Tachy-brady arrhythmias: the critical role of adenosine-induced sinoatrial conduction block in post-tachycardia pauses.

BACKGROUND

In patients with sinoatrial nodal (SAN) dysfunction, atrial pauses lasting several seconds may follow rapid atrial pacing or paroxysmal tachycardia (tachy-brady arrhythmias). Clinical studies suggest that adenosine may play an important role in SAN dysfunction, but the mechanism remains unclear.

OBJECTIVE

To define the mechanism of SAN dysfunction induced by the combination of adenosine and tachycardia.

METHODS

We studied the mechanism of SAN dysfunction produced by a combination of adenosine and rapid atrial pacing in isolated coronary-perfused canine atrial preparations by using high-resolution optical mapping (n = 9). Sinus cycle length and sinoatrial conduction time (SACT) were measured during adenosine (1-100 μM) and DPCPX (1 μM; A1 receptor antagonist; n = 7) perfusion. Sinoatrial node recovery time was measured after 1 minute of "slow" pacing (3.3 Hz) or tachypacing (7-9 Hz).

RESULTS

Adenosine significantly increased sinus cycle length (477 ± 62 ms vs 778 ± 114 ms; P<.01) and SACT during sinus rhythm (41 ± 11 ms vs 86 ± 16 ms; P<.01) in a dose-dependent manner. Adenosine dramatically affected SACT of the first SAN beat after tachypacing (41 ± 5 ms vs 221 ± 98 ms; P<.01). Moreover, at high concentrations of adenosine (10-100 μM), termination of tachypacing or atrial flutter/fibrillation produced atrial pauses of 4.2 ± 3.4 seconds (n = 5) owing to conduction block between the SAN and the atria, despite a stable SAN intrinsic rate. Conduction block was preferentially related to depressed excitability in SAN conduction pathways. Adenosine-induced changes were reversible on washout or DPCPX treatment.

CONCLUSIONS

These data directly demonstrate that adenosine contributes to post-tachycardia atrial pauses through SAN exit block rather than slowed pacemaker automaticity. Thus, these data suggest an important modulatory role of adenosine in tachy-brady syndrome.