The Noise Of Heartbeats

Tuesday, 05 September, 2000

Adelaide University engineers are using complex mathematical processing to remove the noise from heartbeats, an approach that promises benefits for some of the most serious health problems.

Hypertension is a major health problem of the western world. It kills in excess of 40,000 people in the USA, contributed to the deaths of more than 200,000 and added to the problems of a further 58 million who suffer from the condition, in one year alone. That translates to one in four people. If the over 65 are considered, the figure rises to two in every three.

One diagnostic tool used is the sound of the heartbeat. Recorded as a phonocardiogram (PCG), heart sounds can reveal circulation problems, heart murmurs, faulty valves and coronary artery disease.

Unfortunately, sound recordings have difficulties. They are hard to hear in noisy surroundings and the heart generates noise of its own. There are sounds generated by breathing, by the contact of stethoscope on skin and by a foetus if a woman is pregnant. This noise can mask the real sounds of normal or abnormal heart function.

When a phonocardiogram is displayed as a trace on a screen, the noise is shown as random spikes, interfering with the record of the heart's activity.

Researchers at the university's Centre for Biomedical Engineering are working on the Heard Heart Sound Biomonitor Project. The research involves the sophisticated use of wave forms. The simplest wave form is a sine wave; a repeating regular series of hills and valleys. In comparison, the heartbeat wave form is a complex one.

The two wave forms can be related by Fourier's theorem which maintains that any signal, no matter what it looks like, can be copied exactly by adding together many sine waves of different frequencies. It follows that a complex signal can be broken down into its sine wave components. If the signal has unwanted frequencies, these can then be filtered out.

However, for heartbeat signals there is no way to tell what is noise and what is pure heartbeat so filtering doesn't work. Complex signals like heartbeat waveforms also require millions of sine waves to describe them faithfully. A heartbeat is a sharp and short-lived pulse, whereas a sine wave is a smoothly varying signal that repeats.

Engineers are therefore using another kind of waveform termed a 'wavelet.' Wavelets repeat for a short time, quickly declining to zero. They come in a variety of shapes and, since under certain conditions signals can be made up from many wavelets, signals can be represented as a sum of wavelets.

A wavelet's shape is not far different from a heartbeat signal. Researchers have found they can decompose a heartbeat signal using only a few wavelets. The advantage is that the true signal breaks down into a few big wavelet components, whereas noise breaks down into millions of tiny wavelets that can be filtered out.

The results, displayed on a computer screen or as a printout, show both normal and pathological conditions, free from the masking noise.

The research project aims to produce a system where heartbeat sounds recordings can be easily, cheaply, reliably, and repeatably recorded in order to promote its use as a diagnostic tool.

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