11. Respiratory Failure

Question 1

What is the definition of respiratory failure?

Answer 1

Impairment in gas exchange causing hypoxaemia with or without hypercapnia

Question 2

What is type 1 respiratory failure?

Answer 2

Low paO2<8kPa or O2 sat <90% breathing room air at sea level
pCO2 normal or low
G exchange impaired at level of alveolar-capillary membrane
Can progress to type 2

Question 3

What is type 2 respiratory failure?

Answer 3

Low paO2 and high PaCO2>6.5kPa breathing room air at sea level
Reduced ventilators effort or inability to overcome increased resistance to ventilation of entire lung

Question 4

What is hypoxaemia?

Answer 4

Low pO2 in arterial blood

Question 5

what are the effects of hypoxaemia?

Answer 5

Impaired CNS function, confusion, irritability, agitation
Tachypnaea
Tachycardia
Cardiac arrhythmias and cardiac ischaemia
Hypoxic vasoconstriction of pulmonary vessels
Cyanosis

Question 6

What are some causes of hypoxaemia?

Answer 6

Low inspired O2 - high altitude
Ventilation:perfusion mismatch
Diffusion defect - problems of the alveolar capillary membrane
Intra-lung shunt - acute respiratory distress syndrome
Hypoventilation - respiratory pump failure

Question 7

What are the compensatory mechanisms that decrease hypoxia in chronic hypoxaemia?

Answer 7

Increase EPO secreted by kidney, raised Hb
Increased 2,3 DPG - shifts Hb sat curve so oxygen released more freely
Increased capillary density

Question 8

What can chronic hypoxic vasoconstriction of pulmonary vessels result in?

Answer 8

Pulmonary hypertension
Right heart failure
Cor pulmonale

Question 9

What is the most common cause of hypoxaemia?

Answer 9

Ventilation:perfusion mismatch

Question 10

What happens when V:Q <1?

Answer 10

PaO2 is low
Initially PaCO2 rises until/unless there is compensatory hyperventilation, then PaCO2 will be either normal or low
Hyperventilation induced peripheral chemoreceptors firing secondary to hypoxaemia

Question 11

What happens when V:Q>1?

Answer 11

PaO2 rises and PaCO2 falls

Question 12

How can PE lead to V:Q<1?

Answer 12

Embolus results in redistribution of pulmonary blood flow
Blood is diverted to unaffected areas
Leads to V:Q ratio<1 if hyperventilation cannot match increased perfusion - causes hypoxaemia

Question 13

What is a typical ABG in PE?

Answer 13

Low PaO2 - may be normal if hyperventilation can compensate
Low PaCO2
High pH

Question 14

What can cause V:Q mismatch?

Answer 14

Asthma
Pneumonia
RDS in newborn 
Pulmonary oedema
Pulmonary embolisms

Question 15

What would ABG show in hypoxaemia caused by diffusion defect?

Answer 15

pO2 low
pCO2 normal or low
Type 1 respiratory failure initially

Question 16

What can cause diffusion impairment?

Answer 16

Fibrotic lung disease - thickened alveolar membrane slows gas exchange
Pulmonary oedema - fluid in interstitial space increases diffusion distance

Question 17

What are the causes of diffuse lung fibrosis?

Answer 17

Idiopathic pulmonary fibrosis
Asbestosis
Extrinsic allergic alveolitis
Pneumoconiosis

Question 18

What is a shunt in the respiratory system?

Answer 18

When alveolus have no ventilation at all but it is still perfused

Question 19

What is the most common cause of intra pulmonary shunt?

Answer 19

Acute respiratory distress syndrome

Question 20

What happens to the lungs in acute respiratory distress syndrome?

Answer 20

Increased vascular permeability, oedema, fibrin-exudation
Heavy, red lungs showing congestion and oedema, alveoli contain fluid and lined by hyaline membranes
Loss of surfactant resulting in alveolar atelectasis, lung becomes stiff and less compliant

Question 21

What does hypoventilation of the lung always cause?

Answer 21

Hypercapnia

Therefore causes type 2 respiratory failure

Question 22

What are some common causes of acute hypoventilation?

Answer 22

Opiate overdose
Head injury
Very severe acute asthma

Question 23

What are the common causes of chronic hypoventilation?

Answer 23

Severe COPD

Question 24

What can cause disorders of ventilation?

Answer 24

Problems with:
- central control
- motor neurons
- peripheral neuropathy
- neuromuscular junction
Muscle weakness
Muscle fatigue
Chest wall disorders
End stage COPD
End stage restrictive lung disease
Severe asthma exacerbation

Question 25

Give examples of central disorders that can lead to hypoventilation

Answer 25

Central sleep apnea
Narcotic overdose
Sedatives
Medullary disorders
Hypothyroidism
CNS trauma/brainstem herniation

Question 26

Give some examples of disorders of neuromuscular junction that can cause hypoventilation

Answer 26

Myasthenia gravis
Organophosphate toxicity
Botulism

Question 27

Which motor disorders can cause hypoventilation?

Answer 27

Tetanus
ALS (amyotrophic lateral sclerosis)
Spinal cord injury at C3 level

Question 28

Which diseases/conditions of muscle weakness or fatigue can cause hypoventilation?

Answer 28

COPD
Asthma
Malnutrition
Diaphragmatic dysfunction
Muscular dystrophy
Severe restrictive lung disease

Question 29

Name some chest wall disorders that can cause hypoventilation

Answer 29

Scoliosis
Kyphosis
Kyphoscoliosis

Question 30

What are the effects of acute hypercapnia?

Answer 30

Respiratory acidosis
Impaired CNS function - drowsiness, confusion, coma, flapping tremors
Peripheral vasodilation
Cerebral vasodilation - headache

Question 31

What are the effects of chronic hypercapnia?

Answer 31

Respiratory acidosis compensated by retention of HCO3- by kidney
Acclimation to CNS effects - CSF pH normalised
Vasodilation mild but may still be present