Electrical activity of the heart

Question 1

How much longer does contraction last than in skeletal muscle?

Answer 1

15 times longer- due to slow calcium channels.

Question 2

What is the refractory period? Why does this occur?

Answer 2

The period of time after an action potential where a second impulse cannot cause a second contraction of cardiac muscle.
To prevent excessive frequent contraction and to allow adequate filling time.

Question 3

What are myocardial cells supplied by? Where do they exit the heart? Where do most drain?

Answer 3

Coronary arteries- behind the aortic valve cusps in very first part of the aorta.
Into a single vein called the coronary sinus– into right atrium.

Question 4

Cells that do not function in contraction constitute what network? Are in electrical contact with cardiac myocytes via what?

Answer 4

The conducting system of the heart- gap junctions.

Question 5

What do gap junctions enable?

Answer 5

Action potentials to spread from one cell to another- ions can travel directly to other cells.

Question 6

Resting membrane potential of SA node? Due to what?

Answer 6

-55 to -60 mV- due to slow Na+ inflow not found anywhere else in the body.

Question 7

The SA node undergoes what? Known as what? What can occur at threshold?

Answer 7

Slow depolarisation= pacemaker potential. Action potential.

Question 8

How many stages for myocardial action potential? What 3 ion channel mechanisms contribute to pacemaker potential?

Answer 8

4 stages. Progressive reduction in K+ permeability, F-type channels- open when potential is at negative values (inward Na+ current) and Ca2+ channels contributing to inward current which is depolarising boost= T-type Ca2+ channels.

Question 9

Why does the AV node delay the impulse?

Answer 9

To allow the atria to empty blood into ventricles, has less gap junctions and AV fibres are smaller than atrial fibres.

Question 10

Why is there rapid conduction from AV node through the ventricles?

Answer 10

To allow coordinated ventricular contraction, very large fibres, high permeability at gap junctions and spread from endocardium to pericardium.

Question 11

What is automaticity?

Answer 11

The ability of the SA node for spontaneous, rhythmic self-excitation.

Question 12

What does AV node modified cardiac cells conduct action potentials with?

Answer 12

Low resistance.

Question 13

After AV node is excited, action potential progresses down what? Only electrical connection between atria and ventricles? Bundle of His divides into what?

Answer 13

Interventricular septum= bundle of His. AV node and bundle of His.
Right and left bundle branches- separate at apex and enter walls of both ventricles

Question 14

Fibres make contact with what other fibres that rapidly distribute the impulse through much of the ventricles?

Answer 14

Purkinje fibres.

Question 15

What is the parasympathetic innervation of the heart? Controlled by what which binds to what receptors? Decreasing HR known as what? Decreasing contraction force? Decreases what also?

Answer 15

Via the vagus nerve. Each bind to muscarinic receptors.
Negatively chronotropic.
Negatively inotropic.
Cardiac output.

Question 16

What is the sympathetic innvervation of the heart? Controlled by what 2 things? 3 things increased?

Answer 16

Postganglionic fibres
Adrenaline and noradrenaline.
Positively chronotropic, positively inotropic and increases CO.

Question 17

What does an ECG measure?

Answer 17

The currents generated in the extracellular fluid by the changes occurring simultaneously in many cardiac cells. Changes in voltage over time.

Question 18

What are the 5 stages of myocyte action potential?

Answer 18

Phase 0= rapid depolarisation, inflow of Na+.
Phase 1= partial depolarisation, inward Na+ deactivated and outflow of K+.
Phase 2= plateau, slow inward Ca2+ current.
Phase 3= repolarisation, K+ outflow, Ca2+ current deactivated.
Phase 4= pacemaker potential, slow Na+ inflow, slowing of K= outflow.

Question 19

How many electrodes are used to measure an ECG? Where are the V1-V6 leads placed? What are the other 6 electrodes?

Answer 19

12 electrodes. 6 unipolar chest leads.

3 bipolar leads and 3 unipolar arm leads.

Question 20

Where are the 3 bipolar leads placed? What does the right leg act as? These also known as?

Answer 20

Lead 1= between the right and left arm. 
Lead 2= from right arm to left leg. 
Lead 3= from left arm to left leg. 
As a ground electrode. 
Standard limb leads.

Question 21

The negative poles are known as what? The positive poles are known as what?

Answer 21

Reference electrodes. Recording electrodes.

Question 22

3 unipolar arm leads also known as what? They do what?

Answer 22

Augmented leads (aVR, aVL and aVF.) 
Bisect the angles of the triangle by combining two electrodes as reference e.g. aVL- right wrist and foot= combined as negative pole. Point towards recording electrode on left wrist.

Question 23

The 6 unipolar chest leads are known as what? Where is each one placed?

Answer 23

Precordial leads.
V1= 4th intercostal space at right border of sternum.
V2= 4th intercostal space at left border of sternum.
V3= Midway between V2 and V4.
V4= 5th intercostal space at mid-clavicular line.
V5= anterior axillary line at same level as V4.
V6= Mid-axillary line on same level as V4 and 5.

Question 24

How is the 4th intercostal space found?

Answer 24

Using angle of Louis- bony lump below top of sternum. To the right= 2nd intercostal space. Move downwards over 2 more ribs.

Question 25

What is the P wave? In what leads is it seen?

Answer 25

Atrial depolarisation- seen in every lead apart from aVR.

Question 26

What is the PR interval?

Answer 26

Time taken for atria to depolarise and electrical activation to get through AV node.

Question 27

What is the QRS complex? Positive/ negative if on left?

What if it is on the right?

Answer 27

Ventricular depolarisation.

Positive. Negative.

Question 28

What is the ST segment? What is the T wave? What is the QT interval?

Answer 28

The interval between depolarisation and repolarisation.
Ventricular repolarisation.
Time of depolarisation and repolarisation.

Question 29

How does a faster HR affect the QT interval?

Answer 29

Faster HR= shorter QT.

Question 30

T waves are positive in every lead apart from which one?

Answer 30

aVR. Sometimes V1 and/ or V2 depending on trace.

Question 31

What is tachycardia? Bradycardia? Dextrocardia?

Answer 31

Increased heart rate. Decreased heart rate.

Heart on right side of chest instead of left.

Question 32

How does acute anterolateral myocardial infarction affect ST segments?

Answer 32

They are raised in anterior (V3-V4) and lateral (V5-V6) leads.

Question 33

How does an acute inferior myocardial infarction affect ST segments?

Answer 33

They are raised in inferior (2,3 and aVF) leads.

Question 34

When does atrial repolarisation usually occur at same time as?

Answer 34

QRS complex.

Question 35

Electrical impulses in heart move in how many dimensions? ECG measures voltage in how many?

Answer 35

3 dimensions. 1 dimension.

Question 36

If impulse is towards the electrode, it looks what? If away from electrode, looks what? How does impulse from atria compare to the ventricles?

Answer 36

Big. Small or even negative.

Smaller- less myocytes.

Question 37

Each small square equals what? Each big square represents what?

Answer 37

40ms. 0.2s.