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Cardiovascular phase 1 > 2.3 Electrical conduction > Flashcards

2.3 Electrical conduction Flashcards

1
Q

What are the typical settings of an ECG?

A

Speed = mm/sec
Voltage = mm/mV
25cm = 10seconds

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2
Q

Calculate EGC heart rate

A

Rate (bpm) = 300 / number of large squares between cardiac cycles

Rate (bpm) = (cycles in 10 seconds) x 6

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3
Q

What is ohms law calculation?

A

Voltage = current x resistance

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4
Q

What does the P wave on an ECG represent?

A

Atrial depolarisation. Duration is less than 0.12s.

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5
Q

What does the QRS complex on an ECG represent?

A

Ventricular depolarisation. Duration is 0.08-0.1s.

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6
Q

What does the T wave on an ECG represent?

A

Ventricular repolarisation.

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7
Q

What is a normal PR interval value?

A

120 - 200 ms
0.12-0.2 seconds.
2-5 small squares

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8
Q

What does a long PR interval suggest?

A

Delayed AV conduction

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9
Q

What the normal value of QRS complex?

A

Less than 120ms
3 small squares

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10
Q

What does a prolonged QRS complex indicate?

A

QRS > 120ms
Bundle branch block

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11
Q

What is the normal values of QT interval?

A

Men: 350 - 440ms
Women: 350 - 460ms

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12
Q

What might an elevated ST segment be associated with?

A

Myocardial infarction

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13
Q

ECG: where would you place lead 1?

A

Right arm (-ve) to left arm (+ve).

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14
Q

ECG: where would you place lead 2?

A

Right arm (-ve) to left leg (+ve).

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15
Q

ECG: where would you place lead 3?

A

Left arm (-ve) to left leg (+ve).

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16
Q

What is Einthoven’s triangle?

A

An imaginary formation of the 3 limb leads in a triangle shape.

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17
Q

ECG: where would you place lead aVR?

A

Left arm and left leg (-ve) to right arm (+ve).

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18
Q

ECG: where would you place lead aVF?

A

Right arm and left arm (-ve) to left leg (+ve).

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19
Q

ECG: where would you place lead aVL?

A

Right arm and left leg (-ve) to left arm (+ve).

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20
Q

ECG chest leads: In which intercostal space would you place V1 and V2?

A

The 4th intercostal space. V1 is right of the sternum and V2 in left.

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21
Q

ECG chest leads: In which intercostal space would you place V3-V6.

A

The 5th intercostal space. V3 is left of the sternum, V4 is in the mid-clavicular line, V5 is left of V4 and V6 is under the left arm.

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22
Q

What are the average systolic and diastolic pressures for the pulmonary circulation?

A

25 and 10 mmHg.

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23
Q

What are the average systolic and diastolic pressures for the systemic circulation?

A

120 and 80 mmHg.

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24
Q

What would an absent P wave on an ECG be a sign of?

A

Atrial fibrillation

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25
Q

What part of the ECG does the plateau phase of the cardiac action potential coincide with?

A

QT interval.

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26
Q

What does the PR interval represent?

A

The slow conduction between the AVN and the His-Purkinje system.

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27
Q

What ECG lead yields complexes that are normally inverted compared to the anterior and inferior leads?

A

Lead aVR.

28
Q

What is the difference between bipolar and unipolar leads?

A

A bipolar lead has 2 separate conductors: one positive and one negative whereas the unipolar lead is a single conductor lead which measures the voltage between a positive electrode and a combined reference electrode (negative).

29
Q

What are the 3 bipolar limb leads?

A

|, ||, |||

30
Q

What are the 3 unipolar limb leads?

A

aVL, aVF, aVR

31
Q

What are the 6 unipolar chest leads and where placed?

A

V1- V6
V1 + V2 = septal (beside sternum on 4th intercostal space)
V3 + V4 = anterior between 4th intercostal space
V5 + V6 = Lateral between 4th intercostal space.

32
Q

What do the lateral, anterior and septal chest leads correspond to?

A

Lateral - circumflex artery

Anterior - Left anterior descending artery

Septal - left anterior descending / right coronary artery.

33
Q

What pump maintains the negative resting potential of a membrane?

A

Na+/K+ pump.

K+ ions diffuse outwards and excess of anions inside the cell generates a negative potential inside the cell.

34
Q

What is the purpose of the Nernst equation?

A

It is used to determine a membranes potential.

35
Q

Give the Nernst equation.

A

E = 60log(conc outside/conc inside)

36
Q

Why does the myocyte membrane pump require active transport?

A

K+ pumps into the cells
Na+ and Ca2+ pumped out of cells against concentration and electrical gradient so require ATP, therefore active transport process.

37
Q

What membrane channels are responsible for the plateau period in the cardiac AP?

A

Voltage gated Ca2+ ‘slow’ channels.

38
Q

Briefly describe the cardiac action potential in 5 steps.

A

Na+ channels open; influx of Na+ into cell; depolarisation.

When the Na+ channels close, a small number of K+ leave the cell resulting in partial repolarisation.

Ca2+ channels open and there is Ca2+ inflow. K+ channels are also open and there is K+ outflow. This results in the plateau period.

Ca2+ channels close and K+ channels remain open. K+ leaves the cell resulting in repolarisation.

Maintaining the resting potential (approx -90mV). Na+ inflow, K+ outflow.

39
Q

Why is the electrical activity required?

A

Calcium transportation.
The contraction of the heart muscle requires the delivery of Ca2+ ions into the cytoplasm. This is known as excitation-contraction coupling (conversion of electrical activity into muscle contraction).

40
Q

Where is Ca2+ released from in excitation contraction coupling?

A

The T tubules and the sarcoplasmic reticulum.

41
Q

What happens in phase 4 of the cardiac action potential?

A

Resting potential
Na+ inflow and slowing of K+ outflow. Slow depolarisation begins = innate contractility.

42
Q

What 2 channels are closed during the refractory period in a cardiac action potential?

A

Fast Na+ and Ca2+ channels

43
Q

What is the function of the refractory period?

A

It prevents excessively frequent contractions and allows adequate time for the heart to fill.

44
Q

Briefly describe the electrical conduction pathway in the heart.

A
  1. The SAN generates an electrical impulse.
  2. This generates a wave of contraction in the atria.
  3. Impulse reaches AVN.
  4. There is a brief delay to ensure the atria have fully emptied.
  5. The impulse then rapidly spreads down the Bundle of His and Purkinje fibres.
  6. The purkinje fibres then trigger coordinated ventricular contraction.
45
Q

Why is there rapid conduction in the bundle of his and purkinje fibres?

A

The fibres have a large diameter

46
Q

Describe excitation-contraction coupling.

A
  1. Na+ depolarises membrane.
  2. A small amount of Ca2+ is released from T tubules.
  3. Ca2+ channels in sarcoplasmic reticulum open.
  4. Ca2+ flows into cytosol. Cytosolic Ca2+ conc raised.
  5. Ca2+ binds to troponin C, this pulls tropomyosin and exposes the myosin binding site on actin.
  6. Cross bridge cycling begins.
  7. After depolarisation, Ca2+ is returned to SR. K+ outflow = repolarisation.
47
Q

What effect does myocardial contraction have on the A-band of a sarcomere?

A

No effect, it stays the same length.

48
Q

What effect does myocardial contraction have on the I-band and H-zone of a sarcomere?

A

They get shorter.

49
Q

Describe actin (thin filament).

A

A globular protein, single polypeptide. It polymerises with other actin monomers to form a double stranded helix. Together they form F actin.

50
Q

Describe myosin (thick filament).

A

2 heavy polypeptide chains and 4 light chains. The myosin heads have 2 binding sites; one for actin and one for ATP.

51
Q

Describe tropomyosin.

A

An elongated molecule made of 2 helical peptide

52
Q

What is the relevance of troponin on muscle contraction

A

Troponin binds to Ca2+ and transmits information via structural changes throughout the actin-tropomyosin filaments.

53
Q

What is the function of troponin I?

A

Troponin I, together with tropomyosin, inhibits actin and myosin binding.

54
Q

What is the function of troponin T?

A

Troponin T binds to tropomyosin.

55
Q

What is the function of troponin C?

A

Troponin C has a high affinity for Ca2+. TnC drives away TnI and so allows cross bridge formation.

56
Q

What is the phase 4 slope in the cardiac action potential affected by:

A

Autonomic tone
Drugs
Hypoxia
Electrolytes
Age

57
Q

What is the significance of the AV node?

A

Transmits the cardiac impulse between atria and ventricles

It delays the impulse to all the atria to empty

Limit dangerous tachycardia

58
Q

What is the conduction rate in the heart of atrial and ventricular muscle?

A

0.3 - 0.5 m/s

59
Q

What is the conduction rate in the heart of the purkinje fibres?

A

4m/s

60
Q

What is the significance of the His-purkinje system?

A

Rapid conduction to allow coordinated ventricular contraction. Very large diameters. High permeability at gap junctions

61
Q

What are chronotropic effects?

A

Those that change the heart rate. Positive chronotropic = increased heart rate.

62
Q

What are inotropic effects?

A

Those that alter the force of muscular contractions.

63
Q

What affect does parasympathetic stimulation have on heart rate?

A

Decreases heart rate (-ve chronotropic). Cardiac output therefore decreases with parasympathetic stimulation. (CO=HRxSV).

64
Q

What affect does sympathetic stimulation have on force of contraction?

A

Increases force (+ve inotropic).

65
Q

What phase of the cardiac action potential coincides with diastole?

A

Phase 4.

66
Q

Give 4 factors that affect the gating of ion channels.

A

Voltage, drugs, hormones, temperature.

67
Q

What is the resting potential of the SA node?

A

-55 to -60 mV.

Cardiovascular phase 1 (9 decks)

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