I magine a patient who is admitted to your intensive care unit (ICU) with the diagnosis of gastrointestinal bleeding. Even before the patient is admitted, he is hypovolemic from blood loss. As in most cases, he is intravascularly depleted because of inadequate oral intake due to vomiting and diarrhea. Because of vomiting and diarrhea, he is not getting the fluids needed to help replace the lost volume.

Upon admission, laboratory work is drawn—a complete metabolic profile, complete blood cell count with differential prothrombin time, partial thromboplastin time, and blood type and cross-match. This amounts to approximately 40 mL of blood drawn from a patient who is already volume depleted.

We start administering the intravenous fluids, transfused with packed red blood cells. Because the patient is actively bleeding, we will recheck his hemoglobin and hematocrit levels every 4 hours. In a 24-hour period, that means anywhere from 80 to 100 mL...

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