Transcranial Doppler identification of changing autoregulatory thresholds after autoregulatory impairment

OBJECTIVE: Transcranial Doppler (TCD) flow velocity (FV) assessment may pro vide a useful index of autoregulatory impairment after severe head injury. It may define a therapeutic end point against which cerebral perfusion pres sure (CPP) can be titrated. This study examines the relationship between ce rebral blood flow (CBF) and TCD FV indices in a laboratory model before and after autoregulatory impairment. METHODS: CPP, CBF, and middle cerebral artery TCD FV were measured continuo usly in nine anesthetized and ventilated sheep. CPP was decreased by hemorr hagic hypotension. The process was repeated after impairment of autoregulat ion by cisternal infusion, which maintained CPP at 0 mm Hg for 15 minutes. Points of significant change (i.e., breakpoints) from baseline values for e ach of the measured flow parameters were identified by using a ratio of var iance technique. RESULTS: Before any significant change in CBF or systolic TCD, diastolic TC D FV decreased (mean breakpoint, 69 mm Hg; range, 56-78 mm Hg) as CPP was r educed. This divergence of diastolic and systolic TCD FV, which occurred be fore autoregulatory failure, was associated with an increasing TCD pulsatil ity index (mean breakpoint, 63 mm Hg; range, 53-70 mm Hg). At diastolic TCD FV congruent with 10 cm/s, systolic TCD FV (mean breakpoint, 48 mm Hg; ran ge, 46-53 mm Hg) and CBF (mean breakpoint, 49 mm Hg; range, 47-51 mm Hg) de creased rapidly, indicating autoregulatory failure. After autoregulatory im pairment, the breakpoints for all four indices shifted to higher CPP values (mean, 16 mm Hg). CONCLUSION: TCD FV assessment identified two CPP thresholds of autoregulato ry loss. Before autoregulatory failure, an earlier phase of autoregulatory disturbance may be detected by divergent systolic and diastolic TCD FVs. It is important to note that this phase may be detected before CBF decreases. These TCD FV breakpoints depend on the state of autoregulatory impairment and may provide potential targets for CPP-directed therapy.