Surgical Critical Care: Acid-Base Disorders – Basic Evaluation

Evaluation of Primary Disturbance

Normal Blood Gas Values

  Arterial (ABG) Venous (VBG)
pH 7.40 (7.35-7.45) 7.37 (7.32-7.42)
pO2 80-100 mmHg 35-45 mmHg
pCO2 40 (35-45) mmHg 46 (41-51) mmHg
HCO3 24 (22-26) mEq/L 26 (24-28) mEq/L
Base Excess 0 (-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

Advanced Evaluation

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

Compensation

  • For Metabolic Etiologies – Respiratory Compensation Occurs Rapidly Over Minutes-Hours
    • By Changes in Excretion of Acid & Reabsorption of Bicarbonate
  • For Respiratory Etiologies – Renal Compensation Occurs Slowly Over Days-Weeks
    • By Changes in Ventilation

Base Excess

  • Definition: The Amount of Strong Acid Required to Restore a Liter of Blood to Normal pH
    • Blood at Standard Conditions (Fully Oxygenated, PCO2 40 mmHg & Temperature 37 C)
  • Used as an Indicator of the Degree of a Metabolic Disturbance
  • Interpretation:
    • More Positive Values (> +2): Metabolic Alkalosis
    • More Negative Values (< -2): Metabolic Acidosis

Other Testing