How do you read an arterial blood gas step by step?
NCLEX-RN topic: Fluid and electrolyte imbalances
- Read pH first: below 7.35 is acidosis, above 7.45 is alkalosis.
- ROME: Respiratory Opposite, Metabolic Equal names the cause.
- The lungs adjust CO2 within minutes; the kidneys adjust HCO3 slowly.
- Normal pH with abnormal PaCO2 and HCO3 means full compensation.
- Vomiting and gastric suction cause alkalosis; diarrhea causes acidosis.
Key facts
- Normal ABG ranges used here: pH 7.35–7.45, PaCO2 35–45 mm Hg, HCO3 22–26 mEq/L, PaO2 80–100 mm Hg.
- ROME: Respiratory Opposite (PaCO2 moves opposite to pH), Metabolic Equal (HCO3 moves the same way as pH).
- The lungs adjust CO2 within minutes; the kidneys adjust HCO3 slowly, so respiratory problems take longer to compensate.
Show all 7 key factsShow fewer
- Uncompensated: pH abnormal and only one of PaCO2 or HCO3 abnormal. Partially compensated: pH and both values abnormal. Fully compensated: pH normal, both values abnormal.
- Respiratory acidosis comes from hypoventilation (opioids, sedation, COPD, asthma attack); respiratory alkalosis comes from hyperventilation (anxiety, pain, fever, ventilator settings).
- Metabolic acidosis comes from ketoacidosis, lactic acidosis, severe diarrhea or kidney disease; metabolic alkalosis comes from vomiting, gastric suction, diuretics, hypokalemia or excess bicarbonate.
- Rising CO2 first causes headache, sluggishness or anxiety; higher or fast-rising levels cause confusion, then seizures and coma.
Quick check1 question · 2 more at the end
A client who has had severe diarrhea for 3 days has these arterial blood gas results: pH 7.29 (7.35–7.45), PaCO2 30 mm Hg (35–45 mm Hg), HCO3 16 mEq/L (22–26 mEq/L). How should the nurse interpret these results?
Correct answer: C.
Start with the pH: 7.29 is acidosis. HCO3 is low and moves in the same direction as the pH, so the cause is metabolic (Metabolic Equal); severe diarrhea removes bicarbonate. PaCO2 is also low because the lungs are blowing off CO2 to compensate, but the pH is still abnormal, so the compensation is partial.
A Respiratory acidosis would need a PaCO2 above 45 mm Hg; here PaCO2 is low.
B Uncompensated would mean PaCO2 is still normal; here it is already below 35 mm Hg.
C Correct: pH and HCO3 are both low (metabolic acidosis), and the low PaCO2 shows the lungs are compensating while the pH is still abnormal.
D Full compensation would bring the pH back into the 7.35–7.45 range.
Full lesson
Each part starts with its main idea. Open a part when you want the details.
The short answer: Check pH first (7.35–7.45): low is acidosis, high is alkalosis. Then find the cause. If PaCO2 (35–45 mm Hg) moves opposite to pH, the problem is respiratory; if HCO3 (22–26 mEq/L) moves the same way as pH, it is metabolic. Finally decide whether the other value is compensating.
1 What does each ABG value tell you?An arterial blood gas shows how acidic the blood is and whether the lungs or kidneys are causing it. 1 min
An arterial blood gas shows how acidic the blood is and whether the lungs or kidneys are causing it. Ranges vary slightly between labs, so always use the range printed on the report.
| Value | Normal | What it reflects |
|---|---|---|
| pH | 7.35–7.45 | Below 7.35 = acidosis; above 7.45 = alkalosis |
| PaCO2 | 35–45 mm Hg | CO2 is an acid controlled by the lungs |
| HCO3 | 22–26 mEq/L | Bicarbonate is a base controlled by the kidneys |
| PaO2 | 80–100 mm Hg | Oxygen dissolved in arterial blood |
The lungs change CO2 quickly by changing the rate and depth of breathing. The kidneys change HCO3 slowly by keeping or excreting bicarbonate.
2 How do you name the imbalance in three steps?Read the values in the same order every time so you never skip a step. 1 min
Read the values in the same order every time so you never skip a step.
- pH: below 7.35 is acidosis; above 7.45 is alkalosis.
- Find the cause (ROME): if PaCO2 is abnormal and moves opposite to the pH, the cause is respiratory. If HCO3 is abnormal and moves the same way as the pH, the cause is metabolic.
- Compensation: look at the other value.
- Uncompensated: pH abnormal, other value still normal.
- Partially compensated: pH abnormal, other value moving out of range in the correcting direction.
- Fully compensated: pH back in range, both PaCO2 and HCO3 abnormal.
Example: pH 7.31, PaCO2 33, HCO3 17. The pH is low, HCO3 is low (same direction), and PaCO2 is low to compensate, so this is partially compensated metabolic acidosis.
3 What causes each imbalance and how does it look?Each imbalance has a short list of typical causes that you can match to the client's story. 1 min
Each imbalance has a short list of typical causes that you can match to the client's story.
| Imbalance | Typical causes | Clues at the bedside |
|---|---|---|
| Respiratory acidosis | Opioids or sedatives, anesthesia, COPD, asthma attack, pulmonary edema | Slow or shallow breathing, headache, drowsiness, confusion |
| Respiratory alkalosis | Anxiety, pain, fever, head injury, ventilator rate too high, early asthma or pulmonary embolism | Fast breathing, light-headedness, numb or tingling hands, palpitations |
| Metabolic acidosis | Ketoacidosis, lactic acidosis from poor perfusion, severe diarrhea, kidney disease | Rapid deep breathing, confusion, hypotension |
| Metabolic alkalosis | Vomiting, gastric suction, diuretics, hypokalemia, too much bicarbonate | Slow breathing, confusion |
4 What the nurse doesThe nurse treats the client and the cause, not just the number on the report. 1 min
The nurse treats the client and the cause, not just the number on the report.
- Assess: airway, breathing and circulation first, then respiratory rate and depth, level of consciousness, vital signs and intake and output. Early CO2 retention can show only as headache, sluggishness or anxiety.
- Priority action: report an abnormal ABG or signs of worsening right away, and start the treatment that fixes the cause.
- By imbalance:
- Respiratory acidosis: improve ventilation (cough and deep breathing, reverse oversedation as prescribed, nebulizers, BiPAP or CPAP); intubation if these fail.
- Respiratory alkalosis: treat the cause (pain, anxiety), reassure the client and coach slow abdominal breathing with a longer exhale.
- Metabolic acidosis: IV fluids, glucose control and circulatory support as prescribed; IV sodium bicarbonate is often used when pH falls below 7.1.
- Metabolic alkalosis: stop the cause (treat vomiting, stop suction or diuretics as prescribed) and correct low potassium.
- Evaluate: improvement means ABG and electrolytes returning to normal, a regular breathing pattern and adequate oxygenation.
Common mistakes
The traps students fall into most. Read these before you practice.
Watch out: Vomiting and gastric suction cause alkalosis, not acidosis; diarrhea causes acidosis.
Watch out: A normal pH with abnormal PaCO2 and HCO3 is full compensation, not a normal ABG.
Watch out: For hyperventilation, coach slow abdominal breathing, not a paper bag; rebreathing is no longer recommended because it can cause hypoxemia.
Watch out: Rapid breathing in metabolic acidosis is the lungs compensating, not the cause of the problem.
How to remember it
One short rule and one picture of the stomach cover most ABG questions.
Memory hook: "ROME, then look at the partner." Respiratory Opposite, Metabolic Equal names the cause; the partner value tells you the compensation. Vomiting and gastric suction drain acid, so they push toward alkalosis; diarrhea drains bicarbonate, so it pushes toward acidosis.
Self-check:
- Interpret pH 7.48, PaCO2 30, HCO3 24.
- Explain why a client with COPD can have a normal pH with a high PaCO2.
- Name two causes of metabolic alkalosis and one nursing action for each.
Show answers
- Uncompensated respiratory alkalosis.
- pH 7.48 is above 7.45, so it is alkalosis.
- PaCO2 30 is low and moves opposite to the pH, so the cause is respiratory.
- HCO3 24 is still normal, so the kidneys have not compensated yet.
- The kidneys have fully compensated.
- COPD can cause hypoventilation, so CO2 builds up (respiratory acidosis).
- The kidneys slowly keep more bicarbonate, so HCO3 rises above 26.
- pH back in range with both values abnormal is fully compensated respiratory acidosis.
- Any two of these:
- Vomiting: treat the vomiting as prescribed.
- Gastric suction: stop the suction as prescribed.
- Diuretics: stop the diuretic as prescribed.
- Hypokalemia: correct the low potassium as prescribed.