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Heart Rate Variability (HRV) has been frequently cited as an indicator of homeostatic status. Low levels of HRV are associated with aging, disease, or increased risk of death. Many variables have been used to characterize the homeostatic level. The authors based this study on an alphanumerical classification for the levels of homeostasis, structured on three linear variables: Heart Rate, RMSSD and HF ms2. First, they sought validation of this classification by analyzing a bigdata with more than 30 million pieces of data collected from world literature. It was possible to confirm the clinical validity of this alphanumeric classification by comparing it with the expected values. It has been mentioned that traditional techniques for HRV analysis in time and frequency domains are often not sufficient to characterize the complex dynamics of the heartbeat, since the mechanisms involved in cardiovascular regulation probably interact with each other in a nonlinear manner. Thus, the primary objective of the present study was to verify whether or not there are correlations between the variables of the non-linear domain with variables and indices of the linear domain and also with the degree of global clinical impairment or homeostatic level of individuals. An unselected sample of patients, with varying levels of homeostatic impairment, was analyzed. Three specific nonlinear variables were considered for the correlations: Approximate Entropy (ApEn), Sample Entropy (SampEntr) and the α1 component of the Detrended Fluctuation Analysis (DFA α1). It was found, contrary to expectations, that the variables ApEn, SampEntr and DFA α1 were not useful in characterizing the homeostatic level, since they do not differentiate between healthy and highly compromised individuals. Also, in general, no significant correlation was detected between variables in the non-linear domain and variables in the linear domain. Regarding the parasympathetic, sympathetic and stress indexes, only DFA α1 presented a correlation, detecting a significant negative moderate correlation with the sympathetic index and the stress index. The findings of the present study allow for a better understanding of the conflicting reports in the literature and justify further investigations.

Characterization of Homeostatic Level Based on Non-Linear Variables of Heart Rate Variability