Question 1 · Hard
A 58-year-old man in the ICU is sedated and paralyzed, and his ventilator delivers a fixed minute ventilation that is not changed during the events below. He has a severe lactic acidosis. Arterial results before treatment: pH 7.12, pCO2 38 mm Hg [5.1 kPa], HCO3- 12 mmol/L, pO2 96 mm Hg [12.8 kPa]. He is given three 50-mL ampules of 8.4% sodium bicarbonate (150 mmol) as a rapid IV push. Fifteen minutes later, from the same arterial line: pH 7.16, pCO2 52 mm Hg [6.9 kPa], HCO3- 18 mmol/L, pO2 94 mm Hg [12.5 kPa]. By the Henderson-Hasselbalch relationship, the main reason the pH rose so little is:
- AThe second sample was venous, not arterial
- BCO2 from the buffered bicarbonate was not exhaled
- CThe bicarbonate moved straight into the cells
- DMinute ventilation fell after the dose
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Answer: B · CO2 from the buffered bicarbonate was not exhaled
pH depends on the ratio of HCO3- to dissolved CO2 (0.03 x pCO2), not on bicarbonate alone. When bicarbonate buffers hydrogen ions it forms carbonic acid, which becomes CO2 and water; normally the lungs exhale this CO2, which is what makes bicarbonate an effective open buffer. With ventilation fixed, the extra CO2 stayed in the blood: pCO2 rose from 38 to 52 mm Hg, so the ratio rose only from about 10.5 (12/1.14) to 11.5 (18/1.56), and pH only from 7.12 to 7.16. The unchanged pO2 from the same arterial line rules out a venous sample, and the measured 6 mmol/L rise in bicarbonate shows it did not simply leave the plasma. When bicarbonate is given to a patient who cannot raise his own breathing, ventilation has to be increased to clear the CO2.
- A: The pO2 from the same arterial line is unchanged, which a venous sample would not show.
- C: Plasma bicarbonate rose by 6 mmol/L, so the dose did not simply vanish into cells.
- D: The ventilator settings were fixed, so ventilation did not change.