DETECTION OF IMPAIRED CEREBRAL AUTOREGULATION USING SPECTRAL-ANALYSISOF INTRACRANIAL-PRESSURE WAVES

Successful resuscitation following severe traumatic brain injury (TBI) requires rapid evaluation of intracranial pressure (ICP), cerebrovasc ular reactivity (autoregulation), and cerebral metabolism. During impa ired autoregulation, inadequate cerebral blood flow (CBF) can lead to ischemia while excessive CBF can result in elevated ICP. Without infor mation regarding the state of autoregulation, treatment of either situ ation may ameliorate one problem but exacerbate the other. It has been hypothesized that fast Fourier transform (FFT) analysis of arterial b lood pressure (BP) and ICP waves can differentiate states of intact an d impaired autoregulation. BP and ICP waves were recorded in canines b efore and after ischemic injury during arterial normotension, hyperten sion, and hypotension induced with dopamine or nitroprusside infusion. Transfer functions (TFn) were calculated from FFT spectra as ratios o f ICP and BP harmonic peak amplitudes to distinguish states of vasorea ctivity. During normotension and hypertension, autoregulation was inta ct and TF1 averaged 0.05. During hypotension, TF1 averaged 0.22 (8x ba seline, p < 0.010). During impaired autoregulation following ischemic injury, TF1 averaged 0.50 (18x baseline, p < 0.010; 2x nitroprusside l evels, p < 0.01). This large difference in TF relative to baseline ext ended over a large range of BP (60 < BP < 180 mm Hg). Based on these d ata and previous results, it was estimated that TF can differentiate i mpaired autoregulation from effects solely related to elevated ICP or active vasodilation for ICP < 30-40 mm Hg. This suggests that for spec ific, but widely applicable physiologic conditions, spectral analysis can identify states of impaired autoregulation and, as an adjunct to t raditional monitoring techniques, aid in acute resuscitation and preve ntion of secondary injury in TBI.