VentilationPerfusion Relationships in the Lung

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Ventilation/Perfusion Relationships in the Lung
Experimental Biology Refresher Course

• Robb Glenny, M.D.
• University of Washington

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Overview of Approach

• Review nomenclature and abbreviations
• Begin with lung as single unit
• Relationships between PO2, PCO2, alveolar
  ventilation, and blood flow
• Alveolar gas equation
• No Alveolar-arterial differences in PO2
• Introduce additional lung units with mismatch
  between V and Q
• Alveolar-arterial differences in PO2

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Overview of Approach

• Discuss passive and active mechanisms matching
  regional V and Q
• Provide clinical examples

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Schematic lung unit

VA
(ml/min)
Mixed venous
PA
Pa Ca

Q
(ml/min)
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Nomenclature
gas
PAO2
Partial Pressure
Alveolar
CvO2
venous
Content
PaCO2
arterial
FIO2
Fraction
inhaled
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PAO2

Q

Q
where PB is barometric pressure
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Q
PACO2
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Alveolar PO2 and PCO2

• Determined by the ratio between ventilation and
  blood flow V/Q.
• PO2 and PCO2 are inversely related through
  alveolar ventilation.
• Increasing V/Q produces higher PAO2 and lower
  PACO2.
• Hyperventilation defined as PACO2 lt 40
• Decreasing V/Q produces lower PAO2 and higher
  PACO2.
• Hypoventilation defined as PACO2 gt 40

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Alveolar Gas Equation

VO2
PAO2 PIO2 -
PB

VA

PaCO2
where R is respiratory quotient
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Hypoxemia

• Less than normal partial pressure of Oxygen in
  arterial blood
• Normal PaO2 90 mmHg
• Age dependent
• 70 mmHg at 70 years of age
• Five causes of hypoxemia
• 2 are due to decreased alveolar PO2

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Low PIO2 -gt hypoxemia

• Altitude
• FIO2 0.21 everywhere
• Barometric pressure (PB)
• Seattle 760 mmHg
• Denver 630 mmHg
• Mt. Rainier 430 mmHg
• Mt. Everest 225 mmHg
• PAO2 30 mmHg on Rainier without compensatory
  mechanisms

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Compensation for Altitude
PaO2

PaCO2
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Hypoventilation ?hypoxemia
PAO2
PaO2

Q
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Alveolar-arterial O2 difference

• One measure of efficiency of gas exchange across
  alveolar-capillary membrane.
• Helpful in determining cause of hypoxemia and
  making clinical decisions.
• In conditions where hypoxemia is due to low
  alveolar PO2, the A-a O2 difference is normal.

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where CC is content in end capillary blood
PvO2
PAO2
PAO2
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Ventilation-Perfusion Mismatch

• Differences in airway geometry and lung expansion
  produce uneven regional ventilation.
• Differences in vascular geometry and hydrostatic
  pressures produce uneven regional blood flow.
• V/Q ratios vary across regions within normal
  healthy lung.
• Pathologies that lead to regional changes in
  ventilation (e.g. asthma) or changes in perfusion
  (e.g. pulmonary embolism) will increase V/Q
  mismatch.

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Dead space
Shunt
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Review/Introduce

• Partial pressures of O2
• Content of O2
• Hgb-oxygen dissociation curve
• Determinants of Alveolar PO2 and PCO2.

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Hemoglobin-Oxygen Dissociation Curve
Plasma without Hgb
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PO2 100
PO2 40


Vol. 1 liter Hgb 15 gm/dL sat 70 CO2
14.8 ml/dL O2 148 ml
Vol. 1 liter Hgb 15 gm/dL sat 98 CO2
20.7 ml/dL O2 207 ml
Vol. 2 liters Hgb 15 gm/dL O2 355 ml CO2
17.7 ml/dL sat ? PO2 ?
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PO2 100
PO2 40


Vol. 1 liter Hgb 15 gm/dL sat 70 CO2
14.8 ml/dL O2 148 ml
Vol. 1 liter Hgb 15 gm/dL sat 98 CO2
20.7 ml/dL O2 207 ml
Vol. 2 liters Hgb 15 gm/dL O2 355 ml CO2
17.7 ml/dL PO2 52 sat 82
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PvO2
CvO2
PcO2PvO2
CcO2 CvO2
PcO2
CcO2
PaO2
CaO2
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Shunt
CaO2 is a weighted average of the venous and
capillary O2 contents
CaO2 Qs/Qt CvO2 (1-Qs/Qt) CcO2
Shunt
Non-shunt
Where QS is blood flow through shunt regions of
lung and QT is total blood flow through lungs
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Formal Shunt Equation
Measure CvO2 and CaO2 Estimate CcO2 from alveolar
gas equation
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Normal Shunt Fraction

• Normal shunt 5.
• venous return from bronchial circulation
• Thebesian veins in haert
• Normal A-aO2 difference 10 mmHg (age dependent).

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Increased FIO2 in Shunt

• Arterial PO2 increases little with increased FIO2.

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PIO2 150 mmHg
PvO2 40 mmHg
CvO2 15ml/dl
PAO2 100
PvO2 40 mmHg
CvO2 15ml/dl
PcO2 100
CcO2 20 ml/dl
PaO2 54 mmHg
CaO2 17.5 ml/dl
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PIO2 713 mmHg
PvO2 40 mmHg
CvO2 15ml/dl
PAO2 673
PvO2 40 mmHg
CvO2 15ml/dl
PcO2 673
CcO2 22 ml/dl
PaO2 70 mmHg
CaO2 18.5 ml/dl
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Confirming Quantifying Shunts

• Bubble study
• immediate intracardiac
• 4 beats intrapulmonary
• Perfusion scan
• more sensitive
• quantitative
• Clinical significance
• CNS emboli

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Shunt recap

• Blood that does not exchange gas.
• Normal shunt fraction 5
• Shunt contributes to hypoxemia.
• Pathologic conditions include pneumonia,
  atelectasis, and intracardiac shunts
• Little change in PaO2 with increasing FIO2.

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Increased FIO2 in V/Q mismatch

• V/Q mismatch can be differentiated from shunt by
  the response to increasing FIO2.

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PIO2 713 mmHg
PvO2 40 mmHg
CvO2 15ml/dl
PAO2 500 mmHg
PAO2 673
CvO2 21ml/dl
PcO2 673
CcO2 22 ml/dl
PaO2 600 mmHg
CaO2 21.5 ml/dl
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Ventilation-Perfusion Mismatch

• Regional V/Q ratios vary throughout lung
• Pathologic conditions include asthma, emphysema,
  and atelectasis
• Low V/Q regions contribute to hypoxemia.
• Hypoxemia responsive to increasing FIO2.
• Regions with V/Q gt 1.0 do not contribute to
  hypoxemia.

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Effect of Dead space on PaO2
PIO2 150 mmHg
PvO2 40 mmHg
PAO2 150
PAO2 100
PcO2 100
PaO2 100 mmHg
CcO2 20 ml/dl
CaO2 20 ml/dl
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Mechanisms matching regional V and Q

• Passive
• shared effect of gravity
• both V and Q increase down the lung
• shared geometry of airways and vasculature
• airways and pulmonary arteries branch together
• Active
• hypoxic pulmonary vasoconstriction
• crucial role in diseased lungs
• ? role in healthy lung

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Hypoxic Pulmonary Vasoconstriction
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Five Causes of Hypoxemia
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Hypoxemia
? (A-a)O2 difference
Normal
Increased
FIO2 1.0
Hypoventilation or Altitude
responsive
?response
Shunt
Low V/Q
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Clinical correlates

• 24 year old man found down in street minimally
  responsive, transported to ER.
• RR 12 b/min, HR 96 b/min, BP 90/60 mmHg, Hgb-O2
  sat 86
• Arterial blood gas PaO2 55 mmHg, PaCO2 70
  mmHg.

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Hypoxemia (PO2 lt 90 mmHg)
(A-a)O2 difference
Normal
Increased
FIO2 1.0
Hypoventilation or Altitude
responsive
? response
Shunt
Low V/Q
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Clinical correlates

• 24 year old woman very short of breath.
• RR 32 b/min, HR 126 b/min, BP 156/90 mmHg,
  Hgb-O2 sat 86
• Arterial blood gas PaO2 55 mmHg, PaCO2 32
  mmHg.
• Put on FIO2 of 1.0, PaO2? 600 mmHg.

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Hypoxemia (PO2 lt 90 mmHg)
(A-a)O2 difference
Normal
Increased
FIO2 1.0
Hypoventilation or Altitude
responsive
? response
Shunt
Low V/Q
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Summary of Approach

• Begin with lung as single unit where
• PAO2 and PACO2 are determined by the ratio
  between ventilation and blood flow.
• PAO2 and PACO2 are inversely related through
  alveolar ventilation.
• V/Q is uniform throughout and there is no
  difference in PO2 between alveolus and blood
  leaving alveolus
• V/Q mismatch leads to A-a O2 difference
• Five causes of hypoxemia
• A-a O2 difference and response to increased FiO2
  helps assess causes.