Spirometry and Blood Gas Analysis

Question 1

What are the spirometry reference ranges?

Answer 1

• FEV1 of 85% predicted is considered normal

Question 2

What are the features of obstructive lung disease in spirometry? (Asthma and COPD)

Answer 2

• FEV1/FVC ratio <70%
• Reversibility testing done using salbutamol (15%/400ml reversibility in FEV1 is suggestive of asthma)

Question 3

What are the features of restrictive lung disease in spirometry?

Answer 3

• FEV1 and FVC reduced
• FEV1/FVC ratio >70%
• Causes include ILD, kyphoscoliosis/chest wall abnormality, previous pneumonectomy, neuromuscular disease, obesity and poor effort/technique

Question 4

What is transfer factor and how is it used? (TLCO, KCO, DLCO)

Answer 4

• Single breath of small concentration of CO
• CO has very high affinity to Hb
• Measure concentration in expired has to derive uptake in the lungs
• Affected by alveolar surface area, pulmonary capillary blood volume, Hb concentration and ventilation perfusion mismatch
• Reduced in emphysema, ILD, pulmonary vascular disease and anaemia (increased in polycythaemia)

Question 5

What is oximetry?

Answer 5

• Oximetry is non-invasive measurement of saturation of Hb by oxygen. The main causes of hypoxaemia include hypoventilation (e.g. drugs, neuromuscular disease), ventilation/perfusion mismatch (e.g. COPD, pneumonia), shunts (e.g. congenital heart disease) and low inspired oxygen (e.g. altitude, flight).

Question 6

Interpreting ABGs

Answer 6

• Always look at the pO₂ for respiratory failure
• Then look at the pCO₂ (type 1 vs type 2)
• Then consider the acid base balance

​- Acute respiratory acidodis – elevated pCO₂, normal bicarbonate, acidosis

• Compensated respiratory acidosis – elevated pCO₂, elevated bicarbonate (renal compensation), non-acidotic
• Acute on chronic respiratory acidosis – elevated pCO₂, elevated bicarbonate, acidosis

Question 7

What is the significance of the alveolar-air equation?

Answer 7

• If the difference between alveolar and arterial oxygen partial pressures is >2-4kPa this suggest V/Q mismatch

Question 8

Peak flow

Answer 8

• Peak of a persons expiratory flow
• Simple way of demonstrating obstruction
• Useful in asthma