Biomedical Instrumentation Problem Sheet 2

Heart Soundsin Biosignal Processing Module

Centre for Doctoral Training in Healthcare Innovation

11/18 May 2012Institute of Biomedical EngineeringDepartment of Engineering ScienceUniversity of Oxford

1How a heart soundsS1 Atrial contractionS2Ventricular contraction S3 Blood returning to the ventricleS4 Ventricle is too full to contain the bloodNormalAbnormalS1 + S2Lub - DubS1 + S2 + S3 Ken Tuck YS4 + S1 + S2 Ten Nes - SeeAnimation of heart valves opening and closing with sound

LUBDUB

2Heart soundsHow heart sounds change during illness

http://commons.wikimedia.org/wiki/File:Phonocardiograms_from_normal_and_abnormal_heart_sounds_with_pressure_diagrams.pngHeart soundsHow to record heart sounds?Acoustic Stethoscope

Chest piece transmits sound to the listener via air-filled hollow tubesThe chestpiece usually consists of two sides that can be placed against the patient for sensing sound; a diaphragm (plastic disc) or bell (hollow cup). If the diaphragm is placed on the patient, body sounds vibrate the diaphragm, creating acoustic pressure waves which travel up the tubing to the listener's ears. If the bell is placed on the patient, the vibrations of the skin directly produce acoustic pressure waves traveling up to the listener's ears. The bell transmits low frequency sounds, while the diaphragm transmits higher frequency sounds.

This two-sided stethoscope was invented by Rappaport and Sprague in the early part of the 20th century.

One problem with acoustic stethoscopes was that the sound level is extremely low.4Heart soundsHow to record heart sounds?Electronic Stethoscope - microphone

Place a microphone in the chestpiece

Moving magnetic coil electrical signalThe simplest and least effective method of sound detection is achieved by placing a microphone in the chestpiece.

This method suffers from ambient noise interference and has fallen out of favor5Heart soundsHow to record heart sounds?Electronic Stethoscope - piezoelectric

Connect one end of the crystal with the diaphragm

Plates squeezed electric signalPlace a piezoelectric crystal at the head of a metal shaft, the bottom of the shaft making contact with a diaphragm. Or use a piezo-electric crystal placed within foam behind a thick rubber-like diaphragm6Heart soundsHow to record heart sounds?Electronic Stethoscope - capacitorBodyConductive platesDistance between platesElectromagnetic diaphragm with conductive inner surface

Vibrations electric signalOr use an electromagnetic diaphragmwith a conductive inner surface to form a capacitive sensor. This diaphragm responds to sound waves identically to a conventional acoustic stethoscope, with changes in an electric field replacing changes in air pressure. This preserves the sound of an acoustic stethoscope with the benefits of amplification.7Heart soundsPhone apps

Ascultation library of heart soundsiStethoscope turn your iPhone into a stethoscopePlug-in stethoscopeHeart soundsOther uses of stethoscopes:

Heart soundsLung soundsBlood flow in veins and arteries blood pressure measurementsStomach soundsInternal sounds of machinery9Heart sounds

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