ECG Interpretation Flashcards

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

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.

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?

Answer: 75 bpm

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

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

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.

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

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.

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

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.

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?

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.