Which of the following is a small upward curve that represents the sinoatrial node impulse, wave of depolarization through the atria and resultant contraction?

• A. QRS complex
• B. P wave
• C. U wave
• D. T wave

Answer: (B)

Think of the ECG tracing in terms of where in the heart electrical activity is happening. The first small upward deflection represents atrial depolarization, which is the electrical impulse starting in the sinoatrial node and triggering atrial contraction. This deflection is the P wave. It sits just before the ventricular spike (the QRS complex) and after it ends, the ventricles repolarize (the T wave). The U wave, when seen, is a smaller, less consistent deflection not tied to the initial atrial depolarization. So the small upward curve that corresponds to the SA node impulse and atrial contraction is the P wave.