At what point does atrial pacing occur on an ECG?

Atrial pacing is indicated on an electrocardiogram (ECG) when there is a small spike immediately preceding the P wave. This spike represents the electrical impulse generated by the atrial pacemaker, signaling the initiation of atrial contraction. Since atrial activation occurs prior to the depolarization of the ventricles, this spike will appear before the P wave on the ECG tracing.

In this context, the other choices refer to various parts of the ECG waveform that do not correlate with the timing of atrial pacing. The QRS complex represents ventricular depolarization and follows the P wave, the T wave represents ventricular repolarization that occurs after the QRS complex, and the QT interval spans from the start of the QRS complex to the end of the T wave, which again does not align with the timing of atrial pacing.

Thus, the timing of the atrial pacing spike, which occurs before the P wave, clearly identifies it as the accurate answer.