PROBLEM SESSION ANSWERS: Problem 8

Step 1: Evaluate respiratory ventilation mechanics:

Restrictive disease: Yes, because low Total Lung Capacity (all lung volumes are low) Obstructive disease: No, because FEV-1 / VC is ³ 80%

Does her restrictive disease contribute to her problem? Not directly (but see below>, because she is not hypoventilating at rest ( her P_A-CO2 is not above normal )

Step 2: Evaluate her AaDO₂ on air at rest:

P_a-O2 = 75 mmHg (measured)

P_A-O2 = P_I-O2 - P_A-CO2/ R = 150 - 38 / 0.8 = 150 - 47.5 = 102.5 mmHg

AaDO2 = P_A-O2 - P_a-O2 = 102.5 - 75 = 27.5 mmHg

Conclusion: AaDO2 is much higher than normal, so at least one of the following must be present: (1) diffusion limitation, (2) shunt, (3) ventilation / perfusion nonuniformity.

Step 3: Evaluate the three AaDO2 possibilities:

Diffusion limitation: D_CO is only about half the predicted value, so D_O2 is about half its normal value also. But it would have to be less than one-third its normal value to account for hypoxia at rest, so low DO2 is not the primary cause in this case, although it may contribute to the hypoxia in exercise when the time available for equilibration in the lungs is less.

Shunt: Note that when breathing pure O₂, P_a-O2 is slightly larger than 570 mmHg, so shunt does not contribute.

Ventilation / Perfusion Nonuniformity: Since the other two possible causes are ruled out, this must be the problem. One would have to perform direct tests in order to be more specific (washout, scans, etc.).

Step 4: Evaluate anemia

At rest breathing air, blood O₂ content is normal even though P_a-O2 is below normal. Also, when breathing pure O₂, her C_a-O2 is above the expected level. So she is actually slightly polycythemic.

Question: What might have caused the polycythemia?

Step 5: Evaluate stagnant hypoxia

C_a-O2 - C_v-O2 = 50 ml O2/L at rest, both breathing air and breathing oxygen.

Since this difference is normal, she does not have stagnant hypoxia (inadequate cardiac output)

Question: Does her larger extraction during exercise indicate a cardiac limitation? Answer: No, a larger extraction is normal during exercise.

Cause of her cyanosis on exercise:

Probably mainly due to more severe V'_A/Q mismatch, made worse by the low DO2. It is not due to hypoventilation, as determined from P_a-CO2.