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Difficulty: Very hardPhysiological Adaptation

A nurse in the emergency department evaluates the arterial blood gas (ABG) results of a client who intentionally ingested a toxic amount of acetylsalicylic acid 8 hours ago. The client is lethargic, diaphoretic, and breathing deeply at a rate of 34 breaths/minute.

ABG results (on room air):
- pH: 7.42
- PaCO2: 24 mm Hg
- HCO3-: 14 mEq/L
- PaO2: 95 mm Hg

Based on these clinical findings, which conclusion about the client's physiological acid-base status is most accurate?

  1. A
    Primary metabolic acidosis that has been completely compensated by the respiratory system.
  2. A mixed acid-base disorder consisting of concurrent primary respiratory alkalosis and primary metabolic acidosis.Answer
  3. C
    Primary respiratory alkalosis that has been completely compensated by the renal system.
  4. D
    Uncompensated respiratory alkalosis resulting from direct stimulation of the respiratory center.

Answer

The client is experiencing a mixed acid-base disorder consisting of concurrent primary respiratory alkalosis and primary metabolic acidosis.
A normal pH (7.35-7.45) accompanied by significantly abnormal PaCO2 and HCO3- values moving in opposite pH directions indicates a mixed acid-base disorder. Physiological compensatory mechanisms never completely normalize the pH. Salicylate toxicity causes two concurrent primary disorders: direct stimulation of the medullary respiratory center (primary respiratory alkalosis) and uncoupling of oxidative phosphorylation, which leads to the accumulation of organic acids (primary metabolic acidosis).

Step-by-Step Solution

1
Analyze the pH to determine the primary direction of the acid-base imbalance.
The pH is 7.42, which is within the normal range (7.35-7.45) but leans slightly toward the alkalotic side.
Evaluating the pH is the first step in the standard ABG interpretation process.
2
Analyze the PaCO2 and HCO3- values.
The PaCO2 is 24 mm Hg (indicating respiratory alkalosis) and the HCO3- is 14 mEq/L (indicating metabolic acidosis).
This determines which systems are contributing to the acid-base derangement.
3
Evaluate for physiological compensation versus a mixed acid-base disorder.
Physiological compensatory mechanisms do not completely normalize the pH. A normal pH accompanied by significantly abnormal opposing PaCO2 and HCO3- values indicates a mixed acid-base disorder.
This differentiates a simple compensated disorder from a complex mixed disorder.
4
Correlate the ABG interpretation with the pathophysiology of salicylate toxicity.
Aspirin overdose causes direct medullary stimulation (primary respiratory alkalosis) and uncouples oxidative phosphorylation leading to organic acid accumulation (primary metabolic acidosis).
Clinical context confirms the presence of two concurrent primary disorders, matching the ABG findings.

Key Concept

Advanced Interpretation of Mixed Acid-Base Disorders in Salicylate Toxicity
Estimated Time:1m 30s
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Topics

Physiological Adaptation

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