Ventilation–Perfusion (V/Q) Matching

Distribution

•    Apex of lung:

o    High V/Q (ventilation > perfusion)

o    More dead space effect

o    Higher PaO₂, lower PaCO₂

•    Base of lung:

o    Low V/Q (perfusion > ventilation)

o    More physiological shunt

o    Lower PaO₂, higher PaCO₂

Clinical relevance

•    V/Q mismatch → most common cause of hypoxaemia

•    Examples:

o    Pulmonary embolism: high V/Q (dead space)

o    Pneumonia, pulmonary oedema: low V/Q (shunt effect)

Control of Respiration

Chemoreceptors

•    Central (medulla):

o    Respond to ↑ CO₂ (via pH in CSF)

o    Main driver in healthy individuals

•    Peripheral (carotid and aortic bodies):

o    Respond to ↓ PaO₂ (<8 kPa), also pH and CO₂

Chronic hypercapnia

•    Seen in advanced COPD

•    Central chemoreceptors become desensitised

•    Hypoxic drive predominates — excess O₂ can suppress ventilation

Pulmonary Function Tests (PFTs)

Obstructive pattern

•    ↓ FEV1

•    ↓ FEV1/FVC ratio (<0.7)

•    ↑ TLC, ↑ RV (due to air trapping)

•    Examples: asthma, COPD, bronchiectasis

Restrictive pattern

•    ↓ TLC

•    Normal or ↑ FEV1/FVC ratio

•    Examples: interstitial lung disease (ILD), severe kyphoscoliosis, neuromuscular disorders, obesity

Gas Transfer (Diffusing Capacity for Carbon Monoxide, DLCO)

Reduced DLCO

•    Interstitial lung disease

•    Emphysema (alveolar destruction)

•    Pulmonary embolism (reduced perfusion)

•    Anaemia (reduced haemoglobin)

Increased DLCO

•    Asthma (increased pulmonary blood volume)

•    Polycythaemia (increased Hb)

•    Pulmonary haemorrhage

Adaptation to High Altitude

•    Initial response: hyperventilation → respiratory alkalosis

•    Renal compensation: increased bicarbonate excretion

•    ↑ 2,3-DPG → right shift of O₂ dissociation curve

•    ↑ EPO → ↑ RBC production (polycythaemia)

Arterial Blood Gases (ABGs)

Type 1 respiratory failure (hypoxaemic)

•    PaO₂ <8 kPa

•    Normal or low PaCO₂

•    Causes: pneumonia, PE, pulmonary oedema, ARDS, asthma

Type 2 respiratory failure (hypercapnic)

•    PaO₂ <8 kPa

•    PaCO₂ >6.5 kPa

•    Causes: COPD exacerbation, severe asthma with fatigue, CNS depression (e.g., opioids), neuromuscular disease (e.g., Guillain–Barré), obesity hypoventilation syndrome

Extra Revision Pearls

•    Asthma: obstruction is fully or partially reversible (post-bronchodilator improvement in FEV1 ≥12% and 200 mL)

•    FEV1/FVC normal or high + low TLC → think restrictive

•    Raised DLCO in asthma is a classic distinguishing clue from emphysema

•    V/Q mismatch responds to O₂ therapy, shunt (e.g., large consolidated pneumonia) less responsive

•    At altitude, PaO₂ drops, but PaCO₂ also drops due to hyperventilation