Learn PPG
What Is an Interbeat Interval?
An interbeat interval is the time from one heartbeat to the next, usually in milliseconds. IBI is the generic name. On an ECG the same gap is an R-R interval. On a PPG recording it is the time between matching points on two pulses, which Garmin calls a beat-to-beat interval (BBI) and some papers call a pulse rate interval (PRI). The point you mark, and any beat you miss, both land in the HRV math.
Measure the time between pulses.
Measure from one pulse peak to the next, using the same reference point on each beat.
The interbeat interval is the gap between beats. Heart rate is how many of those gaps fit in a minute. Heart rate variability is how the gaps change. The series has several names, and they are not four different measurements.
IBI means interbeat interval (also written inter-beat interval). Garmin's Enhanced BBI white paper (November 2023) uses IBI for the time series that RMSSD and SDNN are computed from, then splits the name by sensor. Intervals taken from the ECG, R wave to R wave, are R-R intervals, which that paper abbreviates RRI. Intervals taken from PPG, between successive pulse waves at the sensor, are beat-to-beat intervals, abbreviated BBI. The wrist is the site Garmin names for that optical gap.
PRI shows up as a third label for the optical series. Hirayama and colleagues recorded a finger pulse oximeter next to a chest ECG and called the oximeter gaps pulse rate intervals, PRI, and the ECG gaps RRI (Yonago Acta Medica, 2023, doi:10.33160/yam.2023.02.010). In that paper PRI is the pulse-to-pulse gap. It is a different object from the PR interval on an ECG, which sits inside one beat, from the P wave to the QRS. A file labeled PRI needs a check of which of those the author meant. This page uses PRI the way Hirayama and colleagues do.
The number also depends on which point on the pulse you mark. Park, Seok, Kim, and Shin list three common choices in an open review of PPG features (Frontiers in Physiology, 2022, doi:10.3389/fphys.2021.808451): the pulse onset, often called the foot; the systolic peak; and the point where the first derivative is largest. Those landmarks do not slide together when the wave changes shape. A delay that is the same on every beat drops out when you subtract neighbors. A delay that changes from beat to beat stays in the series. The pulse-rate variability article (/blog/pulse-rate-variability-vs-hrv) works through why rest and movement belong in separate piles. Pulse transit time (/learn/pulse-transit-time) is the travel piece of that delay.
A missed peak or an extra peak enters two successive differences. Shaffer and Ginsberg write that two artifacts inside a 5-minute segment can distort RMSSD, as they can distort SDNN (Frontiers in Public Health, 2017, doi:10.3389/fpubh.2017.00258). RMSSD is the short-window metric most watches print (/learn/rmssd). The Fenix 6 comparison (/accuracy/garmin-hrv) is one published nocturnal check of a watch HRV number, with the manufacturer's one-night BBI note kept separate from the peer-reviewed table. A signal-quality gate (/learn/signal-quality-index) is how a pipeline decides a segment is worth timing. This page names the interval. It does not read a rhythm or offer a personal range.
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Frequently Asked Questions
What do IBI, BBI, RR, and PRI mean?
They name the gap between beats. IBI is the generic interbeat interval. RR, or RRI, is that gap on an ECG, from one R wave to the next. BBI is Garmin's name for the same gap taken from a PPG pulse wave. PRI, in Hirayama et al. 2023 (doi:10.33160/yam.2023.02.010), is the pulse-oximeter gap. PRI is not the ECG PR interval, which is the time from the P wave to the QRS inside one beat.
Is a PPG interval the same as an ECG R-R interval?
The formula can use either series. The PPG gap is timed at the pulse, after the pressure wave has traveled, so a changing delay stays in the differences. Agreement with ECG is a resting question and a movement question, kept apart in /blog/pulse-rate-variability-vs-hrv.
Does the peak or the foot change the interval?
Yes, when the wave shape changes. Park et al. 2022 (doi:10.3389/fphys.2021.808451) list the foot, the systolic peak, and the steepest point on the upstroke as three places the interval is measured. Pick one landmark and keep it for the whole recording.
Why do artifacts distort RMSSD?
RMSSD is built from neighboring gaps. One missed or extra beat changes two successive differences. Shaffer and Ginsberg (2017, doi:10.3389/fpubh.2017.00258) note that two artifacts in a 5-minute segment can distort RMSSD. The metric page is /learn/rmssd.