Acid-Base Balance

Acid-Base Balance

Arterial blood gas (ABG) interpretation is a core nursing competency tested heavily on the NCLEX. Understanding acid-base physiology allows nurses to quickly identify imbalances, anticipate complications, and intervene effectively. This guide provides a systematic approach to ABG analysis along with clinical applications. For comprehensive physiological review, see the Physiological Integrity study guide.

Normal ABG Values

Component	Normal Range	Reflects
pH	7.35 - 7.45	Overall acid-base status
PaCO2	35 - 45 mmHg	Respiratory component (regulated by the lungs)
HCO3 (Bicarbonate)	22 - 26 mEq/L	Metabolic component (regulated by the kidneys)
PaO2	80 - 100 mmHg	Oxygenation status
SaO2	95% - 100%	Oxygen saturation

Key principle: pH below 7.35 is acidosis; pH above 7.45 is alkalosis. PaCO2 and pH have an inverse relationship (when CO2 goes up, pH goes down). HCO3 and pH have a direct relationship (when HCO3 goes up, pH goes up).

Step-by-Step ABG Interpretation (The Tic-Tac-Toe Method)

Use this systematic approach for every ABG question:

1. Step 1: Evaluate the pH. Is it acidotic (less than 7.35), alkalotic (greater than 7.45), or normal (7.35-7.45)? If normal, determine which side of 7.40 it falls on to identify the primary disorder in compensated states
2. Step 2: Evaluate the PaCO2. Is it high (greater than 45, indicating respiratory acidosis), low (less than 35, indicating respiratory alkalosis), or normal?
3. Step 3: Evaluate the HCO3. Is it high (greater than 26, indicating metabolic alkalosis), low (less than 22, indicating metabolic acidosis), or normal?
4. Step 4: Match the abnormal value to the pH. The component (respiratory or metabolic) that matches the direction of the pH change is the primary disorder. The opposing component, if abnormal, represents compensation
5. Step 5: Determine compensation. Is the body compensating? If both the respiratory and metabolic components are abnormal, the system opposing the primary disorder is attempting to compensate

Respiratory Acidosis

ABG pattern: pH less than 7.35, PaCO2 greater than 45 mmHg

Mechanism: The lungs fail to eliminate adequate CO2 (hypoventilation), causing carbonic acid buildup.

Causes:

• COPD, asthma, pneumonia, pulmonary edema
• Respiratory depression from opioids, sedatives, or anesthesia
• Chest trauma, pneumothorax
• Neuromuscular diseases (Guillain-Barre, myasthenia gravis)
• Airway obstruction

Signs: Dyspnea, headache, confusion, drowsiness, tachycardia, dysrhythmias

Treatment: Improve ventilation (bronchodilators, suctioning, positioning, mechanical ventilation if needed), reverse respiratory depressants with naloxone if opioid-related. In COPD patients, administer low-flow oxygen (1-2 L/min) to avoid suppressing hypoxic drive.

Respiratory Alkalosis

ABG pattern: pH greater than 7.45, PaCO2 less than 35 mmHg

Mechanism: Excessive CO2 elimination through hyperventilation.

Causes:

• Anxiety, pain, fear
• Fever, sepsis
• Hypoxia (early response)
• Mechanical overventilation
• Salicylate (aspirin) overdose (early phase)

Signs: Lightheadedness, numbness and tingling (paresthesias), confusion, tachypnea, tetany

Treatment: Address the underlying cause. For anxiety-related hyperventilation, encourage slow breathing techniques. Adjust ventilator settings if mechanically ventilated. Treat underlying infection or pain.

Metabolic Acidosis

ABG pattern: pH less than 7.35, HCO3 less than 22 mEq/L

Mechanism: Bicarbonate loss or excess acid accumulation.

Causes (MUDPILES mnemonic):

• M - Methanol ingestion
• U - Uremia (renal failure)
• D - Diabetic ketoacidosis
• P - Propylene glycol
• I - Isoniazid, Iron
• L - Lactic acidosis (shock, hypoxia, exercise)
• E - Ethylene glycol ingestion
• S - Salicylate (aspirin) overdose (late phase)

Other causes include severe diarrhea (bicarbonate loss) and renal tubular acidosis.

Signs: Kussmaul respirations (deep, rapid breathing -- respiratory compensation), fatigue, confusion, nausea, warm flushed skin, headache

Treatment: Treat the underlying cause, administer IV sodium bicarbonate for severe acidosis (pH less than 7.1), correct dehydration, insulin for DKA.

Metabolic Alkalosis

ABG pattern: pH greater than 7.45, HCO3 greater than 26 mEq/L

Mechanism: Excess bicarbonate or loss of hydrogen ions.

Causes:

• Prolonged vomiting or nasogastric suctioning (loss of HCl)
• Excessive sodium bicarbonate administration
• Diuretic therapy (loop and thiazide diuretics cause potassium and hydrogen ion loss)
• Hyperaldosteronism
• Excessive antacid use

Signs: Confusion, tremors, muscle cramps, hypoventilation (respiratory compensation), tingling, dysrhythmias (often associated with hypokalemia)

Treatment: Replace potassium and chloride (isotonic saline), discontinue offending medications, administer antiemetics for vomiting, acetazolamide may be used to increase bicarbonate excretion.

Compensation Mechanisms

The body attempts to normalize pH through compensation:

• Respiratory compensation (for metabolic disorders): Occurs within minutes to hours. The lungs adjust ventilation rate and depth to retain or blow off CO2
• Metabolic (renal) compensation (for respiratory disorders): Occurs over hours to days. The kidneys adjust reabsorption or excretion of bicarbonate and hydrogen ions

Status	pH	Primary Abnormality	Compensating Value
Uncompensated	Abnormal	Abnormal	Normal
Partially compensated	Abnormal	Abnormal	Abnormal (moving toward compensation)
Fully compensated	Normal (but leaning toward one side of 7.40)	Abnormal	Abnormal

Mixed Acid-Base Disorders

When two primary disorders occur simultaneously, both PaCO2 and HCO3 move in the same direction relative to the pH. For example, a patient with COPD (respiratory acidosis) who develops renal failure (metabolic acidosis) will have severely depressed pH with both elevated PaCO2 and decreased HCO3. Recognition of mixed disorders requires clinical correlation with the patient's history.

Nursing Interventions Summary

Imbalance	Priority Intervention
Respiratory acidosis	Improve ventilation: suction, position upright, bronchodilators, possible intubation
Respiratory alkalosis	Reduce hyperventilation: treat anxiety, adjust ventilator, treat underlying cause
Metabolic acidosis	Treat cause: insulin for DKA, fluids for lactic acidosis, sodium bicarbonate for severe cases
Metabolic alkalosis	Replace electrolytes (K+, Cl-), stop offending agents, administer isotonic saline

NCLEX ABG Focus Points

• Always follow a systematic approach to interpret ABGs -- never guess
• Kussmaul respirations are the hallmark compensation for metabolic acidosis
• COPD patients on chronic respiratory acidosis: use caution with high-flow oxygen
• Vomiting causes metabolic alkalosis; diarrhea causes metabolic acidosis
• The MUDPILES mnemonic is a high-yield recall tool for metabolic acidosis causes
• Full compensation returns pH to normal but does not correct the underlying values

Use the Acid-Base Balance Cheat Sheet for quick ABG reference during your review. Build confidence with Physiological Integrity flashcards and explore key terms in the Physiological Integrity Glossary.