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Difficulty: MediumPhysiological Adaptation

A nurse is performing a postoperative assessment on a client who is utilizing a patient-controlled analgesia (PCA) pump with morphine following a right total knee arthroplasty. The client is lethargic, difficult to arouse, and has a respiratory rate of 8 breaths per minute with shallow chest excursion. Based on these assessment findings, which arterial blood gas (ABG) result should the nurse anticipate?

  1. pH 7.26, PaCO2 52 mm Hg, HCO3- 25 mEq/LAnswer
  2. B
    pH 7.48, PaCO2 28 mm Hg, HCO3- 23 mEq/L
  3. C
    pH 7.31, PaCO2 39 mm Hg, HCO3- 16 mEq/L
  4. D
    pH 7.37, PaCO2 54 mm Hg, HCO3- 30 mEq/L

Answer

The ABG result showing pH 7.26, PaCO2 52 mm Hg, and HCO3- 25 mEq/L correctly represents acute uncompensated respiratory acidosis.
The correct ABG values reflect acute uncompensated respiratory acidosis. Morphine is a central nervous system depressant that can cause hypoventilation. When a client breathes too slowly and shallowly, they retain carbon dioxide (PaCO2 > 45 mm Hg). This excess CO2 lowers the blood pH (< 7.35). Because this is an acute event, the kidneys have not yet compensated by retaining bicarbonate, so the HCO3- remains normal (22-26 mEq/L).

Step-by-Step Solution

1
Identify the primary physiological derangement based on the clinical assessment.
The client has a respiratory rate of 8 breaths per minute and shallow breathing due to opioid (morphine) administration, indicating severe hypoventilation.
Recognizing the clinical state is required to predict the corresponding acid-base imbalance.
2
Determine the effect of hypoventilation on arterial blood gases.
Hypoventilation leads to the retention of carbon dioxide (PaCO2 > 45 mm Hg), which forms carbonic acid in the blood, resulting in a decreased pH (< 7.35).
Carbon dioxide acts as an acid in the body; its accumulation directly causes respiratory acidosis.
3
Evaluate the chronicity of the event to determine compensation.
Because this is an acute postoperative event, the kidneys have not had sufficient time (which typically takes 2-3 days) to retain bicarbonate (HCO3-). Therefore, the HCO3- level will remain within the normal range (22-26 mEq/L).
Differentiating acute from chronic acid-base imbalances relies on the presence or absence of metabolic compensation.

Key Concept

Opioid-induced respiratory depression causes hypoventilation, leading to carbon dioxide retention and acute uncompensated respiratory acidosis.
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Topics

Physiological Adaptation

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