Question

Which of the following statements regarding Cardiac Resynchronization Therapy (CRT) is correct?

• A. There is mortality benefit only when defibrillator is present along with CRT
• B. Equal benefit for both RBBB and LBBB
• C. Increases synchrony without increasing oxygen demand
• D. Improves coronary perfusion through mechanical pacing

Hint:

Standard indications for CRT include: HFrEF (LVEF $\leq$ 35%), symptomatic heart failure (NYHA class II-IV) despite optimal medical therapy, sinus rhythm, and a wide QRS complex (ideally $\geq$ 150 ms) with an LBBB morphology.

Answer 1

The Correct Option is C

Concept:
Cardiac Resynchronization Therapy (CRT), also known as biventricular pacing, is a specialized treatment for patients with heart failure with reduced ejection fraction (HFrEF) who have significant electrical conduction delays (typically manifested as a widened QRS complex).
When the electrical signal is delayed (like in a Left Bundle Branch Block), the left and right ventricles contract out of sync (dyssynchrony), making the heart an inefficient pump.
Explanation:
• CRT uses a specialized pacemaker with an extra lead placed via the coronary sinus to pace the lateral wall of the left ventricle simultaneously with the right ventricle.

• This restores mechanical synchrony to the ventricles, eliminating the paradoxical septal motion and allowing the heart to squeeze forcefully as a unified unit.

• The major physiological advantage of CRT over pharmacological positive inotropes (like dobutamine or milrinone) is how it improves cardiac output. While inotropes increase cardiac contractility at the cost of vastly increased myocardial oxygen demand and energy consumption (often worsening long-term outcomes), CRT improves stroke volume simply by making the mechanical contraction highly efficient.

• Therefore, CRT increases mechanical synchrony and cardiac output WITHOUT increasing myocardial oxygen demand, representing a highly favorable physiological state for a failing heart.

• Option (A) is incorrect. CRT alone (CRT-P, pacemaker only) provides substantial mortality and morbidity benefits by reversing heart failure remodeling. While a combined defibrillator (CRT-D) offers additional protection against sudden cardiac death, mortality benefit is not exclusive to it.

• Option (B) is incorrect. CRT provides massive benefits for patients with typical Left Bundle Branch Block (LBBB). Evidence for its benefit in Right Bundle Branch Block (RBBB) or non-specific intraventricular conduction delays is much weaker and controversial.

• Option (D) is incorrect. The primary mechanism is mechanical efficiency through synchronous septal and lateral wall contraction, not directly altering coronary perfusion.
Final answer:
CRT is unique because it improves the heart's pumping efficiency and synchrony without imposing an additional oxygen demand on the myocardium.