Introduction

The heart functions as an electrical circuit that pumps blood throughout the body and sustains life. It has a complex anatomy, comprising chambers and regions in which electrical signals are generated and directed to drive cardiac contraction. Exercise can both aid and hinder cardiac function, depending on the heart's baseline health. In conditions that affect the left bundle branch block, the heart isn't functioning at its normal capacity, which can result in changes to exercise tolerance. 

The heart system

Electrical conductivity

The heart has four chambers: two atria and ventricles, with one of each located to the left and right. The heartbeat is a sequence of events known as the cardiac cycle. Each phase of the cycle depends on which chambers are filling (diastole) or emptying (systole). The cardiac cycle is triggered and maintained by the sino-atrial (SA) node – referred to as the natural pacemaker – and the atrioventricular (AV) node.¹ The sinoatrial node maintains the normal sinus rhythm of 60-100 beats per minute in a healthy person.¹² The heart’s contraction starts when the specialised cells of the SA node produce an electrical impulse that spreads to the atria, causing them to contract (atrial systole) and empty blood into the ventricles.¹⁰ The signal then reaches the AV node, where it is briefly delayed to allow complete ventricular filling before triggering ventricular contraction (ventricular systole). Once the ventricles have filled, the electrical signal is rapidly conducted from the AV node through the bundle of His, the left bundle branch, and the Purkinje fibres, causing both ventricles to contract simultaneously.² This mechanism allows the heartbeat to provide sufficient time for the atria to empty and the ventricles to fill with blood with each beat. The signals that move throughout these specialised areas all work to contract the heart muscles and pump the appropriate amount of blood throughout the body, as and when required.³ Issues in the left bundle branches of the heart can cause problems with conduction, resulting in reduced efficiency of the heart and leading to less blood being pumped than the body requires.

Electrocardiogram (ECG)

The heart's electrical activity can be monitored using an electrocardiogram (ECG), which displays waves related to the different electrical stages of the cardiac cycle. ECGs are a crucial tool for detecting cardiac abnormalities, including left bundle branch block.⁴ A standard 12-lead ECG has 10 electrodes that are applied to various sites on the body, four on the limbs – one placed on each wrist and one placed above each foot – and six on the torso.¹¹ These leads measure the heart’s electrical potential (voltage) changes through the skin and produce waves (called a trace) that clinicians can then interpret.¹¹

These ECG waves relate to the different electrical stages of the cardiac cycle. For example, the QRS wave complex (because there is more than one) occurs before ventricular systole, the P wave occurs before atrial systole, and the T wave occurs before ventricular diastole.¹³ In healthy individuals, ECG waves look like peaks, with the QRS complex having tall, narrow peaks, and P and T waves having small bump-like peaks.¹¹ However, in conditions that affect the heart’s electrical conduction, such as left bundle branch block, the ECG is abnormal, where the QRS complex appears wide (broad), as well as changes in the S and R waves.¹⁴ 

Image: Diagram demonstrating the cardiac cycle. Electrically, the cycle starts during atrial systole (at the top). However, mechanically, the cycle begins during ventricular diastole/isovolumetric relaxation. The ECG waves in the image are directly related to specific parts of the cardiac cycle as explained above. Image source: Phases of the Cardiac Cycle by Usien6 via Wikimedia Commons, licensed under CC BY-SA 4.0

Left bundle branch block

Left bundle branch block is a condition that affects the heart’s conduction system, resulting in delayed electrical impulses through the heart required for contraction. The right bundle branches of the heart are still functioning normally, indicating that the heart has some contractility and isn’t an immediate threat to life. The left bundle branches found in the left ventricles of the heart receive their blood supply from arteries within the heart, and oftentimes, a blockage in one of these main arteries results in left bundle branch block.⁵

In the Western world, the incidence of bundle branch block increases markedly with age and affects both genders. Commonly, people have medical conditions affecting the heart or other major organ systems. These include hypertension, coronary artery disease, left ventricular hypertrophy and heart failure. In those with heart failure, there is an increased mortality risk of around 25%.⁶ After the onset of left bundle branch block, people can often find themselves developing other heart abnormalities like arrhythmias, which can further pose a risk to overall health.⁷

Because left bundle branch block has been well studied in medicine, several treatment options are available, depending on the underlying condition. First-line treatment of left bundle branch block associated with chest pain is with medications, typically beta-blockers or a drug called ivabradine. These are medications often taken daily to improve cardiac function (strength and workload). Other options include surgical methods to rewire the heart's electrical system to restore normal function of the left bundle branches.⁶ Alongside this, physiotherapy can be undertaken to improve exercise tolerance over time. 

Exercise tolerance

Left bundle branch block can occur as a result of angina during exercise in cases where there is already long-standing structural heart disease. In these cases, people will often report having symptoms of angina that include chest pain and tightness, shortness of breath and dizziness. Exercise-induced left bundle branch block in the absence of anginal symptoms is also observed but is rare, with only 1.1% of patients undergoing treadmill stress testing demonstrating this phenomenon.⁸ 

Among individuals with active lifestyles, those with left bundle branch block demonstrated reduced aerobic capacity and a higher incidence of pulseless electrical activity (PEA)/electromechanical dissociation, in which ventricular electrical activity is detected on ECG recordings but no effective heartbeat is present. This also means that the heart's overall pumping capacity is often reduced, resulting in a reduced left ventricular ejection fraction in these individuals.⁹ Signs of this reduced tolerance may show in the form of increased breathlessness during activities, fatigue both during and long after the event and a decline in physical function of the body.

Summary

Left bundle branch block is an uncommon disorder of the heart that can be very disruptive to quality of life. Left bundle branch block, often associated with other cardiovascular disorders, can weaken the heart’s pumping ability, reducing exercise tolerance. In some cases, left bundle branch block can be induced by exercise in individuals with weakened hearts, making physical activity of any type more difficult to complete. Even walking short distances can result in increased shortness of breath, chest pain, and chest tightness. There are treatment options available for those suffering from left bundle branch block, which include medical and surgical options as well as a phased return to exercise with regular physiotherapy sessions. The heart may not fully recover from this, depending on individual circumstances; however, currently available treatment options can be beneficial for improving overall health and well-being.