Arrhythmias and Their Electrocardiograms

The electrocardiograms below are lead II, except that for the premature atrial contraction. This is V5, which appears similar to lead II.


First Degree AV Block.
Note the PR interval is greater than 0.2 seconds. This can occur in healthy individuals with significant vagal tone, or it can reflect underlying heart disease.

QUESTION: What type of person has extra vagal tone, and why is this?   Answer

QUESTION: Why is the effect produced by the vagus nerve, and what is the neurotransmitter?    Answer

QUESTION: What is the cellular effect of the neurotransmitter?   Answer

Second Degree AV Block.
Here some P waves are not followed by QRS waves. The pattern shown is probably a benign example that could occur in a well-trained individual with lots of vagal tone. But second degree AV block also can represent underlying heart disease.

Third Degree AV Block.
The block is complete here and probably reflects significant damage to the conduction system by an infarction. But remember that the bundle branches also can act as pacemakers. How fast is their intrinsic rate? Note that the resulting QRS complex is quite misshapen and prolonged. The patient now likely will need a pacemaker.

QUESTION: Why is the QRS complex misshappen and prolonged?   Answer

Premature Ventricular Contraction.
Here an occasional action potential begins in an ectopic focus in the ventricles. Since the ventricles necessarily are not activated in their normal, efficient sequence, the extra QRS wave is misshapen and prolonged. It is not unusual to see these in otherwise healthy young individuals, or they can be generated by a damaged area of the heart. Note the pause after the premature contraction.

Premature Atrial Contraction.
Compare this with the premature ventricular contractions. Why does the QRS wave appear normal? How about the P wave? Observe that a premature contraction occurring in the atria resets the SA node, which does not occur with a premature ventricular contraction. As with ventricular premature contractions, these can occur in healthy or diseased hearts.

Premature Ventricular Contraction, Two Ectopic Foci.

Bundle Branch Block (no ECG)
If a bundle branch stops conducting action potentials, perhaps due to a myocardial infarction, the ventricles beat asynchronously. This is because the action potential conducts first into one ventricle and then from there into the second ventricle. As you would expect, the QRS complex is misshappen and prolonged, and the heart sounds are substantially split.

QUESTION: Why are the heart sounds split?   Answer

Sinus Bradycardia (no ECG)
Technically this is the condition in which the heart rate is less than 60 beats per minute. However, symptoms rarely develop unless the heart rate is 50 beats per minute or less. The electrocardiogram is otherwise normal.

Sometimes it can be due to hypothyroidism or drug effects (e.g; beta blockers, calcium channel blockers, digoxin). Also, athletes with well trained hearts can have bradycardia, although, of course, this is completely healthy.

The most common pathological form of sinus bradycardia is the sick sinus syndrome, which almost always appears in elderly patients with cardiovascular disease. With a dysfunctional or degenerating SA node, these patients have symptoms such as syncope, dyspnea and fatigue.

Supraventricular Tachycardia.
A number of circumstances can cause the AV node or the atrial tissue above it to generate action potentials very rapidly in a condition called supraventricular tachycardia. Often this is due to re-entry, which involves a pathway that carries the action potential immediately in a circle back into the AV node. The result is many closely spaced action potentials entering the ventricles. The tissue involved may be close to the AV node. But longer re-entry pathways occur too.

The rapid beating of the heart usually occurs in sudden bouts, not uncommonly in otherwise healthy, young individuals. Since the onset is sudden and the heart rate rapid, this type of supraventricular tachycardia is often called paroxysmal supraventricular tachycardia. A rapidly beating heart like this may not have time to fill normally, so the blood pressure may be low and person may feel faint. Excessive caffiene, smoking or alcohol increase the risk.

In Wolff-Parkinson-White Syndrome, supraventricular tachycardia arises from an anatomically abnormal accessory pathway in which some cardiac muscle interconnects an atrium with the underlying ventricular tissue. This allows an action potential to re-enter the atrium. Due to its anatomical basis, this type of tachycardia can be very tenacious. Ablation of the abnormal pathway with a catheter using radio frequency waves to heat the tissue is the usual solution.

Ventricular Tachycardia.
Here an ectopic focus drives the ventricles at a rapid rate. This is due to a damaged portion of the ventricle and is a serious matter because it is prone to lapse into ventricular fibrillation.

Atrial Fibrillation.
Here the action potential is moving continuously in an erratic pattern over the atria. Note the absence of P waves, the wavy baseline, and especially the random spacing of the QRS waves. The heart may be pumping blood fairly effectively in a heart like this. A serious problem, however, is the tendency of the blood to form a clot in the fibrillating atria. Should this dislodge and enter the blood, it could cause a stroke. While atrial fibrillation can occur in an otherwise healthy individual, it is much more common in the elderly, who may have an enlarged atrium due to valve or other problems.

QUESTION: Why are the QRS waves spaced randomly?   Answer

Ventricular Fibrillation.
The action potential is following a continuous, chaotic pattern over the heart, which never relaxes and thus is not pumping blood.

Quick Quiz

Identify the ECG:

Question A
Question B
Question C
Question D
Question E
Question F
Question G
Question H
Question I
Question J
Question K

Fill in Answer



Correct Answer

What is the arrhythmia in which only the P-R interval is longer than normal?

What type of premature contraction has a QRS complex that appears normal?

What arrhythmia has nearly random intervals between successive heart beats?

Name a location where the action potential (that causes ventricular contraction) begins in third degree AV block.

Name the location where the action potential (that causes ventricular contraction) begins in second degree AV block.

Name the location where the action potential (that causes ventricular contraction) begins in first degree AV block.

Name the arrhythmia if broad, misshapen QRS complexes occur regularly at 30 times a minute.

Name the arrhythmia if broad, misshapen QRS waves occur at 150 beats per minute.

(Spelling must be correct)