Arthur M. Lam

Comparison of Static and Dynamic Cerebral Autoregulation Measurements

BACKGROUND AND PURPOSE Cerebral autoregulation can be evaluated by measuring relative blood flow changes in response to a steady-state change in the blood pressure (static method) or during the response to a rapid change in blood pressure (dynamic method). The purpose of this study was to compare the results of the two methods in humans with both intact and impaired autoregulatory capacity. METHODS Using intraoperative transcranial Doppler sonography recordings from both middle cerebral arteries, we determined static and dynamic autoregulatory responses in 10 normal subjects undergoing elective surgical procedures. The changes in cerebrovascular resistance were estimated from the changes in cerebral blood flow velocity and arterial blood pressure in response to manipulations of blood pressure. Static autoregulation was determined by analyzing the response to a phenylephrine-induced rise in blood pressure, whereas rapid deflation of a blood pressure cuff around one thigh served as a stimulus for testing dynamic autoregulation. Both measurements were performed in patients with intact autoregulation during propofol anesthesia and again in the same patients after autoregulation had been impaired by administration of high-dose isoflurane. RESULTS There was a significant reduction in autoregulatory capacity after the administration of high-dose isoflurane, which could be demonstrated using static (P < .0001) and dynamic (P < .0001) methods. The correlation between static or steady-state and dynamic autoregulation measurements was highly significant (r = .93, P < .0001). CONCLUSIONS These data show that in normal human subjects measurement of dynamic autoregulation yields similar results as static testing of intact and pharmacologically impaired autoregulation.

Comparison of flow and velocity during dynamic autoregulation testing in humans.

Background and Purpose 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. Methods 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. Results 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. Conclusions 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. 1994;25:793‐797.)

Dynamic and Static Cerebral Autoregulation during Isoflurane, Desflurane, and Propofol Anesthesia

Background Although inhalation anesthetic agents are thought to impair cerebral autoregulation more than intravenous agents, there are few controlled studies in humans. Methods In the first group (n = 24), dynamic autoregulation was assessed from the response of middle cerebral artery blood flow velocity (Vmca) to a transient step decrease in mean arterial blood pressure (MABP). The transient hypotension was induced by rapid deflation of thigh cuffs after inflation for 3 min. In the second group (n = 18), static autoregulation was studied by observing Vmca in response to a phenylephrine-induced increase in MABP. All patients were studied during fentanyl (3 micrograms.kg-1.h-1)/nitrous oxide (70%) anesthesia, followed by, in a randomized manner, isoflurane, desflurane, or propofol in a low dose (0.5 MAC or 100 micrograms.kg-1.min-1) and a high dose (1.5 MAC or 200 micrograms.kg-1.min-1). The dynamic rate of regulation (dROR) was assessed from the rate of change in cerebrovascular resistance (MABP/Vmca) with the blood pressure decreases using computer modeling, whereas the static rate of regulation (sROR) was assessed from the change in Vmca with the change in MABP. Results Low-dose isoflurane delayed (dROR decreased) but did not reduce the autoregulatory response (sROR intact). Low-dose desflurane decreased both dROR and sROR. During 1.5 MAC isoflurane or desflurane, autoregulation was ablated (both dROR and sROR impaired). Neither dROR nor sROR changed with low- or high-dose propofol. Conclusions At 1.5 MAC, isoflurane and desflurane impaired autoregulation whereas propofol (200 micrograms.kg-1.min-1) preserved it.

Monitoring electrophysiologic function during carotid endarterectomy: a comparison of somatosensory evoked potentials and conventional electroencephalogram.

There is no consensus as to the most appropriate monitor for detecting ischemia during carotid endarterectomy. Accordingly, simultaneous 16-channel continuous electroencephalogram (EEG) and somatosensory evoked potential (SSEP) monitoring were performed in 64 normocapnic patients undergoing carotid endarterectomy and anesthetized with isoflurane or halothane-nitrous oxide (supplemented with fentanyl). Recordings were obtained before, during, and for 15 min after cross-clamping of the internal carotid artery. Internal shunt was not used in any patient, regardless of EEG and SSEP changes. Significant amplitude reduction in the cortical component of the primary negative peak (greater than 50%) in SSEP occurred in 6 patients, and an increase in central conduction time (CCT) (greater than 1 ms) occurred in 5 patients. Major EEG changes occurred in 6 patients, 4 of whom also had SSEP changes. Two patients had transient neurologic deficits postoperatively, with both having SSEP changes (amplitude reduction greater than 50%), whereas one had EEG changes. Based on these observations, the relative sensitivity and specificity for EEG and SSEP (amplitude reduction greater than 50%) in detecting postoperative neurologic deficits were 50% and 92% for EEG and 100% and 94% for SSEP, respectively, differences that were not statistically significant. Regarding SSEP, the use of latency change (CCT) as a criterion was associated with a sensitivity of 0% (P = 0.046 from sensitivity of amplitude) and a specificity of 87% (P = 0.17 from specificity of amplitude).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of the Valsalva Maneuver on Cerebral Circulation in Healthy Adults: A Transcranial Doppler Study

BACKGROUND AND PURPOSE Knowledge is limited about the effects of the Valsalva maneuver on cerebral circulation because of the poor temporal resolution of traditional cerebral blood flow measurements. The purpose of this study was to investigate changes in cerebral blood flow during the Valsalva maneuver and to explore its potential use for the evaluation of cerebral autoregulation. METHODS Using transcranial Doppler ultrasonography, we simultaneously recorded systemic arterial blood pressure in the radial artery and flow velocities in both middle cerebral arteries in 10 healthy adults during the Valsalva maneuver. Goslings pulsatility index was calculated for all phases of the Valsalva maneuver. Autoregulatory capacities were estimated from the change in cerebrovascular resistance (flow velocity in relationship to blood pressure) during phase II and changes in the velocity-pressure relationship in phase IV relative to phase I. RESULTS The characteristic changes in blood pressure (phases I to IV) were seen in all subjects, accompanying distinct changes in cerebral blood flow velocity. The relative changes in mean velocity during phases II and IV were significantly greater than those in mean blood pressure. Compared with the baseline value, velocity decreased by 35% in phase IIa, then rose by 56.5% in phase IV (corresponding changes in blood pressure were -10.2% and +29.8%, respectively). During phase II, the pulsatility and cerebrovascular resistance decreased by 19.9%. The increase in cerebral blood flow velocity in phase IV was significantly higher than in phase I (P < .0004), and there was no corresponding significant difference in blood pressure. CONCLUSIONS These results demonstrated that in healthy humans the Valsalva maneuver causes characteristic changes in systemic blood pressure as well as in flow velocity in the middle cerebral artery, reflecting the sympathetic and cerebral autoregulatory responses, respectively. Analysis of these changes may provide an estimate of autoregulatory capacity.

Ketamine Does Not Increase Cerebral Blood Flow Velocity or Intracranial Pressure During Isoflurane/Nitrous Oxide Anesthesia in Patients Undergoing Craniotomy

Ketamines effect on cerebral hemodynamics is controversial.We hypothesized that ketamine would not increase intracranial pressure (ICP) and cerebral blood flow (CBF) velocity in anesthetized, ventilated patients. Twenty patients requiring craniotomy for brain tumor or cerebral aneurysm were studied. After induction with thiopental, anesthesia was maintained with isoflurane and nitrous oxide in oxygen. During controlled ventilation (PaCO2 34 +/- 1 mm Hg); middle cerebral artery blood flow velocity (VMCA), mean arterial blood pressure (MAP), bilateral frontooccipital processed electroencephalogram (EEG), and ICP were measured before and for 10 min after intravenous ketamine 1.0 mg/kg. Cerebral arteriovenous oxygen content difference (AVDO2) and cerebral perfusion pressure (CPP) were calculated. After ketamine, MAP, CPP, PaCO2, and AVDO2 were unchanged. ICP decreased from 16 +/- 1 mm Hg to 14 +/- 1 mm Hg (mean +/- SE; P < 0.001) and VMCA decreased from 44 +/- 4 cm/s to 39 +/- 4 cm/s (P < 0.001). Total EEG power decreased (P < 0.02). These results suggest that ketamine can be used in anesthetized, mechanically ventilated patients with mildly increased ICP without adversely altering cerebral hemodynamics. (Anesth Analg 1995;81:84-9)

Cerebral autoregulation in pediatric traumatic brain injury

Objective: The aims of this study were to document the incidence of impaired cerebral autoregulation in children with traumatic brain injury using transcranial Doppler ultrasonography and to examine the relationship between autoregulatory capacity and outcome in children following traumatic brain injury. Design: Prospective cohort study. Setting: Harborview Medical Center (level I pediatric trauma center) in Washington state. Patients: Thirty-six children <15 yrs old with traumatic brain injury: Glasgow Coma Scale score <9 (n = 12, group 1), Glasgow Coma Scale score 9–12 (n = 12, group 2), and Glasgow Coma Scale score 13–15 (n = 12, group 3). Interventions: Cerebral autoregulation testing was conducted during extracranial surgery. Mean middle cerebral artery flow velocities were measured using transcranial Doppler as mean arterial pressure was increased to whichever variable was greater: 20 above baseline or a set value (80 mm Hg for <9 yrs and 90 mm Hg for 9–14 yrs). Autoregulatory capacity was quantified by the Autoregulatory Index. Autoregulatory Index <0.4 was considered impaired cerebral autoregulation. Discharge outcome using the Glasgow Outcome Scale score was considered good if the Glasgow Outcome Scale score was ≥4. Measurements and Main Results: Twenty-four (67) of 36 children had an Autoregulatory Index ≥0.4. The incidence of impaired cerebral autoregulation was 42 (five of 12) in group 1, 42 (five of 12) in group 2, and 17 (two of 12) in group 3. Ten (42) of the 24 children with intact cerebral autoregulation had a good outcome compared with only one of 12 (8) children with impaired cerebral autoregulation (p = .04). Six of 12 (50) children with impaired cerebral autoregulation had hyperemia compared with one of 24 (4) children with intact cerebral autoregulation (p < .01). Hyperemia was associated with poor outcome (p = .01). Conclusions: The incidence of impaired cerebral autoregulation was greatest following moderate to severe traumatic brain injury. Impaired cerebral autoregulation was associated with poor outcome. Hyperemia was associated with impaired cerebral autoregulation and poor outcome.

Direct cerebrovasodilatory effects of halothane, isoflurane, and desflurane during propofol-induced isoelectric electroencephalogram in humans

Background The effect of volatile anesthetics on cerebral blood flow depends on the balance between the agents direct vasodilatory action and the indirect vasoconstrictive action mediated by flow‐metabolism coupling. To compare the intrinsic action of volatile anesthetics, the effect of halothane, isoflurane, and desflurane on flow velocity in the middle cerebral artery during propofol‐induced isoelectricity of the electroencephalogram was examined.

The Influence of Propofol with and without Nitrous Oxide on Cerebral Blood Flow Velocity and CO2 Reactivity in Humans

The cerebrovascular response to CO2 has been reported to be preserved during propofol anesthesia, but no comparison with awake control values has been made, and the additional influence of N2O has not been investigated. Using the noninvasive technique of transcranial Doppler ultrasonography, this study investigated the cerebrovascular response to varying levels of PaCO2 while awake and during anesthesia with propofol and propofol/N2O. Seven adults without systemic diseases undergoing nonneurologic surgery were studied. A pulsed-wave Doppler monitor was used to measure the mean middle cerebral artery flow velocity (Vmca) during varying levels of PaCO2 (25-55 mmHg) under the following conditions: 1) awake; 2) propofol 2.5 mg.kg-1 bolus followed by continuous infusion of 150 micrograms.kg-1.min-1; and 3) propofol as in the condition above plus 70% N2O. During the awake study condition, hypocapnia was induced by voluntary hyperventilation, and hypercapnia was induced with rebreathing of 7% CO2 in a closed circuit. During the anesthetized study conditions, hypocapnia and hypercapnia were induced by adjustment of minute ventilation. A minimum of five to six simultaneous Vmca and PaCO2 measurements were obtained under each of the study conditions. Systemic blood pressure was monitored via a radial arterial catheter, and phenylephrine was administered if mean arterial blood pressure decreased below 60 mmHg (phenylephrine was used in three of five patients in the propofol-N2O group). Linear regression and analysis of covariance were used for statistical analysis of Vmca-PaCO2 relationships.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of Impaired Cerebral Autoregulation by the Valsalva Maneuver

BACKGROUND AND PURPOSE Transcranial Doppler sonography has recently been used to describe cerebral hemodynamics during the Valsalva maneuver in normal human subjects. Since some changes in flow velocity during the Valsalva maneuver seem to reflect the brains autoregulatory response to a decrease in cerebral perfusion pressure during the strain, we hypothesized that this method could identify vascular territories with impaired autoregulatory capacity. METHODS Eight patients with unilateral (n=7) or bilateral (n=1) severe obstruction of the internal carotid artery and impaired vascular responses to the CO2 reactivity test and to dynamic autoregulation testing were studied. We compared changes in flow velocities and blood pressures during defined phases of the Valsalva maneuver in the patients with the results in a group of 17 normal volunteers. We defined two indices to evaluate autoregulatory capacity based on the response to the Valsalva maneuver. RESULTS During the Valsalva maneuver, changes in flow velocity in the middle cerebral arteries ipsilateral to the lesions showed characteristic abnormalities compared with the normal pattern. Autoregulatory indices of these vessels as defined by the Valsalva maneuver were significantly different from those with normal vascular reactivity to CO2 (P<.0001). There were good correlations between an index based on the changes in flow velocity and blood pressure in phase II and the results of the CO2 test (r=.78; P<.0001) or those of dynamic autoregulatory testing (r=.6; P<.0001). CONCLUSIONS Vascular territories with severely impaired vasomotor reactivity due to carotid obstruction can be identified by transcranial Doppler sonography by their pattern of flow velocity changes if their autoregulatory capacity is challenged during the Valsalva maneuver.