Normalized entropy applied to the analysis of interindividual and gender-related differences in the cardiovascular effects of stress

Nonlinear dynamic methods are considered to be a potential tool for studying the complex behavior of the cardiovascular system. In the present study, interindividual and gender-related differences in cardiovascular (CV) responses to various stress stimuli were studied using conventional CV variables such as heart rate (HR) and blood pressure (BP), as well as a recently introduced criterion of system complexity, normalized entropy (E/H, where E is entropy and H is system energy). A group of healthy students (n=270) of both genders (17-20 years of age) were subjected to noise exposure, mental arithmetic, arithmetic against noise and examination stress. Results showed that CV reactivity depended upon the kind of stress imposed and the gender of the subject. HR and BP stress-induced responses did not differ between men and women. However, men had higher absolute BP levels at baseline and during exposure to stressors. Stress-induced pressor responses lasted longer in men than in women. Changes in the complexity degree of CV signals, as assessed by E/H, were more pronounced and prolonged than those of HR and BP. Unlike the latter, E/H changed significantly in all stress situations for each subject tested and could be divided into two types of stress-induced response. These results allow one to conclude that E/H can better quantitate individual differences in CV stress reactivity in comparison with HR and BP. These findings suggest that stress-induced changes in CV functioning are more varied than can be revealed by applying conventional CV measures.