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