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This page describes products and general use; it is not medical advice.
Short answer: on a pulse oximeter, the figure people call the “pulsatility index” is normally labelled perfusion index (PI) or peripheral perfusion index (PPI). It is a unitless ratio of the pulsing (arterial) part of the light signal to the non-pulsing part. It is not oxygen saturation (SpO2), and it does not measure tissue perfusion directly.
Who uses it: clinicians who watch trends on bedside and perioperative monitors, researchers studying peripheral circulation, and buyers or students comparing monitors that display a PI value.
What it is
The perfusion index is the ratio of the pulsatile portion of the pulse wave (arteries) to the non-pulsatile portion, taken from the photoplethysmography signal that a pulse oximeter already records. A review of adult studies describes it as the ratio of pulsatile to non-pulsatile light absorption, and states that it is unitless and “does not measure direct tissue perfusion” (Sun et al., 2024). For the underlying optical method, see photoplethysmography.
A naming note: “pulsatility index” is also used in Doppler ultrasound, which is a different measurement. The value shown on a pulse-oximeter screen is the perfusion index.
How the oximeter derives it
The sensor shines light through a fingertip or other peripheral site. Arterial blood pulses with each heartbeat, so part of the absorbed light varies with the pulse and part stays steady. PI compares the varying part with the steady part. Elshal et al. (2021) describe the plethysmographic peripheral perfusion index as the ratio between pulsatile and non-pulsatile portions in the peripheral circulation, and name cardiac output and the balance between the sympathetic and parasympathetic nervous systems as its main determinants.
What changes the number
| Factor | Effect on the displayed value | Source |
|---|---|---|
| Vascular tone (pain, stress, anesthesia, vasoactive drugs) | Changes peripheral vascular tone, and with it PI | Sun et al., 2024 |
| Body temperature | Listed as a factor that influences PI; the review also notes poor signal with cold extremities in research settings | Sun et al., 2024; Elshal et al., 2021 |
| Peripheral vascular disease | Listed as a factor that influences PI | Sun et al., 2024 |
| Motion artifact | Can corrupt the signal | Sun et al., 2024 |
| Device algorithm | Different monitors use different PI algorithms, so values are not directly interchangeable | Sun et al., 2024 |
| Measurement site | The middle finger is the most common monitoring site in clinical trials | Sun et al., 2024 |
Typical values and why published ranges differ
Two sources give different-looking ranges because they express the ratio differently. Sun et al. (2024) report that PI in healthy adults is skewed, ranging from 0.3 to 10 as a unitless number. Elshal et al. (2021) cite a suggested normal range of 0.2 to 20% and one study with a median of 4.3 (quartiles 2.9 to 6.2). Because the distribution is highly skewed and the relationship between vascular tone and PI is complex and non-linear, the reviews say it is difficult to judge PI as an absolute value; serial readings in the same person over time are more informative than a single number.
What it is not
- Not SpO2. Oxygen saturation is a separate reading; see pulse oximeter normal readings.
- Not a direct tissue perfusion measurement (Sun et al., 2024).
- Not “higher is always better.” The narrative review cautions that a high PI does not always indicate good perfusion and suggests combining PI with other perfusion measures.
- Not a diagnosis. Research uses, such as a low PPI (below 0.2) being associated with mortality in septic patients, are study findings, not rules for interpreting a reading at home.
Equipment context
PI appears on some multiparameter patient monitors and some fingertip or handheld oximeters; whether a given device displays it, and how it computes it, is set out in that device’s instructions for use. For the wider category, see diagnostic and patient monitoring equipment and diagnostic, monitoring and imaging supplies.
How to choose
Whether a monitor or oximeter displays PI, and how it calculates it, is set out in that device’s instructions for use (IFU); rely on the IFU, not on the label alone. Because different monitors use different PI algorithms (Sun et al., 2024), values from different devices are not directly interchangeable, so compare readings only on the same device.
Safe use and handling basics
- Follow the device IFU for sensor placement and care. The middle finger is the most common monitoring site in clinical trials (Sun et al., 2024).
- Keep the measurement site still: motion artifact can corrupt the signal (Sun et al., 2024).
- Body temperature influences PI, and cold extremities can give a poor signal in research settings (Sun et al., 2024; Elshal et al., 2021). Consider this before reading anything into a low number.
- Look at serial readings in the same person rather than one value (Sun et al., 2024; Elshal et al., 2021).
Common mistakes
- Treating PI as oxygen saturation or as a direct measure of tissue perfusion.
- Reading a single value as normal or abnormal, given how skewed the published distribution is.
- Comparing values between monitors with different algorithms.
- Ignoring motion or a cold finger when a reading looks low.
FAQ
Is pulsatility index the same as perfusion index on a pulse oximeter?
In practice yes: oximeter displays label the value perfusion index (PI) or peripheral perfusion index. The Doppler ultrasound “pulsatility index” is a different measurement.
Does perfusion index measure oxygen level?
No. It reflects signal pulsatility, not oxygen saturation, and it does not measure direct tissue perfusion.
What is a normal perfusion index?
Published ranges are wide and skewed, from 0.3 to 10 (unitless) in one review and 0.2 to 20% in another. Trends within one person are more useful than a single value.
Sources
- Sun X, He H, Xu M, Long Y. Peripheral perfusion index of pulse oximetry in adult patients: a narrative review. Eur J Med Res 2024;29:457. PMC11389527. Read 2026-11-08.
- Elshal MM, Hasanin AM, Mostafa M, Gamal RM. Plethysmographic Peripheral Perfusion Index: Could It Be a New Vital Sign? Front Med 2021;8:651909. Frontiers in Medicine. Read 2026-11-08.
