Guide — Fundamentals
Ventilation-Perfusion (V/Q) Matching
Gas exchange only works when airflow and blood flow meet in the same lung units. This guide builds the V/Q ratio from the ground up — the normal value and its regional gradient, the two extremes of dead space and shunt, hypoxic pulmonary vasoconstriction, and the five mechanisms of hypoxemia.
10 min read · Fundamentals
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Overview
Efficient gas exchange requires ventilation (V) and perfusion (Q) to be matched within the same lung units. The whole-lung ratio is about 0.8 — alveolar ventilation of roughly 4 L/min divided by a cardiac output of roughly 5 L/min. Mismatch between V and Q is the single most common cause of hypoxemia.
Key Concepts
The normal ratio and its gradient. Overall V/Q is about 0.8, but the upright lung is not uniform. The West zones describe a regional gradient: the apex has a higher V/Q (relatively over-ventilated) and the base a lower V/Q (relatively over-perfused). Both ventilation and perfusion are greater at the base — but perfusion increases more, which is why the base ends up with the lower ratio.
The two extremes of mismatch. Every gas-exchange defect lives somewhere between dead space and shunt:
| Extreme | V/Q | Physiology | Examples | Effect |
|---|---|---|---|---|
| Dead space | High or infinite | Ventilation without perfusion | Pulmonary embolus (alveolar) + conducting airways (anatomic) = physiologic dead space | Wasted ventilation; raises PaCO₂ unless minute ventilation compensates |
| Shunt | Low or zero | Perfusion without ventilation | Atelectasis, consolidation, pneumonia, pulmonary edema, ARDS, intracardiac shunt | Hypoxemia that does NOT correct with supplemental oxygen |
Hypoxic pulmonary vasoconstriction (HPV). Alveoli with low oxygen constrict their local vessels, diverting blood toward better-ventilated lung and improving matching. This is unique to the pulmonary circulation — the exact opposite of the systemic response to hypoxia, where vessels dilate.
The five mechanisms of hypoxemia. Every hypoxemic patient fits one (or more) of these five buckets. The A–a gradient and the response to oxygen are what separate them:
| Mechanism | A–a gradient | Corrects on O₂? | Note |
|---|---|---|---|
| V/Q mismatch | Elevated | Corrects | Most common cause of hypoxemia |
| Shunt | Elevated | Does NOT correct | Perfusion without ventilation |
| Hypoventilation | Normal | Corrects | PaCO₂ elevated |
| Diffusion limitation | Elevated | Corrects | For example, interstitial fibrosis |
| Low inspired PO₂ / altitude | Normal | Corrects | High altitude, low FiO₂ |
Assessment & Findings
- Shunt presents as refractory hypoxemia despite a high FiO₂ — it needs recruitment and PEEP, not just more oxygen.
- Dead space presents as a rising PaCO₂ (or a rising minute ventilation needed to hold CO₂) and a widened EtCO₂-to-PaCO₂ gradient — classic with pulmonary embolism.
- The A–a gradient sorts the mechanisms.A normal A–a points to hypoventilation or a low FiO₂; an elevated A–a points to V/Q mismatch, shunt, or diffusion limitation.
RT Priorities & Interventions
- Shunt physiology. Recruit and stabilize alveoli with PEEP/CPAP and lung-protective ventilation, and treat the cause — drain the edema, treat the pneumonia.
- V/Q mismatch. Supplemental oxygen usually corrects it.
- Dead space. Support ventilation, investigate for pulmonary embolism, and watch the response to increased minute ventilation.
- Positioning.In unilateral lung disease, “good lung down” improves matching because perfusion follows gravity to the dependent, better-ventilated lung.
Common Pitfalls
- Treating shunt hypoxemia with escalating FiO₂ alone — it will not correct; it needs PEEP and recruitment.
- Forgetting that ordinary day-to-day hypoxemia is usually V/Q mismatch, not pure shunt.
- Ignoring the EtCO₂-to-PaCO₂ gap as a dead-space clue.
Board Exam Pearls
- Normal V/Q is about 0.8.
- Shunt = perfusion without ventilation, refractory to oxygen; dead space = ventilation without perfusion.
- The “does it correct on 100% oxygen?” test isolates shunt: shunt will not correct, while V/Q mismatch, diffusion limitation, and hypoventilation will.
- HPV diverts blood away from hypoxic alveoli.
- Pulmonary embolism increases alveolar dead space and widens the EtCO₂-to-PaCO₂ gradient.
FAQ
How do I tell shunt from V/Q mismatch?
Apply oxygen. V/Q mismatch hypoxemia improves substantially with supplemental oxygen; a true shunt barely responds because that blood never contacts ventilated alveoli. Shunt requires recruitment and PEEP and treatment of the underlying collapse or alveolar filling.
What is the difference between anatomic, alveolar, and physiologic dead space?
Anatomic dead space is the conducting airways (about 150 mL). Alveolar dead space is ventilated alveoli that are not perfused (for example, distal to a pulmonary embolus). Physiologic dead space is the sum of the two — total wasted ventilation.
Why does the lung base have a lower V/Q than the apex?
Gravity increases both ventilation and perfusion toward the base, but perfusion increases more, so the base is relatively over-perfused (lower V/Q) and the apex relatively over-ventilated (higher V/Q).
What is hypoxic pulmonary vasoconstriction?
A protective reflex in which alveoli with low oxygen constrict their local vessels, shifting blood toward better-ventilated lung and improving V/Q matching — the opposite of how systemic vessels respond to hypoxia.
Put it to work
Grade the oxygenation defect with the P/F ratio — the index that tracks how badly V/Q mismatch and shunt are degrading gas exchange.
Open the P/F Ratio calculator →Related Resources
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Ventilation, perfusion, and gas exchange chapters.
- West JB, Luks AM. West's Respiratory Physiology: The Essentials. 11th ed. Wolters Kluwer; 2021.