Respiratory Failure

Question 1

What is hypoxaemia?

Answer 1

Low O2 in blood

Question 2

What is hypoxia?

Answer 2

O2 deficiency at tissue level

Question 3

What is the normal range for O2 sats?

Answer 3

94-98%

Question 4

What is the normal range for pO2?

Answer 4

9.3-13.3 kPa

Question 5

What characterises type 1 respiratory failure?

Answer 5

Low pO2 (less than 8 kPa) or O2 sats < 90%
pCO2 normal or low

Question 6

What characterises type 2 respiratory failure?

Answer 6

Low pO2 and high pCO2

Question 7

What may cause hypoxia?

Answer 7

1) Low inspired pO2
2) Hypoventilation (resp pump failure)
3) V/Q mismatch
4) Diffusion defect
5) Right to left shunt

Question 8

Where is central cyanosis seen?

Answer 8

In oral mucosa, tongue, lips

Question 9

Where is peripheral cyanosis seen?

Answer 9

In fingers and toes

Question 10

What is the difference between ventilation and perfusion in the lung?

Answer 10

Ventilation = AIR that reaches alveoli
Perfusion = BLOOD that reaches alveoli

Question 11

In hypoventilation, what happens to the amount of O2 entering the blood and CO2 entering alveolus per minute?

Answer 11

Remains unchanged as the metabolic rate is the same - perfusion

Question 12

Why does hypoventilation cause hypoxaemia?

Answer 12

Alveolar pO2 level falls as the alveoli are poorly ventilated, therefore arterial pO2 falls

Question 13

True or False:

Hypoventilation ALWAYS causes hypercapnia

Answer 13

True

Alveolar + arterial pCO2 increases

Question 14

Which kind of respiratory failure does hypoventilation cause?

Answer 14

Type 2 respiratory failure

both hypoxia and hypercapnia

Question 15

What could cause acute hypoventilation?

Answer 15

• Opiate overdose
• Head injury
• V severe acute asthma

Question 16

What could cause chronic hypoventilation?

Answer 16

Severe COPD

Question 17

What compensatory mechanisms are seen in chronic hypoxaemia?

Answer 17

• Increased EPO secreted by kidney to raise Hb

- Increased 2,3-DPG (shifts curve to right - favours O2 release at tissues)

Question 18

What does chronic hypoxic vasoconstriction of pulmonary vessels result in?

Answer 18

Pulmonary hypertension -> right HF -> cor pulmonale

Question 19

What are the effects of hypercapnia?

Answer 19

Respiratory acidosis
Impaired CNS function
Peripheral vasodilation and cerebral vasodilation (headache)

Question 20

What are the effects of chronic CO2 retention on central chemoreceptors?

Answer 20

Low CSF pH corrected by choroid plexus cells which secrete HCO3 in to CSF

pH of CSF returns to normal so central chemoreceptors are no longer stimulated but the pCO2 in the blood is still high

Question 21

In chronic type 2 respiratory failure, why may treatment with O2 worsen hypercapnia?

Answer 21

1) O2 removes stimulus for hypoxic respiratory drive, alveolar ventilation drops worsening hypercapnia
2) Correction of hypoxia removes pulmonary hypoxic vasoconstriction, leading to increased perfusion of poorly ventilated alveoli and diverting blood away from better ventilated alveoli

Question 22

What is the V/Q ratio of optimal gas exchange?

Answer 22

1

Question 23

What happens to alveolar pO2 and pCO2 when V/Q ratio is less than 1?

Answer 23

PO2 falls and PCO2 rises

Question 24

What happens to alveolar pO2 and pCO2 when V/Q ratio is more than 1?

Answer 24

PO2 rises and PCO2 falls

Question 25

Give an example of when V/Q ratio would be greater than 1

Answer 25

Eg in hyperventilation due to anxiety

Question 26

When does V/Q mismatch occur?

Answer 26

In disorders where some alveoli are being poorly ventilated eg asthma and pneumonia

Question 27

Describe V/Q mismatch

Answer 27

If the V/Q ratio drops to <1, i.e. the perfusion is higher than the ventilation, the pO2 falls and the pCO2 in the alveolus will rise.

Hypoxic vasoconstriction can occur, diverting blood to better ventilated parts of the lung. However, these alveoli are unlikely to be able to take up much more O2 than they already are.

As a result, the pO2 remains low, which acts as a stimulus, causing hyperventilation, resulting in either normal or low CO2 levels.

Question 28

Give an example of where a V/Q mismatch occurs due to reduced perfusion

Answer 28

Pulmonary embolism