L12: Understanding the ECG Flashcards by Elizabeth Watson

Define depolarisation?

Change within a cell of electrical charge distribution
Leading to less negative charge inside the cell
Happens through movement of ions (Na+, K+, Cl-)

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Define repolarisation?

Change within a cell of electrical charge distribution

Leading to more negative charge inside the cell

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Define resting membrane potential?

Separation of charge across cell membrane

Measure electrical imbalance between the inside and outside of the cell measured in mV

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Define action potential?

Brief reversal of cell membrane electrical polarity that is then propagated from cell to cell

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What is meant by deflection?

Deviation from straight line, upwards deflection or downwards deflection

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What do ECG allow?

Electrocardiograms provide information about cardiac electrical activity from 12 separate views of the heart

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Describe the electrical conduction across the heart?

Sinoatrial node
Intrinic rhythm–> spontaneously generate AP
Specialised cardiac myocytes–> pacemaker cells
Wave of depolarisation across the atria through gap junctions–> excitation-contraction coupling
Passes to the Atrioventricular node (AV node)
Slows electrical conduction down
Allowing time for atria to contract
Passes to Bundle of His–> passes through the Annular Fibrosis
Divides into Right Bundle branch of His and Left Bundle branch of His
Pass to purkinje fibres which result in depolarisation of the ventricles from the base upwards

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Where is the sinoatrial node located?

Upper part of right atrium

Junction between RA and SVC

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Where is the AV node located?

Interatrial septum

near Tricuspid valve

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Where is the Bundle of His located?

Interventricular septum

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What is the annulus fibrosis?

Fibrous skeleton of the heart
Anchor point for cardiac valves and myocardium
Insulator between atria and ventricles
Four fibrous rings joined together

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What are the difference between the SA node and the AV node?

SA node–> fast depolarisation, 60-100bpm, sets HR and rhythm–> Sinus rhythm
AV node–> Slows conduction, allows atrial contraction to take place, Intrinsic firing rate slower 40-60bpm

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Do the left and right bundle branches have intrinsic activity?

Intrinsic firing

Much slower 20-40bpm–> not sustainable

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What does an ECG do?

Measure changes in electrical potential produced in the myocardium during the cardiac cycle via a series of leads

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What does the ECG actually measure?

Electrical activity transmitted to the chest wall

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What does ECG lead mean?

Can mean:

Cable used to connect the electrode to the recorder
View of the heart

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How is an ECG recorded?

12 different views of the heart
10 different electrodes used
- 4 electrodes on limbs –> Right arm, right leg, left arm and left leg
- 6 electrodes on chest–> V1-6

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What is the function of the right leg electrode?

Grounding electrode

What are the lead limb views?

Measure the difference in voltage between two electrodes on the skin surface
Bipolar–> negative and positive electrodes
Limb lead 1–> Difference between RA and LA–> LA +ve electrode
Limb lead 2–> Difference between RA and LL–> LL +ve electrode
Limb lead 3–> Difference between LA and LL–> LL+ve electrode
Current goes towards +ve electrode so upwards deflection on ECG

What are the augmented limb leads?

Unipolar –> only +ve electrode
aVR, aVL, and aVF
Other electrode is actually the average of the other two remaining limb electrodes
e.g. aVR is the difference between the right limb and the average of LA and LL electrode
ECG recorder does the actual switching of and rearranging of electrode

Why is it important to understand which electrode is positive?

Depolarisation (current) moves towards electrode
Viewed from positive electrode by convention
Upwards deflection on ECG

What are the 6 vertical limb lead views?

Lead 2, lead 3 and aVF–> inferior view of the heart
Lead 1 and aVL–> left lateral view of the heart
aVR–> right lateral view of the heart

What are the remaining horizontal views of the heart?

Precordial or chest leads

What are the precordial chest leads?

Unipolar leads
Other electrode is position in the centre of the chest (average of limb electrodes) known as the ground/reference lead
+ve electrode V1-6 placed around the chest
Measures electrical activity travelling from back to front and right to left

What determines the shape of the deflection?

Depolarisation towards +ve electrode--> upward deflection Depolarisation away +ve electrode--> downward deflection Obliquely towards electrode--> smaller upright QRS complex Wave at 90 degree to electrode--> biphasic or no electrode

What does SA node depolarisation produce on lead 2 ECG?

No deflection | Insufficient signal to register on ECG

What happens to atrial depolarisation on lead 2 ECG?

P wave--> upwards deflection--> towards +ve electrode--> 80-100ms Depolarisation along atrial muscle fibres and internodal pathways Downwards and left

What produces the P-R (P-Q) interval?

Isoelectric segment Delay of depolarisation at AV node Atrial contraction Signal is very small Depolarisation of Bundle of His through the annular fibrosus also contributes (down middle and back up cancels out) Q wave- depolarisation from left bundle towards right- away from electrode -ve deflection)

What does depolarisation of interventricular septum on lead 2 ECG look like?

Q wave Depolisation spreads from left to right Small downward deflection as moving obliquely

What does depolarisation of the ventricles look like on lead 2 ECG?

R wave Large upwards deflection Depolarisation towards electrode Large due to number of cardiomyocytes

What does the last part of depolarisation of ventricle look like on lead 2 ECG?

S wave Small downwards deflection Depolarisation of purkinje fibres along the base of heart Small--> not directly away at an oblique angle

How long does complete ventricular depolarisation take?

80-120ms

What is the T wave on lead 2 ECG?

Ventricle repolarisation Epicardial surface of heart Spreads away from electrode--> opposite direction

Which leads are best at looking at the left side of the heart?

Lead 1 and aVL Left lateral wall problems of ventricle Muscle necrosis --> MI on left side--> left coronary artery blockage

Which leads are best at looking at inferior surface of the heart?

Lead 2, 3, and aVF Diaphragmatic surface Muscle necrosis due to right coronary artery occlusion

Which leads are best at looking at the right ventricle and interventricular spetum?

V1 and V2

Which leads are best at looking at the apex and anterior wall of the RV and LV?

V3 and V4

Which leads are best at looking at the LV?

V5 and V6

Which leads are best at looking at LAD occlusion (widow maker)?

V3 and V4 | aVL

How do you calculate the regular heart rate from an ECG?

1 small square = 0.04 s 1 large square= 0.20 s Count the number of cycles on ECG and over how many squares Multiple no. squares by 0.04 or 0.20 (depends on small or large squares) Divide 60 by answer, then multiple this answer by the number of cycles OR 300 small squares = 60 seconds 300/ number of squares for 1 heart beat

How do you calculate the heart rate if it is irregular?

Calculate the number of QRS complexes in 6 seconds then multiply by 10= total HB in 60 seconds

How long should a normal PR interval be?

0.12-0.20 s (3-5 squares) | Prolonged PR interval--> Delayed conduction through AV node and Bundle of His--> heart block

How long should the QRS complex be?

0.12s 3 small boxes Widened--> Usually depolarisation arising in ventricle Not spreading via His-Purkinje fibre--> takes more time

What does the QT interval show?

Time for ventricular depolarisation and repolarisation Varies with HR Calculation corrected for HR Upper limit--> 0.44-0.45 seconds Prolonged= Prolonged ventricle repolarisation - Risk of arrythmias