• Implies ventricular depolarisation is slow (<0.12s maximum); anything higher is considered ‘wide/broad.’

• Bundle Branch Block – Ventricle unable to be depolarised due to block
• Hyperkalaemia – Causes slow transmission in cells
• Drugs – Antiarrhythmics, antidepressants etc.
• Pacemaker with ventricular stimulation
• Wolf-Parkinson-White Syndrome

• Classified into net positions (net positive or negative, based on the sum of the positive or negative areas from the baseline.)
• No specific calculations, assessment is based upon visual inspection, as per below:

Septal q-wave seen in lateral leads (V5-V6, aVL, & Lead I) due to depolarisation of ventricular septum Isolated & large Q-wave is seen Lead III which amplitude variate with ventilations (respiratory Q-wave).

• Small Q-waves (do not fulfil pathology) may be seen in all limb leads, plus V4-V6.
• V1-V3 must only display small q-waves only (not Q-waves)

Common cause in AMI – Q-wave infarction is caused if AMI leaves pathological Q-waves; must be in two (2) contiguous leads to be abnormal. These causes include:

• AMI
• Left side pneumothorax
• Dextrocardia
• Perimyocarditis
• Cardiomyopathy
• Amyloidosis
• Bundle Branch Block / Fascicular Blocks
• Wolf-Parkinson-White (WPW) Syndrome
• Ventricular hypertrophy
• Pulmonary Embolism

High amplitudes in R-waves can suggest ventricular enlargement or hypertrophy; this is determined by;

• V5 & V6 R-wave is < 26mm high
• V5 & V6 R-wave is < 35mm high
• V1 S-wave is < 35mm high
• aVL R-wave is < 12mm high
• Lead I, II, & III R-wave is < 20mm high

If R-wave in V1 is larger than S-wave in V1 then R-wave should be < 5mm high

Should be natural QRS complex progression of amplitude (height) from V1 to V6 – Due to the contraction direction to left and downward, V1 & V2 should be negative.

• V1 to V2 should be negative due to contraction direction
• V3 to V4 should be positive then taper off in V5
• V5 to V6 should tamper off

Mostly positive in most leads, with amplitude decreasing with increased age. The ST segment transition to T-wave should be smooth. It can be asymmetrical, as it has a slight upslope (1st half) and downslope (2nd half).

T-Wave changes are easily misinterpreted (mostly inverted T-waves), so take caution.

• Women have more asymmetrical T-wave and distinct ST-T transition with lower amplitude
• T-Wave's must be concordant (same direction as net previous QRS complex) to be considered normal.
  • A concordant positive T-wave follows a positive QRS complex
    
  • A discordant T-wave is positive after a negative QRS; this is considered abnormal
• Negative T-waves are also termed 'inverted T-waves'

Usually show inverted T-waves due to different electrical vector (inversion may be present in all leads); should normalise during puberty.

• Some late adolescent can display inversions in V1-V4 (Persisting Juvenile T-wave)
• If all T-wave remain inverted in adulthood it is called ‘Idiopathic global T-wave inversion’

Progression in T-waves should follow R-wave progression rule.

< 6mm in limb leads / 8-10mm in V2-V3 – Outside these ranges are abnormal caused by hyperkalaemia; deafferented with hyperacute T-waves seen in ACS (broad-based, high & symmetric).

V1 concordant is normal (as V1 QRS is negative); can occur in L3, V2 & aVL – Two inversions in two contiguous leads is a medical concern.

• Inversion with ST deviation are possible ischemia (usually invert after ischaemic event)
• Post-AMI the T-wave can be symmetric with varying depth (>10mm or <1mm)
• T-wave inversion can be chronic after AMI
• Secondary T-wave inversions can occur from many conditions – They are symmetrical

Low amplitude common in post-ischemia; common in V1-V3 if stent is in LAD or L2, L3, & aVF (RAD/Cx)

Has a positive & negative deflection – Biphasic has no significance and classified as per positive or inverted.

Primary ST-T Change:

Caused by abnormal repolarisation

• Ischemia
• Electrolyte Disorders
• Tachycardia
• Increased Sympathetic Tone
• Drugs

Secondary ST-T Change:

Abnormal depolarisation causing abnormal repolarisation

• Bundle branch blocks
• Pre-excitation
• Ventricular hypertrophy
• Premature ventricular complex
• Pacemakers stimulated beats

Represents total time for depolarisation and repolarisation (beginning of QRS to end of T-wave). Prolonged QR duration leads to dangerous ventricular arrhythmias – Inversed to heart rate (ie. High HR = Short QT).

• Congenital cause of prolonged QT includes genetics, & long-QT syndrome
• Acquired cause of prolonged QT includes medications, & electrolyte disorders

Explains the inverse relationship of QT & heart rate – This creates the corrected QT duration (QTc) calculated automatically – Cannot be used if bradycardia or tachycardia.

Note: All variables are in seconds (s)

• Males: < 0.450s
• Females: < 0.460s
• Short QTc Syndrome can be < 0.390s – uncommon

• Antiarrhythmics (-mides, -olol, & Amiodarone)
• Psychiatric medications
• Antibiotics
• Hyperkalaemia
• Hypocalcaemia
• Hypomagnesemia
• Cerebrovascular Bleeding
• Ischemia
• Cardiomyopathy
• Bradycardia
• Hypothyroidism
• Hypothermia

• Hypocalcaemia
• Digoxin treatment
• Malignant Arrythmias

Difference between the shortest & longest QT interval. This is due to the variations of QT interval in different leads,

• Long QT dispersion is increased risk group due to higher incidence of ventricular arrythmias

References

College of Pre-Hospital Care. (2015). 12-Lead ECG Analysis: Self-Directed Learning Package. Version 3. St John Ambulance Ltd.  

Curtis, K., & Ramsden, C. (2016). Emergency and trauma care for Nurses and Paramedics (2nd ed.). Elsevier Australia.

DeLaune, S. C., Ladner, P. K., McTier, L., Tollefson, J., & Lawrence, J. (2016). Australian and New Zealand fundamentals of nursing (1st ed.). Cengage Learning Australia Pty Limited.

ECG & ECHO Learning. (2020). Clinical ECG Interpretation. https://ecgwaves.com/topic/ecg-normal-p-wave-qrs-complex-st-segment-t-wave-j-point/

Life in the Fast Lane. (2020). ECG Library. https://litfl.com/ecg-library/

St John WA Ltd. (2017). Electrocardiography (ECG). Clinical Resources. https://clinical.stjohnwa.com.au/clinical-skills/assessment/vital-signs/electrocardiography-(ecg)

WikiEM. 2020. The Global Emergency Medicine Wiki. https://www.wikem.org