L12: Understanding the ECG Flashcards
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-)
Define repolarisation?
Change within a cell of electrical charge distribution
Leading to more negative charge inside the cell
Define resting membrane potential?
Separation of charge across cell membrane
Measure electrical imbalance between the inside and outside of the cell measured in mV
Define action potential?
Brief reversal of cell membrane electrical polarity that is then propagated from cell to cell
What is meant by deflection?
Deviation from straight line, upwards deflection or downwards deflection
What do ECG allow?
Electrocardiograms provide information about cardiac electrical activity from 12 separate views of the heart
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
Where is the sinoatrial node located?
Upper part of right atrium
Junction between RA and SVC
Where is the AV node located?
Interatrial septum
near Tricuspid valve
Where is the Bundle of His located?
Interventricular septum
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
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
Do the left and right bundle branches have intrinsic activity?
Intrinsic firing
Much slower 20-40bpm–> not sustainable
What does an ECG do?
Measure changes in electrical potential produced in the myocardium during the cardiac cycle via a series of leads
What does the ECG actually measure?
Electrical activity transmitted to the chest wall
What does ECG lead mean?
Can mean:
- Cable used to connect the electrode to the recorder
- View of the heart
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
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
