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Ventilation/Perfusion Relationships in the Lung
Experimental Biology Refresher Course Ventilation/Perfusion Relationships in the Lung 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 PAO2 CvO2 PaCO2 FIO2 gas Partial Pressure Alveolar venous
Content PaCO2 arterial FIO2 Fraction inhaled
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VA VA PAO2 Q Q VO2 PAO2 = PIO2 - • PB VA
where PB is barometric pressure
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VA • • VCO2 • Q PACO2 VA • PACO2 VCO2 • VA α
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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 < 40 Decreasing V/Q produces lower PAO2 and higher PACO2. Hypoventilation defined as PACO2 > 40
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Alveolar Gas Equation PaCO2 R PAO2 = FIO2 •(PB-47)- R = VCO2 VO2 PACO2
VA α VO2 PAO2 = PIO2 - • PB • VA PaCO2 VA • = PAO2 = FIO2 •(PB-47)- PaCO2 R 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 -> 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
Q • VA PaO2 • VCO2 PaCO2 VA •
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Hypoventilation →hypoxemia
VA • 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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Shunt Dead space
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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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PAO2 = 0.21 X (PB- PH2O)- PACO2/R
~ 100 mmHg PaO2 = 100 mmHg CaO2 = solubility x PaO2 0.3 ml/dL 0.003 ml O2/dL plasma/mmHg CaO2 = 1.39 x Hgb x %sat 1.39 ml O2/g Hgb 20 ml/dL
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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 = 98
CO2 = 20.7 ml/dL O2 = 207 ml Vol. = 1 liter Hgb = 15 gm/dL % sat = 70 CO2 = 14.8 ml/dL O2 = 148 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 = 98
CO2 = 20.7 ml/dL O2 = 207 ml Vol. = 1 liter Hgb = 15 gm/dL % sat = 70 CO2 = 14.8 ml/dL O2 = 148 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 PcO2=PvO2 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 CcO2 - CaO2 Qs/Qt = CcO2 - CvO2
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 = 673 PAO2 ~ 500 mmHg 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 > 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
PACO2 A-aO2 difference FIO2 = 1.0 Hypoventilation ↑ nl Low FIO2 ↓ Shunt ↓ response V/Q mismatch Diffusion limitation
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Hypoxemia ? (A-a)O2 difference Increased Normal FIO2 = 1.0 Hypoventilation or Altitude responsive response Low V/Q Shunt
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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 < 90 mmHg)
(A-a)O2 difference Increased Normal FIO2 = 1.0 Hypoventilation or Altitude responsive response Low V/Q Shunt
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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 < 90 mmHg)
(A-a)O2 difference Increased Normal FIO2 = 1.0 Hypoventilation or Altitude responsive response Low V/Q Shunt
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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.
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