Session 7 Flashcards
Describe the conduction of the potential over the heart
Activity starts at SAN, depolarisation spreads over the atria, to the AVN where there is a delay of about 120ms. After the delay, the activity spreads down the septum (through Bundle of His). From endocardial to epicardial surface until all ventricular cells are depolarised. Repolarisation after 280ms, relaxes from outside in.
Describe a myocardial action potential
Starts with depolarisation and ends with repolarisation.
Describe the signals produced from depolarisation and repolarisation
Depol moving towards the electrode - upwards signal
Depol moving away from the electrode - downwards signal
Repol moving towards the electrode - downward signal
Repol moving away from the electrode - upwards signal
What determines the amplitude of the signal?
How much muscle is depolarising and how directly towards the electrode the excitation is moving.
Describe atrial depolarisation
Looking up towards apex from left abdomen. Will produce a small upward deflection –> small muscle, little blip, moves towards electrode.
Describe the spread from the septum
Excitation spreads about halfway down the septum, then out across the axis of the heart. Produces a small downward deflection –> down because its moving away, small because not moving directly away
Describe the spread through ventricular myocardium
Depolarisation spreads through the ventricular muscle along an axis slightly to the left of the septum. Produces a large upward deflection (upward because it’s moving towards the electrode, large because there’s a lot of muscle)
Describe the end of depolarisation
Spreads to base of ventricles. Produces small downward deflection (down because it’s moving away, small because it’s not moving directly away).
Describe ventricular repolarisation
After 280ms repolarisation begins on epicardial surface. Spreads through ventricular myocardium in the opposite way to depolarisation. Medium upward deflection.
What do P, Q, R, S & T represent on an ECG?
P = atrial depolarisation Q = septal depolarisation spreading to ventricle R = main ventricular depolarisation S = end ventricular depolarisation T = ventricular repolarisation
What is happens when you use a negative and a positive electrode?
Negative electrode gets inverted and so you get amplification e.g. lead II (positive lead bottom left & positive top right gives you two views from the bottom left)
What view does lead I give you? Explain why.
From the LHS.
Positive electrode top left, negative electrode top right leading to a view from the left hand side due to inversion of the negative electrode
What view does lead III give? Explain why.
Looking straight up (from the umbilicus upwards).
Positive electrode bottom left, negative top left. Looks up due to inversion of the negative electrode.
What can comparisons of the leads help us do?
Localise the abnormalities and detect changes in the electrical axis
What are augmented leads?
When there are two negatives connected, therefore you need to convert two negatives to one. After that you convert the negative signal into a positive one and combine it with the actual positive to give one view.
What do you look at in an ECG?
Rate, rhythm, axis, P wave, R segment (AVN), QRS complex, Q-T interval and the T wave.
How do you measure heart rate from an ECG?
When is tachycardia and bradycardia
300/big boxes over two QRS complexes (for abnormal - 30 big boxes x 10)
Tachycardia - above 100, bradycardia - below 60
What is the significance of the rhythm of an ECG?
Relates to timing of components of ECG. Can be judged from any lead.
What would you see in atrial fibrillation?
No P wave. Irregular atrial contraction - ventricles can still contract. Irregular rhythm.
What can the P-R interval be indicative of? Explain.
1st degree heart block - prolonged
2nd degree heart block - erratic
3rd degree heart block - no relationship between P wave and QRS complex. Ventricle beats irregularly –> no connection of atrial and ventricular contraction
What is the normal value for the PR interval?
0.12-0.2 secs (3-5 small squares)
What does the QRS complex tell you?
Tells you about the axis of the heart and the pattern of conduction through the ventricles (need to compare different leads). Electrical axis relates to the main spread of depolarisation through the wall of the ventricle (R wave). Combination of RV and LV generates a single vector normally pointing left.
When would the shape of the QRS complex change?
If there was damage to the conducting pathways as this would alter route of spread and changes the shape of the QRS complex (bundle branch block).
How would damaged myocardium appear on an ECG?
Current flows in systole produce extra signals in ST segment
What happens if there is a shortage of oxygen to the heart?
Get angina and an ST depression.
What is a STEMI?
ST elevation due to myocardial infarction.
What are the features of myocardial infarction?
ST elevation
Pathological Q waves
Inverted T waves
What is a pathological Q wave?
Damaged myocardium is replaced by scar tissue which cannot conduct as well thus producing Q waves. More than 0.04s (one small square) wide, >2mm deep. Remains after MI resolves.
What are the causes of first degree heart block?
Myocarditis – inflammation of the heart muscle
Low levels of potassium in the blood (hypokalaemia)
Low levels of magnesium in the blood (hypomagnesemia)
What are the causes of second degree heart block?
Hypok Faulty metab in AVN Digoxin toxicity Coronary artery disease MI
What are the causes of third degree heart block?
Hypok
Faulty cell metab low in bundle His
MI (ischaemia of AVN)
