Ventilation/Perfusion Relationships Charles L. Webber, Jr., Ph.D.

Learning Objectives: You should be able to:

• Describe the importance of ventilation/perfusion matching at the alveolar level in maintaining proper levels of systemic arterial blood gases.
• Explain how ventilation/perfusion matching is physiologically achieved in gravity fields operating on fluids (air/blood) of such different mass.
• Generate an alveolar PO2-PCO2 diagram that identifies the three alveolar types, showing the continuum of ventilation/perfusion ratios.
• Identify in vivo pulmonary reflexes that help to correct for vascular shunts or airway obstructions causing ventilation/perfusion disturbances.

Ventilation/Perfusion Match    Alveolar PO2-PCO2 Diagram    Compensations for V/Q Mismatch    MainMenu

Concept of Ventilation/Perfusion Matching

1. Introductory Statements
   • ventilation/perfusion inequality is the most common clinical cause of arterial hypoxemia
   • arterial hypoxia ( PaO2) leads directly to arterial hypoxemia ( CaO2)

    

2. Important Concepts
   • ideally, ventilation and perfusion must be exactly matched
     • ventilation must be distributed to perfused areas
     • perfusion must be distributed to ventilated areas
   • the ratio of ventilation to perfusion (V A/Q ) is the critical factor governing gas exchange
     • regions of high ventilation should have high blood flows (base of lung)
     • regions of low ventilation should have low blood flows (apex of lung)
   • one lung is represented by many regional V A/Q ratios, not a single V A/Q value
     • for V A= 6 L/min and Q = 6 L/min, it is inappropriate to calculate: V A/Q = 1 Lair/Lblood
     • by coincidence, regional V A/Q ÷ 1.0 represents an ideal match between V A and Q
     • inadequate gas exchange occurs when regional V A/Q << 1 or V A/Q >> 1

Ventilation/Perfusion Match    Alveolar PO2-PCO2 Diagram    Compensations for V/Q Mismatch    MainMenu

Derivation of the Alveolar P02-PCO2 Diagram

1. Regional Variations in Ventilation, Perfusion ad Vent/Perf Ratio (Fig. 13)
   • base of lung: high V A, higher Q , low V A/Q < 1 (wasted perfusion)
   • apex of lung: low V A, lower Q , high V A/Q > 1 (wasted ventilation)
   • middle of lung: moderate V A, moderate Q , ideal V A/Q = 1

      

    

2. Regional Variations in Alveolar Gas Tensions

   

   • base of lung: low PAO2 & high PACO2 due to low V A/Q < 1
   • apex of lung: high PAO2 & low PACO2 due to high V A/Q > 1
   • middle of lung: average PAO2 & average PACO2 due to ideal V A/Q = 1

      

    

3. Alveolar PAO2-PACO2 Diagram

   

   • shunt alveolar unit: V A/Q = 0, PAO2 = 40 mm Hg, PACO2 = 46 mm Hg
     • Q >> V A (wasted perfusion)
   • dead space alveolar unit: V A/Q = infinity, PAO2 = 150 mm Hg, PACO2 = 0 mm Hg
     • Q << V A (wasted ventilation)
   • ideal alveolar unit: V A/Q = 1, PAO2 = 100 mm Hg, PACO2 = 40 mm Hg
     • Q = V A (idealized matching)

      

    

4. Continuum of Ventilation/Perfusion Ratios < Ratios

   

   • the alveolar PO2-PCO2 diagram represents a continuum of V A/Q ratios
   • in the normal lung, regional V A/Q ratios range from ÷ 0.5 to ÷ 5
     • basal alveoli with lower ratios tend toward shunt
       (V A/Q ÷ 0.5)
     • apical alveoli with higher ratios tend toward dead space
       (V A/Q ÷ 5)
     • basal alveoli with intermediate ratios are more idealized
       (V A/Q ÷ 1)
   • alveolar dead space (÷10 mL) is attributed to all alveoli with
     • V A/Q ratios > 1
   • functional pulmonary vascular shunts are attributed to all alveoli with
     • V A/Q ratios < 1

Ventilation/Perfusion Match    Alveolar PO2-PCO2 Diagram    Compensations for V/Q Mismatch    MainMenu

Compensations for Ventilation/Perfusion Mismatching

1. Hypocapnic Bronchoconstriction
   • regional abnormality:  V A/Q (wasted ventilation)
   • regional effect:  PACO2 (alveolar hypocapnia)
   • reflex response:  Rairway (bronchoconstriction)
   • regional result:  V A and  V A/Q (compensation)

    

2. Hypoxic Vasoconstriction
   • regional abnormality:  V A/Q (wasted perfusion)
   • regional effect:  PAO2 (alveolar hypoxia)
   • reflex response:  Rpul vasc (vasoconstriction)
   • regional result:  Q and  V A/Q (compensation)

Ventilation/Perfusion Match    Alveolar PO2-PCO2 Diagram    Compensations for V/Q Mismatch    MainMenu