Board Questions

1. When does PaO2 play an important role in oxygen delivery? 

a) Always as it represents dissolved oxygen available to tissues
b) Never as it represents a very small portion of oxygen actually delivered
c) In interstitial lung disease when you have a large diffusion abnormality 
d) In the setting of profound anemia

2. What accounts for the normal A-a gradient? 

a) Right to left shunt through Thebesian veins
b) Right to left shunt through bronchial veins
c) Intrapulmonary shunt through areas of microatelectasis
d) All of the above

3. Which of the following is NOT a direct contributing factor to oxygen delivery? 

a) Tidal volume
b) Cardiac Output
c) Oxygen saturation
d) Hemoglobin

4. Your patient is a 6 yo M who was in a house fire and suffered significant smoke inhalation. He is intubated and sedated with ventilator settings of PIP 25, PEEP 5, RR 16, Vt 8 cc/kg and FiO2 45%. His oxygen saturation is 99%. Venous blood gas from the R IJ CVL reveals a pH of 7.2, PaCO2 of 40, PO2 of 60, HCO3 of 14, and a lactate of 8. What is the most likely explanation for his current clinical state?

a) Respiratory acidosis from smoke inhalation
b) Metabolic acidosis from cyanide poisoning
c) Methemoglobinemia
d) Metabolic acidosis from dehydration due to surface burns and large insensible losses 



ANSWERS & EXPLANATIONS

1. D- PaO2 becomes important for oxygen delivery in the setting of profound anemia as  CaO2= Hgb(Sat)(1.34) + 0.003(PaO2). Hence, if Hgb is extremely low, then a high PaO2, even though it is multiplied by 0.003, can represent a significant contribution to total oxygen content and thereby, oxygen delivery. Otherwise, in normal circumstances (ie Hgb of 10), the PaO2 represents a very minimal contribution to total arterial oxygen content.

2. D- There is normally a A-a gradient of approximately 5-10 that increases with age. This is due to all of the reasons listed, including intrinsic extracardiac shunt from veins draining the heart and bronchi that drain directly to the left side of the circulation as well as areas of microatelectasis that lead to intrapulmonary shunt. West Zone III is an example of lung that has relative intrapulmonary shunt (by definition, a more specific form of VQ mismatch). 

3. A- while tidal volume is important for adequate ventilation (minute ventilation=RR X Vt), it is not a direct contributing factor for oxygen delivery. For example, you could be on an oscillator with no significant tidal volumes and still deliver oxygen well. Similarly, you could be on VV ECMO with no significant tidal volume and still deliver oxygen well. Oxygen saturation (which clearly can be affected by tidal volume), cardiac output, and hemoglobin all directly contribute to oxygen delivery. 

4. B- The patient has a venous PO2 of 60, corresponding to a ScvO2 of approximately 90%. Given this evidence of very limited extraction (99-90=9%), elevated lactate, and metabolic acidosis, particularly in the setting of a house fire, this is most consistent with cyanide toxicity. Cyanide can occur as a result of housefires and leads to impaired oxygen extraction (directly affecting the mitochondrial oxygen transport chain via cytochrome c oxidase), thus resulting in unusually impaired oxygen extraction and high ScvO2's, elevated lactate levels, and metabolic acidosis. 

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