It is well known now that CAD and depression often co-occur; however, it is not clear whether depression precedes CAD, CAD precedes depression, or if both occur simultaneously.

There have been a number of studies evaluating the physiological effects depression has and thereby affects cardiovascular health. One speculation is that depression adversely affects autonomic nervous system functioning and heart rate variability which in turn affects cardiac health (Carney et al., 2005; Carney & Freeland, 2009; Krittayaphong et al., 1997). Medically healthy but depressed patients often have significantly elevated resting catecholamine concentrations and altered hypothalamic-pituitary adrenal axis and autonomic nervous system functioning (Carney & Freeland, 2009; Wyatt et al., 1971).

Low heart rate variability (HRV) is a sign of increased sympathetic nervous system modulation, meaning increased heart rate and stress response. According to a study by Krittayaphong et al. (1997), patients with high depression scores exhibited much lower heart rate variability.

Depression impacts heart rate variability and results in a decrease in the parasympathetic but increased sympathetic nervous system activity. This has an effect on vascular walls and presents a cardiac risk factor. Also, ventricular premature contractions (VPCs) are found to be a predictor of CAD events. Those physiological changes may result in myocardial ischemia, ventricular fibrillation, ventricular tachycardia and sudden cardiac death (Carney & Freeland, 2009).