Formalisation of Electrocardiogram (ECG) - Using Event-B for Critical Device Software Systems - page 244

when assessing an abnormality in the ECG signal and a flowchart describing the
steps in the ECG interpretation protocol. The protocol consists of an evaluation (or
diagnosis) part and a treatment part, to be performed in the successive way. During
the application of guidelines and protocols, as soon as the possibility of a more se-
rious disease is uncovered, the recommendation is to leave the protocol without any
further actions.
10.4 Basic Overview of Electrocardiogram (ECG)
The electrocardiogram (ECG or EKG) [ 13 , 16 ] is a diagnostic tool that measures
and records the electrical activity of the heart precisely in the form of signals. Clin-
icians can evaluate the conditions of a patient's heart from the ECG and perform
further diagnosis. Analysis of these signals can be used for interpreting diagnosis of
a wide range of the heart conditions and to predict the related diseases. The ECG
records are obtained by sampling the bioelectric currents sensed by several elec-
trodes, known as leads. A typical one-cycle ECG tracing is shown in Fig. 10.1 .
Electrocardiogram term is introduced by Willem Einthoven in 1893 at the meeting
of Dutch Medical Society. In 1924, Einthoven received the Nobel Prize for his life's
work in developing the ECG [ 4 , 9 , 10 , 13 , 16 , 18 , 19 ].
The normal electrocardiogram (ECG or EKG) is depicted in Fig. 10.1 . All kinds
of segments and intervals are represented in this ECG diagram. The depolarisation
and repolarisation of the ventricular and atrial chambers are presented by deflection
of the ECG signal. All these deflections are denoted by alphabetic order (P-QRS-
T). Letter P indicates the atrial depolarisation, and the ventricular depolarisation is
represented by QRS complex. The ventricular repolarisation is represented by T-
wave. The atrial repolarisation appears during the QRS complex and generates a
very low amplitude signal which cannot be uncovered from a normal ECG signal.
10.4.1 Differentiating the P-, QRS- and T-waves
Sequential activation, depolarisation, and repolarisation are deflected distinctly in
the ECG due to anatomical difference of the atria and ventricles. Even all sequences
are easily distinguishable when they are not in a correct sequence: P-QRS-T. The
QRS-complex is easily identifiable between P- and T-waves because it has char-
acteristic waveform and dominating amplitude. This amplitude is about 1000 µm
in a normal heart and can be much greater in the ventricular hypertrophy. Nor-
mal duration of the QRS-complex is 80-90 ms. In case of non-existence of the
atrial hypertrophy; an amplitude and duration of the P-wave are about 100 µm and
100 ms, respectively. The T-wave has about twice of the amplitude and duration of
the P-wave. The T-wave can be differentiated from the P-wave by observing that the
T-wave follows the QRS-complex after about 200 ms. In the ECG signal several pa-
rameters are used to evaluate the conditions of a patient's heart from the ECG. The