Estimation of vessel flow and diameter during cerebral vasospasm using transcranial Doppler indices

Cold stimulation of the oral cavity redistributes blood towards the brain in healthy volunteers

BACKGROUND

The aim of this study was to analyze cold stimulation-induced changes in cerebral and cardiac hemodynamics.

METHODS

Upon ingestion of an ice cube, the changes in resistance index, mean flow velocity and flow index of the middle cerebral arteries (MCA) were assessed using transcranial Doppler sonography. Extracranial duplex sonography was used to measure the mean flow velocity and resistance index of the right internal carotid artery (ICA). The change in mean arterial pressure, heart rate, root mean square of successive differences (RMSSD) and end-tidal carbon dioxide pressure were analyzed additionally. These changes were compared to sham stimulation.

RESULTS

Compared with sham stimulation, cooling of the oral cavity resulted in significant changes in cerebral and cardiac hemodynamics. The cold stimulation decreased the resistance index in the MCA (-4.5% ± 5.4%, p < 0.0001) and right ICA (-6.3% ± 15.6%, p = 0.001). This was accompanied by an increase in mean flow velocity (4.1% ± 8.0%, p < 0.0001) and flow index (10.1% ± 43.6%, p = 0.008) in the MCA. The cardiac effects caused an increase in mean arterial pressure (1.8% ± 11.2%, p = 0.017) and RMSSD (55% ± 112%, p = 0.048), while simultaneously decreasing the heart rate (-4.3% ± 9.6%, p = 0.0001).

CONCLUSION

Cooling of the oral cavity resulted in substantial changes in cerebral and cardiac hemodynamics resulting in a blood flow diversion to the brain.

Translational approach towards determining the role of cerebral autoregulation in outcome after traumatic brain injury

Cerebral autoregulation is impaired after traumatic brain injury (TBI), contributing to poor outcome. In the context of the neurovascular unit, cerebral autoregulation contributes to neuronal cell integrity and clinically Glasgow Coma Scale is correlated to intactness of autoregulation after TBI. Cerebral Perfusion Pressure (CPP) is often normalized by use of vasoactive agents to increase mean arterial pressure (MAP) and thereby limit impairment of cerebral autoregulation and neurological deficits. However, current vasoactive agent choice used to elevate MAP to increase CPP after TBI is variable. Vasoactive agents, such as phenylephrine, dopamine, norepinephrine, and epinephrine, clinically have not sufficiently been compared regarding effect on CPP, autoregulation, and survival after TBI. The cerebral effects of these clinically commonly used vasoactive agents are incompletely understood. This review will describe translational studies using a more human like animal model (the pig) of TBI to identify better therapeutic strategies to improve outcome post injury. These studies also investigated the role of age and sex in outcome and mechanism(s) involved in improvement of outcome in the setting of TBI. Additionally, this review considers use of inhaled nitric oxide as a novel neuroprotective strategy in treatment of TBI.

Alterations in cerebral blood flow in children with congestive heart failure due to ventricular septal defect

OBJECTIVE

We aimed to investigate the effect of ventricular septal defect (VSD) and heart failure on cerebral blood flow (CBF) in children, whether heart failure treatment improves CBF, and if there is any relationship between CBF and serum N-terminal pro-brain natriuretic peptide (NT-proBNP) level.

METHOD

Forty children with VSD (13 with heart failure) aged between 1 and 36 months were studied. The control group comprised 25 healthy children in the same age group. Maximum, minimum, and mean blood flow velocities and pulsatility indices of the right and left middle cerebral arteries were assessed using transcranial Doppler ultrasonography. Measurements of CBF and serum NT-proBNP levels were repeated in patients with heart failure a month post-heart failure treatment initiation. The groups were also compared in terms of defect diameters, cardiac structural changes, left ventricular systolic function, and findings related to pulmonary hypertension determined by echocardiography. Correlations between echocardiographic measurements and CBF parameters were analyzed.

RESULTS

Although no significant difference was found between patient and control groups for CBF, right and left maximum CBF velocities significantly increased posttreatment in patients with heart failure (P = .04 and P = .01, respectively). Serum NT-proBNP levels in children with VSD associated with heart failure were significantly higher than those in children with VSD alone (P = .04) or in healthy children (P < .001). NT-proBNP levels were negatively correlated with right and left maximum CBF velocities (r = -0.39, P = .013 and r = -0.32, P = .043, respectively).

CONCLUSION

Although no significant difference was found in CBF velocity among the study groups, increase in the CBF velocity post heart failure treatment and negative correlations between CBF velocity and both the VSD diameter and NT-proBNP levels indicate that the hemodynamic status due to VSD associated with heart failure has an effect on CBF.

Argon does not affect cerebral circulation or metabolism in male humans

OBJECTIVE

Accumulating data have recently underlined argon´s neuroprotective potential. However, to the best of our knowledge, no data are available on the cerebrovascular effects of argon (Ar) in humans. We hypothesized that argon inhalation does not affect mean blood flow velocity of the middle cerebral artery (Vmca), cerebral flow index (FI), zero flow pressure (ZFP), effective cerebral perfusion pressure (CPPe), resistance area product (RAP) and the arterio-jugular venous content differences of oxygen (AJVDO2), glucose (AJVDG), and lactate (AJVDL) in anesthetized patients.

MATERIALS AND METHODS

In a secondary analysis of an earlier controlled cross-over trial we compared parameters of the cerebral circulation under 15 minutes exposure to 70%Ar/30%O2 versus 70%N2/30%O2 in 29 male patients under fentanyl-midazolam anaesthesia before coronary surgery. Vmca was measured by transcranial Doppler sonography. ZFP and RAP were estimated by linear regression analysis of pressure-flow velocity relationships of the middle cerebral artery. CPPe was calculated as the difference between mean arterial pressure and ZFP. AJVDO2, AJVDG and AJVDL were calculated as the differences in contents between arterial and jugular-venous blood of oxygen, glucose, and lactate. Statistical analysis was done by t-tests and ANOVA.

RESULTS

Mechanical ventilation with 70% Ar did not cause any significant changes in mean arterial pressure, Vmca, FI, ZFP, CPPe, RAP, AJVDO2, AJVDG, and AJVDL.

DISCUSSION

Short-term inhalation of 70% Ar does not affect global cerebral circulation or metabolism in male humans under general anaesthesia.

Preoperative mannitol infusion improves perioperative cerebral oxygen saturation and enhances postoperative recovery after laparoscopic cholecystectomy

Objectives:

To test the effect of preoperative mannitol infusion on perioperative decreased cerebral oxygen saturation (rSO₂) during laparoscopic cholecystectomy.

Methods:

Forty patients scheduled for laparoscopic cholecystectomy were enrolled in this study conducted at Dammam Hospital of the University, Dammam, Kingdom of Saudi Arabia from December 2013 to June 2014. Patients received either 0.5 g/kg of 20% intravenous mannitol infusion over 10 minutes before induction of anesthesia (group M), or an equal volume of normal saline instead (group C). Primary outcome variable was rSO₂. Other variables included extubation time, clinical assessment of consciousness recovery using the Modified Observer’s Assessment of Alertness/Sedation Scale (OAA/S), and the mini-mental state examination (MMSE) for cognitive evaluation.

Results:

Anesthesia induction increased rSO₂ in both groups. Pneumoperitoneum decreased rSO₂ in group C, but not in group M. This drop in rSO₂ in the group C reached its maximum 30 minutes after extubation, and was significantly less than the preinduction value. Time to extubation in group M was significantly shorter compared to group C (p=0.007). The OAAS in group M at 10 min after extubation was significantly higher compared to group C. No differences were found between the 2 groups in cognitive function as measured by MMSE score.

Conclusion:

Preoperative mannitol infusion maintains perioperative rSO₂ during laparoscopic cholecystectomy and shortens extubation time with earlier resurgence of OAAS.

Cerebral blood flow and transcranial doppler sonography measurements of CO2-reactivity in acute traumatic brain injured patients

BACKGROUND

Cerebral blood flow (CBF) measurements are helpful in managing patients with traumatic brain injury (TBI), and testing the cerebrovascular reactivity to CO(2) provides information about injury severity and outcome. The complexity and potential hazard of performing CBF measurements limits routine clinical use. An alternative approach is to measure the CBF velocity using bedside, non-invasive, and transcranial Doppler (TCD) sonography. This study was performed to investigate if TCD is a useful alternative to CBF in patients with severe TBI.

METHOD

CBF and TCD flow velocity measurements and cerebrovascular reactivity to hypocapnia were simultaneously evaluated in 27 patients with acute TBI. Measurements were performed preoperatively during controlled normocapnia and hypocapnia in patients scheduled for hematoma evacuation under general anesthesia.

MAIN FINDING AND CONCLUSION

Although the lack of statistical correlation between the calculated reactivity indices, there was a significant decrease in TCD-mean flow velocity and a decrease in CBF with hypocapnia. CBF and TCD do not seem to be directly interchangeable in determining CO(2)-reactivity in TBI, despite both methods demonstrating deviation in the same direction during hypocapnia. TCD and CBF measurements both provide useful information on cerebrovascular events which, although not interchangeable, may complement each other in clinical scenarios.

Relative blood flow changes measured using calibrated frequency-weighted Doppler power at different hematocrit levels

In theory, the power of a trans-cranial Doppler signal may be used to measure changes in blood flow and vessel diameter in addition to velocity. In this study, a flow index (FI) of relative changes in blood flow was derived from frequency-weighted Doppler power signals. The FI, plotted against velocity, was calibrated to the zero intercept with absent flow to reduce the effects of non-uniform vessel insonation. An area index was also calculated. FIs were compared with actual flow in four silicone tubes of different diameter at increasing flow rates and increasing hematocrit (Hct) in a closed-loop phantom model. FI values were strongly correlated with actual flow, at constant Hct, but varied substantially with changes in Hct. Percentage changes in area indexes, relative to the 4-mm tube, were strongly correlated with tube cross-sectional area. The implications of these results for in vivo use are discussed.

Melatonin does not attenuate dynamic cardiovascular and cerebrovascular reflex responses to acute hypotension in healthy men

Background

Melatonin has been shown to attenuate the reflex sympathetic increases that arise in response to orthostatic challenges. We tested the hypothesis that the attenuated sympathetic increase induced by melatonin premedication may weaken the arterial blood pressure (ABP) preserving the capability during acute hypotension, thereby altering dynamic cerebral autoregulation and causing a further decrease in cerebral blood flow (CBF).

Methods

Acute hypotension was induced in 12 healthy subjects by releasing bilateral thigh cuffs before and after an oral dose of melatonin (0.2 mg/kg). Heart rate (HR), arterial blood pressure (ABP), Modelflow estimate of cardiac output (CO), total peripheral resistance (TPR) and cerebral blood flow velocity (CBFV) by transcranial Doppler were measured.

Results

Steady state HR, the mean arterial pressure and CBFV were not altered 60 minutes after melatonin ingestion. Reduced systolic arterial pressure (ΔSAP), changes in HR (ΔHR), CO (ΔCO), and TPR (ΔTPR), ΔHR/ΔSAP and percentage restoration of SAP were not affected after a temporal decrease in ABP induced by thigh cuff release. In the cerebral circulation, melatonin did not affect changes in CBFV, cerebrovascular resistance index, the rate of regulation and percentage restoration of CBFV following a sudden decrease in ABP.

Conclusions

Contrary to our hypothesis, melatonin did not affect the rapid vasodilatory and recovery responses of cardiovascular and dynamic cerebral autoregulation. These results suggest that melatonin premedication may not impair ABP and CBF preserving capability induced by sudden postural changes or hemorrhage.

Reversal of cerebral vasospasm via intravenous sodium nitrite after subarachnoid hemorrhage in primates

OBJECT

Subarachnoid hemorrhage (SAH)-induced vasospasm is a significant underlying cause of aneurysm rupture-related morbidity and death. While long-term intravenous infusion of sodium nitrite (NaNO(2)) can prevent cerebral vasospasm after SAH, it is not known if the intravenous administration of this compound can reverse established SAH-induced vasospasm. To determine if the intravenous infusion of NaNO(2) can reverse established vasospasm, the authors infused primates with the compound after SAH-induced vasospasm was established.

METHODS

Subarachnoid hemorrhage-induced vasospasm was created in 14 cynomolgus macaques via subarachnoid implantation of a 5-ml blood clot. On Day 7 after clot implantation, animals were randomized to either control (saline infusion, 5 monkeys) or treatment groups (intravenous NaNO(2) infusion at 300 μg/kg/hr for 3 hours [7 monkeys] or 8 hours [2 monkeys]). Arteriographic vessel diameter was blindly analyzed to determine the degree of vasospasm before, during, and after treatment. Nitric oxide metabolites (nitrite, nitrate, and S-nitrosothiols) were measured in whole blood and CSF.

RESULTS

Moderate-to-severe vasospasm was present in all animals before treatment (control, 36.2% ± 8.8% [mean ± SD]; treatment, 45.5% ± 12.5%; p = 0.9). While saline infusion did not reduce vasospasm, NaNO(2) infusion significantly reduced the degree of vasospasm (26.9% ± 7.6%; p = 0.008). Reversal of the vasospasm lasted more than 2 hours after cessation of the infusion and could be maintained with a prolonged infusion. Nitrite (peak value, 3.7 ± 2.1 μmol/L), nitrate (18.2 ± 5.3 μmol/L), and S-nitrosothiols (33.4 ± 11.4 nmol/L) increased significantly in whole blood, and nitrite increased significantly in CSF.

CONCLUSIONS

These findings indicate that the intravenous infusion of NaNO(2) can reverse SAH-induced vasospasm in primates. Further, these findings indicate that a similar treatment paradigm could be useful in reversing cerebral vasospasm after aneurysmal SAH.

Cerebral blood flow and autoregulation after pediatric traumatic brain injury

Traumatic brain injury is a global health concern and is the leading cause of traumatic morbidity and mortality in children. Despite a lower overall mortality than in adult traumatic brain injury, the cost to society from the sequelae of pediatric traumatic brain injury is very high. Predictors of poor outcome after traumatic brain injury include altered systemic and cerebral physiology, including altered cerebral hemodynamics. Cerebral autoregulation is often impaired after traumatic brain injury and may adversely impact the outcome. Although altered cerebrovascular hemodynamics early after traumatic brain injury may contribute to disability in children, there is little information regarding changes in cerebral blood flow and cerebral autoregulation after pediatric traumatic brain injury. This review addresses normal pediatric cerebral physiology and cerebrovascular pathophysiology after pediatric traumatic brain injury.

Hypohydration and prior heat stress exacerbates decreases in cerebral blood flow velocity during standing

Hypohydration is associated with orthostatic intolerance; however, little is known about cerebrovascular mechanisms responsible. This study examined whether hypohydration reduces cerebral blood flow velocity (CBFV) in response to an orthostatic challenge. Eight subjects completed four orthostatic challenges (temperate conditions) twice before (Pre-EU and Pre-Hyp) and following recovery from passive heat stress ( approximately 3 h at 45 degrees C, 50% relative humidity, 1 m/s air speed) with (Post-EU) or without (Post-Hyp) fluid replacement of sweat losses (-3% body mass loss). Measurements included CBFV, mean arterial pressure (MAP), heart rate (HR), end-tidal CO(2), and core and skin temperatures. Test sessions included being seated (20 min) followed by standing (60 s) then resitting (60 s) with metronomic breathing (15 breaths/min). CBFV and MAP responses to standing were similar during Pre-EU and Pre-Hyp. Standing Post-Hyp exacerbated the magnitude (-28.0 +/- 1.4% of baseline) and duration (9.0 +/- 1.6 s) of CBFV reductions and increased cerebrovascular resistance (CVR) compared with Post-EU (-20.0 +/- 2.1% and 6.6 +/- 0.9 s). Standing Post-EU also resulted in a reduction in CBFV, and a smaller decrease in CVR compared with Pre-EU. MAP decreases were similar for Post-EU (-18 +/- 4 mmHg) and Post-Hyp (-21 +/- 5 mmHg) from seated to standing. These data demonstrate that despite similar MAP decreases, hypohydration, and prior heat stress (despite apparent recovery) produce greater CBFV reduction when standing. These observations suggest that hypohydration and prior heat stress are associated with greater reductions in CBFV with greater CVR, which likely contribute to orthostatic intolerance.

Cerebral and muscle tissue oxygenation in acute hypoxic ventilatory response test

Eight men were exposed to progressive isocapnic hypoxia for 10 min to test the hypothesis that (i) cerebral and muscle tissue would follow similar deoxygenation profiles during an acute hypoxic ventilatory response (AHVR) test; and (ii) strong cerebrovascular responsiveness to hypoxia would be related to attenuated cerebral deoxygenation. End-tidal O(2) concentration was reduced from normoxia (approximately 102 mmHg) to approximately 45 mmHg while arterial oxygen saturation (SpO2 %) declined from 98+/-1% to 77+/-7% (P<0.001). Near-infrared spectroscopy (NIRS)-derived local cerebral tissue (frontal lobe) deoxyhemoglobin increased 5.55+/-2.22 microM, while oxyhemoglobin and tissue oxygenation index decreased 2.57+/-1.99 microM and 6.2+/-3.4%, respectively (all P<0.001). In muscle (m. vastus lateralis) the NIRS changes from the initial normoxic level were non-significant. Cerebral blood velocity (V(mean), transcranial Doppler) in the middle cerebral artery increased from 53.4+/-10.4 to 60.6+/-11.6 cms(-1) (P<0.001). In relation to the decline in SpO2 % the mean rate of increase of V(mean) and AHVR were 0.33+/-0.19 cms(-1)%(-1) and 0.52+/-0.20l min(-1)%(-1), respectively. We conclude that cerebral, but not muscle, tissue shows changes reflecting a greater deoxygenation during acute hypoxia. However, the changes in NIRS parameters were not related to cerebrovascular responsiveness or ventilatory chemosensitivity during graded hypoxia.

Trends in monitoring patients with aneurysmal subarachnoid haemorrhage

After aneurysmal subarachnoid haemorrhage (SAH), the clinical outcome depends upon the primary haemorrhage and a number of secondary insults in the acute post-haemorrhagic period. Some secondary insults are potentially preventable but prevention requires prompt recognition of cerebral or systemic complications. Currently, several neuro-monitoring techniques are available; this review describes the most frequently used techniques and discusses indications for their use, and their value in diagnosis and prognosis. None of the techniques, when considered in isolation, has proved sufficient after SAH. Furthermore, the use of multi-modality monitoring is hampered by a lack of clinical studies that identify combinations of specific techniques in terms of clinical information and reliability. However, ischaemia at the tissue level can be detected by intracerebral microdialysis technique. Used together with the conventional monitoring systems, for example intracranial pressure measurements, transcranial Doppler ultrasound and modern neuro-imaging, direct assessment of biochemical markers by intracerebral microdialysis is promising in the advancement of neurointensive care of patients with SAH. A successfully implemented monitoring system provides answers but it also raises valuable new questions challenging our current understanding of the brain injury after SAH.

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 > or =4.

MEASUREMENTS AND MAIN RESULTS

Twenty-four (67%) of 36 children had an Autoregulatory Index > or =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.

Effect of acute exposure to 3660 m altitude on orthostatic responses and tolerance

Orthostatic reflexes were examined at 375 m and after 60 min of exposure in a hypobaric chamber at 3660 m using a 20-min 70 degrees head-up tilt (HUT) test. Mean arterial blood pressure, R wave-R wave interval (RRI), and mean cerebral blood flow velocity (MFV) were examined with coarse-graining spectral analysis. Of 14 subjects, 7 at 375 m and 12 at 3660 m were presyncopal. Immediately on arrival to high altitude, breathing frequency and MFV increased, and endtidal PCO2, RRI, RRI complexity, and the parasympathetic nervous system indicator decreased. MFV was similar in HUT at both altitudes. The sympathetic nervous system indicator increased with tilt at 3660 m, whereas parasympathetic nervous system indicator decreased with tilt at both altitudes. Multiple regression analysis of supine variables from either 375 or 3660 m and the time to presyncope at 3660 m indicated that, after 1 h of exposure, increased presyncope at altitude was the result of 1). ineffective peripheral vasoconstriction, despite increased cardiac sympathetic nervous system activity with HUT, and 2). insufficient cerebral perfusion owing to cerebral vasoconstriction as the result of hypoxic hyperventilation-induced hypocapnia.

Transcranial Doppler ultrasound study of the effects of nitrous oxide on cerebral autoregulation during neurosurgical anesthesia: a randomized controlled trial

OBJECT

Nitrous oxide has an adverse effect on cerebrovascular hemodynamics. Increased intracranial pressure, cerebral blood flow (CBF), cerebral metabolic rate of O2 (CMRO2), and reduced autoregulation indices have been reported, but their magnitudes are still being debated. This study was designed to evaluate the effect of N2O on CBF and autoregulatory indexes during N2O-sevoflurane anesthesia in a prospective randomized controlled series of patients.

METHODS

Two groups of 20 patients were studied on the basis of the use of N2O in the anesthetic gas mixture. The transient hyperemic response test, which relies on transcranial Doppler ultrasound techniques, was used to assess cerebral hemodynamics. The time-averaged mean flow velocity, considered to be an index of actual CBF, increased significantly (p < 0.001) after introduction of N2O. The hyperemic response, considered as the index of autoregulatory potential, decreased significantly after introduction of N2O into the gas mixture (p < 0.001).

CONCLUSIONS

The increase in CBF and the reduction in autoregulatory indices suggest caution in using N2O during sevoflurane anesthesia, especially in patients with reduced autoregulatory reserve and during neurosurgical interventions. Transcranial Doppler ultrasonography is an efficacious method to evaluate the effects of anesthetic agents on CBF.

Positron Emission Tomographic Cerebral Perfusion Disturbances and Transcranial Doppler Findings among Patients with Neurological Deterioration after Subarachnoid Hemorrhage

OBJECTIVE

After aneurysmal subarachnoid hemorrhage, approximately 30% of patients experience delayed neurological deficits, related in part to arterial vasospasm and dysautoregulation. Transcranial Doppler (TCD) ultrasonography is commonly used to noninvasively detect arterial vasospasm. We studied cerebral perfusion patterns and associated TCD indices for 25 patients who developed clinical signs of delayed neurological deficits.

METHODS

Patients were treated in a neurosurgical intensive care unit and were studied if they exhibited delayed focal or global neurological deterioration. Positron emission tomographic cerebral blood flow (CBF) studies and TCD studies measuring the mean flow velocity (FV) of the middle cerebral artery and the middle cerebral artery FV/internal carotid artery FV ratio (with the internal carotid artery FV being measured extracranially at the cranial base) were performed. Glasgow Outcome Scale scores were assessed at 6 months.

RESULTS

A markedly heterogeneous pattern of CBF distribution was observed, with hyperemia, normal CBF values, and reduced flow being observed among patients with delayed neurological deficits. TCD indices were not indicative of the cerebral perfusion findings. The mean CBF value was slightly lower for patients who did not survive (32.3 ml/100 g/min), compared with those who did survive (36.0 ml/100 g/min, P= 0.05).

CONCLUSION

Among patients who developed delayed neurological deficits after aneurysmal subarachnoid hemorrhage, a wide range of cerebral perfusion disturbances was observed, calling into question the traditional concept of large-vessel vasospasm. Commonly used TCD indices do not reflect cerebral perfusion values.

Middle cerebral artery blood velocity during intense static exercise is dominated by a Valsalva maneuver

Lifting of a heavy weight may lead to "blackout" and occasionally also to cerebral hemorrhage, indicating pronounced consequences for the blood flow through the brain. We hypothesized that especially strenuous respiratory straining (a Valsalva-like maneuver) associated with intense static exercise would lead to a precipitous rise in mean arterial and central venous pressures and, in turn, influence the middle cerebral artery blood velocity (MCA V(mean)) as a noninvasive indicator of changes in cerebral blood flow. In 10 healthy subjects, MCA V(mean) was evaluated in response to maximal static two-legged exercise performed either with a concomitantly performed Valsalva maneuver or with continued ventilation and also during a Valsalva maneuver without associated exercise (n = 6). During static two-legged exercise, the largest rise for mean arterial pressure and MCA V(mean) was established at the onset of exercise performed with a Valsalva-like maneuver (by 42 +/- 5 mmHg and 31 +/- 3% vs. 22 +/- 6 mmHg and 25 +/- 6% with continued ventilation; P < 0.05). Profound reductions in MCA V(mean) were observed both after exercise with continued ventilation (-29 +/- 4% together with a reduction in the arterial CO(2) tension by -5 +/- 1 Torr) and during the maintained Valsalva maneuver (-21 +/- 3% together with an elevation in central venous pressure to 40 +/- 7 mmHg). Responses to performance of the Valsalva maneuver with and without exercise were similar, reflecting the deterministic importance of the Valsalva maneuver for the central and cerebral hemodynamic response to intense static exercise. Continued ventilation during intense static exercise may limit the initial rise in arterial pressure and may in turn reduce the risk of hemorrhage. On the other hand, blackout during and after intense static exercise may reflect a reduction in cerebral blood flow due to expiratory straining and/or hyperventilation.

Transcranial Doppler assessment of cerebral vasospasm

This review summarizes the use of transcranial Doppler (TCD) for assessment of cerebral vasospasm. The basic hemodynamic principles are presented, and used as a basis for discussing findings and interpretation methods. The need for additional information and measurements to correctly interpret TCD velocities is analyzed, and the use of a special extracranial Doppler technique is recommended. The advantages and limitations of the 'Lindegaard Index' (LI) are discussed. The recent advances in the use of TCD for cerebral autoregulation testing are opening up a new and promising avenue in diagnosis, monitoring and treatment of cerebral vasospasm.

Online monitoring of cerebral hemodynamics during hemodialysis

BACKGROUND

Several factors, including anemia, diabetes, and hypertension, potentially could disturb the cerebral autoregulation mechanism in hemodialysis (HD) patients. This study examined the effect of hemodynamic and rheological changes on mean cerebral blood flow (CBF) velocity (MV) during HD.

METHODS

Continuous online monitoring of MV and pulsatility index in the middle cerebral artery were performed in 18 HD patients by transcranial Doppler ultrasound during the entire HD period (range, 3 to 4 hours). In addition, blood pressure, hematocrit (Hct), and relative decrease in blood volume were continuously monitored. Blood samples were obtained at the beginning and end of HD to measure hemorheological variables.

RESULTS

After HD, Hct increased significantly from 33.6% +/- 5.9% to 41.4% +/- 5.7% (P < 0.001). Blood and plasma viscosity increased significantly from 3.33 +/- 0.77 to 4.36 +/- 1.3 mPa.s (P < 0.001) and from 1.35 +/- 0.29 to 1.54 +/- 0.38 mPa.s (P < 0.001), respectively. The change in MV (DeltaMV) was not significantly different from zero and correlated significantly with change in Hct. During HD, mean arterial pressure (MAP) in 15 patients changed within the normal range (group I), whereas 3 patients developed hypotension (group II) and their MAP decreased from 99 +/- 5 to 60 +/- 8 mm Hg (P < 0.05). In both groups, DeltaMV were not significant.

CONCLUSION

Results of this study suggest that CBF does not appear to be diminished significantly during HD.

Continuous cerebral autoregulation monitoring by cross-correlation analysis: evaluation in healthy volunteers

OBJECTIVE

In a former study, we applied cross-correlation (CC) analysis to recordings of arterial blood pressure (BP), intracranial pressure (ICP), and intracranial blood flow velocity (FV). A lack of significant time delay and a positive correlation coefficient of slow oscillations between these parameters was interpreted as indicative of impaired cerebral autoregulation, whereas a significant time delay and a negative correlation was regarded as preserved autoregulation. To test this hypothesis, cross-correlation was applied on recordings of BP and FV (CC [BP --> FV]) in healthy volunteers with a presumably preserved cerebral autoregulation.

DESIGN

Study of a diagnostic test.

SUBJECTS

A total of 17 healthy volunteers.

MEASUREMENTS AND MAIN RESULTS

BP was recorded by using a tonometric device, and bilateral FV in the middle cerebral arteries (MCA) was measured by transcranial Doppler sonography. Signals were sampled at a resting horizontal position for 29 mins. Cluster analysis showed a mean +/- sd time delay for CC [BP --> FV(MCA right)] of 6.45 +/- 2.1 secs, and for CC [BP --> FV(MCA left) ] of 6.09 +/- 1.8 secs. The mean correlation coefficient was -.33 +/-.17 for the left and -.36 +/-.09 for the right side. In about 30%, differing results with a correlation coefficient between -.2 and.2 and a time delay near zero were found. Cross-correlation between left and right FV showed a mean time delay of 0.09 +/- 0.18 secs, with a mean correlation coefficient of.82 +/-.16. CONCLUSION Spontaneous slow oscillations of BP and FV were detected, and cross-correlation analysis showed a negative correlation and a positive time delay in about 70% of the examinations. These findings corroborate the hypothesis that CC [BP --> FV] might be able to assess the status of cerebral autoregulation continuously. The observed time delay between BP and FV oscillations is in good agreement with former studies on the dynamic properties of cerebral autoregulation.

Non-invasive quantitative monitoring of cerebral blood flow in aneurysmal subarachnoid haemorrhage with 99mTc-ECD

The purpose of this prospective study was to detect symptomatic cerebral vasospasm in aneurysmal subarachnoid haemorrhage (SAH) by a non-invasive mean cerebral blood flow (mCBF) quantification using 99mTc-ethyl cysteinate dimer. Measurement of mCBF without blood sampling and single photon emission tomography (SPECT) were performed at 1 and 7 days after surgery in 35 consecutive SAH patients, of whom 16 were examined at day 30 as well. A decrease in mCBF of more than 10% on day 7 versus day 1 was considered to indicate vasospasm. On visual interpretation of SPECT, a perfusion decrease which appeared newly on day 7 was considered to indicate vasospasm. In total, nine of 35 patients had cerebral vasospasm confirmed by computed tomography (CT) and/or angiography. The mCBF measurement showed a 77.8% (7/9) sensitivity, a 88.5% (23/26) specificity, a 70.0% (7/10) positive predictive value, and a 92.0% (23/25) negative predictive value. SPECT yielded a 33.3% (3/9) sensitivity, a 73.1% (19/26) specificity, a 30.0% (3/10) positive predictive value, and a 76.0% (19/25) negative predictive value. On SPECT, decreased perfusion was observed in most of the patients at clipping sites, which might represent post-operative transient abnormal perfusion and should not be read as vasospasm. In conclusion, this mCBF measurement is more accurate than visual interpretation of SPECT for detecting vasospasm.

Transcranial Doppler in stroke

Transcranial Doppler (TCD) has been extensively used in various clinical situations, and in the last two decades has established its role in the management of patients with cerebrovascular disease and stroke. Based on the Doppler principle, it uses ultrasound waves to insonate the blood vessels supplying the brain to obtain hemodynamic information. Anatomic abnormalities of vascular occlusion, stenosis and spasm can be indirectly derived. Intracranial arterial disease is an important cause of ischemic stroke and TCD can detect these with a fair amount of sensitivity and specificity. In hemodynamically significant extracranial internal carotid artery disease, TCD shows significant abnormalities in flow dynamics of the anterior circulation and abnormalities of cerebral vasomotor reactivity. A distinct advantage of TCD is the ability to monitor blood flow in a blood vessel over prolonged periods of time, which has shown microembolic signals in acute ischemic stroke, carotid artery disease, atrial fibrillation and during angiography. In acute ischemic stroke, TCD can be used to elucidate stroke mechanisms, plan and monitor treatment, and determine prognosis. In an era when stroke is increasingly being recognized as an emergency requiring immediate treatment, TCD may be capable of providing rapid information about the hemodynamic status of the cerebral circulation, within the time frame of the rather small 'therapeutic window'. TCD predicts vasospasm with a high degree of sensitivity and specificity and because of its non-invasive nature repeated assessments can be performed after subarachnoid hemorrhage.

Evaluation of the cerebral hemodynamic response to rhythmic handgrip

The response of the cerebral circulation to exercise has been studied with transcranial Doppler ultrasound (TCD) because this modality provides continuous measurements of blood velocity and is well suited for the exercise environment. The use of TCD as an index of cerebral blood flow, however, requires the assumption that the diameter of the insonated vessel is constant. Here, we examine this assumption for rhythmic handgrip using a spectral index designed to measure trends in vessel flow. Nineteen normal subjects were studied during 5 min of volitional maximum rhythmic right handgrip at 1 Hz. TCD velocities from both middle arteries (left and right), blood pressure, and end-tidal PCO(2) were recorded every 10 s. A spectral weighted sum was also calculated as a flow index (FI). Averages were computed from the last 2 min of handgrip. Relative changes in velocity, FI, and pressure were calculated. The validity of FI was tested by comparing the change in diameter derived from equations relating flow and diameter. Mean blood pressure increased 23.8 +/- 17.8% (SD), and velocity increased 13.3 +/- 9.8% (left) and 9.6 +/- 8.3% (right). Although the mean change in FI was small [2.0 +/- 18. 2% (left) and 4.7 +/- 29.7% (right)], the variation was high: some subjects showed a significant increase in FI and others a significant decrease. Diameter estimates from two equations relating flow and luminal area were not significantly different. Decreases in FI were associated with estimated diameter decreases of 10%. Our data suggest that the cerebral blood flow (CBF) response to rhythmic handgrip is heterogeneous and that middle cerebral artery flow can decrease in some subjects, in agreement with prior studies using the Kety-Schmidt technique. We speculate that the velocity increase is due to sympathetically mediated vasoconstriction rather than a ubiquitous flow increase. Our data suggest that the use of ordinary TCD velocities to interpret the CBF response during exercise may be invalid.

Assessment of middle cerebral artery diameter after aneurysmal subarachnoid hemorrhage by transcranial color-coded duplex sonography

This study examined if vasospasm after aneurysmal subarachnoid hemorrhage could be visualized by middle cerebral artery (MCA) diameter changes in transcranial color-coded duplex sonography (TCCS). Comparative measurements between mean blood velocity (MBV) and MCA diameter were carried out in 17 patients in 76 instances. At two depth ranges (proximal, 60 55 mm: distal, 50-45 mm) two observers assessed the MCA diameter as indicated by the visualized blood flow column. At both points of measurement, the diameter differences between the two observers were within the ¿ 2 S.D. range of the mean difference indicating interobserver agreement. In 17 instances, MBV was > 120 cm/s indicating vasospasm but MBV did not correlate with absolute or relative diameter changes. MCA diameter assessment in TCCS seems reproducible. Because TCCS imaging is influenced by several factors comparative angiographic studies are necessary to clarify the TCCS findings.

The role of transcranial Doppler in the management of patients with subarachnoid haemorrhage--a review

Introduced 15 years ago, transcranial Doppler (TCD) recordings of blood-velocity in patients with recent subarachnoid haemorrhage (SAH) have two objectives: to detect elevated blood velocities suggesting cerebral vasospasm (VSP) and to identify patients at risk for delayed cerebral ischemic deficits (DID). The pathophysiological cascade causing DID is complex. Discrepancies between blood velocities and DID (presuming that there actually is an "ischemic threshold" for blood velocity in absolute terms, which seems most unlikely) have been demonstrated, particularly in patients with elevated intracranial pressure (ICP) levels. Furthermore, the vessel showing the highest blood velocity is not always the one perfusing the area where ischemic symptoms arise, nor does the site of the greatest subarachnoid blood clot always relate to the ischemic brain region. Moreover, it is probable that the complex haemodynamic changes following SAH and the subsequent development of VSP may be underestimated if only considering the crude intracranial artery blood velocities. Cerebral blood flow measurements combined with TCD to assess both flow and velocity have emphasised this point. Despite these findings and ignoring the basic principles of cerebral haemodynamics, cerebral vasospasm is still being assessed from the intracranial velocity measurement alone. The addition of at least a careful measurement from the extracranial internal carotid artery--using the same TCD equipment and taking only a few short minutes to perform--allows a much more accurate assessment of the degree and the effects of vasospasm. This probably explains why the clinical value of TCD is still debated. There is still uncertainty as to the best method to prevent and to treat VSP, and the overall outcome after SAH depends on so many factors besides VSP. Conclusive evidence may therefore be hard to obtain, and it appears sound to conclude that even with advanced investigation technology available, proper selection, pre- peri- and postoperative care and timing of surgery remain cornerstones in the management of these patients,--equal in importance to their treatment in the operating room or in the interventional angiography suite.

Oscillations in cerebral blood flow detected with a transcranial Doppler index

Although transcranial Doppler ultrasound (TCD) has been used to detect oscillations in CBF, interpretation is severely limited, since only blood velocity and not flow is measured. Oscillations in vessel diameter could, therefore, mask or alter the detection of those in flow by TCD velocities. In this report, the authors use a TCD-derived index of flow to detect and quantify oscillations of CBF in humans at rest. A flow index (FI) was calculated from TCD spectra by averaging the intensity weighted mean in a beat-by-beat manner over 10 seconds. Both FI and TCD velocity were measured in 16 studies of eight normal subjects at rest every 10 seconds for 20 minutes. End tidal CO2 and blood pressure were obtained simultaneously in six of these studies. The TCD probe position was meticulously held constant. An index of vessel area was calculated by dividing FI by velocity. Spectral estimations were obtained using the Welch method. Spectral peaks were defined as peaks greater than 2 dB above background. The frequencies and magnitudes of spectral peaks of FI, velocity, blood pressure, and CO2 were compared with t tests. The Kolmogorov-Smirnov test was used to further confirm that the data were not white noise. In most cases, three spectral peaks (a, b, c) could be identified, corresponding to periods of 208+/-93, 59+/-31, and 28+/-4 (SD) seconds for FI, and 196+/-83, 57+/-20, and 28+/-6, (SD) seconds for velocity. The magnitudes of the spectral peaks for FI were significantly greater (P<0.02) than those for velocity. These magnitudes corresponded to variations of at least 15.6%, 9.8%, and 6.8% for FI, and 4.8%, 4.2%, and 2.8% for velocity. The frequencies of the spectral peaks of CO2 were similar to those of FI with periods of 213+/-100, 60+/-46, and 28+/-3.6 (SD) seconds. However, the CO2 spectral peak magnitudes were small, with an estimated maximal effect on CBF of (+/-) 2.5+/-0.98, 1.5+/-0.54, and 1.1+/-0.31 (SD) percent. The frequencies of the blood pressure spectral peaks also were similar, with periods of 173+/-81, 44+/-8, and 26+/-2.5 (SD) seconds. Their magnitudes were small, corresponding to variations in blood pressure of (+/-) 2.1+/-0.55, 0.97+/-0.25, and 0.72+/-0.19 (SD) percent. Furthermore, coherence analysis showed no correlation between CO2 and FI, and only weak correlations at isolated frequencies between CO2 and velocity, blood pressure and velocity, or blood pressure and FI. The Kolmogorov-Smirnov test distinguished our data from white noise in most cases. Oscillations in vessel flow occur with significant magnitude at three distinct frequencies in normal subjects at rest and can be detected with a TCD-derived index. The presence of oscillations in blood velocity at similar frequencies but at lower magnitudes suggests that the vessel diameters oscillate in synchrony with flow. Observed variations in CO2 and blood pressure do not explain the flow oscillations. Ordinary TCD velocities severely underestimate these oscillations and so are not appropriate when small changes in flow are to be measured.