Does Sample Entropy Reflect Nonlinear Characteristics of Cardiovascular Murmurs?

Sample entropy has been applied successfully for quantification of cardiovascular murmurs, but does sample entropy reflect nonlinear and chaotic characteristics of the murmurs? Seven digital audio recordings from subjects with audible carotid murmurs were recorded. Sample entropy was calculated from periods with and without murmurs and compared with sample entropy from surrogates with linear properties. None of the sample entropy measures from the recordings differed significantly from the surrogates with known linear properties. The sample entropy increased significantly in periods with audible murmurs compared to periods without. A similar difference was found between the surrogates of periods with and without murmurs. This confirms that sample entropy is suited for quantification of cardiovascular murmurs, but there is no evidence that nonlinear signal components of the murmursare present or affect the sample entropy.