10 Most Common ECG Diagnoses and Findings

1. Normal Sinus Rhythm (Baseline Reference)

• Rate: 60-100 bpm
• P wave: upright in leads I and II, precedes every QRS
• PR interval: 120-200 ms
• QRS: <120 ms, axis -30° to +90°
• QT interval: <450 ms (men), <470 ms (women)

2. Sinus Tachycardia

• Rate: >100 bpm with a normal P-QRS-T sequence
• P waves are upright and precede each QRS (1:1 relationship)
• Most common dysrhythmia seen on ECG overall - present in ~40% of emergency patients with acute illness (e.g. PE, thyrotoxicosis, sepsis)
• Not a primary arrhythmia - always look for the underlying cause

3. Atrial Fibrillation (AF)

• Absent P waves - replaced by irregular, chaotic fibrillatory (f) waves, best seen in V1
• Irregularly irregular ventricular response (RR intervals vary with no pattern)
• Narrow QRS unless aberrant conduction or bundle branch block co-exists
• When AF is associated with pulmonary hypertension, the ECG may show: R/S ratio in V1 >1, right axis deviation, and ST depressions in V1-V3 (Tintinalli's Emergency Medicine)
• Common in mitral stenosis: ECG shows AF, left atrial enlargement, or right axis deviation

4. Left Ventricular Hypertrophy (LVH)

• Voltage criteria - Several exist; all have poor sensitivity (30-50%) but good specificity (85-95%)
• Cornell criterion: S in V3 + R in aVL >28 mm (men) or >20 mm (women)
• Sokolow-Lyon: S in V1 + R in V5 or V6 >35 mm
• Associated with ST depression and T wave inversion in lateral leads ("strain pattern")
• LV hypertrophy + left atrial enlargement are the most common findings in dilated cardiomyopathy (Tintinalli's)

5. Right Bundle Branch Block (RBBB)

• QRS duration ≥120 ms
• rsR' pattern ("M-shape") in V1 - the classic finding
• Wide, slurred terminal S wave in leads I, V5, V6
• ST-segment downsloping and T wave inversion in right precordial leads (V1-V3) - discordant with QRS
• Normal axis; septal Q waves present in lateral leads
• Can be isolated or combined with left anterior fascicular block (bifascicular block)

RBBB - note the rSR' in V1 and wide terminal S wave in V5 (Goldman-Cecil Medicine)

6. Left Bundle Branch Block (LBBB)

• QRS duration ≥120 ms
• Broad, notched (M-shaped) complex in I, aVL, and left precordial leads (V5-V6)
• Small r waves and broad, deep S waves in right precordial leads (V1-V3)
• ST and T waves discordant (opposite to) the QRS throughout the precordium
• Axis usually normal or deviated left
• New LBBB in a patient with chest pain is treated as a STEMI equivalent

D: Left posterior fascicular block (right axis deviation ~120°). E: LBBB - broad notched R waves in lateral leads, deep S in right precordial leads (Goldman-Cecil Medicine)

7. ST-Elevation Myocardial Infarction (STEMI)

• ST elevation ≥1 mm in ≥2 contiguous limb leads, or ≥2 mm in ≥2 contiguous precordial leads
• Reciprocal ST depression in opposite leads
• Hyperacute T waves - earliest finding (tall, broad, asymmetric)
• Pathological Q waves - develop within hours to days (>40 ms wide or >25% of R wave height)
• Localizing territory:
  • Inferior (II, III, aVF) = RCA occlusion
  • Anterior (V1-V4) = LAD occlusion
  • Lateral (I, aVL, V5-V6) = LCx occlusion
• STEMI management target: door-to-balloon <90 min; primary PCI preferred over thrombolysis (Textbook of Family Medicine)
• New LBBB or posterior STEMI (ST depression V1-V3 with tall R waves) are STEMI equivalents

8. Non-Specific ST-T Wave Changes

• ST depression or T wave inversion that does not meet ischemia criteria
• T wave flattening or subtle inversion
• Causes are extensive: ischemia, LVH strain, electrolyte disturbance (hypokalemia), digoxin effect, myocarditis, pericarditis, cardiomyopathy
• Must always be interpreted in clinical context
• Sinus tachycardia + nonspecific ST-T changes are the most common ECG findings in pulmonary embolism - they do NOT confirm or exclude the diagnosis

9. First-Degree AV Block (and Fascicular Blocks)

• PR interval >200 ms (>5 small squares)
• Every P wave conducts to a QRS - no beats dropped
• Often benign; seen in athletes, with inferior MI, or drug effect (beta-blockers, digoxin)

• Left anterior fascicular block (LAFB): Left axis deviation (around -60°), normal QRS duration, small Q waves in I/aVL, small R waves in II/III/aVF
• Left posterior fascicular block (LPFB): Right axis deviation (~+120°), delayed R-wave progression, requires exclusion of other causes of right axis deviation

10. Ventricular / Atrial Premature Beats

• Premature Ventricular Complexes (PVCs): Wide, bizarre QRS (>120 ms), no preceding P wave, full compensatory pause. Very common - seen in up to 30% of monitored patients. Usually benign in isolation but complex forms (couplets, bigeminy, multifocal) warrant evaluation
• Premature Atrial Complexes (PACs): Early, narrow QRS preceded by an abnormal P wave; often conducted with a slight PR change or blocked entirely
• Both are documented in ECG recording tables from Braunwald's Heart Disease and Goldman-Cecil as top findings in long-term Holter monitoring

Summary Table