ECG Interpretation

ECG Grid

• each 1 mm horizontal box represents 0.04 seconds
• each heavier line occurs at 0.20 second intervals
• vertically, 10mm corresponds to 1 mV
• normal paper speed is 25 mm/sec

Normal Intervals

• PR: 0.12-0.20 sec
• QRS: 0.06-0.10 sec
• QT: < 0.44 sec

P Wave

• Duration: 0.08 - 0.11 sec
• Morphology: Upright in I, II; upright or inverted in aVF; inverted or biphasic in III, aVL, V1, V2; small notching may be present
• Amplitude: Limb leads < 2.5 mm; V1: positive deflection < 1.5 mm and negative deflection < 1 mm
• Normal sinus P wave means an upright complex in I, II, aVF
• P wave is often notched in the limb leads and may be biphasic in V1

PR Interval

• Duration: 0.12 - 0.20
• PR segment: usually isoelectric; may be displaced ina direction opposite to the P wave; elevation is usually < 0.5 mm; depression is typically < 0.8 mm
• Measures from the intiation of P wave to the start of QRS
• shortens with tachycardia
• measures the AV conduction time

QRS complex

• Duration: 0.06 - 0.10
• Axis: -30 to + 105
• Transition Zone: precordial leads with equal positive and negative deflection V2-V4.
• Q wave is due to septal depolarization
• Q wave: small Q waves (duration <0.04 sec and amplitude < 2mm) are common in most leads except aVR, V1 and V2
• R wave is caused by depolarization of the LV; RV depolarization is obscured by the larger mass of the left chamber
• S wave is due to the terminal depolarization of the high lateral wall
• A normal QRS lasts 0.06-0.10 sec and measures ventricular conduction time
• If the entire complex is negative, it is labelled a QS wave
• Low case letters (q, r, s) signify low amplitude waves less than 0.5 mV (5mm on graph)

ST Segment

• normally an electrically neutral segment; usually isolelectric
• J point refers to the point of initiation of the ST segment
• compare it to the TP interval to determine if the ST is depressed, isoelectric, or elevated

T Wave

• amplitude: usually < 6 mm in limb leads and < 10 mm in precordial leads
• represents ventricular repolarization
• this process is slower than depolarization so the T wave is wider than the QRS normally
• the shape is usually asymmetric because the initial upslope occurs more slowly
• in normal circumstances the T-wave amplitudes proceeds in the same direction as the QRS

QT Interval

• defined as the duration from the start of the Q wave to the end of the T wave
• when a BBB exists, it is more accurate to measure as the JT interval
• QTc - QT ÷ square root of the RR (seconds)
• normal values are < 0.44 sec; 0.33-0.44 sec; varies inversely with HR
• it essentially measures depolarization plus repolarization times; ie. total systole
• best to measure in leads with the most prominent T waves

U wave

• Morphology: upright in all leads except aVR
• Amplitude: 5-25% of the height of the T wave (usually < 1.5mm)

Determining Rate

• rate as measured per thick graphic line (0.20 sec) proceeds as follows:
• 300, 150, 100, 75, 60, 50, 43, 37

Axis

• Left Axis (S>R in lead II)
• Right Axis (S>R in lead I)

LAD, Causes of

• -30 to -90 degrees
• Normal variant
• Left anterior hemiblock
• Left bundle branch block
• LVH
• Inferior MI
• Elevated diaphragm

RAD, Causes of

• +90 to +180 degrees
• Normal variant (vertically positioned heart)
• RVH
• Left posterior fascicular block
• Lateral MI
• COPD

LBBB

• QRS duration exceeds 0.12 sec
• broad, slurred, monophasic R wave in leads I, V5, V6
• Loss of the normal small Q in I, V5, V6
• ST and T waves are oppositely directed to the major deflection at the QRS

LBBB, etiologies of

• CAD
• HTN
• Cardiomyopathy
• Degeneration of conduction system

RBBB

• QRS duration exceeds 0.12 sec
• RSR patter in V1 or V2
• Wide S wave in I, V5, V6

Etiologies of RBBB

• CAD
• HTN
• Cardiomyopathy
• Degeneration of conduction system
• cor pulmonale (tall peaked P waves, RAD, RVH)

LVH

• Cornell Criteria: R wave in aVL and S wave in V3; > 24 in males, > 20 in females
• Succinct alternative: R wave in V5 or V6 exceeds 27 mm; OR S wave in V1 plus R in V5 or V6 exceeds 35 mm; OR R wave in AVL exceeds 11 mm

LVH, Causes of

• Thin chest wall
• Left mastectomy
• normal variant in some blacks
• L anterior hemiblock
• HTN
• AS
• Hypertrophic CM
• AR, MR
• Coarctation of the aorta
• PDA
• Athletic heart

RVH

• R wave exceeds S wave in V1 and RAD is present
• Persistent S waves in V5 and V6
• ST depression and T wave inversion in V1, V2, V3

Causes of RVH

• COPD
• Pulmonary HTN/PE
• MS/MR
• Chronic LV failure
• TR
• ASD

Atrial Enlargement, Left

• p wave duration exceeds 120 msec in elad 2 and biphasic; negative terminal defleciton in V1 exceeds 40 msec and greater than 1mm deep

Atrial Enlargement, Right

• P wave peaked in lead II greater than 2.5mm or greater than 1.5 mm height in V1

Pathologic Q waves

• Duration exceeds 40 msec or height exceeds 25% of R wave height
• Small q in I, aVL, V6 are normal
• isolated Q in III, aVR, and V, may also be normal

Poor R Wave progression

• R wave less than 3 mm in V3
• Causes: old anteroseptal MI, cardiomyopathy, LVH, RVH, LBBB, faulty lead placement

ST Elevation, Causes

• Acute MI (upward convex)
• coronary spasm
• pericarditis (upward concave)
• normal early repolarization; seen in V2-V5 in young adults, J point elevation 1-4mm, ratio of ST elevation to T wave height < 25%

ST depression, Causes

• ischemia
• digitalis influence
• hypokalemia

T Wave inversion, Causes

• MI/ischemia
• pericarditis
• cardiomyopathy
• intracranial bleed
• normal variant in III, aVF, V1, aVL, aVR
• juvenile T wave syndrome (persistent T wave inversions in V1-V4)

ECG evolution in MI

• Hyperacute tall T → ST elevation → q wave begins, T wave inverts → deeper Q wave days later and T wave still inverted → Q wave persists, T wave normalizes weeks later