Ventilation–perfusion mismatch

10 min read Last updated April 9, 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

Test yourself…

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 &#039,refractory hypoxaemia&#039, 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 &#039,loss of hypoxic drive&#039,.

  • 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

0 comments