Calculation of integer heart rate variability (HRVi) permits monitoring over extended periods. We asked whether continuous monitoring of HRVi or integer pulse pressure (PP) variability (PPVi) could predict intracranial hypertension, defined as ICP >20 mm Hg, cerebral hypoperfusion, defined as CPP<60 mm Hg, mortality or functional outcome after severe traumatic brain injury. Dense integer data collected during continuous intensive care unit monitoring for periods of 1 to 11 days on 25 patients admitted to our Level I trauma center with Glasgow Coma Scale <9 provided 1,715,000 data points over a mean 106±62 hours. PP, HRVi, and PPVi increased in response to increasing ICP when CPP >60 mm Hg (P<0.001), but HRVi and PPVi decreased when CPP <60 mm Hg and P<50 mm Hg, even with ICP <20 mm Hg (P<0.001). ICP up to 40 mm Hg still evoked an increase in HRVi and PPVi (P<0.001), but both were suppressed with CPP <50 mm Hg and ICP >40 mm Hg (actual or impending brain death). Mean HRVi and PPVi predicted in-hospital mortality (sensitivity, 67%; specificity, 91% to 100%). Combining HRVi and PPVi as an "autonomic index" (AI) best predicted long-term functional outcome [Area Under the Curve: 0.84±0.08 for AI <0.5]. Our data show that HRVi and PPVi can be monitored and calculated automatically and can provide useful prognostic information in patients with severe traumatic brain injury, particularly when combined into a single index.
Adult Area Under Curve