Pacemaker activity can be terminated with a single stimulus at the proper time. 1 This phenomenon (annihilation) is reported in patients with modulated parasystolic rhythms and idioventricular and junctional escape rhythms. 2,3 We report a case of annihilation in sinus node cells.A 68-year-old female patient with a history of hypertension and an earlier admission for syncope was re-admitted with orthopnea and congestive heart failure, which was caused by a tachyarrhythmia with a heart rate between 130 and 150 bpm of unknown duration. Electric cardioversion was attempted after drug preloading; we did not manage to achieve a stable sinus rhythm and rate control therapy was initiated.On the previous admission, the patient's only medication was enalapril for hypertension. Resting ECG did not show sinus bradycardia or any arrhythmia indicative of sinoauricular block. The patient was monitored for syncope and the arrhythmia shown in the Figure below was detected. The first part of the recording shows sinus node activity with a fixed Figure. An atrial premature beat in the second half of the sinus cycle causes sinus arrest with a slow escape rhythm (continuous rhythm strips).cycle length of 700 msec, which is then terminated by an early atrial extrasystole arising in the second half of the sinus node cycle with a coupling time of 520 msec and is followed by a slow escape rhythm. Sinus node activity did not return for at least 12.20 seconds, but the return was not documented. A VVI pacemaker was implanted and the patient has been asymptomatic in the last 5 years since then.Stimuli of critical amplitude and phase can lead to annihilation of normal rhythmic activity of the sinus node and can cause syncope.The exact mechanism of sinus node annihilation is not known. The latest studies suggest that a critical stimulus amplitude-phase combination plays an important role in the impulses reaching the central cells of the sinus node complex and in annihilating their activity. 4 References 1. Jelife J, Antzelevich C: Phase resetting and annihilation of pacemaker activity in cardiac tissue. Science 1979;206:695-697. 2. Castellanos A, Luceri RM, Moleiro F, Kayden DS, Trohman RG, Zaman L, Mayerburg RJ: Annihilation, entrainment and modulation of ventricular parasystolic rhythms. Am J Cardiol 1984;54:317-322. 3. Tenczer J, Littmann L, Rohla M: Termination of escape rhythms by properly timed extrastimulus. The role of pacemaker annihilation. Orvosi Hetilap 1988;18:925-929. 4. Tsalikakis DG, Zhang HG, Fotiadis DI, Kremmydas GP, Michalis LK: Phase response characteristics of sinoatrial node cells. Comput Biol Med 2007;37:8-20.
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