Final answer:
Not every QRS wave has a Q, R, or S, particularly in cases of ECG abnormalities like a second-degree or partial block or a third-degree block.
Step-by-step explanation:
Not every QRS wave has a Q, R, or S. In some cases, a QRS complex may be missing one or more of the waves. This can occur due to certain ECG abnormalities such as a second-degree or partial block or a third-degree block. In these cases, half of the P waves may not be followed by the QRS complex and T waves, while the other half are.