Heart rate variability (HRV) refers to the beat-to-beat variation in the time intervals between heart contractions. This phenomenon reflects physiological processes that are trained in many biofeedback applications. HRV is routinely monitored using an electrocardiograph (ECG) or photoplethysmograph (PPG), supplemented by a respirometer. This article explains the importance of inspecting raw signals, describes the effects of prescription medications and social drugs, identifies common sources of signal contamination, and recommends practical precautions to increase recording fidelity.

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