HYPERTONIC SOLUTIONS AND INTRACRANIAL-PRE SSURE

The properties of the endothelium differ between the brain and the rem ainder of the body. In most non-CNS tissues the size of the junctions between endothelial cells averages 65 Angstrom. Proteins do not cross these gaps, while sodium does. In the brain, the junction size is only 7 Angstrom, which is too small to allow crossing by sodium. Investiga tions with changes in osmotic and oncotic pressure have demonstrated t hat: (1) reducing osmolality results in edema formation in all tissues including normal brain; (2) a decrease in oncotic pressure is only as sociated with peripheral edema but not in the brain; (3) in case of br ain injury, a decrease in osmolality elicits edema in the part of brai n which remained normal; (4) similarly, a decrease in oncotic pressure does not cause an increase in brain edema in the injured part of the brain. The determinant factor of water exchange in the brain is mediat ed through the osmolality and not the oncotic pressure. The use of hyp ertonic solutions (Ringer lactate or NaCl) for intravascular fluid res uscitation of patients suffering from hypovolemic head trauma has gain ed popularity. A research survey in regard with this observation can b e summarized as follows : NaCl 7.5% (2400 mOsm/l) is becoming the most popular hypertonic solution because of its favorable systemic and cer ebral effects. It improves myocardial contractility, precapillary dila tation, and reactive venoconstriction, and it has a plasmatic expansio n factor of 3.8. In regard to the brain tissue, it improves the PO2 an d the cerebral blood flow (CBF) as a result of decreasing cerebrovascu lar resistance. Finally, it reduces the cortical water content of inta ct blood-brain barrier area. The overall consequence is reduction of i ntracranial pressure (ICP). Although the homeostasis of the cerebral i ntracellular compartment remains unknown, it is possible that brain ce lls are able to resist important osmolar overload. NaCl 7.5%/dextran 7 0.6% is clinically at this moment the most studied hypertonic/hyperonc otic agent in prehospital emergencies. Its effects on cerebral homeost asis are identical to NaCl 7.5%. However, the addition of a colloid ag ent has the advantage of prolonging the systemic effects without affec ting the brain. The plasmatic expansion factor is 4.5, which is slight ly superior to NaCl 7.5%. Mannitol improves CBF by maintaining autoreg ulation as a result of changes in viscosity and reactive cerebrovascul ar constriction. It generates an osmotic gradient which reduces the ce rebral volume and subsequently the ICI? In the presence of a cryogenic cerebral lesion, its reductive effects on brain water are superior to the hypertonic/hyperoncotic solution. Because mannitol has less spect acular systemic responses than the other solutions, it is not indicate d for resuscitation following hemorrhagic shock. In conclusion, it is important to note that hypotension and hypoxemia represent the most de terminant factors of secondary cerebral insults. Therefore, in the pre sence of patients with head injury and especially hemorrhagic shock, i t is essential to ensure a cerebral perfusion pressure (CPP) of >80 mm Hg. Hypertonic solutions have gained popularity in these clinical sit uations because of their combined effects on ICP mean arterial pressur e (MAP) and CPP. However, the therapeutic approach to polytraumatized patients with small intravascular volume (4-6 ml/kg) of hypertonic sol utions should not be a substitute for the usual volemic resuscitation technique. The clinical indication for these solutions should be limit ed to the initial resuscitation maneuvers in traumatized patients. Pro longed use of hypertonic solutions for the purpose of intravascular re suscitation would only contribute to increasing the side effects and e ventually counteract the initial beneficial advantages.