Session 7 - Hypoxia And Respiratory Failure

Question 1

What is the difference between hypoxaemia and hypoxia?

Answer 1

Hypoxaemia - low pO2 in the blood

Hypoxia - low O 2 at a tissue level

Question 2

What is the normal range for oxygen saturation of the blood?

Answer 2

94-98%

Question 3

What is the normal range for the partial pressure of oxygen in the blood?

Answer 3

9.3-13.3 kPa

Question 4

What is respiratory failure?

Answer 4

Impairment in gas exchange causing hypoxia with or without hypercapnia.

Question 5

What is the difference between type 1 and type 2 respiratory failure?

Answer 5

Type 1 respiratory failure:

• low pO2
• normal pCO2 (or low)

Type 2 respiratory failure:

• low pO2
• high pCO2

Question 6

What are the possible causes of hypoxaemia?

Answer 6

1. Low inspired pO2
2. Hypoventilation - respiratory pump failure (failure of muscles in chest wall)
3. Ventilation/Perfusion mismatch
4. Diffusion defect - problems of the alveolar-capillary membrane (e.g. pleural effusion)
5. Right to left shunt(e.g. cyanosis heart disease)

Question 7

What is the optimal ventilation/perfusion ratio?

Answer 7

1

Question 8

What is hypoventilation?

Answer 8

When the entire lung is poorly ventilated meaning alveolar ventilation (minute volume) is reduced.

Question 9

What happens to pO2 and pCO2 of the blood in hypoventilation?

Answer 9

Alveolar pO2 falls —> arterial pO2 falls —> hypoxaemia

Alveolar pCO2 rises —> arterial pCO2 rises —> hypercapnia

Question 10

Hypoventilation always causes hypercapnia. Therefore it always causes what type of respiratory failure?

Answer 10

Type 2 respiratory failure, as there will be both hypoxia and hypercapnia

Question 11

Give some examples of causes of acute hypoventilation.

Answer 11

Opiate overdose
Head injury
Very severe asthma attack

Question 12

Give an example of a cause of chronic hypoventilation.

Answer 12

Severe COPD

Acute exacerbation of COPD may occur due to lower respiratory tract infection

Question 13

Why is chronic hypoventilation better tolerated than acute hypoventilation which requires immediate treatment?

Answer 13

Chronic hypoxia and chronic hypercapnia has a slow onset and progression. This allows time for compensation.

Question 14

Give some causes of chronic type 2 respiratory failure.

Answer 14

Myopathy of muscles of respiration
Motor neurone disease
Severe obesity
Kyphoscoliosis
Lung fibrosis
Late stages of COPD

Question 15

What are the effects of hypoxaemia?

Answer 15

Impaired CNS function, confusion, irritability
Cyanosis
Cardiac arrhythmias
Hypoxic vasoconstriction of pulmonary vessels

Question 16

What is the difference between central and peripheral cyanosis?

Answer 16

Central cyanosis:

• seen in oral mucosa, tongue, lips
• indicates hypoxaemia

Peripheral cyanosis:

• seen in fingered, toes
• indicates poor local circulation

Question 17

In chronic hypoxaemia, what compensatory mechanisms are used by the body to increase oxygen delivery?

Answer 17

Increased erythropoietin secretion by the kidney —> raises Hb (polycythemia)
Increased 2,3-BPG

Question 18

What does chronic hypoxic vasoconstriction of pulmonary vessels result in?

Answer 18

Pulmonary hypertension

Right heart failure (Cor Pulmonale)

Question 19

What are the effects of hypercapnia?

Answer 19

Respiratory acidosis
Impaired CNS function; drowsiness, confusion, coma, flapping tremors
Peripheral vasodilation - warm hands, bounding pulse
Cerebral vasodilation - headache

Question 20

How is chronic respiratory acidosis compensated?

Answer 20

Retention of HCO3- by the kidneys

Question 21

What is the effect of chronic hypercapnia on central chemoreceptors?

Answer 21

The central chemoreceptors ‘reset’ to the new higher CO2 level

Question 22

How do the central chemoreceptors become ‘reset’ in chronic hypercapnia?

Answer 22

CO2 diffuses into the CSF —> CSF pH drops —> stimulates chemoreceptors.
Low CSF pH is corrected by choroid plexus cells which secrete [HCO3-] into the CSF
The CSF returns to normal meaning the central chemoreceptors are no longer stimulated
The pCO2 in the blood is still high but the central chemoreceptors and now unresponsive
Central chemoreceptors have ‘reset’ to a new higher CO2 level

Question 23

Why might treatment for hypoxia worsen hypercapnia?

Answer 23

1. O2 therapy removes the stimulus for hypoxic respiratory drive. Alveolar ventilation drops —> causes worsening hypercapnia.
2. Correction of hypoxia removes pulmonary hypoxic vasoconstriction. This leads to increased perfusion of poorly ventilated alveoli, diverting blood away from better ventilated alveoli.

Question 24

Describe how oxygen therapy should be given to patients suffering from type 2 respiratory failure.

Answer 24

Oxygen is life saving. It must be given, but pCO2 needs to be monitored. Controlled oxygen theraoy is given with a target saturation of 88-92%.
If oxygen therapy causes rise in pCO2 - need ventilatory support

Question 25

What happens to the alveolar pO2 and pCO2 when the V/Q ration is <1?

Answer 25

Alveolar pO2 decreases and pCO2 increases

Question 26

What happens to the alveolar pO2 and pCO2 when the V/Q ratio is >1?

Answer 26

Alveolar pO2 increases and pCO2 decreases

Question 27

What happens in hypoxic vasoconstriction?

Answer 27

Blood is diverted from poorly ventilated areas to better ventilated areas.

Question 28

Give examples of disorders that cause V/Q mismatch.

Answer 28

Occurs in disorders where some alveoli are being insufficiently ventilated. For example: - asthma (variable airway narrowing) - pneumonia (exudate in affected alveoli) - RDS in newborn (some alveoli don’t expand) - pulmonary oedema (fluid in alveoli) - pulmonary embolism

Question 29

Why does pulmonary embolism cause type 1 respiratory failure?

Answer 29

1. The embolus results in a redistribution of pulmonary blood flow 2. The blood is diverted to unaffected areas of the pulmonary circulation 3. Leads to V/Q ratio <1 if hyperventilation cannot match the increased perfusion 4. This causes hypoxaemia 5. The hyperventilation is sufficient to get rid of the excess CO2

Question 30

Why do diffusion defects affect O2 more than CO2?

Answer 30

CO2 is more soluble than O2 so can diffuse more easily

Question 31

Diffusion defects (e.g. pulmonary oedema, fibrotic lung disease) will cause what type of respiratory failure?

Answer 31

Type 1 respiratory failure

Question 32

What are the causes of diffuse lung fibrosis?

Answer 32

Idiopathic fibrosis alveolitis Asbestos Extrinsic allergic alveolitis Pneumoconiosis