Session 7 - The ECG Flashcards
What does the P wave represent?
-Atrial depolarisation
What does the QRS complex represent?
-Ventricular depolarisation
What does the T wave represent?
-Ventricular repolarisation
What is the PR interval?
- Start of P to start of Q
- Represents time taken for electrical activity to spread from atria to ventricles
What is the ST segment?
-From the end of S to the start of T
What is the QT segment?
- From the start of Q to the end of T
- Represents the time taken for depolaisation and repolarisation
What is the RR interval?
-The time between 2 QRS complexes ie one beat
Describe the pattern of spread of excitation over the heart
- Spontaneous AP generated in SAN
- Spreads over atria where it passes to AVN at the interventricular septum
- Delay 120ms
- AVN passes excitation, from endocardium to epicardium, over the surface of the ventricles, down the septum (bundle of his) to the apex and finally up towards the base of the heart
In relation to an ECG, what is a lead?
-An imaginary line between 2 electrodes
How is an ECG preformed?
-Electrodes placed onto the surface of the body which detects the electrical signals
How many electrodes are there in a 12-lead ECG?
-10
Where are the electrodes placed in a 12-lead ECG?
-4 limb leads -> RA, LA, LL and RL
-6 chest leads:
V1 -> 4th intercostal right, V2-> 4th intercostal left, V4->5th intercostal mid-clav, V3-> 5th rib, V5 -> 5th intercostal ant axillary, V6-> 5th intercostal mid-axillary
Do all of the limb leads record electrical signals?
-No, RL acts as an earth
In what direction do you take a view from?
-From positive to negative
What acts as the negative electrodes for the chest leads?
-The heart acts as theoretical -ve
NB, it is called the central terminal
Which electrodes measure electrical activity in a vertical plane and which in a horizontal plane?
- Limb leads look vertically
- Chest leads look horizontally
How many leads are generated from the limb and chest leads respectively?
- 6 from the limb
- 6 from the chest
What are the 6 views/leads generated from the limb leads?
- L1 -> LA to RA
- L2-> LL to RA
- L3-> LL to LA
- aVF-> RA +LA - LL
- aVR->LA+LL-RA
- aVL->LL+RA-LA
If electrical activity is travelling towards a view the deflection will be…
…positive
If electrical activity is travelling away from a view, the deflection will be…
…negative
What is the purpose of an ECG?
-To record the electrical activity of the heart from different angles to record pathology
Depolarisation towards a view causes the deflection to be…
Upwards
Depolarisation away from a view causes the deflection to be…
-Negative
Repolarisation towards a view causes the deflection to be…
…negative
Repolarisation away from a view causes the deflection to be…
…positive
The area under the deflection is proportional to…
…the amount/size of depolarisation
When reading ECG paper:
i) 1 large square=
ii) 1 small square =
iii) 5 large squares =
iv) 300 large squares =
i) 0.2s
ii) 0.04s
iii) 1s
iv) 1 minute
What is cardiac axis?
-The overall direction of electrical activity when the ventricles are contracting
Where does cardiac axis usually lay?
-Between -30 to +90 with an overall direction towards L2
What degrees is aVF?
-+90
What degrees is aVL?
-30
What is right axis deviation?
-The overall direction of electrical activity has been distorted to the right between +90 and +180
How is RAD detected on an ECG?
-Looking at leads 1, 2 and 3, L1 will have a negative deflection and L£ has the largest positive deflection
What conditions are associated RAD?
- Right ventricular hypertrophy
- Associated with pulmonary conditions
What is LAD?
-Overall direction of electrical activity has been distorted to the left between -30 to -90
How can you detect LAD on an ECG?
-Looking at leads 1, 2 and 3, L1 will have the most +ve deviation and L3 the most negative
What type of defect are associated with LAD?
-Conduction defects rather than hypertrophy
How do you calculate HR from an ECG?
1) count the number of large squares between the RR
2) divide 300 by that number
What are the 4 catagories to examine when interpreting an ECG?
-Rate, rhythm, axis, waveform
How do you determine rhythm looking at an ECG?
-See if the RR intervals are evenly spaced
How do you determine cardiac axis when interpreting an ECG?
-Look at leads 1, 2 and 3
Normal = L2 most positive
LAD= L1 most positive
RAD = L3 most positive
What are the characteristics of a normal P waveform?
- Rounded, smooth and upright
- Regular
- Proceed every QRS complex
What is an absent P wave with an irregular rhythm indicative of?
-Atrial fibrillation
What are the characteristics of a normal PR interval?
-Consistent length of 0.12-0.2s (3-5 tiny squares)
What does a prolonged PR interval suggest?
-Heart block
What is a shortened PR interval indicative of?
-Wolf Parkinson White syndrome (acessory electrical pathway between atria and ventricles leading to premature ventricular contraction)
What are the characteristics of a normal QRS complex?
- Width is approx 0.12s (3 tiny squares)
- Negative deflection, large positive deflection, negative deflection
What does a wide QRS complex indicate?
-Suggests depolarisation originated in the ventricles
What are the characteristics of a normal of a ST segment?
-Should be level with PR and TP
What does an ST depression suggest?
-Ischaemic myocardial tissue
Is an ST depression specific?
-No can occur in MI, coronary artery insufficiency, digoxin toxicity, tachycardia, anxiety
When does an ST depression become significant?
->1mm (one tiny square)
What does an ST elevation suggest?
-MI (significant above 1mm)
What is the RR interval used for?
-To calculate HR
What is the QT-interval dependant on?
-Heart rate
What is a lengthened QT interval indicative of?
-Marker for potential ventriculat tacharrythmias eg torsades des pointes
What is tousardes des pointes?
-A polymorphic ventricular tachyarrythmia which can lead to ventricular fibrillation and sudden cardiac death
What are the characteristics of the T wave?
- Always preceed QRS complex
- area under curve=to that of QRS complex
- Smooth and rounded
What does a tall and narrow T wave indicate?
-Hyperkalaemia
Why, if the same amount of myocardium is being repolarised as depolarised, are the QRS complex and the T waves different shapes?
- Depolarisation is a co-ordinated contraction event
- Repolarisation is less co-ordinated so occurs slower over a longer time
What are the possible causes of an inverted T wave?
- Smoking
- Anxiety
- Tachycardia/shock
What are the Q, R and S signals in a QRS complex?
- Q depolarisation of surface
- R depolarisation of septum and towards apex
- S depolarisation towards base
