Author + information
- Received June 16, 2008
- Revision received August 7, 2008
- Accepted August 14, 2008
- Published online November 25, 2008.
- Sanjiv M. Narayan, MD, FRCP, FACC⁎,⁎ (, )
- Jason D. Bayer, MS†,
- Gautam Lalani, MD⁎ and
- Natalia A. Trayanova, PhD†
- ↵⁎Reprint requests and correspondence:
Dr. Sanjiv M. Narayan, University of California, San Diego, Cardiology/111A, 3350 La Jolla Village Drive, San Diego, California 92161
Objectives The purpose of this study was to determine whether abnormalities of calcium cycling explain ventricular action potential (AP) oscillations and cause electrocardiogram T-wave alternans (TWA).
Background Mechanisms explaining why heart failure patients are at risk for malignant ventricular arrhythmias (ventricular tachycardia [VT]/ventricular fibrillation [VF]) are unclear. We studied whether oscillations in human ventricular AP explain TWA and predict VT/VF, and used computer modeling to suggest potential cellular mechanisms.
Methods We studied 53 patients with left ventricular ejection fraction 28 ± 8% and 18 control subjects. Monophasic APs were recorded in the right ventricle (n = 62) and/or left ventricle (n = 9) at 109 beats/min.
Results Alternans of AP amplitude, computed spectrally, had higher magnitude in study patients than in controls (p = 0.03), particularly in AP phase II (p = 0.02) rather than phase III (p = 0.10). The AP duration and activation restitution (n = 11 patients) were flat at 109 beats/min and did not explain TWA. In computer simulations, only reduced sarcoplasmic reticulum calcium uptake explained our results, causing calcium oscillations, AP amplitude alternans, and TWA that were all abolished by calcium clamping. On prospective follow-up for 949 ± 553 days, 17 patients had VT/VF. The AP amplitude alternans predicted VT/VF (p = 0.04), and was 78% concordant with simultaneous TWA (p = 0.003).
Conclusions Patients with systolic dysfunction show ventricular AP amplitude alternans that prospectively predicted VT/VF. Alternans in AP amplitude, but not variations in AP duration or conduction, explained TWA at ≤109 beats/min. In computer models, these findings were best explained by reduced sarcoplasmic reticulum calcium uptake. Thus, in heart failure patients, in vivo AP alternans may reflect cellular calcium abnormalities and provide a mechanistic link with VT/VF.
- calcium cycling
- electrical restitution
- ventricular action potentials
- computer modeling
- conduction velocity
- T-wave alternans
- sudden cardiac arrest
Sudden cardiac arrest (SCA) from ventricular arrhythmias (ventricular tachycardia [VT]/ventricular fibrillation [VF]) claims >300,000 lives per year in the U.S. (1). Although patients with systolic dysfunction are at high risk for SCA (1), few cellular or tissue-level mechanisms have been identified in humans to explain this arrhythmic propensity.
T-wave alternans (TWA) from the electrocardiogram (ECG) identifies patients with systolic dysfunction at risk for VT/VF (2). However, its mechanistic basis is unclear. Until recently, it was thought that TWA is caused by a steep slope (>1) of the action potential duration (APD) rate-response curve (restitution) that, at very rapid rates (3), may lead to APD alternans (4). In the presence of conduction slowing or premature beats, APD alternans may develop an opposite phase in adjacent myocardium (discordant alternans), leading imminently to VT/VF (4). However, these fast-rate mechanisms do not explain clinical TWA, which predicts outcome only if present at slow rates (≤109 beats/min) and is most predictive at ≤90 beats/min (2,5). Moreover, spontaneous clinical VT/VF often initiates in the setting of nonelevated heart rates (6). Clinical (7) and theoretical (8) data show that human APD does not fluctuate at such slow rates.
Animal studies show that fluctuations in cytosolic calcium may also cause APD alternans and arrhythmias (4,9–11). Although such alternans occurs at fast rates, most reports have not studied failing hearts with calcium overload (12), in which calcium oscillations may hypothetically occur at slower rates. Theoretically, TWA may also result from oscillations in myocyte activation time (AT) (i.e., conduction) (4,13) or sodium channel inactivation (14,15).
We hypothesized that, in patients with systolic heart failure, abnormal calcium handling causes oscillations of cytosolic calcium at modest heart rate elevation, leading to fluctuations in AP amplitude that link TWA with VT/VF. We tested this hypothesis using ventricular monophasic APs in patients with systolic dysfunction, together with computational modeling to study whether AP amplitude alternans is best explained by abnormalities in cellular calcium, sodium, or conduction, and prospective follow-up to determine whether AP amplitude alternans predicts spontaneous VT/VF.
This study was approved by the joint Institutional Review Board of the Veterans Affairs and University of California Medical Centers, San Diego, and patients provided written informed consent. We recruited 53 study patients with left ventricular ejection fraction (LVEF) ≤40% undergoing programmed ventricular stimulation and 18 control patients (LVEF >40%) undergoing ablation of paroxysmal supraventricular tachycardias. We excluded patients with sustained VT/VF or SCA, within 30 days of an acute coronary syndrome or 6 weeks of coronary revascularization, and those with permanent pacemakers.
Pacing at electrophysiologic study
Patients were studied in the post-absorptive state. A 7-F monophasic action potential (MAP) catheter (EP Technologies, Sunnyvale, California) was advanced transvenously to the apex of the right ventricle (RV) (n = 62), or retrogradely across the aortic valve to the left ventricle (LV) (n = 6 study patients and n = 3 control subjects). MAPs were recorded simultaneously at 2 sites in 11 of these patients. After digitization at 1 kHz from our recorder (Bard, Billerica, Massachusetts), electrograms were analyzed at 0.05 to 500 Hz (MAPs), 30 to 500 Hz (most intracardiacs), and 0.05 to 100 Hz (ECG).
Research was performed before programmed stimulation (study patients) or ablation (control patients) in the population for whom we recently reported that APD restitution did not predict VT/VF (7). First, we studied MAPs and simultaneous TWA during right atrial pacing at 109 beats/min for >90 s (16) (Figs. 1 and 2).⇓ Second, in patients with dual-site MAPs, we measured AT to the second catheter after RV apical pacing, using 10 drive train beats at cycle length 500 ms then single extrastimuli coupled at 400 ms, in 20-ms steps to 300 ms, and then in 10-ms steps to refractoriness. The clinical procedure was then performed, and implantable cardioverter-defibrillators were implanted according to clinical guidelines.
Analysis of AP alternans
Electrograms were exported at 16-bit digital resolution for offline analysis using custom software written (by S.M.N.) in Labview (National Instruments, Austin, Texas).
The AP alternans was computed spectrally as performed for TWA. Using validated software (17), 64 contiguous APs were selected ≥20 beats after the onset of pacing, baseline corrected to a 10-ms segment starting 20 ms before AP onset, and aligned to phases 0 to I. Successive APs were represented as 2-dimensional matrices R (n, t), where n indicates beat number (0 ≤ n ≤ 63) and t the time sample. A fast-Fourier transform was used to compute power spectra across beats (arrows in Fig. 1) for each t, then spectra were summated across the AP (Figs. 1, 2, and 3).⇓
The AP amplitude was defined for phase II or III as the height above baseline. The magnitude of AP amplitude alternans was represented by the dimensionless k-score:where ΣT is spectral magnitude at 0.5 cycles/beat, and μnoise and σnoise are the mean and SD of noise. Thus, k-score scales with the SD of noise. The noise window (0.33 to 0.49 Hz) was selected adjacent to alternans frequency (avoiding the 0.125 to 0.25 Hz respiratory peak), as with TWA. A k >0 indicates that alternans exceeds noise (5). AP amplitude was also measured as the absolute voltage of alternation, Valt (in μV), that is, not scaled by noise SD, as:We studied AP amplitude for the entire AP, in other words, from the end of the alignment window (phases 0 to I) to the time of APD at 90% repolarization (APD90), and for phase II (defined as the first half of this interval) and phase III (second half). We did not study phase I alternans because upstroke velocity is slower in MAPs than in single cell APs and potentially unreliable (18), and because aligning beats to phases 0 to I minimizes phase I alternans.
Alternans of APD
APD90 was measured for these 64 beats with validated software (7). After identifying phase II (maximum plateau) and phase IV (diastolic) voltages, APD90 spanned the interval from phase 0 maximum slope (dV/dt) to 90% voltage recovery. Diastolic interval (DI) spanned the interval from APD90 of the prior beat to AP onset of the current beat. APD90 alternans was defined if variations in ≥5 beats had opposite sign (19).
Measurement of TWA alternans
We analyzed ECG TWA using HearT-wave (Cambridge Heart, Bedford, Massachusetts) simultaneously with AP measurements. Each TWA report was classified by consensus from 3 blinded reviewers as positive, negative, and indeterminate (5). We compared positive and negative TWA against alternans of AP amplitude alternans and APD alternans. For outcome analyses, we grouped positive and indeterminate tests as “abnormal” TWA (2).
Analysis of activation restitution
In 11 patients, we measured AT from the RV apex to the second MAP site (LV, n = 9; RV outflow tract, n = 2) after progressively early extrastimuli (Fig. 4). The AT is the difference in AP onset between sites. We used pairs (DI, AT) to plot AT restitution (20) as best-fit straight lines for flat AT restitution (arbitrarily, absolute slope <0.2), and for lengthening AT (conduction slowing) at short DI. We report DI where AT begins to lengthen (4).
To study mechanisms explaining AP and TWA, a human LV transmural wedge preparation was developed with dimensions shown in Figure 5A. Membrane kinetics were represented by the Ten Tusscher–Panfilov (TTP) model of human ventricular tissue (14). Transmural heterogeneity in ion channel expression between the 3 cell layers was modeled according to Table 1 of Weiss et al. (21). Intracellular and extracellular components were coupled in 3 dimensions using a bidomain formulation and assigned conductivity values to produce transmural conduction velocities of 0.4 m/s in endocardial and mid-myocardial (M-cell) layers, and 0.28 m/s in epicardium (22).
As in the clinical protocol, the endocardium was paced at cycle length of 550 ms for 84 beats. Three sets of simulations were performed: 1) control; 2) Iup, the sarcoplasmic reticulum calcium uptake current, scaled by 25% to 200%; and 3) τh,j, the time constants of fast and slow sodium channel inactivation, scaled by 25% to 200%. These parameters were selected because 33% to 60% down-regulation of Iup protein has been reported in failing human myocytes (Table 2 of Hasenfuss et al. ) and abnormal sodium inactivation may potentiate alternans (14,15). We modified Iup or τh,j simultaneously in all 3 cell layers of the LV wedge preparation before pacing.
Pseudo-ECGs were computed at a centrally located point 3 cm from the epicardium (24). Microvolt TWA was assigned if the voltage difference between successive T waves alternated in sign and exceeded 1/500 of normal T-wave amplitude (25). Spectral analysis was used to quantify TWA and AP amplitude alternans (17), with AP measurements taken at the center of the endocardial, mid-myocardium, and epicardial regions in the model.
LV dysfunction patients were followed up prospectively for 949 ± 553 days using 6-monthly device interrogations, electronic medical records, and telephone questionnaires until the first event or July 27, 2007. No patient was lost to follow-up. Arrhythmia detection was programmed uniformly in implantable cardioverter-defibrillator recipients (n = 45), and events were verified by consensus. The primary end point was appropriate device therapy or sustained VT/VF. The secondary end point was the combined occurrence of ventricular arrhythmias or all-cause mortality.
Statistical analysis and sample size considerations
Continuous data are presented as mean ± SD, and were compared between study and control subjects using the 2-tailed t test. Paired variables were compared using the paired, 2-tailed t test. The Fisher exact test was applied to contingency tables. Statistics were analyzed using SPSS software (SPSS Inc., Chicago, Illinois). Significance was assessed at a 2-tailed alpha level of 0.05.
For the prospective study, we split the study population into a training set (first, n = 26) and a validation set (last, n = 27), based on alphabetical ordering (no patients were related). Receiver-operator characteristic (ROC) analysis on the training set was used to define optimum cutpoints that were used to construct Kaplan-Meier survival curves for the validation set.
Baseline characteristics are listed in Table 1.
Clinical ventricular AP alternans
Figure 1A shows RV APs in a study patient who had positive TWA. Figure 1C shows 64 consecutive superimposed beats, successively colored red, blue, red, and so on. Baseline correction and alignment was effective, yet phases II and III segregate into even (blue) and odd (red) groups; in other words, AP amplitudes alternate. At each time point (arrows; only 3 shown for clarity), a fast-Fourier transform across all 64 beats resulted in the spectrum (Fig. 1D) in which alternans is the magnitude at 0.5 cycles/beat (every other beat). Here, k-score = 10.4. Figure 1B shows small magnitude oscillations in APD90 (3 ± 1 ms) in this patient.
Alternans of amplitude is not uniform within APs
Figure 2 illustrates nonuniform alternans within the LV AP in a study patient. Across the entire AP, alternans k-score = 2.23. However, oscillation between even/odd beats was more evident in phase II than in phase III (Fig. 2C). On comparison, amplitude alternans was present in phase II (k-score = 7.89) yet was undetectable in phase III (k-score = −0.38) (Fig. 2D). The APD90 did not oscillate (Fig. 2B), yet simultaneous TWA was positive (Fig. 2E).
Overall, AP amplitude alternans k-score was nonsignificantly higher in phase II than in phase III in study patients (3.7 ± 4.4 vs. 2.1 ± 3.7, p = 0.057 paired t test), but not in control subjects (−0.4 ± 0.8 vs. 0.2 ± 1.0, p = 0.47).
AP amplitude alternans between ventricular sites
In patients with dual-site recordings (n = 11; 6 with cardiomyopathy), AP amplitude alternans was similar between RV apex, RV outflow tract, and LV apex. Alternans k-scores for the entire AP were similar for RV apex versus RV outflow tract (1.71 ± 2.30 vs. 1.93 ± 3.39, p = 0.76) or LV (−0.36 ± 1.44 vs. −0.11 ± 0.39, p = 0.79). We therefore pooled single-RV and -LV site data. In patients with dual-site recordings, we used data from the RV apex.
AP amplitude alternans separated study from control subjects
The AP amplitude alternans was more prevalent, that is, k-score >0 (72% vs. 33%, p = 0.04), with higher magnitude (k-scores = 3.26 ± 4.20 vs. −0.31 ± 0.95, p = 0.03) in study patients than in control patients. The AP amplitude alternans had higher magnitude in the study group than in the control group in phase II (k-score = 3.66 ± 4.48 vs. −0.44 ± 0.77, p = 0.02) and, nonsignificantly, in phase III (k-score = 2.13 ± 3.69 vs. −0.25 ± 0.96, p = 0.10) (Table 2). Results were similar for Valt, although they may not have reached significance because Valt may be elevated artifactually if the noise SD is high (Table 2).
AT (conduction) restitution
For patients with ischemic cardiomyopathy, Figure 4A shows preserved activation restitution, namely, AT prolongs only for short DI <40 ms (slope −0.66), whereas Figure 4B shows broad activation restitution; in other words, AT prolonged for DI up to 120 ms (slope −0.37). The DI range for which conduction slowed was <120 ms in study subjects and <60 ms in control subjects (Table 2).
Thus, AT did not prolong for DI corresponding to ≤109 beats/min where TWA is measured (DI ≈ 270 to 290 ms) (Figs. 1, 2, and 3), so that conduction dynamics are unlikely to explain TWA.
APD alternans was presented intermittently for more beats in study patients (16 ± 13 beats) than in control patients (4 ± 5 beats, p = 0.015). Neither the number of alternating beats nor the very small APD alternans amplitude (Table 2) separated groups. The number of beats showing APD alternans poorly predicted the primary end point (area under ROC curve: 0.45).
Relationship of TWA to intracardiac measures
TWA was positive in 33 patients. Concordance between TWA and AP amplitude alternans (positive if k-score ≥1.47) was 78% (kappa = 0.48, p = 0.002). The absolute values of AP amplitude alternans and simultaneous TWA (averaged across repolarization) varied linearly, and AP amplitude alternans was 1 to 2 orders of magnitude larger than TWA (AP amplitude alternans magnitude = 38.5 × TWA magnitude − 26.5; p = 0.03), as in animal studies (26). The magnitude of AP amplitude alternans agreed with studies of human unipolar electrograms (27). TWA correlated poorly with APD alternans (p = 0.60).
Mechanisms for AP amplitude alternans
Insights from Computational Modeling
The AP amplitude and TWA arose simultaneously when Iup was reduced beyond the threshold ≥70% (Fig. 5B), and resembled clinical AP alternans (compare Fig. 5D and Figs. 1, 2, and 3B). Indeed, Iup reduction ≥74% caused sustained alternans in intracellular calcium, AP amplitude, and TWA (Fig. 5B). At 74% Iup reduction, modeled k-scores, Valt, and intermittent APD90 alternans (Figs. 5B and 5D, Table 3) agreed with clinical observation (Table 2).
To further define the mechanism for AP amplitude alternans in phases II and III, we scaled sarcoplasmic reticulum release current, Irel, and L-type calcium current, ICaL, from 25% to 200%. The ICaL reduction ≥48% produced alternans in AP amplitude and intracellular calcium (data not shown). However, on clamping ICaL, [Ca]i, or sarcoplasmic reticulum calcium (CaSR) in a single endocardial cell with reduced Iup (Fig. 6A), only clamping CaSR completely abolished AP alternans (Fig. 6B). As suggested by the modeling study of Livshitz and Rudy (10), we found that clamping [Ca]i abolished alternans in APD, but not in diadic subspace calcium and CaSR. Therefore, alternans in AP phase I and II amplitude persisted under this constraint by fluctuations in the calcium-dependent inactivation component of ICaL.
AP amplitude alternans (and TWA) did not occur in any layer of control LV wedge, nor in cardiomyopathy with increased or decreased sodium inactivation (data not shown). Because APs in Figure 5B suggest phase I alternans, we studied whether altering transient outward current, Ito, magnitude, and time constants for voltage-dependent inactivation and activation by 25% to 200% could cause AP alternans. We were unable to demonstrate this (data not shown).
Finally, transmural conduction velocity did not vary by >0.1 m/s in any LV wedge layer, and beat-to-beat variations were not observed at 109 beats/min with any modeled ionic changes.
AP indexes and outcome
We assessed outcome in LV dysfunction patients, of whom 17 experienced the primary end point and 15 died. AP amplitude alternans predicted outcome when applied to the entire AP or phase II; we report the former because clinical surrogates (unipolar electrograms) exist for the entire MAP (27,28).
Patients reaching the primary end point had greater AP amplitude alternans than did those who did not (k-score = 5.4 ± 4.1 vs. 1.8 ± 3.6, p = 0.02). Patients reaching the secondary end point also had greater AP amplitude alternans than those who did not (k-score = 4.4 ± 4.9 vs. 1.6 ± 3.7, p = 0.06).
From ROC analysis of the training set (n = 26), AP amplitude alternans k-score ≥1.47 best predicted the primary end point and was defined as “positive” (area under curve: 0.85). Confirming its validity, a nearly identical cutpoint (≥1.48) was obtained by ROC analysis of the entire LV dysfunction population (area under curve: 0.86). Using k-score ≥1.47, AP amplitude alternans was 91% concordant between sites in patients with dual-site recordings (kappa = 0.79). ROC analysis of the training set for AP amplitude alternans in phase II yielded a similar cutpoint (k-score ≥1.51; area under curve: 0.88).
On Kaplan-Meier analysis of validation patients (n = 27; 8 arrhythmic events, 9 deaths) (Fig. 7), AP amplitude alternans k-score ≥1.47 separated patients with from those without the primary end point with 100% sensitivity, 37% specificity, 100% negative predictive value, and 40% positive predictive value (p = 0.04). The primary end point was also predicted by positive (p = 0.03) and abnormal (p = 0.02) TWA in all study patients. APD alternans at 109 beats/min did not predict the primary end point (p = 0.53).
In patients with systolic heart failure, alternans of RV or LV AP amplitudes developed at modest heart rate elevations and predicted malignant arrhythmias on long-term follow-up. Notably, while AP amplitude alternans (particularly in phase II) explained TWA, we found that APD and intraventricular conduction did not oscillate at slow heart rates. In computational models, reduced Iup to levels reported in failing myocytes produced alternans that agreed with our clinical data, and resulted from calcium oscillations. Moreover, clamping sarcoplasmic calcium abolished AP amplitude and T-wave oscillations. These results link malignant human ventricular arrhythmias with abnormal calcium cycling, and suggest the novel clinical use of AP amplitude alternans to indicate calcium overload and arrhythmic risk in heart failure patients.
Potential mechanisms for AP alternans
AP amplitude alternans was more prevalent in heart failure patients than in control patients and in those with subsequent VT/VF than in those without. This finding was particularly true in AP phase II compared with phase III. These findings agree with data that human ventricular APD does not alternate at slow rates (7,8), and confirm human unipolar electrogram alternans reported by Selvaraj et al. (27), with differences likely explained by the wider spatial integration of unipolar electrograms and the largely epicardial alternans reported by Selvaraj et al. (27).
Computational modeling showed that reduced Iup, to levels observed in failing myocytes (29), explained AP amplitude and TWA and linked them to cytosolic calcium oscillations. Although somewhat counterintuitive, calcium oscillations may alter AP amplitude but not APD because varying constituents of calcium homeostasis have opposing effects on APD. Positive calcium-voltage (Ca-Vm) coupling (where calcium transients prolong APD) may be opposed by negative Ca-Vm coupling (from operation of the sodium-calcium exchanger in reverse mode, sarcoplasmic reticulum calcium release inactivation of ICaL, or intracellular calcium-related enhancement of the slow component of the delayed rectifier current) (11,30). In addition, our results may be influenced by defining phase III as the second half of the AP that may include the late AP plateau. Conversely, oscillations of APD arise at rapid rates (9,11) and reflect decreased rapid component of the delayed rectifier current and sodium-calcium exchanger activity (10).
The absence of alternans with increased sodium inactivation is consistent with Tusscher et al. (14). Unlike that study, we did not observe alternans with decreased sodium channel inactivation since APD and activation restitution were flat at the pacing cycle length of 550 ms. Indeed, even in patients with infarct scar and broad activation restitution (Fig. 4B), and computationally, AT prolongation required shorter DI (i.e., rates faster than ≤109 beats/min). We acknowledge, however, that higher spatial resolution mapping is needed to definitively address regional conduction slowing.
Relationship between AP and TWA
These data show that AP amplitude alternans predicts spontaneous VT/VF, and extend the elegant studies of Selvaraj et al. (27) and Christini et al. (31) linking intracardiac alternans with TWA. Our results also suggest that intracardiac alternans from implanted devices (32) may indicate calcium overload and electrical instability in heart failure patients.
Concordance between AP amplitude and TWA agrees with prior reports (31), suggesting shared mechanisms. Nevertheless, TWA also likely reflects spatial nonuniformities in repolarization, as noted by alternans of the activation-recovery interval (27) and colocalization of TWA near scar (33). Modeling showed that AP amplitude alternans was most marked in endocardial cells, which are characterized by a smaller maximum Ito conductance than other cells (0.073 vs. 0.294 nanosiemens per picofarads) (14), and may explain phase II alternans. However, Selvaraj et al. (27) found unipolar LV alternans in the epicardium; detailed intraoperative mapping may help to resolve these inconsistencies.
Magnitude relationship of AP and TWA
AP amplitude alternans was 2 orders of magnitude larger than TWA, as in animals (26). Selvaraj et al. (27) observed similar magnitude alternans in human unipolar activation-recovery intervals (40 to 70 μV peak differences in Figs. 1 and 2 of Selvaraj et al. ).
First, clinically, although this is the largest reported series of patients with ventricular MAPs to date, the population is small for outcome analysis, which reduces the precision of Kaplan-Meier estimates (computed for the entire study duration). Second, our results are limited by spatial resolution. Although that likely does not detract from our findings, additional sampling may refine analyses of regional repolarization and conduction dynamics. Third, although AP amplitude alternans k-scores differed significantly between groups, differences in Valt were not significantly different. This finding is explained by the large Valt range in study patients (Table 2) that likely reflects artifactual exaggeration of Valt when noise SD is high, whereas k-scores are scaled to noise. Fourth, cardiac motion, respiratory artifact, or baseline wander could theoretically influence our analyses. However, our APs had consistent shape on raw superimposed tracings (Figs. 1, 2, and 3), whereas artifacts should not alternate, alternans showed high signal-to-noise ratio (optimal k-score = 1.5 vs. k-score ≥3 in exercise TWA) (5), and artifact would not track outcome. Fifth, cellular mechanisms could be influenced by medications and blood chemistry, yet these factors were similar between groups (Table 1). Sixth, our sample is also predominantly male.
A limitation of our modeling is the lack of realistic geometry and fiber anisotropy, which alters repolarization and the pseudo-ECG. However, that should not impact our findings that link cellular calcium with repolarization oscillations (shown in single cells as well). Moreover, Iup is dependent on various factors including sarcoplasmic reticulum Ca2+-adenosine triphosphatase, phospholamban, calmodulin, and calcium/calmodulin-dependent protein kinase II, yet in the TTP model, Iup is a functional representation of all of these factors. In vitro studies typically report sarcoplasmic reticulum Ca2+-adenosine triphosphatase protein levels, yet tissue inhibitors (e.g., dephosphorylated phospholamban) may translate reported 33% to 60% protein reductions (23) into greater functional reductions in Iup. We are, therefore, comfortables that modeled Iup reductions (≥70%) are pathophysiologically plausible. Last, modeled phase III Valt was larger than observed clinically. Nevertheless, modeling agreed with the observation that AP amplitude alternans was less prevalent in phase III than phase II, and that APD alternans was intermittent and only a few milliseconds in magnitude. The discrepancy may reflect a steeper descent of AP phase III in the TTP model (Figs. 5 and 6) than in clinical MAPs (Figs. 1 to 4), enabling substantial changes in phase III amplitude with less effect on APD. The TTP model may ineffectively represent the sensitivity of repolarizing potassium currents to intracellular calcium alterations in heart failure (11) that cause negative Ca-Vm coupling and the more gradual phase III descent seen clinically. For these reasons, we believe that these phase II/III differences do not detract from the similarity between modeled and clinical AP alternans (e.g., Fig. 5D vs. Figs. 1, 2, and 3B), and our mechanistic conclusions.
In patients with systolic heart failure, in vivo alternans of RV and LV AP amplitudes at modest heart rate elevations predicted VT/VF and explained TWA. Computational models showed that reduced CaSR uptake resulted in calcium oscillations that explained electrophysiologic alternans, and were eliminated by calcium clamping. These data suggest that AP amplitude alternans in heart failure patients is linked to malignant arrhythmias via abnormal calcium handling, and provides a novel clinical index of calcium overload and arrhythmic risk.
The authors are grateful to Kathleen Mills, BA, for coordinating and administering questionnaires for this study, and to Elizabeth Greer, RN, Stephanie Yoakum, RNP, and Stanley Keys, RCVT, for help with data collection.
This study was funded by the Doris Duke Charitable Foundation, the National Institutes of Health (HL83359 and HL70529), and the American Heart Association (0265120Y) to Dr. Narayan, and by the National Institutes of Health (HL063195, HL082729, and HL067322) to Dr. Trayanova. Dr. Narayan has received speaking honoraria from Cambridge Heart, Inc., St. Jude Medical, Boston Scientific, and Medtronic Corporation. Bruce Stambler, MD, served as Guest Editor for this article.
- Abbreviations and Acronyms
- action potential
- action potential duration
- action potential duration at 90% repolarization
- activation time
- intracellular calcium
- sarcoplasmic reticulum calcium
- diastolic interval
- L-type calcium current
- sarcoplasmic reticulum calcium release current
- transient outward current
- sarcoplasmic reticulum calcium uptake current
- left ventricle/ventricular
- left ventricular ejection fraction
- monophasic action potential
- receiver-operator characteristic
- right ventricle/ventricular
- sudden cardiac arrest
- time constants of fast and slow sodium channel gating
- Ten Tusscher-Panfilov
- T-wave alternans
- absolute voltage of alternation
- ventricular fibrillation
- ventricular tachycardia
- Received June 16, 2008.
- Revision received August 7, 2008.
- Accepted August 14, 2008.
- American College of Cardiology Foundation
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