Perfusion index (PI) checker

What PI means on a pulse oximeter

Oximeters that display it show PI as a percentage next to SpO₂ and pulse rate. It tells you how strong the pulse signal is at the sensor, not how much oxygen is in your blood. A higher PI means a stronger pulsatile signal. A very low PI means the oximeter has less signal to work with, so the SpO₂ and pulse readings beside it deserve a second look. For what the SpO₂ and pulse numbers themselves mean, see the pulse oximeter readings chart.

What the perfusion index measures

A pulse oximeter shines light through tissue. Part of the detected signal pulses with each heartbeat as arterial blood volume changes (the AC component). The rest is a steady baseline from venous blood and other tissue (the DC component). $\mathrm{PI} = \mathrm{AC} \div \mathrm{DC} \times 100$. It's a unitless ratio that reflects how strong the pulsatile signal is at the measurement site. It isn't a direct measure of tissue perfusion (Sun et al., 2024). For the signal-processing details, see the PPG AC/DC ratio explained and the perfusion index explainer.

Published reference values

PI varies widely between people, and its distribution is skewed. Lima et al. (2002) measured 108 healthy adults with a Philips monitor and found a range of 0.3–10.0, a median of 1.4 and an interquartile range of 0.7–3.0. In critically ill patients, a PI of 1.4 best separated poor from adequate peripheral perfusion as judged by core-to-toe temperature difference.

In 10,000 healthy newborns measured with a Masimo monitor at 1–120 hours of age, the median was 1.70 (IQR 1.18–2.50), the 5th centile 0.70 and the 95th centile 4.50. The authors suggested that values under 0.70 may indicate illness and that values under 0.50 (the 1st centile) indicate definite underperfusion (de-Wahl Granelli & Östman-Smith, 2007). For clinical context, read perfusion index in clinical practice and neonatal perfusion index.

What changes PI

A narrative review lists cardiac output, blood volume, perfusion pressure, vascular tone and microcirculatory failure as the main determinants of PI. Pain, stress, peripheral vascular disease and body temperature also affect it. Poor sensor contact, nail polish, ambient light and movement degrade the measurement. Different devices calculate PI differently, so reference values should be device-specific (Sun et al., 2024).

PI and wearable accuracy

When the pulsatile signal is small, optical sensors have less to work with. In healthy volunteers during controlled desaturation, one oximeter's $A_{\mathrm{rms}}$ was 1.64% during low perfusion versus 1.37% at PI above 1 (Sharma et al., 2024). In ICU patients the SpO₂–SaO₂ difference improved only marginally with higher PI (Thijssen et al., 2020). In an animal sepsis model, PI didn't usefully predict which readings were biased (Hummler et al., 2006). Compare published device results on wearable accuracy, and browse datasets in the research hub.