RRI
RRI — the RR interval (inter-beat interval) — is the time between two successive heartbeats, measured between the peaks of the R waves on the ECG. The sequence of RR intervals is the raw material of heart-rate-variability analysis: it tracks small, millisecond changes from beat to beat, which ordinary heart rate — the number of beats per minute — averages out.
Heart rate versus RR intervals
A heart rate of 60 beats per minute does not mean that the heart beats exactly once per second. One interval may be 950 ms, the next 1040 ms, the following 1010 ms. These small fluctuations result from breathing, blood-pressure regulation and the balance between the sympathetic and parasympathetic nervous systems. Heart-rate-variability analysis studies exactly this sequence of intervals.
How RR intervals are obtained
- ECG — the reference method: R waves are sharp, so intervals can be timed with millisecond accuracy. A sampling rate of 250–500 Hz or higher is recommended.
- Chest straps — record the electrical signal and give intervals close to ECG quality at rest.
- Optical sensors (PPG) in watches and rings measure pulse-to-pulse intervals from blood volume changes; they are less accurate during movement and are affected by skin perfusion and fit.
Quality and artefacts
Only intervals between normal sinus beats (NN intervals) are suitable for HRV analysis. Extrasystoles, missed or extra detections, movement and electrode problems create false intervals that strongly distort SDNN, RMSSD, pNN50 and spectral indices. Modern software detects and corrects such intervals; if more than a few percent of beats are abnormal, the HRV results become unreliable and atrial fibrillation or frequent extrasystoles should be considered.
What the RR series tells
- The mean of the intervals gives heart rate (MeanRR).
- Their spread gives SDNN, MaxRR − MinRR (ΔX) and the Baevsky indices.
- The beat-to-beat differences give RMSSD, pNN50, SD1.
- The rhythmic oscillations give the frequency-domain indices (HF, LF, VLF).
- The structure and complexity give nonlinear indices (DFA, entropy, correlation dimension).
An RR series with frequent irregular jumps, pauses or chaotic variation may indicate extrasystoles or atrial fibrillation. If you notice palpitations, an irregular pulse, dizziness or fainting, consult a doctor and have an ECG recorded.
References
- Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. Heart rate variability: standards of measurement, physiological interpretation and clinical use. Circulation. 1996;93(5):1043–1065. doi:10.1161/01.CIR.93.5.1043
- Tarvainen MP, Niskanen JP, Lipponen JA, Ranta-aho PO, Karjalainen PA. Kubios HRV — heart rate variability analysis software. Comput Methods Programs Biomed. 2014;113(1):210–220. doi:10.1016/j.cmpb.2013.07.024
- Shaffer F, Ginsberg JP. An overview of heart rate variability metrics and norms. Front Public Health. 2017;5:258. doi:10.3389/fpubh.2017.00258
This article is for general information and does not replace a consultation with a doctor.
Published on 2 July 2019