Hypoxaemia & Respiratory Failure

Question 1

What is the difference between hypoxia and hyoxaemia?

Answer 1

Hypoxia = low O₂ at the tissue level

Hypoxaemia = low PO₂ in blood

Question 2

What are the normal ranges of O₂ saturation and PaO₂ - at what point is tissue damage likely?

Answer 2

Normal:

• O₂ saturation = 94-98%
• PaO₂ = 9.3- 13.3 kPa

Tissue damage likely when:

• O₂ saturation = <90%
• PaO₂ = <8 kPa

Question 3

What type of things cause hypoxaemia?

Answer 3

• Low inspired pO₂ e.g. at altitude
• Hypoventilation
• V/Q Mismatch
• Diffusion defect (problems with alveolar capillary membrane)
• Intra- Lung Shunt
• Cyanotic heart disease (R→L lung shunt so blood not oxygenated at the lungs)

Question 4

What are some of the effects of hypoxaemia?

Answer 4

• Impaired CNS function: confusion, irritability, agitility
• Cardiac arrythmia & ischemia
• Hypoxic vasoconstriction of pulmonary vessels
• Cyanosis

Question 5

Distinguish between central and peripheral cyanosis

Answer 5

Central: seen in oral mucosa, tongue, lips - indicated hypoxaemia O₂ sat <85%

Peripheral: in fingers and toes- poor local circulation

Question 6

What compensatory mechanisms occur in chronic hypoxaemia?

Answer 6

• Increased EPO secretion from kidneys → raised Hb
• Increased 2,3 DPG shift Hb saturation curve → right so O₂ dissociates more freely
• Vasoconstriction of pulmonary vessels to keep V/Q optimal

Question 7

In chronic hypoxia, vasoconstriction of pulmonary vessles occurs. What are some of the consequences of this?

Answer 7

• Pulmonary hypertension
• Right sided heart failure
• Cor Pulmonale (right sided heart hypertrophy)

Question 8

There are 2 types of respiratory failure, Type 1 and Type 2. What is the difference between the 2?

Answer 8

Type 1 Respiratory Failure: is low pO₂ (<8kPa) with a normal/ low pCO₂

Type 2 Respiratory Failure: is low pO₂ (<8kPa) with a high pCO₂ of >6.7 kPa (50mmHg)

Question 9

How does poor ventilation of the lung lead to hypercapnia?

Answer 9

• Entire lung poorly ventilated (Hypoventilation)
• Alveolar ventilation is reduced
• Alveolar pO₂ fall → arterial pO₂ falls → hypoxaemia
• Alveolar pCO₂ rises → arterial pCO₂ rises → hypercapnia

Question 10

What are some of the effects of hypercapnia?

Answer 10

• Respiratory acidosis
• Impaired CNS function: dowsiness, confusion, coma, flapping tremors
• Peripheral vasodilation - warm hands, bounding pulse
• Cerebral vasodilation - headache

Question 11

What is acute hypoventilation and what are some of the common causes?

Answer 11

Hypoventilation that develops over hours and days and requires urgent ventilatory support

Causes:

• Opiate overdose
• Head injury
• Very severe, acute asthma

Question 12

What is chronic hypoventilation and what are some of the common causes?

Answer 12

Chronic hypoxia that develops gradually, allows some time for compensation so is slightly better tolerated

Common causes:

• Severe COPD
• Acute exacerbations from lower respiratory tract infection

Question 13

How can the curvature of the back affect ventilation?

Answer 13

Scoliosis, Kyphosis or Kyphoscoliosis impair the ability of the chest to expand

Question 14

What are the effects of chronic hypercapnia?

Answer 14

• Respiratory acidosis compensated by the kidney (retains HCO₃^-)
• Acclimation to CNS effects
• Vasodilation is mild but still present → pink puffer

Question 15

What is the effect of chronic CO₂ retentionb on central chemoreceptors?

Answer 15

1. CO₂ diffuses to CSF → pH drops → stimulates central chemoreceptors
2. CSF acidity is harmful to neurones to choroid plexus cells secrete HCO₃^-
3. pH of CSF returns to normal → central chemoreceptors now unresponsive to the high pCO₂
4. Central chemoreceptors have ‘reset’ to the higher level of CO₂
5. The persistant hypoxia has to stimulate peripheral chemoreceptors to increase ventilation not the hypercapnia

Question 16

In someone with chronic type 2 respiratory failure, why can it be dangerous to give them O₂

Answer 16

• In chronic respiratory failure, central chemoreceptors reset to the new level of hypercapnia
• Respiratory drive is only driven by hypoxia by peripheral chemoreceptors
• If giving O₂ the hypoxia is corrected and therefore there is no stimulus for respiratory drive
• Resp rate and depth reduces → alveolar ventilation drops → hypercapnia worsens
• Hypoxia correction removes pulmonary hypoxic vasoconstriction → leads to increased perfusion of poorly ventilated alveoli → diverting blood away from well ventilated alveoli

Question 17

In someone with chronic type 2 respiratory failure, why can it be dangerous to give them O₂. What should do to treat a patient like this?

Answer 17

Give controllex oxygen therapy with a target saturation of 88-92%

Question 18

In a V/Q mismatch, why is hyperventilation insufficient to correct the hypoxia?

Answer 18

Hyperventilation will increase the amount of dissolved O₂ which is only a small proportion of O₂ in blood. Most O₂ is bound to Hb which is already saturated, increasing ventilation does not increase the amount bound to Hb

Question 19

What kind of things can cause hypoxia due to diffusion impairment?

Answer 19

• Thicker diffusion barrier → lung fibrosis
• Legnthened diffusion pathway → pumonary oedema
• Total area for diffusion reduced → emphysema

Question 20

Which gas is most effected by problems with diffusion impairment and why?

Answer 20

O₂ is most affected as it diffuses less readily than CO₂

Question 21

Which kind of respiratory failure does diffusion barrier impairment cause?

Answer 21

Type 1 Respiratory Failure as CO₂ is normal but O₂ is low

Question 22

What kind of things cause diffuse lung fibrosis?

Answer 22

• Idiopathic Fibrosing Alveolitis
• Asbestosis
• Extrinsic allergic alveolitis
• Penumoconiosis

Question 23

What is a pulmonary shunt?

Answer 23

Physiological effect where deoxygenated blood bypasses poorly perfused alveoli and mixes with oxygenated pulmonary veins