Higher breathing frequency in high than in low symptom reporters explains group differences in heart rate and vagally-mediated heart rate variability, but not in heartbeat-evoked potentials

Background: The perception-filter model of medically-unexplained symptoms (MUS) posits three stages contributing to symptom generation: (I.) increased bodily signal strength (II.) decreased filter function, (III.) increased perception. The aims of this study were to test for these model assumptions based on cardiac perception and to clarify if respiration plays a role in potentially altered cardiac perception in MUS.
Methods: The upper and lower decile of a web-based study (N=486) was identified as extreme groups of high (HSR) and low symptom reporters (LSR). Mean heart rate (HR) and heart rate variability (HRV) were assessed as indicators of signal strength (I.). Heartbeat-evoked potentials (HEPs) assessed during rest and while performing a heartbeat perception task reflect attentional resources allocated towards heartbeats and served as index of filter function (II.). Cardiac interoceptive accuracy (IAc) were considered indicators of perception (III.).
Results: HSR showed higher HR and lower HRV (RMSSD) than LSR (I.), as well as higher respiratory frequency. HSR exhibited a stronger increase of HEPs when attention was focused on heartbeats than LSR (II.). When controlling for respiratory frequency, the differences in HR and HRV were no longer significant, whereas the difference in HEPs remained significant. There were no group differences in IAc (III.).
Discussion: Although MUS might be associated with altered signal strength (I.), this effect is mainly associated with higher breathing frequency in HSR. In contrast, dysfunctional filter function (II.) is independent of breathing frequency. Future studies should investigate if respiratory biofeedback might reduce bodily signal strength and reduce MUS distress.