Flashcards in CVS - ECG Deck (32)
What is 'sinus rhythm?'
The is when the rhythm of the heart's contraction is set by the sinoatrial node. It has the fastest rhythm in the heart - around 60-100
Where is the left anterior division of the Bundle of His found?
What are Purkinje fibres?
Fine branches of the Bundles of His. They send a rapid spread of depolarisation throughout the ventricular myocardium
Outline the cardiac cycle?
- SA node depolarises and impulse spreads through atria
- impulse held up at AV node and spreads to the ventricle via the Bundle of His
- spreads rapidly down left and right bundles and Purkinje fibres
- myocytes of the intraventricular septum are depolarised first
- apex and RV and LV are depolarised next
- base of the ventricles are last to be depolarised, then repolarisation of the ventricles happens in the reverse order
What does an ECG record?
Records changes on extracellular surface of cardiac myocytes during wave of depolarisation and repolarisation. It does this from the surface of the body using electrodes pasted on the skin
If the impulse moves towards a positive electrode, will the deflection be positive or negative?
What direction would the electrical impulse move in if the complex is tall and positive?
From the top of the heart to the bottom - towards the apex
What direction would the electrical impulse be going in if there is a small positive complex?
Across the heart, slightly towards the electrode
What direction is the electrical impulse travelling in if there is a large negative complex?
From left ventricle to right atrium, directly away from the electrode
Where is the sinoatrial node located?
Top right hand corner of the right atrium, near junction of superior vena cava and right atrium
How does atrial depolarisation spread across the heart?
Spreads along atrial muscle fibres and intermodal pathways throughout left and right atria. Produces a small upward deflection on ECG (p-wave).
Why is there a delay at the AV node?
It allows time for atrial contraction to fill the ventricle. This forms the flat line section of the ECG between the P wave and the QRS complex
What is the function of the fibrous ring between the atria and ventricles?
It ensures that there is no direct contact between atrial and ventricular myocytes, so that depolarisation can only travel via the bundle of His
How does the interventricular septum depolarise?
It goes from left to right. This produces a small downward deflection in the ECG wave because it is moving obliquely away from the electrode ('Q wave')
What does depolarisation of the apex and ventricular wall look like on an ECG trace?
A large positive deflection, because the depolarisation is moving directly towards the electrode, and there is a large amount of muscle mass so there is more electrical activity.
How would left ventricle hypertrophy affect the R wave?
It would be correspondingly taller
What is the T wave caused by?
A wave of repolarisation of the ventricles moving away from the electrode, which spreads in the opposite direction to depolarisation
How many electrodes are attached in a 12 lead ECG?
10 electrodes - 4 on limbs, 6 on chest
What are 'leads' in an ECG?
Views of the heart
How many views of the heart do the four limb leads provide?
6 views in the vertical plane
Which leads are better for looking at the inferior surface of the heart?
Leads II, III and AVF
Which leads are better for looking at the left side of the heart?
Leads I and aVL
What view of the heart does the chest leads provide?
6 views of the heart in the horizontal plane
Why do different chest leads have different shapes of QRS complex?
- Right heart leads have no Q waves, a small R wave and large S waves.
- Septal heart leads have larger R waves and smaller S waves
- Left heart leads have large R waves and small S waves.
This is due to their differing proximities to parts of the heart
What is the horizontal axis on an ECG?
Time - 5 large squares = one second
What is the vertical axis on an ECG?
How can heart rate be calculated from a single PQRST wave?
Count number of large boxes between complexes (count R-R interval). Divide 300 by number of boxes. This is the heart rate
How is heart rate calculated from an ECG if the rhythm is irregular?
Count number of QRS complexes in 6 seconds then multiply by 10
How long should the PR interval be?
0.12 - 0.20 seconds