Interbeat Interval (IBI) in PPG Explained
The interbeat interval (IBI) is the time in milliseconds between consecutive heartbeats, detected in PPG as the period between successive systolic peaks or foot points. IBI is the primary input for heart rate variability (HRV) analysis and serves as a non-invasive surrogate for R-R intervals from ECG.
IBI extraction from PPG requires robust peak detection algorithms. Common approaches include simple threshold-based peak picking, derivative-based methods (finding zero-crossings of the first derivative), and template-matching techniques. The choice of fiducial point (systolic peak, foot, or mid-point of the rising slope) affects absolute IBI values and must be consistent across analysis pipelines.
The accuracy of PPG-derived IBI relative to ECG R-R intervals depends heavily on signal quality. Under resting conditions, well-designed algorithms achieve mean absolute errors of 2–5 ms. During physical activity, motion artifacts can introduce false peaks and missed detections, inflating IBI error to 15–50 ms, which severely compromises HRV metrics derived from short recordings.
Signal quality indices (SQI) are routinely applied before IBI extraction to flag corrupted segments. Beat-to-beat IBI outlier detection using physiological plausibility constraints (e.g., rejecting IBIs outside 300–2000 ms or deviating >20% from the local mean) further improves reliability. For clinical HRV applications, a minimum of 5 minutes of artifact-free IBI data is required per guidelines from the European Society of Cardiology and the North American Society of Pacing and Electrophysiology.
Frequently Asked Questions
Is PPG-derived IBI equivalent to ECG R-R intervals?
PPG IBI closely approximates ECG R-R intervals but introduces a fixed pulse transit time delay and is more susceptible to noise. For clinical HRV analysis, ECG remains the gold standard.
What sampling rate is needed for accurate IBI extraction?
A minimum of 25 Hz is required, with 100–250 Hz recommended for high-fidelity timing. Higher sampling rates reduce quantization error in peak localization.
How does wrist placement affect IBI accuracy?
Wrist sensors are more susceptible to motion artifacts than finger-based sensors, particularly during activities involving arm movement. This can degrade IBI accuracy by 10–30% during walking or running.