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Chemistry

Acid-Base

Blood gas analysis is a critical tool for assessing a patient’s respiratory and metabolic status

  • Underlying biochemical principles
  • Proper testing techniques
  • Accurate interpretation of results

Biochemical Theory and Pathways

  • Fundamentals
    • Acids donate \(H^+\), bases accept \(H^+\)
    • pH: A measure of acidity/alkalinity
    • Buffers: Resist pH changes
  • Key Players
    • The relationship of \(pH\), \(HCO_3^-\), and Pa\(CO_2\) in the bicarbonate buffer system (Henderson-Hasselbalch equation)
    • \(O_2\) and \(CO_2\): Transport mechanisms and influencers
    • Acid-Base Control
      • Lungs and ventilation (fast response)
      • Kidneys, \(HCO_3^-\) regulation (slow response)
      • Understanding the concepts of respiratory and metabolic balance will help to understand the disease state
  • Understanding Arterial Blood Gasses (ABGs) requires understanding of the underlying physiology
    • Understanding acid production and elimination
    • Understanding acid-base balance within the body

Laboratory Test Procedures

  • Measurements
    • \(pH\): Potentiometry with glass electrode
    • Pa\(O_2\): Amperometry with Clark electrode
    • Pa\(CO_2\): Potentiometry with Severinghaus electrode
  • Calculations
    • \(HCO_3^-\) is calculated using the Henderson-Hasselbalch equation
    • Base Excess (BE) and Anion Gap (AG) are also calculated
  • Considerations
    • Sample handling: anaerobic with prompt processing
    • Quality Control: Calibrate instruments correctly
    • Troubleshooting and interfering substances

Test Result Interpretation

  • Normal Values to Know
    • pH: 7.35-7.45
    • Pa\(CO_2\): 35-45 mmHg
    • \(HCO_3^-\): 22-26 mEq/L
    • Pa\(O_2\): 80-100 mmHg
    • Sa\(O_2\): 95-100%
    • Base Excess: -2 to +2 mEq/L
  • Steps to interpretation
    • Is pH normal, acidemic, or alkalemic?
    • Match primary disorder by abnormal levels of Pa\(CO_2\) or \(HCO_3^-\)
    • Is there compensation? Are these values appropriate
    • What is the Pa\(O_2\)/Sa\(O_2\)?
  • Important Formulas
    • Henderson-Hasselbalch Equation
      • \(pH = pKa + \log \left( \frac {[A^-]} {[HA]} \right)\)
      • \(pH = 6.1 + \log \left( {\frac {[HCO_3^-]} {0.03 \times pCO_2}} \right)\)
    • Anion Gap Calculation
      • [\(Na^+\)] + [\(K^+\)] - [\(Cl^-\)] - [\(HCO_3^-\)]

Disease State Correlation

  • Respiratory Acidosis
    • pH < 7.35, Pa\(CO_2\) > 45
    • Causes: COPD, drug overdose
  • Respiratory Alkalosis
    • pH > 7.45, Pa\(CO_2\) < 35
    • Causes: Anxiety, hypoxia
  • Metabolic Acidosis
    • pH < 7.35, \(HCO_3^-\) < 22
    • Causes: DKA, renal failure
  • Metabolic Alkalosis
    • pH > 7.45, \(HCO_3^-\) > 26
    • Causes: Vomiting, diuretic use
  • Additional Clinical Considerations
    • Use Anion Gap calculation to distinguish causes of metabolic acidosis
    • Consider Hypoxemia

In Summary

  • ABGs require an understanding of basic chemistry, as well as an understanding of the clinical implications for each result
  • The Henderson-Hasselbalch equation can be used to further identify any potential disturbance with the values that were collected
  • The kidneys, lungs, and chemical buffers all work together to maintain homeostasis in the body