Biomedical Engineering Reference
In-Depth Information
1.8.5
The Impossibility of Conventional Standard
Phonocardiography Technology
It is regulated in the conventional industrial standard that a phonocardio-
graph can detect a heart sound signal with frequency between 20 and 600 Hz,
and the heart sound signal is that vibration signal deriving from the heart
and blood vessels and transmitted to the body surface, which contains any
cardiac murmur.
The phonocardiograph comprises a heart sound microphone, an equalizer
(in the case of using a direct conductive microphone, such as an accelerometer
microphone or a velocity microphone), a heart sound recorder, an electrocar-
diographic signal power supply, and it records the heart sound signal and the
electrocardiographic signal simultaneously.
The measurement of the phonocardiograph in accordance with the inter-
national industrial standard is for detecting actuation abnormalities of valves
andtheexistenceofanintracardiacshunt.
However, in regard to the very minute signal driving from the coronary
artery stenoses during diastole and having a wide frequency band between 200
and 1200 Hz, it is impossible to match the acoustic impedance between the
heart sound microphone and body surface if the weight of the microphone
is not 5 g or less and ideally 1 g [78]. Therefore, it is impossible for the
phonocardiograph, in accordance with the international industrial standard,
to detect the very minute acoustic vibration signal between 200 and 1200 Hz,
as mentioned above.
1. In conventional practice, this very minute vibration signal in the fre-
quency band between 200 and 1200 Hz is outside the international in-
dustrial standard, and the signal is outside the frequency range of the
measured object of the phonocardiograph. As the vibration intensity of
such a very minute vibration signal is very weak, it needs to be ampli-
fied to over 100 dB in order to make the signal the measured object.
In the case of using an acoustic vibration sensor, such as a microphone
and an accelerometer, as the detecting sensor of the phonocardiograph
for the above amplification, the matching of acoustic impedance between
the sensor and the body surface cannot be undertaken, since the weight
of the sensor becomes over 200 g.
2. Although the sensor technology by the displacement gauge principle is
the conventional practice, satisfactory detection cannot be carried out
because the sensitivity is insucient and the vibration signal of the object
is buried in the noise.
3. The time resolution is also insucient in conventional practice, using a
vibration sensor such as a microphone with vibrating plate resonance or
an accelerometer with charge generation based on enclosure resonance.
4. There is no system that can perceive and detect very minute displacement
vibration signals such as the coronary artery stenotic, diastolic and other
murmurs. To detect the very minute signal, transmission of the stenotic
