Intervals for Assessing Volume, Osmolality, and GCS Change

We used the acquisition times of two sequential head CT scans acquired within 48 hours of one another to define intervals over which changes in brain volume and corresponding changes in GCS, individual osmolytes, and total serum osmolality were assessed. Figure 1 illustrates how these assessment intervals were defined. We used patients’ initial assessment intervals for our primary analysis but also employed repeated measures statistical techniques to analyze all available assessment intervals. In these patients, we routinely monitor serum osmolality and chemistry panels at least every 6 hours and calculate serum osmolar gaps.(31)

Volume change assessment intervals were defined for each patient as the time between subsequent computed tomography (CT) scans. Cerebrospinal fluid volume was measured on each CT scan and the change in volume was calculated. The corresponding changes in serum osmolality, serum chemistries, and Glasgow Coma Scale (GCS) neurologic examinations for each assessment interval were determined using values collected nearest the acquisition time of the CT scans. For GCS, we used the hourly neurologic assessment immediately prior to CT scan. Only the initial assessment intervals were used for Spearman correlations and linear and ordinal regression models. All assessment intervals were used for generalized estimating equation models.