Which statement about deflections on an ECG is true?

Which statement about deflections on an ECG is true?

• A. A depolarization wave moving toward a positive electrode yields a positive deflection
• B. A depolarization wave moving away from a positive electrode yields a positive deflection
• C. All leads show identical tracings
• D. The ECG cannot indicate direction of impulse toward the heart

Answer: (A)

Deflections on an ECG reflect the projection of the heart’s depolarization vector onto each lead’s axis. When the depolarization wave moves toward the positive electrode of a lead, the recorded deflection is positive (upward). If it moves away from the positive electrode, the deflection is negative (downward). Because each lead views the heart from a different angle, tracings differ across leads; they are not identical, and the direction of the impulse can be inferred from which leads show positive versus negative deflections. Therefore, the true statement is that a depolarization wave moving toward a positive electrode yields a positive deflection. The other options don’t fit: moving away typically yields a negative deflection, leads do not all show identical tracings, and the ECG can indicate the direction of impulse toward the heart.

Deflections on an ECG reflect the projection of the heart’s depolarization vector onto each lead’s axis. When the depolarization wave moves toward the positive electrode of a lead, the recorded deflection is positive (upward). If it moves away from the positive electrode, the deflection is negative (downward). Because each lead views the heart from a different angle, tracings differ across leads; they are not identical, and the direction of the impulse can be inferred from which leads show positive versus negative deflections. Therefore, the true statement is that a depolarization wave moving toward a positive electrode yields a positive deflection. The other options don’t fit: moving away typically yields a negative deflection, leads do not all show identical tracings, and the ECG can indicate the direction of impulse toward the heart.