A 34-year-old male is recovering from head trauma sustained in a motor vehicle accident. He is currently in the intensive care unit on mechanical ventilation. His most recent arterial blood gas analysis shows:
Which of the following additional findings do you most expect in this patient?

Question

A 34-year-old male is recovering from head trauma sustained in a motor vehicle accident.  He is currently in the intensive care unit on mechanical ventilation.  His most recent arterial blood gas analysis shows:
 Which of the following additional findings do you most expect in this patient?

Quizplus · Accepted Answer

While being mechanically ventilated this patient is vulnerable to forced hyper- or hypoventilation. Ventilation is defined as the tidal volume multiplied by the respiratory rate. Hyperventilation (caused by increased tidal volume and/or respiratory rate) results in excessive CO₂ losses and respiratory alkalosis, while hypoventilation (caused by decreased tidal volume and/or respiratory rate) results in CO₂ retention and respiratory acidosis. The patient described is experiencing respiratory alkalosis as evidenced by his increased arterial blood pH (normal = 7.4) and his decreased PaCO₂ (normal = 40 mmHg). His serum bicarbonate level is decreased due to attempted renal compensation for the respiratory alkalosis. Specifically, the kidneys retain increased amounts of H⁺ (protons) and excrete increased amounts of bicarbonate (HCO₃-) in an attempt to normalize the serum pH. The increased amount of HCO₃- in the urine alkalinizes the urine. (Choice A) Renal compensation for respiratory or metabolic acidosis involves low urine bicarbonate excretion. (Choice C) Increased serum aldosterone would result in increased H⁺ and K⁺ losses in the urine and serum alkalosis. "Contraction alkalosis" occurs in states of intracellular volume contraction, where increased levels of aldosterone function to restore intravascular volume but also cause increased urinary proton loss. (Choice D) A high serum anion gap occurs in metabolic acidoses resulting from diabetic ketoacidosis, renal failure, lactic acidosis, and methanol, ethylene glycol or salicylate poisoning. (Choice E) A high serum albumin level may indicate hemoconcentration, as occurs in dehydration. Educational objective: The kidney compensates for respiratory alkalosis by preferentially excreting bicarbonate in the urine. The result is an alkalinized urine (increased urine pH).

A 34-Year-Old Male Is Recovering from Head Trauma Sustained in a Motor