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Difficulty: HardPhysiological Adaptation

An older adult client with a 3-day history of severe, persistent vomiting is admitted to the medical unit. The client appears lethargic, and the nurse notes a respiratory rate of 10 breaths per minute. Arterial blood gas (ABG) results reveal a pH of 7.50, PaCO2 of 54 mm Hg, and HCO3- of 38 mEq/L. Which of the following physiological adaptations best explains this client's respiratory pattern?

  1. A compensatory decrease in alveolar ventilation designed to retain carbon dioxide and lower blood pHAnswer
  2. B
    A primary accumulation of carbon dioxide that directly depresses the central respiratory drive
  3. C
    A renal compensatory mechanism that retains excess bicarbonate in response to primary hypoventilation
  4. D
    A systemic response to profound fluid volume deficit that impairs pulmonary perfusion and oxygenation

Answer

The correct response describes a compensatory decrease in alveolar ventilation designed to retain carbon dioxide and lower the blood pH.
The correct answer accurately links the clinical findings to the underlying physiological process. Severe vomiting causes a loss of gastric acid, resulting in metabolic alkalosis (indicated by the pH of 7.50 and HCO3- of 38 mEq/L). The body's natural physiological adaptation to metabolic alkalosis is respiratory compensation. The respiratory center decreases the rate and depth of breathing (hypoventilation, seen here as 10 breaths per minute) to retain carbon dioxide. This retained carbon dioxide forms carbonic acid in the blood (reflected by the elevated PaCO2 of 54 mm Hg), which helps to lower the pH back toward the normal range.

Step-by-Step Solution

1
Analyze the ABG results to determine the primary acid-base imbalance.
pH is 7.50 (alkalosis). HCO3- is 38 mEq/L (alkalosis). PaCO2 is 54 mm Hg (acidosis).
Matching the pH direction with the abnormal values identifies the primary disorder as metabolic alkalosis.
2
Correlate the primary disorder with the client's clinical history.
Severe vomiting causes significant loss of gastric acid (hydrochloric acid), leading directly to metabolic alkalosis.
Connecting the lab values to the etiology confirms the diagnosis of primary metabolic alkalosis.
3
Evaluate the body's compensatory mechanisms for this specific imbalance.
The respiratory system compensates for metabolic alkalosis by retaining acid (PaCO2) to lower the pH toward the normal range.
The body attempts to restore the 20:1 ratio of bicarbonate to carbonic acid.
4
Identify the physical manifestation of this respiratory compensation.
To retain PaCO2, the respiratory center decreases the rate and depth of breathing (hypoventilation), resulting in the observed respiratory rate of 10 breaths per minute.
This confirms that the slow respiratory rate is a deliberate physiological adaptation, not a sign of respiratory failure.

Key Concept

Acid-Base Imbalances and Compensatory Mechanisms
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Topics

Physiological Adaptation

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