ECG: Electrical Activity of Heart — NEET Biology

Correct answer: B — S1, S2 and S5 only

Let's evaluate each statement: S1: ECG is a graphical representation of the electrical activity of the heart during a cardiac cycle. This statement is correct. As per NCERT, "ECG is a graphical representation of the electrical activity of the heart during a cardiac cycle." S2: The P-wave represents the electrical excitation (depolarisation) of the atria, which leads to atrial contraction. This statement is correct. NCERT states, "The P-wave represents the electrical excitation (or depolarisation) of the atria, which leads to the contraction of both the atria." S3: Any deviation from the standard ECG shape for a given lead configuration always indicates a severe cardiac arrest. This statement is incorrect. NCERT states, "any deviation from this shape indicates a possible abnormality or disease." The term "always" and "severe cardiac arrest" are overstatements and too restrictive. S4: The QRS complex represents the repolarisation of the ventricles, marking the end of systole. This statement is incorrect. The QRS complex represents ventricular depolarisation, not repolarisation. Ventricular repolarisation is represented by the T-wave, and the end of the T-wave marks the end of systole. S5: By counting the number of QRS complexes in a specific time period, one can accurately determine the heart beat rate of an individual. This statement is correct. NCERT mentions, "Obviously, by counting the number of QRS complexes that occur in a given time period, one can determine the heart beat rate of an individual." Therefore, statements S1, S2, and S5 are correct. Option A is incorrect because S3 is false. Option B is correct because S1, S2, and S5 are true. Option C is incorrect because S3 and S4 are false. Option D is incorrect because S4 is false. The core concept tested is the understanding of ECG components and their clinical relevance.