CSI: Cardiac Stability Index Explained
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
CSI in this tool is the coefficient of variation of your beat-to-beat intervals: how much the intervals vary compared with their average length. Because it is divided by the mean interval, it lets you compare rhythm evenness at different heart rates. Lower means the intervals were more alike; higher means more variation.
How this tool calculates it
CSI = standard deviation of the RR intervals ÷ mean RR interval. For example, if your intervals average 800 ms with a standard deviation of 40 ms, CSI is 0.05. 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.
Why camera values look high
With a phone camera, beat timing is only accurate to a few milliseconds and some peaks are misplaced, so the measured spread is larger than the true one. In practice readings around 0.3 are typical on this page; lower values mean a cleaner, steadier signal.
Lower (well under 0.3)
Intervals were fairly even and the signal was clean.
Around 0.3
A typical camera reading.
Clearly higher
Intervals varied a lot: often movement, pressing too hard, cold fingers or poor light; occasionally genuinely irregular beats.
CSI and heart rate variability
Some variation between beats is healthy: the heart speeds up slightly on each inhale and slows on each exhale, and people with good fitness often show more of this at rest. So lower is not automatically "better" in physiology. In this tool, however, CSI is mainly a check on how steady and clean the reading was, and it feeds the Stress (130 × CSI), Energy, Relaxation and Alarm indices.
Health note
If you notice a persistently irregular pulse, palpitations, dizziness or shortness of breath, see a doctor. A browser camera reading cannot detect or rule out arrhythmias such as atrial fibrillation.
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
CSI is the spread of your beat intervals divided by their average; typical camera readings sit around 0.3.
Ready to measure?
The ONDA app works with your iPhone camera, Apple Watch and Apple Health.