A few selected resources on heart rate variability (HRV) with comments:

HRV 101: A few of my own slide decks as an intro to HRV (including some info on acquiring heart or pulse rate during MRI) – English (a shorter but more recent [Jan 2020] one) or German.

New “consensus” paper: Quigley et al. (2024) Publication guidelines for human heart rate and heart rate variability studies in psychophysiology—Part 1: Physiological underpinnings and foundations of measurement. Psychophysiology. – finally extending / replacing the papers from 25 years ago, the 1996 “Task Force” paper Heart rate variability – standards of measurement, physiological interpretation, and clinical use. European Heart Journal 17, 354-381 and the 1997 Berntson et al. paper Heart rate variability – origins, methods, and interpretive caveats. Psychophysiology, 34(6), 623-648.

A set of very useful tutorials and resources by Dan S. Quintana and colleagues: Considerations in the assessment of heart rate variability in biobehavioral research (2014, Frontiers in Psychology), Guidelines for reporting articles on [psychiatry and] HRV (2016, Translational Psychiatry), and Statistical considerations for reporting and planning heart rate variability case-control studies (2017, Psychophysiology).

Tools for cardiac data:

1. Kubios (with all features free up to v2.2) is my preferred toolbox for HRV data preprocessing and analysis: it is quite flexible with the input types, offers a very robust ECG R-peak detection (disappointingly paywalled from v3, so get your hands on a v2.2), and outputs all relevant data/results. I learned to appreciate the shortcoming of not having access to the script level (e.g., for allowing batch processing) as – for reliable HRV results – each R-peak needs to be visually checked anyway, so the Kubios GUI offers handy tools to artefact-check, correct, or interpolate faulty peaks/intervals. NB: Chapter 2 of the “old” Kubios User’s guide (e.g., version 2.2) is an excellent primer on HRV and its analysis.
2. HEPLAB is a plugin for the Matlab-based EEGLAB toolbox. It also offers handy and robust R peak detection, GUI-based artefact correction, and scripting possibilities (but AFAIK no calculation of HRV values).