USC ECG Learning Center

• Duration: 80-110ms
• Morphology: Upright in I, II; upright or inverted in aVF; inverted or biphasic in III, aVL, V1, V2.
• Amplitude: <2.5mm

• In lead V1, positive deflection <1.5mm and negative deflection <1mm

• Duration 120-200ms

• Duration 60-100ms
• Axis: -30&deg to +90&deg
• Normal Q waves: small (<40ms in duration and <2mm in height, in most leads)

• Usually isoelectric (flat) but may vary by approximately 1mm above or below.

• Morphology: Upright in I, II, V3-V6. Inverted in aVR and V1. Maybe be upright, flat, or biphasic in other leads.
• Amplitude: Usually <6mm (limb leads) or <10mm (precordial leads)

• Corrected QT interval (QTc) duration: 300-460ms
• To calculate QTc: QTc = QT/sqrt(RR interval)

• Normal P wave morphology and axis
• Every P wave is followed by a QRS complex, and vice-versa
• Atrial rate is 60-100 bpm and regular

• Normal P wave morphology and axis
• Gradual change in PP interval
• Longest and shortest PP intervals vary by >160ms or 10%

• Normal P wave
• Rate <60 bpm

• Normal P wave
• Rate >100 bpm

• Conducted: Abnormal P wave that is premature relative to normal PP interval, and QRS complex is similar in morphology to QRS complex present during sinus rhythm.
• Non-conducted: Premature and abnormal P wave that is not followed by a QRS complex.
• With aberrant intraventricular conduction: Premature P wave followed by a QRS with abnormal morphology.

• P wave axis or morphology different from sinus node
• Three or more beats in succession at a rate of 100-180 bpm (up to 240)
• Regular rhythm
• Normal QRS follows each P wave

• Regular rhythm, rate >100 bpm
• P wave not easily identified

• Rapid regular atrial undulations at 240-340 per minute
• Typical atrial flutter morphology usually present in the inferior leads II, III, and aVF (”sawtooth” appearance)
• QRS complex may be normal or aberrant

• P waves absent
• Atrial activity is totally irregular, causing random oscillation of the baseline.
• Ventricular rhythm is irregular.

• Uniform: A wide QRS complex that is premature relative to the normal RR interval and not preceded by a P wave; morphology of VPC’s is the same.
• Multiform: VPC’s with >1 morphology

• Succession of 3 or more premature ventricular beats at a rate of >100 per minute.
• RR interval is usually regular but may be slightly irregular at its initiation.

• Rapid, irregular, and chaotic ventricular rhythm with undulating baseline and no distinct QRS complexes.

• In lead V1, biphasic P wave with a large and wide terminal portion (1 box wide and 1 box deep).
• In lead II, a humped P wave with >40ms between the first and second atrial components (humps).

• Biphasic P wave in lead V1, with the first (positive) component larger than the second (negative), or
• P wave height larger than 2.5mm in any limb lead

• PR interval >200ms (may be as long as 800ms)
• Each P wave followed by a QRS complex.

• Progressive prolongation of the PR interval and shortening of the RR interval until a P wave is blocked (i.e. not followed by a QRS complex)

• Regular sinus or atrial rhythm with intermittent nonconducted P waves
• PR intervals in the conducted beats are constant.

• Regular sinus or atrial rhythm with two P waves for each QRS complex (i.e. every otehr P wave is nonconducted)
• Note: Can be Mobitz type I or II 2nd degree AV block.

• Atrial and ventricular rhythms are independent of each other.
• Atrial rate is usually faster than the ventricular rate.

• Prolonged QRS duration (>120 ms)
• Broad R waves in leads V5 and V6 that are usually notched or
• Dominant S wave (either rS or QS) in V1 and/or V2
• Note:
  • LBBB interferes with determination of QRS axis and ECG diagnoses of ventricular hypertrophy and acute MI.

• Prolonged QRS duration (>120 ms)
• Secondary R wave (R’) in leads V1 and V2, with R’ usually taller than the initial R wave.
• Wide S wave in lead 1
• Note:

• RBBB does not interfere with the ECG diagnosis of ventricular hypertrophy of acute MI.
• In RBBB, mean QRS axis is determined by the initial unblocked 60-80ms of the QRS, and it should be normal unless left fascicular blocks are present.

• Limb leads only: Amplitude of the entire QRS complex (R+S) <5mm in all limb leads.
• Limb and precordial leads: Amplitude of the entire QRS complex (R+S)<5mm in all limb leads and <10mm in all precordial leads.
• Possible causes include: chronic lung disease, pericardial effusion, obesity, pleural effusion, restrictive cardiomyopathies, etc.

• Mean QRS axis is >-30°
• Possible causes include: LBBB, LVH, inferior wall MI, etc.

• Mean QRS axis is >90°
• Possible causes include: RVH, dextrocardia, lead reversal, etc.

• Cornell criteria: R wave in aVL + S wave in V3
  • >24mm in males
  • >20mm in females.
• Precordial leads: R V5 or V6 + S V1 > 35mm
• R aVL
  • > 11mm in males
  • > 9mm in females

• T wave abnormalities (flipped/inverted), usually seen in leads I, aVL, V5, and V6

• Right axis deviation (mean QRS axis >90&deg) or
• R wave > S wave in lead V1 or
• R wave > 7mm in lead V1

• T wave abnormalities (flipped/inverted) in V1-V3

• Abnormal Q waves

• Duration >30ms for most leads (>40ms in leads III, aVL, aVF, and V1)
• Indicates necrosis

ST segment elevation (acute myocardial injury)

• >1-2mm elevation in two more more contiguous leads
• Usually upwardly convex
• Can last 48 hours to 4 weeks after MI

T wave inversions

• Indicates ischemia
• May persist indefinitely

Determining age of infarct from ECG:

• Acute MI: Abnormal Q waves, ST elevation.
• Recent MI: Abnormal Q waves, isoelectric ST segments, inverted T waves.
• Old MI: Abnormal Q waves, isoelectric ST segments, normal T waves.

Myocardial infarction vs. ischemia

• Infarction: Abnormal Q waves; ST segment elevation or depression; T waves inverted, normal, or upright.
• Ischemia: ST segment depression; T wave inversion; Q waves absent.

• ST segment elevation in leads V1-V6
• Abnormal Q waves (occur approximately 1 hour after onset)

• ST segment elevation in leads V1-V4
• Abnormal Q waves (occur approximately 1 hour after onset)

• ST segment elevation in leads V1-V3
• Abnormal Q waves (occur approximately 1 hour after onset)

• ST segment elevation in leads I and aVL
• Abnormal Q waves (occur approximately 1 hour after onset)

• ST segment elevation in at least two of leads II, III, aVF.
• Abnormal Q waves (occur approximately 1 hour after onset)

• R wave in V1 and V2 (Q waves from the opposite side)
• R wave > S wave
• ST segment depression in V1 and V2

• Same as recent/acute, but no ST segment elevation.

• Same as recent/acute, but no ST segment elevation.

• Same as recent/acute, but no ST segment elevation.

• Same as recent/acute, but no ST segment elevation.

• Same as recent/acute, but no ST segment elevation.

• Same as recent/acute, but no ST segment elevation is characteristic of acute posterior injury.

• Corrected QT interval (QTc) >420-460ms.
• (QTc = QT divided by the square root of the preceding RR interval)

• QT interval more than half of the cardiac cycle.

• Sagging ST segment depression with upward concavity (”scooped out” ST)

• Can cause almost any type of cardiac arrhythmia, except bundle branch blocks.
• Typical abnormalities include: atrial and junctional premature beats, atrial fibrillation with complete heart block, 2nd or 3rd degree AV block, supraventricular tachycardia, etc.

• Moderate (K⁺ = 5.5-7.5 mEq/L):

• Tall, peaked T waves
• Wide, flattened P waves
• QRS widening

Extreme (K⁺ > 7.5 mEq/L):

• Disappearance of P waves
• Markedly widened QRS

• Prominent U waves
• Flattened T waves
• ST segment depression

• QT shortening

• Prolonged QT segment

• Can include one or more of the following:

• Marked sinus bradycardia
• Bradycardia alternating with tachycardia
• Atrial fibrillation
• Others…