Final answer:
A PAC often deforms the shape of the QRS complex because it represents a premature depolarization of the atria, leading to abnormal ventricular depolarization. The T wave, which denotes ventricular repolarization, may appear different before a PAC, but it is the QRS complex that is directly affected as it reflects the subsequent ventricular depolarization.
Step-by-step explanation:
The question asks about the effect of a Premature Atrial Contraction (PAC) on the shape of various waves in an ECG reading. A PAC typically occurs when there is an ectopic focus in the atria that depolarizes before the normal sinus rhythm should initiate a heartbeat. This premature depolarization leads to a P wave that may appear different than normal P waves and is followed by abnormal QRS complex, given that ventricles are not fully repolarized when this premature signal arrives.
The T wave represents the repolarization of the ventricles, and thus, one might observe an abnormal or different-shaped T wave preceding a PAC because the ventricles may still be in the process of repolarizing when the premature atrial signal arrives. However, it is the QRS complex that is mainly affected because it reflects the subsequent ventricular depolarization, which is now occurring prematurely. Therefore, the option that a PAC often deforms the shape of the QRS complex is correct. It does not typically affect the preceding T wave, the preceding U wave, or the following T wave as directly as it impacts the QRS complex.