6.4. Cardiovascular reflexes and heart rate variation

The results of the standard cardiovascular reflex tests agree well with the traditional measures of the ambulatory ECG recordings, both demonstrating diminished values of various parameters in different subgroup of epilepsy patients compared to those of the control subjects. The dynamic measures of HRV did not correlate with the findings of the standard cardiovascular reflex tests. Dynamic measures and fractal analysis of HRV have been developed to quantify complex HR dynamics and they are suggested to show abnormal patterns of RR interval not easily detected by commonly used statistics of HRV (Mäkikallio et al. 1996). Whereas the conventional methods typically detect quantitative properties, the dynamic measures if HRV detect rather qualitative properties of the HR series (Myllylä et al. 2002)

HRV can easily be analyzed from 24-hour ECG-recordings. There are numerous indexes to describe RR interval fluctuation during a 24-hour period (Myllylä et al. 2002). The advantages of this method are good reproducibility and the information concerning the LF bands of the spectral HRV gathered over longer time periods than the standard cardiovascular reflex tests can be provided (Kleiger et al. 1987, Huikuri et al. 1990, Öri et al. 1992, Myllylä et al. 2002). However, because of the great inter-individual variation in the measures of HRV used in this study, these parameters can only be used to assess autonomic dysfunction at a group level, as is often the case with the standard cardiovascular reflex tests as well (Bannister & Mathias 1999).

In the present study, autonomic dysfunction was detected using both cardiovascular reflex tests and analysis of HRV. These two methods have not been previously used in the assessment of autonomic cardiovascular regulation in the same patients with TLE. Although cardiovascular reflex tests are methodologically standardized and feasible, they may not be as sensitive as an analysis of HRV in detecting autonomic disturbances in patients with TLE. In addition, with dynamic measures, tonic, long-term oscillations of HRV can be revealed. In various other conditions, especially, the dynamic measures of HRV has been associated with an increased risk of sudden cardiac fatalities (Bigger et al. 1996, Huikuri et al. 1996, Mäkikallio et al. 1996, Mäkikallio et al. 1997). The problems concerning the method of the analysis of HRV are great intra- and interindividual variation of the measured values and its susceptibility to artifacts (Myllylä et al. 2002).

Despite the statistical data suggesting the predictive power of the various HRV indices especially in heart diseases, none are widely used in clinical practice to guide preventive therapy for individual patients because no trial has adequately linked the reliability of any of these variables of clinical outcome with intervention (Myllylä et al. 2002).