Acid-Base Balance and Blood Gas Interpretation

Introduction

• Importance: Critical for maintaining homeostasis in the body.
• Definitions:
  • Acids: Substances that give up a hydrogen ion.
  • Bases: Substances that accept a hydrogen ion.
  • pH: Measure of acidity or alkalinity; inverse relationship with H+ ions.

Acidosis and Alkalosis

• Acidosis: Accumulation of acid or loss of base.
• Alkalosis: Accumulation of base or loss of acid.

Homeostatic Processes

• Lungs: Control carbon dioxide (CO2) levels.
  • CO2 buildup increases carbonic acid, affecting pH and causing acidosis.
• Kidneys: Retain/excrete bicarbonate (HCO3).
• Buffer System: Quickly maintains proper pH through bicarbonate-carbonic acid ratio (20:1).
  • Maintains blood pH between 7.35 and 7.45.

Arterial Blood Gas (ABG)

• Purpose: Assesses acid-base balance and oxygenation status.
• Measured Components:
  • Arterial pH
  • PaCO2
  • Bicarbonate (HCO3)
  • PaO2
  • Oxygen saturation
  • Base excess
• Procedure: Blood drawn from radial, brachial, or femoral artery using a heparinized syringe, placed on ice, analyzed quickly.

Normal Values

• pH: 7.35-7.45
• PaCO2: 35-45 mmHg
• Bicarbonate (HCO3): 22-26 mEq/L
• PaO2: 80-100 mmHg
• SaO2: 95-100%

Interpreting ABGs

• Step 1: Check pH for acidosis (<7.35) or alkalosis (>7.45).
• Step 2: Assess PaCO2 for respiratory involvement.
• Step 3: Assess HCO3 for metabolic involvement.

Examples

• Respiratory Acidosis:
  • pH < 7.35, PaCO2 > 45, HCO3 normal
  • Indicates CO2 retention (hypoventilation, CNS depression).
• Respiratory Alkalosis:
  • pH > 7.45, PaCO2 < 35, HCO3 normal
  • Indicates excess CO2 excretion (hyperventilation, fever).
• Metabolic Acidosis:
  • pH < 7.35, PaCO2 normal, HCO3 < 22
  • Indicates low bicarbonate levels.
• Metabolic Alkalosis:
  • pH > 7.45, PaCO2 normal, HCO3 > 26
  • Indicates high bicarbonate levels.

Compensation

• Mechanism: Body attempts to normalize pH by compensating.
• Example: Metabolic acidosis with compensatory respiratory alkalosis.

Mnemonic: ROME

• Respiratory Opposite, Metabolic Equal
  • Respiratory involves CO2 moving opposite to pH.
  • Metabolic involves HCO3 moving in the same direction as pH.

Oxygenation

• PaO2: Indicates dissolved oxygen in blood; low values suggest respiratory dysfunction.
• SaO2: Indicates oxyhemoglobin saturation.

Conclusion

• Understanding acid-base balance and ABG interpretation is essential for evaluating patient homeostasis.
• Endnote: Watch additional videos for more information.