Acid-Base Disorders Overview

Overview

This lecture covers the core concepts, pathophysiology, causes, complications, diagnosis, and treatment of acid-base disorders, focusing on metabolic and respiratory acidosis and alkalosis.

Primary Acid-Base Disorders

• Four main disorders: metabolic acidosis, metabolic alkalosis, respiratory acidosis, respiratory alkalosis.
• Metabolic acidosis: low pH and low bicarbonate (HCO₃⁻).
• Metabolic alkalosis: high pH and high bicarbonate.
• Respiratory acidosis: low pH and high CO₂.
• Respiratory alkalosis: high pH and low CO₂.

Metabolic Acidosis

• Anion gap (AG): Na⁺ - (Cl⁻ + HCO₃⁻); AG > 12 = high anion gap metabolic acidosis (AGMA), AG ≤ 12 = normal AG (NAGMA).
• AGMA caused by accumulation of organic acids; examples: diabetic ketoacidosis (DKA), uremia, lactic acidosis, toxic alcohols.
• NAGMA caused by loss of bicarbonate (renal or GI losses), e.g., diarrhea, renal tubular acidosis.
• Metabolic acidosis complications: hyperkalemia, arrhythmias, compensatory hyperventilation, hypotension (if pH < 7.15).

Metabolic Alkalosis

• Caused by loss of protons from GI tract (vomiting, NG suction) or kidneys (diuretics, hyperaldosteronism).
• Leads to increased bicarbonate and elevated pH.
• Complications: hypokalemia, risk for arrhythmias (Torsades), possible tetany and hypoventilation.

Respiratory Acidosis

• Due to hypoventilation and CO₂ retention; pH drops as CO₂ rises.
• Causes: CNS depression (drugs, stroke), neuromuscular disorders, airway obstruction (COPD, asthma).
• Complications: increased intracranial pressure, lethargy, compensatory renal bicarbonate retention in chronic cases.

Respiratory Alkalosis

• Due to hyperventilation, causing low CO₂ and high pH.
• Causes: anxiety, pain, hypoxemia (high altitude, lung disease), pregnancy, early sepsis, aspirin toxicity.
• Complications: cerebral vasoconstriction (risk of syncope), renal loss of bicarbonate.

Diagnosis Approach

• Start with arterial blood gas: pH < 7.35 = acidosis; pH > 7.45 = alkalosis.
• Differentiate metabolic (HCO₃⁻ abnormal) vs. respiratory (CO₂ abnormal).
• For metabolic acidosis: calculate anion gap and consider AGMA vs. NAGMA.
• For metabolic alkalosis: assess volume status and urine chloride.
• For respiratory disorders: use history, physical, and response to treatments (e.g., naloxone for opioid overdose).

Key Terms & Definitions

• Anion Gap (AG) — Measure of unmeasured anions; AG > 12 suggests AGMA.
• AGMA — High anion gap metabolic acidosis, typically due to increased acids.
• NAGMA — Normal anion gap metabolic acidosis, usually from bicarbonate loss.
• Compensatory Mechanisms — Body responses (altered ventilation or renal adjustments) to maintain pH.
• Hypoventilation — Reduced breathing, leading to CO₂ retention (respiratory acidosis).
• Hyperventilation — Increased breathing, leading to CO₂ loss (respiratory alkalosis).

Action Items / Next Steps

• Review diagnostic algorithms for acid-base disorders.
• Practice calculating anion and osmolar gaps.
• Study causes and treatments for each acid-base disorder.
• Complete assigned readings on renal and pulmonary acid-base regulation.