ObjectivesTo study sequential changes in heart rate, respiratory rate, blood pressure, heart rate power spectra, and plasma catecholamine concentrations in patients with acute brain injury and correlate these variables with the severity of neurologic dysfunction and patient outcome.DesignProspective, clinical study.SettingPediatric intensive care unit.PatientsThirty-seven pediatric patients with acute brain injury caused by trauma, anoxia/ischemia, hemorrhage, or infection.InterventionsNone.Measurements and Main ResultsWe found significant associations between low-frequency (0.01 to 0.15 Hz) heart rate power and severity of neurologic dysfunction (as assessed by the admission Glasgow Coma Scale) (p less than .001) and patient outcome (as assessed by the Glasgow Outcome Scale) (p equals .05). The admission (p equals .05) and maximum (p less than .001) values for low-frequency heart rate power and the minimum value for high-frequency (0.15 to 0.50 Hz) heart rate power obtained during hospitalization (p equals .001) predicted an increased likelihood of survival. Ten brain-dead patients had significantly decreased low-frequency heart rate power (p equals .008) and plasma norepinephrine (p equals .015), epinephrine (p equals .03), and dopamine (p equals .04) concentrations when compared with six non-brain-dead patients with a Glasgow Coma Scale score of 3.ConclusionsOur results imply that autonomic nervous system control of heart rate is disrupted in proportion to the degree of neurologic insult in children after acute brain injury. Thus, heart rate power spectral analysis and plasma catecholamine concentrations may prove to be useful adjuncts in determining severity of neurologic injury and prognosis for recovery in children suffering from brain injury. In addition, these techniques may aid in the determination of brain death.(Crit Care Med 1996; 24:234-240)