NCLEX-RN cheat sheet · Fluids, electrolytes and acid-base
Acid-base and ABGs
Normal values, the four steps, compensation and the four imbalances
AI-checkedChecked by AI against current guidelines, with spot-checks by a registered nurse.
The lungs and kidneys share the job of acid-base balance. A blood gas looks like a jumble of numbers, but four steps, taken in the same order every time, make it readable.
Two printable pages, free. No sign-up.
At a glance
Normal adult blood gas
| Value | Normal | Managed by |
|---|---|---|
| pH | 7.35 to 7.45. Below is acidosis; above is alkalosis | Both |
| PaCO2 (acid) | 35 to 45 mmHg | Lungs: fast, by changing the breathing |
| HCO3 (base) | 22 to 26 mEq/L | Kidneys: slow, over hours to days |
| PaO2 | 80 to 100 mmHg | Lungs |
Follow policy: Critical values, and when to escalate at once, depend on your lab's policy and the client's condition; pH under 7.25 or over 7.60 is not a universal critical value. Severe acid, like a pH near 7.10, is life-threatening.
Four steps, same order every time
| Step | What it means |
|---|---|
| 1. pH | Acidosis under 7.35, or alkalosis over 7.45? |
| 2. PaCO2 | Moves opposite to the pH? The lungs are the cause: respiratory |
| 3. HCO3 | Moves the same way as the pH? The cause is metabolic |
| 4. Compensation | Check it (table below) |
Memory aid: ROME: respiratory opposite, metabolic equal. A high CO2 is acid; a high HCO3 is base.
Compensation
| Read it as | pH | Cause value | Compensating value |
|---|---|---|---|
| Uncompensated | Abnormal | Abnormal | Still normal |
| Partially compensated | Still abnormal | Abnormal | Shifted to pull the pH back toward normal |
| May be fully compensated, or a mixed disorder: compare with the expected response | Normal | Abnormal | Abnormal |
Speed: The lungs compensate quickly for metabolic problems; the kidneys take longer for respiratory ones.
The four imbalances
| Imbalance | Gas | Common causes | Clues | Care |
|---|---|---|---|---|
| Respiratory acidosis | Low pH, high PaCO2: breathing too little | Oversedation, asthma attack, COPD, fluid in the lungs | Headache, sleepiness, then confusion, seizures, coma. Oxygen saturation can stay normal | Improve the breathing: clear the airway, reverse oversedation, nebulizers as ordered |
| Respiratory alkalosis | High pH, low PaCO2: breathing too much | Anxiety, panic, pain, fear, head injury, early asthma or a clot in the lung; early aspirin overdose | Dizziness, chest tightness, tingling, palpitations | Treat the cause, reassure, coach slow belly breathing: in about 4 seconds, out about 8. No paper bag |
| Metabolic acidosis | Low pH, low HCO3 | DKA, lactic acid from poor oxygen delivery, severe diarrhea, kidney disease | Fast, deep breathing can occur, but isn't always the first clue. Confusion, low pressure | Notify the provider quickly. Treat the cause: fluids, glucose control, circulatory support |
| Metabolic alkalosis | High pH, high HCO3 | Vomiting, NG suction, diuretics, too much bicarbonate, low potassium | Breathing slows to hold CO2; confusion can occur | Stop the suction, diuretic or bicarbonate if possible; correct a low potassium |
Read these gases
| Client and gas | Read it as |
|---|---|
| pH 7.30, PaCO2 52 mmHg, HCO3 24 mEq/L | pH acid, CO2 high (opposite), HCO3 normal: uncompensated respiratory acidosis |
| pH 7.25, PaCO2 30 mmHg, HCO3 14 mEq/L | pH acid, HCO3 low (same way), CO2 low but pH still off: partially compensated metabolic acidosis |
| Client with COPD: pH 7.38, PaCO2 56 mmHg, HCO3 32 mEq/L | pH normal on the acid side, both values off: fits fully compensated respiratory acidosis, if it matches the expected response |
| Drowsy after morphine: pH 7.29, PaCO2 58 mmHg, HCO3 25 mEq/L | Low pH, high CO2, normal HCO3: uncompensated respiratory acidosis |
| Prolonged NG suction: pH 7.50, PaCO2 44 mmHg, HCO3 32 mEq/L | High pH, high HCO3, normal CO2, from losing stomach acid: uncompensated metabolic alkalosis |
Priority and safety calls
- ★ Sleepy or confused after sedation? Rising CO2 can cause it, and the oxygen saturation can stay normal. Improve the breathing and reverse oversedation.
- Coach deep breaths and coughing; if that fails, expect a noninvasive ventilation mask, or intubation.
- ★ Some clients with COPD have chronic, compensated respiratory acidosis, tolerated because it built up slowly; many have a normal CO2. A sudden rise is the danger: sleepiness can start with a mild rise, so report new drowsiness at once.
- Hyperventilation: no paper bag; it can drop the oxygen dangerously. If severe symptoms continue, a small dose of a short-acting benzodiazepine may be prescribed.
- Early asthma or a clot in the lung can start as alkalosis, then turn to acidosis as the breathing muscles tire.
- Metabolic acidosis with confusion or low pressure: notify the provider quickly, so a blood gas is drawn and the cause treated.
- ★ Before a radial blood gas, do the Allen test: press both the radial and ulnar arteries, release the ulnar; color and warmth should return to the hand.
How the NCLEX tests it
- Normal pH 7.35 to 7.45; PaCO2 35 to 45 mmHg; HCO3 22 to 26 mEq/L.
- Four steps: pH, CO2, HCO3, compensation. Respiratory opposite, metabolic equal.
- Trap: reading CO2 backwards. A high CO2 is acid.
- Trap: calling a normal pH normal when both other values are off.
- Classic stems: oversedation (respiratory acidosis), DKA with fast, deep breathing (metabolic acidosis), vomiting or suction (metabolic alkalosis).
- A panicking client with tingling hands: respiratory alkalosis. Coach slow breathing, not a paper bag.
Practice it next
This sheet comes from Acid-base balance and blood gases in our NCLEX-RN course. Try free practice questions, or see the lessons it was written from.
Quick self-quiz
- Normal arterial pH is 7.35 to ___.
Show answer
7.45 - Normal PaCO2 is 35 to ___ mmHg.
Show answer
45 - Normal HCO3 is 22 to ___ mEq/L.
Show answer
26 - ROME: respiratory opposite, metabolic ___.
Show answer
equal - Before a radial blood gas, do the ___ test.
Show answer
Allen - Prolonged NG suction causes metabolic ___.
Show answer
alkalosis
Sources
- Open RN, Nursing Fundamentals 2e, 15.5 Acid-Base Balance
- Fundamentals of Arterial Blood Gas Interpretation (PubMed Central)
More fluids, electrolytes and acid-base cheat sheets
Educational use only. This sheet is a study aid for the NCLEX-RN, not a substitute for clinical judgment, your facility’s policy or the provider’s orders. Where practice varies, follow facility policy and state law. NCLEX® and NCLEX-RN® are registered trademarks of NCSBN, which does not endorse these materials.