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Difficulty: Very hardNephrology

Match each of the following patient clinical vignettes with the arterial blood gas and basic metabolic panel profile that most accurately reflects their expected mixed acid-base disorder.

  • A 24-year-old pregnant patient at 10 weeks gestation with intractable hyperemesis gravidarum who has been unable to tolerate oral intake for 4 days; urinalysis is strongly positive for ketones.pH: 7.437.43, PaCO2PaCO_2: 4444 mmHg, HCO3−HCO_3^-: 2828 mEq/L, Na+Na^+: 138138 mEq/L, Cl−Cl^-: 8686 mEq/L
  • A 68-year-old male with severe chronic obstructive pulmonary disease and baseline hypercapnia who recently started high-dose furosemide for cor pulmonale.pH: 7.417.41, PaCO2PaCO_2: 6060 mmHg, HCO3−HCO_3^-: 3737 mEq/L, Na+Na^+: 140140 mEq/L, Cl−Cl^-: 9393 mEq/L
  • A 45-year-old male with profuse, watery diarrhea for 5 days who subsequently develops profound hypotension (blood pressure 70/40 mmHg) and mottled extremities.pH: 7.227.22, PaCO2PaCO_2: 2020 mmHg, HCO3−HCO_3^-: 88 mEq/L, Na+Na^+: 138138 mEq/L, Cl−Cl^-: 110110 mEq/L
  • A 19-year-old female presenting to the triage desk with tinnitus and tachypnea 8 hours after an intentional overdose of an over-the-counter analgesic.pH: 7.477.47, PaCO2PaCO_2: 2020 mmHg, HCO3−HCO_3^-: 1414 mEq/L, Na+Na^+: 142142 mEq/L, Cl−Cl^-: 108108 mEq/L

Answer

The clinical vignettes represent complex mixed acid-base disorders that can be systematically diagnosed by calculating the anion gap, determining the delta-delta gap, and utilizing physiological respiratory compensation formulas to align each patient with their exact laboratory profile.
Each vignette correlates strictly to a specific mixed acid-base disturbance. The hyperemesis patient has a mixed HAGMA and metabolic alkalosis, identified by a high anion gap and a ΔAG\Delta AG indicating a measured HCO3−HCO_3^- much higher than expected. The COPD patient on diuretics has a primary chronic respiratory acidosis with an independent metabolic alkalosis, evidenced by a HCO3−HCO_3^- that exceeds normal chronic compensation. The patient with diarrhea and shock has a mixed NAGMA and HAGMA, proven by an expected HCO3−HCO_3^- based on the delta gap that is still higher than the actual measured HCO3−HCO_3^-. Finally, the salicylate toxicity patient matches the profile demonstrating the classic mixed primary respiratory alkalosis and primary HAGMA.

Step-by-Step Solution

1
Calculate the anion gap (AG=Na+−[Cl−+HCO3−]AG = Na^+ - [Cl^- + HCO_3^-]) for each laboratory profile.
Profiles 1, 3, and 4 have an elevated anion gap (>12>12), indicating the presence of a high anion gap metabolic acidosis (HAGMA). Profile 2 has a normal anion gap (1010).
Identifying a HAGMA is the foundational first step in uncovering mixed metabolic disorders.
2
Calculate the delta gap (ΔAG=Measured AG−12\Delta AG = Measured\ AG - 12) and expected HCO3−HCO_3^- (24−ΔAG24 - \Delta AG) for profiles with a HAGMA.
Profile 1 has a measured HCO3−HCO_3^- higher than expected (revealing a concomitant metabolic alkalosis). Profile 3 has a measured HCO3−HCO_3^- lower than expected (revealing a concomitant normal anion gap metabolic acidosis).
The delta gap uncovers hidden, concurrent metabolic derangements that the anion gap and pH alone cannot show.
3
Assess respiratory compensation for primary metabolic or respiratory disorders.
Profile 2 shows a measured HCO3−HCO_3^- higher than the expected chronic compensation for its elevated PaCO2PaCO_2. Profile 4 shows an alkalemic pH with low PaCO2PaCO_2, confirming a primary respiratory alkalosis with a superimposed HAGMA.
Applying specific compensation rules (such as Winter's formula or chronic PaCO2PaCO_2 adaptation curves) confirms whether the respiratory component is purely compensatory or a primary independent disorder.

Key Concept

Diagnosis of mixed acid-base disorders using the anion gap, delta-delta gap, and expected respiratory compensation.
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Topics

Nephrology

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