However, there is also much evidence suggesting vagal involvement in LF. The LF decreases with strenuous exercise and vagal blockade with atro-pine, and it increases with sleep at night. The LF is decreased in patients with severe congestive heart failure, a state known as the loss of vagal-cardiac restraint. Additionally, the LF may result from Mayer waves in BP through baroreflex control of the heart rate, which is mainly vagally mediated. In the present study, we observed that the postural decrease in the HF was smallest in patients who died from cardiac causes (difference not significant; Table 2) who showed the greatest LF rise. Given that postural vagal withdrawal itself could be a factor reducing the LF in the upright position, LF rise may partly reflect impaired postural vagal response due to a decreased vagal reserve for postural withdrawal.
It is important to note that a postural increase in the LF has been observed in less than half of subjects. In the present study, we found that the distribution of postural LF response was similar between patients with CAD and age-matched and gender-matched healthy control subjects. In both groups, an LF rise was observed in about one third of subjects (Fig 2). A similar observation was reported in an earlier study with healthy subjects. The study also reported that an LF rise is characteristic among subjects susceptible to neurally mediated syncope, a condition in which β-adrenergic overactivation plays an important pathophysiologic role in its early stage. These data indicate that the LF rise is not a consequence of disease but an individual characteristic of postural autonomic response.
If the postural response of the LF is determined by the interactions between increased β-adrenergic activation and impaired cardiac vagal response, the patients showing LF rise may reflect those patients with greater dependence on β-adrenergic activation than on vagal withdrawal in the autonomic neural regulation of the postural heart rate response. Such characteristics of autonomic function may adversely affect the prognosis of stable patients with CAD.