Diagnostic potential in state space parameters of lung sounds

The goal of this study was to investigate state space parameters of the lung sounds of healthy subjects and subjects with symptoms of asthma under different respiratory conditions. Our main objective was to elucidate the diagnostic potential of these parameters, which included embedding dimension (m), time delay (tau) and Lyapunov exponents (lambda). Lung sounds were acquired over the right lower lobe from six healthy subjects, ages 10-26 years, and from eight children with symptoms of asthma recorded pre- and post-bronchial provocation via methacholine challenge (MCh) and post-bronchial dilation (BD). Inspiratory air flows during recordings were 7.5, 15, or 22.5 mL/s per kg (+/-20%). With increasing flow for sounds recorded from healthy subjects, mean values of tau decreased. Percent of breaths with positive lambda decreased when heart sounds were excluded. For the patients who exhibited bronchoconstriction, values of tau increased and percent of positive lambda decreased post-MCh, and returned to pre-MCh values post-BD. Thus, both tau and presence of positive lambda may prove valuable in developing a model that will predict changes in respiratory status using lung sounds.