[Fractal analysis and arterial hypertension]

Fractal analysis is a new tool allowing to study the complexity of cardiovascular variability. This approach may be useful for investigating regulatory mechanisms. In fractal terms, homeostasis required an optimal organisation of complexity (beta coefficient close to 1): conversely, some pathological situations have proven to be associated with an altered organisation of fractals (beta coefficient different from 1). Our study aimed at determining if hypertension was associated with an alteration of the fractal component of blood pressure (BP) or heart rate (HR) variability. Eighty-eight subjects referred for high BP entered the study. BP was measured in ambulatory conditions by an automatic device during 24 hours (ABP). BP and HR were then recorded beat-to-beat by a Finapres and an ECG during 30 minutes at rest. The beta coefficient was obtained by coarse-graining spectral analysis from the BP and RR interval time series. This method has proven its interest for breaking down cardiovascular variability into an harmonic component and a non harmonic one, this latter containing fractal elements. Spontaneous baroreflex sensitivity was assessed by the method of sequences. The percentage of fractals contained in BP and RR signals remained relatively stable despite the increasing severity of hypertension. A significant but loose correlation was found between the beta coefficient of RR and ABP (r = 0.23, p = 0.053 with systolic ABP; r = 0.25, p = 0.03 with diastolic ABP). No correlation could be disclosed between ABP and the beta coefficient of BP. The beta coefficient of RR was significantly correlated with the spontaneous baroreflex sensitivity (-0.59, p < 0.0001). It is concluded that it is rather the complexity of HR signal than that of BP which is altered during hypertension. This may suggest that some regulatory processes are lost or less efficient. The correlation reported between the baroreflex gain and the beta coefficient of RR interval may reflect, in fact, an alteration of the parasympathetic drive. Globally, these results emphasise the importance of HR and probably cardiac output in the pathophysiology of high BP.