Hypoxia and Respiratory failure

Question 1

Describe type 1 respiratory failure. (2)

Answer 1

Low p(O2) with a normal or low p(CO2) - V/Q mismatch, diffusion defect.

Question 2

Define respiratory failure. (2)

Answer 2

p(O2) of 8kPa or less when breathing air at sea level.

Question 3

Describe type 2 respiratory failure. (2)

Answer 3

Low p(O2) with a high p(CO2) > 6.7kPa - hypoventilation.

Question 4

Describe the effects of hypoxia. (5)

Answer 4

Impaired CNS function
Central cyanosis due to lots of unsaturated Hb.
Cardiac arrhythmias
Peripheral vasoconstriction

Question 5

Describe the effects of hypocapnia. (6)

Answer 5

Respiratory acidosis 
Impaired CNS function (drowsiness, confusion)
Peripheral vasodilation (warm hands, bounding pulse) 
Central vasodilation (headache).

Question 6

Describe what you need to maintain normal p(O2). (5)

Answer 6

Normal inspired p(O2)
Normal alveolar ventilation 
Ventilation perfusion matches 
Normal alveolar capillary membrane 
No R > L shunts.

Question 7

Describe how a R > L shunt would cause hypoxia. (2)

Answer 7

Inadequate oxygenation of blood because not all of it goes to the lungs - bypass option.

Question 8

Describe acute causes of hypoventilation. (5)

Answer 8

Respiratory centre depression - head injury, overdose
Difficulty ventilating lungs - acute asthma, RDS.
Mechanical chest wall problems - rib #

Question 9

Describe chronic causes of hypoventilation. (4)

Answer 9

Respiratory muscle weakness due to neurovascular damage
Mechanical chest wall problems - obesity, scoliosis
Hard to ventilate lungs - COPD.

Question 10

Explain which type of respiratory failure hypoventilation causes. (1)

Answer 10

Type 2 - low p(O2) and high p(CO2).

Question 11

Describe adaptations that those who live at high altitudes age to survive. (3)

Answer 11

Polycythaemia
Increased capillary density in tissues
Increased 2,3-BPG

Question 12

Describe the effects on the body of chronic CO2 retention. (2)

Answer 12

Renal compensation of respiratory acidosis

Central chemoreceptors being reset through importing HCO3- into CSF.

Question 13

Describe two possible mechanisms to explain why uncontrolled oxygen therapy might make patients in type 2 respiratory failure worse. (6)

Answer 13

With chronic hypercapnia, central chemoreceptors are reset, so only peripheral chemoreceptors left to create hypoxic drive - remove this hypoxic drive and cause hypoventilation which will increase p(CO2).

O2 therapy worsens V/Q mismatch by removing hypoxia induced vasoconstriction which increases perfusion of bad alveoli, which will increase p(CO2).

Question 14

Describe causes of hypoxia due to V/Q mismatch. (2)

Answer 14

Low ventilation in asthma, CPOD, RDS.

Low perfusion in PE.

Question 15

Describe how hypoxia due to V/Q mismatch leads to type 1 respiratory failure. (3)

Answer 15

Low p(O2) prompts hyperventilation, but can’t fix hypoxaemia because the Hb available is saturated. Leads to low p(O2) but high p(CO2) because all CO2 is blown off.

Question 16

Describe causes of diffusion impairment and how this can cause type 1 respiratory failure. (4)

Answer 16

Barrier thickening due to fibrosis
Pathway lengthening due to pulmonary oedema
SA for diffusion is reduced like in emphysema.
CO2 can diffuse much more easily than O2 so p(CO2) is normal but p(O2) is low.

Question 17

Describe an example where there is more than one mechanism causing hypoxia.(3)

Answer 17

RDS
Lung stiffness causes hypoventilation
Collapse of alveoli leads to V/Q mismatch.

Question 18

Describe how asthma can lead to type 1 failure progressing to type 2. (2)

Answer 18

Initially type 1, but as more airways become narrower and exhaustion sets in, it becomes type 2.