Respiratory heart rate variability: Insights into mechanisms, measurements and interpretations

The analysis of heart rate variability (HRV) has gained considerable attention in recent years, in particular in the psychobiological field to study brain-body interaction. HRV comprises different frequency components, including a prominent modulation generated by respiratory activity, which represents the only HRV component with a precise physiological origin. This respiratory component of HRV is often measured by, and sometimes conflated with, the high-frequency (HF) component of frequency-domain HRV analysis. This perspective aims to clarify the physiological basis, psychobiological implications and interpretative framework of the respiratory component of HRV, referred to as Respiratory Heart Rate Variability (RespHRV, previously termed respiratory sinus arrhythmia). Its primarily, but not exclusively, central origin is highlighted, as mediated by the respiratory modulation of cardiac vagal neuron activity by neurons that generate the respiratory command. The methods to quantify RespHRV are outlined and compared, with focuses on HF-HRV and peak-to-valley metrics, and with an emphasis on the necessity to accurately monitor respiratory activity alongside cardiovascular recordings. These clarifications are intended to facilitate the subsequent interpretations of RespHRV variations and to avoid any misinterpretation. Overall, we advocate for the systematic integration of direct respiratory recordings in both human and animal studies to strengthen the physiological validity of RespHRV analysis and improve its interpretation in psychophysiological and clinical research.