ECG

What is an ECG

An ECG (electrocardiogram) is used to record the electrical activity of the heart from different angles.
ECGs are recorded by placing electrodes on a patient
AN ECG lead is a graphical representation of the heart’s electrical activity, generated from several electrodes.

Wave/Segment	What It Represents	Normal Features	Key Abnormalities
P wave	Atrial depolarisation — the electrical activation of the atria, starting at the SA node and spreading through atrial muscle	Duration <0.12 s (3 small boxes), amplitude ≤2.5 mm, upright in leads I, II, aVF	Tall P (P pulmonale — right atrial enlargement), broad/notched P (P mitrale — left atrial enlargement), inverted P (ectopic atrial rhythm)
PR interval	Time from start of atrial depolarisation (P wave) to start of ventricular depolarisation (QRS) — includes AV node conduction delay	0.12–0.22 s	Prolonged → first-degree AV block; Short → pre-excitation (WPW)
QRS complex	Ventricular depolarisation — electrical activation of the ventricles via His-Purkinje system	Duration <0.12 s; Q wave small in some leads; R wave progression across V1–V6	Wide → BBB, ventricular rhythm; Pathologic Q → old MI; Poor R progression → anterior MI
ST segment	Early phase of ventricular repolarisation — period when ventricles are fully depolarised	Normally isoelectric (flat)	ST elevation → STEMI, pericarditis; ST depression → ischemia, digoxin effect
T wave	Ventricular repolarisation — recovery phase of ventricles	Upright in most leads (except aVR, V1); smooth	Inverted → ischemia, LVH strain; Tall/peaked → hyperkalemia; Flat → hypokalemia
U wave(optional)	Possibly after-potentials from Purkinje fibres	Small, follows T wave	Prominent in hypokalemia, bradycardia

Definition: The cardiac electrical axis is the net direction of electrical depolarisation in the ventricles, viewed in the frontal plane (limb leads).
It reflects the average vector of ventricular depolarisation during the QRS complex.
Measured in degrees, with 0° aligned with lead I’s positive pole.

Left Axis Deviation
- Left ventricular hypertrophy (LVH)
- Left anterior fascicular block
- Inferior MI
- Ventricular pacing
- LBBB
Right Axis Deviation
- Right ventricular hypertrophy (RVH)
- Pulmonary embolism
- Chronic lung disease / cor pulmonale
- Left posterior fascicular block
- Lateral MI
- RBBB (sometimes)
Extreme Axis
- Severe ventricular rhythm
- Ventricular tachycardia
- Hyperkalemia

Represents: Atrial depolarisation (right atrium first, then left)
Normal:
- Duration ≤0.12 s (≤3 small boxes)
- Amplitude ≤2.5 mm (limb leads)
- Upright in II, inverted in aVR, biphasic in V1
Abnormal:
- P pulmonale (RAE): Tall, peaked (>2.5 mm in limb leads)
- P mitrale (LAE): Broad (>0.12 s), M‑shaped in II, deep terminal negative in V1
- Ectopic atrial: Abnormal shape, axis
- Junctional rhythm: Inverted/absent P, may occur before/during/after QRS
Key associations: Chamber enlargement, valvular disease, chronic lung disease, arrhythmias

Represents: Time from start of atrial depolarisation (P wave) to start of ventricular depolarisation (QRS) → includes AV node delay.
Normal: 0.12–0.20 s (3–5 small boxes).
Abnormal:
- Prolonged (>0.20 s): 1st‑degree AV block.
- Progressive lengthening then dropped QRS: Mobitz I (Wenckebach).
- Fixed PR + random dropped QRS: Mobitz II (dangerous, may progress to complete block).
- No P–QRS relationship: 3rd‑degree (complete) AV block.
- Short PR (<0.12 s): Pre‑excitation (WPW), junctional rhythm.
Key associations: AV node disease, accessory pathways, conduction delay.

Represents: Ventricular depolarisation via His–Purkinje system.
Normal: Duration <0.12 s (≤3 small boxes), narrow, upright in most limb leads.
Abnormal:
- Wide (≥0.12 s): Bundle branch block, ventricular rhythm, hyperkalemia.
- Pathological Q waves: ≥0.04 s wide & ≥25% of R amplitude → old MI.
- Bundle branch block patterns:
  - RBBB: rSR′ in V1, wide S in V6.
  - LBBB: Broad notched R in V6, deep S in V1.
- Voltage changes:
  - Tall → LVH, RVH.
  - Low → pericardial effusion, obesity, COPD.
Key associations: Infarction, chamber hypertrophy, conduction defects.

Represents: Early ventricular repolarisation (plateau phase of action potential) when ventricles are fully depolarised.
Normal: Isoelectric (flat), duration ~0.08–0.12 s; compare with PR segment as baseline.
Abnormal:
- Elevation:
  - ≥1 mm in ≥2 contiguous limb leads OR
    ≥2 mm in ≥2 contiguous chest leads → STEMI.
  - Widespread concave elevation + PR depression → pericarditis.
  - Benign early repolarisation (young, healthy adults).
- Depression:
  - Horizontal/downsloping → ischemia, digoxin effect, LVH strain.
  - Upsloping (less specific) → may be normal with exercise.
Key associations: Acute coronary syndromes, pericarditis, electrolyte disturbances, normal variants.

Represents: Ventricular repolarisation (phase 3 of the cardiac action potential).
Normal:
- Upright in I, II, V3–V6.
- Inverted in aVR, may be inverted in V1 (normal variant).
- Smooth, asymmetrical (gradual upstroke, steeper downstroke).
Abnormal:
- Tall peaked → Hyperkalemia, early STEMI (hyperacute T waves).
- Inverted → Ischemia, ventricular strain (LVH/RVH), post-MI changes, pericarditis.
- Flattened → Hypokalemia, nonspecific change.
- Biphasic → Ischemia (type 1: up then down) or hypokalemia (type 2: down then up).
- Symmetrical tall/inverted → Often pathological (e.g., ischemia).
Key associations: Electrolyte imbalances, acute coronary syndromes, ventricular hypertrophy, CNS events.

Represents:
- Thought to be after-depolarisations of the ventricles, possibly from Purkinje fibres or papillary muscle repolarisation.
- Not always visible; usually small and follows the T wave.
Normal:
- Upright in most leads (same polarity as T wave).
- Best seen in V2–V3.
- Amplitude ≤1–2 mm.
Abnormal / Prominent:
- Hypokalemia → tall, prominent U wave.
- Bradycardia → slower heart rates make U waves more obvious.
- Hypothermia, antiarrhythmic drugs (e.g., sotalol, amiodarone), digitalis effect.
- May be fused with T wave in severe cases, risking arrhythmias (torsades de pointes).
Inverted U waves:
- Can indicate ischemia, LVH, or raised intracranial pressure.