Calculated Serum Osmolality Can Lead to a Systematic Bias Compared to Direct Measurement

Agreement of measured and calculated serum osmolality during the infusion of mannitol or hypertonic saline in patients after craniotomy: a prospective, double-blinded, randomised controlled trial

Background

Mannitol and hypertonic saline are used to ameliorate brain edema and intracranial hypertension during and after craniotomy. We hypothesized that the agreement of measured and calculated serum osmolality during the infusion of hypertonic saline would be better than mannitol. The objective was to determine the accuracy of serum osmolality estimation by different formulas during the administration of hyperosmolar agent.

Methods

A prospective, randomized, double-blinded, controlled trial was conducted in a 30-bed neurosurgical intensive care unit at a university hospital. Thirty-five adult patients requiring the use of hyperosmolar agents for prevention or treatment of brain edema after elective craniotomy were enrolled, and randomly assigned 1:1 to receive 125 mL of either 20 % mannitol (mannitol group) or 3.1 % sodium chloride solution (hypertonic saline group) in 15 min. Serum osmolality, serum sodium and potassium concentration, blood urea nitrogen and blood glucose concentration were measured during the study period. The primary outcome was the agreement of measured and estimated serum osmolality during the infusion of the two experimental agents. We used Bland and Altman’s limits of agreement analysis to clarify the accuracy of estimated serum osmolality. Bias and upper and lower limits of agreement of bias were calculated.

Results

For each formula, the bias was statistically lower in hypertonic saline group than mannitol group (p < 0.001). Within group comparison showed that the lowest bias (6.0 [limits of agreement: −18.2 to 30.2] and 0.8 [−12.9 to 14.5] mOsml/kg in mannitol group and hypertonic saline group, respectively) was derived from the formula ‘2 × ([serum sodium] + [serum potassium]) + [blood urea nitrogen] + [blood glucose]’.

Conclusions

Compared to mannitol, a better agreement between measured and estimated serum osmolality was found during the infusion of hypertonic saline. This result indicates that, if hypertonic saline is chosen to prevent or treat brain edema, calculated serum osmolality can be used as a reliable surrogate for osmolality measurement.

Trial registration

ClinicalTrials.gov identifier: NCT02037815

Correlation of measured and calculated serum osmolality during mannitol or hypertonic saline infusion in patients after craniotomy: a study protocol and statistical analysis plan for a randomised controlled trial

INTRODUCTION

Brain oedema is a major complication after craniotomy. Hyperosmolar agents have been used as the medical treatment for this condition. Measurement and estimation of serum osmolality during hyperosmolar agent infusion is of clinical importance to evaluate clinical efficacy, adjust dosage and avoid side effects. However, several studies have shown that calculated serum osmolality may lead to a systematic bias compared with direct measurement. In the present study, mannitol or hypertonic saline (HS) will be used in patients after elective craniotomy. We aim to determine the accuracy of serum osmolality estimation during the application of hyperosmolar agent.

METHODS AND ANALYSIS

The study is a prospective, randomised, double-blinded, controlled, parallel-group design. Adult patients requiring the use of hyperosmolar agents for the prevention or treatment of postoperative brain oedema are enrolled and assigned randomly to one of the two treatment study groups, labelled as 'M group' and 'HS group'. Patients in the M and HS groups receive intravenous infusion of 125 mL of either 20% mannitol or 3.1% sodium chloride solution, respectively. Data will be collected immediately before the infusion of study agents, 15, 30, 60, 120, 240 and 360 min after the start of infusion of experimental agents, which includes serum osmolality, concentration of serum sodium, potassium, urea and glucose. Serum osmolality will be measured by means of freezing point depression. Estimated serum osmolality will also be calculated by using four formulas published previously. Osmole gap is calculated as the difference between the measured and the estimated values. The primary endpoint is the correlation of measured and estimated serum osmolality during hyperosmolar agent infusion.

ETHICS AND DISSEMINATION

The study was approved by the International Review Board (IRB) of Beijing Tiantan Hospital, Capital Medical University. Study findings will be disseminated through peer-reviewed publications and conference presentations.

TRIAL REGISTRATION NUMBER

ClinicalTrials.gov identifier: NCT02037815.

Hypertonic saline solutions for treatment of intracranial hypertension

PURPOSE OF REVIEW

This review aims to provide an update on recent knowledge gained on hypertonic saline solutions for the treatment of intracranial hypertension. Explanatory approaches to the mechanisms underlying the edema-reducing effects of the solutions are outlined, practical aspects of use are presented, and trials that assessed their clinical utility are highlighted.

RECENT FINDINGS

With an established trauma system, hypertonic saline added to conventional fluid resuscitation did not improve long-term outcome in multiple injury with hypotension and brain trauma. In intensive care, hypertonic saline reduced intracranial hypertension after subarachnoid haemorrhage, brain trauma, and a variety of other brain diseases, including cerebral edema in acute liver failure.

SUMMARY

Hypertonic saline solutions have evolved as an alternative to mannitol or may be used in otherwise refractory intracranial hypertension to treat raised intracranial pressure. With high osmolar loads, the efficacy of the solution is enhanced, but no simple relationship between the saline concentration and the clinical effects of a solution is established. Caution is advised with high osmolar loads because they carry increased risks for potentially deleterious consequences of hypernatremia or may induce osmotic blood-brain barrier opening with possibly harmful extravasation of the hypertonic solution into the brain tissue.