Structural heterogeneity promotes triggered activity, reflection and arrhythmogenesis in cardiomyocyte monolayers

Non-technical summary  The heartbeat depends on the spread of electrical waves through the cardiac muscle. If that spread becomes disturbed, arrhythmias and death may ensue. Patients with heart disease are predisposed to cardiac arrhythmias by unidentified mechanisms. Using both experiments and computer models we demonstrate that structural defects in the heart leading to contiguous areas of physical narrowing and expansion of the musculature can alter the spread of the waves, in such a way that some waves may return abnormally along the same narrow pathway as the original electrical wave (reflection), leading to extra beats and arrhythmia initiation. The possibility of reflection is enhanced when structural defects combine with alterations in the sodium channels responsible for the electrical waves, such as seen in inherited and acquired cardiac electrical diseases. Our results provide a novel mechanism whereby a substrate (structural heterogeneity) and a trigger (abnormal sodium channel activity) combine to promote life-threatening arrhythmia initiation. Abstract  Patients with structural heart disease are predisposed to arrhythmias by incompletely understood mechanisms. We hypothesized that tissue expansions promote source-to-sink mismatch leading to early after-depolarizations (EADs) and reflection of impulses in monolayers of well-polarized neonatal rat ventricular cardiomyocytes. We traced electrical propagation optically in patterned monolayers consisting of two wide regions connected by a thin isthmus. Structural heterogeneities provided a substrate for EADs, retrograde propagation along the same pathway (reflection) and reentry initiation. Reflection always originated during the action potential (AP) plateau at the distal expansion. To determine whether increased sodium current (INa) would promote EADs, we employed adenoviral transfer of Nav1.5 (Ad-Nav1.5). Compared with uninfected and adenoviral expression of green fluorescent protein (Ad-GFP; viral control), Ad-Nav1.5 significantly increased Nav1.5 protein expression, peak and persistent INa density, AP upstroke velocity, AP duration, conduction velocity and EAD incidence, as well as reflection incidence (29.2%, n= 48 vs. uninfected, 9.4%, n= 64; and Ad-GFP, 4.8%, n= 21). Likewise, the persistent INa agonist veratridine (0.05–3 μm) prolonged the AP, leading to EADs and reflection. Reflection led to functional reentry distally and bigeminal and trigeminal rhythms proximally. Reflection was rare in the absence of structural heterogeneities. Computer simulations demonstrated the importance of persistent INa in triggering reflection and predicted that the gradient between the depolarizing cells at the distal expansion and the repolarizing cells within the isthmus enabled retrograde flow of depolarizing electrotonic current to trigger EADs and reflection. A combination of a substrate (structural heterogeneity) and a trigger (increased persistent INa and EADs) promotes reflection and arrhythmogenesis.