PI or Pi on a pulse oximeter (perfusion index)
PI, the perfusion index, compares the part of the light signal that pulses with each heartbeat to the part that stays steady. It says how strong the pulsing signal is at the spot being measured, so it tells you how much the oximeter has to work with; it is not an oxygen reading and not a pulse rate.
Check your oximeter’s manual — makers differ.
What it is for
Use it to judge the signal: a very small PI means little pulse at the finger, so SpO2 and PR are harder to measure. Beurer’s PO 45 manual says “the perfusion index (PI) may lie between 0.3% and 20%, and varies depending on the patient, measurement location and state of health” and that “a very low PI value can impair the measurement”. Masimo’s MightySat manual describes Pi as “a noninvasive measure of peripheral perfusion”. Neither maker gives a PI value at which to seek help.
These pages report what each maker’s manual says and quote it wherever health is involved. They do not diagnose anything, they are not medical advice, and they do not tell you whether an oxygen or pulse reading is right for you or what to do about it; only your own oximeter’s manual and your doctor or nurse can do that for the person you are looking after.
- What it looks like
- A smaller third number marked PI or Pi, usually with a % sign, beside or below SpO2 and PR.
- Names makers use
- PI % (Beurer PO 45); Pi (Masimo MightySat Rx); Perfusion index (value in percent) (Beurer PO 45 display key)
Where makes differ
The scale each model shows differs: Masimo lists a Pi display range of 0.02% to 20% for the MightySat, while Beurer lists 0.3 – 20 % for the PO 45. Nonin’s 9590 does not show PI, but its 2022 leaflet makes the same point about weak signals: “Operation of this device below the minimum amplitude of 0.3% modulation may cause inaccurate results.” The Masimo MightySat Rx can also show PVi, which Masimo describes as a measure of how Pi changes with breathing.
What it does not mean
PI is not oxygen saturation, and a high PI is not a better health score. It is not comparable between makers either: Masimo and Beurer quote different ranges and do not say they compute it the same way.
Other symbols with the same shapes
Sources
No published standard we could check defines what a fingertip pulse oximeter’s display shows, so this meaning is taken from 3 makers’ own manuals and support pages, each fetched and read on the date shown:
- Masimo: Masimo MightySat Rx fingertip pulse oximeter — operator’s manual (LAB-9121C, PDF) — General Description for Perfusion Index (Pi): “the ratio of the pulsatile blood flow to the non-pulsatile or static blood in peripheral tissue”, a noninvasive measure of peripheral perfusion; PVi described as a measure of the changes in Pi during the respiratory cycle. Checked 2026-09-29.
- Masimo: Masimo MightySat Rx fingertip pulse oximeter — home care manual (PDF) — Specifications, display ranges: Pi (Perfusion Index) 0.02% to 20%; the MightySat Rx measures SpO2, PR and Pi. Checked 2026-09-29.
- Beurer: Beurer PO 45 pulse oximeter — instructions for use (2020-06-12, German/English, PDF linked from beurer.com) — Display: 5. Perfusion index (value in percent); “Evaluating perfusion index”: PI may lie between 0.3% and 20%, “A very low PI value can impair the measurement”; technical data PI 0.3 – 20 %. Checked 2026-09-29.
- Nonin: Nonin Onyx Vantage 9590 — instructions for use leaflet (P/N 114847-000-02, 11/2022, PDF) — Warnings: “Operation of this device below the minimum amplitude of 0.3% modulation may cause inaccurate results.” Checked 2026-09-29.
Written in our own words; quotations are the makers’ wording.