Tachogram

A tachogram plots the time between R waves, beat after beat. Beat number (or time) runs along the bottom, and the length of each RR interval RR interval RR interval The time from one heartbeat to the next, measured in milliseconds between the R peaks of an ECG. HRV is calculated from a series of these intervals. More about RR interval runs up the side. Some versions plot instantaneous heart rate instead, which shows the same pattern upside down: a long interval is a slow beat.

If the heart kept perfect time, the line would be flat. It never is. At rest, the most obvious pattern is usually a regular wave that rises and falls with your breathing. Intervals get shorter as you breathe in and longer as you breathe out, a pattern called respiratory sinus arrhythmia respiratory sinus arrhythmia Respiratory sinus arrhythmia The normal rise in heart rate as you breathe in and fall as you breathe out. It is driven by the vagus nerve and makes up much of resting HRV. More about Respiratory sinus arrhythmia . Slower waves from blood pressure control can sit underneath it.

The tachogram is the raw material for HRV analysis. Time-domain metrics such as RMSSD RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next. It mainly reflects vagal activity. More about RMSSD summarize how much the line jumps from point to point. Frequency-domain analysis, as the Task Force standards describe it, breaks the same line into its fast and slow rhythms. The line is also the easiest place to spot a lone spike that may be a missed or extra beat rather than a real change.

The lesson What HRV is, in one picture uses a tachogram to show what HRV looks like.