Lecture 11: CVS, ECG Flashcards
Memorise the diagram slide 9
Ya
The primary function of the heart is to generate pressure gradients that drive blood flow.
Heart muscle contraction depends in electrical activity (action potential firing)
Electrical activity (APs)–> mechanical activity (contraction)–> pressure differences –> blood flow (volume changes)
Heart rate is determined by the fastest depolarising cells.
That is the sinoatrial node which has 70-80 APs/minute.
Conduction of electrical activity in the heart.
Activity from the SA node spread through the heart in 2 ways, what are they?
1) via gap junctions to adjacent cells
2) via the specialised conduction system to other areas (ie bundle of his and purkinje fibres)
How is the spread of electrical activity happen in the heart
- Depolarisation initiated at SA node
- Depolarisation spreads through atria
- AV node delay
- Depolarisation travels down bundle of his
- Depolarisation spreads through septum
- Depolarisation spreads through bulk of ventricles (inside out)
- Depolarisation spreads through base of left ventricle (a small area is slow to depolarise)
- REpolarisation spreads through ventricles (outside-in)
The elecocardiogram
Rhythmic, synchronised electrical activity of the heart.
Can reveal clinical abnormalities in
-the regulation of heart rate and rhythm
-the conduction of electrical activity in the heart
Doctors will put you on an ECG if you come into hospital for a heart issue.
A limitation- the ECG measures electrical NOT contractile activity of the heart. Ie you could have a normal ECG but a really weak contraction.
Using limb leads. What is the reason you can do that
Your body is like a big back of salt that can carry electrical signals throughout the body. Currents in body are conducted to surface by movement of ions in the extracellular fluid.
You can detect electrical activity in organs without having to contact the organs directly
The standard ECG waveform
Slide 19
Reversal of membrane polarity during an AP
A cell at rest will have inside that is negative and outside that is positive.
But at the peak of the action potential there is a small section when the inside of the cell become positive and the outside becomes negative. So you can pick up which cells are actively firing AP and which cells are at rest.
If firing and non-firing cells are next to each other, this will create a dipole (negative pole and positive pole). As the cells are surrounded by ionic fluids, currents will flow between the regions of opposite charge (creating a vector: arrow pointing towards positi d pole, indicating the overall strength and direction of a dipole).
To understand the ECG, break the electrical activity of the heart down into vectors, which would explain why some bits are pos, and some bits are neg.
Understanding the ECG Recording
Depolarisation moving towards the positive electrode will give a positive line, and depolarisation going away from the electrode will have a negative deflection/line.
The larger the mass of tissue involve, the bigger the deflection in the ECG because the signal is bigger.
If there is no movement (vector) of depolarisation/hyperpolarisation, the trace/line returns to zero
Waveforms of the normal ECG
- Depolarisation initiated at SA node. This tissue would normally be positive on outside, but when AP Is fired its negative on outside. However this part is too small to register on the ECG. Slide 27
- Depolarisation spreads across the atria to the right, that means its spreading towards the positive electrode, so you are going to get an upwards deflection (p wave)
- AV nodal delay- mass of depolarisation tissue too small to register on ECG trace, trace stays at baseline.
- Depolarisation travels down bundle of his and purkinje fibres-can’t see on ECG
- Depolarisation spreads through septum: vector goes from the right hand side to the left hand side. It is moving away from the positive electrode, because of this you get a negative deflection. Only small mass of tissue so just a small deflection.
- The depolarisation spreads through bulk of ventricles. The ventricles depolarise from the inner wall of the heart to the outside, which means the vector of depolarisation, so is travelling towards the positive electrode, so it is a positive deflection on the ECG trace (large mass if tissue, large deflection)
- Depolarisation spreads through that last bit, the base of the ventricle. So the depolarisation spreads from the inner wall of the heart to the outer wall, so vector is pointing away from the positive electrode, so negative deflection on the ECG. Because its a small block of tissue, it will be a small deflection
- Heart REpolarisation, spreads through ventricle outside in.
Because the heart is repolarising, you will see a positive charge on the outside and towards the electrode so you will get a positive deflection.
Electrode placement and the ECG.
The signal amplitude depends in the position of the electrodes relative to the vector if depolarisation.
Look at slide 33, it will help with understanding why you see a positive and negative deflection depending on we’re the electrode is placed.
If the vector is running parallel to the line of electrodes then you will see the bigger/ clearer deflections
Human ECG limb lead position
Each lead position is optimised for detecting a different vector if depolarisation.
Different leads are better at detecting different abnormalities.
Unipolar limb lead positions
Gah fark me dead thus is difficult
Abnormalities of heart date and rhythms
Tachycardia- abnormally fast heart rate
Bradycardia- slower than normal heart rate
Sinus rhythm- rhythm generated by the SA node, ie 70-80 beats per min?
Inter-beat variability is normal- very regular heartrate can be an autonomic dysfunction. Ie hrt rate should increase when bodies demands increase.
Heart block (AV block)
Impaired conduction through the AV node
Degrees of heart block
1-slow do
Haven’t finished
