Fig. 16.34A . This arrhythmia occurs in various types of chronic heart disease. The atria do not contract and relax sequentially during each cardiac cycle, and thus they do not contribute to ventricular filling. Instead, the atria undergo a continuous, uncoordinated rippling motion. P waves do not appear on the ECG; instead, the tracing shows continuous irregular fluctuations in potential called f waves. The AV node is activated at intervals that may vary considerably from cycle to cycle. Hence, no constant interval occurs between successive QRS complexes or between successive ventricular contractions. Because the strength of ventricular contraction depends on the interval between beats (see ), the volume and rhythm of the pulse are irregular. In many affected patients, the atrial reentry loop and the pattern of AV conduction are more regular than they are in atrial fibrillation. The rhythm is then referred to as atrial flutter.
•Fig. 16.34 Atrial(A)andventricular(B)fibrillation.
Atrialfibrillationandflutterarenotusuallylife-threatening;somepeoplewiththesedisturbancescanfunctionnormally.However,becausetheatriadonotcontractandrelaxrhythmically,bloodclotstendtoformintheatria.Suchclots,ifdislodged,maythentraveltothepulmonaryorsystemicvascularbeds.Patientswithatrialfibrillationorflutteraregenerallytreatedwithanticoagulantdrugssuchasdicumaroltopreventtheformationofsuchclots.Ventricularfibrillation,incontrast,leadstolossofconsciousnesswithinafewseconds.Theirregular,continuous,uncoordinatedtwitchingoftheventricularmusclefiberspumpsnoblood.Deathensuesunlessimmediateresuscitationisachievedortherhythmspontaneouslyrevertstonormal,whichrarelyoccurs.Ventricularfibrillationmaysupervenewhentheentireventricle,orsomeportionofit,isdeprivedofitsnormalbloodsupply.Itmayalsooccurasaresultofelectrocutionorinresponsetocertaindrugsandanesthetics.OntheECG(see
Fig.16.34B ),thefluctuationsinpotentialarehighlyirregular.
Ventricular fibrillation is often initiated when a premature impulse arrives during the vulnerable period of the cardiac cycle. This period coincides with the downslope of the T wave on the ECG. During this period, the excitability of cardiac cells varies spatially. Some fibers are still in their effective refractory periods, others have almost fully recovered their excitability, and still others are able to conduct impulses, but only at very slow conduction velocities. As a consequence, the action potentials are propagated over the chambers in many irregular wavelets that travel along circuitous paths and at various conduction velocities. As a region of cardiac cells becomes excitable again, it is ultimately activated by one of the wave fronts traveling around the chamber. Hence, the process is self-sustaining.
•Fig. 16.35 Electrocardiogramsrecordedfromanormalsubject(A) andfromapatientwithlongQTsyndrome(B). Q SQ S P PT T R R 1 sec. Normal Q SQ S P P T R R Long QT A B