Disruption of ventricular activation by subthreshold delayed afterdepolarizations in RyR2-R420Q catecholaminergic polymorphic ventricular tachycardia

Background

Catecholaminergic polymorphic ventricular tachycardia (CPVT) carries increased risk of ventricular arrhythmias due to altered Ca²⁺ regulation associated with mutations in the ryanodine receptor (RyR2). Increased Ca²⁺ leak is believed to result in diastolic Ca²⁺ waves and delayed afterdepolarization (DADs) in cardiac myocytes, but it is uncertain how these cellular events induce ventricular tachycardia in the whole heart. We utilised a transgenic mouse model of human RyR2-R420Q (R420Q) CPVT mutation and a range of electrical and optical mapping technologies to examine the role of DAD-induced conduction abnormalities.

Methods

Heterozygous R420Q and wildtype (WT) control hearts were perfused on a Langendorff apparatus. Electrical activity was monitored using volume conducted ECG electrodes and monophasic action potential (MAP) electrode recordings. Left ventricular activation and membrane potential changes were recorded using an 8 × 8 multielectrode array and optical mapping, respectively.

Results

ECG recordings showed spontaneous ventricular arrhythmias in isolated R420Q hearts. More severe arrhythmias occurred in R420Q hearts following rapid electrical pacing combined with isoproterenol stimulation. Ventricular activation time was not different between genotypes, regardless of stimulation frequency or isoproterenol. Phase differences in local activation times were greater in R420Q hearts during 10 Hz pacing with isoproterenol, suggesting local conduction slowing. Optical mapping experiments revealed subthreshold DADs occurring in R420Q hearts during diastolic pauses. DADs prolonged the subsequent action potential and were associated with conduction slowing during the second beat after the DAD, but not the first beat. 2D tissue simulations revealed that direct inactivation of I_Na during DADs, or indirectly via cycle length dependent refractory mechanisms could account for local conduction slowing.

Conclusions

Increased activation dispersion could arise from subthreshold DADs in R420Q mouse hearts and may contribute to conduction block. This could increase the propensity for re-entrant arrhythmias in CPVT without directly triggering ectopic beats.