Propagation of the impulse is abnormal in Fig. 16.32B, resulting in a completely different configuration of the QRS complex and T wave compared to normal ventricular deflections. This abnormal impulse originates from an ectopic focus in the ventricles. The time interval between the premature QRS complex and the preceding normal QRS complex is shortened, while the interval after the premature QRS complex and the next normal QRS complex is prolonged by a compensatory pause. The interval from the QRS complex just before the premature excitation to the QRS complex just after it is approximately equal to the duration of two normal cardiac cycles.

In Fig. 16.33, there are two types of paroxysmal tachycardias shown. In A, it is a supraventricular tachycardia where P waves precede each QRS complex. In B, it is a ventricular tachycardia where P waves are not readily observed.

A compensatory pause typically follows a premature ventricular depolarization. This pause occurs because the ectopic ventricular impulse does not disrupt the natural rhythm of the SA node. There are two possible reasons for this: either the ectopic ventricular impulse does not conduct in a retrograde direction through the AV conduction system, or the SA node has already fired at its natural interval before the ectopic impulse could reach and prematurely depolarize it. Similarly, the SA nodal impulse generated just before or after the ventricular extrasystole generally does not affect the ventricle because the AV junction and possibly the ventricles are still refractory from the premature ventricular excitation.

Paroxysmal supraventricular tachycardia and paroxysmal ventricular tachycardia are usually indistinguishable (Fig. 16.33A), hence the term paroxysmal supraventricular tachycardia refers to both types. In this tachycardia, the impulse often circulates an entry loop that involves atrial and AV junctional tissue. The QRS complexes are frequently normal as ventricular activation proceeds over the usual pathways. Paroxysmal ventricular tachycardia, as the name implies, originates from an ectopic focus in the ventricles. The ECG shows repeated bizarre QRS complexes that reflect abnormal intraventricular impulse conduction (see Fig. 16.33B). Paroxysmal ventricular tachycardia is more dangerous than supraventricular tachycardia because it often precedes ventricular fibrillation, a life-threatening arrhythmia described in the next section.

Unlike sinus tachycardia which has gradual rate changes, tachycardias originating from an ectopic focus begin and end abruptly. These ectopic tachycardias are commonly referred to as paroxysmal tachycardias. Episodes of paroxysmal tachycardia can last for only a few beats or for extended periods of time, ranging from hours to days, with episodes often recurring. Paroxysmal tachycardias can occur due to rapid firing of an ectopic pacemaker, triggered activity secondary to afterpotentials reaching threshold, or repetitive circling of an impulse within a reentry loop.

Under certain conditions, cardiac muscle undergoes irregular contractions that are ineffective in propelling blood. This arrhythmia is known as fibrillation and can involve either the atria or the ventricles. Fibrillation is thought to be a reentry phenomenon where the reentry loop breaks into multiple, irregular circuits.

The electrocardiographic changes in atrial fibrillation are illustrated.
Fig. 16.32B . Propagation of the impulse is abnormal, and the configuration of the QRS complex and T wave is entirely different from the normal ventricular deflections because the premature excitation originates at some ectopic focus in the ventricles. The time interval between the premature QRS complex and the preceding normal QRS complex is shortened, whereas the interval after the premature QRS complex and the next normal QRS complex is prolonged by a compensatory pause. The interval from the QRS complex just before the premature excitation to the QRS complex just after it is virtually equal to the duration of two normal cardiac cycles.
•Fig. 16.33 Paroxysmaltachycardias.A, supraventriculartachycardia;PwavesprecedeeachQRScomplex.B, ventriculartachycardia;Pwavesnotreadilyobserved.
As noted, a compensatory pause usually follows a premature ventricular depolarization. This pause occurs because the ectopic ventricular impulse does not disturb the natural rhythm of the SA node. There are two possible reasons for this: The ectopic ventricular impulse is not conducted in a retrograde direction through the AV conduction system, or the SA node had already fired at its natural interval before the ectopic impulse could have reached and depolarized it prematurely. Likewise, the SA nodal impulse generated just before or after the ventricular extrasystole generally does not affect the ventricle because the AV junction and perhaps also the ventricles are still refractory from the premature ventricular excitation.
Fig.16.33A )areusuallyindistinguishable,andthereforethetermparoxysmal supraventricular tachycardia referstobothtypes.Inthistachycardia,theimpulseoftencirclesareentryloopthatincludesatrialandAVjunctionaltissue.TheQRScomplexesarefrequentlynormalbecauseventricularactivationproceedsovertheusualpathways.Asitsnameimplies,paroxysmalventriculartachycardiaoriginatesfromanectopicfocusintheventricles.TheECGischaracterizedbyrepeatedbizarreQRScomplexesthatreflecttheabnormalintraventricularimpulseconduction(see
Fig.16.33B ).Paroxysmalventriculartachycardiaismuchmoreominousthansupraventriculartachycardiabecausetheformerisfrequentlyaprecursorofventricularfibrillation,alethalarrhythmiadescribedinthenextsection.
In contrast to the gradual rate changes that characterize sinus tachycardia, tachycardias that originate from an ectopic focus typically begin and end abruptly. Such ectopic tachycardias are generally called paroxysmal tachycardias. Episodes of paroxysmal tachycardia may persist for only a few beats or for many hours or days, and episodes often recur. Paroxysmal tachycardias may result from (1) rapid firing of an ectopic pacemaker, (2) triggered activity secondary to afterpotentials that reach threshold, or (3) an impulse that circles a reentry loop repetitively.
Under certain conditions, cardiac muscle undergoes an irregular type of contraction that is ineffectual in propelling blood. Such an arrhythmia is termed fibrillation, and the disturbance may involve either the atria or the ventricles. Fibrillation probably represents a reentry phenomenon in which the reentry loop fragments into multiple, irregular circuits.
The electrocardiographic changes in atrial fibrillation are shown in