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:

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

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The ROME Mnemonic

ROME is the most efficient way to interpret ABGs:

Respiratory Opposite

• In respiratory disorders, pH and CO2 move in opposite directions
• pH ↓ + CO2 ↑ = Respiratory Acidosis
• pH ↑ + CO2 ↓ = Respiratory Alkalosis

Metabolic Equal

• In metabolic disorders, pH and HCO3 move in the same direction
• pH ↓ + HCO3 ↓ = Metabolic Acidosis
• pH ↑ + HCO3 ↑ = Metabolic Alkalosis

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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

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The Four Primary Acid-Base Disorders

1. Respiratory Acidosis (pH ↓, CO2 ↑)

Cause: Hypoventilation - the lungs cannot eliminate CO2

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

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:

• Methanol ingestion
• Uremia (kidney failure)
• Diabetic ketoacidosis (DKA)
• Propylene glycol, Paraldehyde
• Isoniazid, Iron, Infection (sepsis)
• Lactic acidosis
• Ethylene glycol
• Salicylates (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 for severe acidosis (pH < 7.1)
• Monitor potassium (may shift with pH changes)

4. Metabolic Alkalosis (pH ↑, HCO3 ↑)

Cause: Loss of acid or excess bicarbonate

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

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ABG Interpretation Table

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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.

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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

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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)