Study Notes: Systematic Approach to a 12-Lead EKG

A 12-lead EKG provides a snapshot of the heart’s electrical activity from multiple angles. A systematic approach ensures nothing is missed — especially life-threatening arrhythmias and ischemic changes.

PART 1: EKG BASICS & FOUNDATIONAL CONCEPTS

1. Waveform Components

Component	Represents	Key Notes
P Wave	Atrial depolarization	Initiated by SA node
PR Segment	AV node conduction delay	Flat line after P wave
PR Interval	SA node firing → ventricles ready	0.12–0.20 sec normal
QRS Complex	Ventricular depolarization	<0.12 sec normally
ST Segment	Ventricles fully depolarized	Look for elevation/depression
T Wave	Ventricular repolarization	Tall/flat/inverted = pathology
QT Interval	Depolarization + repolarization	Risk for Torsades if prolonged

2. Leads & Heart Surfaces

Region	Leads	Artery / Wall
Inferior	II, III, aVF	RCA
Lateral	I, aVL, V5, V6	Circumflex / LAD
Anterior	V1–V4	LAD
Right Ventricular	V1, V2, aVR	RCA

Lead System

• 3 Bipolar limb leads (I, II, III)

• 3 Augmented limb leads (aVR, aVL, aVF)

• 6 Precordial (V1–V6)

3. EKG Paper & Time Values

Box Type	Time (width)	Voltage (height)
1 small box	0.04 sec	0.1 mV
1 large box (5 small)	0.20 sec	0.5 mV

PART 2: SYSTEMATIC INTERPRETATION STEPS

STEP 1: RATE & RHYTHM

A. Rate

• Normal: 60–100 bpm

• Bradycardia: <60 bpm

• Tachycardia: >100 bpm
  Methods:

1. Count R waves in 10 seconds × 6

2. 300 ÷ number of large boxes between R waves

B. Rhythm

• Regular or irregular?

• QRS narrow (<0.12s) or wide (>0.12s)?

• Sinus rhythm = Upright P in II, inverted in aVR, every P followed by QRS

C. Rhythm Differential Summary

Type	Common Causes
Narrow Regular Tachycardia	Sinus Tachycardia, SVT, Atrial Flutter (2:1)
Narrow Irregular Tachycardia	A-fib, A-flutter variable, MAT
Wide Regular Tachycardia	V-tach (until proven otherwise), SVT + BBB, WPW
Wide Irregular Tachycardia	Torsades, A-fib + WPW, A-fib + BBB
Bradycardia	Sinus Brady, AV Block, Junctional, Ventricular

STEP 2: ST SEGMENT (Ischemia & Infarction)

A. ST Elevation (STEMI)

• Measure at J-point

• Significant:

  • ≥1 mm in 2 contiguous leads

  • ≥2 mm in V2–V3 (men), ≥1.5 mm (women)
    Causes: STEMI, early repolarization, pericarditis, vasospasm, PE, LV aneurysm, LBBB (use Sgarbossa criteria)

B. ST Depression

• ≥0.5 mm in 2 contiguous leads

• Types: Horizontal (ischemia), Upsloping (less severe), Downsloping (digoxin)

• Causes: NSTEMI, posterior MI, LVH strain, digoxin, LBBB

C. J Wave (Osborne)

• Small positive deflection at QRS end

• Seen in: early repol, hypothermia, hypercalcemia, Brugada

STEP 3: T WAVES

Finding	Interpretation
Normal inversion	V1–V2, III
Inverted in aVL / V2–V3	Ischemia (Wellens’ Syndrome = LAD lesion)
Hyperacute (tall, broad)	Early STEMI, Prinzmetal’s angina
Biphasic (pos→neg)	Wellens Type A
Biphasic (neg→pos)	Hyperkalemia
Flat	Ischemia or Hypokalemia
Peaked (narrow)	Hyperkalemia, Hypermagnesemia, de Winter’s pattern

STEP 4: QRS COMPLEX

A. Width

0.12 sec = wide
→ think BBB, hyperkalemia, VT, WPW, pacemaker, TCA toxicity

B. Bundle Branch Blocks

• LBBB: Deep S in V1 + M-shaped in V5/V6

• RBBB: RSR' in V1/V2 + slurred S in V5/V6

C. Pathologic Q Waves

0.04 sec wide, >2 mm deep, or >25% QRS height
→ Indicates infarction (old or new)

D. Low Voltage

• Limb: <15 mm total, Precordial: <30 mm

• Seen in effusion, COPD, obesity, HF, amyloidosis

E. R Wave Progression

• Poor progression = anterior MI

• Dominant R in V1/V2 = posterior MI

F. Ventricular Hypertrophy

Type	Criteria	Notes
LVH	(R in V5/V6 + S in V1/V2) > 35 mm	Common in HTN
RVH	(R in V1/V2 + S in V5/V6) > 10 mm + RAD	Suggests pulmonary disease or RV strain

STEP 5: QT INTERVAL

• QTc > 450 ms (men), > 460 ms (women) = prolonged

• Rough guide: QT < ½ R-R interval

Causes of Prolonged QT:
Anti-arrhythmics, Antibiotics, Antipsychotics, Antidepressants, Anti-emetics, low K⁺/Mg²⁺/Ca²⁺, ischemia
Short QT: Hyperkalemia, Hypermagnesemia, Digoxin

STEP 6: P WAVES & PR INTERVAL

A. Atrial Enlargement

Finding	Lead II	Lead V1	Cause
Right Atrial Enlargement	Tall P >2.5 mm	Large positive deflection	Pulmonary HTN
Left Atrial Enlargement	Notched “M-shaped” P	Deep negative deflection	Mitral disease

B. PR Interval

Finding	Interval	Interpretation
Short PR	<0.12 sec	WPW, PACs
Prolonged PR	>0.20 sec	AV block
Mobitz I	Gradual lengthening → dropped QRS
Mobitz II	Fixed PR, sudden dropped QRS
3rd Degree	Atria & ventricles beat independently

STEP 7: AXIS DETERMINATION

Use Leads I and aVF (“Thumbs Rule”):

Finding	Lead I	aVF	Axis	Common Causes
Normal	+	+	0° to +90°	Normal
Left Axis Deviation	+	–	–30° to –90°	LBBB, LVH, Inferior MI
Right Axis Deviation	–	+	+90° to +180°	RBBB, RVH, Lateral MI
Extreme Deviation	–	–	–90° to ±180°	VT, severe RVH, dextrocardia

KEY TAKEAWAYS

1. Always assess life-threatening patterns first – arrhythmias, STEMI, VT.

2. Interpret in order: Rate → Rhythm → ST Segment → T Waves → QRS → QT → P Waves → Axis.

3. Compare with previous EKGs for dynamic changes.

4. Correlate with symptoms (chest pain, syncope, SOB).