Win a Copy of All You Really Need to Know to Interpret Arterial Blood Gases, by Lawrence Martin, M.D.


The question below was posted Sunday, 9/24/00. It took a week for the winning answer to arrive. The winner of the blood gas book is:

Dusty Oliver
Green Valley, Arizona

The next question (with ABG book as prize) will be posted in November, and will not be about blood gases.

Here is the question, with answers.

PART I of 2-part Question.

You and your significant other take a hot air balloon ride one fine autumn morning. Standing in the balloon's open basket, you are enthralled as Jacques, the pilot and guide, points out features of the countryside from an altitude of 1000 feet. Twenty minutes into the ride, a strong updraft suddenly lifts your balloon higher and higher. Jacques tries to bring the balloon down by letting out some air, but finds the exhaust valve is "stuck" closed and won't open. The balloon goes ever higher.

At 18,000 feet (1/2 atmosphere) Jacques finally is able to open the air release valve, and just in time. By then you are cold, scared and feeling light headed. Just before descent starts from this altitude, what is your PaO2?

To answer this question assume that you maintain normal heart and lungs throughout your ordeal. Also, assume that at maximum altitude your: alveolar ventilation has doubled without changing metabolic CO2 production; respiratory quotient is one; body temperature is normal.

ANSWER

Calculate Alveolar PO2 (PAO2) as follows:

PAO2 = FIO2(B.P. - 47) - PCO2
(the respiratory quotient is given as one, so there is no 'fudge factor' for PCO2)

PAO2 = .21(380-47)-20
This part tripped up some people. The FIO2 is .21 throughout the breathable atmosphere. 18,000 feet altitude is 1/2 atmosphere (given), so the barometric pressure is 1/2 of 760 or 380 mm Hg. Also, water vapor pressure is body temperature-dependent, so is unchaged from sea level at 47 mm Hg. Finally, PCO2 is 1/2 normal, since alveolar ventilation has doubled but metabolic CO2 production is unchanged.

Thus, PAO2 = 49.93 or 50 mm Hg

Since heart and lungs are normal, then alveolar-arterial PO2 difference is normal, or about 10 mm Hg. Thus arterial PO2 (PaO2) is about 40 mm Hg. (Any value close to this was accepted.)

PART II of 2-part Question.

Of the following 5 statements, which combination of statements is true as you begin your descent?

a) Your pH is alkalemic.

b) Your bicarbonate is low.

c) The FIO2 is the same as at sea level.

d) Your alveolar PO2 is above normal.

e) Water vapor pressure in your airways is the same as at sea level.

CHOOSE THE CORRECT COMBINATION FROM THE LIST BELOW (only one of these is correct).

1) a, b

2) c, d

3) a, c

4) a, b, c

5) a, c, e

6) b, c, e

7) a, b, c, e,

8) a, b, c, d

9) a, b, d, e

10) all of the statements are true

ANSWER

7) a, b, c, e are all true. d of course is false, since the alveolar PO2 is only 50 mm Hg. Note that with hyperventilation, pH goes up AND bicarbonate falls; the latter is NOT a compensatory mechanism, but occurs simply from mass action of lowering PCO2. Bicarbonate falls about 2 mEq/L for every 10 mm Hg fall in PCO2.

Return to Introduction/ Return to Part I, Questions 1-30/ Go to Part II, Questions 31-60/ Answer Sheet

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