Mixed acid-base disorders

Mixed acid-base disorders

Mixed acid-base disorders

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Intro to Mixed Disorders - When pH Goes Wild

  • Defined as the simultaneous presence of two or more primary acid-base disturbances.
  • Compensation is a physiological response, not a cure; it aims to buffer pH changes but never fully corrects pH to 7.40.
  • If the expected compensatory response is not what is observed (either over or under-compensated), suspect a mixed disorder.

⭐ A normal pH (~7.40) accompanied by an abnormal $pCO_2$ and $HCO_3^-$ is the classic sign of a mixed acid-base disorder.

Diagnostic Approach - The 5-Step Shuffle

A systematic approach is key to diagnosing mixed acid-base disorders. Follow these five steps to deconstruct any ABG.

  • Step 1: Check pH. Acidemia (pH < 7.35) or Alkalemia (pH > 7.45).
  • Step 2: Determine Primary Disturbance. Is it respiratory (PaCO₂ driven) or metabolic (HCO₃⁻ driven)?
  • Step 3: Calculate Expected Compensation. For Metabolic Acidosis, use Winter's Formula: $\text{Expected PaCO₂} = (1.5 \times \text{HCO₃⁻}) + 8 \pm 2$.
  • Step 4: Compare Actual vs. Expected. If the actual value is outside the expected range, a mixed disorder is present.
  • Step 5: Calculate Anion Gap. For metabolic acidosis, calculate $Na⁺ - (Cl⁻ + HCO₃⁻)$. Normal is 8-12 mEq/L.

⭐ In High Anion Gap Metabolic Acidosis (HAGMA), a delta gap (ΔAG / ΔHCO₃⁻) ratio significantly different from 1-2 suggests another underlying metabolic disorder. A ratio >2 points towards a concurrent metabolic alkalosis.

Common Mixed Patterns - The Usual Suspects

Recognizing classic pairings of acid-base disturbances is key. The patient's history is often the biggest clue to the presence of a mixed disorder, as compensation alone rarely normalizes pH.

Clinical ScenarioPrimary Disorder 1Primary Disorder 2Key Lab Findings
Salicylate ToxicityRespiratory AlkalosisMetabolic Acidosis↓ pCO₂, ↓ HCO₃⁻, ↑ Anion Gap
Sepsis / ShockRespiratory AlkalosisMetabolic Acidosis↓ pCO₂, ↓ HCO₃⁻, ↑ Lactate
Cardiopulmonary ArrestRespiratory AcidosisMetabolic Acidosis↑ pCO₂, ↓ HCO₃⁻, ↑ Lactate
Vomiting + Diuretic UseMetabolic AlkalosisContraction Alkalosis↑ HCO₃⁻, ↓ Cl⁻, Volume Depletion

Triple Disorders - The Ultimate Puzzle

Acid-Base Balance: Disorders, Causes, and Compensation

  • Scenario: A patient with Diabetic Ketoacidosis (DKA) and underlying COPD presents with vomiting.
  • Component Disorders:
    • DKA: ↑ Anion Gap Metabolic Acidosis.
    • Vomiting: Metabolic Alkalosis (loss of H⁺).
    • COPD: Respiratory Acidosis (↑ PaCO₂).
  • Lab Picture: The pH may be deceptively near-normal due to opposing metabolic forces. Expect a high PaCO₂ and a high anion gap.
    • Anion Gap = $Na^+ - (Cl^- + HCO_3^-)$ > 12 mEq/L.

⭐ In a triple disorder, the pH can be deceptively normal. Always calculate the anion gap and assess PaCO₂ to unmask all underlying conditions.

High-Yield Points - ⚡ Biggest Takeaways

  • Suspect a mixed disorder when compensation is inadequate or excessive, or if pH is normal with abnormal pCO₂ and HCO₃⁻.
  • If actual pCO₂ deviates from Winter's formula prediction, a mixed respiratory disorder is present.
  • A disproportionate anion gap to bicarbonate change suggests a co-existing metabolic disturbance.
  • The delta-delta gap (ΔAG/ΔHCO₃⁻) unmasks a second metabolic disorder in high AG metabolic acidosis.
  • Classic causes: salicylate toxicity (respiratory alkalosis + metabolic acidosis) and sepsis (lactic acidosis + respiratory alkalosis).

Practice Questions: Mixed acid-base disorders

Test your understanding with these related questions

A 17-year-old male presents to your office complaining of polyuria, polydipsia, and unintentional weight loss of 12 pounds over the past 3 months. On physical examination, the patient is tachypneic with labored breathing. Which of the following electrolyte abnormalities would you most likely observe in this patient?

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Flashcards: Mixed acid-base disorders

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Decreased PCO2 (respiratory alkalosis) results in _____ HCO3- reabsorption

TAP TO REVEAL ANSWER

Decreased PCO2 (respiratory alkalosis) results in _____ HCO3- reabsorption

decreased

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