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Chemistry

Result Interpretation

Interpreting blood gas results requires a systematic approach to assess the patient’s acid-base and oxygenation status

  • Key Parameters
  • Step-by-Step Interpretation
  • Acid-Base Disorders
  • Compensation
  • Clinical Scenarios

Key Parameters

  • pH: 7.35-7.45
    • Primary indicator of acid-base balance
    • \(<\) 7.35: Acidemia
    • \(>\) 7.45: Alkalemia
  • Pa\(CO_2\): 35-45 mmHg
    • Partial pressure of carbon dioxide in arterial blood
    • Reflects the respiratory component of acid-base balance
    • \(>\) 45 mmHg: Respiratory Acidosis
    • \(<\) 35 mmHg: Respiratory Alkalosis
  • \(HCO_3^-\): 22-26 mEq/L
    • Bicarbonate concentration in arterial blood
    • Reflects the metabolic component of acid-base balance
    • \(<\) 22 mEq/L: Metabolic Acidosis
    • \(>\) 26 mEq/L: Metabolic Alkalosis
  • Pa\(O_2\): 80-100 mmHg
    • Partial pressure of oxygen in arterial blood
    • Indicates oxygenation status
    • \(<\) 80 mmHg: Hypoxemia
  • Sa\(O_2\): 95-100%
    • Oxygen saturation
    • Indicates the percentage of hemoglobin saturated with oxygen
    • \(<\) 95%: Hypoxemia
  • Base Excess (BE): -2 to +2 mEq/L
    • Represents the amount of acid or base needed to restore normal pH
    • \(<\) -2 mEq/L: Metabolic Acidosis
    • \(>\) +2 mEq/L: Metabolic Alkalosis
  • Anion Gap: 8-16 mEq/L (with \(K^+\)), 10-20 mEq/L (without \(K^+\))
    • Anion Gap = [\(Na^+\)] + [\(K^+\)] - [\(Cl^-\)] - [\(HCO_3^-\)]
    • Used to assess metabolic acidosis

Step-by-Step Interpretation

  1. Examine the pH:
    • Is the pH within the normal range (7.35-7.45)?
    • If not, is it acidemic (< 7.35) or alkalemic (> 7.45)?
  2. Evaluate the Respiratory Component (Pa\(CO_2\)):
    • Is the Pa\(CO_2\) within the normal range (35-45 mmHg)?
    • If not, is it elevated (> 45 mmHg, indicating respiratory acidosis) or decreased (< 35 mmHg, indicating respiratory alkalosis)?
  3. Assess the Metabolic Component (\(HCO_3^-\)):
    • Is the \(HCO_3^-\) within the normal range (22-26 mEq/L)?
    • If not, is it decreased (< 22 mEq/L, indicating metabolic acidosis) or elevated (> 26 mEq/L, indicating metabolic alkalosis)?
  4. Determine the Primary Disorder:
    • Match the pH abnormality with the corresponding primary disturbance:
      • Acidemia + Elevated Pa\(CO_2\): Respiratory Acidosis
      • Acidemia + Decreased \(HCO_3^-\): Metabolic Acidosis
      • Alkalemia + Decreased Pa\(CO_2\): Respiratory Alkalosis
      • Alkalemia + Elevated \(HCO_3^-\): Metabolic Alkalosis
  5. Evaluate Compensation:
    • Is there evidence of compensation by the other system?
    • Respiratory Compensation: The lungs will try to compensate for metabolic disorders by adjusting the Pa\(CO_2\)
    • Renal Compensation: The kidneys will try to compensate for respiratory disorders by adjusting the \(HCO_3^-\)
    • Compensation is present if the non-causative parameter is also abnormal
  6. Assess Oxygenation:
    • Is the Pa\(O_2\) within the normal range (80-100 mmHg)?
    • Is the Sa\(O_2\) within the normal range (95-100%)?
    • If not, is there evidence of hypoxemia?
  7. Calculate Anion Gap (if Metabolic Acidosis is present):
    • Anion Gap = [\(Na^+\)] + [\(K^+\)] - [\(Cl^-\)] - [\(HCO_3^-\)]
    • Normal Anion Gap: 8-16 mEq/L (with \(K^+\)), 10-20 mEq/L (without \(K^+\))
    • Elevated Anion Gap: Suggests the presence of unmeasured acids (e.g., ketoacids, lactic acid)
  8. Determine if Expected Compensation is Appropriate:
    • Once the primary disorder is determined, calculate the “expected” range for the compensating parameter
    • If the measured value doesn’t fall in that range, there may be a mixed acid-base disorder

Acid-Base Disorders

  • Respiratory Acidosis
    • Definition: Decreased pH and Increased Pa\(CO_2\)
    • Causes: Hypoventilation due to COPD, drug overdose, neuromuscular disorders
    • Compensation: Increased \(HCO_3^-\) (renal compensation)
  • Respiratory Alkalosis
    • Definition: Increased pH and Decreased Pa\(CO_2\)
    • Causes: Hyperventilation due to anxiety, pain, hypoxia
    • Compensation: Decreased \(HCO_3^-\) (renal compensation)
  • Metabolic Acidosis
    • Definition: Decreased pH and Decreased \(HCO_3^-\)
    • Causes: Increased acid production (DKA, lactic acidosis), bicarbonate loss (diarrhea), impaired acid excretion (renal failure)
    • Compensation: Decreased Pa\(CO_2\) (respiratory compensation)
    • Assess Anion Gap: To determine the cause of metabolic acidosis
  • Metabolic Alkalosis
    • Definition: Increased pH and Increased \(HCO_3^-\)
    • Causes: Loss of acid (vomiting), increased bicarbonate intake, mineralocorticoid excess
    • Compensation: Increased Pa\(CO_2\) (respiratory compensation)

Compensation

  • Respiratory Compensation for Metabolic Acidosis
    • Expected Pa\(CO_2\) = 1.5 x [\(HCO_3^-\)] + 8 ± 2
  • Respiratory Compensation for Metabolic Alkalosis
    • Expected Pa\(CO_2\) = 0.7 x [\(HCO_3^-\)] + 20 ± 5
  • Metabolic Compensation for Respiratory Acidosis
    • Acute: \(HCO_3^-\) increases by 1 mEq/L for every 10 mmHg increase in Pa\(CO_2\)
    • Chronic: \(HCO_3^-\) increases by 3.5 mEq/L for every 10 mmHg increase in Pa\(CO_2\)
  • Metabolic Compensation for Respiratory Alkalosis
    • Acute: \(HCO_3^-\) decreases by 2 mEq/L for every 10 mmHg decrease in Pa\(CO_2\)
    • Chronic: \(HCO_3^-\) decreases by 5 mEq/L for every 10 mmHg decrease in Pa\(CO_2\)

Clinical Scenarios

  • Patient with COPD and Pneumonia: Low pH, high Pa\(CO_2\), high \(HCO_3^-\). Diagnosis: Partially compensated respiratory acidosis
  • Patient with DKA: Low pH, low \(HCO_3^-\), low Pa\(CO_2\), high anion gap. Diagnosis: Fully compensated metabolic acidosis with an elevated anion gap
  • Patient with Hyperventilation Due to Anxiety: High pH, low Pa\(CO_2\), low \(HCO_3^-\). Diagnosis: Partially compensated respiratory alkalosis
  • Patient with Prolonged Vomiting: High pH, high \(HCO_3^-\), high Pa\(CO_2\). Diagnosis: Fully compensated metabolic alkalosis

Key Terms

  • Acidemia: A condition in which the blood pH is below the normal range
  • Alkalemia: A condition in which the blood pH is above the normal range
  • Hypoxemia: Low oxygen levels in the blood
  • Hypercapnia: Elevated carbon dioxide levels in the blood
  • Acidosis: A condition in which there is too much acid in the body fluids
  • Alkalosis: A condition in which there is too much base in the body fluids
  • Arterial Blood Gas (ABG): A test that measures the levels of oxygen and carbon dioxide in arterial blood
  • Partial Pressure of Oxygen (Pa\(O_2\)): A measure of the oxygen level in arterial blood
  • Partial Pressure of Carbon Dioxide (Pa\(CO_2\)): A measure of the carbon dioxide level in arterial blood
  • Bicarbonate (\(HCO_3^-\)): A major buffer in the blood
  • Base Excess (BE): A measure of the amount of acid or base needed to restore normal pH
  • Oxygen Saturation (Sa\(O_2\)): A measure of the percentage of hemoglobin saturated with oxygen
  • Respiratory Acidosis: Acidosis caused by increased carbon dioxide levels
  • Metabolic Acidosis: Acidosis caused by decreased bicarbonate levels or increased acid production
  • Respiratory Alkalosis: Alkalosis caused by decreased carbon dioxide levels
  • Metabolic Alkalosis: Alkalosis caused by increased bicarbonate levels or decreased acid levels
  • Anion Gap: The difference between measured cations and anions in serum or plasma
  • Compensation: The physiological response to an acid-base disturbance that attempts to restore normal pH
  • Estimated Glomerular Filtration Rate (eGFR): An estimate of GFR based on serum creatinine, age, sex, and ethnicity
  • Body Surface Area (BSA): The measured or calculated surface of a human body
  • Modified Allen Test: A test to assess collateral circulation in the hand