MRCOG Part 1 · Biophysics · Electrocardiography (ECG)

Principles and Interpretation of ECG: MRCOG Part 1 questions and key facts

A standard twelve-lead surface electrocardiogram records cardiac electrical activity from multiple anatomical viewpoints. Electrodes placed across the limbs and precordium map specific myocardial territories: leads two, three, and augmented vector foot assess the inferior surface; leads one, augmented vector left, V5, and V6 capture the lateral wall; and chest leads V1 to V4 cover the septal and anterior regions. Accurate precordial landmarking is essential, including locating the fourth intercostal space at the sternal borders for V1 and V2, and the fifth intercostal space for V4, V5, and V6. Standard ventricular depolarisation is assessed by the QRS duration, which normally ranges between 0.06 and 0.10 seconds. In addition, acute shifts in extracellular calcium and potassium concentrations induce characteristic alterations in QT interval duration and T-wave morphology.

The Uterio bank has 12 questions on this topic. 2 arebelow in full; the syllabus detail is in the Biophysics study guide.

How the exam tests Principles and Interpretation of ECG

  • Anatomical territories reflected by specific lead groups including inferior, lateral, septal, and anterior walls.
  • Bony landmarks and anatomical line references for positioning chest leads V1, V2, and V4 to V6.
  • Normal, borderline, and prolonged duration limits of the ventricular depolarisation QRS complex.
  • Characteristic QT interval and T-wave alterations caused by calcium and potassium derangements.

High-yield facts

  1. Normal QRS duration is 0.06 to 0.10 seconds, which corresponds to up to approximately two and a half small squares on standard recording paper.
  2. A QRS duration of 0.10 to 0.12 seconds is borderline and can represent incomplete bundle branch block.
  3. A QRS duration of 0.12 seconds or greater defines a wide complex, indicating delayed ventricular conduction such as bundle branch block.
  4. Chest lead V1 is positioned in the fourth intercostal space at the right sternal border.
  5. Chest lead V2 is positioned in the fourth intercostal space at the left sternal border.
  6. Precordial leads V1 and V2 view the interventricular septum, whereas leads V3 and V4 view the anterior left ventricular wall.
  7. Precordial lead V4 is positioned in the fifth intercostal space in the mid-clavicular line.
  8. Precordial leads V5 and V6 are placed in the fifth intercostal space along the anterior axillary and mid-axillary lines, respectively.
  9. Leads II, III, and aVF record electrical activity from the inferior surface of the heart resting on the diaphragm.
  10. Leads I, aVL, V5, and V6 capture electrical activity from the lateral surface of the myocardium.
  11. Hypocalcaemia prolongs the QT interval primarily by lengthening the ST segment, whereas hypercalcaemia causes shortening of the QT interval.
  12. Hyperkalaemia characteristically causes tall, peaked T waves, whereas hypokalaemia results in flattened T waves and prominent U waves.

Common pitfalls

  • Placing chest lead V1 at the left sternal border instead of the right; both V1 and V2 occupy the fourth intercostal space on opposite sides.
  • Confusing calcium effects on the QT interval: hypocalcaemia prolongs the interval via ST lengthening, whereas hypercalcaemia shortens it.
  • Reversing potassium T-wave changes: hyperkalaemia produces tall, peaked T waves, whereas hypokalaemia causes flattened T waves with U waves.

Practice questions: Principles and Interpretation of ECG

Single best answer, exactly as the exam asks. Commit to an option before opening the explanation.

Question 1

Biophysics

What ECG finding is characteristic of hypocalcaemia?

  1. AShort QT interval
  2. BLong QT interval
  3. CPeaked T waves
  4. DWide QRS complex
  5. EST elevation
Show answer and explanation

Correct answer: B. Long QT interval

Key takeaway

Hypocalcaemia characteristically prolongs the QT interval on an ECG, primarily by extending the ST segment. Long QT interval reflects the prolonged myocyte action potential.

Hypocalcaemia, a lower than normal level of calcium in the blood, affects cardiac myocyte action potentials, leading to a characteristic ECG change. A low calcium concentration increases cell excitability, making an action potential more likely.

Long QT interval is the characteristic ECG finding in hypocalcaemia. Extracellular calcium influences the sodium channels, and a reduced calcium concentration lowers the threshold for these channels. Reduced extracellular calcium also prolongs the plateau phase (phase 2) of the cardiac myocyte action potential, as calcium influx is reduced, and this manifests as a prolonged QT interval on the ECG. Prolongation is mainly in the ST segment component, the isoelectric component between the end of QRS and start of T wave; the T wave itself is often of normal duration.

A short QT interval is associated with hypercalcaemia, the opposite condition.

Peaked T waves are typically seen in hyperkalaemia (high potassium levels).

Wide QRS complexes can be seen in various conditions such as bundle branch blocks or ventricular arrhythmias, not specifically hypocalcaemia. The same is true for ST elevation which could be normal, variant or pathological.

Question 2

Biophysics

What is the normal duration of the QRS complex?

  1. A0.04-0.12 seconds
  2. B0.06-0.10 seconds
  3. C0.12-0.20 seconds
  4. D0.20-0.24 seconds
  5. E0.24-0.30 seconds
Show answer and explanation

Correct answer: B. 0.06-0.10 seconds

Key takeaway

The normal duration of the QRS complex is 0.06–0.10 seconds (60–100 ms, up to ~2.5 small squares on a standard 25 mm/s ECG). A QRS duration of ≥ 0.12 seconds (120 ms) is considered prolonged ("wide QRS") and implies delayed ventricular conduction — most commonly a bundle branch block.

The QRS complex on a 12-lead electrocardiogram reflects depolarisation of the two ventricles via the His–Purkinje conduction system. Because this network propagates the signal very fast, normal ventricular depolarisation completes in under 100 ms.

  • Normal QRS duration — 0.06–0.10 s (60–100 ms), up to ~2.5 small squares on 25 mm/s ECG paper.
  • Borderline / upper limit of normal — 100–120 ms (0.10–0.12 s). Some sources include the upper part of this range as "incomplete" bundle branch block depending on morphology.
  • Wide ("prolonged") QRS — ≥ 0.12 s (120 ms). This is the accepted clinical threshold for bundle branch block, ventricular pacing, hyperkalaemia-induced slowing, and ventricular-origin rhythms.

A range of 0.04–0.12 s is too wide: its upper bound (120 ms) reaches the "prolonged" threshold, so the range stretches into abnormal territory. Durations of 0.12–0.20 s, 0.20–0.24 s and 0.24–0.30 s all lie entirely in the prolonged range and would represent pathology, not a normal QRS.

Some texts quote a normal QRS of up to 0.12 s; this more permissive definition includes the borderline zone rather than the strict normal range.

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What the syllabus covers under principles and interpretation of ecg

  • Cardiac Cycle and ECG Waveform
  • ECG Lead Placement and Recording
  • Normal ECG Features

Read the full section in the Biophysics guide, or check the next exam dates.