CCT ECG Fundamentals and Leads 2 — Questions and Answers

Question 1: In a standard 12-lead ECG, lead aVR views the heart from which angle?

Right shoulder, looking toward the heart's base from above-right (Correct answer)
Left shoulder, looking toward the lateral wall
Left foot, looking toward the inferior wall
Anterior chest, looking at the anterior wall

Correct answer: Right shoulder, looking toward the heart's base from above-right

Lead aVR is the augmented unipolar lead placed at the right arm, providing a view from the right shoulder looking down and toward the heart's base and right side. It is often inverted relative to other leads.

The augmented limb leads (aVR, aVL, aVF) are unipolar leads that use the Wilson central terminal as their negative reference. Lead aVR views the heart from the right shoulder at approximately -150° in the frontal plane hexaxial reference system. Because it looks at the heart from the opposite direction of most other leads, aVR typically shows predominantly negative deflections (inverted P wave, negative QRS, inverted T wave) in normal sinus rhythm. aVR is now recognized as an important lead for diagnosing certain conditions including left main or proximal LAD occlusion when ST elevation is seen here.

Question 2: Which leads make up the inferior leads on a standard 12-lead ECG?

II, III, and aVF (Correct answer)
I, aVL, and V5-V6
V1-V4
aVR, aVL, and aVF

Correct answer: II, III, and aVF

Leads II, III, and aVF are the inferior leads because their positive electrodes are located near the left foot, providing a view of the inferior wall of the left ventricle, which is supplied by the right coronary artery in most patients.

The 12-lead ECG is organized into anatomical groupings. Inferior leads (II, III, aVF) view the inferior (diaphragmatic) surface of the left ventricle. Lateral leads (I, aVL, V5, V6) view the lateral wall. Anterior leads (V1-V4, with V1-V2 also called septal leads) view the anterior wall and septum. This grouping is clinically essential for localizing myocardial infarction — ST elevation in the inferior leads indicates an inferior STEMI, typically from RCA occlusion.

Question 3: A properly performed 12-lead ECG should be recorded at what standard paper speed?

25 mm/second (Correct answer)
10 mm/second
50 mm/second
100 mm/second

Correct answer: 25 mm/second

The international standard paper speed for ECG recording is 25 mm/second, which makes each small square (1 mm) equal to 0.04 seconds and each large square (5 mm) equal to 0.20 seconds.

Standard ECG paper speed of 25 mm/second provides the familiar grid where: 1 small square = 1 mm = 0.04 seconds, and 1 large square = 5 mm = 0.20 seconds. At this speed, 5 large squares = 1 second, and 300 large squares = 1 minute (useful for the 300 rule for heart rate calculation). Some countries (notably the UK) occasionally use 50 mm/second for better resolution of rapid rhythms. If a different speed is used, it must be clearly noted on the tracing, as interval measurements will be incorrect if the interpreter assumes the standard speed.

Question 4: What does the standard calibration signal on an ECG represent, and what should its amplitude be?

1 mV electrical signal that should deflect the stylus exactly 10 mm (2 large squares) upward (Correct answer)
0.5 mV signal deflecting 5 mm upward
2 mV signal deflecting 20 mm upward
1 mV signal deflecting 5 mm upward

Correct answer: 1 mV electrical signal that should deflect the stylus exactly 10 mm (2 large squares) upward

The standard calibration (1 mV) signal should produce a 10 mm deflection on standard gain. This allows the interpreter to verify that the gain setting is correct and that ECG amplitudes can be accurately measured.

Every 12-lead ECG should begin with a calibration mark (usually a 1 mV square wave signal) to verify the gain setting. At standard gain (1x or 10 mm/mV), a 1 mV signal produces a 10 mm deflection. If voltage is very high (e.g., LVH), the gain may be reduced to half-standard (5 mm/mV) and must be noted. If voltage is low (pericardial effusion, obesity), the gain may be doubled (20 mm/mV). All amplitude measurements (QRS voltage, ST deviation) are meaningless without knowing the gain setting.

Question 5: In Einthoven's triangle, which statement correctly describes the relationship between leads I, II, and III?

Lead II = Lead I + Lead III (Einthoven's Law) (Correct answer)
Lead I = Lead II + Lead III
Lead III = Lead I + Lead II
All three leads are independent with no mathematical relationship

Correct answer: Lead II = Lead I + Lead III (Einthoven's Law)

Einthoven's Law states that at any given moment, the voltage in Lead II equals the sum of Lead I and Lead III (II = I + III). This mathematical relationship is the basis of the bipolar limb lead system.

Einthoven's Law (II = I + III) arises from the geometric arrangement of the electrodes forming an equilateral triangle around the heart. Lead I measures RA to LA, Lead II measures RA to LL, and Lead III measures LA to LL. Because these three vectors form a closed triangle, they are not independent — any two can be derived from the third. This law is practically useful for verifying lead placement: if the ECG shows an unexpected axis or unusual morphology, checking whether Einthoven's Law holds can help identify electrode reversal errors.

Question 6: Which precordial lead is typically located at the 4th intercostal space, right sternal border?

V1 (Correct answer)
V2
V3
V4

Correct answer: V1

V1 is placed at the 4th intercostal space at the right sternal border. It provides a view of the right side of the heart, the interventricular septum, and is particularly useful for identifying right bundle branch block and P wave morphology.

The precordial electrode positions are standardized: V1 is at the 4th ICS right sternal border; V2 at the 4th ICS left sternal border; V3 between V2 and V4; V4 at the 5th ICS midclavicular line; V5 at the anterior axillary line (same horizontal level as V4); V6 at the midaxillary line (same level as V4-V5). V1 and V2 are the septal leads — they face the right ventricle and septum and are key for evaluating septal depolarization, RBBB, WPW delta waves, and P wave morphology (biphasic P in V1 is seen in left atrial enlargement).

CCT ECG Fundamentals and Leads 2

In a standard 12-lead ECG, lead aVR views the heart from which angle?