ABSTRACT
Ambulatory monitoring of electrocardiograms (ECGs)
was facilitated by the introduction of Holter method
in the early '60s. Since this time, many researchs
have been undertaken in order to improve the overall
performances of the recorder and analyser, two unseparated
parts of this method.
The principal drawbacks of Holter method are wellknown:
. Recording of ECG is based on the assumption that
the arrhythmia of interest will be occurred in monitoring
period.
. Analysis and arrhythmia detection could be processed
only after the monitoring period. This is a timeconsuming
and tedious job, and subjected to human
error.
.Process "Recording-Play-back - Analysis - Diagnosis"
takes usually many days.
In this paper, the design of a microprocessor - based
portable recorder will be described. The analysis and
classification of arrhythmia are processed immediately
upon their occurrence according to a look-up table,
which could be programmed to suitaparticularneed.
By refering
recorder in
tinguished:
SYSTEM DESCRIPTION
to the block diagram of
figure 1, different sections
this portable
may be dis-
ECG Amplifier
First of all, the ECG signal is picked up by a bipolar
electrodes system before routing to the input of an
ECG amplifier. Depending on the nature of the ECG
signal, whether it is paced or not, the output of amplifier
is sent directly to a pace-maker pulse reject
circuit or simply bypassing this latter. This fonction
is fulfilled by means of a switch.
The ECG amplifier is constructed around three low
power operational amplifiers in classical configuration,
providing no isolation. All discret components
are calculated in order to have a frequency response
from 0.05 Hz to 40 Hz, this band of frequency is
judged enough for our application. Furthermore, all
artefacts of high frequency such as EMGs are greatly
attenuated by this choice.
Pace-maker pulse reject circuit
The pacing pulse is simply filtered out by a circuit
with slow rise time, passing only the slow varying
parts of ECG signal.
Signal conditioning section
This section consists of an R wave detector and
associated logic circuits, whose principal output is a
pulse presenting a R Wave. Its on-time is lengthened
by a refractory period of approximately 200ms,
preventing the detector from the false triggering on
tall T waves.
The heart of R wave detector is a bandpass filter,
centering about 20 Hz, thus the frequency components
of R wave are enhanced in respect to the others.
At the same time, the slow varying base line is stabilized,
this is frequently occurred in ambulatory
monitoring. A threshold circuit is then used to detect
the presence of a R wave. An accompanied logic
signal indicates the site of detected R wave, ventricular
or supraventricular.
Upon the occurrence of two successive R waves,
the microprocessor uses a real-time timer to calculate
the RR interval. It is the primary parameter
using by detection algorithm to classify arrhythmia.
Recording section
An analog delay circuit, FM modulation circuit and
a cassette recorder are the principal parts of this
section. The recording of arrhythmia on cassette tape
is under controlled of a logic signal coming from
microprocessor,

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