A surprisingly common misconception is that metabolic acidosis is the most common acid-base disorder in the ICU. Metabolic alkalosis is the most common acid-base disorder. True, in the acute phase metabolic acidosis is a lot more common but in the long run (due to chloride losses, diuretics and hypoalbuminemia) alkalosis is a lot more common.
After hearing the opposite one too many times, a frustrated norwegian professor of anesthesia and intensive care named Erik Waage Nilsen spent five minutes of data base work on extracting 22.583 base excess measurements from all the arterial blood gasses obtained at his ICU for the last three years. He then published the frequency histogram below.
(Antall = Number N, baseoverskudd = base excess)
He also noted that if there is any bias, it will be towards acidosis as we obviously do more arterial blood gasses in the early acute phase.
Found it in a norwegian medical journal, here (in norwegian).