Human Physiology, 14th edition (2016)

440 Chapter 13

Arrhythmias Detected

by the Electrocardiograph

Arrhythmias, or abnormal heart rhythms, can be detected

and described by the abnormal ECG tracings they produce.

Although proper clinical interpretation of electrocardio-

grams requires information not covered in this chapter, some

knowledge of abnormal rhythms is interesting in itself and

is useful in gaining an understanding of normal physiology.

A heartbeat occurs whenever a normal QRS complex is

seen, and the ECG chart paper moves at a known speed, so the

cardiac rate (beats per minute) can be easily obtained from an

ECG recording. A cardiac rate slower than 60 beats per minute

indicates bradycardia; a rate faster than 100 beats per minute

is described as tachycardia ( fig. 13.34 ).

Both bradycardia and tachycardia can occur normally.

Endurance-trained athletes, for example, often have heart rates

ranging from 40 to 60 beats per minute. This athlete’s brady-

cardia occurs as a result of higher levels of parasympathetic

inhibition of the SA node and is a beneficial adaptation. Acti-

vation of the sympathetic division of the ANS during exercise

or emergencies (“fight or flight”) causes a normal tachycardia.

Abnormal tachycardia occurs if the heart rate increases when

the person is at rest. This may be due to abnormally fast pacing by

the atria (caused, for example, by drugs), or to the development of

abnormally fast ectopic pacemakers —cells located outside the SA

node that assume a pacemaker function. This abnormal atrial tachy-

cardia thus differs from normal, or sinus, (SA node) tachycardia.

Ventricular tachycardia results when abnormally fast ectopic pace-

makers in the ventricles cause them to beat rapidly and indepen-

dently of the atria. This is very dangerous because it can quickly

degenerate into a lethal condition known as ventricular fibrillation.

Flutter and Fibrillation

Extremely rapid rates of electrical excitation and contrac-

tion of either the atria or the ventricles may produce flutter or

Figure 13.33 Depression of the ST segment as a result
of myocardial ischemia. This is but one of many ECG changes
that alert trained personnel to the existence of heart problems.

Normal Ischemia

P

R

R

T P T

S

S

Q Q

CLINICAL APPLICATION

Cerebrovascular accident, also called stroke, is the third

leading cause of death in the United States and the sec-

ond worldwide. There are two categories of stroke: isch-

emic stroke, caused by blockage of a cerebral artery by a

thrombus and usually the result of atherosclerosis; and hem-

orrhagic stroke, caused by bleeding from a cerebral artery,

often because of an aneurism. Hypertension is the major risk

factor for stroke; others include atrial fibrillation, high blood

cholesterol, and diabetes. Ischemic stroke can be treated

with anticoagulant and thrombolytic drugs, but these are

most effective if delivered soon after the ischemic injury. This

is because of excitotoxicity (chapter 7, section 7.7), a process

whereby neurons die as a result of the ischemia-induced

impairment in the removal of glutamate from the synaptic

clefts. This results in excessive inflow of Ca^2 1 through the

NMDA receptors, causing neuron death. There is presently no

effective way to prevent excitotoxicity and its consequences.

Figure 13.34 Some arrhythmias detected by the ECG. In ( a ) the heartbeat is paced by the normal pacemaker—the SA node
(hence the name sinus rhythm ). This can be abnormally slow (bradycardia—42 beats per minute in this example) or fast (tachycardia—
125 beats per minute in this example). Compare the pattern of tachycardia in ( a ) with the tachycardia in ( b ). Ventricular tachycardia is
produced by an ectopic pacemaker in the ventricles. This dangerous condition can quickly lead to ventricular fibrillation, also shown in ( b ).

(a) Sinus tachycardia

Sinus bradycardia

(b) Ventricular fibrillation

Ventricular tachycardia