Biomedical Engineering Reference
In-Depth Information
mathematical perspective, this pacemaker is an example of a nonlinear oscilla-
tor. Thus, if the rhythm is perturbed, for example by delivering a shock to the
atria, then in general the timing of subsequent firings of the sinus node may be
reset (i.e., they occur at different times than they would have if the shock had
not been delivered), but the frequency and amplitude of the oscillation will re-
main the same. A wave of excitation initiated in the sinus node travels through
the atria, then through the atrioventricular node, then through specialized Pur-
kinje fibers to the ventricles. The wave of electrical excitation is associated with
a wave of mechanical contraction so that the cardiac cycle is associated with
contraction and pumping of the blood through the body. The right and left atria
are comparatively small chambers and act as collection points for blood. The
right atrium collects blood from the body and the left atrium collects blood from
the lungs. The right ventricle pumps blood to the lungs to be oxygenated,
whereas the left ventricle pumps blood that has returned to the heart from the
lungs to the rest of the body. The right atrium and right ventricle are separated
by the tricuspid valve, which prevents backflow of blood during ventricular con-
traction. Similarly, the left atrium and left ventricle are separated by the mitral
valve. In order to pump the blood, the ventricles are comparatively large and
muscular.
The electrical events associated with cardiac activity can be easily moni-
tored in an electrocardiogram, which is a measurement of the potential differ-
ence between two points on the surface of the body. Since the heart generates
waves of electrical activation that propagate through the heart during the cardiac
cycle, the deflections on the electrocardiogram reflect cardiac activity. In the
normal electrocardiogram, there are several main deflections, labeled the P
wave, the QRS complex, and the T wave (Figure 2A) (1). The P wave is associ-
ated with electrical activation of the atria, the QRS complex is associated with
electrical activation of the ventricles, and the T wave is associated with repolari-
zation of the ventricles. The duration of the PR interval reflects the conduction
time from the atria to the ventricles, typically 120 to 200 ms. The duration of the
QRS complex reflects the time that it takes for the wave of excitation to activate
the ventricles. Because of the specialized Purkinje fibers, the wave of activation
spreads rapidly through the ventricles so that the normal duration of the QRS
complex is less than 100 ms. The time interval from the beginning of the QRS
complex to the end of the T wave, called the QT interval, reflects the duration of
the time the ventricles are in the contraction phase. The duration of the QT in-
terval depends somewhat on the basic heart rate. It is shorter when the heart is
beating faster. For heartbeats in the normal range the QT interval is typically of
the order of 300-450 ms. The rate of the heart is often measured by time inter-
vals between two consecutive R waves. Abnormally fast heart rates, faster than
about 90 beats per minute, are called tachycardia, and abnormally slow heart
rates, slower than about 50 beats per minute, are called bradycardia.
