John A. Vest

β-Blockers Restore Calcium Release Channel Function and Improve Cardiac Muscle Performance in Human Heart Failure

Background—Chronic &bgr;-adrenergic receptor (&bgr;-AR) blockade improves cardiac contractility and prolongs survival in patients with heart failure; however, the mechanisms underlying these favorable responses are poorly understood. Stress-induced activation of the sympathetic nervous system results in protein kinase A (PKA)-mediated phosphorylation of the calcium (Ca2+) release channel/cardiac ryanodine receptor (RyR2), required for cardiac excitation-contraction (EC) coupling, activating the RyR2 channel, and increasing cardiac contractility. The hyperadrenergic state of heart failure results in leaky RyR2 channels attributable to PKA hyperphosphorylation and depletion of the stabilizing FK506 binding protein, FKBP12.6. We tested the hypothesis that improved cardiac muscle function attributable to &bgr;-AR blockade is associated with restoration of normal RyR2 channel function in patients with heart failure. Methods and Results—We assessed the effects of &bgr;-AR blockade on left ventricular volume using isolated perfused hearts and &bgr;-agonist responsiveness using muscle strips from patients undergoing transplantation. Twenty-four human hearts were examined, 10 from patients with heart failure treated with &bgr;-AR blockers (carvedilol, metoprolol, or atenolol), 9 from patients with heart failure without &bgr;-AR blocker treatment, and 5 normal hearts. RyR2 PKA phosphorylation was determined by back-phosphorylation, FKBP12.6 in the RyR2 macromolecular complex was determined by coimmunoprecipitation, and channel function was assayed using planar lipid bilayers. &bgr;-AR blockers reduced left ventricular volume (reverse remodeling) and restored &bgr;-agonist response in cardiac muscle from patients with heart failure. Improved cardiac muscle function was associated with restoration of normal FKBP12.6 levels in the RyR2 macromolecular complex and RyR2 channel function. Conclusions—Improved cardiac muscle function during &bgr;-AR blockade is associated with improved cardiac Ca2+ release channel function in patients with heart failure.

Defective Cardiac Ryanodine Receptor Regulation During Atrial Fibrillation

Background—Ca2+ leak from the sarcoplasmic reticulum (SR) may play an important role in triggering and/or maintaining atrial arrhythmias, including atrial fibrillation (AF). Protein kinase A (PKA) hyperphosphorylation of the cardiac ryanodine receptor (RyR2) resulting in dissociation of the channel-stabilizing subunit calstabin2 (FK506-binding protein or FKBP12.6) causes SR Ca2+ leak in failing hearts and can trigger fatal ventricular arrhythmias. Little is known about the role of RyR2 dysfunction in AF, however. Methods and Results—Left and right atrial tissue was obtained from dogs with AF induced by rapid right atrial pacing (n=6 for left atrial, n=4 for right atrial) and sham instrumented controls (n=6 for left atrial, n=4 for right atrial). Right atrial tissue was also collected from humans with AF (n=10) and sinus rhythm (n=10) and normal cardiac function. PKA phosphorylation of immunoprecipitated RyR2 was determined by back-phosphorylation and by immunoblotting with a phosphospecific antibody. The amount of calstabin2 bound to RyR2 was determined by coimmunoprecipitation. RyR2 channel currents were measured in planar lipid bilayers. Atrial tissue from both the AF dogs and humans with chronic AF showed a significant increase in PKA phosphorylation of RyR2, with a corresponding decrease in calstabin2 binding to the channel. Channels isolated from dogs with AF exhibited increased open probability under conditions simulating diastole compared with channels from control hearts, suggesting that these AF channels could predispose to a diastolic SR Ca2+ leak. Conclusions—SR Ca2+ leak due to RyR2 PKA hyperphosphorylation may play a role in initiation and/or maintenance of AF.

A clinical feasibility study of atrial and ventricular electromechanical wave imaging

BACKGROUND Cardiac resynchronization therapy (CRT) and atrial ablation procedures currently lack a noninvasive imaging modality for reliable treatment planning and monitoring. Electromechanical wave imaging (EWI) is an ultrasound-based method that has previously been shown to be capable of noninvasively and transmurally mapping the activation sequence of the heart in animal studies by estimating and imaging the electromechanical wave, that is, the transient strains occurring in response to the electrical activation, at both high temporal and spatial resolutions. OBJECTIVE To demonstrate the feasibility of transthoracic EWI for mapping the activation sequence during different cardiac rhythms in humans. METHODS EWI was perfor`med in patients undergoing CRT and a left bundle branch block (LBBB) during sinus rhythm, left ventricular pacing, and right ventricular pacing, as well as in patients with atrial flutter (AFL) before intervention, EWI findings from patients with AFL were subsequently correlated with results from invasive intracardiac electrical mapping studies during intervention. In addition, the feasibility of single-heartbeat EWI at 2000 frames/s is demonstrated in humans for the first time in a patient with both AFL and right bundle branch block (RBBB). RESULTS The electromechanical activation maps demonstrated the capability of EWI to localize the pacing sites and characterize the bundle branch block activation sequence transmurally in patients with CRT. In patients with AFL, the EWI propagation patterns obtained with EWI were in excellent agreement with those obtained from invasive intracardiac mapping studies. CONCLUSIONS Our findings demonstrate the potential capability of EWI to aid in the assessment and follow-up of patients undergoing CRT pacing therapy and atrial ablation, with preliminary validation in vivo.

Differences in Repeating Patterns of Complex Fractionated Left Atrial Electrograms in Longstanding Persistent as Compared with Paroxysmal Atrial Fibrillation

Background— Complex fractionated atrial electrograms (CFAE) are morphologically more uniform in persistent longstanding as compared with paroxysmal atrial fibrillation (AF). It was hypothesized that this may result from a greater degree of repetitiveness in CFAE patterns at disparate left atrial (LA) sites in longstanding AF. Methods and Results— CFAEs were obtained from recording sites outside the 4 pulmonary vein (PV) ostia and at a posterior and an anterior LA site during paroxysmal and longstanding persistent AF (10 patients each, 120 sequences total). To quantify repetitiveness in CFAE, the dominant frequency was measured from ensemble spectra using 8.4-second sequences, and repetitiveness was calculated by 2 novel techniques: linear prediction and Fourier reconstruction methods. Lower prediction and reconstruction errors were considered indicative of increasing repetitiveness and decreasing randomness. In patients with paroxysmal AF, CFAE pattern repetitiveness was significantly lower (randomness higher) at antral sites outside PV ostia as compared with LA free wall sites (P<0.001). In longstanding AF, repetitiveness increased outside the PV ostia, especially outside the left superior PV ostium, and diminished at the LA free wall sites. The result was that in persistent AF, there were no significant site-specific differences in CFAE repetitiveness at the selected LA locations used in this study. Average dominant frequency magnitude was 5.32±0.29 Hz in paroxysmal AF and higher in longstanding AF, at 6.27±0.13 Hz (P<0.001), with the frequency of local activation approaching a common upper bound for all sites. Conclusions— In paroxysmal AF, CFAE repetitiveness is low and randomness high outside the PVs, particularly the left superior PV. As evolution to persistent longstanding AF occurs, CFAE repetitiveness becomes more uniformly distributed at disparate sites, possibly signifying an increasing number of drivers from remote PVs.

Analysis of disease progression in patients with transthyretin cardiac amyloidosis

Background In transthyretin (TTR) cardiac amyloidosis, myocardial deposition of liver-derived TTR fibrils results in heart failure and death. Wild-type TTR (ATTRwt) amyloidosis is an acquired disease, whereas familial amyloidotic cardiomyopathy (FAC) is a hereditary form of the disease caused by mutations in the TTR gene resulting in the deposition of both mutant and wild-type TTR. Published data on disease progression in TTR cardiac amyloidosis is limited. To explore this further, we analyzed natural history data in ATTRwt amyloidosis patients to characterize disease progression in this population and compared this to our previously reported data in FAC.

Non-invasive Electromechanical Wave Imaging of atrial, supraventricular and ventricular cardiac conduction disorders in canines and humans

Electromechanical Wave Imaging (EWI) is a novel ultrasound-based imaging modality for the mapping of the electromechanical wave (EW), i.e., the transient deformations occurring in immediate response to the electrical activation. The correlation between the EW and the electrical activation has been established in canines in previous studies. However, the methods used previously to map the EW required the reconstruction of images over multiple cardiac cycles, precluding the application of EWI for non-periodic arrhythmia such as fibrillation. In this study, we present a new unfocused sequences based on flash emissions to image the entire heart at very high frame rates (2000 fps) during free breathing in a single heartbeat and compare it to the automatic composite technique (ACT), which is based on acquisition spanning multiple heart cycles and is used to image normal human subjects during sinus rhythm. Feasibility of flash-beam sequences is assessed by imaging the atria and ventricles of closed-chest, conscious canines during sinus rhythm and during right-ventricular pacing following atrioventricular dissociation, i.e., a non-periodic rhythm and during ventricular fibrillation in one open-chest canine. These results indicate that EWI can be used for the characterization of non-periodic arrhythmia in conditions close to the clinical setting, in a single heartbeat, and during free-breathing.