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Europace 2003 5(1):95-102; doi:10.1053/eupc.2002.0281
© 2003 by European Society of Cardiology
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Mode switching failure during atrial flutter: the ‘2:1 lock-in’ phenomenon

M. Goethals, W. Timmermans, P. Geelen, J. Backers and P. Brugada

Cardiovascular Centre Aalst, OLV Ziekenhuis Moorselbaan 164, Aalst, 9300, Belgium

AIMS: To evaluate incidence and mechanism of a special form of automatic mode switching (MS) failure in patients with atrial flutter.

METHODS AND RESULTS: Retrospectively the charts of 134 patients implanted with dual chamber pacemakers with MS algorithms were reviewed. Seven patients (5.2%) were identified that presented with sustained rapid ventricular pacing resulting from atrial flutter with failure of automatic MS. Since this form of MS failure implies 2:1 tracking of atrial flutter, it was coined ‘2:1 lock-in’. A theoretical timing model was developed to clarify the mechanism of this special form of MS failure. Prerequisites for the ‘2:1 lock-in’ phenomenon are: (1) the sum of the AV delay and the post ventricular blanking (PVAB) must be longer than the cycle length of the atrial flutter, (2) the tachycardia detection rate must be higher than half the atrial flutter rate and (3) the maximum tracking rate (MTR) must be higher than half the atrial flutter rate. Recommendations for programming in order to avoid this specific form of MS failure are made accordingly and parallel algorithms for flutter detection are discussed.

CONCLUSION: ‘2:1 lock-in’ is a typical form of MS failure in patients with atrial flutter and the mechanism is closely linked to the typical atrial sensing windows.

Key Words: Mode switching failure, atrial flutter, dual chamber pacemakers

Correspondence: Marc A. Goethals, Cardiovascular Centre, OLV Ziekenhuis Aalst, Moorselbaan 164, 9300 Aalst, Belgium. Tel.: 32 53 724439; Fax: 32 53 724185; E-mail: marc.goethals{at}olvz-aalst.be


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