Basics of ECG Flashcards
Describe the electrical conduction in the heart
The SA node generates action potentials:
These action potentials spread across the atria
They reach the AV node (conduction is delayed)
They spread rapidly through the bundle of His and its branches and the network of purkinje fibres to spread across the ventricles
Physics of the ECG
Potential differences between different sites on the body surface are detected by electrodes placed on the sin coupled to a sensitive recording device
An ECG is a recording of the potential differences between time
Describe how deflections are produced on an ECG
Lead one electrode acts as the recording (positive) electrode
- When depolarisation moves TOWARDS the recording electrode it will generate an UPWARD deflection on the ECG
- When depolarisation moves AWAY from the recording electrode it generates a DOWNWARD deflection on the ECG
- If there is no movement then there will be no deflections
where is the lead one axis
From RA (-ve) to LA (+ve)
Where is the lead two axis?
RA (-ve) to LL (+ve)
Where is the lead 3 axis
LA (-ve) to LL (+ve)
Which limb is the earthed limb
Right leg
what does the P wave represent
atrial depolarisation
what does the QRS complex represent and how long should it be
ventricular depolarisation
less than 3 small boxes
what does the t wave represent
ventricular repolarisation
When is the PR interval, what does it indicate, how long is it normally?
From the start of the P wave to the start of QRS interval
Time for SA node impulse to reach the ventricles, and indicates there is a delay in conduction of the AV node
0.12 - 0.2 seconds
When is the ST segment and what does it represent?
From the end of the QRS complex to the start of the T wave
Represents ventricular contraction (systole)
When is the QT interval, what does it indicate, what is the normal time for it?
Start of QRS complex to the end of T wave
Reflects time for ventricular depolarisation and repolarisation
Normally (depending on HR) 0.36 - 0.44s
What does the TP segment represent
Relaxation of the ventricles (diastole)
What leads are a 12 lead ECG comprised of
- 3 standard limb leads (I, II, III)
- 3 augmented voltage leads (aVR, aVL, aVF)
- 6 chest leads (V1 - 6) - precordial
What does a 12 lead ECG allow you to look at?
To look at the heart from different directions which allows you to:
Determine axis of heart in thorax
Look for any ST segment or T wave changes in relation to specific heart regions
Look for any voltage changes
What leads are the lateral leads, and which direction does it look at the heart from?
Leads I and aVL
Views heart from the left
Which leads are the inferior leads and what direction are they looking at the heart from?
Leads II, III and aVF
Looks at heart from an inferior direction
Where are each of the precordial leads placed?
V1 - 4th intercostal space on right of the sternum
V2 - 4th intercostal space on left of sternum
V3 - midway between V2 - 4
V4 - fifth intercostal space mid clavicular line
V5 - same horizontal level as V4, anterior axillary line
V6 - same horizontal level as V4, mid axillary line
Which areas of the heart are each of the precordial leads viewing?
V1 and V2 - looking at interventricular septum
V3 and V4 - looking at anterior of the heart
V5 and V6 - looking at lateral aspect (left ventricle) of heart
Steps of analysing an ECG
- Verify patient name and DOB
- Check date and time ECG taken
- Check calibration of ECG paper
- usually 25mm/sec - determine axis if possible
- Work out rate and rhythm
- is electrical activity present
- is rhythm regular or irregular
- what is heart- 300/number of larger squares
(regular) - count QRS complexes in 30
large squares x 10 (irregular)- P waves present
- Check PR interval (0.12 - 0.2)
- if each P wave followed by QRS
- If QRS duration normal (<0.10 sec)
- 300/number of larger squares
- Look at individual leads for ST or T wave changes
Describe each of the waves in lead II (seen best in lead II)
P wave - atrial depolarisation
Q wave - left to right
depolarisation of the interventricular septum
R wave - depolarisation of the main ventricular mass
S - depolarisation of the ventricles at bases of heart
T wave - ventricular repolarisation
When does systole and diastole happen?
Systole - ventricular contraction in the ST segment
Diastole - ventricles relaxing in the TP segment
