Final answer:
The letters PQRST in an ECG represent specific electrical events of the heart's cycle: the P wave for atrial depolarization, the QRS complex for ventricular depolarization, and the T wave for ventricular repolarization. The QRS complex masks the atrial repolarization which occurs at the same time.
Step-by-step explanation:
The letters PQRST as used to describe the waves seen in an Electrocardiogram (ECG) provide a systematic approach to interpret the heart's electrical activity during one heartbeat. The P wave signifies the depolarization of the atria, which leads to atrial contraction. The QRS complex signifies the depolarization of the ventricles, which is much larger due to the mass of the ventricular muscle and leads to ventricular contraction. The ventricles start contracting as the QRS complex culminates in the peak of the R wave.
After this, the T wave represents the repolarization of the ventricles, indicating the start of ventricular relaxation. It's important to note that the repolarization of the atria occurs simultaneously with the QRS complex but is not seen on an ECG because it is overshadowed by the more prominent activity of the ventricles.