Acid-Base Analysis Overview

Overview

This lecture synthesizes the traditional approach to acid-base analysis, summarizing the key steps, concepts, and algorithms needed to diagnose complex acid-base disorders using arterial blood gases (ABGs).

ABG Basics and Parameters

• ABG measures five key parameters: pH, PaCO₂, PaO₂, HCO₃⁻ (bicarbonate), and O₂ saturation.
• The relationship among pH, PaCO₂, and HCO₃⁻ is defined by the Henderson-Hasselbalch equation.

Acid-Base Disorders: Definitions and Types

• Acidemia: pH < 7.35; Alkalemia: pH > 7.45.
• Acidosis/Alkalosis: Processes pushing pH towards acidemia or alkalemia.
• Respiratory disorders: Linked to PaCO₂ changes (lungs).
• Metabolic disorders: Linked to HCO₃⁻ changes (kidneys, GI, etc.).

Compensation Mechanisms and Formulas

• The body compensates for metabolic acidosis by hyperventilating (↓PaCO₂); for metabolic alkalosis by hypoventilating (↑PaCO₂).
• Compensation for respiratory disorders is by renal adjustment of HCO₃⁻, depending on acute or chronic status.
• Compensation never fully normalizes pH; formulas predict expected compensation.

Anion Gap and Delta Ratio

• Anion gap (AG) = Na⁺ – (Cl⁻ + HCO₃⁻); adjust for hypoalbuminemia.
• Normal adjusted AG: 8–12 mEq/L; varies by lab.
• Elevated AG signals unmeasured anions, suggesting certain metabolic acidoses.
• Delta ratio: (Change in AG from normal) / (Change in HCO₃⁻ from normal); used to identify mixed metabolic disorders.
  • Low ratio: concurrent normal gap acidosis.
  • High ratio: metabolic alkalosis present.

Mixed and Complex Disorders

• Multiple acid-base processes can occur, resulting in a normal pH but abnormal PaCO₂/HCO₃⁻.
• Specialized tables/algorithms assist in identifying mixed disorders with normal pH.

Differential Diagnosis Algorithms

• For elevated AG metabolic acidosis: check lactate, ketones, renal function, then osmolal gap.
• For normal AG acidosis: assess saline infusion, renal function, serum K⁺, urine pH, and urine anion gap.
• For metabolic alkalosis: assess volume status, blood pressure, and K⁺.
• For respiratory disorders: consider obstructive lung disease, drugs, or obesity hypoventilation.

Master Algorithm for Acid-Base Analysis

• 1. Check pH for acidemia/alkalemia.
• 2. Compare direction of PaCO₂/HCO₃⁻ changes to determine primary disorder.
• 3. Assess if compensation is appropriate; if not, suspect additional disorders.
• 4. Always check AG (adjusted for albumin).
• 5. If AG elevated, calculate Delta ratio for mixed metabolic issues.
• 6. Use flowcharts to generate differential diagnoses; apply the law of parsimony.

Key Terms & Definitions

• ABG — Arterial Blood Gas, a test for acid-base status.
• Anion Gap (AG) — Difference between serum Na⁺ and sum of Cl⁻ + HCO₃⁻.
• Delta Ratio — Ratio between AG change and HCO₃⁻ change.
• Compensation — Physiological adjustment by lungs or kidneys in response to acid-base disturbances.
• Law of Parsimony — The simplest explanation is preferred in differential diagnosis.

Action Items / Next Steps

• Practice interpreting ABG cases using the master algorithm.
• Review tables for AG, Delta ratio, and compensation formulas.
• Prepare for next lecture on common pitfalls in acid-base analysis.