Critical Volume of Human Myocardium Necessary to Maintain Ventricular Fibrillation

Abnormal QT intervals, long QT or short QT, have been epidemiologically linked with sudden cardiac death because of ventricular fibrillation (VF). Consequently, Food and Drug Administration recommends testing all pharmacological agents for QT toxicity as a risk factor for cardiac toxicity. Such tests assess QT/QTc interval, which represents ventricular depolarization and repolarization. However, the current QT toxicity analysis does not account for the well-known anisotropy in cardiac tissue conductivity. Mines demonstrated in 1913 that cardiac wavelength (λ) determines inducibility of reentrant arrhythmia, where both repolarization time or action potential duration and conduction velocity determine λ=action potential duration×conduction velocity. We aimed to determine the role of anisotropic wavelength in inducibility of VF in explanted human left ventricular preparations. We tested the hypothesis that 3-dimensional cardiac wavelength, which takes into account anisotropic cardiac tissue conductivity, can accurately predict VF sustainability.

We conducted panoramic optical mapping of coronary perfused human left ventricular wedge preparations subjected to pharmacologically induced shortening and prolongation of action potential duration, by I_K,ATP agonist pinacidil and antagonist glybenclamide, respectively. This measured action potential duration, conduction velocity, and thus determined pacing cycle length-dependent wavelengths in longitudinal (λ_L), transverse (λ_TV), and transmural (λ_TM) directions using S1S1 pacing protocol, from which wavelength volume (V_λ) was determined, as V_λ=λ_L×λ_TV×λ_TM, and compared with tissue volume_. We tested a hypothesis that tissue volume/V_λ ratio can predict VF sustainability.

At baseline, at pacing rate of 240 beats per minute, the wavelengths were λ_L=9.6±0.6 cm, λ_TV=4.2±0.3 cm, and λ_TM=5.8±0.2 cm, respectively (n=7), and thus V_λ=246.4±42.1 cm³. Administration of pinacidil at escalating concentrations progressively decreased V_λ, and VF became sustained, when tissue volume/V_λ was above safety factor κ=4.4±0.6 (n=9) during rapid pacing. Treatment with glybenclamide decreased V_T/V_λ below κ at any pacing rate and prevented VF sustainability.

Sustained VF was only sustained in ventricular volume exceeding critical V_λ=λ_L×λ_TV×λ_TM.