11 Diffusion / Hypoxemia and Hypercapnia Flashcards Preview

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Flashcards in 11 Diffusion / Hypoxemia and Hypercapnia Deck (22)
1

Bulk flow

  • Bulk flow
  • During expiration,...

  • Bulk flow
    • Oxygen and carbon dioxide are transported through the conducting airways by “bulk flow”
    • That is, there is actual movement of the gas along a pressure gradient
  • During expiration,...
    • This process is reversed
    • Gas flow progressively increases as it moves toward the mouth

2

Bulk flow

  • During inspiration, as gas moves from the mouth through the conducting airways, this pressure gradient (the difference between airway and alveolar pressure)...
  • This occurs for two reasons
  • This causes...
  • At the level of the alveoli,...

  • During inspiration, as gas moves from the mouth through the conducting airways, this pressure gradient (the difference between airway and alveolar pressure)...
    • Progressively falls
  • This occurs for two reasons
    • (1) As gas flows through the airways, total gas or airway pressure decreases because of losses to viscous or frictional forces
    • (2) As gas travels from the trachea to the respiratory bronchioles, the diameter of individual airways decreases, but total cross-sectional airway diameter increases dramatically
  • This causes...
    • A progressive drop in total airway resistance, which is accompanied by a decrease in airway pressure
    • As airway pressure falls, so does the rate of gas flow
  • At the level of the alveoli,...
    • Bulk flow stops
    • The movement of O2 and CO2 depend solely on the process of diffusion

3

Diffusion

  • Diffusion
  • O2 and CO2 move...
  • O2 dissolves in and diffuses through...
  • CO2 diffuses...

  • Diffusion
    • The net movement of molecules from a region in which a particular gas exerts a high partial pressure to a region in which a lower partial pressure is present
    • The process whereby O2 and CO2 are exchanged across the alverolar-capillary interface
  • O2 and CO2 move...
    • Between the alveoli and the pulmonary capillary blood along partial pressure gradients
  • O2 dissolves in and diffuses through...
    • The alveolar epithelium
    • The capillary endothelium
    • The plasma (where some remains dissolved)
    • The erythrocyte (where it combines with hemoglobin)
  • CO2 diffuses...
    • In the opposite direction

4

Fick's law

  • Fick's law
  • The surface area available for gas diffusion in an average size adult
  • The rate of gas transfer will fall with...

  • Fick's law for diffusion
    • Describe the factors that determine the rate at which O2 and CO2 move across the alveolar-capillary interface
      • The more surface area of contact you have b/n gas and blood, the more O2 and CO2 will be able to move across it
      • The greater the gradient of partial pressures b/n alveolar gas and capillary blood, the faster the molecules will move
      • The thicker the membrane, the less the molecules will move
    • Vgas = [ A x D x (P1 - P2) ] / T *don't need to memorize*
      • Vgas = the rate at which gas passes through the gas-liquid interface
      • A = surface area of the interface
      • D = diffusion coefficient of the particular gas
      • P1 – P2 = partial pressure difference of the gas across the interface (between alveolar gas and plasma)
      • T = thickness of the interface (diffusion distance)
  • The surface area available for gas diffusion in an average size adult
    • ~ 100 square meters
  • The rate of gas transfer will fall with...
    • Any process that decreases the gas-liquid interface
    • E.g. destruction of lung parenchyma or alveolar capillaries

5

Diffusion coefficient

  • Diffusion coefficient (D) 
  • DCO2 vs. DO2

  • Diffusion coefficient (D)
    • Directly proportional to the solubility of the gas in the tissues and fluids that it must traverse
    • Inversely related to the square root of the molecular weight (MW) of the gas
      • I.e. the larger the molecule, the slower it moves
    • D α solubility / √MW
  • DCO2 vs. DO2
    • Although CO2 has a higher molecular weight than O2 (1.17:1), its solubility is 24 times that of O2
    • These factors cause DCO2 to be approximately 20 times DO2

6

Partial pressure gradient

  • Gas molecules
    • May be considered to exist in 2 forms in a liquid
    • The molecules in these phases are...
  • The partial pressure gradient
  • PO2
  • PCO2
  • The thickness of the alveolar-capillary barrier 
  • Gas transfer will be decreased by...

  • Gas molecules
    • May be considered to exist in 2 forms in a liquid
      • Gas --> partial pressure
        • ​The partial pressure of a gas in a liquid is equal to the partial pressure of the gas outside the liquid
      • Dissolved in liquid --> concentration
        • ​The concentration of a gas in a liquid is determined by both its partial pressure and its solubility
    • The molecules in the gas phase and the liquid phase are in equilibrium
  • The partial pressure gradient
    • Important factor determining gas transfer
  • PO2
    • O2 moves from an alveolar PO2 of approximately 100 mmHg to a mixed venous PO2 of about 40 mmHg
  • PCO2
    • The partial pressure of CO2, however, only falls from about 46 to 40 mmHg during the movement of blood through the lungs.
  • The thickness of the alveolar-capillary barrier
    • Normally between 0.2 – 0.5 μm
  • Gas transfer will be decreased by...
    • Any process that increases this diffusion distance
    • E.g. interstitial fibrosis, interstitial edema

7

Gas partial pressure and concentration

  • The partial pressure of O2 and CO2 in the end-capillary blood is normally equal to...
  • The concentration of dissolved O2 and CO2

  • The partial pressure of O2 and CO2 in the end-capillary blood is normally equal to...
    • The partial pressure in the alveolar gas
  • The concentration of dissolved O2 and CO2
    • Dissolved O2 (ml/dl) = 0.003 x PaO2
    • Dissolved CO2 (ml/dl) = 0.067 x PaCO2
       

8

Diffusion of O2

  • Time an erythrocyte spends in alveolar capillaries
  • O2 during this time in the lungs
  • PO2
  • PO2 under resting conditions

  • Time an erythrocyte spends in alveolar capillaries
    • An average of 0.75 seconds
  • O2 during this time in the lungs
    • O2 diffuses across the alveolar-capillary interface along its partial pressure gradient
    • Most of the O2 enters RBCs and combines with hemoglobin
    • Only a small proportion remains in a gaseous form or dissolved in the plasma
  • PO2
    • Only the gaseous O2 contributes to the PO2 of the blood
    • A large amount of O2 can be transferred before alveolar and capillary PO2 become equal
    • Even so, this equilibration occurs very rapidly
  • PO2 under resting conditions
    • Alveolar and capillary PO2 normally equilibrate within 0.25 second
    • ​Equilibrium occurs even when transit time is reduced
    • That is, the maximum amount of oxygen has normally been transferred to the erythrocytes and plasma by about one-third of the total blood transit time

9

Diffusion of O2

  • During strenuous exercise
  • The time required for alveolar-capillary equilibration increases in the presence of disorders that...
  • If the diffusion impairment is severe, there may be...

  • During strenuous exercise
    • Transit time is markedly reduced by the increase in cardiac output
    • Alveolar and blood PO2 equilibrate long before the blood leaves the alveoli
  • The time required for alveolar-capillary equilibration increases in the presence of disorders that...
    • Decrease the surface area available for diffusion
    • Increase the thickness of the diffusion barrier
  • If the diffusion impairment is severe, there may be...
    • Insufficient time for equilibration to occur
    • A gradient between alveolar and end-capillary PO2
      • This gradient will be further increased during exercise

10

Diffusion of CO2

  • Equilibration of PCO2 between the capillary blood and alveolar gas takes...
  • In the lungs, CO2 molecules move...
  • Most CO2 molecules come from...
    • Only a small percentage...

  • Equilibration of PCO2 between the capillary blood and alveolar gas also takes approximately 0.25 second
    • This may seem strange given the much greater diffusion coefficient for CO2
    • However, the partial pressure gradient for CO2 is much less than that for O2
    • Incomplete equilibrium of PCO2 is not clinically relevant because of the small partial pressure gradient
  • In the lungs, CO2 molecules move along the partial pressure gradient from the blood to the alveolar gas
  • Most CO2 molecules come from the RBCs
    • Only a small percentage leave the dissolved form in the plasma

11

Clinical assessment of diffusion

  • In the pulmonary function laboratory, gas diffusion between the alveoli and the capillary blood is assessed by...
  • As capillary blood moves through the tissues, the movement of oxygen and carbon dioxide...

  • In the pulmonary function laboratory, gas diffusion between the alveoli and the capillary blood is assessed by measuring the diffusing capacity for carbon monoxide (DLCO)
  • As capillary blood moves through the tissues, the movement of oxygen and carbon dioxide are reversed

12

Partial pressure vs. concentration

  • When O2 or another gas enters the blood...
  • Partial pressure
  • Concentration of the gas in the blood

  • When O2 or another gas enters the blood...
    • Some molecules remain in gaseous form
    • Others actually dissolve and enter the liquid phase
  • Partial pressure
    • The pressure exerted by the gaseous molecules
  • Concentration of the gas in the blood
    • The number of dissolved molecules is reflected

13

Partial pressure vs. concentration

  • Gas molecules move...
  • At equilibrium,...
  • Gases that are highly soluble will have...

  • Gas molecules move...
    • Between the alveoli and the blood
    • Between the blood and the tissues
    • Along their partial pressure gradient until equilibrium occurs
  • At equilibrium,...
    • The partial pressure of a gas is the same in the alveoli, blood, and all tissues and organs
    • The concentration of a gas, however, depends both on its partial pressure and its solubility
  • Gases that are highly soluble will have...
    • A much higher blood and tissue concentration than poorly soluble gases, even if their partial pressures are the same

14

Partial pressure vs. concentration

  • The rate at which the partial pressure of a gas equilibrates between the alveoli, blood, and tissues
  • For example, O2 and CO2 molecules

  • The rate at which the partial pressure of a gas equilibrates between the alveoli, blood, and tissues
    • Inversely related to its solubility
  • For example, O2 and CO2 molecules
    • Diffuse between the alveolar gas and the pulmonary capillary blood along their partial pressure gradients
    • Both are relatively insoluble
      • Equilibration between the partial pressure in blood and alveolar gas occurs very rapidly
      • Equilibration occurs quickly between the partial pressure of O2 and CO2 in the blood and tissues

15

Partial pressure vs. concentration

  • The greater the gas solubility...
  • For example, clinical anesthesia depends on...
  • Therefore, the onset of action of an anesthetic gas...

  • The greater the gas solubility...
    • The longer it takes to reach partial pressure equilibrium
  • For example, clinical anesthesia depends on...
    • Achieving an adequate partial pressure (not concentration) of the anesthetic gas in the CNS
  • Therefore, the onset of action of an anesthetic gas...
    • Varies inversely with its solubility
    • Insoluble anesthetics act quickly, whereas more soluble drugs have a more delayed effect

16

Mechanisms of arterial hypoxemia

  • A decrease in PAO2 causes hypoxemia without an increase in PA-aO2
    • Hypoventilation
    • Reduced barometric pressure (high altitude)
    • Reduced FIO2
  • Disorders that affect the airways, parenchyma, or blood vessels cause an increase in PA-aO2
    • Ventilation-Perfusion inequality
    • Shunt
    • Diffusion impairment

17

Mechanisms of arterial hypoxemia:
Hypoventilation

  • Hypoventilation increases PACO2
    • According to the alveolar air equation, an increase in PACO2 (and PaCO2) causes PAO2 (and PaO2) to fall
  • Since PaO2 decreases only because of the fall in PAO2, there is no increase in the normal PA-aO2
  • PAO2 = (PB – PH2O) FIO2 – PACO2 / R

18

Mechanisms of arterial hypoxemia:
Reduced PB or FIO2

  • According to the alveolar air equation, a decrease in total gas (barometric) pressure (PB) or FIO2 will decrease PAO2
  • This causes PaO2 to fall, and therefore PA-aO2 does not increase

19

Mechanisms of arterial hypoxemia:
Ventilation-perfusion mismatching

  • Low V/Q lung units cause both a decrease in PaO2 and an increase in the normal PA-aO2

20

Mechanisms of arterial hypoxemia:
Shunt

  • The addition of mixed venous blood to the arterial circulation causes a fall in PaO2 and an increase in the normal PA-aO2

21

Mechanisms of arterial hypoxemia:
Impaired diffusion

  • lack of equilibrium between alveolar and end-capillary PO2
  • This may result from thickening of the diffusion barrier or from a decrease in the surface area available for gas exchange
  • If sufficiently severe, impaired diffusion leads to a decrease in PaO2 and an increase in the PA-aO2
  • These abnormalities worsen with exercise

22

Hypercapnia

  • Arterial hypercapnia always results from...
  • PaCO2 equation
  • Alveolar ventilation may be inadequate for two reasons

  • Arterial hypercapnia always results from...
    • Inadequate alveolar ventilation (hypoventilation)
    • Decreased minute ventilation
  • PaCO2 equation
    • PAO2 = (PB – PH2O) FIO2 – PACO2 / R
  • PCO2 equation
    • PCO2 = K x VCO2 / (VE - VD)
  • Alveolar ventilation may be inadequate for two reasons
    • (1) Abnormally low VE
      • May result from respiratory center depression, neuromuscular disease, severe chronic lung disease, or “restrictive” chest wall disorders (e.g. severe kyphoscoliosis and massive obesity)
    • (2) Inadequate increase in VE
      • The PaCO2 will rise if VE is inadequate to compensate for an acute increase in either physiologic dead space or CO2 production

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