Ventilation–perfusion mismatch

10 min read Last updated April 8, 2026

Clinical approach to hypoxaemia: where V/Q mismatch fits

  • Major mechanisms of hypoxaemia: low inspired O2, hypoventilation, diffusion limitation, V/Q mismatch, shunt
    • V/Q mismatch is the commonest cause of hypoxaemia in anaesthesia/ICU
    • Key discriminator at the bedside: response to increased FiO2 (V/Q mismatch improves; true shunt improves little)
  • Pattern recognition
    • Low V/Q (units underventilated relative to perfusion): hypoxaemia; CO2 often normal/low initially due to increased overall ventilation
    • High V/Q (units underperfused relative to ventilation): wasted ventilation (dead space); tends to increase PaCO2 if minute ventilation cannot rise
    • Shunt (V/Q = 0): maximal low V/Q; refractory hypoxaemia
    • Dead space (V/Q → ∞): maximal high V/Q; inefficient ventilation
  • Immediate management principles
    • Increase FiO2 and recruit lung (PEEP, positioning) for low V/Q; treat cause (bronchospasm, atelectasis, pulmonary oedema, pneumonia)
    • For high V/Q/dead space: optimise perfusion (treat hypotension/low CO), treat PE, reduce excessive PEEP/overdistension, reduce apparatus dead space

Common perioperative scenarios

  • Atelectasis after induction: low V/Q and shunt-like units; improves with recruitment and PEEP
    • Mechanisms: reduced FRC, absorption atelectasis (high FiO2), compression atelectasis (obesity, pneumoperitoneum)
  • One-lung ventilation: large shunt fraction; hypoxic pulmonary vasoconstriction (HPV) is key defence
    • Optimise: correct tube position, FiO2, CPAP to non-dependent lung, PEEP to dependent lung (avoid overdistension), maintain CO and avoid excessive volatile dose
  • Pulmonary embolism: high V/Q and increased physiological dead space; may cause hypoxaemia via V/Q scatter and low mixed venous O2
    • Clues: increased PaCO2–ETCO2 gradient, rising ETCO2 variability, hypotension, RV strain

Core definitions and normal values

  • V/Q ratio = alveolar ventilation (V̇A) / pulmonary perfusion (Q̇)
  • Whole-lung average V/Q ≈ 0.8 (V̇A ~4 L/min, Q̇ ~5 L/min)
  • Extremes: shunt V/Q = 0; dead space V/Q → ∞
  • Top vs bottom of lung (upright): both ventilation and perfusion increase downwards, but perfusion increases more → V/Q is higher at apex and lower at base

Why V/Q mismatch causes hypoxaemia (key physiology)

  • V/Q mismatch creates a wide distribution of alveolar PO2 (PAO2) and PCO2 (PACO2) across lung units
  • Low V/Q units dominate arterial oxygenation because blood leaving them has low PO2 and cannot be fully compensated by high V/Q units
    • Reason: Hb–O2 dissociation curve is flat at high PO2; extra PAO2 in high V/Q units adds little O2 content
    • CO2 is less affected: CO2 dissociation curve is more linear and CO2 diffuses readily; increased ventilation of other units can often maintain PaCO2
  • V/Q mismatch increases A–a gradient (PAO2 − PaO2); hypoventilation alone does not (on room air, assuming normal lungs)

Alveolar gas equation and A–a gradient (exam-useful)

  • Alveolar gas equation: PAO2 = FiO2 × (Patm − PH2O) − (PaCO2 / R)
    • At sea level: (Patm − PH2O) ≈ 760 − 47 = 713 mmHg (≈ 95 kPa)
    • Respiratory quotient R ≈ 0.8 (varies with diet/metabolism)
  • A–a gradient rises with: V/Q mismatch, shunt, diffusion limitation; also increases with age
    • Rule of thumb (room air): expected A–a ≈ (Age/4) + 4 mmHg (varies by source; use as estimate)

Low V/Q vs shunt vs diffusion limitation: oxygen response

  • Low V/Q: improves substantially with increased FiO2 (because some ventilation reaches those units)
  • True shunt: limited improvement with increased FiO2; best treated by recruitment/PEEP and addressing cause
  • Diffusion limitation: worsens with exercise and low alveolar PO2; improves with increased FiO2 (increases diffusion gradient)

High V/Q and dead space (clinical relevance)

  • High V/Q = relatively overventilated units (e.g., reduced perfusion); contributes to wasted ventilation and increased VD/VT
  • Physiological dead space = anatomical + alveolar dead space; alveolar dead space rises in PE, low CO states, overdistension (high PEEP), emphysema
  • Bohr equation: VD/VT = (PaCO2 − PECO2) / PaCO2 (Enghoff modification uses PaCO2 to reflect V/Q mismatch and shunt effects)
  • PaCO2–ETCO2 gradient increases with increased dead space and V/Q mismatch (also affected by low CO and rapid shallow breathing)

Mechanisms that worsen V/Q mismatch perioperatively

  • Reduced FRC (supine, anaesthesia, obesity, pregnancy) → airway closure in dependent lung → low V/Q and shunt
  • High FiO2 → absorption atelectasis (nitrogen washout) → low V/Q/shunt
  • Positive pressure ventilation and high PEEP can overdistend some units → high V/Q (dead space) while other units collapse → low V/Q (V/Q scatter)
  • Pulmonary vasoconstriction/vasodilation changes distribution of Q̇: HPV reduces perfusion to hypoxic units (protective); vasodilators and some anaesthetics can blunt HPV
    • HPV is strongest at PAO2 ~ 60–70 mmHg; very low PAO2 can reduce HPV (vascular collapse/other factors)

Classic causes (high-yield lists)

  • Low V/Q causes: asthma/COPD exacerbation, mucus plugging, pneumonia, pulmonary oedema, atelectasis, endobronchial intubation, ARDS (heterogeneous)
  • High V/Q / dead space causes: pulmonary embolism, low cardiac output/shock, emphysema (capillary loss), excessive PEEP/overdistension, pulmonary vascular disease
Define ventilation–perfusion (V/Q) mismatch and give normal values. How does V/Q vary from apex to base in an upright lung?

Examiners usually want definitions, extremes, and gravitational distribution.

  • V/Q ratio = alveolar ventilation (V̇A) divided by pulmonary perfusion (Q̇)
  • Whole-lung average V/Q ≈ 0.8 (V̇A ~4 L/min; Q̇ ~5 L/min)
  • Extremes: shunt V/Q = 0 (perfusion without ventilation); dead space V/Q → ∞ (ventilation without perfusion)
  • Upright lung: both V̇A and Q̇ increase towards bases, but Q̇ increases more → V/Q higher at apex and lower at base
Explain why V/Q mismatch causes hypoxaemia but often does not cause hypercapnia initially.

This is a common viva: link V/Q scatter to O2 content, Hb curve, and CO2 handling.

  • Low V/Q units produce blood with low PO2; when mixed with other blood, arterial PO2 falls
  • High V/Q units cannot compensate much for O2 because Hb is already near-saturated; extra PO2 adds little to O2 content (flat top of Hb–O2 curve)
  • CO2 is less affected because CO2 dissociation curve is more linear and CO2 diffuses readily; increased ventilation in better units can excrete extra CO2
  • Hypercapnia occurs when overall alveolar ventilation cannot rise enough (fatigue, severe airflow obstruction, CNS depression, high dead space, low minute ventilation)
A patient is hypoxaemic. How do you use the response to oxygen therapy to distinguish V/Q mismatch from shunt?

They want the concept of 'refractory hypoxaemia' and why.

  • V/Q mismatch (including low V/Q): PaO2 improves significantly with increased FiO2 because some ventilation reaches perfused alveoli
  • True shunt: limited PaO2 improvement with increased FiO2 because shunted blood bypasses ventilated alveoli
  • Recruitment (PEEP, positioning, treating collapse) reduces shunt fraction and improves oxygenation more effectively than simply increasing FiO2
Describe the alveolar gas equation and how V/Q mismatch affects the A–a gradient.

Often asked as a calculation framework plus interpretation.

  • PAO2 = FiO2 × (Patm − PH2O) − (PaCO2 / R)
  • A–a gradient = PAO2 − PaO2; increases with V/Q mismatch, shunt, diffusion limitation; normal/near-normal with pure hypoventilation (on room air, normal lungs)
  • V/Q mismatch lowers PaO2 disproportionately compared with PAO2 because of mixing of blood from units with different V/Q
What is hypoxic pulmonary vasoconstriction (HPV)? How does it influence V/Q matching and what factors inhibit it?

Common FRCA viva: definition, purpose, and perioperative modifiers.

  • HPV is constriction of pulmonary arterioles in response to low alveolar PO2, diverting blood away from poorly ventilated alveoli
  • It improves V/Q matching and reduces shunt fraction (e.g., during one-lung ventilation)
  • Inhibitors/attenuators: high pulmonary artery pressures, alkalosis, hypothermia, vasodilators (e.g., nitrates), and volatile anaesthetics (dose-dependent; clinical effect often modest at ≤1 MAC)
  • HPV can be worsened by very low mixed venous PO2 (less O2 reserve) and by high FiO2 causing absorption atelectasis (increasing shunt-like units)
Explain the difference between anatomical dead space, alveolar dead space, and physiological dead space. Give perioperative causes of increased dead space.

Definitions plus examples; mention PE and overdistension.

  • Anatomical dead space: conducting airways that do not participate in gas exchange
  • Alveolar dead space: ventilated alveoli with little/no perfusion (high V/Q)
  • Physiological dead space = anatomical + alveolar dead space
  • Perioperative causes: pulmonary embolism, low cardiac output, excessive PEEP/overdistension, emphysema/capillary loss, hypotension, high intrathoracic pressures reducing pulmonary blood flow
A ventilated patient has a rising PaCO2–ETCO2 gradient. Explain the physiology and list causes.

Frequently examined: link to dead space and V/Q mismatch.

  • ETCO2 reflects CO2 from well-perfused alveoli; PaCO2 reflects mixed arterial CO2. Increased dead space means more exhaled gas comes from poorly perfused alveoli with low CO2 → ETCO2 falls relative to PaCO2
  • Causes: pulmonary embolism, low cardiac output, high PEEP/overdistension, severe COPD with V/Q heterogeneity, hypotension, cardiac arrest/low pulmonary blood flow
  • Also consider technical: sampling line leak/obstruction, high fresh gas flows diluting sample (less common with mainstream capnography)
Why does atelectasis after induction cause hypoxaemia? Include the roles of FRC, closing capacity, and FiO2.

High-yield perioperative physiology.

  • Anaesthesia and supine position reduce FRC; if FRC falls below closing capacity, dependent airways close during tidal breathing → low V/Q and shunt
  • High FiO2 promotes absorption atelectasis: oxygen is absorbed faster than it is replaced, especially behind closed airways → alveolar collapse
  • Management: recruitment manoeuvres, appropriate PEEP, avoid unnecessarily high FiO2 once stable, optimise positioning
Compare V/Q mismatch and diffusion limitation as causes of hypoxaemia. How can you distinguish them clinically/physiologically?

They want the concept of exercise effect and oxygen response.

  • V/Q mismatch: due to uneven matching of ventilation and perfusion; common; increases A–a gradient; improves with FiO2
  • Diffusion limitation: impaired transfer across alveolar-capillary membrane (fibrosis) or reduced transit time (exercise); increases A–a gradient; worsens with exercise; improves with FiO2 by increasing diffusion gradient
  • In practice, many diseases have both; V/Q mismatch usually predominates except in severe interstitial lung disease or extreme exercise
In one-lung ventilation, what are the determinants of oxygenation and how do you manage hypoxaemia?

A classic FRCA viva scenario; structure your answer: check, optimise, rescue.

  • Determinants: shunt fraction through non-ventilated lung, HPV effectiveness, distribution of perfusion (dependent lung), lung volumes/atelectasis in dependent lung, cardiac output and mixed venous O2 content
  • Immediate steps: confirm DLT/bronchial blocker position (fibreoptic), increase FiO2, optimise ventilation (tidal volume, rate), suction, treat bronchospasm
  • Improve V/Q: apply CPAP to non-dependent lung; apply PEEP to dependent lung (avoid overdistension that increases dead space and diverts blood)
  • Support HPV: avoid excessive volatile concentration, maintain normocapnia (or mild hypercapnia depending on context), avoid alkalosis, maintain temperature and haemodynamics
A patient with COPD has worse oxygenation when given high-flow oxygen. Explain using V/Q mismatch concepts.

Focus on V/Q effects rather than only 'loss of hypoxic drive'.

  • High FiO2 can worsen V/Q mismatch by inhibiting HPV, increasing perfusion to poorly ventilated units (low V/Q) → increased venous admixture and lower PaO2 than expected
  • Also causes: absorption atelectasis (nitrogen washout) and Haldane effect (oxygenation reduces Hb CO2 carriage → PaCO2 rises)
  • Net effect can be rising PaCO2 and sometimes worsening oxygenation; titrate oxygen to target saturations and treat airflow obstruction
Describe how positive pressure ventilation and PEEP can both improve and worsen gas exchange in terms of V/Q.

They want recruitment vs overdistension and the idea of V/Q scatter.

  • Improves: recruitment of collapsed dependent alveoli → increases V̇A in low V/Q units and reduces shunt
  • Worsens: overdistension compresses capillaries → reduces Q̇ to some units (high V/Q, dead space) and can reduce cardiac output → lower mixed venous O2 and worse oxygenation
  • Optimal PEEP balances recruitment against overdistension; effects are disease- and patient-specific

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