CCP Blood Gas & Acid-Base Interpretation 3 — Questions and Answers
Question 1: Which of the following causes a LEFT shift of the oxyhemoglobin dissociation curve, increasing hemoglobin's affinity for oxygen?
- Hyperthermia
- Alkalosis and hypothermia (Correct answer)
- Acidosis and hypercarbia
- Increased 2,3-DPG
Correct answer: Alkalosis and hypothermia
Alkalosis and hypothermia both increase hemoglobin's affinity for oxygen, shifting the dissociation curve to the left and impairing oxygen release to tissues.
Question 2: A perfusionist notes SvO2 dropping from 75% to 55% during stable CPB. Without changing pump flow, which blood gas change would MOST likely explain this?
- Rising PaO2 from 200 to 300 mmHg
- Falling hemoglobin from 9 to 6 g/dL (Correct answer)
- Rising pH from 7.38 to 7.45
- Falling PaCO2 from 40 to 30 mmHg
Correct answer: Falling hemoglobin from 9 to 6 g/dL
A fall in hemoglobin reduces oxygen delivery (DO2 = CO × CaO2), forcing tissues to extract more oxygen and lowering SvO2.
Question 3: What is the anion gap in a patient with Na+ 140, Cl- 102, HCO3- 18 mEq/L?
- 8 mEq/L
- 16 mEq/L
- 20 mEq/L (Correct answer)
- 24 mEq/L
Correct answer: 20 mEq/L
Anion gap = Na+ − (Cl- + HCO3-) = 140 − (102 + 18) = 20 mEq/L, which is elevated above the normal range of 8–12 mEq/L.
Question 4: During CPB, a blood gas taken from the arterial line shows PaO2 450 mmHg. The perfusionist should:
- Increase FiO2 to maximize oxygen delivery
- Reduce FiO2 on the oxygenator to avoid hyperoxia-related free radical damage (Correct answer)
- Immediately increase pump flow
- Add bicarbonate to normalize oxygen content
Correct answer: Reduce FiO2 on the oxygenator to avoid hyperoxia-related free radical damage
Excessive hyperoxia during CPB generates reactive oxygen species that contribute to reperfusion injury; FiO2 should be reduced to maintain PaO2 in an appropriate range (150-200 mmHg).
Question 5: A patient presents with pH 7.48, PaCO2 48 mmHg, HCO3- 35 mEq/L after prolonged NG suction. This is BEST described as:
- Respiratory alkalosis with metabolic compensation
- Metabolic alkalosis with partial respiratory compensation (Correct answer)
- Combined alkalosis
- Metabolic alkalosis without compensation
Correct answer: Metabolic alkalosis with partial respiratory compensation
Loss of gastric HCl causes metabolic alkalosis; the elevated PaCO2 represents partial respiratory compensation (hypoventilation) to retain CO2.
Question 6: Base excess (BE) in a blood gas report indicates:
- The amount of CO2 that needs to be removed to normalize pH
- The amount of acid or base (in mEq/L) needed to restore pH to 7.40 at normal PaCO2 (Correct answer)
- The difference between measured and predicted bicarbonate
- The total buffer capacity of whole blood
Correct answer: The amount of acid or base (in mEq/L) needed to restore pH to 7.40 at normal PaCO2
Base excess is the amount of strong acid (if positive) or strong base (if negative) required to restore 1 liter of blood to a pH of 7.40 at PaCO2 of 40 mmHg and 37°C.
Question 7: A blood gas during CPB shows PaO2 of 80 mmHg on FiO2 1.0. This most likely indicates:
- Normal oxygenator function
- Significant oxygenator failure or severe shunting (Correct answer)
- Hypothermia-induced CO2 retention
- Adequate but conservative oxygen delivery
Correct answer: Significant oxygenator failure or severe shunting
PaO2 of only 80 mmHg on 100% FiO2 indicates a large intrapulmonary shunt or oxygenator failure; expected PaO2 should exceed 400 mmHg on FiO2 1.0.
CCP Blood Gas & Acid-Base Interpretation 3
Which of the following causes a LEFT shift of the oxyhemoglobin dissociation curve, increasing hemoglobin's affinity for oxygen?