Question

In an ECG, why is there an absence of electrical activity between the P to Q wave, and S to T wave?

Answer 1

The flat line present between the end of P wave and beginning of Q wave is called PQ or PR segment. It serves as isoelectric line or baseline as it reflects the slow impulse conduction through AV node. Since, there is no electrical activity in atria or ventricles, it comes as flat line in ECG between P wave (atrial systole or depolarization) and Q wave (beginning of ventricular contraction or depolarization).

There is also an isoelectric line between S wave (end of ventricular systole or depolarization) and beginning of T wave (ventricular diastole or repolarization) because it reflects the zero potential between ventricular depolarization and ventricular repolarization. Since the potential is zero i.e, there is no electrical activity, ECG shows ST segment as isoelectric line.