Paradoxical atrial undersensing by a dual chamber pacemaker during atrial fibrillation

This report describes paradoxical atrial undersensing by a dual chamber pacemaker in a patient with paroxysmal atrial fibrillation. Atrial undersensing was present only when the device was programmed to a high sensitivity but sensing normalized when a lower sensitivity was programmed. This unusual response should be differentiated from the recently documented lock-in behavior of pacemakers delivering managed ventricular pacing. (Cardiol J 2012; 19, 2: 207–209

Usefulness of the 12-lead electrocardiogram in the follow-up of patients with cardiac resynchronization devices. Part I

Cardiac resynchronization therapy (CRT) has added a new dimension to the electrocardiographic evaluation of pacemaker function. During left ventricular (LV) pacing from the posterior or posterolateral coronary vein, a correctly positioned lead V1 registers a tall R wave and there is right axis deviation in the frontal plane with few exceptions. During simultaneous biventricular stimulation from the right ventricular (RV) apex and LV site in the coronary venous system, the QRS complex is often positive (dominant) in lead V1 and the frontal plane QRS axis usually points to the right superior quadrant and occasionally the left superior quadrant. The reported incidence of a dominant R wave in lead V1 during simultaneous biventricular pacing (RV apex) varies from 50% to almost 100% for reasons that are not clear. During simultaneous biventricular pacing from the posterior or posterolateral coronary vein with the RV lead in the outflow tract, the paced QRS in lead V1 is often negative and the frontal plane paced QRS axis is often directed to the right inferior quadrant (right axis deviation). A negative paced QRS complex in lead V1 during simultaneous biventricular pacing with the RV lead at the apex can be caused by incorrect placement of the lead V1 electrode (too high on the chest), lack of LV capture, LV lead displacement, pronounced latency (true exit block), conduction delay around the LV stimulation site, ventricular fusion with the intrinsic QRS complex, coronary venous LV pacing via the middle or anterior cardiac vein, unintended placement of two leads in the RV and severe conduction abnormalities within the LV myocardium. Most of these situations can cause a QS complex in lead V1 which should be interpreted (excluding fusion) as reflecting RV preponderance in the depolarization process. Barring the above causes, a negative complex in lead V1 is unusual and it probably reflects a different activation of a heterogeneous biventricular substrate (ischemia, scar, His-Purkinje participation). The latter is basically a diagnosis of exclusion. With a non-dominant R wave in lead V1, programming the V-V interval with LV preceding RV may bring out a diagnostic dominant R wave in lead V1 representing the contribution of LV stimulation to the overall depolarization process. In this situation the emergence of a dominant R wave confirms the diagnosis of prolonged LV latency (exit delay) or an LV intramyocardial conduction abnormality near the LV pacing site but it rules out the various causes of LV lead malfunction or misplacement. (Cardiol J 2011; 18, 5: 476–486

Usefulness of the 12-lead electrocardiogram in the follow-up of patients with cardiac resynchronization devices. Part II

The interval from the pacemaker stimulus to the onset of the earliest paced QRS complex (latency) may be prolonged during left ventricular (LV) pacing. Marked latency is more common with LV than right ventricular (RV) pacing because of indirect stimulation through a coronary vein and higher incidence of LV pathology including scars. During simultaneous biventricular (BiV) pacing a prolonged latency interval may give rise to an ECG dominated by the pattern of RV pacing with a left bundle branch block configuration and commonly a QS complex in lead V1. With marked latency programming the V-V interval (LV before RV) often restore the dominant R wave in lead V1 representing the visible contribution of the LV to overall myocardial depolarization. When faced with a negative QRS complex in lead V1 during simultaneous BiV pacing especially in setting of a relatively short PR interval, the most likely diagnosis is ventricular fusion with the intrinsic rhythm. Fusion may cause misinterpretation of the ECG because narrowing of the paced QRS complex simulates appropriate BiV capture. The diagnosis of fusion depends on temporary reprogramming a very short atrio-ventricular delay or an asynchronous BiV pacing mode. Sequential programming of various interventricular (V-V) delays may bring out a diagnostic dominant QRS complex in lead V1 that was previously negative with simultaneous LV and RV apical pacing even in the absence of an obvious latency problem. The emergence of a dominant R wave by V-V programming strongly indicates that the LV lead captures the LV from the posterior or the posterolateral coronary vein and therefore rules out pacing from the middle or anterior coronary vein. In some cardiac resynchronization systems LV pacing is achieved with the tip electrode of the LV lead as the cathode and the proximal electrode of the bipolar RV as the anode. This arrangement creates a common anode for both RV and LV pacing. RV anodal capture can occur at a high LV output during BiV pacing when it may cause slight ECG changes. During LV only pacing (RV channel turned off) RV anodal pacing may also occur in a more obvious form so that the ECG looks precisely like that during BiV pacing. RV anodal stimulation may complicate threshold testing and ECG interpretation and should not be misinterpreted as pacemaker malfunction. Programming the V-V interval (LV before RV) in the setting of RV anodal stimulation cancels the V-V timing to zero. (Cardiol J 2011; 18, 6: 610–624

Diagnostic challenge of artifactual electrocardiographic tachyarrhythmias

Electrocardiographic artifacts may generate recordings mimicking supraventricular and ventricular tachyarrhythmias. This report describes the diagnostic challenge presented by Holter or loop recordings in two patients, one with pseudo-atrial flutter and the other with pseudo- -polymorphic ventricular tachycardia