Multifractal property assessment in the very low frequency range, in subjects with different progression of aortic valve stenosis disease

Aortic valve stenosis is the third commonest heart disease in developed countries. When the opening of the aortic valve is narrowed, the ability of the heart to pump blood is severely limited. This inhibits the ability of the circulation system to adapt to changing external requirements. This can drastically change state of the nonlinear system and therefore should be clearly visible in signals coming from it. Aortic valve replacement is connected with a high non-postoperative mortality. It would be interesting to predict the mortality risk based on pre-operative heart rate variability properties. We analyzed 418 heart rate variability recordings from subjects before the operation, who had different stages of aortic valve stenosis (age 22-82, male and female, ejection fraction 10-95%, aortic gradient 28-218 mmHg, where 352 subjects had critical aortic valve stenosis >;70 mmHg). In our analysis, besides classical linear heart rate variability measures and echocardiography, we applied Multiscale Multifractal Analysis (MMA) [1] - a method yielding the dependence of the local Hurst exponent as a function of the parameter q and of the scale (the Hurst surface). It is designed to analyze correlation properties of the signal at very low frequencies. We discuss characteristic patterns in the shape of the Hurst surface and abnormalities in heart rate variability of the patients and observe important changes in comparison with the Hurst surface obtained for a group of healthy subjects analyzed in our earlier studies. However, no well-defined correlation of the properties of the Hurst surface with the level of ejection fraction or aortic gradient was obtained.