Acid-Base Balance Nursing Guide 2026: ABG Interpretation, ROME Mnemonic, and Clinical Application

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Acid-base balance is the physiological foundation of everything in acute care nursing. Respiratory failure, kidney disease, diabetic emergencies, shock, overdose, severe vomiting, diarrhea — all produce acid-base disturbances that nurses must recognize, interpret, and respond to. Arterial blood gas (ABG) interpretation is not an ICU-only skill; any nurse who cares for acutely ill patients must understand what pH, PaCO2, and bicarbonate values mean and what clinical action they require. This guide builds a systematic, step-by-step approach to ABG interpretation that works for every clinical scenario.

Acid-base balance refers to the body's maintenance of blood pH within the narrow normal range of 7.35 to 7.45. The body's acid-base state is primarily regulated by the lungs (controlling CO2 levels through ventilation) and the kidneys (controlling bicarbonate [HCO3-] levels through retention or excretion). When either system fails or is overwhelmed, acid-base disturbances occur.

ABG Normal Values: The Reference Foundation

Parameter	Normal Range	What It Measures
pH	7.35–7.45	Blood acidity/alkalinity; below 7.35 = acidosis; above 7.45 = alkalosis
PaCO2	35–45 mmHg	Arterial CO2 pressure; the respiratory component; CO2 is an acid — high CO2 = more acid; regulated by the lungs
HCO3- (bicarbonate)	22–26 mEq/L	Serum bicarbonate; the metabolic component; HCO3- is a base — low HCO3- = less base = more acidic; regulated by the kidneys
PaO2	80–100 mmHg	Arterial oxygen pressure; reflects oxygenation (not acid-base directly, but critical for clinical context); below 60 mmHg = hypoxemia requiring intervention
SaO2	95–100%	Arterial oxygen saturation on hemoglobin; reflects functional oxygenation

The 5-Step ABG Interpretation Method

Step 1: Assess the pH — Is the patient acidotic or alkalotic?

pH below 7.35 = acidosis. pH above 7.45 = alkalosis. pH 7.35–7.45 = normal (but compensation may still be occurring — don't stop at Step 1).

Step 2: Assess the PaCO2 — Is the respiratory system contributing to the problem?

PaCO2 above 45 = hypoventilation → respiratory acidosis (CO2 retention). PaCO2 below 35 = hyperventilation → respiratory alkalosis (CO2 blowing off). PaCO2 35–45 = normal respiratory contribution.

Step 3: Assess the HCO3- — Is the metabolic system contributing to the problem?

HCO3- below 22 = metabolic acidosis (bicarbonate deficit). HCO3- above 26 = metabolic alkalosis (bicarbonate excess). HCO3- 22–26 = normal metabolic contribution.

Step 4: Determine which system matches the pH abnormality

The PRIMARY problem is the system whose abnormality matches the pH direction. If pH is low (acidosis) AND PaCO2 is high (respiratory acidosis direction): primary problem is respiratory acidosis. If pH is low (acidosis) AND HCO3- is low (metabolic acidosis direction): primary problem is metabolic acidosis. If BOTH are abnormal in the same direction, the problem is mixed.

Step 5: Assess for compensation

Compensation is the opposite system's response to restore pH toward normal. Compensation is never complete — the pH moves toward normal but doesn't normalize unless the underlying problem resolves. Partial compensation: pH still abnormal but moving toward normal. Full compensation: pH within normal range despite abnormal CO2 and/or HCO3-.

The Four Primary Acid-Base Disturbances

Disturbance	pH	Primary Problem	Common Causes	Compensation
Respiratory Acidosis	Below 7.35	PaCO2 above 45 (CO2 retention; hypoventilation)	COPD exacerbation, opioid overdose (respiratory depression), severe asthma, neuromuscular disease, mechanical ventilation settings, sleep apnea	Kidneys retain HCO3- to buffer the acid; HCO3- rises above 26 with compensation; takes hours to days
Respiratory Alkalosis	Above 7.45	PaCO2 below 35 (CO2 loss; hyperventilation)	Anxiety/hyperventilation, mechanical ventilation (overventilation), pregnancy (progesterone-driven hyperventilation), fever, pain, early sepsis (respiratory compensation for metabolic acidosis)	Kidneys excrete HCO3-; HCO3- falls below 22 with compensation; takes hours to days
Metabolic Acidosis	Below 7.35	HCO3- below 22 (bicarbonate loss or acid accumulation)	DKA, lactic acidosis (sepsis, shock), renal failure, salicylate overdose, severe diarrhea (HCO3- loss in stool), methanol/ethylene glycol poisoning	Lungs increase ventilation to blow off CO2; PaCO2 falls below 35 with compensation; begins within minutes, maximum effect in hours
Metabolic Alkalosis	Above 7.45	HCO3- above 26 (bicarbonate gain or acid loss)	Prolonged vomiting (HCl loss), nasogastric suctioning (HCl removal), diuretic use (loop and thiazide diuretics cause Cl- and H+ loss), excessive antacid use, hypokalemia (K+ and H+ are exchanged; hypokalemia drives H+ into cells and HCO3- increases)	Lungs hypoventilate to retain CO2; PaCO2 rises above 45 with compensation; less reliable than metabolic compensation

The ROME Mnemonic: Respiratory Opposite, Metabolic Equal

ROME: Respiratory Opposite — In respiratory disturbances, pH and PaCO2 move in opposite directions. Respiratory acidosis: pH ↓, PaCO2 ↑ (opposite). Respiratory alkalosis: pH ↑, PaCO2 ↓ (opposite). Metabolic Equal — In metabolic disturbances, pH and HCO3- move in the same direction. Metabolic acidosis: pH ↓, HCO3- ↓ (equal/same). Metabolic alkalosis: pH ↑, HCO3- ↑ (equal/same). ROME identifies which system caused the primary problem by which direction the abnormal parameter matches the pH change.

Clinical Examples: Applying the 5 Steps

Example 1: ABG: pH 7.22, PaCO2 58, HCO3- 25. Step 1: pH 7.22 = acidosis. Step 2: PaCO2 58 = elevated = respiratory acidosis direction. Step 3: HCO3- 25 = normal. Step 4: pH matches PaCO2 direction = primary respiratory acidosis. Step 5: HCO3- is normal = uncompensated respiratory acidosis. Clinical picture: likely acute (no time for renal compensation); immediate concern for respiratory failure — causes include opioid effect, acute COPD, neuromuscular failure. This patient may need ventilatory support.

Example 2: ABG: pH 7.32, PaCO2 30, HCO3- 15. Step 1: pH 7.32 = acidosis. Step 2: PaCO2 30 = low = respiratory alkalosis direction. Step 3: HCO3- 15 = low = metabolic acidosis direction. Step 4: pH is acidic; HCO3- is low (metabolic acidosis direction) = primary metabolic acidosis. Step 5: PaCO2 30 (low) = respiratory compensation — lungs are hyperventilating to blow off CO2 to compensate for the metabolic acidosis = partially compensated metabolic acidosis. Clinical picture: consider DKA, lactic acidosis (sepsis/shock), renal failure. The respiratory compensation is intact (lungs working to buffer) but incomplete.

Nursing Priorities for Acid-Base Disturbances

For any significant acid-base disturbance, nursing priorities are: notify the provider; identify the underlying cause (assess clinical context — is this a DKA patient? A patient on opioids? A post-cardiac arrest patient?); address the underlying cause with ordered interventions; support ventilation if respiratory acidosis is severe (position patient, prepare for potential intubation, withhold respiratory depressants); monitor electrolytes (acidosis and alkalosis both cause electrolyte shifts — particularly potassium); repeat ABG per provider order to assess response to treatment.

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