Differential interaction of adrenergic and cholinergic effects on AV junctional automaticity and AV conduction

Physiology of the escape rhythm after radiofrequency atrioventricular junctional ablation

The physiology of the escape rhythm (ER) and its response to pharmacological modulation under varying autonomic conditions were studied 48 patients undergoing radiofrequency ablation of the atrioventricular junction (AVJ) for refractory atrial fibrillation. The QRS morphology and cycle length (CL) of the baseline ER were measured 15 minutes postablation. The CL of the ER was measured in response to doses of isoproterenol, atropine, adenosine, lidocaine, and verapamil. The ER QRS was narrow (QRS < 120 ms) in 20 patients and wide (QRS > 120 ms) in 28 patients. Of the 28 patients with wide QRS ER, 11 patients had a new bundle branch block (8 patients new right bundle branch block [RBBB] and 2 patients new left bundle branch block [LBBB]). The ERCL was similar in both narrow and wide ERs (1,593 +/- 376 ms and 1,516 +/- 296 ms, P = 0.44). In 23 patients receiving isoproterenol infusion, the ER CL decreased with increasing doses from 1 mcg/min to 2 mcg/min (1,378 +/- 200 to 1,240 +/- 229 ms, P < 0.001), but did not decrease further at 3 mcg/min (1,201 +/- 192 ms, P = 0.48 vs 2 mg/min). Seven patients received 0.02 mg/kg of atropine, and ER decreased significantly (1,572 +/- 408 ms to 1,319 +/- 333 ms, P = 0.028). In 30 patients who received intravenous boluses of adenosine (6-18 mg), the ER did not change significantly. In 28 patients who received 150 mg of lidocaine, the ER increased from 1,462 +/- 286 ms to 1,715 +/- 467 ms (P < 0.001), and one patient developed transient asystole. Nineteen patients received 7.5 mg of verapamil, and the ER did not change (1,488 +/- 313 ms to 1,513 +/- 666 ms, P = 0.80). There was no significant difference in response to isoproterenol, adenosine, lidocaine, or verapamil between the patients with wide and narrow QRS ERs. We conclude that patients may have stable ERs immediately following AVJ ablation even when a wide complex ER results. The ER is responsive to sympathetic stimulation and vagal blockade. The ER is prolonged after lidocaine but not after verapamil, suggesting response to sodium but not to calcium channel blockade. These data are consistent with an ER originating in the distal compact AV node or proximal His bundle.

Characterization of junctional rhythm after atrioventricular node ablation

BACKGROUND

Catheter ablation of the atrioventricular (AV) node with radiofrequency current (RFC) is associated with the short-term onset of a junctional escape rhythm (JER) in nearly all patients. However, the origin of the JER and short-term thermal effects of RFC on this junctional pacemaker activity are ill defined.

METHODS AND RESULTS

Short-term and noninvasive long-term follow-up studies were performed to examine the electrophysiological characteristics of the underlying JER in 45 patients who had undergone AV nodal ablation with RFC. Baseline characteristics and responses to overdrive ventricular pacing and intravenous atropine followed by an incremental isoproterenol infusion were determined. Short- and long-term responses were compared. HV intervals before and after ablation were 49 +/- 9 and 48 +/- 9 milliseconds, respectively (P = NS). Follow-up was 11 +/- 8.3 months. JER cycle length was 1526 +/- 298 milliseconds in the short-term setting and was present in 44 patients (98%) in the long-term setting, measuring 1426 +/- 223 milliseconds (P < .005). Junctional recovery times increased exponentially as overdrive pacing rates increased-there was no difference between short-term and long-term responses. Drug responses within each study were all significant when compared with baseline. However, there was no significant difference between short- and long-term responses, except at the highest dose of isoproterenol. Intravenous atropine (1 mg) caused an 8.6 +/- 9.3% decrease in JER cycle length in the short-term setting compared with a 7.6 +/- 7.3% decrease in the long-term setting. The decreases in JER cycle length with isoproterenol infusion (short-term versus long-term) were 10.1 +/- 9.6% versus 9.6 +/- 7.4% with 1 microgram/min, 15.8 +/- 11.7% versus 17.4 +/- 8.5% with 2 micrograms/min, 17.9 +/- 11.2% versus 21.4 +/- 9.1% with 3 micrograms/min (all P = NS), and 20.6 +/- 12.1% versus 24.8 +/- 9.1% with 4 micrograms/min (P < .01).

CONCLUSIONS

Radiofrequency ablation of the AV node is associated with development of a JER that is stable in the long-term setting. The lack of change in HV interval after ablation locates the junctional pacemaker proximal to the central fibrous body. The pattern of drug responses suggests an origin within the proximal His bundle at its junction with the AV node rather than the AV node itself. The overall similarity between short- and long-term characteristics of junctional pacemaker activity mitigates against any reversible thermal effects of RFC on this pacemaker focus.

Quantitative effects of sympathetic and vagal nerve stimulations on sinus and AV junctional rhythms

Responses of the sinus node and atrioventricular (AV) junctional pacemakers to autonomic denervation and to individual stimulations of the right and left stellate and both vagi were studied in 33 anesthetized dogs. Autonomic denervation depressed sinus node automaticity by only 18% from control, whilst AV junctional automaticity was reduced by 48.5% from control. Sympathetic and parasympathetic stimulation frequency-response curves (0.25, 0.5, 1, 2, 4, 8, 16 and 32 Hz) were obtained. In the sinus node the chronotropic responses to sympathetic stimulations reflect a bilaterally asymmetrical innervation with a right sided preponderance. In contrast, sinus slowing in response to either right or left vagal stimulations were indistinguishable when lower frequencies of stimulation were used. At 4 Hz and higher frequencies there is a right vagal preponderance. The AV junctional chronotropic responses suggest that this major subsidiary pacemaker receives a bilaterally symmetrical autonomic innervation. The chronotropic responses to individual nerve stimulations expressed as percent changes in sinus rate and AV junctional rate from their respective controls after autonomic denervation show that the AV junction is far more responsive than the sinus node to both sympathetic and parasympathetic stimulations. To allow for more meaningful comparisons the data were normalized using the respective maximum increase and maximum decrease of sinus node and AV junctional rates to left and right sympathetic and parasympathetic stimulations as the 100% reference. These normalized curves show that 50% of the maximal chronotropic responses were always achieved at a lower stimulus frequency in the AV junction than in the sinus node; shift of the AV junctional response curves to the left of the sinus node response curves by a 0.2 (sympathetic) and 0.3 (parasympathetic) log units was observed. These studies further showed that sympathetic activity in the AV junction is an absolute prerequisite to maintain regular AV junctional rhythms especially during the bradycardic episodes evoked in the study of vagal stimulus frequency-response curves.