Innervation of the Heart and ECG (09/05)

Question 1

what are the two types of nerve fibres the heart receives?

Answer 1

parasympathetic (rest & digest)

sympathetic (fight & flight)

Question 2

how are parasympathetic nerve fibres transmitted?

Answer 2

via the vagus nerve (CN X)

Question 3

what is parasympathetic stimulation controlled by

Answer 3

acetylcholine, which binds to muscarinic receptors

Question 4

what does parasympathetic stimulation do?

Answer 4

• decreases heart rate (negatively chronotropic)
• decreases force of contraction (negative inotropic)
• decreases cardiac output (by up to 50%)
• decreased parasympathetic stimulation leads to increased heart rate

Question 5

how is sympathetic stimulation transmitted through the heart?

Answer 5

through postganglionic fibres that innervate the entire heart

Question 6

what is sympathetic stimulation controlled by

Answer 6

adrenaline and noradrenaline

Question 7

what does sympathetic stimulation do?

Answer 7

• increases heart rate (positively chronotropic)
• increases force of contraction (positively inotropic)
• increases cardiac output (by up to 200%)

decreased sympathetic stimulation leads to decreased heart rate and decrease in cardiac output up to 30%

Question 8

What does the electrocardiogram (ECG) measure?

Answer 8

it measures the currents generated in the extracellular fluid by changes occurring simultaneously in many cardiac cells

Question 9

recap the phases of myocyte action potential

Answer 9

phase 0: rapid depolarisation, inflow of Na+

phase 1: partial repolarisation, inward current of Na+ deactivated and outflow of K+

phase 2: plateau, slow inward Ca2+ cirrent

phase 3: repolarisation, K+ outflow, Ca2+ current deactivated

phase 4: pacemaker potential, slow Na+ inflow, slow K+ outflow

Question 10

In a normal trace, define:

P Wave:

PR interval:

QRS complex:

ST segment:

T wave:

Answer 10

P Wave: atrial depolarisation

PR interval: time taken for atria to depolarise and electrical activation to get through AV node

QRS complex: ventricular depolarisation

ST segment: interval between depolarisation and repolarisation

T wave: ventricular repolarisation

Question 11

define tachycardia, bradycardiaa and dextrocardia

Answer 11

tachycardia: increased heart rate
bradycardia: decreased heart rate
dextrocardia: heart on the right side of the chest instead of the left

Question 12

define acute antereolateral myocardial infarction

Answer 12

ST segments are raised in anterior (V3

- V4) and lateral (V5-V6) leads

Question 13

why is atrial repolarisation not evident on an ECG?

Answer 13

it occurs at the same time as QRS complex

Question 14

where are the leads positioned?

Answer 14

• standard limb leads (I,II,III) [wrists and left leg]
• augmented leads (aVR, aVL, aVF) bisect the angles of the triangle by combining 2 electrodes as a reference (eg. aVL is b/w right wrist and foot, pointing at left wrist)
• precordial leads (V1 - V6) recording electrodes placed on the chest

Question 15

what are positive and negative electrodes?

Answer 15

negative electrode: reference electrode

positive: recording electrode

Question 16

what do each small and big square represent in an ECG?

Answer 16

small square: 40ms

big square: 0.2s

Question 17

in a normal ECG,

P waves:

T waves:

Answer 17

P waves: are positive in every lead except aVR

T waves: positive in every lead apart from aVR and sometimes V1/V2

Question 18

what is systole?

Answer 18

ventricular contraction + blood ejection (takes 0.3s in total)

Question 19

how does systole work?

Answer 19

1. isovolumetric contraction of ventricles: increase in pressure but no change in volume since valves remain closed (note: isovolumetric contraction + relaxation is the only time when all the valves of the heart are closed)
2. once the pressure in the ventricle exceeds that in the aorta and pulmonary trunk, the aortic and pulmonary valves open and maximal; ejection from ventricles into the arteries occurs (note: ventricles do not completely empty during contraction)

Question 20

what is diastole?

Answer 20

ventricular relaxation + blood filling

takes 0.5s

Question 21

how does diastole work?

Answer 21

1. reduced ejection
2. ventricles begin to relax and aortic and pulmonary valves close. At this time, the atriovetricular valves are closed and there is no blood entering or leaving the ventricles. Ventricular volume not changing is known as isovolumetric ventricular relaxation (decrease in pressure but volume remains unchanged)
3. rapid left ventricle filling and ventricle suction: since blood in the atria is slightly pressurised due to venous return from superior and inferior vena cava and pulmonary vein, pressure is enough to open mitral and tricuspid valves. Since there is also lower pressure in the ventricles, blood rushes down the pressure gradient. This is responsible for 80% of ventricular filling before atrial contraction.
4. slow ventricular filling: since blood keeps flowing into atria from veins, pressure between atria and ventricles are equalising and thus diastasis occurs: little to no net movement of blood, AV node is delaying stimuli from SAN to allow full ventricular filling
5. atrial booster: pressure suddenly increases due to atrial contraction, enabling ventricles to be actively filled, squeezing remaining blood from atria into ventricless

Question 22

What is a typical heart rate? How long does each cardiac cycle last?

Answer 22

72 beats/min

0.8s, with 0.3s of systole and 0.5s of diastole