Summary
Background
Differences in the duration of the excitable gap along the reentry circuit during typical atrial flutter are poorly known.
Aim
To prospectively evaluate and compare the duration and composition of the excitable gap during typical counterclockwise atrial flutter in different parts of the circuit all around the tricuspid annulus.
Methods
The excitable gap was determined by introducing a premature stimulus at various sites around the tricuspid annulus during typical counterclockwise atrial flutter in 34 patients. Excitable gap was calculated as the difference between the longest resetting coupling interval and the effective atrial refractory period.
Results
The duration of the excitable gap, the effective atrial refractory period and the resetting coupling interval differed significantly along the tricuspid annulus. Duration of excitable gap was significantly longer at the low lateral right atrium (79 ± 22 ms) than at the cavotricuspid isthmus (66 ± 23 ms; P = 0.002). The effective atrial refractory period was significantly longer at the cavotricuspid isthmus (160 ± 26 ms) than at the high lateral right atrium (149 ± 29 ms; P = 0.004). Other locations, such as coronary sinus ostium, right atrial septum and atrial roof displayed intermediate values.
Conclusion
The duration of the excitable gap differed significantly along the tricuspid annulus, with a larger excitable gap at the lateral right atrium and a shorter excitable gap at the cavotricuspid isthmus, because of longer refractory periods at the isthmus.
Résumé
Contexte
Les différences dans la durée et la composition de la fenêtre d’excitabilité (EG) le long du circuit de réentrée dans le flutter typique (AFL) sont mal connues chez l’homme.
Objectifs
Évaluer et comparer la durée et composition de la fenêtre d’excitabilité du flutter typique antihoraire à divers sites du circuit de réentrée autour de l’anneau tricuspidien.
Méthodes
L’EG a été déterminé en introduisant un stimulus prématuré à divers sites autour de l’anneau tricuspidien en cours de AFL antihoraire chez 34 patients. EG a été calculé par la différence entre le couplage le plus long recyclant le flutter (RCI) et la période réfractaire atriale (EARP).
Résultats
Les durées de l’EG, EARP et RCI diffèrent significativement le long de l’anneau tricuspidien. L’EG était plus long dans l’oreillette droite latérale basse (79 ± 22 ms) que sur l’isthme cavo-tricuspidien (CTI) (66 ± 23 ms ; p = 0,002). Les EARP étaient plus longues sur le CTI (160 ± 26 ms) que dans l’oreillette droite latérale haute (149 ± 29 ms ; p = 0,004). Les autres localisations comme l’ostium du sinus coronaire, le septum inter-atrial droit ou le toit de l’oreillette droite montraient des valeurs intermédiaires.
Conclusion
La durée de l’EG diffère significativement le long de l’anneau tricuspidien, avec un EG plus large dans l’oreillette droite latérale et un EG plus court sur le CTI du fait de périodes réfractaires plus longues sur l’isthme.
Background
The mechanism of typical atrial flutter AFL in humans is based on a large macro-reentry located in the right atrium, with a counterclockwise reentrant wavefront rotating around the tricuspid annulus and surrounding a central obstacle formed by the inferior vena cava and adjacent areas of functional block .
Reentry in AFL displays an excitable gap (EG) between the tail of the refractory period of the previous wave and the front of the next activation . The EG can be defined as the part of the cycle during which the tachycardia can be advanced by a premature stimulus , and can be clinically evaluated by analysing the resetting response after a premature stimulation .
In humans, the EG during typical AFL has a duration ranging from 30–100 ms, representing 15–40% of the atrial flutter cycle length (AFLCL) , and can be divided into fully and partially excitable parts . The presence of a large EG is particularly relevant for the stability of the arrhythmia, for the ability to stop reentry acutely using antiarrhythmic drugs , and for the possibility of capture of the circuit by pacing for entrainment and termination of the tachycardia .
Determination of the duration and composition of the EG in humans has been performed mainly at the cavotricuspid isthmus (CTI) or at the lateral right atrial wall . To date, only one study has investigated the EG at various parts of the right atrium . The aim of this study was to prospectively evaluate and compare the duration and composition of the EG during typical counterclockwise AFL in different parts of the circuit all around the tricuspid annulus.
Methods
Thirty-four consecutive patients referred for radiofrequency (RF) ablation of typical counterclockwise AFL were prospectively included. Only patients with typical AFL, with negative sawtooth waves in the inferior leads, counterclockwise right atrial activation and successful RF ablation at the CTI were included. Patients with a previous history of atrial RF ablation were not included.
AFL was present at the beginning of the procedure in each patient. One standard quadripolar electrode-catheter (interelectrode distance of 10 mm) and one roving RF catheter (Bard Stinger, with 8-mm tip and 10-mm interelectrode distance at the distal dipole) were inserted percutaneously via the right femoral vein. The quadripolar catheter was positioned under fluoroscopic guidance along the tricuspid annulus at the right atrial lateral wall, to enable stable pacing/detection from the high right atrium (HRA; 10 o’clock position in left anterior oblique view) and the low right atrium (LRA; 8 o’clock), using the proximal and distal dipoles, respectively ( Fig. 1 ). The tip of the RF catheter was then successively placed at different locations along the tricuspid annulus: at the CTI (6 o’clock), at the coronary sinus ostium (CSO; 4 o’clock), at the right atrial septum (Sept; 2 o’clock) and at the atrial roof (AR; 12 o’clock), for resetting manoeuvres ( Fig. 1 ). Filtered and amplified bipolar intracardiac electrograms were recorded on a Cardiolab ® system (Prucka Eng., Houston, TX, USA). Measurements were made at a speed of 100 mm/s. Regularity of AFL was checked at baseline, and patients with any alternans in AFLCL or differences greater than 10 ms between successive or non-successive cycles were excluded.
The EG during AFL was determined once at each atrial pacing site before any RF application, by introducing a premature stimulus and analysing the effect on AFL as previously described . Bipolar pacing and detection were always performed from the same pairs of electrodes from the quadripolar or RF catheter. Briefly, single extrastimuli (bipolar, 20 mA output, 2 ms duration) were delivered every eight sensed atrial complexes with progressively shorter coupling intervals, in 10 ms decrements, beginning 10 ms below the AFLCL and up to atrial refractoriness. Coupling intervals were measured between the intrinsecoid deflection of the last atrial activation and the spike artefact. Analysis of the following atrial intervals allows determination of when the reentry has been resetted (i.e. the tachycardia has been advanced by the premature stimulus ), indicating that the premature paced beat has entered the reentry circuit during the EG. Lack of atrial capture was easily detected by the lack of resetting of AFL and the lack of anticipated atrial beat, which was always clearly visible, at least in the remaining intracardiac leads.
RCI was defined as the longest coupling interval allowing resetting, and EARP as the longest coupling interval failing to result in an atrial depolarization. The duration of the whole EG was then calculated as the difference between RCI and EARP . Furthermore, for each coupling interval leading to resetting, the duration of the PPI was measured between the spike artefact and the intrinsecoid deflection of the following atrial activation. The response pattern characterized the way the tachycardia is transiently entrained by the coupling intervals, leading to resetting, and is drawn from the evolution of the PPI according to the coupling intervals of the premature paced beat. Response pattern was considered as flat (< 10 ms difference between PPI for any coupling interval allowing resetting), increasing (prolongation of PPI by ≥ 10 ms when decreasing coupling intervals) or mixed (flat pattern for at least two successive coupling intervals and then an increasing pattern while still decreasing coupling intervals) . In case of mixed pattern, the duration of the PEG was calculated as the difference between the longest coupling interval leading to an increased PPI and the EARP, whereas duration of the FEG was calculated as the difference between the duration of the PEG and the whole EG (see examples in Figs. 2 and 3 ). A resetting response curve was constructed by plotting the duration of the PPI against the coupling interval, and the mean slope of the ascending part of the curve was evaluated by dividing the increase in PPI by the duration of the PEG .
Finally, to analyse the catheter location with regard to the reentry circuit, we evaluated the differences between PPI and AFLCL, and between AFLCL and RCI.
Statistical analysis
Statistical analysis was performed using the StatView ® programme (Abacus Concepts Inc., version 4.57). Results are expressed as means ± standard deviations (ranges). Statistical comparison between categorical data was performed using Fisher’s exact test, while numerical variables were compared using Student’s unpaired t test. Analysis of variance repeated measures were used to analyse the differences between various pacing locations, and comparisons between sites were then performed using a post-hoc Bonferroni-Dunn test when a significant difference was found in the analysis of variance. A P value < 0.05 was considered statistically significant, except for post-hoc analysis ( P < 0.0033 significant).
Results
Clinical characteristics of the population are listed in Table 1 . Mean AFLCL was 248 ± 27 ms (210–300). RF ablation was successful in every case, with termination of AFL during RF application at the CTI and achievement of complete bi-directional CTI block in each patient. AFL recurred in one patient over a mean follow-up of 15 ± 7 months, who underwent a second and successful procedure.
| Characteristic | |
|---|---|
| Men/women (n/n) | 31/3 |
| Age (years) | 69 ± 8 (53–86) |
| Underlying heart disease | 28 (82) |
| Hypertensive cardiac disease | 3 (9) |
| Coronary artery disease | 11 (32) |
| Idiopathic dilated cardiomyopathy | 4 (12) |
| Valvular | 4 (12) |
| Cor pulmonale | 3 (9) |
| Chronic pericarditis | 1 (3) |
| Miscellaneous a | 2 (6) |
| Antiarrhythmic drugs (amiodarone) | 24 (70) |
| Preserved ejection fraction | 20 (58) |
| Ejection fraction (if altered) | 35 ± 11 (range, 20–50) |
| Previous atrial fibrillation | 14 (41) |
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