📌 Professor’s Notes: Quick Revision
High-yield revision notes for rapid review before university examinations.
- Electrocardiogram (ECG) is a graphical recording of the heart’s electrical activity obtained from electrodes placed on the body surface. It is a simple, noninvasive investigation used to diagnose arrhythmias, myocardial ischemia, myocardial infarction, conduction defects, chamber enlargement, electrolyte disturbances, and to monitor pacemakers and drug effects.
- Standard ECG is recorded at a paper speed of 25 mm/s with a calibration of 1 mV = 10 mm. One small square represents 0.04 second, and one large square represents 0.20 second.
- A 12-lead ECG consists of:
- Six limb leads (I, II, III, aVR, aVL, aVF) that assess the frontal plane.
- Six chest (precordial) leads (V1–V6) that assess the transverse plane.
- Einthoven’s triangle forms the basis of bipolar limb leads, while Wilson’s central terminal serves as the reference for chest leads. Correct placement of chest leads is essential for accurate diagnosis.
- A normal ECG consists of P wave, QRS complex, T wave, U wave (occasionally), PR segment, ST segment, and PR, QRS, QT, PP, and RR intervals.
- P wave: Atrial depolarization.
- QRS complex: Ventricular depolarization.
- T wave: Ventricular repolarization.
- PR interval: 0.12–0.20 s; reflects AV conduction.
- QRS duration: 0.08–0.10 s.
- QT interval: Represents ventricular depolarization and repolarization (electrical systole).
- ECG is based on the concept of a cardiac dipole. During depolarization and repolarization, the heart behaves as a moving electrical vector. A wave moving toward the positive electrode produces an upward deflection, while movement away produces a downward deflection. Wave amplitude depends on the amount of activated myocardium and its alignment with the recording lead.
- In Lead II, the P wave is positive because atrial depolarization moves toward the positive electrode. The Q wave represents septal depolarization, the R wave reflects depolarization of the major ventricular mass, the S wave represents terminal ventricular depolarization, and the T wave is positive because ventricular repolarization proceeds from epicardium to endocardium.
- ECG interpretation should follow a systematic sequence:
- Calculate heart rate (normal 60–100 beats/min).
- Assess rhythm.
- Determine the mean QRS axis (normal –30° to +110°).
- Examine P waves.
- Measure PR interval.
- Assess QRS complex.
- Evaluate ST segment.
- Examine T waves.
- Measure QT interval (QTc).
- Important ECG abnormalities include:
- Bradycardia: Heart rate <60/min.
- Tachycardia: Heart rate >100/min.
- Pathological Q waves: Suggest myocardial infarction.
- ST elevation: Acute myocardial injury/infarction.
- ST depression: Myocardial ischemia.
- Tall T waves: Hyperkalemia.
- Inverted T waves: Ischemia, ventricular hypertrophy, or bundle branch block.
📝 Practice MCQs
🎙️ Frequently Asked Viva Questions
High-yield viva questions with concise answers for rapid university examination preparation.
Q1. Why is ECG considered an indirect recording of cardiac activity?
Answer: ECG records the electrical activity of the heart from the body surface. It does not record mechanical contraction directly.
Q2. Why does the ECG paper move at 25 mm/second?
Answer: This standard speed allows accurate measurement of wave duration and intervals. At 25 mm/s, one small square equals 0.04 s and one large square equals 0.20 s.
Q3. Why is Lead II most commonly used for rhythm analysis?
Answer: Lead II is almost parallel to the normal direction of cardiac depolarization, so it produces clear, upright P waves and QRS complexes.
Q4. What is the physiological significance of Einthoven’s triangle?
Answer: It provides a framework for analyzing cardiac electrical activity in the frontal plane and helps determine the mean electrical axis.
Q5. Why is the P wave smaller than the QRS complex?
Answer: The atria contain much less muscle mass than the ventricles, so atrial depolarization generates a smaller electrical vector.
Q6. Why is the QRS complex the tallest waveform on the ECG?
Answer: It represents ventricular depolarization, which involves the largest mass of myocardium and therefore generates the greatest electrical potential.
Q7. Why is the T wave normally positive despite representing repolarization?
Answer: Ventricular repolarization proceeds from epicardium to endocardium, producing a net electrical vector in the same direction as the main depolarization vector.
Q8. What does the PR interval represent?
Answer: It represents atrial depolarization and impulse conduction through the AV node and His–Purkinje system. Normal duration is 0.12–0.20 second.
Q9. Why is the ST segment normally isoelectric?
Answer: During the ST segment, the ventricles are uniformly depolarized, so there is no significant potential difference on the body surface.
Q10. What is the clinical significance of ST-segment elevation?
Answer: Significant ST elevation usually indicates acute myocardial injury, especially acute myocardial infarction.
Q11. What is the physiological basis of ECG waveforms?
Answer: ECG waveforms are produced by changing cardiac electrical dipoles (vectors) during depolarization and repolarization.
Q12. Why does a depolarization wave moving toward a positive electrode produce an upward deflection?
Answer: The recorded voltage becomes positive when the net electrical vector is directed toward the positive electrode of the lead.
Q13. What is the normal mean QRS axis?
Answer: The normal mean QRS axis lies between −30° and +110° in adults.
Q14. How can heart rate be calculated from the ECG?
Answer: At a paper speed of 25 mm/s, heart rate = 1500 ÷ RR interval (in small squares).
Q15. What is the importance of systematic ECG interpretation?
Answer: A systematic approach helps identify rhythm abnormalities, conduction defects, myocardial ischemia, axis deviation, and other cardiac disorders accurately.
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