respiratory failure

Question 1

what is hypoxia?

Answer 1

reduced level of tissue oxygenation

Question 2

what is hypoxaemia?

Answer 2

decrease in partial pressure of oxygen in the blood

Question 3

why do hypoxia and hypoxaemia not always coexist?

Answer 3

individuals can develop hypoxaemia without hypoxia is there is a compensatory

Question 4

what is arterial oxygen tension?

Answer 4

the partial pressure of oxygen that indicates the dissolved oxygen in plasma (not oxygen bound to Hb)

Question 5

what measures hypoxia and hypoxaemia?

Answer 5

uses an arterial blood gas analyser

Question 6

what law does pulse oximetry use?

Answer 6

Beer-Lambert-Bougeur law

Question 7

what is type 1 respiratory failure?

Answer 7

reduction in PaO2 but no change in PaCO2 - V/Q mismatch

Question 8

what is type 2 respiratory failure?

Answer 8

increase in PaCO2 and reduction in PaO2 - underventilation

Question 9

what are the 5 mechanisms of hypoxaemia?

Answer 9

1) V/Q mismatch
2) right-to-left shunt
3) diffusion impairment
4) hypoventilation
5) low inspired pO2

Question 10

what is the A-a gradient?

Answer 10

the difference between the alveolar O2 level and the arterial O2 level

Question 11

what does the A-a gradient indicate?

Answer 11

the integrity of the alveolocapillary membrane and the effectiveness of gas exchange

Question 12

if hypoxaemia has a widened A-a gradient what is it caused by?

Answer 12

V/Q mismatch, diffusion limitation and shunt

Question 13

if hypoxaemia has a normal A-a gradient what is it caused by?

Answer 13

hypoventilation

Question 14

what is the most common cause of hypoxemia?

Answer 14

V/Q mismatch

Question 15

what is regional heterogeneity of V/Q?

Answer 15

sub atmospheric intrapleural pressure and gravity

Question 16

how does V/Q mismatch cause hypoxaemia?

Answer 16

• ventilation and perfusion are higher at the bases and lower at the apex
• V/Q ratio is higher at the apex and low at the base
• a low V/Q ratio produces hypoxemia by decreasing the PAO2 and subsequent PaO2
• hypoxemia is caused if the compensatory rise in total ventilation is absent

Question 17

what is the compensatory mechanism of a V/Q mismatch?

Answer 17

there is a restriction in perfusion in areas of the lung with reduced ventilation known as hypoxic pulmonary vasoconstriction

Question 18

what are the characteristics of a V/Q mismatch?

Answer 18

• hypoxaemia due to V/Q mismatch can easily be corrected by supplemental oxygen
• widened A-a gradient

Question 19

what are the common causes of a V/Q mismatch?

Answer 19

asthma, COPD, bronchiectasis, cystic fibrosis, ILD and pulmonary hypertension

Question 20

what is a shunt?

Answer 20

blood from the right side of the heart enters the left side without taking part in case exchange

Question 21

what feature of a shunt distinguishes it from other mechanisms?

Answer 21

there is a poor response to oxygen therapy

Question 22

what fraction does the shunt have to reach for hypoxemia to be more common?

Answer 22

50%

Question 23

why is there a lack of hypercapnia in a shunt?

Answer 23

due to stimulation of the respiratory centre by chemoreceptors

Question 24

what are the characteristics of a pulmonary shunt?

Answer 24

• A-a gradient is elevated
• pCO2 is normal
• poor response to oxygen therapy

Question 25

what are the common causes of a shunt?

Answer 25

pneumonia, pulmonary oedema, ARDS, pulmonary arteriovenous communication

Question 26

what is diffusion limitation?

Answer 26

transport across the alveolocapillary membrane is impaired due to a decrease in lung surface area for diffusion, inflammation and fibrosis, low alveolar oxygen and reduced capillary transit time

Question 27

why is hypercapnia uncommon with diffusion limitation?

Answer 27

CO2 is 20x more soluble in O2 and is less likely to be affected by diffusion limitation

Question 28

what are the characteristics of diffusion limitation?

Answer 28

- hypoxaemia shows a good response to oxygen therapy - A-a gradient is elevated - PaCO2 is normal

Question 29

what is the hallmark of hypoventilation?

Answer 29

high PaCO2

Question 30

when does hypoventilation cause significant hypoxaemia?

Answer 30

in the presence of lung disease

Question 31

how do you correct hypoxaemia due to hyperventilation?

Answer 31

oxygen therapy however hypoventilation and hypercapnia can persist

Question 32

what are the causes of hypoventilation?

Answer 32

- impaired central drive: drug overdose, brain stem infarction, primary alveolar hypoventilation - spinal chord: ALS - nerve: Guillan-Barre syndrome - neuromuscular junction: Myasthenia graves - respiratory muscles: myopathy

Question 33

what are the characteristics of hypoventilation?

Answer 33

- hypoxaemia shows good response to oxygen therapy - A-a gradient is normal - PaCO2 is high