ACOUSTIC ANALYSIS OF CARPENTIER-EDWARDS BIOPROSTHETIC HEART VALVES

R. Bedit, J.T.E. McDonnellt and K.A.A. Fox *

t Department of Electrical Engineering, *Cardiovascular Research Unit , The University of Edinburgh, King’s Buildings, Mayfield Road, Edinburgh EH9 3JL, U. K.

ABSTRACT

This paper characterises the spectra of closing sounds produced by the Carpentier-Edwards bioprosthetic heart valve by extracting from the sound spectrum heuristic features of diagnostic significance. Algorithms were tested on sounds recorded from forty patients with the Carpentier-Edwards bioprosthesis (twenty-four aor- tic and sixteen mitral). Results show that normally functioning aortic valves are characterised by four to five dominant peaks, with the major concentration of spec- tral energy occurring between 25Hz and 125Hz, whereas, normally functioning mitral valves are characterised by two to three dominant peaks with the major concen- tration of spectral energy occurring between d.c. and 100Hz. For leaky, regurgitant valves and valves diag- nosed as having stiffening, calcified cusps, a shift in spec- tral energy was observed, to the regions between d.c.and 75Hz and 50Hz and 200Hz respectively. It was found that the frequency occurring at the -10dB level can be used to discriminate between normally functioning, leaky and stiffening aortic prostheses, where mean fre- quencies were observed of 113.6Hz, 53.7 Hz and 238.1Hz respectively. This paper presents the first publication of these findings for the Carpentier-Edwards bioprosthesis.

INTRODUCTION

Previous related work by the authors [l-31 describes in detail the procedures involved regarding the acqui- sition of phonocardiographic signals from patients with implanted bioprosthetic heart valves, as well as the the- ory, suitability and performance of spectral estimation techniques when applied to valvular closing sounds. In this paper, heuristic features of diagnostic significance are extracted from the sound spectrum produced by normally functioning, leaky and stiffening Carpentier- Edwards bioprostheses, in an attempt to uniquely char- acterise the sound spectrum of each of these states of physiological and pathological functionality. Quantita- tive analysis of the spectrum of normally functioning valves should assist in the detection of leaky and stiff- ening valves through changes in the sound spectrum.

DIAGNOSTIC FEATURE SELECTION

The selection of diagnostic features was based on the empirical identification of a number of dominant fre- quency peaks to quantify the natural resonance modes of bioprosthetic heart valves, and to characterise the dis- tribution of energy in the sound spectrum by evaluating bandwidth, energy and root mean square (RMS) mea- surements over the entire spectral profile.

RESULTS AND DISCUSSION

For normally functioning aortic prostheses, Prony’s method of spectral analysis identified four to five domi- nant frequency peaks and one to two weaker secondary peaks. Mean frequencies and amplitudes of these peaks were: F1 = 37.4Hz 62 OdB, F 2 = 77.8Hz @ -7.66dB1 F 3 = 116.5Hz (B -10.04dB, F4 = 171.1Hz ’@ -14.17dB, F 5 = 245.8Hz @ -16.59dB, F6 = 292.4H~ @ -17.0dB. The major distribution of spectral energy occurred in the region between 25Hz and 125Hz, with mean rela- tive energies and RMS energies of 35.73% and 33.49% respectively.

For leaky aortic prostheses, Prony’s method identified four dominant peaks and one weaker secondary peak, with mean frequencies and amplitudes of F1 = 30.6%

17.69dB1 F4 = 149.2Hz @ -19.21dBI F5 = 210.7Hz @ -22.05dB. These results are slightly lower than those pro- duced by normally functioning prostheses, particularly the amplitudes of peaks F 3 to F6. The major distribu- tion of spectral energy occurred in the region between d.c. and 75&, with mean relative energies and RMS en- ergies of 28.67% and 26.3% respectively. The higher fre- quency peaks, F 3 to F6, which characterised normally functioning prostheses have less energy, with the remain- der of the energy being evenly distributed throughout the other frequency bands of the sound spectrum. This concentration of energy in the lowest frequency regions of the sound spectrum is due primarily to the presence of diastolic murmurs produced by incompetent, regurgi- tant aortic prostheses.

@ OdB, F 2 = 55.3Hz @ -7.54dB, F 3 = 114.OHz @ -

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For a o ~ i c prostheses diagnosed as having stiffening, calcified cusps, a shift in spectral energy was observed, with the higher frequency peaks assuming more energy. Mean frequencies and amplitudes of these peaks were: F1 = 181:2Hz ‘3 OdB, F 2 = 147.1Hz @ -0.77dB, F 3 = 101.OHz 69 -0.82dB, F4 = 67.1Hz @ -1.49dB, F 5 = 44.2Hz ‘3 -2.97dB, F 6 = 216.3Hz @ -6.68dB. The major distribution of spectral energy occurred in the region between 50Hz and 200Hz, with mean relative energies and RMS energies of 62% and 61% respectively.

For normally functioning mitral prostheses, Prony’s method identified two to three dominant frequency peaks and one to two weaker secondary peaks. The ma- jor distribution of spectral energy occurred in the region between d.c. and lOOHz with mean relative energies and RMS energies of 39.22% and 36.62% respectively. A similar shift in spectral energy to the region between d.c. and 75Hz was observed for leaky mitral prostheses. Figure 1 shows examples of spectra and distributions of spectral energy produced by Carpentier-Edwards bio- prostheses.

In general, it was observed that for normally function- ing prostheses, there was a correlation between spectral energy and the period of implantation. As the dura- tion of implantation increased, the amount of energy de- creased slightly, having been redistributed in the higher- frequency regions of the sound spectrum. This is a result of the inevitable gradual stiffening associated with the cusps of porcine bioprosthetic heart valves.

CONCLUSIONS

This paper has characterised the spectra of the closing sounds produced by the Carpentier-Edwards biopros- thetic heart valve. Results have shown that normally functioning, leaky and stiffening prostheses each exhibit unique spectral characteristics. For leaky prostheses, a shift in spectral energy was observed, from the higher frequency peaks to the lower frequency peaks. For stiff- ening prostheses, a shift in spectral energy was also ob- served, with the higher frequency peaks assuming more energy. It was found that the frequency occurring at the -10dB can be used to discriminate between normally functioning, leaky and stiffening aortic prostheses.

This study has indicated that features extracted from the sound spectrum offer the potential of diagnosing prostheses as normally functioning, leaky or stiffening. The suitability of these features in the development of classification and neural network algorithms is currently being investigated.

This work has shown that valuable information re- garding the physiological and pathological state of bio- prosthetic heart valves is contained within the sound spectrum. The results obtained are very encouraging and suggest that it is now possible to develop a low-cost prototype system to provide periodic evaluation of aor-

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Figure 1: Characteristics of Carpentier-Edwards bio- prostheses. (a and b) Spectral estimate and energy dis- tribution of normally functioning aortic prostheses, (c and d) Spectral estimate and energy distribution of leaky aortic prostheses, (e and f) Spectral estimate and en- ergy distribution of stiffening aortic prostheses, ( g and h) Spectral estimate and energy distribution of normally functioning mitral prostheses.

tic and mitral bioprosthetic heart valves. Such a system would be non-invasive, passive, atraumatic and accu- rate, detecting impending valvular malfunctions, dys- functions and degenerations.

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REFERENCES

R. Bedi and J. T. E. McDonnell, “Evaluation of Parametric Methods for the Spectral Analysis of Carpentier-Edwards Heart Valve Sounds,” Proceed- ings 20th Annual International Conference IEEE Compuders in Cardiology, pp. 675-678, London 1993.

R. Bedi, J . T. E. McDonnell and K. A. A. Fox, “Forward-Backward Linear Predictive Spec- tral Analysis of Bioprosthetic Heart Valve Sounds,” Proceedings 15th Annual International Conference IEEE Engineering in Medicine and Biology Society, pp. 890-891, San Diego 1993.

R. Bedi, H. P. Sava and J . T . E. McDonnell, “Spec- tral Analysis of Prosthetic Heart Valve Sounds,” Proceedings IEE Colloquium Signal Processing and Modelling in Cardiography, Newcastle 1994, ac- cepted for publication.