First Experience with an Automatic Sensing Algorithm in Single-Lead VDD Stimulation

An "Autosensing" algorithm available in SSI(B) and DDD(R) pacemakers automatically adapts the device's sensitivity to changing intracardiac signals. The atrial sensing function of this algorithm was tested for the first time with a VDD pacing system in which large variations of the atrial signal may occur because the atrial electrodes float in the atrial blood pool. Methods: 15 patients with a VDD pacing system were studied (Unity 292–07, lead 425; Sulzer Intermedics). The atrial sensing threshold was measured, and the atrial sensitivity was programmed with a 2:1 safety margin. The autosensing algorithm and sensitivity profile were temporarily activated, and an ambulatory ECG with continuous marker annotation was recorded. All patients underwent a 30-minute daily life activities protocol. A beat-to-beat analysis of the ambulatory ECG was correlated with the changes in atrial sensitivity. Results: The algorithm changed the baseline sensitivity from 0.57 ± 0.23 mV during the test to 0.39 ± 0.20 mV after the final rest period (P < 0.05). During the test 12.6 ± 10.2 adaptations of the sensitivity occurred (range 0–33). In eight patients atrial undersensing occurred in 4.4%± 7.5% of the cycles (4–458 unsensed P waves]. In these patients, the algorithm continuously adjusted the sensitivity towards more sensitive values, operating 19.1 ± 18.3 changes compared with 5.4 ± 7.3 changes in patients without undersensing (P = 0.009). Oversensing did not occur. Conclusion: The autosensing algorithm effectively optimized atrial sensitivity in VDD pacing. In patients with atrial undersensing the algorithm continuously remained near the most sensitive settings, thus reacting as intended. A faster sensitivity adjustment of the system would be desirable.     

Multiple Atrioventricular Nodal Pathways in Humans:

Multiple AV Nodal Pathways. Introduction : Multiple AV nodal pathway physiology can be demonstrated in certain patients with clinical AV reentrant tachycardia.
Methods and Results : Evidence suggesting multiple AV nodal pathway conduction was present in seven (two males; age range 15 to 75 years) of 78 patients (9%) who underwent electrophysiologic studies for AV nodal tachycardia. The presence of two discrete discontinuities in the AV nodal conduction curves suggested triple AV nodal pathway conduction. Detailed mapping of their retrograde atrial activation sequence was performed along the tricuspid annulus from the coronary sinus ostium to the His-bundle electrogram recording site. Three zones (anterior, middle, and posterior) correspond to the upper, middle, and lower third of the triangle of Koch, respectively. The fast pathway exits were determined as anterior (4/7) or middle (3/7), the intermediate pathway exits as middle (4/7) or posterior (3/7), and the slow pathway exits as middle (1/7) or posterior (6/7). Other evidence suggesting multiple AV nodal pathway conduction includes: (1) triple ventricular depolarizations from a single atrial impulse; (2) sequential dual ventricular echoes; (3) spontaneous transformation between the slow-fast and fast-slow forms of AV nodal reentrant tachycardia; and (4) persistent cycle length alternans during AV nodal reentrant tachycardia. In four patients, all three pathways were shown to be involved in AV nodal echoes or reentrant tachycardia.
Conclusion : Multiple AV nodal pathways are not uncommon and can be identified by careful electrophysiologic elucidation and mapping technique.     

Pleomorphic Ventricular Tachycardia with Antegrade His-Bundle Activation:

His-Bundle Activation During VT. The use of multiple His bundle-right hundle branch recordings in electrophysiologic studies has facilitated definition of the mechanism and elucidation of the direction of impulse propagation in bundle branch reentrant tachycardia, "Mahaim" fiber reciprocating tacbycardia, and retrograde His depolarization in fascieular or ventricular tacbycardias. This report details the electrophysiologic evaluation of pleomorphic ventricular tachycardia in a patient with advanced coronary heart disease. The ventricular tachycardia at baseline revealed variation in the QRS duration without alteration of the elec-trocardiographic (ECG) morphology. Following flecainide administration, a ventricular tachycardia with close resemblance of tbe ECG morpbology to sinus rbytbm was induced. Proximal and distal His-bundle recordings revealed early antegrade His-bundle activation during tbis tacbycardia. Programmed stimulation converted this tachycardia back to tbe clinical ventricular tacbycardia witb intermittent narrowing of the QRS complexes. Early His activation was evident only during the narrower complexes but not in tbe tacbycardia beats witb wide complex. Penetration of the His bundle by ventricular tacbycardia, witb resultant fusion from intramyocardial ventricular activation and His-Purkinje activation, could have accounted for tbe near normalization of tbe QRS morphology during the ventricular tachycardia.     

Catheter Induced Mechanical Stunning of Accessory Pathway Conduction: Useful Guide to Successful Transcatheter Ablation of Accessory Pathways

Established electrophysiological criteria indicating anatomical proximity to an accessory pathway include early ventricular or atrial activation during antegrade or retrograde accessory pathway conduction, recording of accessory pathway potentials, and pace map concordance. This article describes two cases of RF catheter ablation of accessory pathways, during which positioning of the mapping catheter at specific sites on the endocardial aspect of the atrioventricular annulus led to prolongation of accessory pathway refractoriness and/or slowing of conduction. HF energy application at these sites successfully abolished accessory pathway conduction. When observed on an "intentional" basis during catheter mapping, catheter induced stunning of accessory pathway conduction provides evidence of satisfactory electrode-tissue contact in addition to anatomical proximity, and may give additional predictive value to successful transcatheter accessory pathway ablation.