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Bronze Level Electrocardiography

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Aims

1. Brief summary of relevant clinical electrophysiology

2. Indications for taking an electrocardiogram (ECG)

3. How to obtain a diagnostic ECG4. Basic ECG interpretation

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Section 1 – electrophysiology for clinicians

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Unique properties of cardiomyocytes

1. Electrical syncytiumThis means that the cells are coupled together in a way that permits rapid conduction of electrical impulses

2. Automaticity This describes the ability of cardiomyocytes to spontaneously depolarise. Under normal conditions the cells of the sinoatrial node have the fastest rate of spontaneous depolarisation and therefore are the dominant pacemaker cells.

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What is the ECG measuring?

• Electrical activity detected at the body surface– Cardiac tissue– Neuromuscular tissue (= movement)

• Movement artefact such as trembling results in irregular baseline movement as shown below:

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Einthoven’s triangle• Dr Einthoven invented the first practical ECG in

1903• Einthoven’s triangle refers to the imaginary

equilateral triangle formed by the 3 standard limb leads

+ Lead I - - L

ead

II

+

The dots demonstrate the standard electrode positions

Left forelimb Left hindlimb - Lead III +

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Anatomy of the intracardiac conduction system

Sinoatrial node (SAN)

Atrioventricular node (AVN)

Bundle of His

Left bundle branchRight bundle branch

Right Left

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Origin of -QRS-T

+VE(Left hindlimb in lead II)

-VE(Right forelimb in lead II)

Wave of depolarisation moves from sinoatrial node across atria from right to left thereby creating a flow in current towards the positive electrode

ECG:

P

Right Left

P

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Origin of P- RS-T

Small delay as impulse traverses AVN hence trace returns to baseline. Depolarisation of the proximal interventricular septum then creates a small negative deflection – the Q wave.

ECG:

P Q

-VE(Right forelimb in lead II)

+VE(Left hindlimb in lead II)

Right Left

Q

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Origin of P- Q S-T

Wave of depolarisation moves rapidly through the conduction system to the heart apex thereby creating a flow in current towards the positive electrode – the R wave

ECG:

P QR

-VE(Right forelimb in lead II)

+VE(Left hindlimb in lead II)

Right Left

R

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Origin of P- QR -T

Wave of depolarisation moves from the cardiac apex towards the heart base

ECG:

P Q R S

-VE(Right forelimb in lead II)

+VE(Left hindlimb in lead II)

Right Left

S

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Origin of P- QRS-

Wave of depolarisation moves from sinoatrial node across atria from right to left thereby creating a flow in current towards the positive electrode

ECG:

P QRS T

-VE(Right forelimb in lead II)

+VE(Left hindlimb in lead II)

Right Left

T

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Section 2 - Indications for obtaining an ECG

• Common indications:– Document heart rate and rhythm– Dysrhythmia on auscultation

• Less common indications:– Electrolyte abnormalities– Suspected drug toxicity– Suspected cardiac chamber enlargement

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Section 3 - Obtaining a diagnostic ECG

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Patient set up for conscious ECG

• Patient calm and still• Good electrical contact• Clips over bony areas to reduce muscle

artefact• 50mm/s in leads I, II, III, aVL, aVR and aVF• 25mm/s rhythm strip for 1-5 minutes

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Machine set up for conscious ECG

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Set up for monitoring ECG

• Multi-parameter monitors• Tape ensures good contact between

electrode and pad

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Muscle movement artefact

A common artefact seen on ECG is movement artefact caused by electrical activity present in moving muscles being detected by the ECG This results in rapidly undulating baseline movement which does not disrupt the superimposed heart rhythm.

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Now move onto the second slide show