Effects of mannitol on blood volume and central hemodynamics in patients undergoing cerebral aneurysm surgery.

The effects on hemodynamics and blood volume of 500 ml of 20% mannitol administered intravenously in 15 min at the beginning of cerebral aneurysm surgery have been studied in 10 patients. Measurements were made before the infusion of mannitol (control) and at 15-min intervals for 1 hr. Control measurements showed normal hemodynamic data, while blood volume was lower than normal (P less than 0.001). Immediately after the mannitol infusion cardiac index (25%; P less than 0.01), pulmonary capillary wedge pressure (48%; P less than 0.001), and blood volume (43%; P less than 0.001) increased. Thirty minutes after the mannitol infusion, blood volume had returned to control levels, while the cardiac index and pulmonary capillary wedge pressure decreased 21% (P less than 0.01 and P less than 0.05, respectively) below control levels. Forty-five minutes after the mannitol infusion, serum osmolality and urine volume remained high. Our data confirm the presence of hypovolemia in patients with subarachnoid hemorrhage and a transient increase in blood volume associated with the infusion of mannitol. The data emphasize, however, that the hemodynamic response is biphasic, with an initial increase in pulmonary capillary wedge pressure and cardiac index, followed by a hypokinetic circulatory pattern with pulmonary capillary wedge pressure and cardiac index below control levels. The hypokinetic state occurred in spite of return of blood volume to control levels, suggesting that redistribution of blood from central to peripheral circulatory compartments had occurred.