids an invasive procedure.
What the Numbers Mean Clinically
There is no single universally agreed-upon cutoff for PAPi, but the most commonly cited threshold is 2.0. In studies of patients receiving left ventricular assist devices (LVADs), a PAPi below 2.0 achieved optimal sensitivity and specificity for predicting right ventricular failure after surgery. Patients below that threshold had more than three times the odds of needing an additional mechanical support device for the right side of the heart.
Other studies have used cutoffs ranging from 0.88 to 3.3, depending on the patient population and the outcome being predicted. One widely referenced early study found that a PAPi below 1.85 predicted right heart failure with 94% sensitivity and 81% specificity, outperforming several other hemodynamic markers used at the time. The variation in cutoffs partly reflects different disease severities and different definitions of what counts as right heart failure across studies.
In general, higher PAPi values indicate better right ventricular function. A mean PAPi of around 3.4 has been reported in mixed populations of LVAD candidates, with right heart failure occurring in about a third of those patients.
Factors That Can Skew the Result
PAPi is a simple ratio, which makes it easy to calculate but also sensitive to anything that shifts one of its three components independently of actual right ventricular function.
Severe tricuspid regurgitation is one of the most important confounders. When the valve between the right atrium and right ventricle leaks significantly, blood flows backward during each contraction. This raises right atrial pressure and can artificially lower PAPi even if the right ventricle’s forward pumping ability is still partially preserved. In one study, 35% of patients in the low PAPi group had severe tricuspid regurgitation, compared to about 20% in the higher PAPi group.
Elevated pulmonary vascular resistance also affects the result. When the blood vessels in the lungs are stiff or narrowed, diastolic pulmonary artery pressure rises. Since diastolic pressure is subtracted in the numerator, a higher diastolic value shrinks the pulse pressure and pulls PAPi down. Patients with low PAPi have been shown to have meaningfully higher pulmonary vascular resistance and higher diastolic pulmonary artery pressures than those with higher PAPi values.
Whether a patient is on inotropes (medications that boost heart contraction strength) at the time of catheterization also matters. PAPi measured while a patient is on inotropes appears to be more predictive of outcomes than PAPi measured without them, likely because inotropes partially mask right ventricular weakness and reveal the patients whose hearts still cannot generate adequate pulse pressure despite pharmacologic support.
Where PAPi Is Used Most
PAPi was originally developed and validated in the LVAD population, where predicting right heart failure before surgery is critical. An LVAD supports only the left ventricle, so if the right side fails after implantation, the patient may need emergency mechanical support for the right heart as well. PAPi gives surgical teams a quick, single-number estimate of that risk during preoperative catheterization.
Since then, PAPi has been applied more broadly: in acute heart failure, cardiogenic shock, acute pulmonary embolism, and after cardiac surgery. In each setting, the core logic is the same. A low PAPi flags a right ventricle that cannot generate enough forward flow relative to the pressure building behind it, which helps clinicians identify patients who may deteriorate or need more aggressive intervention.