A comparison of the transient hyperemic response test and the static autoregulation test to assess graded impairment in cerebral autoregulation during propofol, desflurane, and nitrous oxide anesthesia.
ABSTRACT The transient hyperemic response (THR) test has been used to assess cerebral autoregulation in anesthesia and intensive care. To date it has not been compared with the static autoregulation test for assessing graded changes in cerebral autoregulation. We compared the two tests during propofol, desflurane, and nitrous oxide anesthesia. Seven subjects were studied. For the THR test, changes in the middle artery blood flow velocity were assessed during and after a 10-s compression of the ipsilateral common carotid artery. Two indices of autoregulation--THR ratio (THRR) and strength of autoregulation (SA)--were calculated. For the test of static autoregulation, changes in the middle cerebral artery flow velocity after a phenylephrine-induces increase in mean arterial pressure were assessed, and the static rate of regulation (sROR) was calculated. The tests were performed before induction and after equilibrium at 0.5 minimum alveolar anesthetic concentration (MAC) and then at 1.5 MAC of desflurane. THRR, SA and sROR decreased significantly (P < 0.001) at 0.5 MAC and then at 1.5 MAC desflurane. CHanges in THRR and SA reflected the changes in sROR with a sensitivity of 100%. Implications: When compared with the established test of static autoregulation, the transient hyperemic response test provides a valid method for assessing graded impairment in cerebral autoregulation.
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ABSTRACT: We investigated the validity of transcranial Doppler recordings for the analysis of dynamic responses of cerebral autoregulation. We found no significant differences in percentage changes among maximal (centerline) blood flow velocity, cross-sectional mean blood flow velocity, and signal power-estimated blood flow during 24-mm Hg stepwise changes in arterial blood pressure. We investigated blood flow propagation delays in the cerebral circulation with simultaneous Doppler recordings from the middle cerebral artery and the straight sinus. The time for a stepwise decrease in blood flow to propagate through the cerebral circulation was only 200 msec. Brief (1.37-second) carotid artery compression tests also demonstrated that the volume compliance effects of the cerebral vascular bed were small, only about 2.2% of normal blood flow in 1 second. Furthermore, transients associated with inertial and volume compliance died out after 108 msec. We also investigated the hypothesis that autoregulatory responses are influenced by hyperventilation using the same brief carotid artery compressions. One second after release, the flow index increased by 17% during normocapnia and 36% during hypocapnia. After 5 seconds, the flow index demonstrated a clear oscillatory response during hypocapnia that was not seen during normocapnia. These results suggest that the intact human cerebral circulation in the absence of pharmacological influences does not function as predicted from pial vessel observations in animals.Stroke 10/1991; 22(9):1148-54. · 6.16 Impact Factor
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ABSTRACT: We compared relative changes in middle cerebral artery velocity and internal carotid artery flow during autoregulation testing to test the validity of using transcranial Doppler recordings of middle cerebral artery velocity to evaluate cerebral autoregulation in humans. Seven human volunteers had dynamic autoregulation tested during surgical procedures that included exposure of the internal carotid artery. The mean arterial blood pressure and middle cerebral artery velocity spectral outline (Vmax), using transcranial Doppler, and ipsilateral internal carotid artery flow, using an electromagnetic flowmeter, were continuously and simultaneously recorded during transient sharp decreases in blood pressure that were induced by rapid deflation of thigh blood pressure cuffs. The resulting responses of velocity in the middle cerebral artery and flow in the internal carotid artery were compared. Moderate decreases in blood pressure evoked responses in cerebral autoregulation. There were no significant (P = .97) differences between the responses in middle cerebral artery velocity and internal carotid artery flow to the blood pressure decreases. Relative changes in Vmax accurately reflect relative changes in internal carotid artery flow during dynamic autoregulation testing in humans. Therefore, alterations in middle cerebral artery diameter do not occur to the extent that they introduce a significant error in making these comparisons.Stroke 05/1994; 25(4):793-7. · 6.16 Impact Factor
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ABSTRACT: A mathematical model of cerebral blood flow and the cerebrospinal fluid circulation is described which permits the study of phenomena caused by dynamic changes in cerebrovascular autoregulatory or cerebrospinal fluid compensatory reserves. A transient decrease in cerebral perfusion pressure was produced by carotid artery compression. Comparison of the computer simulations with clinical and experimental data, reported elsewhere, suggests that the transient hyperaemic response (THR) is proportional to the strength of the autoregulatory response. The relationships between the magnitude and time course of the THR, and the period and level of reduction in CPP were studied. This model suggests that simple clinical tests based on the examination of THR using transcranial Doppler velocity measurements are of potential value for the non-invasive assessment of the autoregulatory reserve.Acta Neurochirurgica 02/1992; 115(3-4):90-7. · 1.55 Impact Factor