Accurate Localization of First and Second Heart Sounds via Template Matching in Forcecardiography Signals

Abstract

Cardiac auscultation is an essential part of physical examination and plays a key role in the early diagnosis of many cardiovascular diseases. The analysis of phonocardiography (PCG) recordings is generally based on the recognition of the main heart sounds, i.e., S1 and S2, which is not a trivial task. This study proposes a method for an accurate recognition and localization of heart sounds in Forcecardiography (FCG) recordings. FCG is a novel technique able to measure subsonic vibrations and sounds via small force sensors placed onto a subject's thorax, allowing continuous cardio-respiratory monitoring. In this study, a template-matching technique based on normalized cross-correlation was used to automatically recognize heart sounds in FCG signals recorded from six healthy subjects at rest. Distinct templates were manually selected from each FCG recording and used to separately localize S1 and S2 sounds, as well as S1-S2 pairs. A simultaneously recorded electrocardiography (ECG) trace was used for performance evaluation. The results show that the template matching approach proved capable of separately classifying S1 and S2 sounds in more than 96% of all heartbeats. Linear regression, correlation, and Bland-Altman analyses showed that inter-beat intervals were estimated with high accuracy. Indeed, the estimation error was confined within 10 ms, with negligible impact on heart rate estimation. Heart rate variability (HRV) indices were also computed and turned out to be almost comparable with those obtained from ECG. The preliminary yet encouraging results of this study suggest that the template matching approach based on normalized cross-correlation allows very accurate heart sounds localization and inter-beat intervals estimation.

Keywords: forcecardiography; heart rate variability; heart sounds; normalized cross-correlation; template matching.

Figure 1

An excerpt of signals from subject #4: ECG (blue line) and heart sounds from FCG (black line). Blue circles mark the locations of R-peaks in the ECG signal.

Figure 2

Examples of template selection. (a) The portion of the HS-FCG signal selected as a template of S1 sound is enclosed within the light green box, while the green dashed line marks the position of the absolute maximum within the S1 template; (b) the portion of the HS-FCG signal selected as a template of S2 sound is enclosed within the light red box, while the red dashed line marks the position of the absolute maximum within the S2 template; (c) the portion of the HS-FCG signal selected as a template of S1–S2 pair is enclosed within the light blue box, while the blue dashed line marks the position of the absolute maximum within the S1–S2 template. The positions of the absolute maxima within the templates were used to re-align the corresponding NCC functions.

Figure 3

Examples of heart sounds templates (red line) selected in FCG signals of subject #4. (a) S1 template, (b) S2 template, (c) S1–S2 template.

Figure 4

Examples of events localization via the template matching method in FCG signals of subject #4: (a) S1 sounds localization: heart sounds signal (black line), NCC function computed by using the S1 template (green line), NCC peaks indicating temporal locations of S1 sounds (red points); (b) S2 sounds localization: heart sounds signal (black line), NCC function computed by using the S2 template (orange line), NCC peaks indicating temporal locations of S2 sounds (violet points); (c) S1–S2 pairs localization: heart sounds signal (black line), NCC function computed by using the S1–S2 template (red line), NCC peaks indicating temporal locations of S1–S2 pairs (black points). Please note that each NCC function was re-aligned according to the relative position of the absolute maximum within the selected template. In the case of the S1–S2 template selected from the HS-FCG signal of subject #4, the absolute maximum of the S1–S2 template coincided with the S1 peak, therefore, also the NCC peaks in panel (c) turned out to be aligned with the S1 peaks in the HS-FCG signal.

Figure 4

Examples of events localization via the template matching method in FCG signals of subject #4: (a) S1 sounds localization: heart sounds signal (black line), NCC function computed by using the S1 template (green line), NCC peaks indicating temporal locations of S1 sounds (red points); (b) S2 sounds localization: heart sounds signal (black line), NCC function computed by using the S2 template (orange line), NCC peaks indicating temporal locations of S2 sounds (violet points); (c) S1–S2 pairs localization: heart sounds signal (black line), NCC function computed by using the S1–S2 template (red line), NCC peaks indicating temporal locations of S1–S2 pairs (black points). Please note that each NCC function was re-aligned according to the relative position of the absolute maximum within the selected template. In the case of the S1–S2 template selected from the HS-FCG signal of subject #4, the absolute maximum of the S1–S2 template coincided with the S1 peak, therefore, also the NCC peaks in panel (c) turned out to be aligned with the S1 peaks in the HS-FCG signal.

Figure 5

Examples of inter-beat intervals estimation in signals of subject #4. Inter-beat intervals were estimated in the ECG signal as the temporal difference between two consecutive R-peaks (double black arrow), and in the heart sounds signals as the temporal difference between two consecutive peaks in the NCC function obtained via: (a) S1 template (double purple arrow); (b) S2 template (double red arrow); (c) S1–S2 template (double green arrow).

Figure 6

Results of statistical analyses on inter-beat intervals obtained from S1 sounds and from ECG: (a) Regression and correlation plot (regression line is depicted in red); (b) Bland–Altman plot.

Figure 7

Results of statistical analyses on inter-beat intervals obtained from S2 sounds and from ECG: (a) Regression and correlation plot (regression line is depicted in red); (b) Bland–Altman plot.

Figure 8

Results of statistical analyses on inter-beat intervals obtained from S1–S2 pairs and from ECG: (a) Regression and correlation plot (regression line is depicted in red); (b) Bland–Altman plot.