USMLE Guide: Acid-Base Disorders

Introduction

In this USMLE Guide, we will explore the topic of acid-base disorders. Acid-base abnormalities are common clinical conditions encountered in various medical settings. It is of utmost importance for medical professionals to have a thorough understanding of acid-base physiology and disorders, as they play a crucial role in patient management and treatment. This guide aims to provide a concise overview of acid-base disorders, including their classification, etiology, clinical manifestations, diagnostic approach, and management.

Table of Contents

• 1. Acid-Base Physiology
• 2. Classification of Acid-Base Disorders
• 3. Respiratory Acidosis
• 4. Respiratory Alkalosis
• 5. Metabolic Acidosis
• 6. Metabolic Alkalosis
• 7. Diagnostic Approach
• 8. Management
• 9. Conclusion

1. Acid-Base Physiology

• Acid-base balance is maintained by the body's buffering systems, respiratory system, and renal system.
• The pH scale measures the acidity or alkalinity of a solution, with 7 being neutral, <7 acidic, and >7 alkaline.
• The main buffering systems in the body include the bicarbonate buffer system, protein buffer system, and phosphate buffer system.
• The respiratory system regulates acid-base balance by controlling the elimination of carbon dioxide (CO2) through ventilation.
• The kidneys regulate acid-base balance by reabsorbing or excreting bicarbonate (HCO3-) and hydrogen ions (H+).

2. Classification of Acid-Base Disorders

Acid-base disorders can be classified based on the primary disturbance:

• Respiratory Acidosis: Primary increase in arterial carbon dioxide levels (PaCO2).
• Respiratory Alkalosis: Primary decrease in arterial carbon dioxide levels (PaCO2).
• Metabolic Acidosis: Primary decrease in serum bicarbonate levels (HCO3-).
• Metabolic Alkalosis: Primary increase in serum bicarbonate levels (HCO3-).

3. Respiratory Acidosis

• Etiology: Impaired alveolar ventilation leading to retention of CO2, such as respiratory depression, airway obstruction, or neuromuscular disorders.
• Clinical Manifestations: Headache, confusion, somnolence, respiratory distress, and ultimately, decreased level of consciousness.
• Diagnostic Findings: Decreased blood pH, increased PaCO2, and compensatory increase in serum bicarbonate.
• Treatment: Address the underlying cause, provide ventilatory support if necessary.

4. Respiratory Alkalosis

• Etiology: Hyperventilation leading to excessive elimination of CO2, often due to anxiety, pain, or hypoxemia.
• Clinical Manifestations: Lightheadedness, paresthesias, tachypnea, and sometimes, tetany.
• Diagnostic Findings: Increased blood pH, decreased PaCO2, and compensatory decrease in serum bicarbonate.
• Treatment: Address the underlying cause, provide reassurance and proper breathing techniques.

5. metabolic acidosis

• Etiology: Primary loss of bicarbonate or increase in non-carbonic acids, such as diabetic ketoacidosis, renal failure, or lactic acidosis.
• Clinical Manifestations: Deep and rapid breathing (Kussmaul respirations), nausea, vomiting, and altered mental status.
• Diagnostic Findings: Decreased blood pH, decreased bicarbonate, and compensatory decrease in PaCO2 (if respiratory compensation occurs).
• Treatment: Treat the underlying cause, administer bicarbonate if severe acidemia is present.

6. metabolic alkalosis

• Etiology: Primary increase in bicarbonate due to excessive loss of acid or gain of bicarbonate, such as vomiting, diuretic use, or excessive antacid ingestion.
• Clinical Manifestations: Weakness, muscle cramps, and sometimes, altered mental status.
• Diagnostic Findings: Increased blood pH, increased bicarbonate, and compensatory increase in PaCO2 (if respiratory compensation occurs).
• Treatment: Treat the underlying cause, administer saline along with potassium if necessary.

7. Diagnostic Approach <a