Heart failure occurs when the heart does not pump as it should. According to the Heart Failure Society of America, nearly 6.5 million Americans over the age of 20 have heart failure, with an estimated 960,000 new cases diagnosed yearly. The impact of heart failure is immense and can include frequent hospitalization, issues with physical capacity, independence, stress for the patient and their familiar, significant financial costs, decreased quality of life, and death.
A large-scale study funded by the Patient-Centered Outcomes Research Institute titled “Cardiac Resynchronization Therapy Using His/Left Bundle Pacing vs. Left Ventricular Epicardial Pacing in Patients with Heart Failure” aims to test the comparative effectiveness of His or left bundle to biventricular pacing in relation to patient-centered outcomes (quality of life and patient activity, HF hospitalization, mortality) in patients with heart failure with reduced ejection fraction (HFrEF) meeting traditional CRT criteria
The study will enroll 2136 patients across 65 states in the U.S. and Canada. A unique aspect of this clinical trial is the focus on patient engagement. This study will continuously engage patients and stakeholders from various fields in each study phase to ensure that the clinical trial is timely, feasible, equitable, and ultimately patient-centered.
What is “Left vs. Left?”
It is a large patient outcome randomized-controlled trial comparing cardiac conduction system pacing to conventional CRT (Cardiac Resynchronization Therapy). The conduction system is paced at either the His bundle or where it branches out into the left bundle branch. You may have heard CRT referred to as bi-ventricular pacing.
What is the cardiac conduction system, and why pace it?
Think of the cardiac conduction system as a branching system of nerves that start at the His bundle and terminates in cardiac muscle cells throughout the ventricles. It choreographs the sequence of ventricular muscle cell contraction throughout the ventricles. All animals and birds have a cardiac conduction system. Investment in something as elaborate and complex as the conduction system suggests it has a critical function.
The His bundle resides atop the ventricular septum at the base of the heart. The base is a structure between the upper chambers (atria) and the lower chambers (ventricles). It is a cologne structure containing four valves and is referred to as the heart’s skeleton. Yet the ventricular base region is the last to contract. The conduction system choreographs ventricular contraction producing a rapid, asynchronous intraventricular contraction from apex to base and endocardium to epicardium.
The ventricular apex contracts first, giving blood kinetic energy. Blood is propped toward the outflow valves, followed by squeezing energy to complete the pump cycle. Even fish and amphibians have an apex to base cardiac contraction.
Both leads of conventional CRT (biventricular stimulation) produce a discordant contraction pattern. Cells at the electrode site are stimulated and contract immediately* followed by a wavefront of cell-to-cell contraction, drawing the myocardium toward the electrode. Maximal fiber strain away from the electrode site, over time, results in hypertrophy (bulking up of muscle) away from the electrode site. With continuous pacing, myocardium away from the electrodes works at high capacity, without relief day or night.
Thus, “Left vs. Left” could be called Choreographed vs. Discordant Ventricular Activation.
Other media about the study:
“Baylor led research awarded $31 Million for Resynchronization Therapy in Patients with Heart Failure,” Baylor College of Medicine, July 26, 2022
* Cells are stimulated by a field strength of at least 1 volt/cm. The voltage field strength falls off with the square of the distance from the electrode.