ECG Acquisition in a Computer - ECG Acquisition and Automated Remote Processing - page 51

4. Spatial recording—it is also called vectorcardiography, which is performed on
3 orthogonal leads.
5. Heart mapping—it is also called surface ECG test, which employs multiple
channel (as many as 80 leads) recording by placing electrodes on heart muscles.
Conventional ECG recording generates traces of 12 leads (3 bipolar standard
leads, 3 augmented limb leads, and 6 chest leads) in a chart recorder, printing 5-6
beats (R-R intervals) per lead for visual examination by a cardiologist. The lead
recordings may be sequential, i.e., the medical professional selects the leads to be
traced by switch positions on the recorder or, simultaneous, where recording of all
12-leads are traced.
Mechanical-type recording systems employ a current activated stylus, which
produces an impression on a thermally sensitive paper moving at 25 mm/s. The
inertia of the mechanical system puts an upper limit of recoding at 80 Hz. Since the
clinical bandwidth of ECG is in the range 0.05-130 Hz, personal computer-based
systems are gradually replacing mechanical recorders. Following, presently many
ECG recorders provide a port for connectivity to a computer for direct acquisition of
ECG samples. Further details of electrocardiography can be found in [ 1 - 3 ].
3.3 ECG Acquisition Systems
3.3.1 ECG Signal Characteristics and Artifacts
In this section, we describe the basic electrical characteristics of ECG signal which
are essential for design and development of a hardware acquisition module for its
collection from a patient. ECG is a low-amplitude (± 3 mV maximum) and non-
stationery signal. Most of the clinically significant information in ECG is found in
the spectral band 0-100 Hz.
Even with utmost care during the record, the ECG signal is contaminated with
several types of unwanted signals, collectively called 'artifacts.' Some of these are
of physiological origin, i.e., generated within human body and the others non-
physiological, i.e., external to the body. Some of these signals have overlapping
spectral band with the ECG signal itself, which means designing a simple band-
pass filter is not sufficient for their elimination. The sources of artifacts are as
follows:
1. Electromyography (EMG) noise: Picked up due to muscular activity of the
patient. Their amplitude and frequency band, 0.1-1 mV and 5 Hz-1 kHz,
respectively, are partly overlapping with ECG signal. EMG noise, if not
properly taken care off, may completely destroy the signal based analysis. For
short-duration clinical testing, the patient is advised to lie on resting condition
so as to minimize the noise. However, for long-term Holter monitoring, this
EMG noise is unavoidable.