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Difficulty: MediumPhysiological Adaptation

A nurse is assessing a client who has been receiving continuous nasogastric (NG) tube suctioning for 48 hours due to a mechanical small bowel obstruction. The client is now reporting severe muscle cramping and tingling in the extremities. The nurse observes that the client's respirations are shallow at a rate of 10 breaths/min. Based on the client's clinical presentation and the physiological effects of gastric fluid loss, which of the following arterial blood gas (ABG) results should the nurse anticipate?

  1. pH 7.51, PaCO2 49 mm Hg, HCO3 34 mEq/LAnswer
  2. B
    pH 7.29, PaCO2 56 mm Hg, HCO3 26 mEq/L
  3. C
    pH 7.32, PaCO2 40 mm Hg, HCO3 18 mEq/L
  4. D
    pH 7.49, PaCO2 29 mm Hg, HCO3 24 mEq/L

Answer

The expected result is pH 7.51, PaCO2 49 mm Hg, HCO3 34 mEq/L, representing metabolic alkalosis with partial respiratory compensation.
The correct answer reflects an elevated pH (alkalosis) and an elevated HCO3 (metabolic origin), which is the direct result of acid loss from NG suctioning. Furthermore, the elevated PaCO2 correctly demonstrates the body's physiological adaptation of hypoventilating (shallow respirations at 10 breaths/min) to retain acid and partially compensate for the alkalosis.

Step-by-Step Solution

1
Identify the primary source of fluid loss and its effect on acid-base balance.
Continuous NG suctioning removes significant amounts of stomach acid (hydrochloric acid).
The loss of acid leaves the body with a relative excess of base (bicarbonate), leading to primary metabolic alkalosis (elevated pH and elevated HCO3).
2
Analyze the client's symptoms.
The client has muscle cramping, tingling, and a decreased respiratory rate (10 breaths/min).
Alkalosis causes a decrease in ionized calcium, resulting in neuromuscular irritability (tingling and cramping). The shallow, slow breathing is the body's expected physiological response.
3
Determine the body's compensatory mechanism for the primary imbalance.
The lungs compensate for metabolic alkalosis by hypoventilating.
Hypoventilation retains carbon dioxide (an acid) to help lower the pH back toward normal, resulting in an elevated PaCO2 level (partial respiratory compensation).
4
Select the ABG values that match these findings.
Identify the option with elevated pH (> 7.45), elevated HCO3 (> 26 mEq/L), and elevated PaCO2 (> 45 mm Hg).
Only the correct option demonstrates metabolic alkalosis with an elevated PaCO2, reflecting appropriate respiratory compensation.

Key Concept

Continuous loss of gastric acid through NG suctioning or vomiting leads to metabolic alkalosis. The body compensates for this imbalance through respiratory hypoventilation to retain CO2.
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Topics

Physiological Adaptation

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