PROBLEM SESSION ANSWERS: Problem 6

Step 1: Calculate the value of P_A-CO2 with the 2x hyperventilation, assuming the rate of CO₂ production remains constant and noting that CO₂ production is the product of alveolar ventilation and the CO₂ fraction of the alveolar gas:

V'_CO2 = V'_A x F_A-CO2

Thus, if alveolar ventilation doubles and CO₂ production remains the same, alveolar CO₂ fraction (and partial pressure) must be reduced to half the normal values, so

P_A-CO2 = 20 mmHg

Step 2: Use the new value of P_A-CO2 in the alveolar air equation:

P_A-CO2 = 85 - 20/0.8 = 85 - 25 = 60 mmHg

Conclusion: 60 mmHg, systemic arterial blood would be about 85-90% saturated. Thus, hyperventilation would definitely help oxygenation. but at the cost of low blood CO₂ and high blood pH.

Note limitation on ventilation increase due to low blood CO₂ .

Note problem of "mountain sickness" and alkaline CSF pH. Actually, what will happen over the next few days is that CSF pH will return towards normal, which will make the inhibition of ventilation increase less.

Note problem of pulmonary hypertension due to increase in pulmonary vascular resistance.