Klimek Acid-Base Made Simple: ABGs, ROME and Ventilators for the NCLEX
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Acid-base is Lecture 1 for a reason. It is the topic students fear most and the one that shows up across the entire NCLEX: respiratory, cardiac, renal, endocrine, post-op. Get it automatic and you free up mental space for everything else.
Here is the whole thing, taught the way that actually sticks.
Step 1: ROME
Respiratory Opposite, Metabolic Equal.
- Respiratory: pH and CO2 move in opposite directions.
- Metabolic: pH and bicarbonate move in the same direction.
Three moves and you are done: look at the pH (acidosis or alkalosis), look at CO2, look at HCO3. Whichever value moved the way ROME predicts tells you the cause.
The normal values you must know cold
| Value | Normal range | What it means |
|---|---|---|
| pH | 7.35 to 7.45 | Below is acidosis, above is alkalosis |
| PaCO2 | 35 to 45 mmHg | The respiratory number (lungs) |
| HCO3 | 22 to 26 mEq/L | The metabolic number (kidneys) |
| PaO2 | 80 to 100 mmHg | Oxygenation, separate question |
Memory hook for the middle two: CO2 runs 35-45, bicarb runs 22-26. Note that reference ranges vary slightly by laboratory.
The four imbalances
Respiratory acidosis (pH down, CO2 up)
The patient is not blowing off CO2. Think anything that slows breathing: opioid overdose, COPD exacerbation, atelectasis, pneumonia, oversedation, neuromuscular weakness, airway obstruction.
Nursing priority: improve ventilation. Position, encourage deep breathing, suction, prepare for possible mechanical support.
Respiratory alkalosis (pH up, CO2 down)
The patient is blowing off too much CO2. Think hyperventilation: anxiety and panic, pain, fever, early sepsis, pulmonary embolism, mechanical over-ventilation, high altitude.
Nursing priority: slow the breathing and treat the cause. Coach breathing, address pain and anxiety, and rule out the dangerous causes rather than assuming panic.
Metabolic acidosis (pH down, HCO3 down)
Acid is being produced or bicarb is being lost. Think DKA, lactic acidosis, shock, renal failure, severe diarrhoea, salicylate toxicity.
Memory hook: the body loses bicarb from below. Diarrhoea equals acidosis.
Metabolic alkalosis (pH up, HCO3 up)
Acid is being lost or bicarb gained. Think vomiting, prolonged nasogastric suction, excessive antacids, diuretics, hypokalaemia.
Memory hook: acid is lost from above. Vomiting equals alkalosis.
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Vomiting vs diarrhoea: the trick they love
The exam will hand you a patient with days of vomiting or days of diarrhoea and ask what you expect on the ABG. One line settles it:
Vomiting loses acid, so the patient goes alkalotic. Diarrhoea loses bicarb, so the patient goes acidotic.
Say it out loud twice. That single sentence is worth points every single exam.
Compensation, without the confusion
The body tries to fix itself. Lungs compensate fast (minutes to hours), kidneys compensate slowly (hours to days).
- Uncompensated: pH abnormal, only one system has moved.
- Partially compensated: pH still abnormal, but both CO2 and HCO3 have shifted.
- Fully compensated: pH back in the normal range, but both values are still abnormal.
On a fully compensated gas, decide the original problem by which side of 7.40 the pH sits on. Below 7.40 the primary problem was acidosis. Above 7.40 it was alkalosis.
Ventilator questions
Ventilator items look intimidating and are usually simple safety questions in disguise.
- High-pressure alarm means something is blocking flow: secretions, kinked tubing, biting, coughing, bronchospasm. Suction and check the tubing.
- Low-pressure alarm means a leak or disconnection. Reconnect and check the cuff.
- Never silence an alarm without assessing the patient first.
- If you cannot fix the problem, manually ventilate with a bag-valve mask and call for help. You never leave the patient to go find someone.
Notice how the never-ever rules do the work here. Assess the patient, not the machine.
Practise the pattern, not the paragraph
Reading this page will not make you fast. Doing 20 ABGs in a row will. Force yourself to call each one within five seconds, then check. Speed on acid-base buys you time later in the exam when the case studies get long.
The NCLEX FastTrack All-in-One Bundle pairs the lectures and full PDF library with a 3,000-question NGN-ready bank, including acid-base case studies and bow-tie items with full rationales, in a day-by-day plan. $169, lifetime access, Pass Guarantee.
See what's inside FastTrack →Frequently asked questions
What is the easiest way to interpret ABGs on the NCLEX?
Use ROME: Respiratory Opposite, Metabolic Equal. Check the pH for acidosis or alkalosis, then see whether CO2 moved opposite to the pH (respiratory) or bicarbonate moved with it (metabolic).
Does vomiting cause acidosis or alkalosis?
Alkalosis. Vomiting loses gastric acid, raising the pH. Diarrhoea does the opposite, losing bicarbonate and causing metabolic acidosis.
What does a high-pressure ventilator alarm mean?
Something is obstructing airflow: secretions, kinked tubing, biting, or bronchospasm. Assess the patient, suction if indicated, and check the tubing. Never silence an alarm before assessing.
Which Klimek lecture covers acid-base?
Lecture 1, which also covers ventilators. It is widely considered the highest-value hour in the entire series because acid-base reasoning appears across every body system on the exam.
You've got this. Learn ROME, drill 20 gases, and acid-base stops being scary.
Nurse June, BSN, RN
Founder, Your Nursing Space
