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214

Overview

IN THIS CHAPTER, you will learn the characteristics of rhythms that originate within the

ventricles, and you will find out what things are common to all ventricular arrhythmias. You will

then learn the names and features of five different arrhythmias that originate in the ventricles.

For each of these arrhythmias, you will learn about the etiology, conduction, and resulting EKG

features (regularity, rate, P waves, PR intervals, and QRS complexes).

Ventricular Rhythms

1. All of the arrhythmias you have learned so far are classified as supraventricular

arrhythmias, because they originate above the ventricles. All supraventricular

arrhythmias have one thing in common: they all have QRS complexes of less than

 second duration. This classification has a very scientific

basis. We know from physiological measurements that an impulse originating above

the ventricles, and that follows the normal conduction pathways, can depolarize the

ventricles in less than 0.12 second. We also learned that it is possible for a supraventricular impulse to produce a QRS that is wider than 0.12 second, but that this would

indicate some kind of delay in through the ventricles and

0.12

conduction

Ventricular Rhythms

8

Ventricular Rhythms 215

thus would be considered an abnormality to be noted. Generally speaking, a normal

supraventricular complex will have a measurement of less

than second.

2. We say that a supraventricular arrhythmia should have a QRS complex of less than

0.12 second, but we also acknowledge that an abnormality could cause the QRS to

be wider than that. However, we can say with certainty that a ventricular impulse

cannot depolarize the ventricles in less than 0.12 second. Hence, a basic rule for ventricular arrhythmias is that the QRS measurement will be 0.12 second or greater. If a

complex measures less than 0.12 second, we know that it must have been initiated by

a impulse. But if it is 0.12 second or greater, it might have originated in the , or it could have originated above the ventricles

but encountered a conduction disturbance along the way.

3. Ventricular arrhythmias are very serious for several reasons. First, the heart was

intended to depolarize from the top down. The were meant

to contract before the in order to pump blood effectively.

When an impulse originates in the ventricles, this process is reversed, and the heart’s

efficiency is greatly reduced. Further, since the ventricles are the lowest site in the

conduction system, there are no more fail-safe mechanisms to back up a ventricular

arrhythmia. Ventricular arrhythmias are the most serious arrhythmias because the

heart has lost its and because it is functioning on its last level

of backup support.

4. In this section, we will be learning five ventricular arrhythmias:

• Premature Ventricular Complex (PVC)

• Ventricular Tachycardia

• Ventricular Fibrillation

• Idioventricular Rhythm

• Asystole

Although their mechanisms differ, each of these arrhythmias originates

in the and thus will have a QRS measurement of

 second or more.

Premature Ventricular Complex (PVC)

5. The first arrhythmia is not a rhythm itself, but is instead a single ectopic beat

originating from an irritable ventricular focus. Since it arises from an irritable focus,

the complex will come than expected in the cardiac cycle

and will interrupt the regularity of the underlying rhythm. PVCs (Figure  61) are

single that come earlier than expected and interrupt

the underlying .

6. Because PVCs originate in the ventricles, the QRS will be than

normal. But another important feature of a ventricular focus is that there is no P wave

preceding the QRS complex. Since the SA node did not precipitate the ventricular depolarization, there will be no P wave. On the EKG, you will see a very wide, bizarre QRS

complex that is not preceded by a wave.

7. One of the things that gives a PVC such a bizarre appearance, in addition to

the width of the QRS complex, is the tendency for PVCs to produce a T wave that

QRS

0.12

supraventricular

ventricles

atria

ventricles

effectiveness

ventricles

0.12

earlier

ectopics

rhythm

wider

P

216 Chapter 8

extends in the opposite direction of the QRS complex (Figure 62). That is, if the QRS is

negative, the T wave will be . This is not a hard-and-fast rule,

but is a very frequent finding that contributes to an overall

appearance of a PVC.

8. PVCs are usually easy to spot because they are wide and bizarre, with a QRS

complex measurement of second or more, and they are not

preceded by a wave (Figure 63). Another feature common to

many PVCs is that the T wave is in the opposite direction of the

complex.

9. PVCs come earlier than expected, but they don’t conduct impulses back through the

AV node and into the atria. Therefore, the atria are not depolarized. This leaves the sinus

node undisturbed; it continues to discharge regularly. The result is that the distance

between the normal complex preceding the PVC and the normal complex following

the PVC will measure exactly twice the distance of one R–R interval in the underlying

rhythm (Figure  64). This feature is called a compensatory pause. If an ectopic is

followed by a pause, it is a good indication that the ectopic is

a . The presence of a compensatory pause helps identify the

ectopic as a PVC.

upright

bizarre

0.12

P

QRS