Influence of the Purkinje-muscle junction on transmural repolarization heterogeneity

Abstract

Aims: To elucidate the properties of the PMJ and myocardium underlying these effects. Transmural heterogeneity of action potential duration (APD) is known to play an important role in arrhythmogenesis. Regions of non-uniformities of APD gradients often overlap considerably with the location of Purkinje-muscle junctions (PMJs). We therefore hypothesized that such junctions are novel sources of local endocardial and transmural heterogeneity of repolarization, and that remodelling due to heart failure modulates this response.

Methods and results: Spatial gradients of endocardial APD in left ventricular wedge preparations from healthy sheep (n = 5) were correlated with locations of PMJs identified through Purkinje stimulation under optical mapping. APD prolongation was dependent on proximity of the PMJ to the imaged surface, whereby shallow PMJs significantly modulated local APD when stimulating either Purkinje (P = 0.0116) or endocardium (P = 0.0123). In addition, we model a PMJ in 5 × 5× 10 mm transmural tissue wedges using healthy and novel failing human ventricular and Purkinje ionic models. Short distances of the PMJ to cut surfaces (<0.875 mm) revealed that APD maxima were localized to the PMJ in healthy myocardium, whereas APD minima were observed in failing myocardium. Amplitudes and spatial gradients of APD were prominent at functional PMJs and quiescent PMJs. Furthermore, increasing the extent of Purkinje fibre branching or decreasing tissue conductivity augmented local APD prolongation in both failing and non-failing models.

Conclusions: The Purkinje network has the potential to influence myocardial AP morphology and rate-dependent behaviour, and furthermore to underlie enhanced transmural APD heterogeneities and spatial gradients of APD in non-failing and failing myocardium.

Keywords: Conduction system; Optical mapping; Purkinje-muscle junction; Repolarization; Transmural heterogeneity.

Figure 1

APD heterogeneity at origins of activation following pacing of PF. (A) Activation time (AT) map derived from the endocardial surface of sheep LV wedge following pacing of PF. Activation origins indicated by arrows. Regions of slowed conduction correspond to intra-trabeculated grooves. (B) Corresponding APD map. AT (C) and APD (D) maps following direct stimulation of the endocardium. (E) Profiles of AT from pixels along the dashed lines in (A and C). (F) Corresponding APD profiles. AP traces extracted from the origin of activation [box with red arrow in (A) and from the local myocardium (box in A)] when pacing PF (G) or endocardium (H).

Figure 2

Surface APD heterogeneity determined by depth of the PMJ. Optical AP upstroke morphology depends on orientation of wave front relative to the imaged surface. Schematics of transmural propagation patterns and optical AP upstrokes measured from pixels corresponding to the surface location of earliest activation from deep (A) and shallow (B) sources. Normalized amplitude of maximal derivative determines v_F*. (C) v_F* map following PF pacing. Arrows indicate origins of activation from Figure 1A. (D). Disparity of PMJ v_F* observed in (C). (E) v_F* vs. local APD differences between all origins and their local myocardium. Dashed line shows v_F* threshold differentiating origins residing at the tissue surface or deeper myocardium. A linear correlation was observed between v_F* and APD differences from origins located between surface and mid-myocardial layers (R = −0.60). (F) Mean ± SD local APD differences at surface origins. Asterisks indicate that data are significantly different from a hypothetical difference of 0.0 ms (n = 9, one-sample t-test, P < 0.05).

Figure 3

Ionic models and heterogeneous tissue wedge. (A) APs derived from human epicardial, endocardial, and PF ionic models of non-failing (left panel) and failing (right panel) hearts. Shown are the last of a train of 200 APs stimulated at 1 Hz. From the time window shown, ionic state variables were captured at time 0 ms for single-cell APD restitution and 3D computations. (B) APD restitution curves from S1S2 stimulation in non-failing (left panel) and failing (right panel) single-cell ionic models. Single S2 pulses, pre-conditioned by the S1 pulse shown in A, were applied over a range of intervals from 230 to 1000 ms. (C) 3D transmural tissue wedge with a junctioning PF.

Figure 4

Conduction across the PMJ. (A) Impulse propagation in non-failing wedge simulations was initiated by point stimulation at the endocardial surface, superficial to the PMJ, for retrograde activation sequences. The PMJ was 2.5 mm from the transmural cut surface. From left to right, panels are surface activation maps and a transmural cross-section of the wedge through the PMJ, APs from nodes positioned in the myocardium at the centre of the PMJ (black solid trace), at a transmural depth of 5 mm in the myocardium (black dashed trace), the distal-most node of the PF (red solid trace), and at the mid-PF (red dashed trace). The right-most panel is an expanded view of upstrokes from APs at the PMJ. (B) Retrograde activation sequences from failing simulations. Shown are corresponding activation maps and AP traces as in (A). Isochrones are spaced 2 ms.

Figure 5

The PMJ contributes to myocardial APD heterogeneity. (A) APD maps from transmural cross-sections through the PMJ of non-failing simulations for retrograde activation sequences corresponding with Figure 2. (B) APD profile across transmural cross-sections from endocardium to epicardium and intersecting the PMJ (solid line). A control profile in the absence of a PF is shown (dashed line). Equivalent APD map (C) and APD profile (D) for a failing simulation. Isobars have 2 ms intervals.

Figure 6

Transmural APD heterogeneity and proximity of the PMJ to the cut surface. (A) Surface APD maps of non-failing simulations with varying PMJ distances from the cut surface following retrograde activation. (B) Equivalent APD maps for failing simulations. Isobars have 2 ms intervals. (C) Transmural APD profiles extracted from nodes along the transmural cut surface intersecting nodes with maximal APD from non-failing simulations and for each PMJ distance (dashed lines in A). Control profiles in the absence of a PF for each PMJ distance are shown (dashed lines). (D) Equivalent transmural APD profiles for failing simulations. APD restitution for nodes localized in the myocardium at the PMJ and mid-wall and nodes from the PF at the PMJ and mid-fibre derived from constant cycle length pacing with retrograde activation sequences. APD restitution was compared between PMJ distances of 0.25 (E) and 2.5 mm (F). Pacing frequencies associated with alternans are indicated by arrows.

Figure 7

Determinants of transmural APD heterogeneity at the PMJ. Transmural APD profiles intersecting the PMJ from simulations with PMJ at a distance of 2.5 mm from the transmural cut surface. Control profiles in the absence of a PF are shown (dashed lines). R_PMJ for 75%, 100, and 125% in non-failing (A) and failing (B) simulations. Corresponding maps are shown in Figures 2, 3, and 5. PMJs with radii of 400, 600, and 800 µm in non-failing (C) and failing (D) simulations were considered. Corresponding figures are shown in Supplementary material online, Figure S2. Intramyocardial coupling was modified by changes in σ_L to 0.09, 0.21, and 0.33 S/m while preserving anisotropy at 4 : 2 : 1 for non-failing (E) and failing (F) simulations.