Case Study 2

Question 1

How is electrical activity propagated through the heart ?

Answer 1

1. ) action potential triggers contraction and is the abrupt reversal of the membrane potential to a positive value.
2. ) Spread of excitation through the atria, ventricles and conducting system is brought about by local electrical currents.
3. ) in the active depolarised zone, the interior of the membrane is positively charged while the resting zone ahead is negative.
4. ) Positive charge flows through the gap junctions and depolarises the next cell.
5. ) externally positive charge flows the opposite direction reducing the charge on the outside of the resting membrane.

Question 2

What are the conduction velocities at each level of the conduction pathways?

Answer 2

Atria: 0.5 m/s 
AV Node: 0.05 m/s 
Bundle of His and bundle branches: 1.0 m/s 
Purkinje fibres: 5.0 m/s 
Ventricles: 0.5 m/s

Question 3

How is the heart regulated by the autonomic nervous system? (sympathetic)

Answer 3

Sympathetic:

1. ) noradrenaline is released
2. ) Noradrenaline binds to beta1-adrenergic receptor
3. ) Sodium (Gna) and calcium conductance (Gca) are increased and so the inward current of calcium and sodium are also increased.
4. ) slope of the pacemaker potential is increased

Sympathetic Part 2:

1. ) noradrenaline is released
2. ) Noradrenaline binds to beta1-adrenergic receptor
3. ) This causes a rise in the intracellular concentration of cyclic AMP
4. ) cyclic AMP increases current of sodium into the cell (if)
5. ) It also activates protein kinase A which phosphorylates the Ca channel and thereby increases the inward calcium current. (Ica)

Question 4

What is the pacemaker potential in SA and AV nodes?

Answer 4

There is no stable resting membrane potential and the inward current of Na (if) makes the cell more positively charged. The reduced iK (outward current of K) makes the cell also more positively charged until it reaches the threshold, at which it fires.

Question 5

How is the heart regulated by the autonomic nervous system? (parasympathetic)

Answer 5

1. ) ACh binds to its receptor
2. ) This causes a fall in the intracellular concentration of cAMP
3. ) Reduces the effects of sympathetic stimulation
4. ) reduced if and Ica
5. ) reduced slope of the pacemaker potential

part 2:

1. ) ACh binds to its receptor
2. )activation of ACh-sensitive potassium channel occurs
3. ) potassium conductance is increased

All in all, this reduces the slope of the pacemaker potential and hyper polarises the SA node. This causes the threshold to be reached later and the heart rate slows down.

Question 6

What is ECG ?

Answer 6

It is a recording go the potential changes at the skin surface that results from the depolarisation and repolarisation of the heart muscles.

Question 7

More specifically, how does and ECG work ?

Answer 7

It works because there is a spread of excitation that creates currents in the extracellular fluid. These currents create small potential differences across the body of about 1 mV. They are recorded using a sensitive voltmeter connected to metal electrodes on the skin surface and so it is recorded on a piece of paper by a computer.

Question 8

What is the paper speed of the ECG?

Answer 8

25 mm/s (1 large division every 0.2 seconds)

Question 9

What does the magnitude of the skin potential difference depend on?

Answer 9

It depends on the mass of the myocardium that is activated.

Question 10

What does ECG not detect the activity of?

Answer 10

It doesn’t detect the activity of the SA and AV nodes. Only the atrial and ventricular muscles.

Question 11

Why is chest lead V1 mostly negative and Why is chest lead V6 mostly positive

Answer 11

It is because the depolarisation of the left ventricle is much stronger than the contraction of the right ventricle. Therefore, both leads see a wave of depolarisation towards V6 since the left ventricle depolarisation cancels out the right ventricle depolarisation. Hencee, V1 is negative S wave (see RS) and V6 is a postive R wave (see QR).

Question 12

Bi polar lead?

Answer 12

single postive and single negative electrode

Question 13

Unipolar lead?

Answer 13

single positive lead and utilize combination of other electrodes to serve as a composite negative electrode.

Question 14

Lead 1 ?

Answer 14

records the potential diffrence between LA (+) and RA (-). It is 0 degrees and S wave is missing.

Question 15

Lead 2?

Answer 15

records the potential diffrence between the RA (-) and the LL (+). It is oriented at 60 degrees. Best one out of the bipolar leads because it follows the current fromt he SA node to the apex.

Question 16

Lead 3?

Answer 16

records the potential diffrence between the LA (-) and the LL (+). It is oriented at 120 degrees

Question 17

aVR (augmented voltage right arm)

Answer 17

The right arm is positive and it is oriented at -150 degrees. Therefore the deflection is negative and there is an opposite direction of flow of the signal of the heart,

Question 18

aVF (augmented voltage left foot)

Answer 18

The left foot is postive and it is oriented at 90 degrees.

Question 19

aVL (augmented voltage left arm)

Answer 19

The left arm is postive and it is oriented at 30 degrees.

Question 20

Chest leads?

Answer 20

V1 - V6 are placed on the chest. Right foot is earthed and LA. RA, and LL are all connected together as a negative electrode. V1 and V2 are in the right ventricle and give off a negative S wave. V3 and V4 are int he interventricular septum and give off a postive R and a negative S wave. V5 and V6 are in the left ventricle and give off a positive R wave.

Question 21

How can the heart rate be estimated from the ECG?

Answer 21

Impulse of 1 mV generates a peak of 10 mm (2 large boxes) and the recorder is run at 25 mm/s, So count the boxes.

Question 22

What happens if the patient has a slow and irregular rhythm?

Answer 22

On the ECG there is no P waves adn they are replaced my several small depolarisations (f waves) and also there are no p waves before the QRS.

Question 23

What is atrial fibrillation and its comaprison to normal heart rhythm?

Answer 23

Normal: waves of depolarisation take well defined routes. Every SA nodal impuls leads to an AV nodal depolarisation which leades to a ventricular depolarisation.

Atrial fibrillation: SA node depolarises normallu but because there is an injury to the atrial muscle, the waves of depolarisation take on abnormal routes. So random dpeolarisations occur and most impulses never reach the AV node and only random ones will. Which will cause the ventricles to contract randomly because there will be random ventricular depolarisation. So the HR is irregularly irregular.

Question 24

What is warfarin?

Answer 24

A medication used as an anticoagulant and used in people to prevent stroke for people who have atrial fibrillation. It decreases blood clotting by blocking the enzyme that reactivates vitamin K and swo without sufficient active vitamin K, clotting factors (II, VII, IX, and X) have a decreased clotting ability. A few days is required for the medication to take full effect.

Question 25

What is holiday heart?

Answer 25

It is the excessive alcohol consumption by healthy adults and so with a small percentage of people it results in an acute alcohol-induced injurt to the heart that distubs the normal pathways through the atria. Therefore, an arrythmia called atrial fibrilaltion occurs where there is no P wave, that are replaced by many smaller depolarisations and an irrefular pulse occurs.

Question 26

What is a cardioversion?

Answer 26

is a procedure used to return an abnormal heartbeat to a normal rhythm. This procedure is used when the heart is beating very fast or irregular. This is called an arrhythmia. So a low energy shock is given under sedation or general anaesthesia. At the SA node. On the other defibrillation is a high energy shock.