An infant with prolonged vomiting secondary to pyloric stenosis has arterial blood gases (ABG) drawn. Which ABG results should a nurse expect when reviewing the laboratory report if the infant has an acid-base imbalance?

Choice A rationale Respiratory alkalosis is primarily caused by hyperventilation, resulting in a low PaCO2. While the pH is elevated in this choice, pyloric stenosis causes a metabolic loss of hydrochloric acid through vomiting. This results in an accumulation of bicarbonate, not a primary respiratory issue. Metabolic compensation for respiratory alkalosis would involve the kidneys excreting bicarbonate, which would lead to a low HCO3 level. This does not match the physiological consequences of gastric acid loss seen in infants with pyloric stenosis. Choice B rationale Respiratory acidosis is characterized by a low pH and a high PaCO2 due to alveolar hypoventilation. Pyloric stenosis specifically leads to the loss of hydrogen and chloride ions from the stomach, which produces an alkalotic state, not an acidotic one. While the body might eventually show metabolic changes, the initial and primary insult in vomiting is the elevation of pH. A decreased pH is scientifically inconsistent with the persistent loss of gastric acid associated with an upper gastrointestinal obstruction. Choice C rationale Pyloric stenosis causes severe vomiting, leading to the loss of gastric hydrochloric acid. This results in metabolic alkalosis, characterized by an elevated pH above 7.45 and an increased HCO3 above 28 mEq/L. To compensate, the respiratory system slows the rate and depth of breathing to retain carbon dioxide, which acts as an acid. This respiratory compensation results in an increased PaCO2. Therefore, an elevated pH, increased HCO3, and increased PaCO2 are the expected findings in this clinical metabolic imbalance. Choice D rationale Metabolic acidosis is defined by a low pH and a low HCO3 level, often caused by diarrhea or renal failure. In pyloric stenosis, the infant is losing acid, which leads to a deficit of hydrogen ions and a relative excess of base. This produces alkalosis, not acidosis. Respiratory compensation for metabolic acidosis would involve hyperventilation to decrease PaCO2, which is the opposite of the hypoventilation expected when the body tries to compensate for a high systemic pH.