L4 Cardiac conduction system and ECG

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27 Terms

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Learning objectives

  • list the pacemaker cells of the heart and understand its electric conduction system

  • Compare the action potentials in heart cells: pacemaker cells vs. non-pacemaker cells

  • Rationalise the regulation of heart rate

  • Understand the principles behind ECG

  • Describe the normal ECG trace and the corresponding electrical events in the heart

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What are some shared characteristics between cardiac muscle and skeletal muscle?

Striated, tropononin-tropomyosin, length-tension relationship, T-tubules

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What are some shared characteristics between cardiac muscle and smooth muscle?

Calcium enters from extracellular fluid (ECF) and sarcoplasmic reticulum (SR), pacemaker activity, gap junctions

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What are some unique characteristics of cardiac muscle?

Syncytium, long action potential (AP)

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Pacemaker AP

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Non-pacemaker AP

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What are the pacemaker cells (4)?

  • Sinoatrial (sinus) node

  • Atrioventricular node

  • Bundle of His

  • Purkinje fibres

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What nerve is activated by the PNS?

Vagus nerve → SA + AV node → acteylcholine → muscarinic receptors

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What happens when PNS system is activated?

↓ HR

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What is activated by SNS?

Noradrenaline → b1 receptors → SA node and myocardium

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What happens when the SNS is activated?

↑ increase HR, ↑ contraction force

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P wave

Atrial depolarisation

  • SA nodes undergo depolarisation

  • Message goes to AV node

  • Message went through interatrial pathway

    • Left/right atrium will contract soon

<p>Atrial depolarisation</p><ul><li><p>SA nodes undergo depolarisation</p></li><li><p>Message goes to AV node</p></li><li><p>Message went through interatrial pathway</p><ul><li><p>Left/right atrium will contract soon</p></li></ul></li></ul>
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QRS complex

Ventricular depolarisation

  • bundle of His/purkinje fibres undergo depolarisation

  • 2 ventricles will contract soon

  • Atrial repolarisation

<p>Ventricular depolarisation</p><ul><li><p>bundle of His/purkinje fibres undergo depolarisation</p></li><li><p>2 ventricles will contract soon</p></li><li><p>Atrial repolarisation</p></li></ul>
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T wave

Ventricular repolarisation

  • ventricles will relax soon

<p>Ventricular repolarisation</p><ul><li><p>ventricles will relax soon</p></li></ul>
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Bipolar limb leads

I,II,III → make up the Einthoven’s triangle

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I

Measures from right arm to left arm

<p>Measures from right arm to left arm</p>
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II

Measures between right arm and left leg

<p>Measures between right arm and left leg</p>
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III

Measures from left arm to left leg

<p>Measures from left arm to left leg</p>
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What happens if there is something wrong at I?

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What happens if there is something wrong at II?

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aVR (unipolar lead)

Between I and III

<p>Between I and III</p>
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aVL (unipolar lead)

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aVF (unipolar lead)

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<p>Sinus arrthymia</p>

Sinus arrthymia

On inspiration: ↑ HR

On expiration: ↓ HR, cardioinhibitory centres in medulla via vagus nerve

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