Correlation dimension is a measure of dimensionality of the space occupied by a set of points that can be used to measure complexity of the RR interval time series. Reconstruction of the attractor was performed. Then the correlation integral was calculated with the number of points in the phase space that are closer than a certain threshold r, repeating the process for a certain range of thresholds. Correlation dimension was computed as the slope of the line fitting the log-log plot of the correlation integral as a function of threshold if the number of points was sufficiently large and r was small. Detrended fluctuation analysis quantifies fractal-like scaling exponents and correlation properties of RR intervals. The RR interval time series was integrated, and detrended within each segment. The root mean square fluctuation of the integrated and detrended data was repeatedly measured over different segment lengths and plotted against different segment lengths on a log-log scale. The scaling exponent DFA a indicates the slope of this regression line. The short-term fluctuation DFA a1 was calculated within range 4#n#16 and long-term fluctuation DFA a2 was calculated within range 16# n#64. Shannon entropy is a measure of signal complexity of the time series dynamics, and is the information entropy of line segment density distributions from the RR intervals. It is measured by the negative sum of the multiplication of the probabilities and base 2 logarithms of probabilities for given line lengths. The probability is the number of length lines divided by the total number of lines. Non-linear dynamics and chaotic indices in HRV were significantly reduced in patients with a propensity for adverse arrhythmic Bortezomib events which exhibit morning surge. Reduced chaos was associated with increased arrhythmias. Thus the changes in linear and nonlinear dynamics in our CHF dogs are similar to that in CHF patients, thereby validating this new arrhythmogenic canine preparation as a valuable model in which to further study molecular, cellular, physiologic and neurohumoral mechanisms in ways not possible in humans. There are numerous factors that may contribute to this time-ofday elevated incidence in ventricular arrhythmias in dogs as well as in humans. Catecholamines, angiotensin, renin, aldosterone and growth hormone have higher plasma concentrations in morning during maximal sympathetic activity in humans. Protective regulators such as bradykinin, magnesium, potassium, vitamin E and C and melatonin are all low in the morning, and may be associated with greater vascular resistance, blood pressure, contractility, coronary artery tone, stroke volume and cardiac output. Additional factors such as acetylcholine, histamine, serotonin, and myocardial oxygen demand exhibit circadian variation and may contribute to increased risk of adverse cardiovascular events in the morning. All these corresponding biochemical changes in the morning in CHF dogs are reflected by attenuated morning HR rise, blunted autonomic oscillation, decreased cardiac chaos, a loss of time-of-daydependent rhythm, and a lack of enhanced chaos at the time of enhanced arrhythmia occurrence. Diurnal variation of HRV was reported in healthy human subjects.