Calcium oscillations and T-wave lability precede ventricular arrhythmias in acquired long QT type 2

Abstract

Background: Alternans of intracellular Ca(2+) (Ca(i)) underlies T-wave alternans, a predictor of cardiac arrhythmias. A related phenomenon, T-wave lability (TWL), precedes torsades de pointes (TdP) in patients and animal models with impaired repolarization. However, the role of Ca(i) in TWL remains unexplored.

Objective: This study investigated the role of Ca(i) dynamics on TWL in a noncryoablated rabbit model of long QT syndrome type 2 (LQT2) using simultaneous measurements of Ca(i) transient (CaT), action potentials (APs), and electrocardiogram (ECG) during paced rhythms and focused on events that precede ventricular ectopy.

Methods: APs and CaTs were mapped optically from paced Langendorff female rabbit hearts (n = 8) at 1.2-s cycle length, after atrioventricular node ablation. Hearts were perfused with normal Tyrode solution, then with dofetilide (0.5 μM), and reduced [K(+)] (2 mM) and [Mg(2+)] (0.5 mM) to elicit LQT2. Lability of ECG, voltage, and Ca(i) signals were evaluated during regular paced rhythm, before and after dofetilide perfusion.

Results: In LQT2, lability of Ca(i), voltage, and ECG signals increased during paced rhythm, before the appearance of early afterdepolarizations (EADs). LQT2 resulted in AP prolongation and multiple (1 to 3) additional Ca(i) upstrokes, whereas APs remained monophasic. When EADs appeared, Ca(i) rose before voltage upstrokes at the origins of propagating EADs. Interventions (i.e., ryanodine and thapsigargin, n = 3 or low [Ca](o) and nifedipine, n = 4) that suppressed Ca(i) oscillations also abolished EADs.

Conclusion: In LQT2, Ca(i) oscillations (Ca(i)O) precede EADs by minutes, indicating that they result from spontaneous sarcoplasmic reticulum Ca(2+) release rather than spontaneous I(Ca,L) reactivation. Ca(i)O likely produce oscillations of Na/Ca exchange current, I(NCX). Depolarizing I(NCX) during the AP plateau contributes to the generation of EADs by reactivating Ca(2+) channels that have recovered from inactivation. TWL reflects CaTs and APs lability that occur before EADs and TdP.

Figure 1

Prolonged repolarization induces TWL Examples of EKG recordings during pacing at 50 beats per min: (A) Control and (B) LQT2. T-wave morphology is constant in A, but changes on a beat-to-beat basis in B. EKG lability (green traces) is plotted before (C) and during LQT2 (D) and is superimposed on signal-averaged EKG (red traces). The Y-axis for lability is expanded 10-fold with respect to signal-averaged EKG. TWL is calculated as the logarithm of maximum EKG lability measured during the repolarization phase and normalized with respect to the amplitude of signal-averaged QRS. TWL is essentially absent in C and highly pronounced in D. Maximum lability occurs at ∼470 ms after the pacing stimulus in this case.

Figure 2

Ca_iO precede EADs Simultaneous recordings of V_m (blue) and Ca_i (red) before (A) and during LQT2 (B,C). A: V_m and Ca_i are shown at slow (top) and fast (bottom) sweep speeds and both exhibit monophasic time-courses. B: V_m and Ca_i are shown at slow (top) and fast (bottom) sweep speeds. In the first 2 APs, LQT2 condition prolonged APD and elicited Ca_i oscillations during the paced beats with no V_m instabilities. The 3^rd AP exhibited multiple Ca_i peaks which were occasionally coincident with EADs. C: An example of TdP onset following the 5^th paced beat. The EKG (top trace) and the optical traces of Vm and Cai (bottom traces) were recorded simultaneously. The Ca_iO precede the first EADs at this site.

Figure 3

Time-dependent oscillations of Ca_i and the evolution of EADs V_m (blue) and Ca_i (red) measurements (left) during pacing (A, B) and ventricular escape rhythms (C, D) and the corresponding phase-plots (right). A. In control, V_m and Ca_i are monophasic and similar in shape. B: LQT2 for 3 min, a 2^nd rise of Ca_i appears during the AP plateau while V_m remains free of EADs. C: LQT2 for 6 min promotes more complex Ca_iO that are associated with a single EAD. In this pixel, the Ca_i upstroke precedes the V_m upstroke. D: LQT2 for 9 min, 2 consecutive EADs follow each AP upstroke. Prominent Ca_i upstrokes precede EAD upstrokes.

Figure 4

Spatial and temporal heterogeneity of CaT and APs during LQT2 A: Isochronal Map of APD₉₀ (A (a), top left) from the anterior surface in LQT2; the field-of-view of the array is shown (see inset d) and isochronal lines are 50 ms apart (see color scale). Ca_i signals recorded at each site are depicted in the symbolic map of the photodiode array (b). Simultaneous V_m and Ca_i from a single-beat are superimposed for pixels labeled 1, 2 and 3 on the maps and are shown at fast sweep speed (c). Marked spatial heterogeneities of Ca_iO appear; with the highest number of Ca_iO found at sites with the longest APDs and decrease in going to sites with shorter APDs. B: EKG, V_m and Ca_i recordings from pixel (1) for 30 s show a gradual APD prolongation associated with an increasing number of Ca_iO and a rise of diastolic Ca_i. The right panel shows a shorter segment of the signals (in green box on left) with better time resolution.

Figure 5

Propagation of V_m and Ca_i upstrokes during an EAD A: Isochronal activation map of an ectopic beat (EAD) occurring during an escape rhythm. The origin of the EAD is at site 1. Isochronal lines are 3 ms apart. Note that another independent wavefront emanates from the base of the heart. B: V_m and Ca_i tracings from sites: 1 and 3 in (A). The 1^rst AP and CaT are monophasic at site 1; during the 2^nd AP, there is a distinct 2^nd Ca_i peak without an EAD (arrow). On the 3^rd beat, an EAD appears with sufficient magnitude to propagate, as in A. C: The temporal relationship between Ca_i and V_m signals are shown at higher resolution at sites 1-3 as labeled in A. At the site of EAD origin (1), Ca_i upstroke precedes V_m upstroke (8 ms); at site 2, V_m is coincident with Ca_i and at site 3, remote from the EAD origin, V_m precedes Ca_i (3 ms).

Figure 6

EKG recordings of complex ectopy: Bigeminy and Trigeminy correspond to Ca_iO A: EKG recording of paced rhythm with bigeminy. Each paced beat (green arrows) is followed by a ventricular ectopic beat (red arrow). T-waves on EKG signals are indicated by black arrows. Note that neither V_m nor Ca_i recover to baseline before the ectopic beat; in this sense, the paced/ectopic beat can be understood as a single complex AP. B: An episode of trigeminy from the same experiment. Two paced beats are followed by an ectopic beat. The optical tracings indicate that alternans between a short monophasic paced AP and a “bigeminal” AP underlies the trigeminal pattern. C: Simultaneous EKG and Ca_i tracing with brief runs of polymorphic VT. Pacing rate was 50 bpm with 2:1 capture. Each run of polymorphic VT corresponds to a single CaT with multiple secondary Ca_iO.

Figure 7

Effect of Reduced SR Ca²⁺ load on Ca_iO and TdP TdP was induced with LQT2 solution then nifedipine (5 μM) was added (A) or external Ca²⁺ was lowered (100 μM) (B) in the perfusate. Both interventions attenuated Ca_iO and terminated TdP despite continuous dofetilide perfusion and marked APD prolongation.