P01.001 Acid-base abnormality

Acid-base balance is crucial for normal physiological function. Understanding the principles of acid-base disturbances is essential for the PLAB exam. Here’s a concise guide with tables and key points to remember:

Basic Concepts

Normal Arterial Blood Gas (ABG) Values:

• pH: 7.35 – 7.45
• PaCO2: 35 – 45 mmHg
• HCO3- (Bicarbonate): 22 – 26 mEq/L

Types of Acid-Base Disorders

Common Causes

Diagnostic Approach

Step-by-Step Analysis:

• Evaluate pH: Determine if the blood is acidotic (<7.35) or alkalotic (>7.45).
• Identify Primary Disorder:
  • Look at PaCO2 and HCO3- to identify if the primary disorder is respiratory or metabolic.
  • Respiratory disorders are indicated by abnormal PaCO2.
  • Metabolic disorders are indicated by abnormal HCO3-.
• Check Compensation: Determine if the body is compensating appropriately (e.g., metabolic compensation for respiratory disorders, and vice versa).

Key Points to Remember

• Compensation Mechanisms: The body uses respiratory compensation for metabolic disorders and renal compensation for respiratory disorders. Full compensation is rare; typically, it’s partial.
• Anion Gap (AG): Useful in metabolic acidosis.
  • AG = Na+ – (Cl- + HCO3-)
  • Normal AG: 8 – 12 mEq/L
  • Increased AG: Suggests addition of unmeasured acids (e.g., ketoacidosis, lactic acidosis).
  • Normal AG Metabolic Acidosis: Usually due to bicarbonate loss (e.g., diarrhea, renal tubular acidosis).

Example Calculation for Anion Gap

• Patient’s Electrolytes: Na+ = 140, Cl- = 100, HCO3- = 15
• AG = 140 – (100 + 15) = 25 (Increased AG, indicating a high anion gap metabolic acidosis)

Sample ABG Interpretation

Example 1:

• ABG: pH 7.30, PaCO2 50 mmHg, HCO3- 24 mEq/L
• Interpretation: Respiratory acidosis (elevated PaCO2 with low pH)

Example 2:

• ABG: pH 7.50, PaCO2 30 mmHg, HCO3- 22 mEq/L
• Interpretation: Respiratory alkalosis (low PaCO2 with high pH)

Example 3:

• ABG: pH 7.28, PaCO2 40 mmHg, HCO3- 16 mEq/L
• Interpretation: Metabolic acidosis (low HCO3- with low pH)