Reduced Disorder in Heart Rate Variability as an Early Finding of Cardiac Involvement in Steinert's Disease: Evidence Provided by the Estimation of Entropy on the Hourly-Qualified Sinusal R-R Intervals

The present study tries to compare the disorder in nonlinear variability of electrocardiographic sinusal R-R intervals (SRRI) in clinically healthy subjects (CHS) and patients affected by Steinert's disease (myotonic dystrophy, MD) in its early stage. The aim is to detect whether a reduced disorder in heart rate (HR) variability might represent an early finding of cardiac involve- ment in MD. The SRRI were provided by the Holter ECG of 15 MD patients (5 males and 10 females, mean age = 40 ± 7 years) at the early stage of their disease, who were lacking documentable signs of cardiac involvement, including bradycardia. The control data were obtained by the Holter ECG of 10 CHS (5M, 5F, mean age = 38 ± 6 years). The disorder in SRRI nonlinear variability was estimated by measuring the Entropy (E) per each hour of the Holter recording. The hourly-qualified series of SRRI, HR and E were thus analyzed using methods of conventional statistics and chronobiology, the latter ones being used for validating and quantifying the circadian rhythm (CR). Despite the comparability of the estimates regarding both the SRRI and HR, the E was found to be significantly decreased in its daily, diurnal and nocturnal mean level in MD patients as compared to CHS. Notwithstanding that the E was seen to maintain in its hourly-qualified values a significant CR. The E, however, was found to exhibit a significant reduction in the entity of its circadian oscillatory mean level. The reduced E in sinusal SRRI is evidence suggesting that a less pronounced disorder is detectable in HR variability of MD patients at the early stage of their disease. Given the fact that the entropic decrease is detectable: 1. in the absence of significant changes for the conventional and rhythmic estimates of SRRI and HR, and 2. in the absence of clinical and instrumental signs of cardiomyopathy, it can be argued that the reduction of the expected disorder in HR variability can be taken as an early finding of cardiac involvement in Steinert's disease. J Clin Basic Cardiol 2001; 4: 67-72.