Left bundle branch block causes relative but not absolute septal underperfusion during exercise

Area of Science:

• Cardiology
• Nuclear Medicine
• Physiology

Background:

• Left bundle branch block (LBBB) is associated with septal perfusion defects in myocardial perfusion imaging.
• These defects are typically seen with exercise stress but not vasodilator stress, questioning their origin.

Purpose of the Study:

• To investigate whether LBBB-induced septal defects are due to inherent structural disease or functional changes.
• To compare spontaneous LBBB with temporary LBBB induced by right ventricular pacing (PM).

Main Methods:

• Regional myocardial blood flow (MBF) was measured using 15O-H2O and PET.
• Ten healthy volunteers, 10 LBBB patients, and 10 PM patients (with and without pacing) were studied at rest and during exercise.

Main Results:

• LBBB patients showed a 19% lower septal/lateral MBF ratio compared to controls during exercise.
• Right ventricular pacing in PM patients decreased the septal/lateral MBF ratio by 17% during exercise.
• Septal MBF in LBBB patients tended to be higher than in controls, suggesting relative lateral hyperperfusion.

Conclusions:

• The septal perfusion defect in LBBB is primarily due to relative lateral hyperperfusion, not absolute septal flow reduction.
• This perfusion pattern is reversible with right ventricular pacing, indicating a functional rather than structural cause.