Mechanisms underlying the antifibrillatory action of hyperkalemia in Guinea pig hearts

Abstract

Hyperkalemia increases the organization of ventricular fibrillation (VF) and may also terminate it by mechanisms that remain unclear. We previously showed that the left-to-right heterogeneity of excitation and wave fragmentation present in fibrillating guinea pig hearts is mediated by chamber-specific outward conductance differences in the inward rectifier potassium current (I(K1)). We hypothesized that hyperkalemia-mediated depolarization of the reversal potential of I(K1) (E(K1)) would reduce excitability and thereby reduce VF excitation frequencies and left-to-right heterogeneity. We induced VF in Langendroff-perfused guinea pig hearts and increased the extracellular K(+) concentration ([K(+)](o)) from control (4 mM) to 7 mM (n = 5) or 10 mM (n = 7). Optical mapping enabled spatial characterization of excitation dominant frequencies (DFs) and wavebreaks, and identification of sustained rotors (>4 cycles). During VF, hyperkalemia reduced the maximum DF of the left ventricle (LV) from 31.5 +/- 4.7 Hz (control) to 23.0 +/- 4.7 Hz (7.0 mM) or 19.5 +/- 3.6 Hz (10.0 mM; p < 0.006), the left-to-right DF gradient from 14.7 +/- 3.6 Hz (control) to 4.4 +/- 1.3 Hz (7 mM) and 3.2 +/- 1.4 Hz (10 mM), the number of DF domains, and the incidence of wavebreak in the LV and interventricular regions. During 10 mM [K(+)](o), the rotation period and core area of sustained rotors in the LV increased, and VF often terminated. Two-dimensional computer simulations mimicking experimental VF predicted that clamping E(K1) to normokalemic values during simulated hyperkalemia prevented all of the hyperkalemia-induced VF changes. During hyperkalemia, despite the shortening of the action potential duration, depolarization of E(K1) increased refractoriness, leading to a slowing of VF, which effectively superseded the influence of I(K1) conductance differences on VF organization. This reduced the left-to-right excitation gradients and heterogeneous wavebreak formation. Overall, these results provide, to our knowledge, the first direct mechanistic insight into the organization and/or termination of VF by hyperkalemia.

Figure 1

DF organization during 7 mM [K⁺]_o. (A and B) Fluorescent signals (left) recorded from pixels located in the fastest (1 and 3) and largest (2 and 4) domains of DF maps (center) generated from control (A) and 7 mM [K⁺]_o. (B) Movies of anterior wall excitation in a representative experiment. Squares indicate a 5 × 5 pixel area in the center of the LV and RV. (C) (Left) TSP of fluorescence along a line of pixels crossing the LV-RV junction (from (x,y) to (x′,y′)) during control (upper) and hyperkalemic (lower) VF in a similar experiment. (Right) Location of SP initiation in the same VF episodes. Black dotted lines and numeric labels on the left indicate activated sites. Yellow lines demarcate the LV-RV boundary. Black dotted lines on the right demarcate the LV-RV junction region from the two chambers. Red dotted line indicates the line of pixels used for TSP plots.

Figure 2

DF organization during 10 mM [K⁺]_o. (A) Volume conductor electrocardiograms recorded during control (left) and 10 mM [K⁺]_o VF (center) and VT (right). (B) Representative DF maps generated during normokalemic (4 mM [K⁺]_o) and 10 mM [K⁺]_o VF (center) and VT (right). (C) Mean rate of SP density in LV-RV junction, LV, and RV regions identified during control and 10 mM [K⁺]_o VF. ^∗p < 0.036 versus control. ^¶p < 0.019 versus LV and LV-RV junction.

Figure 3

Quantification of rotor properties and SP. Individual (open symbols) and mean (solid symbols) values of MRP (A) and MCA (B) of LV rotors identified in control VF (●) and 10 mM [K⁺]_o VF (□) and VT (Δ).

Figure 4

Mechanisms leading to the termination of SP during VF. (A) Termination after the collision of wavefronts (green) and inability to propagate through the narrow isthmus between their respective SPs. (B) Termination of SPs after the fusion of wavelets. (C) Termination of SP after collision of the SP with the base of the heart (SP No. 1) or after drift of SP out of the field of view (SP No. 2).

Figure 5

(A) DF maps of simulated spiral wave activity at [K⁺]_o concentrations ranging from 4.0 to 9.0 mM, in 1.0 mM increments. Numbers on maps indicate DF values at the selected locations. Asterisk (^∗) indicates pixel selected for plotting simulated values in panel B. (B) Simulated LV rotor frequency (■) and experimental values of DF_max during VF (•) or VT (▴) as a function of [K⁺]_o.

Figure 6

(A and B) Simulated spiral waves. Examples of spiral wave reentry during 4.0 mM [K⁺]_o (A, left) and 7.0 mM [K⁺]_o (B, left), with the corresponding TSP (A and B, right) along a selected line of pixels (red dashed line). The dashed yellow line indicates how RV gradually follows a 1:1 propagation from LV in 7.0 mM [K⁺]_o. (C) Snapshots of spiral wave activity before its termination in 9.0 mM [K⁺]_o. The red arrow denotes the direction of propagation, and double red bars indicate the site of collision with the boundary. Yellow lines demarcate the LV/RV boundary. (D) Trajectories followed by the LV spiral tip at 4.0 mM [K⁺]_o (left), 7.0 mM [K⁺]_o (center), and across the LV-RV border before termination (right). Note that the tip meander location is not same as in the snapshot, because of the way it was calculated. One pixel = 0.2 mm.

Figure 7

Ionic determinants of DF organization. DF maps of simulated hyperkalemic VF during artificial clamping to control values of E_K1 and E_Kr (A), G_K1 and G_Kr (B), and I_Kr (C). Asterisk (^∗) indicates pixel location selected for plotting simulated values in panel D. (D) Plot of DF versus [K⁺]_o for the control simulation in Fig. 6 (■) and simulations shown in panels A (•), B (▴), and C (▾).

Figure 8

(A) Representative APs from a point located in the LV during spiral wave simulation at 4.0, 7.0, and 9.0 mM [K⁺]_o. (B) Quantification of APD₈₀ and (C) the availability of sodium current assessed by the product of its inactivation gating variables hj at a single point from LV and RV as a variation of [K⁺]_o from 4.0 to 9.0 mM.