Cardiovascular Respiratory Interactions and Reflexes

Question 1

Describe 2 respiratory influences on the heart.

Answer 1

1 - Mechanical interaction:

• Inspiration increases venous return to the right ventricle, thereby increasing right SV and therefore left SV by Starling’s mechanism.

2 - Neural interaction:

• Known as sinus arrhythmia, where inspiration increases heart rate and expiration decreases heart rate.

Question 2

List the 2 main mechanisms behind sinus arrhythmia.

Answer 2

1 - Central inspiratory neurones exert an inhibitory influence on vagal activity to the heart via action at the nucleus ambiguus directly. This decreases vagal activity, thereby increasing heart rate.

2 - Inspiration activates pulmonary stretch receptors, which exert an inhibitory influence on vagal activity to the heart via action at the nucleus tractus solitarius, which consequently carries the inhibitory influence to the nucleus ambiguus. This decreases vagal activity, thereby increasing heart rate.

Question 3

List 3 other secondary mechanisms behind sinus arrhythmia.

Answer 3

An increase in respiratory rate and heart rate may be due to:

1 - Systemic hypoxia.

2 - Increased exercise.

3 - Increased stress.

• The reverse is also true.

Question 4

Define systemic hypoxia.

Answer 4

A fall in blood PaO2 of <8.1kPa.

Question 5

Which receptors initiate the reflex responses to systemic hypoxia?

Answer 5

Peripheral chemoreceptors.

Question 6

List the locations of peripheral chemoreceptors.

Answer 6

1 - In carotid bodies (bilaterally on the internal carotid arteries near the bifurcation of the common carotid arteries).

2 - In aortic bodies.

Question 7

List the nerves that are responsible for carrying afferent information from peripheral chemoreceptors.

Answer 7

1 - The glossopharyngeal (CNIX) nerve carries afferent information from carotid bodies.

2 - The vagus (CNX) nerve carries afferent information from aortic bodies.

Question 8

List the triggers that stimulate peripheral chemoreceptors.

Answer 8

1 - A decrease in PaO2.

2 - An increase in PaCO2.

3 - A decrease in arterial blood pH.

Question 9

To which area of the brain is afferent information from peripheral chemoreceptors carried?

Answer 9

The nucleus tractus solitarius (in the medulla).

Question 10

List the effects of peripheral chemoreceptor reflex activity on the cardiovascular system when respiration isn’t able to be increased.

What is the purpose of these effects?

Answer 10

1 - Decreases heart rate.

2 - Causes vasoconstriction.

• The purpose of these effects is to conserve oxygen.

Question 11

What is the effect of peripheral chemoreceptor reflex activity on the cardiovascular system when respiration is able to be increased?

What is the purpose of this effect?

Answer 11

• Heart rate increases because the effects of increasing respiration on heart rate dominates.
• The purpose of this effect is to restore PaO2.

Question 12

Describe the chemoreceptor reflex when respiration isn’t able to be increased.

Answer 12

1 - Stimulation of peripheral chemoreceptors stimulates nerves originating in the nucleus tractus solitarius.

2 - Nerves originating in the nucleus tractus solitarius innervate nerves in the rostral ventrolateral medulla (RVLM), the nucleus ambiguus and the inspiratory centres of the brain:

• The RVLM stimulates sympathetic fibres innervating arterioles, causing vasoconstriction.
• The nucleus ambiguus stimulates parasympathetic fibres innervating the heart, decreasing heart rate.
• Inspiratory centres of the brain stimulate respiratory motor neurones, however cannot increase respiration further.

Question 13

Describe the chemoreceptor reflex when respiration is able to be increased.

Answer 13

The pathway is the same as in the condition where respiration cannot increase, however the additional step of increasing respiration antagonises the whole pathway:

1 - Inspiratory motor neurones are able to increase respiration.

2 - This stimulates pulmonary stretch receptors, which, via nerves originating in the nucleus tractus solitarius, have an inhibitory effect on the nucleus ambiguus both directly and via inspiratory centres of the brain (before, these signals were excitatory).

3 - The overall effect in this case is therefore to inhibit parasympathetic drive from the nucleus ambiguus, increasing heart rate.

Question 14

List 5 conditions in which systemic hypoxia might occur and respiration isn’t able to be increased.

Answer 14

1 - When paralysed / under the muscle relaxant.

2 - After high spinal transection.

3 - When under water.

4 - Foetuses in utero.

5 - Severe respiratory diseases.

Question 15

On which other reflex is the chemoreceptor reflex superimposed?

Answer 15

Local effects (of hypoxia).

Question 16

List 5 local effects of hypoxia on the cardiovascular system.

Answer 16

1 - Decreased heart rate.

2 - Decreased contractility.

3 - Cerebral vasodilation.

4 - Muscle vasodilation.

5 - Pulmonary vasoconstriction.

Question 17

Give an example of a risk associated with pulmonary vasoconstriction.

Answer 17

Pulmonary oedema, which also eventually leads to right ventricular failure.

Question 18

List 3 conditions in which systemic hypoxia might occur and respiration is able to be increased.

Answer 18

1 - Hypoxic atmosphere.

2 - High altitude.

3 - Less severe respiratory disease.

Question 19

What is meant by the terms ‘blue bloater’ and ‘pink puffer’?

Answer 19

• In patients with hypoxia who are unable to increase respiration, patients remain hypoxic as, although they have the oxygen-conserving chemoreceptor reflex, they are unable to restore PaO2, so appear blue in colour as oxygen that is available is directed to the brain.
• In patients with hypoxia who are able to increase respiration, patients are able to restore PaO2 by increasing respiration, increasing heart rate (through respiratory mechanisms), and exhibiting generalised vasoconstriction.

Question 20

Describe the diving reflex.

Answer 20

1 - Stimulators of trigeminal afferents, such as cold water on the face, stimulate afferent nerves that synapse at the nucleus tractus solitarius.

2 - Nerve fibres originating in the nucleus tractus solitarius exert an inhibitory influence on central inspiratory neurones, causing expiratory apnoea.

3 - Nerve fibres originating in the nucleus tractus solitarius exert an excitatory influence on vagal activity, decreasing heart rate.

4 - Nerve fibres originating in the nucleus tractus solitarius exert an excitatory influence on sympathetic fibres originating in the rostral ventrolateral medulla, causing arteriolar vasoconstriction (except in the brain).

Question 21

What is the purpose of the diving reflex?

Answer 21

It is an oxygen conserving reflex.

Question 22

List 3 other receptors that evoke similar reflexes to the diving reflex.

Answer 22

1 - Receptors in facial sinuses.

2 - Receptors in the larynx.

3 - Receptors in the pharynx.

Question 23

List 4 stimulators of similar reflexes to the diving reflex.

Answer 23

1 - Sinus washing.

2 - Irritant vapours.

3 - Intubation.

4 - Food caught in the pharynx / larynx.

Question 24

How can expiratory apnoea be treated?

If the respiratory apnoea is due to a diving reflex / similar reflex, what other biological changes will occur upon cessation of the expiratory apnoea?

Answer 24

• Mouth to mouth respiration.
• This will also restore rhythmic beating because the whole reflex is terminated upon cessation of the expiratory apnoea.

Question 25

Why might putting an ice bag on the face help treat tachycardia?

Answer 25

The ice bag will trigger the diving reflex, causing expiratory apnoea and bradycardia.