Ganong's Review of Medical Physiology, 23rd Edition

500 SECTION VICardiovascular Physiology

However, the second succeeding impulse from the SA node
produces a normal beat. Thus, ventricular premature beats are
followed by a compensatory pause that is often longer than
the pause after an atrial extrasystole. Furthermore, ventricular

premature beats do not interrupt the regular discharge of the

SA node, whereas atrial premature beats often interrupt and

“reset” the normal rhythm.

Atrial and ventricular premature beats are not strong

enough to produce a pulse at the wrist if they occur early in

diastole, when the ventricles have not had time to fill with

blood and the ventricular musculature is still in its relatively

refractory period. They may not even open the aortic and pul-

monary valves, in which case there is, in addition, no second

heart sound.

Paroxysmal ventricular tachycardia (Figure 30–14) is in

effect a series of rapid, regular ventricular depolarizations

usually due to a circus movement involving the ventricles.

Torsade de pointes is a form of ventricular tachycardia in

which the QRS morphology varies (Figure 30–15). Tachycar-

dias originating above the ventricles (supraventricular tachy-

cardias such as paroxysmal nodal tachycardia) can be

distinguished from paroxysmal ventricular tachycardia by use

of the HBE; in supraventricular tachycardias, a His bundle H

deflection is present, whereas in ventricular tachycardias,

there is none. Ventricular premature beats are common and,

in the absence of ischemic heart disease, usually benign. Ven-

tricular tachycardia is more serious because cardiac output is

decreased, and ventricular fibrillation is an occasional com-

plication of ventricular tachycardia.

In ventricular fibrillation (Figure 30–15), the ventricular

muscle fibers contract in a totally irregular and ineffective way

because of the very rapid discharge of multiple ventricular

FIGURE 30–14 Top: Ventricular premature beats (VPB). The
lines under the tracing illustrate the compensatory pause and show
that the duration of the premature beat plus the preceding normal
beat is equal to the duration of two normal beats. Bottom: Ventricular
tachycardia.

NNNP N

Comp.

pause

VPB

FIGURE 30–15 Record obtained from an implanted cardioverter–defibrillator in a 12-year-old boy with congenital long QT
syndrome who collapsed while answering a question in school. Top: Normal sinus rhythm with long QT interval. Middle: Torsade de pointes.
Bottom: Ventricular fibrillation with discharge of defibrillator, as programmed 7.5 s after the start of ventricular tachycardia, converting the heart
to normal sinus rhythm. The boy recovered consciousness in 2 min and had no neurologic sequelae. (Reproduced with permission from Moss AJ, Daubert
JP: Images in clinical medicine. Internal ventricular fibrillation. N Engl J Med 2000;342:398.)

Sinus rhythm

Torsade de pointes

Ventricular fibrillation

and sinus rhythm

Discharge