Pulsatility Index (PI) Clinical Interpretation Guidelines

1. Definition and Calculation

The pulsatility index is calculated as (peak systolic velocity – end‑diastolic velocity) ÷ mean velocity in vascular Doppler studies, and as (systolic pressure – diastolic pressure) ÷ right‑atrial pressure for pulmonary‑artery assessment. 1

Abnormal umbilical‑artery PI is defined as > 95th percentile for gestational age; any absent or reversed end‑diastolic velocity (AEDV/REDV) is always abnormal. 1
When PI is > 95th percentile, initiate weekly Doppler surveillance; if AEDV develops, increase monitoring to 2–3 times per week. 1
If REDV is present, immediate hospitalization, administration of antenatal corticosteroids, twice‑daily cardiotocography, and strong consideration of delivery are required. 1
Progression from a normal Doppler to AEDV/REDV typically occurs within 2 weeks if fetal deterioration is imminent; therefore, serial assessments are essential. 1

Abnormal MCA PI is defined as < 5th percentile for gestational age. 3
A cerebro‑placental ratio (MCA PI ÷ umbilical‑artery PI) below the 5th percentile indicates brain‑sparing physiology and warrants intensified surveillance and delivery planning according to gestational age. 3
MCA PI < 5th percentile reflects cerebral vasodilation in response to fetal hypoxemia, prompting increased monitoring frequency. 3
A low cerebro‑placental ratio also contributes to the diagnosis of late‑onset fetal growth restriction. 3

Peak systolic velocity > 200 cm/s or renal‑to‑aortic velocity ratio > 3.5 indicates hemodynamically significant renal‑artery stenosis (≈ ≥ 60 % lumen reduction). 4
In stented renal arteries, higher thresholds apply: PSV ≥ 395 cm/s or RAR ≥ 5.1. 4
Resistive index > 0.80 predicts a poorer response to revascularization, although this relationship remains controversial. 4
A distal parvus‑tardus waveform (acceleration time > 70 ms with loss of early systolic peak) confirms proximal stenosis even when the lesion is not directly visualized. 4
Systemic blood pressure influences PI and RI; using velocity ratios such as RAR improves diagnostic specificity. 4
Technical limitations (obesity, bowel gas, accessory renal arteries) may preclude direct visualization; indirect evidence from distal waveforms should be employed. 4
False‑negative Doppler findings can occur in bilateral stenosis or global hyperemia. 4

PI > 1.4 combined with diastolic velocity < 20 cm/s signals critically elevated intracranial pressure and mandates immediate measures to raise cerebral perfusion pressure (e.g., MAP ≥ 80 mmHg, correction of hypoxemia/acidosis, consider ICP monitoring). 5
In pediatric patients, PI > 1.3 or diastolic velocity < 25 cm/s predicts poor neurological outcome. 5
In moderate TBI (GCS 9‑14), PI > 1.25 with diastolic velocity < 25 cm/s forecasts secondary neurological deterioration within one week. 5
A Lindegaard ratio (MCA velocity ÷ ICA velocity) > 6 identifies severe cerebral vasospasm and guides targeted therapy. 6

Pulmonary‑artery pulsatility index (PAPi) < 2.0 identifies significant right‑ventricular dysfunction across clinical settings. 2
PAPi < 1.0 defines refractory cardiogenic shock and indicates the need to evaluate mechanical circulatory support. 2
PAPi values between 1.0 and 2.0 represent intermediate right‑ventricular dysfunction. 2
When PAPi < 1.0 together with cardiac‑power output < 0.6 W, mechanical circulatory support should be strongly considered. 2
Confirmation of right‑atrial pressure > 15 mmHg helps verify right‑ventricular‑dominant shock. 2
Maintain mean arterial pressure > 65 mmHg with minimal vasopressor use; avoid excessive positive‑pressure ventilation, and promptly correct metabolic acidosis and hypoxemia. 2

A transit‑time flow measurement PI > 5 predicts graft failure and major adverse cardiac events. 7
False‑positive PI results are rare, whereas false‑negative results (low PI despite a poor graft) occur more frequently. 7
Combining PI with anatomic assessments (e.g., fluorescence imaging or epicardial ultrasound) provides a comprehensive evaluation of graft patency. 7

PI should never be interpreted in isolation; integrate Doppler findings with clinical examination, imaging, and other hemodynamic parameters. [5][2]
Waveforms obtained near the placental insertion may show higher end‑diastolic flow than those at the fetal insertion, but this variation rarely alters clinical decisions. 1
When fetal growth restriction is detected before 32 weeks with isolated findings, chromosomal microarray analysis is indicated. 1