9.2 Arterial Blood Gases (ABGs) and Acid-Base Balance

NCLEX-RN Registered Nurse

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9 Chapter 9: Physiological Adaptation

9.1 Fluid Imbalances (Hypovolemia and Hypervolemia) 9.2 Arterial Blood Gases (ABGs) and Acid-Base Balance 9.3 Medical Emergencies: Stroke (CVA) 9.4 Medical Emergencies: Myocardial Infarction (MONA) 9.5 Diabetic Emergencies (Hypoglycemia and DKA) 9.6 Respiratory Conditions (COPD, Pneumonia) 9.7 Cardiac Conditions (Heart Failure) 9.8 Neurological Conditions (Seizures)

9.2 Arterial Blood Gases (ABGs) and Acid-Base Balance

Key Takeaways Explain This Topic AI

Normal ABG values: pH 7.35-7.45, PaCO2 35-45 mmHg, HCO3 22-26 mEq/L, PaO2 80-100 mmHg

ROME mnemonic: Respiratory Opposite (pH and CO2 move opposite), Metabolic Equal (pH and HCO3 move same direction)

Respiratory acidosis (pH down, CO2 up) results from hypoventilation; treat by improving ventilation

Metabolic acidosis (pH down, HCO3 down) results from acid accumulation or bicarbonate loss; treat underlying cause

• Compensation occurs when the unaffected system adjusts to normalize pH
• Last updated: October 2026

Arterial Blood Gases and Acid-Base Balance

Understanding arterial blood gas (ABG) interpretation is a critical nursing skill. Acid-base imbalances can be life-threatening, and rapid identification enables prompt intervention.

Normal ABG Values

Memorize these normal ranges - they form the foundation of ABG interpretation:

• Parameter Normal Range What It Measures
• pH 7.35 - 7.45 Acidity/alkalinity of blood
• PaCO2 35 - 45 mmHg Respiratory component (acid)
• HCO3 22 - 26 mEq/L Metabolic component (base)
• PaO2 80 - 100 mmHg Oxygen level
• SaO2 95 - 100% Oxygen saturation

Memory Tip: The number 7.4 is the "perfect" pH. Below 7.35 is acidosis; above 7.45 is alkalosis.

The ROME Mnemonic

ROME is the most efficient way to interpret ABGs:

• R espiratory O pposite
• In respiratory disorders, pH and CO2 move in opposite directions
• pH ↓ + CO2 ↑ = Respiratory Acidosis
• pH ↑ + CO2 ↓ = Respiratory Alkalosis
• M etabolic E qual
• In metabolic disorders, pH and HCO3 move in the same direction
• pH ↓ + HCO3 ↓ = Metabolic Acidosis
• pH ↑ + HCO3 ↑ = Metabolic Alkalosis
• Step-by-Step ABG Interpretation
• Step 1: Look at the pH

Is it acidic (< 7.35) or alkalotic (> 7.45)?

• If 7.35-7.45, it may be compensated or normal
• Step 2: Determine the primary cause
• Check CO2 (respiratory component)
• Check HCO3 (metabolic component)

Which value matches the pH direction using ROME?

Step 3: Check for compensation

Is the other system trying to correct the imbalance?

• If pH is abnormal but both systems are abnormal = partial compensation
• If pH is normal and both systems are abnormal = full compensation
• The Four Primary Acid-Base Disorders
• 1. Respiratory Acidosis (pH ↓, CO2 ↑)
• Cause: Hypoventilation - the lungs cannot eliminate CO2

• Causes Examples
• CNS Depression Opioid overdose, sedatives, anesthesia
• Neuromuscular Guillain-Barre, myasthenia gravis
• Airway Obstruction COPD, asthma, foreign body
• Pulmonary Pneumonia, pulmonary edema, ARDS

Signs and Symptoms:

• Confusion, drowsiness, lethargy
• Headache
• Tachycardia
• Warm, flushed skin

Nursing Interventions:

• Improve ventilation (position upright, encourage deep breathing)
• Administer bronchodilators if bronchospasm present
• Prepare for intubation if severe
• Administer Naloxone for opioid overdose
• 2. Respiratory Alkalosis (pH ↑, CO2 ↓)
• Cause: Hyperventilation - excessive CO2 elimination

• Causes Examples
• Anxiety/Panic Most common cause
• Pain Acute pain response
• Hypoxia Body compensates by breathing faster
• Fever/Sepsis Increased metabolic demand
• Mechanical Ventilation Rate set too high

Signs and Symptoms:

• Lightheadedness, dizziness
• Tingling in extremities (paresthesias)
• Muscle twitching, tetany
• Confusion

Nursing Interventions:

• Treat underlying cause
• Coach slow, controlled breathing for anxiety
• Rebreather mask or paper bag (use with caution)
• Adjust ventilator settings if applicable
• 3. Metabolic Acidosis (pH ↓, HCO3 ↓)
• Cause: Accumulation of acid or loss of bicarbonate

MUDPILES Mnemonic for Causes:

• M ethanol ingestion
• U remia (kidney failure)
• D iabetic ketoacidosis (DKA)
• P ropylene glycol, Paraldehyde
• I soniazid, Iron, Infection (sepsis)
• L actic acidosis
• E thylene glycol
• S alicylates (aspirin overdose)

Other Causes:

• Severe diarrhea (loss of bicarbonate)
• Renal tubular acidosis

Signs and Symptoms:

• Kussmaul respirations (deep, rapid breathing - compensatory)
• Confusion, lethargy
• Nausea, vomiting
• Warm, flushed skin
• Fruity breath (DKA)

Nursing Interventions:

Treat underlying cause (insulin for DKA, dialysis for renal failure)

IV sodium bicarbonate only for severe acidosis, as ordered (in DKA, routine bicarbonate is not recommended; it is considered only if pH < 7.0)

• Monitor potassium (may shift with pH changes)
• 4. Metabolic Alkalosis (pH ↑, HCO3 ↑)
• Cause: Loss of acid or excess bicarbonate

• Causes Examples
• GI Acid Loss Vomiting, NG suction
• Diuretic Use Loop and thiazide diuretics
• Excess Alkali Antacid overuse, IV bicarbonate
• Hypokalemia Causes hydrogen ion shift into cells

Signs and Symptoms:

• Confusion, irritability
• Muscle twitching, tremors
• Numbness, tingling
• Hypoventilation (compensatory)
• Cardiac arrhythmias

Nursing Interventions:

• Replace fluids and electrolytes (especially potassium and chloride)
• Discontinue or reduce diuretics
• Administer antiemetics for vomiting
• 0.9% NS helps correct chloride-responsive alkalosis
• ABG Interpretation Table

• Disorder pH PaCO2 HCO3
• Respiratory Acidosis ↓ (< 7.35) ↑ (> 45) Normal → ↑
• Respiratory Alkalosis ↑ (> 7.45) ↓ (< 35) Normal → ↓
• Metabolic Acidosis ↓ (< 7.35) Normal → ↓ ↓ (< 22)
• Metabolic Alkalosis ↑ (> 7.45) Normal → ↑ ↑ (> 26)
• Compensation Explained

The body attempts to restore normal pH through compensation:

Respiratory Compensation for Metabolic Disorders:

• Occurs within minutes to hours
• Metabolic acidosis → Lungs blow off CO2 (Kussmaul respirations)
• Metabolic alkalosis → Lungs retain CO2 (hypoventilation)

Metabolic Compensation for Respiratory Disorders:

• Takes 24-48 hours (kidney response is slower)
• Respiratory acidosis → Kidneys retain HCO3
• Respiratory alkalosis → Kidneys excrete HCO3

Exam Tip: If the pH is NORMAL but both CO2 and HCO3 are abnormal, the patient has fully compensated . Look at the value that is MORE abnormal to determine the primary disorder.

• Practice ABG Interpretation
• Example 1: pH 7.28, CO2 58, HCO3 24
• pH is LOW (acidosis)
• CO2 is HIGH (opposite of pH = respiratory)
• HCO3 is normal
• Answer: Uncompensated Respiratory Acidosis
• Example 2: pH 7.32, CO2 30, HCO3 16
• pH is LOW (acidosis)
• HCO3 is LOW (same direction = metabolic)
• CO2 is LOW (respiratory trying to compensate by blowing off CO2)
• Answer: Partially Compensated Metabolic Acidosis
• Key Points for the NCLEX
• Always look at pH first to determine acidosis or alkalosis
• Use ROME to identify respiratory vs. metabolic cause

Kussmaul respirations = deep, rapid breathing seen in metabolic acidosis (the body is trying to blow off CO2 to compensate)

• Opioid overdose causes respiratory acidosis - give Naloxone
• Vomiting and NG suction cause metabolic alkalosis (losing stomach acid)
• Diarrhea causes metabolic acidosis (losing bicarbonate)