Cardiac Resynchronisation Therapy and Cellular Bioenergetics: Effects Beyond Chamber Mechanics

Metabolic flexibility and reverse remodelling of the failing human heart

BACKGROUND AND AIMS

Cardiac resynchronization therapy (CRT) produces long-term reverse remodelling which requires greater adenosine triphosphate delivery to the contractile machinery. Whilst the heart retains some metabolic flexibility in non-ischaemic cardiomyopathy, whether this correlates with reverse remodelling is unknown. This study investigated whether CRT acutely changes cardiac substrate uptake, and whether this translates to favourable reverse remodelling.

METHODS

The effect of CRT on cardiac substrate uptake was assessed via direct coronary flow and arteriovenous measurements, with metabolomic/lipidomic analysis on infusions of insulin/glucose and intralipid. Cardiac function was assessed with left ventricular pressure-volume loops during implantation, and cardiac magnetic resonance before and 6 months following CRT, with and without biventricular pacing.

RESULTS

Regardless of substrate infusion, CRT acutely improved stroke work without increasing O2 uptake on both insulin/glucose (by 34%, P = .05) and intralipid (by 36%, P = .03). This was followed by increased fatty acid (FA) uptake on insulin/glucose (R = 0.89, P = .03) and increased β-hydroxybutyrate uptake (R = 0.81, P = .05) during intralipid infusion. After 6 months, there was a 48% (P < .001) reduction in left ventricular end diastolic volume, beyond that achievable by acutely shortening or lengthening QRS duration. Reverse remodelling significantly correlated with increased FA uptake with CRT on insulin/glucose (R = 0.71, P = .05) driven by long and medium chain uptake, and increased ketone uptake with CRT on intralipid (R = 0.79, P = .05).

CONCLUSIONS

CRT acutely alters the metabolic phenotype of non-ischaemic cardiomyopathy towards a more physiological picture of FA uptake which correlates with reverse remodelling. Retained metabolic flexibility may therefore be critical for subsequent reverse remodelling.

Multipoint Left Ventricular Pacing as Alternative Approach in Cases of Biventricular Pacing Failure

Cardiac resynchronization therapy (CRT) is a cornerstone in the treatment of dyssynchronous heart failure with reduced ejection fraction. However, the phenomenon of non-response has plagued CRT since its initial application. Notwithstanding issues such as failure to capture the left ventricle, lower-than-required pacing delivery percent, and failure to optimize atrioventricular and interventricular delays, there are patients who fail to exhibit an adequate response to CRT in its classical biventricular pacing (BiVP) form. Several modalities have been proposed as a means to remedy this issue, including pacing the conduction system itself-His or left bundle branch pacing, allowing for intrinsic conduction in some myocardial segments, pacing the left ventricle from multiple points in the coronary sinus (multipoint pacing), or even combining the above (e.g., His/left bundle pacing and BiVP leading to His/left bundle-optimized CRT). In the present review, we present recent evidence for the advantages and disadvantages of each modality and attempt to formulate a pathophysiology and simulation-based strategy to determine the best way forward for delivering CRT in non-responders to BiVP.

Concepts of Cardiac Dyssynchrony and Dynamic Approach

Cardiac conduction involves electrical activity from one myocyte to another, creating coordinated contractions in each. Disruptions in the conducting system, such as left bundle branch block (LBBB), can result in premature activation of specific regions of the heart, leading to heart failure and increased morbidity and mortality. Structural alterations in T-tubules and the sarcoplasmic reticulum can lead to dyssynchrony, a condition that can be treated by cardiac resynchronization therapy (CRT), which stands as a cornerstone in this pathology. The heterogeneity in patient responses underscored the necessity of improving the diagnostic approach. Vectocardiography, ultra-high-frequency ECG, 3D echocardiography, and electrocardiographic imaging seem to offer advanced precision in identifying optimal candidates for CRT in addition to the classic diagnostic methods. The advent of His bundle pacing and left bundle branch pacing further refined the approach in the treatment of dyssynchrony, offering more physiological pacing modalities that promise enhanced outcomes by maintaining or restoring the natural sequence of ventricular activation. HOT-CRT emerges as a pivotal innovation combining the benefits of CRT with the precision of His bundle or left bundle branch area pacing to optimize cardiac function in a subset of patients where traditional CRT might fall short.

Electrical Stimulation Increases the Secretion of Cardioprotective Extracellular Vesicles from Cardiac Mesenchymal Stem Cells

Clinical trials have shown that electric stimulation (ELSM) using either cardiac resynchronization therapy (CRT) or cardiac contractility modulation (CCM) approaches is an effective treatment for patients with moderate to severe heart failure, but the mechanisms are incompletely understood. Extracellular vesicles (EV) produced by cardiac mesenchymal stem cells (C-MSC) have been reported to be cardioprotective through cell-to-cell communication. In this study, we investigated the effects of ELSM stimulation on EV secretion from C-MSCs (C-MSCELSM). We observed enhanced EV-dependent cardioprotection conferred by conditioned medium (CM) from C-MSCELSM compared to that from non-stimulated control C-MSC (C-MSCCtrl). To investigate the mechanisms of ELSM-stimulated EV secretion, we examined the protein levels of neutral sphingomyelinase 2 (nSMase2), a key enzyme of the endosomal sorting complex required for EV biosynthesis. We detected a time-dependent increase in nSMase2 protein levels in C-MSCELSM compared to C-MSCCtrl. Knockdown of nSMase2 in C-MSC by siRNA significantly reduced EV secretion in C-MSCELSM and attenuated the cardioprotective effect of CM from C-MSCELSM in HL-1 cells. Taken together, our results suggest that ELSM-mediated increases in EV secretion from C-MSC enhance the cardioprotective effects of C-MSC through an EV-dependent mechanism involving nSMase2.

Multipoint left ventricular pacing as an addition to cardiac resynchronization therapy: a bridge to the holy grail?

Cardiac resynchronization therapy (CRT) constitutes a cornerstone to the treatment of advanced dyssynchronous heart failure (DyssHF); moreover it represents one of the few instances that a revolutionary approach was pursued, yielding previously unfathomable benefits to patients out of realistic therapeutic options. However, as is rather extensively established, nonresponse, or even negative response, to CRT continue to plague its course, precluding favourable effects in up to 40% of recipients, for a multitude of reasons. Given the scope of the issue of nonresponse, attempts to negate it by means of altering CRT delivery mode, and, more specifically, by introducing multipoint left ventricular pacing (MPP) have been focused on. Possible reasons for divergent trial results will be presented, as well as potential criteria for predicting whether MPP activation may reap additional benefits as compared to conventional biventricular pacing (BVP). Finally, an alternative framework for approaching CRT in general will be put forward, including advancements which in the (near) future may once more revolutionise heart failure treatment.

Comparison of left ventricular and biventricular pacing: Rationale and clinical implications

Cardiac resynchronization therapy constitutes a cornerstone in advanced heart failure treatment, when there is evidence of dyssynchrony, especially by electrocardiography. However, it is plagued both by persistently high (~30%) rates of nonresponse and by deterioration of right ventricular function, owing to iatrogenic dyssynchrony in the context of persistent apical pacing to ensure delivery of biventricular pacing. Left ventricular pacing has long been considered an alternative to standard biventricular pacing and can be achieved as easily as inserting a single pacing electrode in the coronary sinus. Although monoventricular left ventricular pacing has been proven to yield comparable results with the standard biventricular modality, it is the advent of preferential left ventricular pacing, combining both the powerful resynchronization potential of multipolar coronary sinus and right-sided electrodes acting in concert and the ability to preserve intrinsic, physiological right ventricular activation. In this review, we aim to present the underlying principles and modes for delivering left ventricular pacing, as well as to highlight advantages of preferential over monoventricular configuration. Finally, current clinical evidence, following implementation of automated algorithms, regarding performance of left ventricular as compared with biventricular pacing will be discussed. It is expected that the field of preferential left ventricular pacing will grow significantly over the following years, and its combination with other advanced pacing modalities may promote clinical status and prognosis of patients with advanced dyssynchronous heart failure.