Resp 8 - Breathlessness and control of breathing

Question 1

Is tidal expiration an active or passive process?

Answer 1

Passive - due to the natural recoil of the lungs

Question 2

State the equation for minute ventilation (VE)

Answer 2

VE = VT x Frequency 
Frequency = 60/TTOT (if you want it per minute) or 1/TTOT for frequency 
TTOT = duration of one inspiration
VT = tidal volume

Question 3

How can this equation be manipulated to include TI?

Answer 3

VE = VT/TI x TI/TTOT
TI is the Inspiratory time 
TE is the expiratory time 
TTOT = TE + TI 
VT/TI mean inspiratory flow

Question 4

What does VT/TI represent?

Answer 4

Neural Drive - mean inspiratory flow (volume over time gives flow)
How powerfully the muscles contract. It is the slope of the upstroke.

Question 5

What does TI/TTOT represent?

Answer 5

Inspiratory Duty Cycle

Proportion of the cycle spent actively ventilating (i.e. breathing in)

Question 6

How do these factors change when there is an increase in metabolic demand?

Answer 6

Increased metabolic demand —> increased ventilation
VT/TI = INCREASE
TTOT = DECREASE (increase frequency)
TTOT is decreased by a combination of reduction in TI and TE

Question 7

What is the normal tidal volume, normal minute ventilation, breathing rate, TI/TTOT?

Answer 7

Tidal Volume = 0.5 L
Minute Ventilation = 6 L/min
Breathing Rate = 12 breaths per minute
TI/TTOT = 40%

Question 8

What changes take place if you use a noseclip?

Answer 8

Breathe more DEEPLY - increase in VT
Breathe SLOWER - decrease in frequency
Ventilation remains the SAME

Question 9

What changes take place when artificial dead space is added?

Answer 9

Compared to with mouthpiece only:
Minute ventilation = INCREASE
VT = INCREASE
Frequency = INCREASE 
VT/TI (neural drive)= INCREASE - to satisfy need for more ventilation
TI/TOTT UNALTERED

Question 10

How is the breathing of someone with COPD different to a normal person?

Answer 10

Breathing is SHALLOWER and FASTER
Difficulty more on expiration
Neural drive is the same - do not breathe harder

Question 11

What changes when we exercise?

Answer 11

Increases neural drive and hence ventilation 
Increases frequency (halving TTOT)

Question 12

Where is the voluntary and involuntary control of breathing located?

Answer 12

Voluntary = Cerebral Cortex 
Involuntary = Medulla

Question 13

How is the metabolic controller reset in sleep?

Answer 13

PCO2 rises

Question 14

Where, in the motor homunculus, is behavioural control of breathing located?

Answer 14

Between the hip and the trunk

Question 15

Which receptors are involved in regulating the involuntary control of breathing?

Answer 15

HYDROGEN ION RECEPTORS found in the carotid bodies and in the metabolic centre itself

Question 16

Where are the peripheral chemoreceptors located?

Answer 16

Carotid bodies (at the junction of the internal and external carotids, supplied by pharyngeal nerve)

Question 17

Where are the pacemakers for respiratory breathing located?

Answer 17

Many pacemakers close together in the brain stem. Inaccessible.
10 groups of neurons in medulla

Question 18

What is the main group of neurons that are involved in generating respiratory rhythm?

Answer 18

The pre-Botzinger complex situated near 4th ventricle

called the ‘gasping centre’

Question 19

Describe the Hering-breuer reflex. Which nerve is involved?

Answer 19

10th nerve
Pulmonary stretch receptors sense lengthening and shortening and terminates inspiration and expiration.
Basically feedback from lung helps terminate inspiration.

Question 20

What are the two parts of the metabolic controller?

Answer 20

• central part in the medullla responding to [H+] in the EC fluid -> SLOW response
• peripheral part at the carotid bifurcation (carotid sinus) - H receptors -> FAST
  Fast and slow responses due to the fact that change of H+ is rapidly sensed in hyperperfused carotid bodies but not in EC fluid

Question 21

Describe the carbon dioxide challenge and what it shows.

Answer 21

Testes the sensitivity of the metabolic respiratory centre to H ions.
Ventilation measured against PCO2.
Changes in PCO2 induced by breathing in and out of 6L bag with 7% CO2.

The slope of the curves obtained indicate the chemosensitivity. Arterial PCO2 rises at a constant rate of about 1 pKa per minute. This causes a rise in minute per ventilation - 30L/min per pKa.

Question 22

How does hypoxia affect the acute CO2 response?

Answer 22

Hypoxia increases the sensitivity (so the slope is greater) of the acute CO2 response. mediated through carotid bodies

Question 23

How does chronic metabolic acidosis affect the PCO2 threshold that gives a minimal drive to breathe?

Answer 23

Increases the threshold (shifts the intercept with the x on the left) but does not alter sensitivity (gradient is the same as in hypoxia). Chronic metabolic alkalosis has the opposite effect.

Question 24

Is the minimal drive to breathe present when asleep?

Answer 24

No it is attributed to wakefulness. In sleep, ventilation would drop to 0 but continuing CO2 production means that in 10-60 seconds, arterial PCO2 has risen sufficiently above the apnoeic point to restant breathing.

Question 25

What can depress the ventilatory response to PCO2? Give a central and a peripheral example.

Answer 25

Depress ventilatory response to CO2 means a flattening of the slope and rised set point.

This is caused by disease affecting metabolic control (tumour, congenital) but more commonly suppressed by drugs - opioid/anaesthetics
Peripheral cause - respiratory muscle weakness
COPD

Question 26

Describe the ventilatory response to a hypoxic challenge.

Answer 26

It increases when alveolar PO2 decreases - the relationship is hyperbolic.
And increases when oxygen saturation decreases - a linear relationship.
There is a 30L/min increase in minute ventilation for a 7kPa change in arterial PO2 –> a similar change of 30L/min would be brought about by 1kPa change in arterial PCO2.

Question 27

Why is this system bad at dealing with altitude where you experience hypoxic hyperventilation?

Answer 27

Altitude = a fall in PaO2 and PaCO2 together.
System is good at dealing like this: fall in ventilation –> fall in PaO2 and rise in PaCO2
The fall in PaCO2 increases sensitivity of carotid bodies to PaCO2 and H+ therefore ventilation is increased and PaO2 increased.

Altitude lowers PCO2 and inhibits the ventilatory response because no rise in PaCO2 but a fall.
So the body needs several days of acclimatisation before metabolic centres adjust.

Question 28

How is neural drive different in people with COPD?

Answer 28

It is the same

Question 29

How do people with obstructive disease maintain a normal minute ventilation despite breathing more shallowly?

Answer 29

Their frequency increases

Question 30

What are the rapid and slow responses to respiratory acidosis?

Answer 30

Rapid - fall in ventilation leads to rise in PCO2 and H+ which stimulates metabolic controls to increase breathing
Slow - renal excretion and retention of weak acids as an important backup if the lungs can’t compensate fully

Question 31

How is metabolic acidosis different to respiratory acidosis?

Answer 31

When the excess H+ comes from metabolism rather than inadequate ventilation.
Example of ause: diabetic ketoacidosis

Question 32

What are the mechanisms for dealing with metabolic acidosis?

Answer 32

Compensatory mechanisms: ventilatory stimulation - hyperventilation, renal excretion of weak acids, renal retention of chloride to reduce strong ion difference
Metabolic alkalosis would be the opposite.

Question 33

Give some central and peripheral causes of hypoventilation.

Answer 33

Central
Anaesthetics, drugs - acute - metabolic centre poisoning
chronic:
Disease of metabolic centre, congenital central hypoventilation syndrome, obesity hypoventilation syndrome, chronic mountain sickness

Peripheral
Acute
Muscle relaxant drugs, myasthenia gravis
Chronic
Neuromuscular with respiratory muscle weakness

Question 34

What are the three types of breathlessness?

Answer 34

• tightness
• increased work and effort
• air hunger

Question 35

What scale is used to measure breathlessness?

Answer 35

Borg scale