12. Interpreting ECGs Flashcards
What is depolarisation?
Change within cell of electric charge distribution leading to less negative charge inside cell
What is repolarisation?
Change within cel of lactic charge distribution leading to more negative charge inside cells
What is deflection?
Deviation from straight line in ECG either an upward or downward wave/peak from the baseline
How are cardiac cells electrically connected?
Through gap junctions
How does the electrical activity spread through the atria?
Initiated SAN - junction of RA and SVC
Depolarisation right atrium and left atrium
Hits AVN - in interatrial estimate near tricuspid valve - slightly slows down in AVN
Goes to Bundle of His - wide, fast, conducting muscle fibres that travel through annulus fibrosis
What is the annulus fibrosis?
Connective tissue that separates the atria from ventricles
Anchors myocardium and cardiac valves
Electrical insulator between atria and ventricles
Consists of 4 fibrous rings
How does the electrical activity spread through the ventricles?
Bundle of His enters inter-ventricular septum where it divides into:
- right bundle branch - excites right ventricle
- left bundle branch - excites left ventricle
Branches terminate in extensive network of muscle fibres (purkinje fibres) which continue to conduct depolarisation wave through the ventricles
Describe the sinoatrial node
Fastest rate of depolarisation in the heart
Intrinsic firing rate 60-100 times/min
Sets heart rate and rhythm (sinus rhythm)
Describe the atrioventricular node
Slows conduction
Gives time for atria to contract before ventricles
Intrinsic firing rate without stimulation is 40-60 times/min
Describe the left and right bundle branches
Ventricular electrical conducting system, cells also have an intrinsic firing rate although not typically manifested
Intrinsic firing rate 20-40 times/min
What does an ECG measure?
Changes in electrical potential produced in successive areas of myocardium during cardiac cycle via a series of leads attached to the body - measures and records these changes over time
Records cardiac electrical activity as transmitted to chest wall and lims
What are the 2 different things called leads in an ECG?
Cable used to connect electrode to ECG recorder
Electrical view of the heart obtained from any one combination of electrodes
What is an electrode?
Conductive pad that is attached to skin and enables recording of electrical currents
How many different electrodes are there?
10 electrodes - 4 on limbs, 6 on chest
Gives 12 views of heart
One on each limb (right leg is a grounding lead)
Precordial (chest) leads
Where are the precordial (chest) leads positioned?
V1 - 4th intercostal (right) V2 - 4th intercostal (left) V3 - between V2 and V4 V4 - midclavicular V5 - 5th intercostal space (anterior axillary line) V6 - 5th intercostal (midaxillary line)
Describe limb lead 1
Bipolar - negative and positive electrodes
Voltage difference between electrode RA and LA
LA is +ve electrode
Describe limb lead 2
Bipolar - negative and positive electrodes
Voltage difference between electrode RA and LL
LL is +ve electrode
Describe limb lead 3
Bipolar - negative and positive electrodes
Voltage difference between electrode LA and LL
LL is +ve electrode
What are the 3 augmented limb leads?
aVR, aVL, aVF
Unipolar - only have a positive electrode
Other electrode represents average of remaining 2 electrodes and is designated neutral or reference
Where are the positive electrodes located for each of the augmented limb leads?
Right arm for aVR
Left arm for aVL
Left leg for aVF
Where is the cardiac view provided by a lead from?
Perspective of the positive electrode
If electrical current (depolarisation) of heart being looked at is travelling to the positive electrode of lead the ECG wave will have positive deflection
If the repolarisation current is travelling towards the positive electrode which way will the deflection be?
Negative (downwards)
Where are the 6 views in the vertical plane from limb leads?
Check diagram on notability
Describe the precordial (chest) leads
Unipolar - pther electrode is average of limb electrodes and positions in the middle of the chest
Positive electrodes placed across chest
Ground lead in centre of chest
Measure electrical activity that is moving in front-back direction and right-left direction - horizontal plane
What does the height or depth of deflection depend on?
How directly depolarisation wave is coming towards (or going away from) positive electrode and number of cells generating the signal
What does a tall upright QRS complex suggest?
Depolarisation wave coming directly towards positive electrode
What does a small upright QRS complex suggest?
Wave obliquely towards electrode
What does biphasic or no complex suggest?
Wave at 90 degrees to electrode
What does a deep negative complex suggest?
Depolarisation wave going directly away from electrode
Describe what happens in regards to electrical conduction during atrial depolarisation?
Spreads akin atrial muscle fibres and internodal pathways
Throughout both right and left atria
Direction: downwards and to the left towards AVN
Lasts 80-100ms
What does the p wave represent?
Atrial depolarisation
Upwards because towards recording positive electrode
Why is there a delay at the AVN and how is it represented on the ECG?
Allows time for atrial contraction to fill ventricle
Isoelectric (flat line) segment after p wave
How long from the start of atrial depolarisation does ventricular depolarisation occur?
120-200ms
How does depolarisation spread through ventricles?
First part to depolarise is muscle in interventricular septum
Depolarisation spreads from left to right
How is the depolarisation of the interventricular septum represented on the ECG?
May produce small downward deflection because moving obliquely away (to the sides)
Termed a Q wave, first downward deflection after P
What causes the R wave?
Depolarisation of apex and free ventricular walls
Upward deflection due to depolarisation moving directly towards electrode
Large because large muscle mass so more electrical activity
What causes the S of the QRS?
Depolarisation spreads upwards to base of ventricles, produces small downward deflection
Downward because moving away and small because not moving directly away
How long does ventricular polarisation take?
80-120ms
Describe ventricular repolarisation
Beings on epicardial surface of heart
Spreads in opposite direction to depolarisation
What causes the T wave?
Ventricular repolarisation
Upwards because it is wave of repolarisation moving away from electrode
What can cause a negative deflection?
When depolarisation moves away form lead or when repolarisation moves towards electrode
What can cause positive deflection?
Wave of depolarisation moving towards electrode or repolarisation moving away from electrode
What are leads 1 and aVL good at looking at?
Left side of heart
Best limb leads for looking at problems of lateral wall of left ventricle
Muscle necrosis due to occlusion of branch of left coronary artery - lateral wall MI
What are leads 2, 3 and aVF good at looking at?
Inferior surface of heart
Best limb leads to detect problems in inferior surface of heart
Muscle necrosis due to occlusion of right coronary artery - inferior MI
What are the 6 views of the heart in the horizontal plane?
V1 and V4 - antero-septal leads
V1 and V2 - face RV and septum (septal leads)
V3 and V4 - faces apex and anterior wall of RV and LV
V5 and V6 - face LV (lateral leads)
How do you work out heart rate from an ECG?
300 divided by how many big squares are between to R peaks if regular
What is the PR interval and how long should it be?
Time between beginning of P wave and beginning of QRS complex
0.12–0.20 seconds
How long should QRS interval be?
<0.12 seconds
What is the QT interval?
From beginning of QRS complex to end of T wave
Time taken for depolarisation and repolarisation of ventricle and varies with heart rate
<0.44-0.45 seconds
Prolonged QTc at risk of arrhythmias
