Arterial Blood Gas (ABG) Analysis

Arterial Blood Gas (ABG) Analysis

David Ray Velez, MD

Table of Contents

Evaluation of Primary Disorder
Respiratory Acidosis
Respiratory Alkalosis
Metabolic Acidosis
Metabolic Alkalosis

Evaluation of Primary Disorder

Normal Blood Gas Values

 Arterial (ABG)Venous (VBG)
pH7.40 (7.35-7.45)7.37 (7.32-7.42)
pO280-100 mmHg35-45 mmHg
pCO240 (35-45) mmHg46 (41-51) mmHg
HCO324 (22-26) mEq/L26 (24-28) mEq/L
Base Excess0 (-2 to +2)0 to +4

Evaluation of Simple Primary Disturbance

  • pH < 7.35 = Acidosis
    • pCO2 > 45 = Respiratory Acidosis
    • HCO3 < 22 = Metabolic Acidosis
  • pH > 7.45 = Alkalosis
    • pCO2 < 35 = Respiratory Alkalosis
    • HCO3 > 26 = Metabolic Alkalosis

Buffering System: H + HCO3 = H2CO3 = CO2 + H2O

Compensation

  • Metabolic Disorders: Respiratory Compensation Occurs Rapidly Over Minutes-Hours (Change in Ventilation)
  • Respiratory Disorders: Renal Compensation Occurs Slowly Over Days-Weeks (Change in Acid Excretion and Bicarbonate Reabsorption)

Simple vs Mixed Disorders

  • Simple Acid-Base Disorder: A Single Disorder with Appropriate Respiratory or Renal Compensation
  • Mixed Acid-Base Disorder: Multiple Disorders Present Simultaneously

Respiratory Acidosis

*See Respiratory Acidosis

Respiratory Acid-Base Disorders are Compensated by Renal Changes in Bicarbonate Reabsorption (Slow Process Over Days-Weeks)

Acuity/Compensation

  • Acute Respiratory Acidosis – High pH Changes are Seen with Minimal Compensation
    • Only Slight Compensation Occurs Over Minutes-Hours from Cellular Buffering of Plasma Proteins
  • Chronic Respiratory Acidosis – pH More Normalized Due to Renal Compensation Over Days-Weeks with Increased Reabsorption of Bicarbonate and Excretion of Acid

Expected Change in Bicarbonate and pH (For Every 10 mmHg Increased in pCO2 Above 40 mmHg)

  • Acute Respiratory Alkalosis:
    • HCO3 Increases 1 mEq/L Above 24
    • pH Decreases 0.08 Below 7.40
  • Chronic Respiratory Alkalosis:
    • HCO3 Increases 3-4 mEq/L Above 24
    • pH Decreases 0.03 Below 7.40
  • Mnemonic: “1-2-3-4-5 Rule”
    • Expected Change in HCO3 (mEq/L) for Every 10 mmHg Change in pCO2 Due to Respiratory Acid-Base Disorders
 AcuteChronic
Respiratory Acidosis (↑ CO2)↑ 1↑ 4
Respiratory Alkalosis (↓ CO2)↓ 2↓ 5

Respiratory Alkalosis

*See Respiratory Alkalosis

Respiratory Acid-Base Disorders are Compensated by Renal Changes in Bicarbonate Reabsorption (Slow Process Over Days-Weeks)

Acuity/Compensation

  • Acute Respiratory Alkalosis – High pH Changes are Seen with Minimal Compensation
  • Chronic Respiratory Alkalosis – pH More Normalized Due to Renal Compensation Over Days-Weeks with Decreased Reabsorption of Bicarbonate

Expected Change in Bicarbonate

  • Acute Respiratory Alkalosis: HCO3 = 24 – 2 x (40 – pCO2) / 10
  • Chronic Respiratory Alkalosis: HCO3 = 24 – 5 x (40 – pCO2) / 10
  • Mnemonic: “1-2-3-4-5 Rule”
    • Expected Change in HCO3 (mEq/L) for Every 10 mmHg Change in pCO2 Due to Respiratory Acid-Base Disorders
 AcuteChronic
Respiratory Acidosis (↑ CO2)↑ 1↑ 4
Respiratory Alkalosis (↓ CO2)↓ 2↓ 5

Metabolic Acidosis

*See Metabolic Acidosis

Anion Gap

  • Anion Gap = Na – (HCO3 + Cl)
  • Gap > 12: Anion Gap Metabolic Acidosis (AGMA)
  • Gap ≤ 12: Non-Anion Gap Metabolic Acidosis (Non-AGMA)

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

Compensation

  • Winter’s Formula
    • Expected pCO2 = (1.5 x HCO3) + 8 ± 2
    • Used to Determine if Respiratory Compensation is Appropriate
  • 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)
  • 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

Delta Gap (ΔΔ)

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

Metabolic Alkalosis

*See Metabolic Alkalosis

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

Expected CO2 = (0.7 x HCO3) + 20 mmHg ± 5