Final answer:
In a third-degree block, P waves don't lead to QRS complexes, causing the heart rate to decrease significantly. An ECG depicts a P wave for atrial depolarization, a QRS complex for ventricular depolarization, and a T wave for ventricular repolarization, with atrial repolarization masked by the QRS complex.
Step-by-step explanation:
The impulse sometimes travels through internodal pathways to depolarize the atria, but the resultant P wave is usually masked in the QRS complex. In a third-degree block, impulses initiated by the SA node may not reach the AV node. Consequently, P waves are not followed by the QRS complex, reflecting a lack of synchronization between atrial and ventricular contractions. This can result in a situation where the atrial rate (P waves) and ventricular rate (QRS complexes) are independent of each other. A third-degree block often causes a significant decrease in heart rate (pulse), as the ventricles may rely on a secondary pacemaker with a lower intrinsic firing rate.
During a normal cardiac cycle, the ECG reveals that the P wave signifies atrial depolarization, which occurs prior to atrial contraction, pushing blood into the ventricles. The large QRS complex marks ventricular depolarization and the onset of ventricular contraction. The atrial repolarization, which is electrically 'hidden' within the QRS complex, cannot be seen on the ECG due to the simultaneous occurrence and greater electrical activity associated with ventricular depolarization. This is further evidenced on an ECG by the T wave, which represents ventricular repolarization, signaling the preparation for the next cycle of cardiac contraction.