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All the ECG’s and line diagrams are the author’s own images except where shown.

Basic Cardiac Electrophysiology &

Introduction to the ECG

Associate Professor Neil Strathmore Royal Melbourne Hospital University of Melbourne

Can the heart beat if removed from the body?

www.bbc.com

Single Cardiac Myocyte: Action Potential + Contraction

Arie Verkerk and Antoni van Ginneken, AMC, Amsterdam, The Netherlands. www.en.ecgpedia.org

Can the heart beat if removed from the body?

• Yes

• Electrical “heart beat” = action potential

• Initiated by spontaneously firing cells

• Heart rate & conduction modified by neural & hormonal input

• Conducted to all cardiac cells

Is the cell membrane polarised? Negative, Positive or Neutral

• Yes – all cells • Inside is NEGATIVE relative to the outside • Approx -90mV in cardiac cells

How do cells maintain their polarisation?

1. Semipermeable Cell Membrane

2. Na-K Pump 3. High intracellular K+

4. Low intracellular Na+

5. Negatively charged proteins

6. All of the above

Na+

Protein

What makes nerve & cardiac cells special?

• They can change their polarisation • Sudden Depolarisation = Action Potential

cvphysiology.com: R.E.Klabunde

How does the action potential happen? • Open Na+ channel • Influx of Na+ • Voltage change from – to +

=Depolarisation • Close Na+ channel • ...... • Open K+ channel • Efflux of K+ • Voltage change from + to –

=Repolarisation • Close K+ channel

Molecular Cell Biology. 4th edition. Lodish H, Berk A, Zipursky SL, et al. New York: W. H. Freeman; 2000. After Hodgkin & Huxley J. Physiol (1952) 117: 500-544

How are cardiac cells different from nerve cells?

• ….. • Ca channels open to prolong the action

potential duration

www.theeplab.com

Na+ Channel Opens

Ca++ Channel Opens

K+ Channel Opens

Can all cardiac cells depolarise spontaneously?

• No: Only “Pacemaker Cells” • Phase 4 of action potential: Slow Na+ influx • Gradual depolarisation until threshold

reached and action potential starts: Ca++ influx

www.cvphysiology.com

Na+ influx

Ca++ influx

Which area of the heart “sets” the heart beat normally?

• Sino-atrial Node • Fastest rate of spontaneous depolarisation • Autonomic nerve stimulation changes rate • Sympathetic system speeds rate • Parasympathetic (vagal) slows rate

www.cvphysiology.com

Can other areas of the heart set the rate?

• Yes • If S-A node is very slow

or fails or conduction is blocked

• Other pacemaker cells take over and set the rate e.g. AV Node, Atrium, Ventricle

• “ESCAPE RHYTHM”

How does the action potential get to the rest of the heart?

• Action potential causes depolarisation in next cell • Impulse moves cell-to-cell through myocardium • Specialised conduction tissue (His-Purkinje fibres)

conducts impulse rapidly

www.cvphysiology.com

What is the normal cardiac conduction sequence?

S-A Node èAtrium èAV Node – slower conduction èHis Bundle èR & L Bundles (Purkinje fibres) - endocardial èVentricle

Can AV Node Conduction be altered?

• Yes • Autonomic nerve

stimulation changes conduction velocity

• Sympathetic speeds up conduction

• Parasympathetic (vagus) slows down conduction

The Action Potential and the ECG

www.textbookofcardiology.org

Single Ventricular Cell Action Potential

Surface recording Multiple cells summated Action Potential moving through the heart

P: Atrium PR: AV Node QRS: Ventricular Depolarisation QT: Action Potential Duration T&U: Ventricular Repolarisation

www.ecgpedia.org

Wilhelm Einthoven 1860-1927 1st practical ECG: 1903 Nobel Prize: 1924

www.ecgpedia.org

www.aviva.co.uk

Reading an ECG • Identify the ECG: Name, Date, Time • Check the scaling: 25mm/sec 10mm/mV

www.geekymedics.com

ECG: 12 leads – information overload!! Look at the specific leads you need for the piece of information you want.

Limb Leads Chest Leads

Reading an ECG: Rate

300/no. of large squares = 300/4 = 75

1 2 7 3 4 5 6 8 9 10 11 12 13 10 sec page 6 X 13 = 76

Current through the heart

Current moving away from

LEAD

Current moving towards LEAD

Current moving perpendicular to

LEAD

Negative Positive

Zero

As the Action Potential (current) passes through the heart it causes an ECG deflection Size and Direction in any LEAD depends on the orientation of the LEAD to the current

1. I 2. II 3. III 4. V1 5. V2 6. V3

On an ECG the P wave represents the atrial depolarisation. This will be

maximal in which leads?

I

II III

V1 V2 V3

Reading an ECG: Rhythm Look for the P Waves: Best seen in Leads II and V1

Reading & References

• The ECG Made Easy: John Hampton • www.cvphysiology.com • www.textbookofcardiology.org • www.ecgpedia.org • www.ecglibrary.com • www.ecg.utah.edu