ALVEOLAR VENTILATION & BLOOD GAS:  Problems

1. The composition of air remains constant at altitudes near the earth's surface, but barometric pressure decreases progressively with height. At 4250 meters (14,000 feet), total barometric pressure averages 450 mmHg. If ventilation remained normal (neither hyper- nor hypoventilation), what would be the expected value of PA-O2? Do you think you could survive at that altitude. (Hint 1: Assume PA-CO2 = 40 mmHg. Why? Hint 2: Do not forget to include the effect of PH2O. Hint 3: To estimate your survival chances, determine the blood O2 saturation at the PA-O2 you have calculated.

2. Suppose you attempted to compensate for the reduced oxygen in Problem 1 by doubling alveolar ventilation rate, while metabolic rate (including O2 consumption and CO2) production remained constant. How much would this hyperventilation increase PA-O2? By how much would blood O2 saturation increase?

3. At the cruising altitude of modern jet planes (10,000 meters), the total barometric pressure is about 200 mmHg. Suppose you are flying in an airplane and a window blows out, causing the cabin pressure to fall to the ambient barometric pressure. Do you think you could survive breathing air? Do you think you could survive breathing pure oxygen?

4. The following measurements were taken on a 23 year old female with a recent history of dyspnea and cyanosis on exertion.

Ventilation Predicted Measured % Predicted
Vital Capacity 3.3 liters 2.3 liters 70%
Residual Volume 0.8 liters 0.6 liters 75%
Total Lung Capacity 4.1 liters 2.9 liters 71%
FEV-1 2.6 liters 1.9 liters 73%
FEV-1/FVC > 80% 83%  
Blood Gases Rest - on air Exercise - on air Rest - 100% O2
Pa-O2 75 mmHg 61 mmHg 580 mmHg
Pa-CO2 38 mmHg 37 mmHg 40 mmHg
Ca-O2 20 ml O2/dl 17 ml O2/dl 24 ml O2/dl
Cv-O2 15 ml O2/dl 8 ml O2/dl 19 ml O2/dl
Diffusion Capacity Predicted Measured
DCO 22 ml/min/mmHg 11 ml/min/mmHg

A. Does this patient have restrictive pulmonary disease?

B. Does she have obstructive pulmonary disease?

C. Is she hypoventilating?

D. Does she have an unusually high AaDO2 at rest breathing air?

E. Is her AaDO2 primarily due to diffusion limitation, shunt, or ventilation to perfusion nonuniformity/mismatch?

F. Is she anemic?

G. Does she have stagnant hypoxia?

H. What is the major cause of her cyanosis on exercise?

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