Osmolality changes during mannitol therapy in cerebral oedema

Introduction

Mannitol is still used for reduction of intracranial pressure (ICP) in cerebral oedema. Its therapeutic effect might be caused by different mechanisms. One of them is an increase in blood-brain osmotic gradient, which is followed by a reduction of the brain water.

Method

Our investigation was prospectively carried out on 60 ICU patients. Fifty-eight patients had acute brain disease with Glasgow Coma Scale (GCS) in the range 11–14 (tumour, 23 patients; trauma, four patients; subarachnoid hemorrhage, eight patients; intracerebral haemorrhage, 21 patients; infection, one patient; epilepsy, one patient) and two patients had spine trauma.

The investigation started at least 24 hours from the beginning of therapy with 20% mannitol. Serum sodium and osmolality were measured before and after the mannitol infusion and before the next dose. Patients were divided into three groups, which differed in doses of mannitol and intervals between infusions (each infusion lasted 20 min): I, 40 g/day (10 g/6 hours); II, 60 g/day (10 g/4 hours); III, 80 g/day (20 g/6 hours).

No other hypertonic agents were used. All samples were taken from the same vein on an extremity where neither mannitol nor any other infuse were applied.

Results

There were no significant changes in blood sodium concentration or osmolality in any of the groups. For parameters, reference ranges, mean values ± standard deviations in samples 1, 2 and 3, see Table 1.

Table 1

Conclusion

This prospective study does not show any evidence that 20% mannitol significantly altered blood sodium or osmolality values in CNS diseases, although the osmolality was around the upper reference range. Therefore, there still remains a question, whether this significant elevation of serum osmolality is sufficient for a reduction of the brain volume or whether the reduction is caused by a different mechanism.