7.7. Possible pathophysiological mechanisms of abnormal short and long term heart rate dynamics

The physiological background of altered short term fractal-like behaviour is not exactly known, but one potential explanation for the altered short-term correlation properties of heart rate dynamics in patients with vulnerability to life-threatening arrhythmia might be altered sympathovagal interaction (Levy 1971). The concept of sympathoexcitation is supported by observations of more complex Poincaré plots of successive RR intervals in heart failure patients (Brouwer et al. 1996, Woo et al. 1994) with high norepinephrine levels (Woo et al. 1994). Similar changes have also been observed upon intravenous infusion of physiological doses of norepinephrine in young healthy adults (Tulppo et al. 1998). Random, uncorrelated beat-to-beat RR interval behaviour may also reset the repolarization dynamics of the myocardium and thereby increase vulnerability to ventricular arrhythmogenesis.

The physiologic relevance of high approximate entropy of RR interval data is also open to speculation. It is possible that a higher sympathetic tone may also explain the reduction in the very low frequency spectral component and the change in beat-to-beat complexity and, consequently, the increase in approximate entropy.

The moderate correlation of the value of low-to-high frequency spectral component rates with both fractal correlation properties and approximate entropy suggests that dynamic fractal behaviour and the complexity of RR interval dynamics are related to neuroautonomic interactions.

The physiological background of altered long term heart rate behaviour (power-law slope) is not known, but the observation of significantly steeper power-law slopes in denervated hearts suggests that it is partly influenced by the autonomic input to the heart (Bigger et al. 1996).The slope has been found to be steeper in elderly subjects than in younger healthy subjects (Bigger et al. 1996, Saul et al. 1987), showing that ageing itself results in progressive changes in the long-term spectral characteristics of heart rate variability. No changes in ultra low frequency power, but a linear decline in very low frequency power have been observed upon ageing (Bigger et al. 1995), which probably also explains the steeper slope of the power-law regression line in the elderly. The altered autonomic modulation of long-term heart rate behaviour with advancing age may arise from age-related changes in various organs and body systems, which may interact with each other and thereby impair the function of the cardiovascular autonomic regulatory systems.

Abnormalities in the autonomic modulation of heart rate have been observed in various cardiovascular and cerebrovascular disorders (Huikuri 1995, Barron et al. 1994, Korpelainen et al. 1996), and it is possible that altered cardiovascular neural regulation expressed by a steep slope of long-term heart rate dynamics may be a sign of an underlying subclinical vascular disease predisposing to mortality. Another potential explanation for the prognostic role of altered long term heart rate behaviour is that it may reflect an impairment in the adaptive systems during acute perturbations, such as myocardial or cerebral ischemic events. This is supported by experimental observations, which have shown that cardiovascular autonomic regulation plays an important role in the occurrence of life-threatening arrhythmias during acute cardiac or cerebral ischemia (Schwartz et al. 1992, Hachinski et al. 1992).