DEFICITS

Characteristics of Fluid Deficits

The fluid deficit equals the maintenance fluid requirement multiplied by the hours since last intake plus unreplaced preoperative external and third space losses. When hypovolemia is present, sufficient fluid should be infused to restore mean arterial pressure, heart rate, and filling pressures to near-normal values before induction. If sufficient time is available, restoration of normal urine flow rate is also desirable. The fluid infusion rate for normal patients should then be set to deliver three to four times the maintenance rate until the calculated deficit has been corrected.

Electrolyte abnormalities are common among hospitalized patients. Surgical urgency often forces us to evaluate and correct preexisting abnormalities that may be directly related to the surgical problem or to comorbid diseases or their therapy. The principles of management are outlined in other sections of this chapter. The induction of anesthesia and the onset of mechanical ventilation, fluid shifts, and stress responses induced by surgical trauma all lead to redistribution of water, protein, and electrolytes. The most common and most studied of these are abnormalities of potassium, calcium, and magnesium.