Investigation of frequency coupling in restricted pulsatile flows modeling aortic stenosis

A quantitative understanding of the frequencies present in sound signals produced by aortic stenosis is crucial to assess the severity of the stenosis. Inspired by this problem, a parametric study is performed to correlate changes in the acoustic spectrum with the restriction percentage in a pulsatile flow. To simulate some of the conditions in an aortic valve, the pulsing frequency is set to 70 beats per minute and the geometric shape of the model valve opening mimics an open tricuspid valve. The dynamic conditions are set to match a range of Reynolds numbers relevant to human aortic valves. Eight different restrictions are tested at eight different Reynolds numbers to provide a parametric study of the relationship between the acoustic spectrum and narrowing severity. The sound signals measured with a contact microphone are analyzed with both the power spectrum and the bicoherence, which provides a measure of quadratic phase coupling between frequencies present in the signal. Across all Reynolds numbers, the energy present in high frequencies increases with increasing restriction percentage. Characteristic frequency bands are identified that uniquely correlate with the restriction, where 100 – 150 Hz appears significant for moderate to severe restrictions, and a band around 270 Hz increases in prominence with increasing severity. The level of phase coupling appears to decrease with increasing restriction, providing a unique quantitative measure that contrasts with the results in the power spectrum.