Which component of the ECG corresponds to the refractory period during which a second ventricular contraction cannot be initiated, and why is a P wave sometimes hidden within this interval?
- A QT interval; it spans ventricular depolarisation and repolarisation, overlapping any atrial activity if heart rate is rapid ✓
- B PR interval; atrial depolarisation may overlap AV nodal conduction
- C ST segment; it represents the absolute refractory period and may obscure a retrograde P wave
- D T wave; ventricular repolarisation corresponds to the relative refractory period only
Correct answer: A. QT interval; it spans ventricular depolarisation and repolarisation, overlapping any atrial activity if heart rate is rapid
Explanation
The QT interval spans from ventricular depolarisation (QRS) through repolarisation (T wave), corresponding to the total ventricular refractory period. At fast heart rates or in junctional rhythms, atrial activity (P wave) can fall within the QT interval and be hidden. The ST segment and T wave together span absolute and relative refractory periods respectively. A P-on-T phenomenon during the relative refractory period can precipitate ventricular fibrillation.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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