Intramural optical mapping of V(m) and Ca(i)2+ during long-duration ventricular fibrillation in canine hearts

Abstract

Intramural gradients of intracellular Ca(2+) (Ca(i)(2+)) Ca(i)(2+) handling, Ca(i)(2+) oscillations, and Ca(i)(2+) transient (CaT) alternans may be important in long-duration ventricular fibrillation (LDVF). However, previous studies of Ca(i)(2+) handling have been limited to recordings from the heart surface during short-duration ventricular fibrillation. To examine whether abnormalities of intramural Ca(i)(2+) handling contribute to LDVF, we measured membrane voltage (V(m)) and Ca(i)(2+) during pacing and LDVF in six perfused canine hearts using five eight-fiber optrodes. Measurements were grouped into epicardial, midwall, and endocardial layers. We found that during pacing at 350-ms cycle length, CaT duration was slightly longer (by ≃10%) in endocardial layers than in epicardial layers, whereas action potential duration (APD) exhibited no difference. Rapid pacing at 150-ms cycle length caused alternans in both APD (APD-ALT) and CaT amplitude (CaA-ALT) without significant transmural differences. For 93% of optrode recordings, CaA-ALT was transmurally concordant, whereas APD-ALT was either concordant (36%) or discordant (54%), suggesting that APD-ALT was not caused by CaA-ALT. During LDVF, V(m) and Ca(i)(2+) progressively desynchronized when not every action potential was followed by a CaT. Such desynchronization developed faster in the epicardium than in the other layers. In addition, CaT duration strongly increased (by ∼240% at 5 min of LDVF), whereas APD shortened (by ∼17%). CaT rises always followed V(m) upstrokes during pacing and LDVF. In conclusion, the fact that V(m) upstrokes always preceded CaTs indicates that spontaneous Ca(i)(2+) oscillations in the working myocardium were not likely the reason for LDVF maintenance. Strong V(m)-Ca(i)(2+) desynchronization and the occurrence of long CaTs during LDVF indicate severely impaired Ca(i)(2+) handling and may potentially contribute to LDVF maintenance.

Fig. 1.

A: schematic diagram of the optical system. LED, light-emitting diode; DM₁ and DM₂, first and second dichroic mirrors, respectively; EmF₁, emission filter; PDA₁ and PDA₂, first and second photodiode arrays, respectively. B: cross-sectional and side views of the of the optrode tissue end. C: view of the detector end of the optrode array overlaid on the PDA mask. Solid circles, recording optical fibers; shaded circles: alignment fibers. Seven intramural recording fibers (fibers 2–8) from each of the five optrodes (optrodes I–V) were arranged in rows. Five epicardial recording fibers (fibers I-1 to V-1) were arranged in another row.

Fig. 2.

Intramural membrane voltage (V_m) and intracellular Ca²⁺ (Ca_i²⁺) measured during regular rhythm. A: raw traces of V_m (blue) and Ca_i²⁺ (red) from five optrodes. The inset in the bottom right corner schematically shows the optrode insertion sites. PM, papillary muscle; Epi, epicardium. B and C: approximate maps of transmural activation during sinus rhythm (B) and Epi pacing at a cycle length (CL) of 350 ms (C). The solid circles schematically depict the locations of the recording sites. Endo, endocardium.

Fig. 3.

Transmural differences of action potentials (APs) and Ca_i²⁺ transients (CaTs). A: average values for the three left venticular layers [Epi, midwall (Mid), and Endo; left] and an example of a transmural map (right) of the AP-CaT distribution during regular pacing (CL: 350 ms). B: similar characteristics for AP duration (APD) at 60% repolarization (APD₆₀) and CaT duration (CaD) between activation and 60% level of signal recovery (CaD₆₀; B). C: CaD₆₀-APD₆₀ difference. *Statistical significance (P < 0.05).

Fig. 4.

APD and CaT amplitude (CaA) alternans during rapid pacing (CL: 150 ms). A: examples of APD and CaA alternans from one optrode. B: superimposed Ca_i²⁺ and V_m traces from two intramural sites together with corresponding values of CaA and APD.

Fig. 5.

Intramural V_m and Ca_i²⁺ during ventricular fibrillation (VF). A: recordings from one optrode located near the apex during early perfused VF. EG, epicardial electrogram. B: V_m and Ca_i²⁺ recorded from the same optrode at 3 min of long-duration VF (LDVF).

Fig. 6.

Intramural V_m and Ca_i²⁺ recordings during late LDVF. A: V_m and Ca_i²⁺ traces from an apical optrode at 5 min of LDVF. B: recordings from an optrode located at the PM region. Insets show selected traces with long CaTs and short APDs.

Fig. 7.

Transmural maps of the activation rate (AR) during VF averaged over six hearts. A: perfused VF. B–E: different stages of LDVF.