Which of the following variables can be calculated from measured ...

[BRWH]

Which of the following variables can be calculated from measured arterial blood gas values? 1. The partial pressure of oxygen at which hemoglobin is 50 saturated (P50) 2. pH 3. PCO2 4. HCO3 5. SaO2
 
  a. 1, 3, and 4 c. 1, 4, and 5
  b. 2, 4, and 5 d. 2 and 3

Question 2

A P50 measurement refers to which of the following?
 
  a. The amount of oxygen diffusion in fluids
  b. The PO2 when hemoglobin is 50 satu-rated with oxygen
  c. The partial pressure of fetal hemoglobin at 50 saturation
  d. The partial pressure of oxygen consumed by the oxygen electrode

[Anna]

Answer to Question 1

ANS: C
The P50 can be derived by using the oxyhemoglobin dissociation curve to look for the PaO2 at 50 hemoglobin saturation. The HCO3 is derived from measurements of pH and PaCO2 by using the Henderson-Hasselbalch equation. The saturation of hemoglobin can be calculated by using an equation empirically derived from the oxyhemoglobin saturation curve. The pH is measured di-rectly by using a Sanz electrode, and the PaO2 is measured directly by using a Clark electrode.

Answer to Question 2

ANS: B
P50 is a convenient way of describing hemoglobin affinity for oxygen because it identifies the PO2 in millimeters of mercury when hemoglobin is 50 saturated with oxygen.

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