Respiratory Failure

Question 1

What is respiratory failure?

Answer 1

Respiratory failure occurs when gas exchange is inadequate, resulting in hypoxia. It is defined as a PaO₂ <8kPa and subdivided into two types according to PaCO₂ level.

Question 2

What is type I respiratory failure? What may cause it?

Answer 2

Defined as hypoxia (PaO₂ <8kPa) with a normal or low PaCO₂.

It is caused primarily by ventilation/perfusion (v/q) mismatch, hypoventilation, abnormal diffusion, right to left cardiac shunts.

Question 3

Give examples of causes of type I respiratory failure

Answer 3

• Pneumonia
• Pulmonary oedema
• PE
• Asthma
• Emphysema
• Pulmonary fibrosis
• ARDs

Question 4

What is type II respiratory failure? What may cause it?

Answer 4

Defined as hypoxia (PaO₂ <8kPa) with hypercapnia (PaCO₂ >6.0kPa).

This is caused by alveolar hypoventilation, with or without v/q mismatch.

Question 5

Give examples of causes of type II respiratory failure

Answer 5

• Pulmonary disease
  
  • Asthma, copd, pneumonia, end-stage pulmonary fibrosis and obstructive sleep apnoea
• Reduced respiratory drive
  
  • Sedative drugs, CNS tumour or trauma
• Neuromuscular disease
  
  • Cervical cord lesion, diaphragmatic paralysis, poliomyelitis, myasthenia gravis and Guillain–Barré syndrome
• Thoracic wall disease
  
  • Flail chest and kyphoscoliosis

Question 6

What are the signs and symptoms of hypoxia?

Answer 6

Dyspnoea; restlessness; agitation; confusion; central cyanosis. If longstanding hypoxia: polycythaemia; pulmonary hypertension; cor pulmonale.

Question 7

What are the signs and symptoms of hypercapnia?

Answer 7

Headache; peripheral vasodilation; tachycardia; bounding pulse; tremor/flap; papilloedema; confusion; drowsiness; coma.

Question 8

What investigations should be ordered for respiratory failure?

Answer 8

• Blood tests: FBC, U&E, CRP and ABG
• CXR
• Sputum and blood cultures (if febrile)
• Spirometry
  
  • COPD, neuromuscular disease and Guillain–Barré syndrome

Question 9

When should an ABG be considered?

Answer 9

• Any unexpected deterioration in an ill patient
• Anyone with an acute exacerbation of a chronic chest condition
• Anyone with impaired consciousness or impaired respiratory effort
• Signs of CO₂ retention
• Cyanosis, confusion, visual hallucinations (signs of ↓PaO₂; S_AO₂ is an alternative)
• To validate measurements from transcutaneous pulse oximetry

Question 10

What are the signs of CO₂ retention?

Answer 10

Bounding pulse, drowsy, tremor (flapping) and headache.

Question 11

What are the reference ranges for the following parameters?

1. pH
2. PaCO₂
3. PaO₂

Note: in kPa

Answer 11

1. pH: 7.35 – 7.45
2. PaCO₂: 4.7 – 6.0 kPa
3. PaO₂: 11 – 13 kPa

Question 12

Differentiate between acidotic and alkalotic blood

Answer 12

Acidotic: pH <7.35

Normal: pH 7.35 – 7.45

Alkalotic: pH >7.45

Question 13

How does pH, pCO₂ and HCO₃^- present in the following conditions?

1. Respiratory acidosis
2. Respiratory alkalosis
3. Respiratory acidosis with metabolic compensation
4. Respiratory alkalosis with metabolic compensation

Answer 13

Question 14

How does pH, pCO₂ and HCO₃^- present in the following conditions?

1. Metabolic acidosis
2. Metabolic alkalosis
3. Metabolic acidosis with respiratory compensation
4. Metabolic alkalosis with respiratory compensation

Answer 14

Question 15

What does a high base excess indicate?

Answer 15

A high base excess (> +2mmol/L) indicates that there is a higher than normal amount of HCO₃^– in the blood, which may be due to a primary metabolic alkalosis or a compensated respiratory acidosis.

Question 16

What does a low base excess indicate?

Answer 16

A low base excess (< -2mmol/L) indicates that there is a lower than normal amount of HCO₃^– in the blood, suggesting either a primary metabolic acidosis or a compensated respiratory alkalosis.

Question 17

Briefly describe both metabolic and respiratory compensation

Answer 17

Respiratory acidosis/alkalosis (changes in CO₂) can be metabolically compensated by increasing or decreasing the levels of HCO₃^– in an attempt to move the pH closer to the normal range.

Metabolic acidosis/alkalosis (changes in HCO₃^-) can be compensated by the respiratory system retaining or blowing off CO₂ in an attempt to move the pH closer to the normal range.

Question 18

How long does respiratory compensation take?

Answer 18

Respiratory compensation for a metabolic disorder can occur quickly by either increasing or decreasing alveolar ventilation to blow off more CO₂ (↑ pH) or retain more CO₂ (↓ pH).

Question 19

How long does metabolic compensation take?

Answer 19

Metabolic compensation for a respiratory disorder, however, takes at least a few days to occur as it requires the kidneys to either reduce HCO₃^– production (to decrease pH) or increase HCO₃^– production (to increase pH).