Deceleration capacity

Deceleration capacity is the average amount, in milliseconds, by which the heart slows when it slows, measured from a 24-hour ECG ECG ECG A recording of the heart's electrical activity, taken with electrodes on the skin. ECG-based RR intervals are the reference standard for measuring HRV. More about ECG by a method called phase-rectified signal averaging.

Why this measure exists

Bauer and colleagues wanted to separate the vagal side of heart rate control from the sympathetic sympathetic Sympathetic nervous system The "fight or flight" branch of the autonomic nervous system. It speeds up the heart and makes it pump harder, but it acts too slowly to drive beat-to-beat HRV. More about Sympathetic nervous system side without a drug or a laboratory. The vagus slows the heart, so they looked only at slow-downs: every beat whose interval is longer than the one before becomes an anchor, the intervals around each anchor are lined up and averaged, and the size of the step in the averaged pattern is the deceleration capacity. Acceleration capacity does the same with speed-ups. In their 2006 Lancet study of 1,455 people after a heart attack, validated in 656 in London and 600 in Oulu, deceleration capacity predicted death over the following two to three years better than ejection fraction or SDNN SDNN SDNN The standard deviation of all NN intervals in a recording, in milliseconds. It measures overall variability, and its value depends strongly on how long the recording lasts. More about SDNN , with areas under the curve of 0.80 and 0.74 against 0.69 and 0.64 for SDNN. Sassi and colleagues’ 2015 position paper summarises it as a better predictor of death than ejection fraction and SDNN after a heart attack, and of more value still when combined with heart rate turbulence heart rate turbulence Heart rate turbulence The brief speed-up and slower recovery of the heart rhythm after a premature ventricular beat, measured from a 24-hour ECG as two numbers: turbulence onset (%) and turbulence slope (ms per beat). More about Heart rate turbulence .

The formula

$$\mathrm{DC} = \frac{X(0)+X(1)-X(-1)-X(-2)}{4}$$

$X(k)$ is the averaged interval $k$ beats away from an anchor, over all anchors in the recording. The formula takes the mean of the anchor interval and the one after it, subtracts the mean of the two before, and halves the difference: the typical size of a slow-down, in milliseconds. The risk bands used since Bauer’s study are high risk at 2.5 ms or below, intermediate from 2.6 to 4.5 ms, and low risk above 4.5 ms; they were set on 24-hour recordings of heart-attack survivors.

Worked example

A line chart of 40 heartbeat intervals with a dot marking each of the 18 intervals that are longer than the one before. Averaging the two intervals before and the two from each dot onward over the 17 dots that have neighbours gives a deceleration capacity of 17.2 milliseconds for this short stretch.

Over these 40 intervals, 18 are slower than the beat before. Averaging the pattern around the 17 anchors that have two neighbours on each side gives a deceleration capacity of 17.2 ms: the breathing swing of the teaching recording slows the heart by about that much each time it slows. Over a day the same averaging runs over tens of thousands of anchors, and the small slow-downs of breathing are joined by the big ones of rest and sleep.

What moves it

Vagal activity Vagal activity Vagal tone The ongoing influence of the vagus nerve on the heart, which slows it down. It is usually estimated from HRV measures such as RMSSD and HF power. More about Vagal tone , which is why it is reduced after a heart attack, and why the rhythm of breathing contributes to it. Bauer’s values come from 24-hour Holter ECGs; Steger and colleagues reported in 2024 that a value from a 5-minute ECG also predicted death over 13 years in 823 adults from the general population.

When it misleads

In a short recording most of the slow-downs are breathing, much as in RMSSD RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next, in milliseconds. It mainly reflects vagal activity. More about RMSSD . Ectopic beats Ectopic beats Ectopic beat A heartbeat that starts outside the heart's normal pacemaker and usually comes early. Ectopic beats are common, but they distort HRV unless removed or corrected. More about Ectopic beat and artifacts artifacts Artifact An error in a heartbeat recording, such as a missed, extra or mistimed beat, often caused by movement or poor sensor contact. Artifacts can badly distort HRV numbers. More about Artifact make false anchors. The cut-offs are risk markers for groups of patients, not a fitness score for one healthy person.

Where you will see it

Kubios HRV prints DC and AC in ms, and modified two-point versions DCmod and ACmod, under its time-domain measures; it divides the four-point sum by 2 rather than Bauer’s 4, so its DC reads twice as large. Holter software and the risk-stratification literature report it; wearables do not. What are nonlinear HRV measures and do I need them? puts it in context.