The deflections that occur on an ECG are labeled with the letters _____.

• A. P, Q, R, S, T and U
• B. A, B, C, D
• C. I, II, III, IV
• D. X, Y, Z

Answer: (A)

Deflections on an ECG trace are labeled with the letters P, Q, R, S, T and, when present, U to correspond to the specific electrical events in the heart: the P wave for atrial depolarization, the QRS complex for ventricular depolarization (composed of the Q, R, and S components), the T wave for ventricular repolarization, and the U wave a less common afterwave sometimes seen. Other letter groups don’t map to these ECG waveforms, since they don’t represent the characteristic deflections of the ECG tracing. So, the sequence P, Q, R, S, T and U is the correct labeling.