How to Read an ECG

The ECG Waveform

The 14-Step Systematic Approach (Harrison's, p. 6812)

Step 1 — Calibration & Technical Quality

• Standard calibration: 1 mV = 10 mm (2 large squares tall)
• Paper speed: 25 mm/s (each small square = 0.04 s; each large square = 0.2 s)
• Check for artifact, poor lead contact, and correct lead placement

Step 2 — Rhythm

• Is it regular or irregular?
• Is there a P wave before every QRS? A QRS after every P?
• Sinus rhythm: P waves upright in leads I, II, aVF

Step 3 — Heart Rate

• Regular rhythm: 300 ÷ number of large squares between R-R intervals
  • 1 box = 300 bpm, 2 = 150, 3 = 100, 4 = 75, 5 = 60, 6 = 50
• Irregular rhythm: Count QRS complexes in a 10-second strip × 6

Step 4 — PR Interval (AV Conduction)

• Normal: 0.12–0.20 s (3–5 small squares)
• Short PR: pre-excitation (WPW), accelerated AV conduction
• Long PR: 1st-degree AV block
• Progressively lengthening → dropped beat: Mobitz I (Wenckebach)
• Fixed PR with dropped beats: Mobitz II

Step 5 — QRS Duration

• Normal: < 0.12 s (< 3 small squares)
• Widened QRS (≥ 0.12 s): bundle branch block, ventricular rhythm, hyperkalemia, drug toxicity

Step 6 — QT/QTc Interval

• Normal QTc: < 440 ms (men), < 460 ms (women)
• Correct for rate using Bazett's formula: QTc = QT ÷ √RR
• Prolonged QTc: risk of Torsades de Pointes (electrolyte abnormalities, drugs, congenital syndromes)

Step 7 — Mean QRS Axis

• Normal axis: −30° to +90°
• Quick method: if QRS is upright in leads I and aVF → normal axis
• Left axis deviation (< −30°): LBBB, inferior MI, left anterior fascicular block
• Right axis deviation (> +90°): RVH, RBBB, left posterior fascicular block, PE

Step 8 — P Waves

• Normal: upright in I, II; inverted in aVR; duration < 0.12 s; amplitude < 2.5 mm
• Peaked P in II (P pulmonale): right atrial enlargement
• Bifid P in II / biphasic P in V1 (P mitrale): left atrial enlargement
• Absent P waves: atrial fibrillation, sinoatrial block

Step 9 — QRS Voltages

• Low voltage (< 5 mm in all limb leads): pericardial effusion, emphysema, obesity, hypothyroidism
• Left ventricular hypertrophy (LVH): S in V1 + R in V5/V6 > 35 mm (Sokolow-Lyon)
• Right ventricular hypertrophy (RVH): dominant R in V1

Step 10 — Precordial R-Wave Progression

• R wave should grow from V1 → V5/V6
• Poor R-wave progression (dominant S beyond V3): anterior MI, LBBB, RVH

Step 11 — Abnormal Q Waves

• Pathologic Q wave: > 0.04 s wide OR > 25% of the R wave height in that lead
• Indicate prior MI (age and territory depend on which leads are affected)

Step 12 — ST Segments

• Normal: isoelectric (flat, at baseline)
• ST elevation: STEMI, pericarditis (saddle-shaped, diffuse), Brugada, early repolarization
• ST depression: NSTEMI/UA, posterior MI (V1–V2), digitalis effect, strain pattern

Step 13 — T Waves

• Normal: upright in I, II, V3–V6; inverted in aVR
• T-wave inversions: ischemia, RVH/LVH strain, RBBB/LBBB, CNS events
• Peaked T waves: hyperkalemia, early ischemia ("hyperacute T waves")

Step 14 — U Waves

• Small positive deflection after T wave (best seen in V2–V3)
• Prominent U waves: hypokalemia, bradycardia, antiarrhythmics
• Inverted U waves: may indicate ischemia or LVH

Quick Reference: Common ECG Patterns

Key principle (Harrison's, p. 6812): Always compare with prior ECGs — subtle changes are often more significant than absolute values. Errors in ECG interpretation most commonly result from skipping steps.

Clinical Assessment

Synthesizing the Clinical Picture

• Splenomegaly
• Prominent/dilated portal vein (USG)
• Iron deficiency anemia (likely from hypersplenism + sequestration)
• Normal upper GI endoscopy (no varices / no oesophageal varices bleed yet)
• Normal liver function tests
• Normal menstrual history currently (prior 4-month amenorrhea likely from anemia-related hypothalamic suppression)

Bailey & Love's (p. 1243): "Non-cirrhotic portal fibrosis, common in tropical countries, is associated with massive splenomegaly and pancytopenia without stigmata of liver dysfunction."

Differential Diagnosis

Most Likely Diagnosis: EHPVO or NCPF

"The clinical hallmarks of EHPVO are hemorrhage from gastric/esophageal varices and hypersplenism with splenic enlargement, thrombocytopenia, and less commonly ascites. Liver function is always preserved unless EHPVO occurs with a coexistent primary liver disorder."

USG Doppler Findings (What to Look For)

• Portal vein diameter (>13 mm = dilated — here already noted as prominent)
• Flow direction (hepatopetal vs. hepatofugal)
• Evidence of cavernous transformation of portal vein (EHPVO)
• Splenic vein diameter
• Hepatic echotexture (to rule out cirrhosis/fibrosis)
• Hepatic veins (to rule out Budd-Chiari)

Investigations Required

Key Points About the Anemia

1. Hypersplenism — splenic sequestration of RBCs → shortened RBC survival → compensatory increased iron utilization
2. Menstrual loss — prior episodes of delayed/prolonged cycles may have contributed (though now normal)

Management Outline (After Confirmation)

1. Iron supplementation — oral (ferrous sulfate) or IV iron if severe/malabsorption
2. Doppler-guided diagnosis — definitive imaging before any intervention
3. If EHPVO confirmed: Thrombophilia workup → anticoagulation if acute/subacute thrombosis → surgical evaluation (Meso-Rex bypass preferred; portosystemic shunt if bypass not feasible)
4. If NCPF: Conservative + endoscopic surveillance; beta-blockers for portal HTN if varices develop
5. Monitor CBC — particularly platelets (hypersplenism marker)
6. GI surveillance endoscopy — at regular intervals even if currently normal, as varices can develop

Summary

This 18-year-old female most likely has Non-Cirrhotic Portal Hypertension (EHPVO or NCPF) presenting with hypersplenism-related iron deficiency anemia and splenomegaly with portal vein prominence, in the absence of liver disease or variceal bleeding. The prior 4-month amenorrhea is best explained by anemia-induced functional hypothalamic suppression. The next critical step is a Doppler ultrasound of the portal venous system and CBC with peripheral smear to characterize the anemia and confirm hypersplenism.