THE EFFECT OF THE REDUCTION OF COLLOID ONCOTIC PRESSURE, WITH AND WITHOUT REDUCTION OF OSMOLALITY, ON POSTTRAUMATIC CEREBRAL EDEMA

Background: It has been asserted that reduction of colloid oncotic pre ssure (COP) can aggravate traumatic brain edema. To explore this issue , the authors measured the effect of COP reduction, with and without a simultaneous decrease in osmolality, on the development of brain edem a after fluid percussion injury (FPI). Methods: Isoflurane-anesthetize d Wistar rats received a 2.7-atm right parasagittal FPI followed by is ovolemic exchange with (1) normal saline (NS); (2) half-normal saline (0.5 NS); (3) whole blood (WB); or (4) hetastarch (Hespan, Dupont). Sh ed blood (16 mi) was replaced with donor erythrocytes suspended in the study fluid. The WB group received heparinized fresh donor WB. Centra l venous pressure was maintained with additional study fluid as requir ed. The specific gravity (SG) of the cortex and subcortex near the imp act site was determined 4.5 h after FPI. The water content of the hemi spheres was also determined using the wet-dry method. To define the st atus of the blood-brain barrier in the non-FPI hemisphere, two additio nal groups (FPI, non-FPI) were studied. Both groups received 30 mg/kg Evans' blue and NS at 4 ml/kg(-1)/h(-1). Four hours after FPI, the con centration of Evans' blue in the hemispheres was determined. Results: After exchange, COP (mmHg +/- SD) decreased in the NS (9.6 +/- 2.1) an d 0.5 NS (8.5 +/- 0.5) groups and was unchanged in the WB (16.7 +/- 3. 3) and hetastarch (18.9 +/- 1.1) groups. Osmolality was unchanged in t he WB group (295 +/- 5 mOsm/kg), increased in the NS (304 +/- 3 mOsm/k g) and hetastarch (306 +/- 2 mOsm/kg) groups, and was decreased in the 0.5 NS group (261 +/- 6 mOsm/kg). The Evans' blue data indicated that FPI resulted in blood-brain barrier damage in both hemispheres. In al l four exchange groups, the SG of both cortical and subcortical tissue was less (indicating greater water content) in the impact hemisphere than in the nonimpact hemisphere. The SG was less in both hemispheres, although it was less in both hemispheres in the NS and 0.5 NS groups than in the WB and hetastarch groups. The lowest SG values were observ ed in the 0.5 NS group. The wet-dry water content determinations yield ed a similar pattern of edema formation. Conclusions: These data, whil e confirming the important edematogenic effect of decreased osmolality , indicate that COP reduction per se can also aggravate brain edema af ter a mild to moderate mechanical head injury.