ECG Flashcards
How does an ECG detect the excitation of the heart?
The wave of depolarisation and repolarization moves across the heart and sets up electrical currents which can be detected by surface electrodes
What is an ECG a record of?
Depolarisation and repolarisation cycle of cardiac muscle obtained from skin surface, the potential differences between distant sites on the body surface are detected by electrodes and coupled to the electrocardiograph
What are the 3 limb leads?
- lead I RA-LA
- lead II RA-LL
- lead III LA-LL
What is the P wave?
Atrial depolarisation
What is the QRS complex?
Ventricular depolarisation (masks atrial repolarisation)
What is the T wave?
ventricular repolarisation
What is the PR interval?
Largely AV node delay
What is the ST segment?
Ventricular systole
What is the TP interval?
diastole
When do the atria contract?
between the P wave and QRS complex
What is the difference between the electrocardiograph and the electrocardiogram?
The electrocardiograph is the machine which the electrodes are coupled to and the electrocardiogram is the recording of the potential differences
What is meant by a dipole?
charges which are separated - a vector with components of magnitude and direction
What information is provided by an ECG?
information on: cardiac rate cardiac rhythm chamber size the electrical axis of the heart - main test for myocardial ischaemia and infarction
How is the magnitude of the electrical vector determined?
by the mass of cardiac muscle involved in the generation of the signal
How is the direction of the electrical vector determined?
the overall activity of the heart at any instant in time and varies over the cardiac cycle
What does the word lead refer to in an ECG?
an imaginary line between 2 or more electrodes, one of which electrodes is the recording (+ve) or seeing electrode
What happens on the electrocardiogram when the depolarisation moves towards the recording electrode?
Deflection is upwards
deflection downwards when depolarisation moves away
What 12 leads comprise a 12 lead ECG?
3 standard limb leads (I, II, III) these are bipolar
3 augmented voltage (aV) leads (aVR, aVL and aVF) these are unipolar
6 chest leads (V1 - V6) the precordial leads
What view of the heart do the precordial leads provide?
A horizontal view
What view of the heart do the limb leads and augmented voltage leads provide?
A vertical view
In lead I which electrode is which?
RA is negative
LA is positive (recording)
In lead II which electrode is which?
RA is negative
LL is positive (recording)
In lead III which electrode is which?
LA is negative
LL is positive (recording)
What direction does lead II see the heart from?
it looks at the heart from the inferior left position so is well placed as the depolarisation of the heart moves in this direction as well
What is the normal duration of the P wave?
less than 0.120s
What is the downward deflection before the R wave?
The Q wave
What is the large upwards deflection?
R wave
What is the downward deflection following the R wave?
S wave
What is the T wave?
The wave of re polarisation moving away from the recording electrode
What does the PR interval indicate?
From start of P wave until start of QRS
time for the SA node impulse to reach the ventricles
0.12-0.2s is normal duration
What is the ST segment?
end of QRS to start of T wave should be isoelectric
What does the QT interval indicate?
from start of QRS to end of T wave primarily reflects time for ventricular depolarisation and repolarisation
How do augmented limb leads observe the heart?
there is one positive electrode viewing the others which are linked as -ve, the augmented leads see the heart from different angles than just the standard limb leads
What is the hexaxial reference system?
The 6 different views of the heart seen in the vertical plane in a 12 lead ECG
Where is the -ve electrode for the precordial leads?
Due to the way the electrodes on the precordium link up the -ve electrode is essentially in the centre of the heart
What view do V1 and V2 look at the heart from?
from the right at the interventricular septum
What view do V3 and V4 look at the heart from?
anterior view
What view do V5 and V6 look at the heart from?
look at the lateral aspect - left ventricle
Where do you position the chest leads?
V1 - 4th intercostal space, right sternal edge
V2 - 4th intercostal space, left sternal edge
V3 - midway between V2 & V4
V4 - 5th intercostal space, midclavicular line
V5 - same horizontal position as V4, anterior auxillary line
V6 - same horizontal level as V4, mid auxillary line
What cardiac event is indicated by the ST segment?
ventricular systole
What cardiac event is indicated by the TP segment?
Ventricular diastole
Why are 12 leads used when 1 lead can determine rate and rhythm?
it allow us to
determine the axis of the heart in the thorax
look at ST segment and T wave changes in relation to specific heart regions
look at voltage criteria changes - to diagnose chamber hypertrophy
6 steps to looking at an ECG
- Verify patient details: name and date of birth
- Check date and time ECG was taken
- Check the calibration of the ECG paper
- Determine the axis (if possible)
- Workout the rate and rhythm
- Look at individual leads for voltage criteria changes OR any ST or T-wave changes
What are the 7 questions to workout rate and rhythm from the rhythm strip of the ECG?
- Is electrical activity present?
- Is the rhythm regular or irregular?
- What is the heart rate?
- P-waves present?
- What is the PR interval?
- Is each P-Wave followed by a QRS complex?
- Is the QRS duration normal?
