The ECG

What are the necessary components of an electrocardiogram?

  • Starting from the patient end, the components of an ECG are:
    • Electrodes
      • These are adherent electrical conductors with a silver/silver chloride layer that connect to the patient’s skin, and detect tiny electrical potentials from the myocardium present in the skin
        • The skin needs to be dry, clean and free from hair
    • Cables
      • To safely conduct the electrical signals to the processing unit, the cables must be insulated electrical conductors with sufficient shielding to prevent significant interference from other instruments
    • Processor
      • This unit processes the incoming signal, using a variety of filters and amplifiers to enhance the desired signal and reduce interference
    • Monitor
      • To display the electrocardiogram

What is Einthoven’s law?

  • Three ECG electrodes placed at specific points on the body form the three corners of a triangle
    • The three ‘sides’ of this triangle are the leads I, II and III which can be used to measure voltage or potential difference (between 0.1 and 2mV) across the myocardium between two electrodes
      • Einthoven’s law states that lead I – Lead II + Lead III = 0
        • The positive (R) deflections and negative (Q) deflections of the QRS complexes in each lead are summed to give a net value for each lead. Einthoven’s law states the sum of these three net values is always zero

Localising a lesion using ECG

We are hugely grateful to Druv Bhagavan on twitter for his permission to share his wonderful images relating to ECGs and in particular, locating cardiac lesions based on the ECG findinds. His account is definitely worth a follow!

Shared with permission from Druv Bhagavan – original tweet here

Our Favourite ECG-related Tweets

If there is anyone you should follow on twitter for ECG wisdom, it’s Sam Ghali – who regularly posts insightful and well-explained cases to keep you up to date and practised in interpreting the most important ECG findings.