Causes of hypoxia

From Iusmphysiology

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(Created page with '*started here on 02/17/11 at 8AM. ==Causes of hypoxia== *stopped here on 02/17/11 at 9AM.')
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==Causes of hypoxia==
==Causes of hypoxia==
 +
 +
===Objectives===
 +
 +
===Definitions===
 +
*Hypoxemia is an abnormally low PO2 in arterial blood.
 +
**Hypox = low oxygen
 +
**emia = blood
 +
**clinically = PO2 < 80
 +
**this is different than hypoxia
 +
*Hypoxia = any state in which the availablity of oxygen to the tissues is impaired.
 +
**You can have low oxygen to the tissues but not be hypoxic.
 +
***Stroke: brain isn't getting blood flow but the blood has normal amount of oxygen.
 +
***CO poisoning or anemia: think about this
 +
 +
===Primary causes of hypoxemia===
 +
*Four primary reasons: hypoventilation, diffusion impairment, shunt, or V/Q mismatch
 +
 +
====Hypoventilation====
 +
*If you breathe too little, PaCO2 goes up, and this is how it is measured clinically.
 +
**Always always High PaCO2 = hypoventilation.
 +
*amount of CO2 produced by tissue and the rate of blow off determines the PACO2.
 +
**PACO2 is inversely proportional to ventilation (VA).
 +
*Usually PAO2 and PaO2 are low
 +
**Give supplemental oxygen.
 +
*Obstruction example:
 +
**If air can't get to alveoli, PAO2 will be low.
 +
**PACO2 is high because airway cannot blow off CO2
 +
**The longer one is obstructed, the lower PAO2 goes and the higher PACO2 goes.
 +
 +
=====Causes=====
 +
*Drugs that depress the respiratory system: morphine, barbituates, etc.
 +
*Injury to the medulla or to the respiratory centers
 +
*Disease states that impaire muscle function like myasthenia gravis
 +
**If skeletal muscle is dpressed diaphragm won't depress and air won't come in.
 +
*One more.
 +
 +
====Diffusion impairment====
 +
*Equilibration of gases won't occur in this case.
 +
*Even though air gases are the same, PaO2 and PaCO2 will be abnormal (won't equal that of the alveolar gas); PaO2 will be low and PaCO2 will be high.
 +
*Gas on Hb '''does not contribute to partial pressures'''.
 +
*You can use CO to measure diffusion capacity = DLco
 +
**Known concentration of CO, breate in over 10 seconds, hold it for 10 seconds, CO is diffusing across, then blow out. 
 +
**Measure CO at the end of expiration.
 +
**This will tell us how much diffused.
 +
*If blow flow is high (cardiac output is high), more Hb per unit time is flowing past so more diffusion will occur.
 +
***think higher heart rate, higher hematocrit, etc.
 +
*Most disease states will reduce DLco
 +
**Emphysmea decreases surface area so diffusion goes down.
 +
**fibrosis decreases because diffusion will occur mmore slowly.
 +
**Edema will decrease because the diffusion distance goes up.
 +
**Anemia wil ldecrease the Hb delivery to the alveoli so it decreases the DLco.
 +
 +
====Shunt====
 +
*A shunt moves deox blood into oxygenated blood.
 +
**The deoxed blood was never exposed to the alveoli so it looks like venous blood (PO2 = 40, PCO2 = 46).
 +
**This shunt is a right to left shunt.
 +
***There is some normal right to left shunt.
 +
*Adolexisis = collapse of the alveoli
 +
**So blood that travels by these alveoli will not go through exchange and will get mixed and can cause significant lower PO2 levels.
 +
*How do we diagnose?
 +
**Put hypovent perosn on oxygen, their PO2 goes up.
 +
**Put shunt person on oxygen and PO2 doesn't go up b/c the blood never sees the high PO2 air.
 +
***This is how you diagnose.
 +
**Normal person on oxygen would go to PAO2 660!
 +
 +
====Ventilation-perfusion mismatch====
 +
*This is the most common cause of hypoxemia.
 +
*Normal ventilation is around 4L / min
 +
*Normal '''perfusion = Q''' (cardiac output) = 5 L / min
 +
*'''Normal V / Q ratio is about 0.8'''
 +
**Normal range is 0.8-1.2
 +
*Deviations means theres an impaired ability to exchange.
 +
 +
====Limits of V/Q mismatch====
 +
*Airway obstruction:
 +
**The partial pressure of gas in the alveoli will approach venous blood.
 +
**This mnakes sense because that's what they are exposed to.
 +
**In this situation, V is changing, going down.
 +
**So V/Q goes down.
 +
**As V/Q goes down, the blood gases will approach that of venous blood.
 +
*Bloodway obstruction:
 +
**Alveolus is seeing air but not blood.
 +
**Gases in the alveolus will approach that of atmospheric air.
 +
**Q goes down (but is in the denominatory)
 +
**V/Q goes up
 +
**As V/Q goes up, the blood gases will appoarch that of atmospheric air.
 +
 +
====V/Q and PO2 - PCO2====
 +
*Normal PO2 = 100, PCO2 = 40.
 +
*As V/Q decreases:
 +
**Gases approach venous: PO2 up and PCO2 down.
 +
*As V/Q increases:
 +
**Gases approach atmospheric air: PO2 goes down and PCO2 goes up.
 +
 +
====Regional gas exchange====
 +
*V/Q at the apex is higher than at the base.
 +
*Blood to base of lungs is higher.
 +
*Alveoli in the apex are more expanded b/c of a little extra negative pressure at the top.
 +
*At the bottom blood flow and ventilation are higher.
 +
**But blood flow is higher than the ventilation.
 +
**So V/Q is low.
 +
*As you move from the base to the apex, both V and Q go down; however, blood flow decreases faster.
 +
*At the apex, the V and Q are both low
 +
**but ventilation is higher than the blood flow.
 +
**So V/Q is higher than at the base.
 +
**This affects the gases and exchange.
 +
 +
 +
*Low V/Q (at the apex) means that the gases are closer to the venous; the base has a higher V/Q so will have gases closer to the atmosphere.
 +
See cartoon.
 +
 +
====V/Q and overall gas exchange====
 +
*At the apex, the V/Q ratio is higher than at the basse.
 +
**With higher V/Q ratio, the gases will approach the atmosphere levels
 +
*At the base the V/Q ratio is lower than the apex.
 +
**At low V/Q the gases approach the venous levels.
 +
**So the base has low Po2 and Higher PCO2.
 +
**And this is where most of the ventilationa nd perfusion occur.
 +
*So, overall, the blood looks like the exhcnage occuring at the base which is approaching venous levels b/c that's where most of the action takes place.
 +
 +
====Compensation for V/Q mismatch====
 +
*With low perfusion, PCO2 goes down.
 +
**So airway smooth muscle sees a lower pH, causing it to constrict to shunt air away.
 +
*Decrease in ventilation, PO2 goes down.
 +
**Blood will be shunted away from low ventilated places.
 +
 +
====Conditions causeing lower V/Q====
 +
*Pneumonia, asthma, pulmonary edema, compliance changes, body position
 +
 +
====Lung volume and airway patency====
 +
*V/P relationship must take iinto account the difference in V/Q at the base and apex.
 +
*At FRC under normal conditions, alveoli are more expanded at the apex than at the base.
 +
*As you inspire, the alveoli become more equal at the base and apex.
 +
*As you approach residual volume, base alveoli approach collapse.
 +
 +
====Lung volume====
 +
*As you breathe in, intrapleurla pressure is most negative at the apex so air goes first to the apex but '''most of the air goes to the base''' b/c they are more compliant.
 +
 +
====closing volume====
 +
*Diagnosing "small airway disease"
 +
*Determine at what volume you see the collapse of the alveoli at the base of the lung.
 +
*Pt breathes 100% oxygen from residual to total lung capacity.
 +
**Concentration of air in trachea after expiring is alveolar air (high Nitrogen, PO2 = 100, PCO2 = 46)
 +
*Recall that first air breathed is in the apex.
 +
*so the air that has the nitrogen in it will go mostly to the apical alveoli.
 +
*Measure the amount of N coming out.
 +
*First (phase 2) part won't have much N.
 +
*Phase 2 has a mixture of nitrogen and not (from base and apex)
 +
*Phase 3 has a mixture
 +
*Phase 4:
 +
**As you get closwer to residual volume, base alveoli collapse and you get more air from the apical alveoli
 +
**So the second increase in nitrogen represents the '''closing volume''' where the base alveoli close
 +
 +
 +
*In small-airway disease, closing volume will occur at much lower volumes.
 +
 +
===P(A-a) gradient and hypoxemia===
 +
*This is the best measure of blood oxygenation
 +
*As gases equilabrate with alveoli, there is some mixture of venous blood that will lower the blood gas concentrations.
 +
**Normally by about 10ish.
 +
*This will increase the A-a gradient
 +
*Diffusion impairment, shunt, and V/Q mismatch '''increase the A-a gradient'''.
 +
*Hypoventilation is not an exchange issue, only a ventilation issue.
 +
**Does not affect the V/Q gradient.
 +
 +
*Stopped at 44:30.
 +
 +
*stopped here on 02/17/11 at 9AM.
*stopped here on 02/17/11 at 9AM.

Revision as of 14:52, 21 February 2011

  • started here on 02/17/11 at 8AM.


Contents

Causes of hypoxia

Objectives

Definitions

  • Hypoxemia is an abnormally low PO2 in arterial blood.
    • Hypox = low oxygen
    • emia = blood
    • clinically = PO2 < 80
    • this is different than hypoxia
  • Hypoxia = any state in which the availablity of oxygen to the tissues is impaired.
    • You can have low oxygen to the tissues but not be hypoxic.
      • Stroke: brain isn't getting blood flow but the blood has normal amount of oxygen.
      • CO poisoning or anemia: think about this

Primary causes of hypoxemia

  • Four primary reasons: hypoventilation, diffusion impairment, shunt, or V/Q mismatch

Hypoventilation

  • If you breathe too little, PaCO2 goes up, and this is how it is measured clinically.
    • Always always High PaCO2 = hypoventilation.
  • amount of CO2 produced by tissue and the rate of blow off determines the PACO2.
    • PACO2 is inversely proportional to ventilation (VA).
  • Usually PAO2 and PaO2 are low
    • Give supplemental oxygen.
  • Obstruction example:
    • If air can't get to alveoli, PAO2 will be low.
    • PACO2 is high because airway cannot blow off CO2
    • The longer one is obstructed, the lower PAO2 goes and the higher PACO2 goes.
Causes
  • Drugs that depress the respiratory system: morphine, barbituates, etc.
  • Injury to the medulla or to the respiratory centers
  • Disease states that impaire muscle function like myasthenia gravis
    • If skeletal muscle is dpressed diaphragm won't depress and air won't come in.
  • One more.

Diffusion impairment

  • Equilibration of gases won't occur in this case.
  • Even though air gases are the same, PaO2 and PaCO2 will be abnormal (won't equal that of the alveolar gas); PaO2 will be low and PaCO2 will be high.
  • Gas on Hb does not contribute to partial pressures.
  • You can use CO to measure diffusion capacity = DLco
    • Known concentration of CO, breate in over 10 seconds, hold it for 10 seconds, CO is diffusing across, then blow out.
    • Measure CO at the end of expiration.
    • This will tell us how much diffused.
  • If blow flow is high (cardiac output is high), more Hb per unit time is flowing past so more diffusion will occur.
      • think higher heart rate, higher hematocrit, etc.
  • Most disease states will reduce DLco
    • Emphysmea decreases surface area so diffusion goes down.
    • fibrosis decreases because diffusion will occur mmore slowly.
    • Edema will decrease because the diffusion distance goes up.
    • Anemia wil ldecrease the Hb delivery to the alveoli so it decreases the DLco.

Shunt

  • A shunt moves deox blood into oxygenated blood.
    • The deoxed blood was never exposed to the alveoli so it looks like venous blood (PO2 = 40, PCO2 = 46).
    • This shunt is a right to left shunt.
      • There is some normal right to left shunt.
  • Adolexisis = collapse of the alveoli
    • So blood that travels by these alveoli will not go through exchange and will get mixed and can cause significant lower PO2 levels.
  • How do we diagnose?
    • Put hypovent perosn on oxygen, their PO2 goes up.
    • Put shunt person on oxygen and PO2 doesn't go up b/c the blood never sees the high PO2 air.
      • This is how you diagnose.
    • Normal person on oxygen would go to PAO2 660!

Ventilation-perfusion mismatch

  • This is the most common cause of hypoxemia.
  • Normal ventilation is around 4L / min
  • Normal perfusion = Q (cardiac output) = 5 L / min
  • Normal V / Q ratio is about 0.8
    • Normal range is 0.8-1.2
  • Deviations means theres an impaired ability to exchange.

Limits of V/Q mismatch

  • Airway obstruction:
    • The partial pressure of gas in the alveoli will approach venous blood.
    • This mnakes sense because that's what they are exposed to.
    • In this situation, V is changing, going down.
    • So V/Q goes down.
    • As V/Q goes down, the blood gases will approach that of venous blood.
  • Bloodway obstruction:
    • Alveolus is seeing air but not blood.
    • Gases in the alveolus will approach that of atmospheric air.
    • Q goes down (but is in the denominatory)
    • V/Q goes up
    • As V/Q goes up, the blood gases will appoarch that of atmospheric air.

V/Q and PO2 - PCO2

  • Normal PO2 = 100, PCO2 = 40.
  • As V/Q decreases:
    • Gases approach venous: PO2 up and PCO2 down.
  • As V/Q increases:
    • Gases approach atmospheric air: PO2 goes down and PCO2 goes up.

Regional gas exchange

  • V/Q at the apex is higher than at the base.
  • Blood to base of lungs is higher.
  • Alveoli in the apex are more expanded b/c of a little extra negative pressure at the top.
  • At the bottom blood flow and ventilation are higher.
    • But blood flow is higher than the ventilation.
    • So V/Q is low.
  • As you move from the base to the apex, both V and Q go down; however, blood flow decreases faster.
  • At the apex, the V and Q are both low
    • but ventilation is higher than the blood flow.
    • So V/Q is higher than at the base.
    • This affects the gases and exchange.


  • Low V/Q (at the apex) means that the gases are closer to the venous; the base has a higher V/Q so will have gases closer to the atmosphere. 
See cartoon.

V/Q and overall gas exchange

  • At the apex, the V/Q ratio is higher than at the basse.
    • With higher V/Q ratio, the gases will approach the atmosphere levels
  • At the base the V/Q ratio is lower than the apex.
    • At low V/Q the gases approach the venous levels.
    • So the base has low Po2 and Higher PCO2.
    • And this is where most of the ventilationa nd perfusion occur.
  • So, overall, the blood looks like the exhcnage occuring at the base which is approaching venous levels b/c that's where most of the action takes place.

Compensation for V/Q mismatch

  • With low perfusion, PCO2 goes down.
    • So airway smooth muscle sees a lower pH, causing it to constrict to shunt air away.
  • Decrease in ventilation, PO2 goes down.
    • Blood will be shunted away from low ventilated places.

Conditions causeing lower V/Q

  • Pneumonia, asthma, pulmonary edema, compliance changes, body position

Lung volume and airway patency

  • V/P relationship must take iinto account the difference in V/Q at the base and apex.
  • At FRC under normal conditions, alveoli are more expanded at the apex than at the base.
  • As you inspire, the alveoli become more equal at the base and apex.
  • As you approach residual volume, base alveoli approach collapse.

Lung volume

  • As you breathe in, intrapleurla pressure is most negative at the apex so air goes first to the apex but most of the air goes to the base b/c they are more compliant.

closing volume

  • Diagnosing "small airway disease"
  • Determine at what volume you see the collapse of the alveoli at the base of the lung.
  • Pt breathes 100% oxygen from residual to total lung capacity.
    • Concentration of air in trachea after expiring is alveolar air (high Nitrogen, PO2 = 100, PCO2 = 46)
  • Recall that first air breathed is in the apex.
  • so the air that has the nitrogen in it will go mostly to the apical alveoli.
  • Measure the amount of N coming out.
  • First (phase 2) part won't have much N.
  • Phase 2 has a mixture of nitrogen and not (from base and apex)
  • Phase 3 has a mixture
  • Phase 4:
    • As you get closwer to residual volume, base alveoli collapse and you get more air from the apical alveoli
    • So the second increase in nitrogen represents the closing volume where the base alveoli close


  • In small-airway disease, closing volume will occur at much lower volumes.

P(A-a) gradient and hypoxemia

  • This is the best measure of blood oxygenation
  • As gases equilabrate with alveoli, there is some mixture of venous blood that will lower the blood gas concentrations.
    • Normally by about 10ish.
  • This will increase the A-a gradient
  • Diffusion impairment, shunt, and V/Q mismatch increase the A-a gradient.
  • Hypoventilation is not an exchange issue, only a ventilation issue.
    • Does not affect the V/Q gradient.
  • Stopped at 44:30.



  • stopped here on 02/17/11 at 9AM.
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