Glossary¶

Core Concepts¶

RR Interval: Time between consecutive heartbeats (R-peaks on ECG), measured in milliseconds. Also called inter-beat interval (IBI).
NN Interval: "Normal-to-Normal" interval — RR intervals after removing artifacts and ectopic beats. HRV metrics are computed from NN intervals, not raw RR intervals.
Heart Rate Variability (HRV): The variation in time intervals between consecutive heartbeats. Higher HRV generally indicates better cardiovascular health and autonomic flexibility.
Tachogram: A plot of RR (or NN) intervals over time. The primary visual tool for inspecting heart rate recordings.

RMSSD (Root Mean Square of Successive Differences): The square root of the mean squared differences between consecutive NN intervals. Primary marker of parasympathetic (vagal) activity. Most robust short-term metric.
SDNN (Standard Deviation of NN intervals): Overall variability of the NN interval series. Reflects total autonomic modulation (both sympathetic and parasympathetic). Strongly influenced by recording length.
pNN50 (Proportion of NN50): Percentage of consecutive NN intervals differing by more than 50 ms. Correlates with RMSSD; reflects parasympathetic tone.
Mean HR (Mean Heart Rate): Average heart rate in beats per minute (BPM), derived from NN intervals: HR = 60000 / mean(NN).
Mean NN: Average NN interval in milliseconds.

VLF (Very Low Frequency): Power in the 0.003–0.04 Hz band. Reflects thermoregulation and hormonal fluctuations. Requires recordings > 5 minutes.
LF (Low Frequency): Power in the 0.04–0.15 Hz band. Reflects both sympathetic and parasympathetic modulation plus baroreflex activity. Not a pure sympathetic marker.
HF (High Frequency): Power in the 0.15–0.4 Hz band. Primarily reflects parasympathetic (vagal) activity and respiratory sinus arrhythmia.
LF/HF Ratio: Ratio of LF to HF power. Previously interpreted as sympathovagal balance, this interpretation is now considered invalid (Quigley et al., 2024). Included for legacy compatibility.

SD1 (Poincaré Plot): Short-term variability from the Poincaré plot (width of the point cloud perpendicular to the line of identity). Mathematically related to RMSSD.
SD2 (Poincaré Plot): Long-term variability from the Poincaré plot (length of the point cloud along the line of identity). Related to SDNN.

Artifact: An anomalous RR interval caused by measurement error, movement, or ectopic beats. Must be detected and corrected before analysis.
Ectopic Beat: A heartbeat originating from an abnormal location in the heart (premature atrial or ventricular contraction). Produces a short RR interval followed by a long one.
Artifact Rate: Percentage of detected artifacts relative to total beats. Critical quality indicator — segments with > 10% should be excluded.
Quality Grade (Quigley 2024): Quality classification based on artifact rate: Excellent (≤ 2%); Good (2–5%); Moderate (5–10%, time-domain only); Poor (> 10%, excluded). Segments with fewer than 50 beats are excluded regardless of artifact rate.
Kubios Algorithm: Artifact detection method by Lipponen & Tarvainen (2019). Classifies artifacts as extra beats, missed beats, or misaligned. Used in NeuroKit2's signal_fixpeaks.

Section: A time range defined by experimental events (e.g., "measurement_start" to "pause_start"). Used for protocol-based analysis.
Segment: A fixed-duration time window (e.g., 5 minutes) used for artifact detection and quality assessment. Generated automatically.
Event Sequence: The order of experimental conditions assigned to a participant (e.g., music → silence → music). Supports counterbalanced designs.
Condition Label: Human-readable name for a condition type (e.g., "condition_a" → "Relaxing Music").
Canonical Event: A standardized event name (e.g., measurement_start) that maps to various raw labels from recording devices (e.g., "Messung Start", "Begin recording").
Project: A folder containing configuration files (config/*.yml) and data. Projects keep settings isolated between studies.

Lipponen, J. A., & Tarvainen, M. P. (2019). A robust algorithm for heart rate variability time series artefact correction using novel beat classification. Journal of Medical Engineering & Technology, 43(3), 173–181. doi:10.1080/03091902.2019.1640306
Quigley, K. S., et al. (2024). Publication guidelines for human heart rate and heart rate variability studies in psychophysiology. Psychophysiology, 61(9), e14604. doi:10.1111/psyp.14604
Task Force of ESC and NASPE (1996). Heart rate variability: Standards of measurement, physiological interpretation, and clinical use. Circulation, 93(5), 1043–1065. doi:10.1161/01.CIR.93.5.1043
Quintana, D. S., et al. (2016). Guidelines for reporting articles on psychiatry and heart rate variability (GRAPH). Translational Psychiatry, 6(5), e803. doi:10.1038/tp.2016.73