No evidence of nonlinear or chaotic behavior of cardiovascular murmurs

The current work examines cardiovascular murmurs for evidence of nonlinear and chaotic characteristics. In recent studies, methods suited for analysis of nonlinear and chaotic systems have been suggested for quantification of cardiovascular murmurs, but neither nonlinear nor chaotic behavior in cardiovascular murmurs have been confirmed. We obtained seven digital recordings from subjects with known carotid artery stenosis and audible carotid murmurs with a digital stethoscope. Measures as time reversibility, the power of third order cumulants, prediction errors from a nonlinear model, sample entropy, the correlation dimension and BDS statistics were calculated and compared to measures from surrogates generated by stationary Gaussian linear processes. Additionally nonlinear measures appropriateness for murmur quantification was evaluated by comparing sample entropy between periods with and without audible murmurs. Except from the power of third order cumulants from a single recording none of the measures from the seven recordings differed significantly from the surrogates with known linear properties. The sample entropy values increased significantly in periods with audible murmurs compared to periods without. Our study confirms that a nonlinear measure as sample entropy is suited for quantification of cardiovascular murmurs, but there is no evidence of either nonlinear or chaotic behavior in the signals.