Biomedical Engineering Reference
In-Depth Information
R
R
R-R INTERVAL
P-R
SEGMENT
Q
S
T
P
S-T INTERVAL
S-T
SEGMENT
FIGURE 3.22
Typical lead II ECG. This electrocardiogram is typical of one that would be recorded from the
body's surface by having a positive electrode on the left leg and a negative electrode on the right arm. The vertical
direction represents voltage, and the horizontal direction represents time. The P, R, and T waves are easily identi-
fied and are the result of the movement of ions in cells in different parts of the heart. Different intervals and
segments have been identified that provide information about the health of the heart and its conduction system.
The R-R interval can be used to determine heart rate.
QRS
INTERVAL
P-R INTERVAL Q-T INTERVAL
(AV) node, where it slows to a rate of about 0.05 m/s to allow time for the ventricles to
completely fill with the blood from the atria. After leaving the AV node, the activation
wavefront moves to specialized conduction tissue, the Purkinje system, which spreads
the wavefront very rapidly (at about 3 m/s) to many cells in both venricles. The activation
wavefront spreads through ventricular tissue at about 0.5 m/s. This results in the simul-
taneous contraction of both ventricles (ventricular systole) so blood is forced from the
heart into the pulmonary artery from the right ventricle and into the aorta from the left
ventricle.
The electrocardiogram (ECG; Figure 3.22) is an electrical measure of the sum of these
ionic changes within the heart. The P wave represents the depolarization of the atria, while
the QRS represents the depolarization of the ventricles. Ventricular repolarization shows up
as the T wave, while atrial repolarization is masked by ventricular depolarization. Changes
in the amplitude and duration of the different parts of the ECG provide diagnostic informa-
tion for physicians. Many biomedical engineers have worked on methods for recording and
analyzing ECGs.
EXAMPLE PROBLEM 3.6
What would be the heart rate given by an ECG in which 10 R-waves occurred in 6.4 s?
Solution
A sequence of 10 R-waves represents 9 R-R intervals (see Figure 3.22) or beats of the heart.
60
ΒΌ
9
beats
6
s
1min
84
bpm
:
4
s
