Metabolic Acidosis

Metabolic Acidosis

David Ray Velez, MD

Table of Contents

Definition
Anion Gap
Causes
Physiologic Changes
Compensation
Delta Gap (ΔΔ)
Treatment

Definition

Definition: An Acid-Base Disorder with Acidosis (Decreased pH) Due to a Metabolic Process (Decreased Bicarbonate or Increased Hydrogen)

Arterial Blood Gas (ABG) Analysis

Anion Gap

Anion Gap = Na – (HCO3 + Cl)

Anion Gap Metabolic Acidosis (AGMA)

  • Definition: Anion Gap > 12
  • Caused by Adding Organic Acids

Non-Anion Gap Metabolic Acidosis (Non-AGMA)

  • Definition: Anion Gap ≤ 12
  • Caused by Adding HCl or Losing HCO3/NaCl – Think GI or Renal Causes

Causes

Anion Gap Metabolic Acidosis (AGMA)

  • Methanol
  • Uremia
  • Diabetic Ketoacidosis (DKA)
  • Propylene Glycol
  • Iron/INH
  • Lactic Acidosis
  • EtOH
  • Rhabdomyolysis or Renal Failure
  • Salicylate (Aspirin) Intoxication (Late Stage)
  • Mnemonic: “MUDPILERS”

Non-Anion Gap Metabolic Acidosis (Non-AGMA)

  • Hyperalimentation
  • Addison Disease
  • Renal Tubular Acidosis (RTA)
  • Diarrhea or Dialysis
  • Acetazolamide
  • Spironolactone
  • Saline: Large Volume Resuscitation with Normal Saline
    • Hyperchloremic Metabolic Acidosis
    • Excess Cl Causes a Decreased SID (Strong Ion Difference) Resulting in Increased H+
    • SID = (Na + K) – (Cl + Lactate)
  • Mnemonic: “HARD-ASS”

Lactic Acidosis

  • Type A: Caused by Tissue Hypoxia or Hypoperfusion
    • Shock
    • Ischemia (Limb/Mesenteric)
    • Seizures
  • Type B: Not Caused by Tissue Hypoxia or Hypoperfusion
    • Liver Failure
    • Medications (Metformin, Linezolid, Epinephrine)
    • Mitochondrial Myopathy and Congenital Defects
    • Thiamine Deficiency

Physiologic Changes of Acidosis

Pulmonary Changes

  • Stimulated Respiratory Drive and Increased Respiratory Rate
  • Right-Shift of the Oxygen-Hemoglobin Dissociation Curve (Decreased Affinity)

Cardiovascular Changes

  • Net Increase in Cardiac Output – Acidosis Directly Decreases Contractility but Sympathoadreal Effects Cause Increased Preload, Heart Rate, and Contractility
  • Increased Risk of Arrhythmia
  • Decreased Systemic Vascular Resistance (SVR) and Arterial Vasodilation
  • Decreased Responsiveness to Catecholamines

Hematologic Changes

  • Coagulopathy from Impaired Clotting Factor Function
  • Decreased RBC Rheology (Flow) – Contributes to Rouleaux Formation
  • Impaired Platelet Aggregation

Renal Changes

  • Decreased Bicarbonate Secretion
  • Diuresis

Electrolyte Changes

  • Hyperkalemia – From Intracellular Shifts
  • Hypercalcemia – From Decreased Calcium Binding to Albumin and Increased Renal Wasting

Neurologic Changes

  • Cerebral Vasodilation and Increased Intracranial Pressure (ICP)
  • Increased Cerebral Blood Flow
  • Nausea and Vomiting

Compensation

Metabolic Acidosis is Compensated by Pulmonary Changes with Increased Ventilation (Decreased CO2)

Winter’s Formula

  • Expected pCO2 = (1.5 x HCO3) + 8 ± 2
  • Used to Determine if Respiratory Compensation is Appropriate
  • Interpretation:
    • Actual PCO2 Within Expected Values: Adequate Respiratory Compensation
    • Actual PCO2 > Expected: Secondary Respiratory Acidosis or Mixed Acid Base Disorder
    • Actual PCO2 < Expected: Secondary Respiratory Alkalosis or Mixed Acid Base Disorder

Other Methods to Approximate Expected pCO2

  • Expected pCO2 = HCO3 + 15
  • Expected pCO2 Approximates the Decimal Digits of Arterial pH (pH 7.27 with pCO2 27 mmHg)

Delta Gap (ΔΔ)

Formula Used in Evaluating Anion Gap Metabolic Acidosis (AGMA) for Additional Disturbances

Delta Gap (ΔΔ)

  • Delta Gap (ΔΔ) = Change in Anion Gap – Change in Bicarb
    • Change in Anion Gap = AG – 12
    • Change in Bicarb = 24 – HCO3
  • Gap Interpretation:
    • Significantly Positive (> 6): Mixed AGMA and Metabolic Alkalosis
      • Decrease in Bicarbonate is Less than Expected
    • Near-Equal (-6 to +6): AGMA Alone
    • Significantly Negative (< -6): Mixed AGMA and Non-AGMA
      • Rise in Anion Gap is Not as Significant as the Observed Decrease in Bicarbonate

Delta Ratio

  • Delta Ratio = Change in Anion Gap / Change in Bicarb
    • Change in Anion Gap = AG – 12
    • Change in Bicarb = 24 – HCO3
  • Ratio Interpretation:
    • < 0.4: Non-AGMA Alone
    • 4-0.8: Mixed AGMA and Non-AGMA
    • 8-2.0: AGMA Alone
    • > 2.0: Mixed AGMA and Metabolic Alkalosis

Treatment

Primarily Managed by Treatment of the Underlying Cause

Can Augment Respiratory Compensation if Mechanically Ventilated

Sodium Bicarbonate

  • Use is Debated and Generally No Survival Improvement
  • General Indications:
    • pH < 7.1
    • pH < 7.2 with Severe AKI
    • Renal Tubular Acidosis
  • Side Effects:
    • Myocardial Depression
    • Left-Shift Oxygen Dissociation Curve (Impairs Oxygen Delivery)
    • Increased Lactate Production
    • Intracellular Acidification
    • Hypokalemia