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Difficulty: MediumPhysiological Adaptation

A nurse is evaluating a client who was admitted with a mechanical small bowel obstruction. For the past 48 hours, the client has been treated with a nasogastric (NG) tube set to continuous low wall suction, yielding high outputs of gastric drainage. The nurse notes the client is becoming lethargic with a respiratory rate of 10 breaths/min and shallow chest excursion. Based on these clinical findings, which of the following arterial blood gas (ABG) results should the nurse anticipate?

  1. pH 7.52, PaCO2 53 mm Hg, HCO3- 35 mEq/LAnswer
  2. B
    pH 7.28, PaCO2 32 mm Hg, HCO3- 15 mEq/L
  3. C
    pH 7.31, PaCO2 54 mm Hg, HCO3- 28 mEq/L
  4. D
    pH 7.48, PaCO2 28 mm Hg, HCO3- 22 mEq/L

Answer

An ABG showing a high pH, high PaCO2, and high HCO3- (pH 7.52, PaCO2 53 mm Hg, HCO3- 35 mEq/L)
The correct ABG result demonstrates a high pH (>7.45) and a high HCO3- (>26 mEq/L), which correctly identifies the primary metabolic alkalosis caused by the continuous suctioning of gastric acid. Furthermore, the elevated PaCO2 (>45 mm Hg) accurately reflects the client's compensatory hypoventilation (retaining acidic CO2 to balance the alkalosis), which aligns with the nurse's assessment of a low respiratory rate and shallow breathing.

Step-by-Step Solution

1
Identify the primary acid-base disturbance based on the clinical scenario.
Prolonged nasogastric suction removes hydrochloric acid from the stomach, leading to a primary metabolic alkalosis.
Loss of gastric acid leaves a relative excess of bicarbonate in the blood, elevating both the pH and the HCO3- levels.
2
Determine the expected physiological compensation for the identified disturbance.
The respiratory system compensates for metabolic alkalosis by hypoventilating (decreasing respiratory rate and depth).
Hypoventilation retains CO2 in the blood. Since CO2 acts as an acid in the body, retaining it helps lower the alkaline pH back toward a normal range.
3
Correlate the client's current symptoms with the expected ABG changes.
The respiratory rate of 10 breaths/min and shallow excursion confirms compensatory hypoventilation, which will result in an elevated PaCO2 on the ABG.
The combination of elevated pH, elevated HCO3- (primary issue), and elevated PaCO2 (compensatory response) points exclusively to partially compensated metabolic alkalosis.

Key Concept

Acid-Base Imbalances and Compensatory Mechanisms
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Topics

Physiological Adaptation

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