CRAT ECG Interpretation 2 — Questions and Answers

Question 1: On a standard ECG recorded at 25 mm/sec, each large box (5 mm) represents what duration of time?

200 ms (0.20 seconds) (Correct answer)
100 ms (0.10 seconds)
400 ms (0.40 seconds)
40 ms (0.04 seconds)

Correct answer: 200 ms (0.20 seconds)

At standard paper speed of 25 mm/sec, each large box (5 mm wide) represents 200 ms or 0.20 seconds; each small box (1 mm) represents 40 ms.

Understanding ECG time scale is foundational: at 25 mm/sec, 1 small box = 1 mm = 40 ms; 5 small boxes = 1 large box = 5 mm = 200 ms; 5 large boxes = 25 mm = 1 second. These values are used for all interval measurements: PR interval (normal 3-5 small boxes = 120-200 ms), QRS duration (normal < 3 small boxes = < 120 ms), and QT interval calculation.

Question 2: The large box method for estimating heart rate from a regular rhythm divides 300 by the number of large boxes between consecutive R waves. If there are 4 large boxes between R waves, what is the approximate heart rate?

75 bpm (Correct answer)
100 bpm
60 bpm
50 bpm

Correct answer: 75 bpm

Using the 300 method: 300 divided by 4 large boxes between R waves equals 75 bpm.

The 300-method works for regular rhythms: memorize the sequence — 1 box = 300 bpm, 2 boxes = 150 bpm, 3 boxes = 100 bpm, 4 boxes = 75 bpm, 5 boxes = 60 bpm, 6 boxes = 50 bpm. This is accurate only for regular rhythms. For irregular rhythms, count all QRS complexes in a 6-second strip and multiply by 10 (or a 10-second strip and multiply by 6).

Question 3: In a 12-lead ECG, which leads are considered the inferior leads?

II, III, and aVF (Correct answer)
I, aVL, and V5-V6
V1, V2, and V3
aVR, V1, and V2

Correct answer: II, III, and aVF

Leads II, III, and aVF view the inferior wall of the left ventricle (supplied by the right coronary artery) and are called the inferior leads.

The 12 leads are grouped by the cardiac region they view: Inferior (II, III, aVF) — inferior LV wall, RCA territory; Lateral (I, aVL, V5, V6) — lateral LV wall, circumflex territory; Anterior/Septal (V1-V4) — anterior LV, LAD territory; aVR — right atrium, right ventricle (reciprocal of inferior/lateral). Changes in specific lead groupings localize the region of ischemia or injury.

Question 4: Which ECG finding is the classic sign of acute ST-elevation myocardial infarction (STEMI)?

ST segment elevation greater than 1 mm in two or more contiguous leads (Correct answer)
ST depression greater than 0.5 mm in multiple leads
New left bundle branch block only
T wave inversion in all leads

Correct answer: ST segment elevation greater than 1 mm in two or more contiguous leads

STEMI requires ST elevation greater than 1 mm (2 mm in V2-V3) in two or more anatomically contiguous leads, representing a current of injury from transmural myocardial ischemia.

STEMI diagnostic criteria: ≥ 1 mm ST elevation in ≥ 2 contiguous limb leads, or ≥ 2 mm in V2-V3 (2.5 mm in men < 40 years, 1.5 mm in women). Contiguous leads share anatomical territory: inferior (II, III, aVF), anterior (V1-V4), lateral (I, aVL, V5-V6). New LBBB may also represent STEMI equivalent. Immediate catheterization (primary PCI) is required within 90 minutes of first medical contact.

Question 5: Left bundle branch block (LBBB) on ECG is confirmed by which set of findings?

QRS duration ≥ 120 ms, broad notched R in lateral leads (I, aVL, V5-V6), and rS pattern in V1 (Correct answer)
QRS duration ≥ 120 ms with RSR' pattern in V1 and wide S waves in I and V6
Short QRS with delta waves
Wide QRS with absent R wave in all leads

Correct answer: QRS duration ≥ 120 ms, broad notched R in lateral leads (I, aVL, V5-V6), and rS pattern in V1

LBBB produces QRS ≥ 120 ms, a broad, notched or slurred R wave in lateral leads (I, aVL, V5, V6), and a deep rS or QS pattern in V1 — the opposite pattern from RBBB.

LBBB results from block of the left bundle branch, forcing the right ventricle to activate first. The wavefront then spreads slowly across the interventricular septum (right to left) into the left ventricle. This produces: QRS ≥ 120 ms, broad monophasic R in I, aVL, V5-V6 (sometimes with notching = M shape), deep QS or rS in V1-V3. ST-T changes are discordant (opposite to main QRS deflection) and are expected in LBBB — making ischemia interpretation difficult.

Question 6: A patient's 12-lead ECG shows ST depression in leads V1 through V4 with tall R waves in V1-V2. Which condition should be suspected?

Posterior wall STEMI with reciprocal changes in anterior leads (Correct answer)
Anterior STEMI
Right bundle branch block
Hyperkalemia

Correct answer: Posterior wall STEMI with reciprocal changes in anterior leads

ST depression with tall R waves in V1-V2 is the mirror image (reciprocal changes) of posterior STEMI — the posterior wall is not directly viewed by standard leads but appears as reciprocal anterior changes.

Posterior MI is a STEMI equivalent not directly visible on standard 12-lead ECG because no standard leads face the posterior wall. Instead, it appears as reciprocal changes in anterior leads: horizontal ST depression and tall, broad R waves in V1-V2 (the mirror of the posterior ST elevation and Q waves). Confirmed by placing posterior leads (V7-V9). Posterior MI is typically caused by circumflex or right coronary occlusion and requires the same emergent reperfusion as other STEMIs.

CRAT ECG Interpretation 2

On a standard ECG recorded at 25 mm/sec, each large box (5 mm) represents what duration of time?