Application to biomedical signals - Practical Biomedical Signal Analysis Using MATLAB - page 172

SA

AV

R

SA

AV

SA

AV

T

P

U

Q

PQ
segment

ST segment

S

QRS
interval

PR interval ST interval

QT interval

0.8 s

FIGURE 4.28: The potential recorded by lead II during the heart cycle. Main
waves, intervals, and segments marked. Above schematic representations of elec-
trical activity propagation during the cycle are marked.

the context of EEG, the same potential difference on the body surface can be due
to different configurations of sources; hence abnormal activity may be masked. The
information on the electromagnetic activity of the heart, its generation and measure-
ment may be found in [Malmivuo and Plonsey, 1995].
In the clinical environment ECG recording is often accompanied by the measure-
ment of respiration and blood flow, since these signals are mutually dependent, e.g.,
respiration rhythm is visible in the heart rate signal.
The ECG is a clinical test in most heart diseases. Other medical conditions such
as diabetes and metabolic disturbances may also have an impact on ECG. In ECG
the activity of autonomous nervous system is reflected, therefore ECG can be used
as an indicator of activation of sympathetic and parasympathetic systems. Among
the heart activity disturbances reflected in ECG are the arrhythmias, bradycardia
(slowing of the rhythm), tachycardia (increase of heart rate), ectopy, fibrillation, my-
ocardial infarction, premature atrial/ventricular contraction, atrial/ventricular flutter,
conduction blocks, cardiac ischemia, hyper- or hypokalemia (excess/deficiency of
potassium), hyper-, hypocalcemia (excess/ deficiency of calcium). The description
of influence of these and other pathological heart conditions on ECG may be found
in [Chee and Seow, 2007].