A 4 year old child presents with vomiting and shows signs of dehydration. What is his arterial blood gas profile likely to show?

In a child presenting with vomiting and signs of dehydration, the arterial blood gas profile is expected to show a high pH and high PCO2 due to the physiological responses to the loss of gastric acid and the resultant metabolic alkalosis.

Vomiting, especially if it is severe or prolonged, leads to the loss of hydrochloric acid from the stomach, which can result in a decrease in the hydrogen ions in the bloodstream, causing an increase in pH (alkalosis). In conjunction with metabolic alkalosis, the respiratory system may attempt to compensate by hypoventilating, which would lead to an accumulation of carbon dioxide (CO2) in the blood, as evidenced by an elevated PCO2 level.

Thus, in this scenario, the expected arterial blood gas profile reflects these changes: the high pH indicates alkalosis, and the high PCO2 reflects respiratory compensation, creating a picture consistent with a child suffering from dehydration and the effects of vomiting.