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Difficulty: MediumPhysiological Adaptation

A client is brought to the emergency department after intentionally ingesting a toxic amount of acetylsalicylic acid. The client exhibits tachypnea and diaphoresis. The initial arterial blood gas (ABG) results are: pH=7.53pH = 7.53, PaCO2=26 mm HgPaCO_2 = 26 \text{ mm Hg}, and HCO3−=22 mEq/LHCO_3^- = 22 \text{ mEq/L}. Based on these findings, which pathophysiological mechanism is the primary cause of this specific acid-base disturbance?

  1. A
    Accumulation of unmeasured acidic anions in the blood plasma
  2. B
    Profound depression of the central nervous system
  3. Direct stimulation of the medullary respiratory centerAnswer
  4. D
    Excessive renal excretion of hydrogen ions

Answer

The primary cause of the current acid-base disturbance is direct stimulation of the medullary respiratory center.
The provided ABG results reveal an elevated pH (alkalosis) and a decreased PaCO2 (respiratory origin) with a normal HCO3-. This indicates acute respiratory alkalosis. In the early phases of acetylsalicylic acid (aspirin) overdose, the salicylate acts directly on the medulla oblongata, heavily stimulating the respiratory center. This leads to marked tachypnea and hyperventilation, blowing off carbon dioxide and causing the specific laboratory findings seen in this client.

Step-by-Step Solution

1
Analyze the pH value to determine the primary acid-base state.
The pH is 7.53, which is greater than the normal range of 7.35-7.45. This indicates alkalemia.
Identifying whether the client is in an acidotic or alkalotic state is the foundational step of ABG interpretation.
2
Evaluate the respiratory component (PaCO2) to identify the etiology.
The PaCO2 is 26 mm Hg, which is below the normal range of 35-45 mm Hg.
A low PaCO2 in the presence of alkalemia confirms a respiratory etiology. The client is exhaling excessive amounts of acidic carbon dioxide.
3
Evaluate the metabolic component (HCO3-) for compensation.
The HCO3- is 22 mEq/L, which is within the normal range of 22-26 mEq/L.
A normal bicarbonate level indicates that the kidneys have not yet initiated a compensatory response to the acute respiratory alkalosis.
4
Correlate the ABG interpretation (acute respiratory alkalosis) with the clinical scenario of acetylsalicylic acid ingestion.
Acetylsalicylic acid (aspirin) acts directly on the brainstem to increase respiratory drive, causing the hyperventilation responsible for these lab values.
Connecting laboratory data to the pathophysiological mechanisms of a toxic ingestion is required to determine the correct underlying cause.

Key Concept

Acid-Base Imbalances and Toxic Ingestions
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Topics

Physiological Adaptation

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