In a structrually normal heart, why does symptomatic LBBB (Left bundle branch block) come and go?

In a structurally normal heart, symptomatic left bundle branch block (LBBB) that comes and goes can be attributed to a few factors related to electrical conduction and autonomic nervous system regulation. Here’s why this might occur:

1. Autonomic Nervous System Influence:

• The autonomic nervous system plays a crucial role in regulating heart rate and rhythm. Sympathetic stimulation (e.g., during exercise or stress) and parasympathetic activity (e.g., during rest or sleep) can impact conduction through the heart’s electrical system. These changes in autonomic tone can transiently worsen or improve the conduction of electrical impulses through the left bundle branch.
• When parasympathetic tone is higher, the conduction velocity may slow down, potentially exacerbating LBBB symptoms.
• When sympathetic tone increases, conduction may improve, and the LBBB could resolve temporarily.

2. Changes in Heart Rate:

• LBBB can be more pronounced at certain heart rates, especially when the heart rate is lower or highly variable. At slower rates, the conduction through the left bundle may become more sluggish, resulting in a transient block, whereas a faster heart rate may help to overcome this by increasing the overall speed of electrical conduction.

3. Electrolyte Imbalances or Acute Triggers:

• Changes in electrolyte levels (e.g., potassium, calcium, or magnesium) or sudden changes in blood pressure can influence conduction pathways. For example, during certain periods of dehydration or metabolic shifts, LBBB might temporarily worsen or become symptomatic.
• Other acute triggers, like illness, stress, or medication changes, could also transiently affect the conduction system.

4. Ischemia or Subclinical Coronary Artery Disease:

• While the heart is structurally normal, subclinical coronary artery disease or microvascular ischemia could occasionally affect conduction pathways, leading to intermittent LBBB. This could be more likely during periods of increased cardiac demand (e.g., exercise or stress).

5. Right Ventricular Pacing or Ventricular Dyssynchrony:

• If there are subtle issues in ventricular dyssynchrony or occasional conduction disturbances in the right ventricle (e.g., from a pacemaker, right ventricular hypertrophy, or other mild abnormality), it could affect left bundle branch conduction intermittently.

6. Other Intermittent Conduction Issues:

• Some people may have a mild, latent conduction defect in the left bundle that occasionally becomes symptomatic in response to certain physiological conditions (e.g., exercise, emotional stress, etc.). This could cause the LBBB to be intermittent, coming and going based on how the conduction system is stressed.

In summary, symptomatic LBBB in a structurally normal heart can be influenced by autonomic nervous system variations, heart rate fluctuations, electrolyte changes, or even transient ischemic episodes. These factors can lead to intermittent impairment in the conduction system, causing LBBB to come and go.