Analysing an ECG Flashcards
Describe the electrical activity of the heart
The electrical activity of the heart begins at the sinoatrial node. The atria contract.
Slight delay at atrioventricular node, then depolarisation passes down bundle of His, down the right and left bundles, through the Purkinje fibres and through the ventricles.
What does a positive spike on an ECG show?
Depolarisation towards ECG electrode
What do P waves represent?
Atrial depolarisation
When does the PR interval start and end and what is a normal PR interval?
- The PR interval begins at the start of the P wave and ends at the beginning of the Q wave
- PR interval: 120-200ms
Why is there a flat line between P and Q?
With atrial depolarisation complete, the impulse is delayed at the AV node
What does the QRS complex represent?
Ventricular depolarisation, begining at the apex.
Normal QRS time
70-100ms
What does the ST segment represent?
The time between ventricular depolarisation and repolarisation (i.e. ventricular contraction).
What does the T wave represent?
The T wave represents ventricular repolarisation, begining at the apex.
When does the QT interval start and end?
What does the QT interval represent?
What is the normal QTc time?
The QT interval begins at the start of the QRS complex and finishes at the end of the T wave.
It represents the time taken for the ventricles to depolarise and then repolarise.
QTc: <440ms (men) and <460ms (women)
Q, R and S wave definitions
Q wave: first negative wave after a P wave
R wave: first positive wave after P wave
S wave: first negative wave after R wave
What are the steps of interpreting an ECG?
- Rate
- Rhythm
- Axis
- P waves
- PR interval
- QRS complex
- QT interval
- ST-segment
- T waves
How would you work out heart rate on an ECG?
If a patient has a regular heart rhythm:
Count the number of large squares present within one R-R interval
Divide 300 by this number to calculate heart rate
If a patient’s heart rhythm is irregular:
Count the number of complexes on the rhythm strip (each rhythm strip is typically 10 seconds long).
Multiply the number of complexes by 6 (giving you the average number of complexes in 1 minute).
Big box = 0.2 seconds
How would you conclude if heart rhythm is regular or irregular?
Mark out several consecutive R-R intervals on a piece of paper, then move them along the rhythm strip to check if the subsequent intervals are similar.
(Can be irregularly irregular or regularly irregular)
Causes of an irregularly irregular heart rhythm
- AF
- Ectopics
Causes of a regularly irregular heart rhythm
- Ectopics
- Second degree heart block
- Sinus arrhythmia
What does cardiac axis describe and what leads do you need to look at to determine cardiac axis?
Cardiac axis describes the overall direction of depolarisation within the heart.
To determine the cardiac axis you need to look at leads I, II and aVF
Draw the ECG vectors