Lung Function Tests

Question 1

What are the main parameters measured by spirometry?

Answer 1

FEV₁, FVC, and FEV₁/FVC ratio — define obstructive, restrictive, or mixed pattern.

Question 2

What test measures TLC, RV, and FRC?

Answer 2

Body plethysmography or helium dilution.

Question 3

What is considered a ‘normal’ spirometry result in terms of Z-score?

Answer 3

≥ LLN, meaning Z > –1.64 (above 5th percentile).

Question 4

What defines obstructive pattern on spirometry?

Answer 4

FEV₁/FVC < LLN (≈ <0.70 if <60 y; <0.65 if >70 y).

Question 5

What defines restrictive pattern on spirometry?

Answer 5

FVC < LLN with FEV₁/FVC ≥ LLN, confirmed by TLC < 80% predicted.

Question 6

What defines mixed pattern on spirometry?

Answer 6

FEV₁, FVC, and FEV₁/FVC all below LLN.

Question 7

What Z-score ranges correspond to mild, moderate, and severe disease?

Answer 7

Mild: –1.64→–2.5, Moderate: –2.5→–4, Severe: < –4.

Question 8

What are the typical normal FEV₁/FVC ratios for adults and elderly?

Answer 8

≈ 0.75–0.80 in adults, ≈ 0.65 in elderly.

Question 9

What TLC range is normal for females and males?

Answer 9

♀ 4–6 L, ♂ 6–8 L.

Question 10

When is TLC considered abnormal?

Answer 10

<80% predicted = restriction, >120% = hyperinflation.

Question 11

What is a normal RV/TLC ratio and what indicates air-trapping?

Answer 11

Normal 0.25–0.45; >0.50 indicates air-trapping.

Question 12

What proportion of TLC is FRC and when is it low?

Answer 12

FRC ≈ 40% of TLC; low in obesity or pregnancy.

Question 13

What indicates significant bronchodilator reversibility?

Answer 13

≥12% and ≥200 mL rise in FEV₁ or FVC (from baseline).

Question 14

What alternate bronchodilator criterion can be used per ERS 2022?

Answer 14

≥10% increase from predicted value.

Question 15

What DLCO values are considered normal?

Answer 15

20–30 mL/min/mm Hg (3.5–5 mmol/min/kPa).

Question 16

What conditions reduce DLCO?

Answer 16

Emphysema, ILD, pulmonary hypertension, anaemia.

Question 17

What conditions increase DLCO?

Answer 17

Asthma, polycythaemia, alveolar haemorrhage, L→R shunt.

Question 18

What is the normal range of KCO and what increases it?

Answer 18

4–6 mL/min/mm Hg/L; increased by ↓ alveolar volume (pneumonectomy/lobectomy) or ↑ blood flow.

Question 19

How does haemoglobin affect DLCO?

Answer 19

Each 10 g/L ↓ Hb ≈ 7% ↓ DLCO (≈30% ↓ if Hb 80 g/L).

Question 20

What flow–volume loop pattern indicates obstruction?

Answer 20

Scooped expiratory limb.

Question 21

What flow–volume loop pattern indicates restriction?

Answer 21

Tall, narrow loop.

Question 22

What loop change indicates variable extrathoracic obstruction?

Answer 22

Flattened inspiratory limb (e.g., vocal cord dysfunction).

Question 23

What loop change indicates variable intrathoracic obstruction?

Answer 23

Flattened expiratory limb (e.g., tracheomalacia).

Question 24

What loop change indicates fixed obstruction?

Answer 24

Flattened both inspiratory and expiratory limbs (e.g., goitre, stenosis).

Question 25

How does obesity affect lung volumes?

Answer 25

↓ ERV and FRC (<80%), TLC ~ normal → V/Q mismatch.

Question 26

How does pregnancy affect respiratory physiology?

Answer 26

↑ TV 30–50%, ↑ VE 20–50%, ↓ FRC 20%, ↓ TLC 5%, PaCO₂ ≈30 mm Hg, FEV₁/FVC unchanged.

Question 27

What is the key diagnostic triad for emphysema?

Answer 27

↓ FEV₁/FVC + ↑ TLC + ↓ DLCO.

Question 28

What is the key diagnostic triad for asthma?

Answer 28

↓ FEV₁/FVC + normal DLCO + reversibility.

Question 29

What is the key diagnostic triad for pulmonary hypertension?

Answer 29

Normal spirometry + ↓ DLCO.

Question 30

What is the alveolar gas equation?

Answer 30

PAO₂ = FiO₂ × (Patm – PH₂O) – (PaCO₂ / R).

Question 31

What is the simplified alveolar gas equation at sea level?

Answer 31

PAO₂ ≈ 150 – 1.25 × PaCO₂.

Question 32

What is the A–a gradient formula?

Answer 32

A–a = PAO₂ – PaO₂.

Question 33

What is the normal A–a gradient?

Answer 33

≈ (Age / 4 + 4) mm Hg; <10 young, <100 on 100% O₂.

Question 34

How does A–a gradient change with age?

Answer 34

Increases ≈ 0.2 mm Hg per year.

Question 35

What is the quick bedside formula for A–a gradient?

Answer 35

A–a = (150 – 1.25 × PaCO₂) – PaO₂.

Question 36

Which two causes of hypoxaemia have a normal A–a gradient?

Answer 36

Low FiO₂ and hypoventilation.

Question 37

What distinguishes V/Q mismatch from shunt?

Answer 37

Both ↑ A–a, but V/Q mismatch improves with O₂; shunt does not.

Question 38

What finding suggests diffusion limitation?

Answer 38

↑ A–a only on exercise (ILD, emphysema).

Question 39

How does obesity alter A–a gradient?

Answer 39

↓ FRC → basal collapse → ↑ A–a.

Question 40

How does pregnancy alter A–a gradient?

Answer 40

Progesterone ↑ VE → ↓ PaCO₂ (~30), ↑ PAO₂ (~105), mild ↑ A–a.

Question 41

What is the methacholine challenge test positive threshold?

Answer 41

≥20% fall in FEV₁ (PC20 ≤8 mg/mL = asthma).

Question 42

What is a positive mannitol challenge?

Answer 42

≥15% fall in FEV₁.

Question 43

What is a positive exercise/eucapnic hyperventilation test?

Answer 43

≥10–15% fall in FEV₁ = exercise-induced bronchoconstriction.

Question 44

What are normal ABG values?

Answer 44

pH 7.35–7.45, PaCO₂ 35–45 mm Hg, HCO₃⁻ 22–26 mmol/L, PaO₂ 80–100 mm Hg.

Question 45

What compensation occurs in chronic respiratory acidosis?

Answer 45

↑ HCO₃⁻ by 4 mmol for each 10 mm Hg PaCO₂ rise.

Question 46

What compensation occurs in acute respiratory alkalosis?

Answer 46

↓ HCO₃⁻ by 2 mmol for each 10 mm Hg PaCO₂ fall.

Question 47

What formula estimates expected PaCO₂ in metabolic acidosis?

Answer 47

Winter’s formula = 1.5 × HCO₃⁻ + 8 ± 2.

Question 48

What respiratory compensation occurs in metabolic alkalosis?

Answer 48

PaCO₂ increases 0.7 mm Hg per 1 mmol HCO₃⁻ rise.

Question 49

What is the key memory cue for LLN?

Answer 49

Z = –1.64 = LLN (5th percentile).

Question 50

What is the key memory cue for bronchodilator response?

Answer 50

12 + 200 = reversibility threshold.

Question 51

What is the key memory cue for alveolar gas equation?

Answer 51

150 – 1.25 × PaCO₂ = PAO₂ (room air).

Question 52

What A–a gradient indicates abnormal gas exchange?

Answer 52

A–a > (Age/4 + 4) mm Hg.

Question 53

When does KCO increase physiologically?

Answer 53

When alveolar volume is reduced (pneumonectomy, lobectomy) or pulmonary blood flow increases.