What are the normal reference ranges for the pulsatility index in various vessels, and how should high or low PI values be interpreted and managed?

Pulsatility Index Interpretation

Definition and Calculation

The pulsatility index (PI) is calculated as (peak systolic velocity - end diastolic velocity) / mean velocity in vascular Doppler studies, or as (systolic pressure - diastolic pressure) / right atrial pressure for pulmonary artery assessment. 1, 2

Normal Reference Ranges by Vessel

Umbilical Artery (Fetal)

• Normal PI values are gestational age-dependent, with abnormal defined as PI >95th percentile for gestational age 1
• Absent or reversed end-diastolic velocity (AEDV or REDV) is always abnormal regardless of PI value 1
• Longitudinal reference ranges show PI decreases progressively throughout pregnancy as placental resistance falls 3

Middle Cerebral Artery (Fetal)

• Normal MCA PI values are gestational age-specific, with abnormal defined as <5th percentile 1
• The cerebroplacental ratio (MCA PI / umbilical artery PI) <5th percentile indicates brain-sparing physiology 1
• Longitudinal monitoring requires conditional reference intervals based on prior measurements rather than cross-sectional norms 3

Renal Artery

• Peak systolic velocity (PSV) >200 cm/s or renal-to-aortic ratio (RAR) >3.5 indicates significant stenosis 1
• Resistive index (RI) >0.80 predicts poor response to revascularization, though this remains controversial 1
• For stented renal arteries, higher thresholds apply: PSV ≥395 cm/s or RAR ≥5.1 1

Cerebral Vessels (Pediatric/Adult)

• In traumatic brain injury, PI >1.4 combined with diastolic velocity <20 cm/s indicates critically elevated intracranial pressure requiring immediate intervention 1
• In children, PI >1.3 or diastolic velocity <25 cm/s predicts poor neurological outcome 1
• Age-dependent normal values exist but pediatric data remain insufficient for universal application 1

Pulmonary Artery

• PAPi <2.0 identifies significant right ventricular dysfunction across all clinical settings 2
• PAPi <1.0 defines refractory cardiogenic shock requiring mechanical circulatory support consideration 2
• PAPi 1.0-2.0 represents intermediate right ventricular dysfunction 2

Interpretation of High PI Values

Umbilical Artery

Elevated umbilical artery PI (>95th percentile) mandates weekly Doppler surveillance initially, escalating to 2-3 times weekly if AEDV develops. 1

• With decreased end-diastolic velocity or severe fetal growth restriction (EFW <3rd percentile), perform weekly umbilical artery Doppler 1
• AEDV requires hospitalization consideration, 2-3 times weekly monitoring, and delivery planning based on gestational age 1
• REDV mandates immediate hospitalization, antenatal corticosteroids, cardiotocography 1-2 times daily, and strong consideration for delivery 1

Renal Artery

• PSV >200-300 cm/s indicates hemodynamically significant stenosis (≥60%) requiring further evaluation 1
• Parvus-tardus waveform distally (acceleration time >70 ms, loss of early systolic peak) confirms proximal stenosis even without direct visualization 1
• RI >0.80 suggests poor revascularization outcomes, though should not be an absolute contraindication 1

Cerebral Vessels (TBI)

PI >1.4 with diastolic velocity <20 cm/s requires immediate measures to improve cerebral perfusion pressure, including mean arterial pressure optimization (≥80 mmHg), correction of hypoxemia and acidosis, and consideration of intracranial pressure monitoring. 1

• In moderate TBI (GCS 9-14), PI >1.25 with diastolic velocity <25 cm/s predicts secondary neurological deterioration within one week 1
• Lindegaard ratio >6 (MCA velocity / ICA velocity) indicates severe vasospasm requiring specific management 1

Interpretation of Low PI Values

Umbilical Artery

• Low or normal PI in fetal growth restriction provides reassurance but requires continued surveillance every 2-4 weeks 1
• If PI remains normal after initial 1-2 weeks of monitoring, extend interval to every 2-4 weeks 1

Middle Cerebral Artery

MCA PI <5th percentile indicates cerebral vasodilation (brain-sparing) in response to hypoxemia, warranting increased surveillance frequency and delivery consideration depending on gestational age. 1

• Cerebroplacental ratio <5th percentile is a contributory parameter for late-onset fetal growth restriction diagnosis 1
• Brain-sparing physiology suggests chronic fetal compromise requiring weekly cardiotocography 1

Pulmonary Artery

PAPi <1.0 combined with cardiac power output <0.6 W constitutes a clear indication for mechanical circulatory support evaluation. 2

• Confirm right atrial pressure >15 mmHg to verify RV-dominant shock 2
• Maintain mean arterial pressure >65 mmHg with minimal vasopressor doses 2
• Avoid excessive positive-pressure ventilation which worsens RV function 2
• Correct metabolic acidosis and hypoxemia immediately 2

Critical Management Pitfalls

Fetal Doppler

• Never rely on a single Doppler assessment; progression from abnormal to AEDV/REDV typically occurs within 2 weeks if deterioration will happen 1
• Waveforms obtained near the placental insertion show higher end-diastolic flow than fetal insertion, but this variation rarely affects clinical decisions 1
• Chromosomal microarray analysis is indicated when fetal growth restriction is detected <32 weeks with isolated findings 1

Renal Doppler

• PI and RI are influenced by systemic blood pressure, not just stenosis severity; use velocity ratios (RAR) to improve specificity 1
• Obesity, bowel gas, and accessory renal arteries limit direct visualization; use distal parvus-tardus waveforms as indirect evidence 1
• False negatives occur with bilateral stenosis or global hyperemia 1

Cerebral Doppler

• Do not interpret PI in isolation; integrate with clinical examination, imaging, and other hemodynamic parameters 1, 2
• Hypertension alone elevates PI without indicating distal vascular disease 4
• In cardiogenic shock, right-heart catheterization should not delay urgent revascularization 2

Coronary Bypass Grafts

• PI >5 on transit-time flow measurement predicts graft failure and major adverse cardiac events 1
• False positives (good graft, high PI) are rare; false negatives (poor graft, low PI) occur more frequently 1
• Combine PI with anatomic assessment (fluorescence imaging or epicardial ultrasound) for complete evaluation 1