Defined as an abnormal heart rhythm or heart rate not physiologically justified – Usually caused by abnormal formation or transmission of impulses.

Typical Behaviours of impulses includes;

  • Bradycardia (Slow)
    • Usually from dysfunctional automaticity of pacemakers or impulse block resulting in slower impulse
  • Tachycardia (Fast)
    • Resulted from impulse spreading out from atria (Supraventricular) or ventricle (Ventricular) resulting in rapid/faster impulse.
Impact of antiarrhythmic drugs on the ECG

For treating heart failure and irregular heartbeat like AF – The drug enhances ventricular contractility but lowers heart rate by making more calcium available and increasing vagus nerve activity (cause diminishes SA node automaticity).

  • ST depression with curved appearance will occur
  • T-wave amplitude diminishes (can appear biphasic) or inverted wave
  • U-wave slightly increase in amplitude
  • QTc interval shortened at therapeutic doses

For treatment of ventricular tachyarrhythmias or AF

  • Prolonged QRS
  • ST depressions may develop
  • T-wave may diminish in amplitude
  • QT & QTc interval may prolong
  • May provoke ventricular arrythmias

For treatment of ventricular tachyarrhythmias or symptomatic paroxysmal supraventricular tachyarrhythmias.

  • Prolonged QRS
  • QT & QTc interval may prolong

Antiarrhythmic drug for serious SVT or VT, and WPW syndrome & AF.

  • Prolonged QRS
  • Wider T-wave
  • Obvious U-wave
  • QT & QTc interval may prolong
  • Sinus bradycardia &/or torsade de pointes may show

Acute treatment of SVT & VT or controlling AF after return to sinus rhythm.

  • PR interval prolonged
  • QT & QTc interval may prolong
Impact of Beta-Blockers on the ECG

Role to reduce effect of catecholamines on the heart resulted from low heart rate, &/or decrease conductivity – aim to block the effect of adrenaline.

The SA node decreases in automaticity (sinus rate decrease)  

  • PR interval prolonged
  • QT & QTc may be shortened
  • Slower heart rate seen
Impact of calcium channel blockers on the ECG

Prevention of calcium from entering the heart cells and arteries decreasing the muscle action – This therefore, cause muscle relaxation – Less forceful contractions, vasodilation etc.

  • PR interval prolonged
  • QT & QTc may be shortened
Impact of electrolyte imbalance on the ECG

Increased (hypernatremia) or decreased (hyponatremia) sodium levels do not affect the ECG as sodium is more related to fluid volume control.

Increased Calcium (Hypercalcaemia)

Caused by hyperthyroidism or malignancies in most causes – other imbalances or diseases can cause this condition;

  • Shortened QT interval (longer QRS duration)
  • Bradycardia may occur

Potential complications can occur;

  • Increased QRS amplitude
  • Diminished T-wave amplitude
  • STE in V1-V2
  • All degrees of AV block
  • Osborn-like waves
Decreased Calcium (Hypocalcaemia)

Caused by issues relating to the pancreas, sepsis, alkalosis or GIT issues.

  • Lengthened QT interval (short QRS duration)
  • Torsade De Pointes can occur

Potential complications can occur;

  • AV block
  • Sinus bradycardia
  • SA block
  • VF

Hypermagnesemia is rare but can cause AV and intraventricular conduction disturbance which can result in asystole or third-degree AV block.

Hypomagnesemia can turn into effects of digoxin and show SVT or VT.

Hyperkalaemia (High Potassium)

High levels of potassium resulting in decrease impulse transmission to the whole heart – Caused by many medical factors and diseases.

Mild (> 6mmol)

  • Pointed T-waves (tall & narrow top)
  • LVH patients can show secondary T-wave inversion (V5-V6, aVL & Lead I)


  • Like mild symptoms but more pronounced
  • Wider P-wave (amplitude decrease)
  • PR prolonged
  • WPW syndrome patient loses delta-wave
  • STE in V1-V3

Severe (> 7.5mmol)

  • Like mild & moderate but more pronounced
  • Wide QRS
  • QRS may fuse with T-wave (forms sine wave)
  • VF may occur after sine wave is formed
Hypokalaemia (Low Potassium)

When levels are < 3mmol from fluid loss, malnutrition, medical/injury or alcohol use – Can be caused by medications such as steroids or diuretics & insulin.

  • Wider T-wave (lower amplitude)
  • ST depression can occur with T-wave inversion
  • P-wave amplitude & duration increase (increase PR interval)
  • U-wave emerge (Usually V2-V3)
  • U-wave can be larger than T-wave in worse case 


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

Page contributors:

60825Thanh Bui, AP60825
Event Medic, Emergency Medical Technician &
Volunteer Development Officer



Andrew Moffat, AP16790
Volunteer Training Manager & Volunteer Development Officer

Want to help improve this article? Visit our Contribute page.

St John Ambulance Western Australia Ltd © Copyright 2020, All Rights Reserved

Privacy Policy | Copyright Statement & Disclaimer