Quick facts
| Test | Range | Unit | Applies to |
|---|---|---|---|
| pH (arterial)[1] | 7.35-7.45 | No unit | Teaching table, population not stated |
| PaCO2 (carbon dioxide)[1] | 35-45 | mm Hg | Teaching table, population not stated |
| HCO3 (bicarbonate)[2] | 22-26 | mEq/L | Age 1 month and older |
| PaO2 (oxygen)[1] | 80-100 | mm Hg | Teaching table, population not stated |
| SaO2 (oxygen saturation)[3] | 95-100 | % | Adults |
Read pH first, then let PaCO2 and HCO3 name the cause
A normal oxygen saturation does not rule out carbon dioxide retention
- SaO2 can stay normal while carbon dioxide builds up, so a normal saturation should not decide whether an acid-base problem needs more assessment. With signs of rising carbon dioxide, assess airway, breathing and circulation and get help.[3]
- Opioids, sedatives, anesthesia and alcohol can reduce ventilation enough to cause respiratory acidosis, and so can COPD, an acute asthma attack or pulmonary edema.[3]
Check hand circulation before the draw and keep the sample bubble-free after it
- Before a wrist draw, the hand's circulation is tested: the wrist arteries are pressed to stop flow for a few seconds, then released to see how quickly flow returns. After the draw, pressure stays on the puncture site for at least 5 minutes.[5]
- UW's lab asks for a capped syringe with no air bubbles, sent to the lab immediately and analyzed within 30 minutes of collection.[6]
The Arterial blood gases flashcard deck covers what each ABG value measures. Find the deck in the free app preview.
Open the free app previewHow it shows up on nursing exams
- Work every ABG item in the same order: pH, then PaCO2, then HCO3, then compensation.[3]
- Fully compensated means the pH is back in range while both PaCO2 and HCO3 sit outside their ranges. Breathing changes compensate quickly for metabolic problems, while kidney compensation for respiratory problems is slower.[3]
- Use the reference values the question or your course gives. Labs set their own ranges, so the cutoffs can vary between tables.[4]
Practice question
A 58-year-old patient on a surgical unit has had nasogastric suction for 2 days after bowel surgery. Heart rate is 96/min, blood pressure 118/72 mm Hg, respiratory rate 16/min, and SpO2 96% on room air. The arterial blood gas shows pH 7.51, PaCO2 43 mm Hg, HCO3 34 mEq/L and PaO2 88 mm Hg. The reference ranges are pH 7.35 to 7.45, PaCO2 35 to 45 mm Hg and HCO3 22 to 26 mEq/L. Which acid-base imbalance do these results show?
- A. Respiratory acidosis
- B. Respiratory alkalosis
- C. Metabolic alkalosis
- D. Metabolic acidosis
Show the answer and why
Correct answer: C. Metabolic alkalosis
- A. Incorrect. The pH is above 7.45, not below 7.35, and PaCO2 is not elevated, so this is not an acidosis.
- B. Incorrect. The pH is high, but PaCO2 43 mm Hg is inside its 35 to 45 range. A respiratory alkalosis needs a low PaCO2 pushing the pH up.
- C. Correct. The pH is above 7.45 and HCO3 is above 26, so pH and HCO3 rose together: metabolic. Losing stomach acid through nasogastric suction is a known cause of metabolic alkalosis.
- D. Incorrect. HCO3 is high, not low, and the pH is above 7.45. Metabolic acidosis needs a pH below 7.35 with a low HCO3.
Sources for this question: [1][2][3]
Memory hook: ROME: Respiratory Opposite, Metabolic Equal. pH first, every time.