PaO2, SaO2 and Oxygen Content

Answers to Clinical Problems - Chapter 5

Lawrence Martin, M.D.

Clinical Problem 5-1.

b) PaO2 unchanged, SaO2 unchanged, CaO2 reduced.

Hemoglobin content is suddenly reduced by half, which will lower CaO2 by half. However, the PaO2 and SaO2 will be unaffected, since their values are independent of the content of hemoglobin present.

Clinical Problem 5-2.

Of the choices given only answer e), lung disease with intra-pulmonary shunting, would be expected to lower PaO2. The other choices represent changes in hemoglobin content and binding and should not (by themselves) lower PaO2.

Clinical Problem 5-3.

Hemoglobin 12 gms%, PaO2 50 mm Hg, pH 7.40. To calculate oxygen content, you first need to find the SaO2. From Figure 5-3, you can see that SaO2 is about 83% (in using the graph ±1% is acceptable). Ignoring the dissolved oxygen fraction (which is very small), the O2 content is

CaO2 = .83 x 12 x 1.34 = 13.35 ml O2/dl

Note that this content falls midway between the oxygen content values for hemoglobin of 10 and 15 gms%.

Clinical Problem 5-4.

The calculation is the same as with the two beakers, except that PO2 is 100 mm Hg instead of 160 mm Hg (PO2 is lower than atmospheric pressure because of the addition of water vapor pressure and PaCO2); thus his dissolved fraction is .3 ml O2/dl instead of .48 ml O2/dl in the beakers. His O2-bound fraction is also slightly lower because a PO2 of 100 mm Hg gives an SaO2 of about 98%. Thus the oxygen content in human blood under these conditions is:

CaO2 = (Hb x 1.34 x SaO2) + (.003 x PaO2)

= (15 x 1.34 x .98) + (.003 x 100)

= 19.7 + .3

= 20.0 ml O2/dl

The dissolved O2 content is .3/20 = 1.5% of the total oxygen content. Stated another way, under these conditions (normal PaO2 and hemoglobin content), hemoglobin carries about 67 times more oxygen than is carried dissolved in the plasma. Clearly, hemoglobin is vital. Under conditions of ambient air and pressure, the content of dissolved oxygen is far too little to meet our metabolic needs.

Clinical Problem 5-5.
 CONDITION PaO2 SaO2 CaO2 Severe Anemia normal normal decreased CO Poisoning normal decreased decreased Severe V-Q decreased decreased decreased High Altitude decreased decreased decreased

Clinical Problem 5-6.

The body needs oxygen molecules, so oxygen content (CaO2) takes precedence over partial pressure in determining degrees of hypoxemia. In this problem the amount of oxygen contributed by the dissolved fraction is negligible and will not affect the answer.

Patient A

CaO2 = .95 x 7 x 1.34 = 8.9 ml O2/dl

Patient B

CaO2 = .85 x 15 x 1.34 = 17.1 ml O2/dl

Patient A, with the higher PaO2, is more hypoxemic.

Clinical Problem 5-7.

a. True

b. True

c. False

d. False

e. False

f. False

g. True

h. False