δ_EPCD: the electrophysiologic coefficient of depressiveness

Abstract

Despite research advances, recently identified biological markers for depression are either non-specific or impractical in daily clinical practice. Hence, we aim to identify a novel biomarker: δ_EPCD, the electrophysiologic coefficient of depressiveness. δ_EPCD must be sensitive and specific to the vulnerability towards depression. It should also detect the presence of a depressive clinical state and be able to quantify its severity. Moreover, it should be easily accessible and cost-effective. Accordingly, combining high-frequency heart rate variability (HF-HRV), which reflects a reduction in vagal tone, and tryptophan metabolism, which influences serotonin synthesis pathway, may have a good diagnostic and prognostic accuracy in depression. δ_EPCD is the multiplication of the intrinsic difference between state 0 (rest) and state 1 (exposure to stress) of HF-HRV and the plasma concentration ratio between quinolinic acid and kynurenine. δ_EPCD theoretically fluctuates between −1000 and 0 where being closer to 0 signifies no vulnerability to depression. Individuals with a score between −16.7 and −167 have a high vulnerability to depression. Finally, individuals with a δ_EPCD closer to −1000 have the most severe forms of depression. δ_EPCD is theoretically conceived to be easy to assess and monitor which makes it a candidate for further evaluation of reliability and validity.

CLINICAL SIGNIFICANCE

• Depression is currently diagnosed based on emotional and behavioural symptoms; however there is currently a rising interest in the field of neurobiological markers that could improve diagnostic accuracy.

• Many current biological approaches are primarily based on single neurobiological markers that are either non-specific or impractical in daily clinical practice.

• Among other neurological effects, depression may modify the parasympathetic nervous system tone and disturb the tryptophan metabolism.

• The electrophysiological coefficient of depressiveness δ_EPCD combines heart rate variability (HRV) and tryptophan metabolism to reflect the intrinsic individual vulnerability towards depression and the inherent severity of an index depressive disorder.

• δ_EPCD is the intrinsic difference between state 0 (without stress) and state 1 (exposed to a stressful task) of the high-frequency heart rate variability multiplied by the intrinsic difference between both states, e.g. state 0 and 1, of the plasma concentration ratio of quinolinic acid over kynurenine.

Keywords:

• Depression
• biological markers
• heart rate variability
• tryptophan metabolism
• Kynurenine pathway

Acknowledgments

We would like to thank Dr Mario Njeim, a colleague cardiologist and electrophysiologist, for his help in English language editing of this manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

Data sharing is not applicable to this article as no new data were created or analysed in this study.