Ventilation-Perfusion (V/Q) Inequality

Definition

• Normal overall V/Q ratio = ~0.8 (VA ~4.2 L/min; Q ~5.5 L/min)
• In a perfect lung, every alveolus would have V/Q = 0.8
• In reality, even normal lungs show regional V/Q variation due to gravity; disease dramatically amplifies this

Physiological Basis: The V/Q Spectrum

Normal V/Q Variation Due to Gravity

• Apex: High V/Q (~3.3) - well-ventilated, poorly perfused. Alveolar PO2 is high (~130 mmHg), PCO2 is low (~28 mmHg). Acts like dead space.
• Base: Low V/Q (~0.6) - relatively poorly ventilated, more heavily perfused. Alveolar PO2 is lower (~89 mmHg), PCO2 higher (~42 mmHg). Acts like a shunt.
• Net effect on arterial PO2: reduces it by only ~4 mmHg in a healthy lung

Why V/Q Inequality Causes Hypoxemia (Mechanism)

• Alveolus A (overventilated, V/Q = 1.3): PO2 = 116 mmHg, SaO2 = 98.2%
• Alveolus B (underventilated, V/Q = 0.3): PO2 = 66 mmHg, SaO2 = 91.7%
• Mixed arterial O2 saturation = 95% (hypoxaemic) - even though overall VA and Q are unchanged

1. Haemoglobin is already ~98% saturated at a PO2 of 104 mmHg (flat part of curve) - raising PO2 further adds negligible O2 content
2. Blood from the underventilated alveolus pulls the mixed saturation down significantly (steep part of curve)

Why V/Q Inequality Causes Hypercapnia (or Why It Often Does NOT)

• Overventilated units can increase CO2 elimination proportionally
• Underventilated units retain CO2, but the overventilated ones can compensate

• Hypoxaemia (cannot be compensated by increased ventilation due to flat ODC)
• Normal or low arterial PCO2 (compensated by hyperventilation)
• Only in severe/advanced disease does hypercapnia supervene

The O2-CO2 Diagram (Riley Analysis)

• Mixed venous blood (V-bar): PO2 ~40 mmHg, PCO2 ~45 mmHg
• Inspired air (I): PO2 ~150 mmHg, PCO2 ~0

• Point i (ideal): where arterial blood and alveolar gas would sit with perfect matching
• Point a (arterial blood): shifts LEFT toward mixed venous (lower PO2, slightly higher PCO2)
• Point A (alveolar gas): shifts RIGHT toward inspired air (higher PO2, lower PCO2)
• Result: Alveolar-arterial PO2 difference (A-a gradient) increases - the hallmark of V/Q inequality

Assessment of V/Q Inequality

1. Arterial PO2

• The lower the PaO2, the greater the mismatch (in general)
• Limitation: sensitive to overall ventilation, cardiac output, and inspired O2

2. Alveolar-Arterial PO2 Difference (A-a Gradient)

• Normal A-a gradient = ~5-15 mmHg (increases with age)
• Formula: A-a gradient = PAO2 - PaO2
  • PAO2 = PIO2 - (PaCO2/R) [Alveolar gas equation]
• Increased A-a gradient = V/Q mismatch, shunt, or diffusion limitation
• A-a gradient is more informative than PaO2 alone because it is less sensitive to hypoventilation
• Key point: V/Q inequality increases the A-a gradient; pure hypoventilation does NOT significantly increase the A-a gradient

3. Physiologic Dead Space (VD/VT - Bohr equation)

• VD/VT = (PaCO2 - PECO2) / PaCO2
• High V/Q units (poorly perfused) are functional dead space
• Normal VD/VT ~0.30; elevated in obstructive lung disease

4. Physiologic Shunt (Venous Admixture)

• Qs/Qt = (CcO2 - CaO2) / (CcO2 - CvO2)
• Represents effect of low V/Q units (and true shunt) combined
• Normal physiologic shunt ~2-5%

5. Multiple Inert Gas Elimination Technique (MIGET)

• Gold standard for quantifying V/Q distributions
• Infuses 6 inert gases of differing solubility; measures retention in arterial blood and excretion in expired gas
• Can identify true shunt (V/Q = 0), dead space (V/Q = ∞), and the full distribution of V/Q ratios
• Too complex for routine clinical use but invaluable in research

V/Q Distribution Patterns in Disease

Effect of Cardiac Output on V/Q Inequality

• In a normal lung (no inequality): cardiac output has no effect on PaO2 or PaCO2
• In a diseased lung with V/Q inequality:
  • Reduced cardiac output (e.g. myocardial infarction): lowers mixed venous PO2 → when this blood transits low V/Q regions, it exaggerates hypoxaemia disproportionate to the V/Q inequality itself
  • Elevated cardiac output (e.g. asthma with beta-agonist use): raises mixed venous PO2 → "hides" degree of V/Q inequality, arterial PO2 is higher than expected

Response to Supplemental Oxygen (Key Exam Point)

Summary of Key Points (Murray & Nadel's Key Points)

1. V/Q inequality is the most common cause of hypoxaemia in lung disease
2. V/Q ratios range from 0 (shunt = mixed venous composition) to infinity (dead space = inspired air composition)
3. Hypoxaemia from V/Q inequality results from the non-linearity of the O2-Hb dissociation curve - high V/Q units cannot compensate for low V/Q units
4. V/Q inequality impairs exchange of all gases (O2, CO2, CO, anaesthetic agents)
5. Pure V/Q inequality causes both hypoxaemia and hypercapnia, but CO2 is usually normalised by compensatory hyperventilation; hypoxaemia cannot be similarly corrected
6. The A-a gradient is the standard clinical measure; MIGET is the research gold standard
7. Cardiac output modulates the severity of hypoxaemia through its effect on mixed venous PO2
8. Two causes of hypercapnia: hypoventilation and V/Q inequality
9. Five causes of hypoxaemia: decreased FiO2, hypoventilation, diffusion limitation, shunt, and V/Q inequality (the most common clinically)

How to Write V/Q Inequality in MD Exam for Maximum Marks

EXAM WRITING STRATEGY

WRITE EXACTLY THIS STRUCTURE (10 marks = ~2.5-3 pages)

Introduction / Definition (2-3 lines max)

"Ventilation-perfusion (V/Q) inequality refers to non-uniform matching of alveolar ventilation to pulmonary blood flow in different lung regions. It is the most common cause of hypoxaemia in clinical practice."

Normal V/Q Ratio (3-4 lines)

• Normal overall V/Q = 0.8 (VA 4.2 L/min / Q 5.5 L/min)
• Apex: V/Q = 3.3 (over-ventilated, under-perfused)
• Base: V/Q = 0.6 (under-ventilated, over-perfused)
• One line: "This normal inequality due to gravity reduces PaO2 by only ~4 mmHg"

V/Q Spectrum (draw a small table - examiners LOVE tables)

Mechanism of Hypoxaemia (THIS is the heart - write clearly)

• Low V/Q units → low alveolar PO2 → blood leaves under-saturated
• High V/Q units → high alveolar PO2 → but Hb already near 100% saturated → cannot compensate
• Mixed arterial blood → net hypoxaemia

Why PCO2 is Usually Normal (separate heading - instant marks)

"V/Q inequality causes initial hypercapnia, but rising PCO2 stimulates chemoreceptors → increased ventilatory drive → PCO2 normalised. However, hypoxaemia persists because increased ventilation to high V/Q (dead space) units cannot raise O2 content due to flat dissociation curve."

A-a Gradient (always mention - 1 mark guaranteed)

• A-a gradient = PAO2 - PaO2 (normal 5-15 mmHg)
• Hallmark of V/Q inequality = raised A-a gradient
• Pure hypoventilation → normal A-a gradient (distinguish!)

Diseases Causing V/Q Inequality (table again)

Response to O2 (one mark - one line)

"V/Q inequality corrects with 100% O2 (distinguishes it from true shunt, which does not respond)."

Conclusion (2 lines - always end with one)

"V/Q inequality is the dominant mechanism of hypoxaemia in virtually all chronic lung diseases. Understanding it is essential for interpreting blood gas results and planning oxygen therapy."

THE GOLDEN EXAM RULES