In many of these procedures, blood loss can be significant and easily exceed calculated blood volume. It is difficult to accurately measure blood loss when it is lost under the drapes or irrigated away. Meticulous weighing of the sponges and observation of suction canisters are important, but blood loss can be continuous from the time of skin incision and seems relentless for an inexperienced clinician. Absolute changes in CVP are a relative guide to the volume in the central compartment, but no one isolated value correlates with a known blood volume. A steady decrease in CVP in association with a downward trend in mean arterial pressures (MAPs) during periods of surgical intervention known to produce significant bleeding should be treated aggressively with crystalloid (20 mL/kg) or colloid (ie, 5% albumin) replacement first, followed by a vasopressor as necessary. Ideally, with an arterial line in place and point-of-care testing, one can follow the trend in hemoglobin prior to initiation of blood transfusion. Persistent hypotension with a normal hematocrit could be due to hypocalcemia, either from parathyroid hormone insufficiency or from citrate (used as an anticoagulant in blood products), which binds ionized calcium if the patient was recently transfused.