Draft

269  When Does Hypoxemia Become Apparent

269.1 Summary

  • When doe hypercapnia cause hypoxemia in SLC?
  • Remember, hypercapnia in general is PaCO2 over 45 mmHg. However, at SLC elevation a PaCO2 of 42 mmHg has the same prognostic significance due to altitude induced hyperventilation.
  • Standard Equation, explained
  • Standard Equation, assumptions
  • Extended Equation Justification
  • Extended Equation, Explained
  • SLC Average values
  • When is hypoxemia noticeable?
  • Empirical measures: Crapo et al.
  • Critical PaCO2 with normal A-a gradient?

269.2 Slide outline

269.2.1 Slide 1

  • When doe hypercapnia cause hypoxemia in SLC?
  • Brian Locke, MD
  • Pulmonary and Critical Care Fellow ### Slide 2
  • Remember, hypercapnia in general is PaCO2 over 45 mmHg. However, at SLC elevation a PaCO2 of 42 mmHg has the same prognostic significance due to altitude induced hyperventilation.
  • More on this later. ### Slide 3
  • Standard Equation, explained
  • PAO2 PiO2 – (PaCO2 / RQ)
  • PiO2 represents O2 arriving at the alveoli, which becomes saturated with H2O through the airways
  • PiO2 (Patm – PH2O) FiO2, PH2O taken to be 47, but varies w/ Temp
  • PaCO2 / RQ represents the volume of O2 in the alveoli removed into the blood ### Slide 4
  • Standard Equation, assumptions
  • PAO2 (Patm – PH2O) FiO2 – (PaCO2 / RQ)
  • Gases obey Dalton’s Law – Total pressure equals the sum of all the partial pressures (PAtm PN2 + PO2 + PCO2 + PH2O). Not quite true but close
  • Alveolar Gases are completely saturated with water vapor. True
  • CO2 in Alveoli is in equilibrium with CO2 in in pulmonary capillaries: PACO2 PaCO2. Essentially always true
  • Inspired gas contains no CO2. Not quite true but close if breathing atm air
  • Amount of gas inspired/utilized is equal to amount of gas expired/created by metabolism (e.g. RQ 1) – not true, but close in most circumstances.
  • Addressed by complete equation on next slide ### Slide 5
  • Extended Equation Justification
  • PAO2 (Patm – PH2O) FiO2 – (PaCO2 / RQ)
  • (Patm – PH2O) FiO2 represents O2 arriving at the alveoli, which becomes saturated with H2O through the airways
  • PaCO2 / RQ represents the volume of O2 in the alveoli removed into the blood
  • However: if RQ / 1, we know that every O2 molecule leaving the alveoli is NOT replaced by 1 molecule of CO2.
  • E.g. RQ 0.8, VO2 might be 0.25 L/min, VCO2 might be 0.20 L/min
  • Thus, at steady state – 0.05 L/min must passively diffuse into the alveoli ### Slide 6
  • Extended Equation Justification
  • However: if RQ / 1, we know that every O2 molecule leaving the alveoli is NOT replaced by 1 molecule of CO2.
  • Vp (passive) |VO2 – VCO2|
  • DOI 10.1093/bjaceaccp/mkh008 ### Slide 7
  • Extended Equation Justification
  • However: if RQ / 1, we know that every O2 molecule leaving the alveoli is NOT replaced by 1 molecule of CO2.
  • Vp (passive) |VO2 – VCO2|
  • When FiO2 (1-RQ) is much less than 1, this term is small and can be safely ignored.
  • DOI 10.1093/bjaceaccp/mkh008 ### Slide 8
  • Extended Equation, Explained
  • PAO2 (Patm – PH2O) FiO2 – (PaCO2 / RQ) + FiO2 PaCO2 (1-RQ)/RQ
  • (Patm – PH2O) FiO2 represents O2 arriving at the alveoli, which becomes saturated with H2O through the airways
  • PaCO2 / RQ represents the volume of O2 in the alveoli removed into the blood
  • FiO2 PaCO2 (1-RQ)/RQ represents the passive flow that must occur to maintain steady state when RQ / 1, because every O2 molecule leaving the alveoli is NOT replaced by 1 molecule of CO2. ### Slide 9
  • SLC Average values
  • PAO2 (Patm – PH2O) FiO2 – (PaCO2 / RQ) + FiO2 PaCO2 (1-RQ)/RQ
  • PAO2 (650 – 47) 0.21 – (PaCO2 / 0.8) + 0.21 PaCO2 (1-0.8)/0.8
  • PAO2 126.63 – (PaCO2 / 0.8) + 0.05 PaCO2
  • PAO2 126.63 – 1.3 PaCO2 ### Slide 10
  • When is hypoxemia noticeable?
  • SpO2 <90% catches people’s attention
  • Corresponds to a PaO2 of ~55 mmHg (depending exactly on the individual’s oxygen hemoglobin dissociation curve.
  • Normal A-a gradient? 4 + (0.25 Age)
  • Critical PAO2 paO2 + (A-a gradient)
  • Critical PAO2 51 + 0.25 Age ### Slide 11
  • Empirical measures: Crapo et al.
  • PaO2 by age:
  • -0.25 mmHg / yr ### Slide 12
  • Critical PaCO2 with normal A-a gradient?
  • Combining critical PAO2 with expected PAO2 by CO2:
  • 51 + (0.25 Age) 126.63 – 1.3 PaCO2
  • 75.63 - 0.25Age 1.3 PaCO2
  • 58.17 - .19 Age PaCO2
  • For someone who is 60?
  • PaCO2 46.77 mmHg ### Slide 13
  • Comparison: at sea level
  • Sea level:
  • PAO2 149.73 – 1.3 PaCO2
  • 51 + (0.25 Age) 149.73 – 1.3 PaCO2
  • 75.95 - .19 Age PaCO2 ### Slide 14
  • TODO: No text extracted from this slide. ### Slide 15
  • How elevation plays in to this
  • pAtm 760 e(Elevation/-7000)
  • Expected PaCO2 by elevation? ### Slide 16
  • Expected compensation (From Grissom)

269.3 Learning objectives

  • When doe hypercapnia cause hypoxemia in SLC?
  • Remember, hypercapnia in general is PaCO2 over 45 mmHg. However, at SLC elevation a PaCO2 of 42 mmHg has the same prognostic significance due to altitude induced hyperventilation.
  • Standard Equation, explained
  • Standard Equation, assumptions
  • Extended Equation Justification

269.4 Bottom line / summary

  • When doe hypercapnia cause hypoxemia in SLC?
  • Remember, hypercapnia in general is PaCO2 over 45 mmHg. However, at SLC elevation a PaCO2 of 42 mmHg has the same prognostic significance due to altitude induced hyperventilation.
  • Standard Equation, explained
  • Standard Equation, assumptions
  • Extended Equation Justification

269.5 Approach

  1. TODO: Outline the initial assessment or decision point.
  2. TODO: Outline the next diagnostic or management step.
  3. TODO: Outline follow-up or escalation criteria.

269.6 Red flags / when to escalate

  • TODO: List red flags that require urgent escalation.

269.7 Common pitfalls

  • TODO: Capture common errors or missed steps.

269.8 References

TODO: Add landmark references or guideline citations.

269.9 Slides and assets

269.10 Source materials