  In this study, an improved second-order difference plot is proposed to
analyze the variability of heart rate variability. Although the variation of
physiological status of cardiovascular system can be shown graphically by the
second-order difference plot, the descriptive ability of existing indicators
for this plot is insufficient. As a result, the physiological information
contained in the second-order difference plot cannot be extracted adequately.
Addressing the problem, the temporal variation measure analysis is presented to
describe distribution patterns of scatter points in the second-order difference
plot quantitatively and extract the acceleration information for variation of
heart rate variability. Experiment results demonstrate the effectiveness of the
temporal variation measure analysis. As a quantitative indicator, the temporal
variation entropy is properly designed and successfully applied in the
recognition and classification of the physiological statuses of the heart.
