Causes of hypoxia

From Iusmphysiology

• started here on 02/17/11 at 8AM.

Contents 1 Causes of hypoxia 1.1 Objectives 1.2 Definitions 1.3 Primary causes of hypoxemia 1.3.1 Hypoventilation 1.3.1.1 Causes 1.3.2 Diffusion impairment 1.3.3 Shunt 1.3.4 Ventilation-perfusion mismatch 1.3.5 Limits of V/Q mismatch 1.3.6 V/Q and PO2 - PCO2 1.3.7 Regional gas exchange 1.3.8 V/Q and overall gas exchange 1.3.9 Compensation for V/Q mismatch 1.3.10 Conditions causeing lower V/Q 1.3.11 Lung volume and airway patency 1.3.12 Lung volume 1.3.13 closing volume 1.4 P(A-a) gradient and hypoxemia

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.