ECG Practice Pathophysiology - Complete Explanation

The Basics of ECG Reading (Foundation)

Heart Rate (HR)

• Count the R-R intervals. A quick rule: 300 ÷ (number of large squares between two R peaks).
• Normal: 60-100 bpm. Below 60 = bradycardia. Above 100 = tachycardia.

Rhythm

• Normal Sinus Rhythm (NSR): Every QRS must be preceded by a P wave. P wave is upright in lead II. Regular intervals.
• Sinus Tachycardia: NSR but HR > 100 bpm.
• Sinus Bradycardia: NSR but HR < 60 bpm.

Axis (Direction the heart's electrical impulse travels)

• Look at leads I and aVF:
  • Both positive → Normal axis (0° to +90°)
  • Lead I positive, aVF negative → Left axis deviation (LAD)
  • Lead I negative, aVF positive → Right axis deviation (RAD)
• Normal: Lead I positive (+) and aVF positive (+)

Blocks (Conduction delays)

• Right Bundle Branch Block (RBBB): Wide QRS (≥0.12s), RSR' pattern ("rabbit ears") in V1-V2. Think "WiLLiaM MaRRoW" - in RBBB, Wide complex with M shape in V1, W shape in V5.
• Left Bundle Branch Block (LBBB): Wide QRS, broad notched R wave (RR') in V5-V6. No septal Q waves.
• AV blocks are graded 1°, 2°, 3° based on PR interval prolongation or dropped beats.

Hypertrophy

• Left Ventricular Hypertrophy (LVH): Tall R wave in V5 or V6 ≥ 26 mm, or R in V5/V6 + S in V1 > 35 mm. Also: S in V1 + R in V5 > 35 mm.
• Right Ventricular Hypertrophy (RVH): Tall R wave in V1, right axis deviation.
• Left Atrial Hypertrophy (LAH): Broad, notched P wave in lead II ("mitral P"), biphasic P wave in V1 with prominent negative terminal component.

Myocardial Infarction (MI) - ST Changes

• ST Elevation = acute injury/STEMI in the territory of that lead group:
  • Leads I, aVL, V5-V6 → Lateral wall (LCx territory)
  • Leads II, III, aVF → Inferior wall (RCA territory)
  • Leads V1-V4 → Anterior wall (LAD territory)
  • V1-V2 only → Septal (proximal LAD/septal branches)
  • V1-V4 → Anteroseptal
• ST Depression in opposite leads = reciprocal change (confirms STEMI)
• Prominent T waves or ST depression alone can indicate ischemia without full infarction

Strip-by-Strip Explanations

Strip 1

• HR: (not clear)
• Rhythm: Normal Sinus Rhythm
• Axis: Normal
• Blocks: No conduction blocks visible in any lead
• Hypertrophy: Groove sign in V1 = 10 mm; R in V4 = 26 mm; P in 4 mm; S = 55 mm
• MI: No ST elevation or depression

• The deep S wave in V1 (10 mm) + tall R in V4/V5 (26-55 mm) crosses the Sokolow-Lyon threshold (>35 mm), indicating Left Ventricular Hypertrophy (LVH).
• LVH happens because the left ventricle has been under pressure (e.g., hypertension, aortic stenosis) and its muscle mass increases. More muscle = bigger electrical signal = taller/deeper waves.
• No ischemia present on this strip.

Strip 2

• HR: ~50 bpm (bradycardia)
• Rhythm: Normal Sinus Rhythm - identical R waves, each preceded by a P wave
• Axis: Normal (Lead I and aVF positive)
• Blocks: No AV conduction blocks observed
• Hypertrophy: None observed
• MI: ST elevation in leads I, II, aVF; ST depression in aVL → Inferior and/or lateral wall MI

• ST elevation in II, III, aVF = Inferior STEMI - the Right Coronary Artery (RCA) is blocked.
• ST depression in aVL = reciprocal change confirming inferior STEMI (mirrors the opposite wall).
• Reciprocal changes are important - they rule out benign causes of ST elevation (like early repolarization).
• Bottom line: Acute inferior MI - the RCA is occluded and the inferior wall is dying.

Strip 3

• HR: ~100 bpm
• Rhythm: Normal Sinus Rhythm
• Axis: Normal
• Blocks: None
• Hypertrophy: None
• MI: ST elevation observed in V2 leads → MI in complete anterior version (anteroseptal)
• Also notes: aVR is positive or biphasic; T waves in DI

• ST elevation in V1-V4 = Anterior/Anteroseptal STEMI - the Left Anterior Descending (LAD) artery is blocked.
• The LAD supplies the front wall of the heart (the largest portion of the left ventricle). LAD occlusion is sometimes called the "widow maker" because it causes massive heart damage.
• Biphasic T waves in aVR can indicate left main or proximal LAD involvement.

Strip 4

• HR: ~150 bpm
• Rhythm: Sinus Tachycardia (identical R waves, each preceded by a P wave)
• Axis: Left Axis Deviation (Lead I positive, aVF negative)
• Blocks: Left Bundle Branch Block (LBBB) - double R (RSR') pattern in V5-V6 leads
• Hypertrophy: None
• MI: ST elevation in V1-V4 → Anteroseptal MI - ST depression in aVL also observed

• LBBB occurs when the left bundle branch (the electrical wire that activates the left ventricle) is blocked. The impulse must go the "long way around," widening the QRS complex and creating a characteristic double-hump (RSR') in left-sided leads.
• Important: LBBB itself can mimic ST changes - new LBBB in a patient with chest pain is treated as a STEMI equivalent (Sgarbossa criteria apply).
• Left axis deviation + LBBB often occur together because LBBB disrupts normal conduction routing.

Strip 5

• Axis/Rhythm: Normal Sinus Rhythm
• Blocks: No blocks to be observed
• Hypertrophy: Right Ventricular Hypertrophy can be observed (Tall R waves in V1, right axis deviation)
• MI: No MI - coronary Q waves absent; prominent ST waves; ST elevation or depression with prominent T waves in II, III, aVF

• RVH produces a tall R in V1 (normally V1 has a small R) because the enlarged right ventricle dominates.
• Causes of RVH: chronic lung disease (cor pulmonale), pulmonary hypertension, pulmonary stenosis, left-to-right shunts.
• Prominent T waves in inferior leads without frank ST elevation can indicate early ischemia or demand ischemia.

Strip 6

• HR: 75 bpm
• Axis/Rhythm: Normal Sinus Rhythm, Normal Axis
• Blocks: No AV conduction blocks present
• Hypertrophy: Left Atrial Hypertrophy - biphasic P wave in V1; broad notched P wave in lead II (the "mitral P wave")
• MI: Persistent ST elevation / prominent T waves; ST depression / prominent inverted waves can be seen

• Left Atrial Hypertrophy (LAH) = the left atrium is enlarged and stressed.
  • Causes: mitral stenosis, mitral regurgitation, LVH, heart failure.
  • The P wave in V1 is biphasic because the atrium is enlarged - the terminal negative component in V1 is diagnostic.
  • Lead II shows a broad, bifid (double-humped) P wave > 0.12 s = "P mitrale."
• Persistent ST elevation (weeks after an event) can indicate a left ventricular aneurysm - the infarcted tissue scars into an outpouching that never repolarizes normally.

Strip 7 (Partially filled / Practice Template)

Strip 8

• Blocks: No blocks can be seen
• Hypertrophy: No hypertrophy can be observed
• MI: ST elevation in leads I, II, aVF (inferior + lateral); ST depression in V1, V5 (reciprocal or posterior involvement); Inferior MI

• ST elevation in I, II, aVF = Infero-lateral MI, involving RCA (inferior) and possibly LCx (lateral branch).
• ST depression in V1-V2 in the context of inferior MI can indicate posterior wall MI - a mirror image. A "posterior MI" has no leads directly overlying it, so you see its mirror: ST depression in V1-V3.

Summary Table

Quick Coronary Artery Memory Aid