J Neurosurg 76:961-966, 1992 The cerebral hemodynamics of normotensive hypovolemia during lower-body negative pressure COLE A. GILLER, PH.D., M.D., BENJAMIN D. LEV1NE, M.D., YVES MEYEa, M.D., JAY C. BUCKEY, M.D., LYNDA D. LANE, M.S., R.N., AND D. JOHN BORCHERS, B.S. Departments of Neurosurgery, Radiology, and Internal Medicine, Southwestern Medical School Dallas, Texas ~" Although severe hypovolemia can lead to hypotension and neurological decline, many patients with neurosurgical disorders experience a significant hypovolemia while autonomic compensatory mechanisms maintain a normal blood pressure. To assess the effects of normotensive hypovolemia upon cerebral hemo- dynamics, transcranial Doppler ultrasound monitoring of 13 healthy volunteers was performed during grad- ed lower-body negative pressure of up to -50 mm Hg, an accepted laboratory model for reproducing the physiological effects of hypovolemia. Middle cerebral artery flow velocity declined by 16% ___ 4% (mean ___ standard error of the mean) and the ratio between transcranial Doppler ultrasound pulsatility and systemic pulsatility rose 22% _+ 8%, suggesting cerebral small-vessel vasoconstriction in response to the sympathetic activation unmasked by lower-bodynegative pressure. This vasoconstrictionmay interfere with the autoregu- latory response to a sudden fall in blood pressure, and may explain the common observation of neurological deficit during hypovolemiaeven with a normal blood pressure. KEY WORDS 9 transeranial Doppler ultrasound 9 lower-body negative pressure 9 hypovolemia sympathetic nervous system I NTRAVASCULARvolume depletion is an untoward state that commonly occurs in a variety of neuro- surgical settings such as stroke, subarachnoid hem- orrhage (SAH), and trauma, as well as during surgery on patients in the sitting position. Although this hypo- volemia may progress to a frank fall in blood pressure, in most cases neurohumoral regulatory mechanisms can preserve blood pressure over significant periods of time. w'3o'33 These homeostatic mechanisms include a powerful systemic vasoconstriction,24 and the conse- quent alterations in cerebral hemodynamics may be of clinical importance both during the normotensive phase of hypovolemia and during any subsequent neurologi- cal deterioration as blood pressure falls. Prolonged stable periods of controlled normotensive hypovolemia are not common in the clinical setting, so there are few investigations of the associated hemody- namics. The laboratory technique of inducing lower- body negative pressure, however, has long provided a standard model for controlled hypovolemia. 4"~6'~7"L9"24" 25,28,3o,33 By applying a measured amount of negative pressure in a chamber sealed around a patient's lower extremities, graded central hypovolemia is easily pro- duced and physiological variables can be readily mon- itored. Physiological parameters change rapidly during lower-body negative pressure, so the method of cerebral hemodynamic monitoring must provide moment-to- moment information. Transcranial Doppler ultrasound has recently emerged as a tool permitting continuous monitoring ~ and has been found useful in such disparate cases as assessment of cerebral vasospasm ~'2 and guid- ance of carotid artery occlusion. ~2 The purpose of this study is to document changes in cerebral hemodynamics during trials of lower-body neg- ative pressure in man, using transcranial Doppler ultra- sound technology, and to use these results as a model for the normotensive hypovolemia found in clinical practice. Clinical Material and Methods Thirteen healthy male volunteers (average age 27 +_ 2 years) underwent graded lower-body negative pressure during continuous monitoring of transcranial Doppler ultrasound signals from the middle cerebral artery (MCA). All subjects had experienced several lower-body negative pressure trials previously. Seven of the subjects were trained athletes and had a measured oxygen up- take (VOw) maximum of 68 _ 7 ml/kg/min (mean __ J. Neurosurg. / Volume 76/June, 1992 961