Six Gases, Sixty Seconds
A six-card gas bank in a three by two grid, each card carrying four hard-aligned stacked values and an empty answer strip along its foot, with a plain timing dial to the right drawn as a circle with one hand and four tick marks at fifteen-second intervals.
Clock rule: sixty seconds for the matrix, then check. Second pass target is forty seconds.
GAS BANK
| pH | PaCO₂ | HCO₃⁻ | PaO₂ | |
|---|---|---|---|---|
| A | 7.26 | 58 mm Hg | 25 mEq/L | 68 mm Hg |
| B | 7.29 | 28 mm Hg | 13 mEq/L | 94 mm Hg |
| C | 7.43 | 31 mm Hg | 20 mEq/L | 90 mm Hg |
| D | 7.53 | 40 mm Hg | 32 mEq/L | 92 mm Hg |
| E | 7.40 | 40 mm Hg | 24 mEq/L | 52 mm Hg |
| F | 7.24 | 22 mm Hg | 9 mEq/L | 96 mm Hg |
Gas F chemistry line: sodium 136 mEq/L, chloride 96 mEq/L, bicarbonate 9 mEq/L.
MATRIX ITEM. One mark per column, per row.
| Gas | Respiratory / Metabolic / Neither | Acidosis / Alkalosis / Neither | Uncompensated / Partly / Fully / Not applicable |
|---|---|---|---|
| A | ☐ ☐ ☐ | ☐ ☐ ☐ | ☐ ☐ ☐ ☐ |
| B | ☐ ☐ ☐ | ☐ ☐ ☐ | ☐ ☐ ☐ ☐ |
| C | ☐ ☐ ☐ | ☐ ☐ ☐ | ☐ ☐ ☐ ☐ |
| D | ☐ ☐ ☐ | ☐ ☐ ☐ | ☐ ☐ ☐ ☐ |
| E | ☐ ☐ ☐ | ☐ ☐ ☐ | ☐ ☐ ☐ ☐ |
| F | ☐ ☐ ☐ | ☐ ☐ ☐ | ☐ ☐ ☐ ☐ |
DROP-DOWN ITEM. Gas C is best described as a [ 1 ], and its pH tells the nurse that the compensating system has [ 2 ]. Slot 1: fully compensated respiratory alkalosis · uncompensated metabolic acidosis · partly compensated respiratory acidosis · a normal gas with no acid-base disorder. Slot 2: returned the pH to the reference range · overcorrected the pH past 7.40 · failed to respond at all · not yet begun to respond.
TREND ITEM. Gas A was drawn at 0900 and gas E at 1300 on the same patient. Is this patient improving? State the value that decides your answer.
CONDENSED RATIONALES (cover with your hand while working)
A — Low pH, high PaCO₂, normal HCO₃⁻. Uncompensated respiratory acidosis. The kidneys have not started. B — Low pH, low HCO₃⁻, low PaCO₂. Partly compensated metabolic acidosis. Lungs working, pH not yet home. C — Normal pH above 7.40, low PaCO₂, low HCO₃⁻. Fully compensated respiratory alkalosis. The pH names the primary. D — High pH, high HCO₃⁻, normal PaCO₂. Uncompensated metabolic alkalosis. Nothing is compensating yet. E — Every acid-base value normal. No disorder. The PaO₂ of 52 mm Hg is critical, so oxygenation is the problem. F — Partly compensated metabolic acidosis, and the gap is 136 − (96 + 9) = 31 mEq/L, so unmeasured acid. Trend — No. The pH corrected while the PaO₂ fell from 68 to 52 mm Hg. Normal pH is not the same as normal patient. Drop-down — Slot 1: fully compensated respiratory alkalosis. Slot 2: returned the pH to the reference range.
Margin notenormal pH is not the same as normal patient.
Watch outa fully compensated gas can still carry a lethal PaO₂. Read the fourth value even when the first three have resolved.
☐ Pass one, sixty seconds ☐ Pass two, forty seconds
Takeawaycompensation status is a third question, and skipping it is how a correctly named disorder still becomes a wrong answer.
Reference ranges vary between institutions and reference laboratories. Follow local protocol.