Physical signs of hypovolemia are insensitive and nonspecific (Table 14-16: Signs and Symptoms of Hypovolemia). A normal blood pressure reading may represent relative hypotension in an elderly or chronically hypertensive patient. Conversely, substantial hypovolemia may occur despite an apparently normal blood pressure and heart rate.
Elderly patients may demonstrate orthostatic hypotension despite a normal blood volume.
Young, healthy subjects can tolerate an acute blood loss equivalent to 20% of their blood volume while exhibiting only postural tachycardia and variable postural hypotension.
Orthostatic changes in central venous pressure, coupled with assessment of the response to fluid infusion, may represent a useful test of the adequacy of blood volume.
Hematocrit is a poor indicator of blood volume because it is influenced by the time elapsed since hemorrhage and the volume of asanguineous fluid replacement. Hematocrit is virtually unchanged by acute hemorrhage; later, hemodilution occurs as fluids are administered or as fluid shifts from the interstitial to the intravascular space. If the intravascular fluid volume has been restored, hematocrit measurement will more accurately reflect red blood cell mass and can be used to guide transfusion.
Blood urea nitrogen and serum creatinine levels may be increased if hypovolemia is sufficiently prolonged. (Both measurements may also be influenced by events unrelated to blood volume.) Although hypovolemia does not cause metabolic alkalosis, ECF volume depletion is a potent stimulus for the maintenance of metabolic alkalosis.
Intraoperative Clinical Assessment. Visual estimation, as seen on operative sponges and drapes, is the simplest technique for quantifying intraoperative blood loss.
Preservation of the blood pressure accompanied by a central venous pressure of 6 to 12 mm Hg in the presence of a volatile anesthetic suggests an adequate blood volume.
During profound hypovolemia, indirect measurement of blood pressure may significantly underestimate true blood pressure, emphasizing the potential value of direct blood pressure measurements in selected patients.
An additional advantage of direct arterial pressure monitoring may be recognition of increased systolic blood pressure variation accompanying positive-pressure ventilation in the presence of hypovolemia. Although pulse pressure variation may be superior to central venous pressure as a means to predict cardiac output increase in response to a fluid challenge, it is inconclusive in approximately 25% of patients.
Urinary output usually decreases precipitously (<0.5 mL/kg/hr) in the presence of moderate to severe hypovolemia.
Oxygen Delivery as a Goal of Management. No intraoperative monitor is sufficiently sensitive or specific to detect hypoperfusion in all patients. In high-risk surgical patients, systemic oxygen delivery of 600 mL/m2/min or above (equivalent to a cardiac index of 3 L/m2/min and a hemoglobin concentration equivalent to 14 g/dL) may result in improved outcome.