CCP Blood Gas & Acid-Base Interpretation 2 — Questions and Answers
Question 1: A patient on CPB has a PaCO2 of 35 mmHg at 37°C (alpha-stat). If body temperature is actually 28°C, what is the temperature-corrected PaCO2?
- Approximately 24 mmHg (Correct answer)
- Approximately 35 mmHg
- Approximately 46 mmHg
- Approximately 55 mmHg
Correct answer: Approximately 24 mmHg
CO2 solubility increases with cooling, so temperature-corrected PaCO2 is lower (~24 mmHg) than the 37°C measurement in alpha-stat management.
Question 2: Which buffer system provides the most immediate response to an acute acid load in the blood?
- Phosphate buffer system
- Protein buffer system
- Bicarbonate-carbonic acid system (Correct answer)
- Hemoglobin buffer system
Correct answer: Bicarbonate-carbonic acid system
The bicarbonate-carbonic acid system is the most important extracellular buffer and responds immediately to acute acid loads due to its open system with pulmonary CO2 elimination.
Question 3: A blood gas shows pH 7.52, PaCO2 28 mmHg, HCO3- 22 mEq/L. What is the primary disorder?
- Metabolic alkalosis
- Respiratory alkalosis with metabolic compensation
- Respiratory alkalosis uncompensated (Correct answer)
- Mixed alkalosis
Correct answer: Respiratory alkalosis uncompensated
Low PaCO2 with high pH and near-normal HCO3- indicates uncompensated respiratory alkalosis; compensation would have lowered bicarbonate further.
Question 4: During hypothermic CPB at 20°C using pH-stat management, the perfusionist adds CO2 to the oxygenator. What is the primary intended effect?
- Increase cerebral vasoconstriction
- Maintain temperature-corrected pH at 7.40 and PaCO2 at 40 mmHg (Correct answer)
- Reduce cerebral oxygen consumption
- Decrease arterial oxygen tension
Correct answer: Maintain temperature-corrected pH at 7.40 and PaCO2 at 40 mmHg
pH-stat management adds CO2 to correct for hypothermia-induced alkalosis, maintaining temperature-corrected pH 7.40 and PaCO2 40 mmHg.
Question 5: In a patient with chronic COPD, a blood gas shows pH 7.36, PaCO2 60 mmHg, HCO3- 33 mEq/L. This represents:
- Acute respiratory acidosis
- Chronic respiratory acidosis with full metabolic compensation (Correct answer)
- Combined respiratory and metabolic acidosis
- Metabolic alkalosis with respiratory compensation
Correct answer: Chronic respiratory acidosis with full metabolic compensation
Elevated PaCO2 with proportionally elevated HCO3- and near-normal pH indicates chronic respiratory acidosis with full renal compensation.
Question 6: The Bohr effect describes which phenomenon important during CPB?
- Increased CO2 shifts the oxyhemoglobin dissociation curve left
- Increased CO2 and acidosis shift the oxyhemoglobin dissociation curve right (Correct answer)
- Hypothermia increases hemoglobin's oxygen affinity shifting the curve left
- Alkalosis decreases hemoglobin's oxygen affinity shifting the curve right
Correct answer: Increased CO2 and acidosis shift the oxyhemoglobin dissociation curve right
The Bohr effect states that increased CO2 and acidosis decrease hemoglobin's oxygen affinity, shifting the oxyhemoglobin dissociation curve rightward and facilitating oxygen unloading.
Question 7: A post-CPB patient has pH 7.30, PaCO2 40 mmHg, HCO3- 19 mEq/L, lactate 6 mmol/L. The expected respiratory compensation (Winter's formula) for this metabolic acidosis would be:
- PaCO2 of 24-28 mmHg
- PaCO2 of 30-34 mmHg (Correct answer)
- PaCO2 of 35-39 mmHg
- PaCO2 of 40-44 mmHg
Correct answer: PaCO2 of 30-34 mmHg
Winter's formula: expected PaCO2 = 1.5 × HCO3- + 8 ± 2 = (1.5 × 19) + 8 ± 2 = 36.5 ± 2; the measured PaCO2 of 40 mmHg is higher, suggesting a concurrent respiratory acidosis.
CCP Blood Gas & Acid-Base Interpretation 2
A patient on CPB has a PaCO2 of 35 mmHg at 37°C (alpha-stat).
If body temperature is actually 28°C, what is the temperature-corrected PaCO2?