Directions: This quiz is designed to test your understanding of information in the review paper, The Four Most Important Equations in Clinical Practice.. For each of the following five numbered statements or questions, there are five lettered responses (a-e), each of which may be true or false. Circle the correct or true responses. Answers immediately follow the quiz.

1. Normal range for PaCO₂ is 35-45 mm Hg. A change in PaCO₂ from normal to 28 mm Hg means the subject

a) is hyperventilating.
b) has excess alveolar ventilation for the amount of CO₂ production.
c) must have hypoxia, anxiety, and/or metabolic acidosis.
d) must be breathing faster than normal.
e) must have acute respiratory alkalosis.

2. The arterial PO₂ is predicted to be reduced to some extent from

a) anemia.
b) ventilation-perfusion (V-Q) imbalance with an increase in the number of low V- Q units.
c) increased PCO₂, while the subject is breathing room air.
d) carbon monoxide poisoning.
e) altitude.

3. To obtain a reasonable idea of the acid-base state of a patient's blood, you would need to know the

a) pH and PaCO₂.
b) pH and PaO₂.
c) PaCO₂ and PaO₂.
d) PaCO₂ and HCO₃^-.
e) pH and SaO₂ (%saturation of hemoglobin with oxygen).

4. Arterial blood gas data (pH, PaCO₂, PaO₂, SaO₂) are related in some simple but important ways. Which of the following are valid relationships?

a) Alveolar PO₂ is related to PaCO₂ by the alveolar gas equation: as PaCO₂ goes up, alveolar PO₂ goes down.
b) PaO₂ is inversely related to blood pH: as pH goes up, PaO₂ goes down.
c) If PaCO₂ increases while HCO₃^- remains unchanged, pH always goes down.
d) PaO₂ is related to SaO₂ on a linear scale (i.e., a straight-line relationship).
e) The SaO₂ is related to hemoglobin-bound arterial oxygen content on a linear scale (i.e., a straight-line relationship).

5. There are some "truisms" in terminology and physiology for proper blood gas interpretation. They include which of the following?

a) "Hyperventilation" and "hypoventilation" are clinical terms, and are not diagnosed by arterial blood gases.
b) The alveolar-arterial PO₂ difference increases with age and with increase in the fraction of inspired oxygen.
c) The arterial PO₂ cannot go above 100 mm Hg while breathing room air at sea level.
d) A continuously negative alveolar-arterial PO₂ difference is incompatible with life.
e) If arterial pH is normal, the patient cannot have a clinically significant acid-base disorder.

A N S W E R S

1. a and b are true.

The patient may have hyperventilation from many causes (including voluntary hyperventilation). The subject may be breathing deeper than normal, rather than faster. And the subject may be hyperventilating to compensate for metabolic acidosis, which would not be a respiratory alkalosis.

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2. b, c and e are true; other responses are false.

Anemia and carbon monoxide poisoning do not affect PaO₂ (except when there is a ventilation-perfusion imbalance and some right to left shunting).

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3. a and d are true.

You need to know two of the three Henderson-Hasselbalch equation variables to assess acid-base status.

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4. a, c and e are true.

PaO₂ and pH are not related in any formal way. The relationship of PaO₂ to SaO₂ is sigmoid-shaped, not straight line.

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5. b and d are true.

Hyperventilation and hypoventilation are specifically not defined by clinical or bedside criteria, but by changes in PaCO₂. The PaO₂ can easily go above 100 mm Hg with hyperventilation and normal lungs. Arterial pH can be normal with two or more acid-base disorders occurring at the same time.

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