HR Acceleration: Average Beat-to-Beat Change
Estimated from a single camera (PPG) reading — an educational estimate, not a medical measurement. State labels (Flow, Focus, Alarm…) are rough, experimental patterns, not emotion detection.
What this number shows
HR Acceleration here is a small number that describes whether your beat-to-beat intervals were, on average, getting longer or shorter from one beat to the next during the last 15 seconds. Despite its name, it is a simple average of successive differences, not a true second derivative of heart rate.
How this tool calculates it
The tool takes the differences between consecutive RR intervals (in ms), averages them (skipping the first one), and divides by 100. The tool works on a rolling 15-second window of fingertip video (about 30 frames per second), finds each pulse peak, keeps only beat-to-beat (RR) intervals between 400 and 1,500 ms that are within 22% of the median, and smooths the displayed numbers so they change gradually.
How to read it
Positive
RR intervals were getting longer on average, which means heart rate was drifting down during the window. This often happens during a slow exhale or as you settle.
Negative
RR intervals were getting shorter, so heart rate was drifting up, for example on an inhale, after moving, or as you get tense.
Near zero
No clear drift. This is the most common value at rest, because the natural speed-up and slow-down with each breath cancels out.
Why it jumps around
Your heart naturally speeds up a little when you breathe in and slows when you breathe out (respiratory sinus arrhythmia). Over a 15-second window that contains only two or three breaths, where the window starts and ends in the breathing cycle strongly affects this value. Camera timing errors of a few milliseconds per beat add more noise. Small values in either direction are best read as "no meaningful change".
Important limits
This is not a standard clinical or sports-science metric, and there are no established normal ranges for it. It is shown as a quick directional indicator only, not as a medical or fitness measurement.
The ONDA app is a separate product: it focuses on pulse, breathing and, with an Apple Watch, coherence. It does not use the formulas on this page.
The ONDA Principle
Positive means beats were spreading out (pulse slowing); negative means they were bunching up (pulse rising).
Ready to measure?
The ONDA app works with your iPhone camera, Apple Watch and Apple Health.