Is a Reduced Entropy in Heart Rate Variability an Early Finding of Silent Cardiac Neurovegetative Dysautonomia in Type 2 Diabetes Mellitus?

The present study estimates the entropy (E), say the amount of disorder, that is detectable in non-linear variability (NLV) of electrocardiographic sinusal R-R intervals (SRRI) in apparently uncomplicated diabetic patients (DP) affected by type 2 diabetes mellitus (NIDDM). The aim is to detect whether a reduction of the expected disorder in heart rate (HR) variability (V) can be taken as an early finding of cardiac dysautonomia in type 2 DP. The SRRI were provided by the Holter ECG of 10 type 2 DP (5 M and 5 F, mean age = 41 ± 5 years), who were metabolically compensated, via a hypoglycaemic diet, and lacking of apparent clinical and instrumental signs of cardiac neurovegetative involvement. Control data were obtained by the Holter ECG of 10 clinically healthy subjects (CHS, 5 M and 5 F, mean age= 38 ± 6 years). The E in SRRI NLV was estimated per each hour of the Holter recording. The hourly-qualified series of SRRI, HR and E were, thus, analyzed via methods of conventional statistics and chronobiology, the latter ones being used for assessing whether or not a reduced disorder could depend on the disappearance of HR circadian rhythm (CR). Notwithstanding the comparability of the conventional and chronobiological estimates regarding both the SRRI and HR, the E was found to be significantly lower in its daily, diurnal and nocturnal mean level in type 2 DP in comparison with CHS. However, the hourly-qualified values of SRRI, HR and E were seen to maintain the expected CR. The reduced E in hourly-qualified series of SRRI suggests that a less pronounced disorder is detectable in HRV of type 2 DP. Importantly, such a reduced disorder seems not to be associated with the disappearance of HR CR, whose abolition is regarded to be a sign of an established cardiac neurovegetative complication in DP. Considering that the investigated DP were lacking of documentable signs of cardiac neuropathy, one can argue that the measurement of E in HRV might be considered as an early tool for detecting a silent cardiac neurovegetative dysautonomia complicating the NIDDM.