Formal Logic Based Heart-Model - Using Event-B for Critical Device Software Systems - page 143

8.3 Background
8.3.1 The Heart System
The human heart is wondrous in its ability to pump blood to the circulatory sys-
tem continuously throughout a lifetime. The heart comprises four chambers: right
atrium, right ventricle, left atrium and left ventricle, each of which contract and
relax periodically. The atria form one unit and the ventricles another. The heart's
mechanical system (the pump) requires impulses from its electrical system to func-
tion. An electrical stimulus is generated by the sinus node (see Fig. 8.2 ), which is a
small mass of specialised tissue located in the right atrium of the heart. The electri-
cal stimulus travels down through the conduction pathways and causes the heart's
lower chambers to contract and pump out the blood. The right and left atria are stim-
ulated first and contract for a short period of time before the right and left ventricles.
Each contraction of the ventricles represents one heartbeat. The atria contract for a
fraction of a second before the ventricles, so their blood empties into the ventricles
before the ventricles contract.
Arrhythmias are caused by cardiac problems that produce abnormal heart
rhythms. In general, arrhythmias reduce haemodynamic performance, including
situations where the heart develops an abnormal rate or rhythm or when normal
conduction pathways are interrupted, and a different part of the heart takes over
control of the rhythm. An arrhythmia can involve an abnormal rhythm increase
(tachycardia: > 100 bpm) or decrease (bradycardia: < 60 bpm), or it may be char-
acterised by an irregular cardiac rhythm, such as that caused by asynchrony of
the cardiac chambers. Irregularities in the heartbeat are called bradycardia and
tachycardia. Bradycardia indicates that the heart rate falls below the expected level
whereas tachycardia indicates that the heart rate goes above the expected heart rate.
An artificial pacemaker can restore synchrony between the atria and the ventri-
cles [ 5 , 12 , 16 , 25 , 27 , 30 ]. Beats per minute (bpm) is the basic unit used to measure
the rate of heart activity.
8.3.2 Basic Overview of Electrocardiogram (ECG)
The ECG (or EKG) [ 16 , 24 ] is a diagnostic tool that measures and records precisely
the electrical activity of the heart in the form of signals. Clinicians can evaluate the
conditions of a patient's heart from the ECG and perform further diagnosis. Analysis
of these signals can be used to diagnose a wide range of heart conditions and to
predict the related diseases. ECG records are obtained by sampling the bioelectric
currents sensed by several electrodes, known as leads. A normal ECG is depicted
in Fig. 8.3 . Electrocardiogram term is introduced by Willem Einthoven in 1893 at a
meeting of the Dutch Medical Society. In 1924, Einthoven received the Nobel Prize
for his life's work in developing the ECG [ 5 , 24 , 27 , 30 ].