Postural Response of Low-Frequency Component of Heart Rate Variability

Postural Response of Low-Frequency Component of Heart Rate VariabilityDecreased heart rate variability (HRV) is associated with an adverse prognosis in patients with coronary artery disease (CAD). This association has been ascribed to the harmful effects of cardiac vagal dysfunction and/or the resulting relative sympathetic overactivity. However, with few exceptions, most earlier studies that reported the prognostic value of HRV have been based on the data obtained from ambulatory monitoring in patients receiving medications. An analysis of HRV under such conditions has limited value as an assessment of autonomic functions due to many confounding factors. These factors may be divided into the following two categories: posture, physical and mental activities, wake-sleep cycle, and many cardiovascular agents could influence autonomic activities directly; and respiration and peripheral autonomic blockades may be an indirect influence by the modification of the relationships between autonomic activities and HRV, Although adjustment of the effects of both categories of factors is apparently important for assessing intrinsic autonomic functions by HRV, there are few studies’ examining HRV under such conditions, and none of them have reported prognostic associations in patients with CAD.

In the present study, we aimed at examining whether HRV assessment under well-controlled conditions predicts the risk for death in stable patients with CAD during a long-term follow-up. We investigated 8-year survival data for a cohort of patients who had undergone a standardized autonomic function test by HRV analysis during head-up tilt testing (HUTT) when they had been admitted to the hospital for elective coronary angiography. The HUTT was performed after the discontinuation of treatment with all medications and with the standardization of conditions, including the time of day, the temperature of the laboratory, and the food and beverage intake of the patients from the previous night. Also, HRV and the patients’ responses to HUTT were assessed under paced breathing. There is much evidence supporting that the power of the high-frequency component (HF) measured under these conditions provides an accurate assessment of cardiac vagal function” and that the postural increase in the power of the low-frequency component (LF) reflects, at least in part, postural β-adrenergic sympathetic activation. These features of the present study allowed us to evaluate the independent associations between the intrinsic autonomic functions and long-term survival in stable patients with CAD.