ECG theory and practice Flashcards
Where does depolarisation spread?
From the endocardium to the epicardium
Where does repolarisation spread?
From the epicardium to the endocardium
What is an ECG?
The ECG is a recording of potential changes, detected by electrodes positioned on the body surface, that allows the electrical activity of the heart to be monitored in a manner that is simple to perform
What is needed to generate detectable potential changes?
Large masses of cardiac tissue
What is extracellular current?
What generates the changes in potential which we can record on the body surface
What is an electrical vector?
the separation of charge with particular direction
Where does the wave of depolarisation spread in terms of the AV node?
Away from the AV node. Spreading to the left of the heart
What allows us to approximate the electrical axis of the heart?
The direction of the vector
What are the components of the electrical vector?
Magnitude and direction
What defection on the ECG does depolarisation generate moving towards the recording electrode?
upward deflection
What deflection on the ECG does depolarisation generate moving away from recording electrode?
downward deflection
What is the lead axis?
The imaginary line between two electrodes
What deflection on the ECG does depolarisation generate when there is no movement away or from recording electrode?
no deflection. Isopotential
What does the 12 lead ECG comprise of?
- three standard limb leads
- three augmented voltage leads
- six chest leads
Which plane of the heart is seen in chest leads?
horizontal
What plane of the heart is see in limb and augmented leads?
vertical
What is lead I
RA to LA
What is lead II
RA to LL
What is lead III
LA to LL
How is the P wave formed?
Atrial depolarization spreads from the SA node inferiorly and to the left - depolarization is moving towards the recording electrode (+ve) in lead II producing a (normally) upward deflection in this lead – the P wave
What is a Q wave?
a downward (negative) deflection preceding an R wave is called a Q wave
What is an R wave?
a deflection upwards (positive) is called an R wave, irrespective of whether, or not, it is proceeded by a Q wave – this can be confusing
What is an S wave?
a downward (negative) deflection following an R wave
How is a Q wave formed?
ventricular depolarization starts in the interventricular septum and spreads from left to right causing the small and narrow Q wave
How is an R wave formed?
subsequently the main free walls of the ventricles depolarize causing a tall and narrow R wave
How is an S wave formed?
finally, the ventricles at the base of the heart depolarize, producing a small and narrow S wave
How is a T wave formed?
It is an upward (positive) deflection because the wave of repolarization is spreading away from the recording electrode (think of it as negative charge moving away from the recording electrode which is the equivalent of positive charge moving towards it!)
What is a T wave?
ventricular repolarisation
What is the normal length for the PR interval?
0.12-1.2 s
What is the PR interval strongly influenced by?
the delay in conduction through the AV node. If conduction from AV node is fast, the PR interval decreases and vice versa
True/False. the ST segment is normally isoelectric
True
What is the normal length for the QT interval?
0.44 in males and 0.46 in females
What is the recording electrode in augmented leads?
One recording lead other two linked.
How mamy views of the heart do we have with a 12 lead ECG?
12 views, 6 from limbs and augmented and 6 from chest
What are V1 and V2 looking at?
interventricular septum
What are V3 and V4 looking at?
anterior of the heart
What are V5 and V6 looking at?
lateral aspect (left ventricle) of the heart
What is the first positive deflection in the QRS complex in V1?
R wave immediately followed by an S wave
What happens to the R ad S wave as you move from V1 to V6?
progressive increase in R wave and progressive decrease in S wave
How long does the P wave last?
0.08-0.1 s
How long does the QRS complex last?
less than 0.1 s
What is the ECG rhythm strip?
Prolonged recording of one lead. Usually lead II. It allows you to detect rhythm disturbance
What are the 6 steps in analysing the ECG?
1- verify patients name and DOB 2-check time and date when ECG was taken 3-Check the calibration of the ECG paper 4- determine the axis if possible 5-work out the rhythm 6- look at individual leads for voltage criteria changes or any ST or T wave changes
What are the 7 steps in determining rhythm?
- 1 Is electrical activity present?
- 2 Is the rhythm regular or irregular?
- 3 What is the heart rate?
- 4 P-waves present?
- 5 What is the PR interval?
- 6 Is each P-Wave followed by a QRS complex?
- 7 Is the QRS duration normal?
true or False. Always use ECG over clinical judgement
False clinical judgment comes first
