10 Ventilation-Perfusion Relationships Flashcards Preview

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Flashcards in 10 Ventilation-Perfusion Relationships Deck (19)
1

Dye and water simulate the movement of oxygen and blood through the lungs

  • Oxygen and carbon dioxide are exchanged by...
  • Diffusion
    • Occurs...
    • Depends in part on...
  • Ventilation
  • At the same time, the pulmonary circulation (perfusion)...
  • Although we can calculate the mean alveolar PO2 and PCO2, the partial pressure of O2 and CO2 in any given alveolus as well as the PO2 and PCO2 of the capillary blood leaving it depend on...

  • Oxygen and carbon dioxide are exchanged by...
    • Diffusion across the alveolar-capillary interface
  • Diffusion
    • Occurs passively
    • Depends in part on the partial pressure gradients of O2 and CO2 between alveolar gas and pulmonary capillary blood
  • Ventilation
    • Brings O2 to the alveoli 
    • Removes CO2 that has diffused from the capillary blood
  • At the same time, the pulmonary circulation (perfusion)...
    • Transports CO2 to the alveolar-capillary interface
    • Removes O2 that diffuses into the capillary blood from the alveoli
  • Although we can calculate the mean alveolar PO2 and PCO2, the partial pressure of O2 and CO2 in any given alveolus as well as the PO2 and PCO2 of the capillary blood leaving it depend on...
    • The RATIO of ventilation and perfusion to that alveolus

2

Dye and water simulate the movement of oxygen and blood through the lungs

  • Analogy of a large water-filled container explains...
  • Powdered dye
  • Fresh water
  • The concentration of dye (O2) in both the container (alveolus) and the effluent water (pulmonary capillary blood) will depend on...
  • In fact, the PO2 and PCO2 of both alveolar gas and end-capillary blood actually
    depend on...

  • Analogy of a large water-filled container explains...
    • The effect of ventilation and perfusion on PO2
  • Powdered dye
    • Continuously added to the water
  • Fresh water
    • Continuously flows through the container
  • The concentration of dye (O2) in both the container (alveolus) and the effluent water (pulmonary capillary blood) will depend on...
    • Rate at which dye is added (ventilation)
    • Rate at which it is removed by the flowing water (perfusion)
  • In fact, the PO2 and PCO2 of both alveolar gas and end-capillary blood actually
    depend on...
    • The ratio of ventilation to perfusion (V/Q)

3

Alveolar PO2 and PCO2 when V/Q is normal, zero, and infinity:
To clarify the effect of the V/Q ratio on PO2 and PCO2, let’s examine what happens when ventilation and perfusion to an alveolus are matched and when either ventilation or perfusion is absent (V/Q = 0 and V/Q = ∞, respectively)

  • If the PO2 of inspired air is 150mmHg and the PCO2 of mixed venous blood is 45mmHg, when ventilation and perfusion are matched...
  • If the airway to this alveolus becomes completely occluded, PAO2 and PACO2 will quickly become equal to...
  • Since the pulmonary capillary blood is not exposed to fresh inspired air, it...
  • If, on the other hand, ventilation is maintained but perfusion to this alveolus stops,...

  • If the PO2 of inspired air is 150mmHg and the PCO2 of mixed venous blood is 45mmHg, when ventilation and perfusion are matched...
    • PAO2 = 100 mmHg
    • PACO2 = 40 mmHg
  • If the airway to this alveolus becomes completely occluded, PAO2 and PACO2 will quickly become equal to...
    • The PO2 and PCO2 of mixed venous blood
  • Since the pulmonary capillary blood is not exposed to fresh inspired air, it...
    • Leaves the alveoli unchanged
  • If, on the other hand, ventilation is maintained but perfusion to this alveolus stops,...
    • PAO2 will increase to 150mmHg 
    • PACO2 will fall to zero (i.e. the partial pressure of these gases in inspired air)

4

The O2 – CO2 diagram

  • Using computer modeling, the alveolar PO2 and PCO2 (and therefore the end-capillary PO2 and PCO2) for every V/Q between zero and infinity can be determined
  • This is illustrated by the O2-CO2 diagram
  • PAO2 decreases and PACO2 increases as V/Q falls
  • The opposite occurs as V/Q increases

5

The distribution of V/Q ratios in the normal lung:
examining the ventilation-perfusion relationships that exist in whole lungs rather than single alveoli

  • Both ventilation and perfusion are greatest in...
  • In an upright subject, for example, ventilation and perfusion...
  • Gravity-induced change in perfusion​ vs. ventilation

  • Both ventilation and perfusion are greatest in...
    • The dependent portions of the lungs
  • In an upright subject, for example, ventilation and perfusion...
    • Increase progressively from the apices to the bases
  • Gravity-induced change in perfusion​ vs. ventilation
    • The gravity-induced change in perfusion is greater than that for ventilation
    • This means that a distribution of V/Q ratios must exist, even in the normal lung

6

High and low V/Q ratios in the normal lung

  • Non-dependent regions of the lung
  • Dependent regions of the lung
  • Mismatching of ventilation and perfusion
  • Any disease that affects ventilation (e.g. asthma, COPD) and/or perfusion (e.g. pulmonary embolism, emphysema) will...
  • Mismatching of ventilation and perfusion leads to...

  • Non-dependent regions of the lung
    • Have high V/Q regions
    • Contribute blood with high PO2 and low PCO2
  • Dependent regions of the lung
    • Have low V/Q regions
    • Contribute blood with low PO2 and high PCO2
  • Mismatching of ventilation and perfusion (distribution of ventilation/perfusion ratios)
    • Occurs even in normal lungs
  • Any disease that affects ventilation (e.g. asthma, COPD) and/or perfusion (e.g. pulmonary embolism, emphysema) will...
    • Increase the degree and extent of V/Q inequality
  • Mismatching of ventilation and perfusion leads to...
    • Impairment of overall gas exchange in the lungs by using a series of two-compartment lung models

7

An “ideal” lung model in which there is uniform ventilation and perfusion

  • Ventilation and perfusion in an “ideal” or “perfect” lung
  • Under these conditions, the gas in both compartments...
  • PA-aO2

  • Ventilation and perfusion in an “ideal” or “perfect” lung
    • Both compartments (groups of alveoli) receive the same amount of ventilation and perfusion
    • All V/Q ratios are the same
  • Under these conditions, the gas in both compartments...
    • The end-capillary blood and the mixed arterial blood
    • The PO2 and PCO2 of the gas in all alveoli and of the capillary blood leaving them is the same
  • PA-aO2
    • There is no difference between the average alveolar PO2 and PO2 of the mixed arterial blood
    • PA-aO2 is zero

8

Two-compartment lung model with non-uniform ventilation

  • Narrowing of one airway causes...
  • An increase in the V/Q leads to...
  • The low V/Q in compartment B causes...
  • Looking at the average alveolar and the mixed arterial PO2...
  • Compared with the “ideal” conditions,...
    • The average alveolar PO2...
    • The mixed arterial PO2...
    • The PA-aO2...

  • Narrowing of one airway causes...
    • Compartment A to receive three times more inspired gas than compartment B
    • Since overall ventilation is unchanged and blood flow remains equally distributed, this non-uniform ventilation causes compartment B to have a low V/Q (0.4) and compartment A to have a high ratio (1.2)
  • The high V/Q in compartment A causes...
    • Increased PO2
    • Decreased PCO2 of both the alveolar gas and the blood
  • The low V/Q in compartment B causes...
    • Decreased PO2
    • Increased PCO2
  • Looking at the average alveolar and the mixed arterial PO2...
    • The PO2 of mixed alveolar gas is determined by allowing for the difference in ventilation between the two compartments
    • PAO2 = [3(114) + 77] /4
  • Compared with the “ideal” conditions,...
    • The average alveolar PO2 has increased from 100 to 105mmHg
    • The mixed arterial PO2 has fallen from 100 to 89mmHg
    • The PA-aO2 is now 16mmHg

9

The relationship between PaO2 and the oxygen content of blood

  • The mixed arterial PO2 is derived by...
  • For example, simply averaging the PO2 of the blood from compartment A and compartment B leads to...
  • Averaging hemoglobin saturations (SO2) of 98.2% and 95.4%, however, leads to...
  • This discrepancy occurs because...
  • For this reason...

  • The mixed arterial PO2 is derived by...
    • Averaging the oxygen contents or hemoglobin saturations, not the PO2, of the blood leaving the two compartments
  • For example, simply averaging the PO2 of the blood from compartment A and compartment B leads to...
    • A value of 96mmHg
  • Averaging hemoglobin saturations (SO2) of 98.2% and 95.4%, however, leads to...
    • A mixed saturation of 96.8%
    • Corresponds to a PaO2 of 89mmHg
  • This discrepancy occurs because...
    • The oxyhemoglobin dissociation curve is not linear
  • For this reason...
    • High V/Q units do not compensate for low V/Q units
    • The presence of low V/Q lung units causes a fall in arterial PO2 and an increase in the PA-aO2

10

The relationship between PaO2 and the oxygen content of blood

  • The “normal” PAaO2 of about 8 -12mmHg results in part from...
  • The same considerations apply to...
  • Low V/Q units can lead to...

  • The “normal” PAaO2 of about 8 -12mmHg results in part from...
    • Low V/Q regions, which are present in normal lungs
  • The same considerations apply to...
    • PCO2
  • Low V/Q units can lead to...
    • Elevation of the PaCO2

11

The relationship between PaCO2 and the CO2 content of blood

  • Due to the nearly linear shape of the CO2 dissociation curve,...
  • If ventilation to compartment B falls to zero
  • Once again, the increase in the PO2 of the blood coming from compartment A will...
  • The component of the “normal” PA-aO2 that is not explained by V-Q mismatching can be attributed to...

  • Due to the nearly linear shape of the CO2 dissociation curve,...
    • High V/Q units can better compensate for low V/Q units
    • Increases in PaCO2 can usually be compensated for by an increase in alveolar ventilation
  • If ventilation to compartment B (V/Q) falls to zero
    • Since overall ventilation and perfusion are unchanged
      • Compartment A has an abnormally high V/Q
      • Compartment B has a V/Q of 0
    • Compartment B has been converted into a shunt
    • Mixed venous blood is added directly to the arterial circulation
  • Once again, the increase in the PO2 of the blood coming from compartment A will...
    • Fail to compensate for the addition of mixed venous blood to the arterial circulation
    • That is, a right to left shunt, like low V/Q lung units, causes a decrease in the PaO2 and an increase in the PAaO2
  • The component of the “normal” PA-aO2 that is not explained by V-Q mismatching can be attributed to...
    • The small amount of anatomic shunting that occurs in normal lungs (i.e. bronchial venous return

12

Two compartment lung model with non-uniform perfusion:
Situation in which blood flow to one compartment has been completely stopped

  • Since overall ventilation and perfusion are unchanged...
  • Compartment A
  • Compartment B

  • Since overall ventilation and perfusion are unchanged...
    • Ventilation to both compartments remains equal
    • Cessation of blood flow to one compartment increases alveolar and physiologic dead space
  • Compartment A
    • Now receives twice as much blood flow
    • This causes compartment A to have an abnormally low V/Q
  • Compartment B
    • Has a V/Q of infinity (i.e. ventilation remains normal but perfusion is zero)
    • Has been converted into a type of dead space
      • Dead space is created when V/Q > 1

13

Two compartment lung model with non-uniform perfusion

  • Anatomic dead space
  • Alveolar dead space
  • As alveolar dead space increases...
  • Physiologic dead space

  • Anatomic dead space
    • All parts of the upper and lower airways that are not in contact with pulmonary capillary blood
  • Alveolar dead space
    • Situation in which the gas filling certain alveoli cannot participate in gas exchange because these alveoli receive no blood flow
    • Acts the same as anatomic dead space
  • As alveolar dead space increases...
    • The minute ventilation required to maintain a given PaCO2 increases
      • PaCO2 will increase if VE is maintained at a constant level
    • Any increase in physiologic dead space will increase the VE needed to maintain a given PCO2
      • ​VA = VE - VD
  • Physiologic dead space
    • The total amount of dead space in the lungs (both anatomic and alveolar)

14

Two compartment lung model with non-uniform perfusion

  • The complete cessation of blood flow to alveoli leads to...
  • Alveolar dead space is generated whenever...
  • Distinguish the effects of high and low V/Q lung units on PaCO2
  • The compartment (B) with a low V/Q contributes blood with...
  • Ventilation to compartment B
  • The lack of sufficient ventilation to compartment A (leading to a low V/Q) that may lead to...

  • The complete cessation of blood flow to alveoli leads to...
    • An increase in alveolar dead space
  • Alveolar dead space is generated whenever...
    • Alveoli receive either excessive ventilation or inadequate perfusion
    • That is, high V/Q lung units increase alveolar and physiologic dead space
  • Distinguish the effects of high and low V/Q lung units on PaCO2
    • High V/Q regions cause an increase in physiologic dead space, which impairs CO2 excretion by the lungs
    • The increased PCO2 actually comes from blood leaving low V/Q regions
  • The compartment (B) with a low V/Q contributes blood with...
    • Increased PCO2
  • Ventilation to compartment B
    • Wasted since gas exchange cannot occur
    • This leads to a marked increase in dead space
  • The lack of sufficient ventilation to compartment A (leading to a low V/Q) that may lead to...
    • An increase in PaCO2 (and a fall in PaO2)

15

Two compartment lung model with non-uniform perfusion

  • Any disturbance in ventilation or perfusion leads to...
  • For example, asymmetric ventilation...
  • The lack of blood flow to compartment B...

  • Any disturbance in ventilation or perfusion leads to...
    • The development of both low and high V/Q regions
  • For example, asymmetric ventilation...
    • Decreases the V/Q of compartment B
    • Leads to a decrease in PaO2 and an increase in PA-aO2
    • Causes compartment A to receive too much ventilation (V/Q = 1.2)
    • Leads to a small increase in physiologic dead space
  • The lack of blood flow to compartment B...
    • Doubles the flow to compartment A
    • Produces a low V/Q lung unit even though there has been no decrease in ventilation

16

Two compartment lung model with non-uniform perfusion:
Summary

  • Any disorder affecting ventilation or perfusion...
  • Low V/Q regions...
  • High V/Q regions...

  • Any disorder affecting ventilation or perfusion leads to both low and high V/Q regions
  • Low V/Q regions and shunts
    • Decrease the PaO2
    • Increase the PA-aO2 and PaCO2
    • Any increase in PaCO2 comes from blood leaving low V/Q units
    • Assessment: PA-aO2
  • High V/Q regions (including regions with no perfusion)
    • Increase alveolar and physiologic dead space
    • Impair CO2 excretion
    • Assessment: physiologic dead space (Bohr equation)

17

The effect of increasing V/Q inequality on gas exchange

  • The lungs have...
  • Each alveolus has...
  • As disease produces alterations in ventilation and perfusion, the number of alveoli with abnormally high and low ratios...
  • The effect of increasing V/Q inequality on PaO2, PaCO2, and alveolar dead space in a computer model in which VCO2 and VE are maintained constant

  • The lungs have...
    • Millions of alveoli, not just two compartments
  • Each alveolus has...
    • Its own V/Q ratio
  • As disease produces alterations in ventilation and perfusion, the number of alveoli with abnormally high and low ratios...
    • Increases
    • This is referred to as increasing V/Q inequality and is typically represented by the symbol “σ”
  • The effect of increasing V/Q inequality on PaO2, PaCO2, and alveolar dead space in a computer model in which VCO2 and VE are maintained constant
    • PaO2 falls,
    • PaCO2 and alveolar dead space increase with increasing V/Q inequality

18

The effect of increasing V/Q inequality on gas exchange:
Several calculations can be used to determine the extent of V/Q inequality

  • PA-aO2
  • Bohr equation
  • PECO2

  • PA-aO2
    • The simplest calculation
    • is the PA-aO2, which increases with low V/Q regions and shunt
  • Bohr equation
    • Another method is to calculate the amount of physiologic dead space
    • Unlike the PA-aO2, this calculation provides information about high V/Q lung units
    • Allows us to calculate the ratio of dead space volume to tidal volume (VD/VT)
    • VD/VT = (PaCO2 – PECO2) / PaCO2
    • Tells us that the difference between arterial and exhaled PCO2 increases with the proportion of dead space in a tidal breath
  • PECO2
    • Decreases as physiologic dead space increases
    • Partial pressure of CO2 in mixed, exhaled gas
    • Usually measured by collecting exhaled gas in a large bag

19

Major points

  • For each alveolus,...
  • When you have a distribution of V/Q ratios (normally),...
    • The low V/Q ratios cause...
    • The high V/Q ratios cause
  • In lung disease,...
  • The main reason that the V/Q mismatching affects arterial blood gases is because...

  • For each alveolus, the ratio of ventilation to perfusion determines the PO2 and PCO2 of each alveolus
  • When you have a distribution of V/Q ratios (normally)
    • The low V/Q ratios cause a drop in the arterial PO2
    • The high V/Q ratios cause an increase in alveolar dead space and total physiologic dead space, which in turn increases the minute ventilation requirement to maintain a normal PCO2
  • In lung disease, this distribution gets wider, and all these problems get worse
    • PO2 decreases, your minute ventilation requirements increases, and your A-a gradient increases
  • The main reason that the V/Q mismatching affects arterial blood gases is because of the non-linear shape of the hemoglobin dissociation curve and the fact that you have to average arterial oxygen contents or saturations rather than PO2s

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