Recording & interpreting the ECG

• What is the ECG?• How does it work?• Procedure for the OSCE• Tips for this station• Questions

Contents

What is the ECG?

• ECG stands for electrocardiogram• Its a recording of the heart’s electrical activity• Useful in demonstrating heart pathology

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How does it work?

How does it work?

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The procedure

1. Relax, but be ready2. Wash your hands using the gel provided3. Introduction, explanation, consent

“Good afternoon. My name is Umar Hussain and I’m a 1st year medical student. I’m here today to record an ECG exam. Basically it will involve me attaching these wires on to your wrists and ankles; the computer will take a recording for about a minute and then we’d be done. At no point should thisexam be painful, but if it does get to that stage then please let me know. Is this ok with you?”

The procedure

4. The patient will already have electrodes attached to them but if not:– Clean the inside of both wrists and ankles with a

mediswab– Attach 5 electrodes to the patient:• 2 for the right wrist• 1 for the left wrist• 1 for the left ankle• 1 for right ankle

The procedure

5. Connect the wires in the following sequence:– Right wrist: Red and white– Left wrist: Black– Right ankle: Green– Left ankle: Brown

The wires should have labels on them (e.g. RA for right arm on the red and white wires) but learn the colours just in case

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The procedure6. Set sampling rate on computer to 400/s7. Set time axis compression to 10:1

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The procedure8. Check the patient is comfortable9. Ask the patient to sit quietly and to remain as still as

possible for about a minute10. Click the start button

The procedure

11. Once you have at least 10 cycles then stop recording

12. Disconnect the leads from the patient12. Thank the patient for their cooperation

Analysing the ECG

• Now you have your recordings• BUT you won’t use them!• There is an ECG trace on a separate computer

ready for you to analyse, for two reasons– Examiners– Good for you (in case you made a mess of your

ECG recording there’s still marks to gain)

Analysing the ECG• Now it’s time to select an ECG recording from the

many on the screen12. Select one of the leads which gives a large and clear

R wave (lead 2)

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Analysing the ECG• If you still find it hard just try to see which lead has a

recording closest to the “textbook ECG”13. Click and hold down the mouse and drag a box over

2 complete cardiac cycles, which you think are best

Analysing the ECG

14. Click on the windows menu and select Zoom Window from the drop down menu

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Zoom Window...

Analysing the ECG

15. You now have a zoomed in view of the two cycles you need to analyse, but to make use of this you need to know the theory...

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The ECG recording

• You will be asked to measure the duration of:– Segments– Intervals

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The ECG recording - Segments

• The P-wave (P segment)– Formed when the atria are depolarised– Contraction of the atria occurs shortly after

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The ECG recording - Segments

• The QRS complex (QRS segment)– Formed when the ventricles are depolarised– Much larger due to larger size of ventricles

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The ECG recording - Segments

• The T wave (T segment)– Formed when the ventricles repolarise– N.B the atria repolarise too but the signal is

shadowed by the much more powerful signal of the QRS complex

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The ECG recording - Segments

• The P-Q segment (P-R segment)– The portion of the recording between P-wave and

QRS complex

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The ECG recording - Segments

• The S-T segment– The portion of the recording between the QRS

complex and the T-wave

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The ECG recording - Intervals

• The P-R interval– Misleading– From beginning of P to beginning of Q

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The ECG recording - Intervals

• The Q-T interval– From beginning of QRS complex to end of T

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The ECG recording - Intervals

• The T-P interval– From end of T to beginning of P on next wave

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Analysing the ECG

16. Place the marker (M) at the beginning of the segment or interval you are asked to measure (e.g. P-R interval)

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Analysing the ECG

17. Move the cursor along the trace to the end of the segment or interval you are asked to measure (e.g. P-R interval)

t=∆0.256 s

Analysing the ECG

18. Now record the time from the top of the window19. Tell the examiner your result and you’re done!!!

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t=∆0.256 s

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Measuring Heart Rate using the ECG• Change the time-axis compression to 50:1• Again, use a lead on the ECG that is clear• Highlight 15 seconds worth of graph

Measuring Heart Rate using the ECG• Count the number of QRS complexes within

the selection• Multiply this number by 4...and you have your

answer!

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The ECG station

• Easy marks• This skill is rarely given alone, so when

practicing get the quickest time possible• Make sure you know the theory inside out• Practice makes perfect• Act as if you would with a real patient

Any Questions?

GOOD LUCK!!!!!