The relationships between autonomic neural activities and HRV also could be influenced by medications and respiratory parameters. An increase in breathing frequency and a decrease in tidal volume reduce the HF without changing the mean cardiac vagal tone. Indeed, in earlier clinical studies with ambulatory monitoring, the HRV measures showing the strongest predictive power were those reflecting global or longterm variability, such as 24-h SD, triangular index,2 and ultra-low and very-low frequency components. The underlying mechanisms of these measures are unclear. Basic physiologic research reported that HRV measures such as the successive difference in R-R intervals and HF reflect more specifically the cardiac vagal function, but these measures had only moderate prognostic power in the clinical studies using ambulatory monitoring. Thus, the prognostic significance of intrinsic autonomic dysfunction in patients with CAD is not clear from these earlier observations.
From this point of view, the present study has strength. It is unique compared to earlier studies concerning the following points: (1) the use of HUTT under a controlled environment and physiologic state; (2) the withdrawal of treatment with medications that might influence autonomic functions and their assessment by HRV; and (3) the use of paced breathing to control the effects of respiration on autonomic assessment by HRV. Much evidence from basic physiologic research of HRV supports the notion that the analysis of HRV under these conditions provides a reliable autonomic functional assessment. The observations of the present study seem to be useful for evaluating the associations between intrinsic autonomic functions and prognosis in patients with CAD. canadian neighborhood pharmacy
Prognostic Value of Reduced HF
The present study partly supports the concept that decreased cardiac vagal activity results in an increased risk for death in patients with CAD. We observed that a decreased HF in both the supine position and during a head-up tilt showed a significant univariate association with an increased risk for noncardiac death.