DFA1
DFA1 — the short-term scaling exponent α1 of detrended fluctuation analysis — is a nonlinear heart-rate-variability index that describes the fractal correlation properties of heartbeat intervals over short scales (about 4–16 beats). At rest it reflects the healthy organisation of heart-rate fluctuations and has strong prognostic value; during exercise it falls with increasing intensity and can be used to track the aerobic threshold.
What DFA measures
Detrended fluctuation analysis asks how the fluctuations of the heartbeat series grow when we look at longer and longer windows. The series is integrated, divided into windows of n beats, a trend line is removed in each window, and the remaining fluctuation F(n) is measured. On a log–log plot F(n) rises along a straight line; its slope is the scaling exponent α.
| α | Character of the signal |
|---|---|
| ≈ 0.5 | Uncorrelated, random ("white noise") |
| ≈ 1.0 | Fractal ("1/f") correlations — typical of healthy physiological regulation |
| ≈ 1.5 | Strongly correlated, smooth ("Brownian noise") |
DFA1 at rest: prognosis
In healthy people at rest α1 is close to 1. A reduction of α1 means a loss of the normal fractal organisation and a more random beat-to-beat behaviour. In patients with reduced left-ventricular function after acute myocardial infarction, α1 below 0.75 was the strongest heart-rate-variability predictor of all-cause mortality, with about a three-fold higher risk. Very high values (around 1.5) indicate an overly smooth, rigid rhythm, as in some heart-failure patients and during strong sympathetic activation.
DFA1 during exercise
As exercise intensity increases, α1 falls progressively from about 1 towards 0.5. Studies with gas-exchange measurements found that the point where α1 crosses 0.75 corresponds closely to the first ventilatory (aerobic) threshold. This allows the aerobic threshold to be estimated from a chest-strap recording and to be used for prescribing low-intensity endurance training. Accurate RR detection (ECG chest strap, few artefacts) is essential, because artefacts strongly distort α1.
Low α1 at rest
- Loss of fractal organisation, more random fluctuations
- After myocardial infarction, heart failure — associated with higher mortality
- Artefacts or frequent ectopic beats can mimic low α1
High α1 at rest
- Overly regular, smooth dynamics
- Strong sympathetic activation, some forms of heart failure
- Very slow breathing patterns may also raise α1
DFA1 is a supplementary index. If it is persistently low at rest together with other reduced HRV indices, or with symptoms such as palpitations, breathlessness or chest pain, discuss the results with a cardiologist.
References
- Peng CK, Havlin S, Stanley HE, Goldberger AL. Quantification of scaling exponents and crossover phenomena in nonstationary heartbeat time series. Chaos. 1995;5(1):82–87. doi:10.1063/1.166141
- Huikuri HV, Mäkikallio TH, Peng CK, Goldberger AL, Hintze U, Møller M. Fractal correlation properties of R-R interval dynamics and mortality in patients with depressed left ventricular function after an acute myocardial infarction. Circulation. 2000;101(1):47–53. doi:10.1161/01.CIR.101.1.47
- Rogers B, Giles D, Draper N, Hoos O, Gronwald T. A new detection method defining the aerobic threshold for endurance exercise and training prescription based on fractal correlation properties of heart rate variability. Front Physiol. 2021;11:596567. doi:10.3389/fphys.2020.596567
- Voss A, Schulz S, Schroeder R, Baumert M, Caminal P. Methods derived from nonlinear dynamics for analysing heart rate variability. Philos Trans A Math Phys Eng Sci. 2009;367(1887):277–296. doi:10.1098/rsta.2008.0232
This article is for general information and does not replace a consultation with a doctor.
Published on 14 June 2019