Introduction to Acid-Base Disorders

Supporting Information

• Support: Subscribe, like, comment, and check links in the description for notes and illustrations.

Key Concepts

• Acidosis: Process by which protons ( H+ ) are increased.
  • Acidemia: Blood pH < 7.35
• Alkalosis: Process by which protons are decreased.
  • Alkalemia: Blood pH > 7.45
• Key Formula: pH = -log[ H+ ]
  • High H+ → low pH → acidic
  • Low H+ → high pH → alkaline

Influencing Molecules

• Carbon Dioxide (CO2) and Bicarbonate ( HCO3- ) also affect pH.
  • Carbonic Anhydrase enzyme catalyzes CO2 + H2O ↔ H2CO3

Modified Henderson-Hasselbalch Equation

• pH = HCO3- / pCO2
• Respiratory Disorders: Change in pCO2
  • Respiratory Acidosis: High pCO2 → Low pH
    • Causes: CNS depression, neuromuscular issues, COPD
  • Respiratory Alkalosis: Low pCO2 → High pH
    • Causes: CNS stimulation, hypoxemia
• Metabolic Disorders: Change in bicarbonate ( HCO3- )
  • Metabolic Acidosis: Low HCO3- → Low pH
    • Causes: Increased H+ or loss of HCO3-

Anion Gap

• Equation: Na+ - (Cl- + HCO3- )
• Normal AGMA: Anion gap ≤ 12
  • Causes: HARDUP mnemonic
• Elevated AGMA: Anion gap > 12
  • Causes: MUDPILES mnemonic
• Delta Ratio: Determines mixed disorders
  • (<1) Pure AGMA
  • (1-2) Mixed AGMA and NAGMA
  • (>2) Mixed AGMA and Metabolic Alkalosis

Consequences: Acidosis vs. Alkalosis

• Cardiovascular System
  • Acidosis: Decreased contractility, hypotension, arrhythmias, resistance to pressors
  • Alkalosis: Vasoconstriction, arrhythmias
• Respiratory Effects
  • Acidosis: Hyperventilation, potential fatigue
  • Alkalosis: Hypoventilation, hypoxemia
• Metabolic Effects
  • Acidosis: Hyperkalemia, insulin resistance
  • Alkalosis: Hypokalemia, hypomagnesemia, low ionized calcium
• CNS Effects
  • Acidosis: Altered mental status, coma
  • Alkalosis: Altered mental status, seizures, tetany

Compensation Mechanisms

• Kidneys: Compensate for respiratory disorders (takes hours to days)
  • Respiratory Acidosis: ↑ H+ excretion, ↑ HCO3- reabsorption
  • Respiratory Alkalosis: ↓ HCO3- reabsorption, ↓ H+ excretion
• Respiratory System: Compensates for metabolic disorders (takes minutes)
  • Metabolic Acidosis: ↑ respiratory rate to ↓ pCO2
  • Metabolic Alkalosis: ↓ respiratory rate to ↑ pCO2

Interpreting ABGs

• Step-by-Step Process
  • 1. Check pH: Determine if acidemia or alkalemia
  • 2. Check pCO2 and HCO3- : Determine if primary disorder is respiratory or metabolic
  • 3. Calculate Anion Gap in metabolic acidosis
  • 4. Calculate Delta Ratio if AGMA to check for mixed disorders
• Normal Values
  • pH: 7.35 - 7.45
  • pCO2: 35 - 45 mmHg
  • HCO3-: 22 - 26 mEq/L
  • Anion Gap: ≤ 12

Practice Problems

• Identify the primary disorder
• Check for compensation
• Calculate anion gap if needed.