Validity of cerebral arterial blood flow calculations from velocity measurements

The influence of nitroglycerin and prostaglandin E1 on dynamic cerebral autoregulation in adult patients during propofol and fentanyl anaesthesia

We investigated dynamic cerebral autoregulation in 24 normocapnic adult patients during propofol and fentanyl anaesthesia. Hypotension was induced, to a mean arterial pressure (MAP) of 60-65 mmHg, using nitroglycerin or prostaglandin E1. Time-averaged mean cerebral blood flow velocity in the right middle cerebral artery was measured continuously using transcranial Doppler sonography. Dynamic autoregulatory response was activated by a sudden decrease in MAP following release of bilateral thigh cuffs (thigh cuff test) and evaluated as a dynamic rate of autoregulation (dRoR in % x s(-1)). The cuff test was repeated to obtain two values of dRoR during baseline and during induced hypotension; the data were then averaged. The mean value of dRoR during baseline and during induced hypotension was 14.2 (2.9) and 14.2 (1.6) % x s(-1), respectively, in the nitroglycerin group, and 14.6 (2.6) and 14.4 (2.4) % x s(-1), in the prostaglandin E1 group. We were unable to demonstrate significant between- or within-group differences in dRoR. Thus, we conclude that nitroglycerin and prostaglandin E1 do not attenuate dynamic cerebral autoregulation.

Perfusion-weighted magnetic resonance imaging in patients with carotid artery disease before and after carotid endarterectomy

OBJECTIVE

The purpose of this study was to investigate the potential of perfusion-weighted magnetic resonance imaging for preoperative and postoperative evaluation of cerebral hemodynamics in patients undergoing carotid endarterectomy for carotid artery stenosis.

METHODS

We examined 26 patients with angiographically proven stenoses (60%-99%) of the internal carotid artery preoperatively. Perfusion imaging studies were performed by bolus-tracking of a dosage of 0.2 mmol/kg body weight of gadolinium diethylenetriaminepentaacetic acid on a 1.5-T scanner using a T2*-weighted fast low-angle shot sequence. The observed signal intensities were converted pixel by pixel into concentration-time curves. In each patient, the hemispheres were compared and the difference between the normalized first moments (NFMs) and the percentage changes of the regional cerebral blood volume (CBV) were calculated. Three months postoperatively, perfusion-weighted magnetic resonance imaging was performed in 13 patients.

RESULTS

In patients with <80% stenosis (n = 10), there was no significant alteration of NFM and regional CBV compared with the contralateral hemisphere (-0.16 +/- 0.7 s, +5.9 +/- 24.6%). In patients with stenoses >or=80% (n = 16), we found an increase in NFM ipsilateral to the stenosis of 1.2 +/- 0.92 s (P < .001) and an increase of CBV of 16.8 +/- 15.2% (P < .005). Three months postoperatively, perfusion parameters were normal in all 13 patients examined.

CONCLUSIONS

Perfusion-weighted magnetic resonance imaging is well suited to evaluate the preoperative and postoperative hemodynamic changes in patients with carotid artery stenosis. This noninvasive, semiquantitative magnetic resonance technique could prove to be a valuable adjunct in identification of patients who might benefit from carotid endarterectomy.

Brain SPECT used to evaluate vasospasm after subarachnoid hemorrhage: correlation with angiography and transcranial Doppler

PURPOSE

The primary objective of this study was to correlate Tc-99m HMPAO and ethyl cysteine dimer perfusion brain SPECT imaging with angiography and transcranial Doppler (TCD) to identify vasospasm after subarachnoid hemorrhage.

METHODS

A retrospective analysis of consecutive patients who had cerebral blood flow SPECT imaging for subarachnoid hemorrhage and aneurysm clipping was made. Flow velocity measurements were correlated using TCD and cerebrovascular angiography.

RESULTS

Of the 129 patients were included in this study, 84 were female and 45 were male, with a mean age of 51.9 years and a median age of 51 years (range, 9 to 84 years). Eighty-nine patients had brain SPECT evidence of hypoperfusion. Concordance was found between SPECT and TCD with vasospasm in 57 of 89 (64%) patients and nonconcordance was evident in 32 patients (36%). Eleven patients who had concordance between SPECT and TCD had nonconcordant results of angiography for vasospasm.

CONCLUSIONS

These findings suggest that all three methods are complementary to each other in the evaluation of patients with vasospasm after subarachnoid hemorrhage. Concordance of 64% between SPECT and TCD is acceptable and explicable by the differences in technique and measurement of cerebral blood flow compared with vascular narrowing, respectively.

Ocular hemodynamics during isometric exercise

The autoregulatory capacity of the human retina is well documented, but the pressure-flow relationship of the human choroid is still a matter of controversy. Recent data, using laser Doppler flowmetry to measure choroidal blood flow, indicate that the choroid has some autoregulatory potential, whereas most data using other techniques for the assessment of choroidal hemodynamics indicate that the choroidal pressure-flow curve is linear. We used a new laser interferometric technique to characterize choroidal blood flow during isometric exercise. Twenty healthy subjects performed squatting for 6 min during normocapnia and during inhalation of 5% CO2 and 95% air. Ocular fundus pulsation amplitude, flow velocities in the ophthalmic artery, intraocular pressure, and systemic hemodynamics were measured in 2-min intervals. To gain information on choroidal blood flow fundus pulsation amplitude was corrected for changes in flow pulsatility using data from the ophthalmic artery and for changes in pulse rate. Ocular perfusion pressure was calculated from mean arterial pressure and intraocular pressure. The ocular pressure-flow relationship was calculated by sorting data according to ascending ocular perfusion pressure values. In a pilot study in 6 healthy subjects comparable ocular pressure flow relationships were obtained when choroidal blood flow was assessed with the method described above and with laser Doppler flowmetry. In the main study isometric exercise caused a significant increase in mean arterial pressure (56%, P < 0.001), pulse rate (84%, P < 0.001), and intraocular pressure (37%, P 0.004), but decreased fundus pulsation amplitude (-36%, P < 0.001). Significant deviations from baseline choroidal blood flow were observed only at ocular perfusion pressures >69% during normocapnia and 70% during hypercapnia. Our data indicate that during isometric exercise the choroid has a high capacity to keep blood flow constant despite changes in perfusion pressure and that this pressure-flow relationship is not altered by moderate changes in arterial carbon dioxide levels.

Transcranial Doppler ultrasonography monitoring of cerebral hemodynamics during performance of cognitive tasks: a review

The examination of blood flow velocity (BFV) changes during the performance of mental tasks is one of the applications of transcranial Doppler (TCD) ultrasonography. The purpose of this review is to summarize the results of the functional TCD literature, to investigate the effects of methodological differences between studies, and to provide guidelines for future research. It is concluded that larger series of more homogeneous groups concerning age and handedness, and stricter criteria for subject selection and laboratory setting are required. The implication of quantitative and qualitative performance measures and psychological parameters (motivation, anxiety, and task anticipation) could also yield important information. We recommend future agreement upon a more standardized methodology. TCD promises to be a useful tool to provide further insight into the cerebral organization and temporal reactivity of the human brain.

Transcranial Doppler blood velocity measurement--the effect of changes in velocity profile

Transcranial Doppler (TCD) units measure blood velocity in the middle cerebral artery (MCA) and are used to examine the effects of pharmacological agents. The units actually measure the average of the maximum blood velocity envelope (aveV(max)) and it is assumed that changes in aveV(max) follow changes in the true mean velocity (aveV(mean)). This may not be true if there are changes in velocity profile. Results from previous TCD studies using acetazolamide (ACZ) and caffeine were examined for evidence for changes in velocity profile. ACZ increased aveV(max) by 21% (95% CI 13 to 29%) and aveV(mean) by 14% (95% CI 9 to 19%). Caffeine decreased aveV(max) by 8% (95% CI 4 to 12%) and aveV(mean) by 5% (95% CI 4% increase to 13% decrease). In both cases, the true change, measured using aveV(mean) was lower, indicating possible changes in velocity profile. We conclude that the possibility of changes in velocity profile must be considered when using TCD to quantify changes in blood velocity.

Estimating normal and pathological dynamic responses in cerebral blood flow velocity to step changes in end-tidal pCO2

The regulation of cerebral blood flow (CBF) following changes in arterial blood pressure (ABP) and end-tidal pCO2 (EtCO2) are of clinical interest in assessing cerebrovascular reserve capacity. Linear finite-impulse-response modelling is applied to ABP, EtCO2 and CBF velocity (CBFV, from transcranial Doppler measurements), which allows the CBFV response to ideal step changes in EtCO2 to be estimated from clinical data showing more sluggish, and additional random variations. The confounding effects of ABP changes provoked by hypercapnia on the CBFV are also corrected for. Data from 56 patients suffering from stenosis of the carotid arteries (with normal or diminished cerebrovascular reactivity to EtCO2 changes--CVRCO2) were analysed. The results show the expected significant differences (p < 0.05) between EtCO2 steps up and down, the significant contribution from ABP variation, and also differences in the dynamic responses of patients with reduced CVRCO2 (p < 0.01 after 10 s). For the latter the CBFV response appears exhausted after about 15 s, whereas for normals CBFV continues to increase. While dispersion of individual step responses remains large, the method gives encouraging results for the non-invasive study of compromised haemodynamics in different patient groups.

The effects of nicardipine on dynamic cerebral autoregulation in patients anesthetized with propofol and fentanyl

We investigated the effects of nicardipine on dynamic cerebral pressure autoregulation in 13 normal adult patients undergoing gynecologic or orthopedic surgery. Anesthesia was induced and maintained with propofol and fentanyl. Hypotension to a mean arterial pressure of 60-65 mm Hg was induced and maintained with a continuous infusion of nicardipine. Time-averaged mean blood flow velocity in the right middle cerebral artery was measured continuously by using transcranial Doppler ultrasonography. The cerebral autoregulatory responses were activated by releasing thigh cuffs. The actual blood flow velocity in the right middle cerebral artery response to acute change in mean arterial pressure was fitted to 1 of 10 computer-generated curves to determine the dynamic rate of cerebral autoregulation (dRoR), and the best fitting curve was used. The autoregulation test was repeated until two values of dRoR were obtained at baseline and during induced hypotension. Nicardipine significantly reduced dRoR values of 13.1% +/- 3.6%/s at baseline to 8.3% +/- 2.6%/s during hypotension (P: < 0.01). During deliberate hypotension induced by nicardipine, the cerebral dynamic autoregulatory response is impaired in normal adult patients.

IMPLICATIONS

During deliberate hypotension induced by nicardipine, the cerebral dynamic autoregulatory response is impaired in normal adult patients.

Correlation of cerebral hemodynamic changes during mental activity and recovery after stroke

OBJECTIVE

To investigate the correlation between changes in cerebral functional activity during mental engagement and the potential for neurologic recovery after stroke.

BACKGROUND

Transcranial Doppler ultrasonography (TCD) makes it possible to detect the dynamic adjustment of cerebral perfusion related to functional neuronal changes.

METHODS

TCD monitoring of flow velocity changes in the middle cerebral artery of 29 ischemic stroke patients was performed during an object recognition task. The study took place within 4 weeks from stroke onset. Based on recovery occurring after 2 months, the patients were divided into four groups depending on the side of hemispheric lesion and the presence or absence of neurologic recovery. Ten healthy subjects served as control subjects.

RESULTS

During the recognition task, control subjects showed a bilateral increase in flow velocity with respect to the rest phase (right side, 7.02 +/- 1.3%; left side, 6.65 +/- 1. 1%), with no side-to-side difference. In patients who experienced recovery, a similar pattern of bilateral activation was observed, irrespective of the side of the lesion. Conversely, in patients with no recovery, the increase of flow velocity was significantly higher on the side contralateral to the brain lesion (p < 0.0001) with respect to the lesion side. Performance during the recognition task was comparable in the four groups of patients.

CONCLUSIONS

These findings suggest that satisfactory recovery from a neurologic deficit requires the persistence of functional activity in the damaged hemisphere despite the presence of an anatomic lesion. The possibility of obtaining early prognostic indications with TCD may be relevant for an early selection of patients with the best probability of benefiting from rehabilitation therapy.

Absence of hemispheric dominance for mental rotation ability: a transcranial Doppler study

Mean blood flow velocity (MFV) of the middle cerebral arteries was monitored in 19 healthy, adult, right-handed subjects during the resting phase and the execution of a series of neuropsychological tests: two right/left discrimination tasks, two mental rotation paradigms (the Ratcliff's test and a cube comparison test) and a phonemic fluency task, which was utilised as an internal control. In the group as a whole, the Ratcliff's test was associated with a significant bilateral increase in MFV versus both the resting state (right: p < .000001, left: p < .000001) and right/left discrimination tasks (task 1: right: p = .003, left: p = .005; task 2: right: p = .001, left: p = .001). The cube comparison in turn produced a significant increase in MFV versus both the baseline conditions (right: p < .000001, left: p < .000001) and the Ratcliff's test (right: p = .01, left: p = .002). As expected, the fluency task was associated with a significant asymmetric increase in cerebral perfusion (left > right: p = .0001). Increasing task difficulty (right/left discrimination < Ratcliffs test < cube comparison) was paralleled by a roughly proportional rise in MFV values (right: r = .424, p < .01; left: r = .331, p = .01). In conclusion, we were able to demonstrate that (1) in addition to the amount of MFV variation due to right/left discrimination (when required), mental rotation per se causes a bihemispheric activation irrespective of the experimental paradigm; (2) the MFV variation is proportional to the difficulty of the tasks.

Using doppler signal power to detect changes in vessel size: a feasibility study using a wall-less flow phantom

The power of a Doppler signal is theoretically proportional to the volume of blood within the sample volume of an ultrasound (US) beam and, hence, may provide a means of detecting in vivo changes in the cross-sectional area of cerebral vessels, such as the middle cerebral artery. The purpose of this study was to examine the relationship between power and vessel size for signals recorded from a wall-less flow phantom. The results demonstrate the importance for the in vitro case of maximising the received signal power for each channel to obtain the true relationship between power and size, and show that a nonproportional relationship observed between the two parameters is primarily caused by high-pass filtering and nonuniform insonation. In addition, an investigation of the reproducibility of power values after transducer repositioning has shown that variation occurs even when extreme care is taken to maximise the received signal intensity. The implications of these results for the in vivo use of the Doppler signal power method are discussed.

Reversal of caffeine withdrawal by ingestion of a soft beverage

Followlng regular use, acute cessation of caffeine is associated with a characteristic withdrawal syndrome. Despite this, caffeine remains popular with its consumers. The aim of this study was to examine the physiologic and psychologic effects of small caffeine doses, administered in the form of a market-leading soft drink, on healthy women who were acutely withdrawn from caffeine. After 48-h abstinence and overnight fast, 11 healthy (22 to 40 years) female volunteers, all regular caffeine users (daily consumption 143 to 773 mg) consumed using a double-blind. randomized, controlled cross-over design either 2 tins of regular or caffeine-free Diet Coke. On both visits a Mars bar was eaten to prevent hypoglycaemia. Thus, the caffeine load was 76 or 10 mg respectively. Following ingestion of regular Diet Coke, there was a l0% fall in middle cerebral artery velocity (95% CI [6%-l4%], p < 0.005 versus caffeine free) and improvement in feelings of pleasure (p < 0.046) and energy (p < 0.037). Intellectual function (4-choice reaction time) was unaffected by caffeine status. On both visits, ingestion of Diet Coke induced a pressor response (maximum rise in systolic pressure +15+/- 2 mm Hg with caffeine and +l2 +/- 2 mm Hg with caffeine-free beverage, both p < 0.001 compared with baseline). In conclusion, in women acutely withdrawn from caffeine, ingestion of a popular soft beverage containing modest amounts of caffeine is associated with demonstrable physiologic and psychologic effects.

Vertigo: analysis by magnetic resonance imaging and angiography

The relationship of vascular disease of the vertebrobasilar artery system to isolated vertigo was examined by magnetic resonance imaging and angiography. Eighty-nine individuals complaining of vertigo were evaluated by standard otoneurologic investigations, and the data were correlated with the vascular patterns of the cervical region and posterior fossa. The age distribution extended from the fourth decade to the ninth decade; the peak occurrence was observed in the eighth. Approximately 85% of the group experienced numerous episodes of vestibular dysfunction from months to years before examination; the remaining segment was examined following the first episode due to severity or persistence of symptoms. The criteria for vascular abnormality proposed by the authors are based upon comparison with previous normal findings. Approximately 52% of the cohort demonstrated abnormal configurations or evidence of diminished flow within the vertebrobasilar artery system. Of this segment, a vertebral artery was most frequently abnormal, in 76%; the basilar artery was judged pathological in 32%, and combined disease of several arteries was evident in 20%.

Transcranial Doppler findings during balloon test occlusion of the internal carotid artery

The authors performed transcranial Doppler ultrasonography (TCD) during internal carotid artery (ICA) balloon test occlusion (BTO) and observed changes in mean flow velocity (Vm) in the middle cerebral artery (MCA), and pulsatility index (PI) while monitoring the stump pressure (Sp) of the internal carotid artery (ICA), and neurologic findings. A group of 17 patients requiring possible temporary or permanent occlusion of the ICA in the course of planned procedures first underwent BTO. A patient who either developed neurologic changes or maintained less than 60% of preocclusion Sp or Vm in the ipsilateral MCA during BTO was considered to have a positive test. Eleven patients had negative results, while in six patients, tests were positive. Mean flow velocity showed a decrease after occlusion in all cases but not to a remarkable extent in some patients. Stump pressure decreased in all negative cases after balloon inflation and than tended to increase progressively during 15 minutes of BTO. Pulsatility index tended to decrease gradually during BTO in all negative patients. However, in positive cases, PI and Sp fell steeply. Only one positive case had a neurologic symptom of severe headache. The decreased PI in the MCA reflected autoregulatory dilation of cerebral vessels to compensate for decreased absolute cerebral blood volume following ICA occlusion. Changes in PI are a good indicator for evaluating blood flow during BTO.

Comparison of transcranial Doppler flow velocity and cerebral blood flow during focal ischemia in rabbits

The predictive value of transcranial Doppler (TCD) cerebral blood flow velocity (CBFV) measurements for cerebral blood flow (CBF) calculations in humans is still controversial, and experimental correlative studies are lacking. The aim of the present study was to validate TCD signals of CBFV during focal cerebral ischemia. Therefore, CBFV determined in the middle cerebral artery (MCA) was compared with values of CBF obtained from autoradiograms of ischemic brain areas. To determine CBFV, a transcranial Doppler ultrasound probe (TCD) adapted to small sample volumes was used in 9 rabbits. CBF was quantified after a final infusion of [14C]-iodoantipyrine in the same animals. For focal cerebral ischemia induction, two threads were flushed upward simultaneously into the internal carotid artery, resulting in a flow reduction in the ipsilateral MCA. After thread occlusion, mean systolic CBFV in the MCA decreased from 49 +/- 9 cm/s to 22 +/- 3 cm/s. CBF in the caudate nucleus was reduced (19 +/- 8 mL/100 g/min) compared to the contralateral nonischemic side (52 +/- 18 mL/100 g/min). The decrease in hemispheric CBF correlated well with the decrease in both mean systolic (r = 0.97) and diastolic (r = 0.94) CBFV in the MCA (p < 0.01). The decrease in CBFV determined by transcranial Doppler ultrasound in the MCA appears to reflect the reduction in CBF in the affected brain hemisphere and can be used as a quantitative in vivo parameter for tissue perfusion.

Acetazolamide-induced cerebral and ocular vasodilation in humans is independent of nitric oxide

Acetazolamide, a carbonic anhydrase inhibitor, is used orally in the treatment of primary and secondary open-angle glaucoma and induces ocular and cerebral vasodilation. Several in vitro studies have shown that carbonic anhydrase pharmacology and the L-arginine-nitric oxide (NO) pathway are closely related. We investigated the role of NO in acetazolamide-induced vasodilation on cerebral and ocular vessels in 12 healthy subjects in the presence or absence of NG-monomethyl-L-arginine (L-NMMA), a NO synthase inhibitor, and in the presence or absence of L-arginine, the precursor of NO. Acetazolamide was administered after pretreatment with either L-NMMA or placebo and either L-arginine or placebo. Pulsatile choroidal blood flow was assessed with laser interferometric measurement of fundus pulsation. In addition, mean blood flow velocity (MFV) in the middle cerebral artery (MCA) and ophthalmic artery (OA) was measured with Doppler sonography. Acetazolamide increased ocular fundus pulsation amplitude (FPA; +27%, P < 0.001) and MFV in the MCA (+38%, P < 0.001) and in the OA (+19%, P = 0.003). Administration of L-NMMA alone reduced FPA (-21%, P < 0.001) and MFV in the MCA (-11%, P = 0. 030) but did not change MFV in the OA. All hemodynamic effects of L-NMMA were reversed by L-arginine. However, neither L-NMMA nor L-arginine altered acetazolamide-induced changes in cerebral or ocular hemodynamic parameters. The present data indicate that acetazolamide-induced hemodynamic changes are not mediated by NO. Which mediators other than NO are involved in the hemodynamic effects as induced by carbonic anhydrase inhibitors remains to be elucidated.

Hemodynamics of cerebrovascular spasm

An understanding of the hemodynamics of cerebrovascular spasm following subarachnoid hemorrhage is important for the diagnosis and treatment of this potentially dangerous condition. An overview model is presented which includes the main elements determining the overall effect of vasospasm. The model included realistic pressure-flow-velocity-diameter relationship encountered in a geometry resembling that of vasospasm of the middle cerebral artery. Viscosity was adjusted to that expected of human blood. Furthermore, a realistic model the cerebral autoregulation was included. The effects of induced hypertension as well as hypotension were studied. It was found that the friction pressure loss in the spastic segment was 3.5 times as high as that predicted by using the Hagen-Poiseuille formula. The reason for this discrepancy was probably the 'inlet length effect' considerably increasing the friction. Furthermore, including the Bernoulli kinetic pressure energy, a formula was proposed that accurately described the experimental data. From this hemodynamic perspective, strong support was found for the present trend to use aggressive hypertensive therapy in patients with vasospasm. The results also confirmed that TCD velocity measurements in the spastic segment when taken alone may not be a good index of the degree and effect of the spasm. These measurements must be combined with other techniques such as extracranial Doppler or CBF to assess the degree of spasm.

Critical closing pressure in cerebrovascular circulation

OBJECTIVE

Cerebral critical closing pressure (CCP) has been defined as an arterial pressure threshold below which arterial vessels collapse. Hypothetically this is equal to intracranial pressure (ICP) plus the contribution from the active tone of cerebral arterial smooth muscle. The correlation of CCP with ICP, cerebral autoregulation, and other clinical and haemodynamic modalities in patients with head injury was evaluated.

METHOD

intracranial pressure, arterial blood pressure (ABP) and middle cerebral artery blood flow velocity were recorded daily in ventilated patients. Waveforms were processed to calculate CCP, the transcranial Doppler-derived cerebral autoregulation index (Mx), mean arterial pressure (ABP), intracranial pressure (ICP), and cerebral perfusion pressure (CPP).

RESULTS

Critical closing pressure reflected the time related changes in ICP during plateau and B waves. Overall correlation between CCP and ICP was mild but significant (R=0.41; p<0.0002). The mean difference between ABP and CCP correlated with CPP (R=0.57, 95% confidence interval (95% CI) for prediction 25 mm Hg). The difference between CCP and ICP, described previously as proportional to arterial wall tension, correlated with the index of cerebral autoregulation Mx (p<0.0002) and CPP (p<0.0001). However, by contrast with the Mx index, CCP-ICP was not significantly correlated with outcome after head injury.

CONCLUSION

Critical closing pressure, although sensitive to variations in ICP and CPP, cannot be used as an accurate estimator of these modalities with acceptable confidence intervals. The difference CCP-ICP significantly correlates with cerebral autoregulation, but it lacks the power to predict outcome after head injury.

Cerebrovascular regulation in the postural orthostatic tachycardia syndrome (POTS)

Patients with the postural orthostatic tachycardia syndrome (POTS) have symptoms of orthostatic intolerance despite having a normal orthostatic blood pressure (BP), which suggests some impairment of cerebrovascular regulation. Cerebrovascular autoregulation refers to the maintenance of normal cerebral blood flow in spite of changing BP. Mechanisms of autoregulation include myogenic, metabolic and neurogenic vasoregulation. Beat-to-beat recording of blood-flow velocity (BFV) is possible using transcranial Doppler imaging. It is possible to evaluate autoregulation by regressing deltaBFV to deltaBP during head-up tilt. A number of dynamic methods, relating deltaBFV to deltaBP during sudden induced changes in BP by occluding then releasing peripheral arterial flow or by the Valsalva maneuver. The deltaBFV to deltaBP provides an index of autoregulation. In orthostatic hypotension, the autoregulated range is typically expanded. In contrast, paradoxical vasoconstriction occurs in POTS because of an increased depth of respiration, resulting in hypocapnic cerebrovascular constriction, and impaired autoregulation.

The cerebral haemodynamics of music perception. A transcranial Doppler sonography study

The perception of music has been investigated by several neurophysiological and neuroimaging methods. Results from these studies suggest a right hemisphere dominance for non-musicians and a possible left hemisphere dominance for musicians. However, inconsistent results have been obtained, and not all variables have been controlled by the different methods. We performed a study with functional transcranial Doppler sonography (fTCD) of the middle cerebral artery to evaluate changes in cerebral blood flow velocity (CBFV) during different periods of music perception. Twenty-four healthy right-handed subjects were enrolled and examined during rest and during listening to periods of music with predominant language, rhythm and harmony content. The gender, musical experience and mode of listening of the subjects were chosen as independent factors; the type of music was included as the variable in repeated measurements. We observed a significant increase of CBFV in the right hemisphere in non-musicians during harmony perception but not during rhythm perception; this effect was more pronounced in females. Language perception was lateralized to the left hemisphere in all subject groups. Musicians showed increased CBFV values in the left hemisphere which were independent of the type of stimulus, and background listeners showed increased CBFV values during harmony perception in the right hemisphere which were independent of their musical experience. The time taken to reach the peak of CBFV was significantly longer in non-musicians when compared with musicians during rhythm and harmony perception. Pulse rates were significantly decreased in non-musicians during harmony perception, probably due to a specific relaxation effect in this subgroup. The resistance index did not show any significant differences, suggesting only regional changes of small resistance vessels but not of large arteries. Our fTCD study confirms previous findings of right hemisphere lateralization for harmony perception in non-musicians. In addition, we showed that this effect is more pronounced in female subjects and in background listeners and that the lateralization is delayed in non-musicians compared with musicians for the perception of rhythm and harmony stimuli. Our data suggest that musicians and non-musicians have different strategies to lateralize musical stimuli, with a delayed but marked right hemisphere lateralization during harmony perception in non-musicians and an attentive mode of listening contributing to a left hemisphere lateralization in musicians.

Cerebral hemodynamic changes in sleep apnea syndrome and effect of continuous positive airway pressure treatment

BACKGROUND AND OBJECTIVE

A clear association among snoring, sleep apnea, and increased risk of stroke has been shown by previous studies. However, the possible role played by sleep apnea in the pathogenesis of cerebrovascular disease is subject to debate. To evaluate the influence of hemodynamic changes caused by obstructive sleep apnea syndrome (OSAS), we investigated cerebrovascular reactivity to hypercapnia in patients with OSAS.

METHODS

The study was performed at baseline and after 1 night and 1 month of nasal continuous positive airway pressure (n-CPAP) therapy, with patients in the waking state (8:00 to 8:30 AM and 5:30 to 6:00 PM) with transcranial Doppler ultrasonography. Cerebrovascular reactivity was calculated with the breath-holding index (BHI).

RESULTS

In the baseline condition, compared with normal subjects, patients with OSAS showed significantly lower BHI values in both the morning (0.57 versus 1.40, p < 0.0001) and the afternoon (1.0 versus 1.51, p < 0.0001). Cerebrovascular reactivity was significantly higher in the afternoon than it was in the morning in both patients (p < 0.0001) and controls (p < 0.05). In patients, the BHI returned to normal values, comparable with those of control subjects, after both 1 night and 1 month of n-CPAP therapy.

CONCLUSIONS

These findings suggest an association between OSAS and diminished cerebral vasodilator reserve. This condition may be related to the increased susceptibility to cerebral ischemia in patients with OSAS, particularly evident in the early morning.

Relationships among cerebral perfusion pressure, autoregulation, and transcranial Doppler waveform: a modeling study

OBJECT

The aim of this study was to analyze how the main values extrapolated from the transcranial Doppler (TCD) waveform (systolic, mean, and diastolic velocity; velocity peak-to-peak amplitude; and pulsatility index [PI]) are affected by changes in intracranial pressure (ICP), systemic arterial pressure (SAP), autoregulation, and intracranial compliance.

METHODS

The analysis was performed using a mathematical model of the intracranial dynamics. This model includes a passive middle cerebral artery, the biomechanics of large and small pial arteries subjected to autoregulatory mechanisms, a collapsing venous cerebrovascular bed, the cerebrospinal fluid circulation, and the ICP-volume relationship. The results indicate that there are approximately three distinct zones characterized by different relationships between cerebral perfusion pressure (CPP) and velocity parameters in patients with preserved autoregulation. In the central autoregulatory zone (CPP > 70 mm Hg) the mean velocity does not change with decreasing CPP, whereas the PI and velocity peak-to-peak amplitude increase moderately. In a second zone (CPP between 4045 and 70 mm Hg), in which vasodilation of small pial arteries becomes maximal, the mean velocity starts to decrease, whereas the PI and velocity amplitude continue to increase. In the third zone, in which autoregulation is completely exhausted (CPP < 40 mm Hg), arterioles behave passively, mean velocity and velocity amplitude decline abruptly, and the PI exhibits a disproportionate rise. Moreover, this rise is quite independent of whether CPP is reduced by increasing ICP or reducing mean SAP. In contrast, in patients with defective autoregulation, the mean velocity and velocity amplitude decrease linearly with decreasing CPP, but the PI still increases in a way similar to that observed in patients with preserved autoregulation.

CONCLUSIONS

The information contained in the TCD waveform is affected by many factors, including ICP, SAP, autoregulation. and intracranial compliance. Model results indicate that only a comparative analysis of the concomitant changes in ultrasonographic quantities during multimodality monitoring may permit the assessment of several aspects of intracranial dynamics (cerebral blood flow changes, vascular pulsatility, ICP changes, intracranial compliance, CPP, and autoregulation).

Mechanical ventilation for ischemic stroke and intracerebral hemorrhage: indications, timing, and outcome

OBJECTIVE

To compare the incidence, indication, and timing of intubation and outcome in patients with cerebral infarction (ISCH) and intracerebral hemorrhage (HEM) requiring mechanical ventilation (MV).

BACKGROUND

Poor outcomes have been reported for ISCH patients requiring MV. Because the target population, pathophysiology, and management of ISCH and HEM patients differ considerably, we compared the characteristics of patients with ISCH and HEM who required MV.

METHODS

A retrospective review of ISCH and HEM stroke patients who underwent MV at a tertiary care academic center from 1994 to 1997 was performed to determine age, sex, type, and location of stroke (anterior or posterior circulation); brainstem dysfunction at intubation (pupillary, corneal, and oculocephalic reflexes); indication for intubation (neurologic deterioration, cardiopulmonary deterioration, or elective intubation for surgery); timing of intubation (on presentation or later); comorbidities; and outcome (hospital disposition).

RESULTS

A total of 230 patients, 74 with ISCH and 156 with HEM (mean age, 61 +/- 16 years; male-to-female ratio, 1.15:1), underwent MV. Intubation rates were 6% for ISCH patients and 30% for HEM patients. Two-thirds of the patients required intubation on presentation (84% were intubated for neurologic deterioration) and 131 patients (57%) died (ISCH, 55%; HEM, 58%). Signs of brainstem dysfunction predicted a higher mortality for both groups. Additionally, early intubation and older age predicted mortality for HEM, and male gender predicted mortality in ISCH. Stroke location and comorbidities did not influence outcome.

CONCLUSIONS

MV in acute stroke is associated with high mortality. Mortality and outcome were similar for ISCH and HEM; however, the factors predictive of outcome may differ and influence decisions about the use of MV in such patients.

Assessment of cerebral pressure autoregulation in humans--a review of measurement methods

Assessment of cerebral autoregulation is an important adjunct to measurement of cerebral blood flow for diagnosis, monitoring or prognosis of cerebrovascular disease. The most common approach tests the effects of changes in mean arterial blood pressure on cerebral blood flow, known as pressure autoregulation. A 'gold standard' for this purpose is not available and the literature shows considerable disparity of methods and criteria. This is understandable because cerebral autoregulation is more a concept rather than a physically measurable entity. Static methods utilize steady-state values to test for changes in cerebral blood flow (or velocity) when mean arterial pressure is changed significantly. This is usually achieved with the use of drugs, shifts in blood volume or by observing spontaneous changes. The long time interval between measurements is a particular concern in many of the studies reviewed. Parallel changes in other critical variables, such as pCO2, haematocrit, brain activation and sympathetic tone, are rarely controlled for. Proposed indices of static autoregulation are based on changes in cerebrovascular resistance, on parameters of the linear regression of flow/velocity versus pressure changes, or only on the absolute changes in flow. The limitations of studies which assess patient groups rather than individual cases are highlighted. Newer methods of dynamic assessment are based on transient changes in cerebral blood flow (or velocity) induced by the deflation of thigh cuffs, Valsalva manoeuvres, tilting and induced or spontaneous oscillations in mean arterial blood pressure. Dynamic testing overcomes several limitations of static methods but it is not clear whether the two approaches are interchangeable. Classification of autoregulation performance using dynamic methods has been based on mathematical modelling, coherent averaging, transfer function analysis, crosscorrelation function or impulse response analysis. More research on reproducibility and inter-method comparisons is urgently needed, particularly involving the assessment of pressure autoregulation in individuals rather than patient groups.

Temporal patterns of evoked cerebral blood flow during reading

This study describes the dynamics of flow activation by reading and investigates the potential use of repeated flow velocity measurements for the lateralization of speech. Using simultaneous transcranial Doppler recordings from both middle cerebral arteries and averaging techniques in 25 healthy volunteers, we describe the changes in blood flow velocity caused by repetitive reading tasks of variable duration in comparison with a resting state. Reading aloud evoked a characteristic temporal flow pattern in both hemispheres, consisting of three relative maxima in flow velocity during and after activation. Flow velocities lower than baseline were common during longer lasting activation. The amplitudes of two of the observed peaks decreased depending on the duration of the task. Reading silently produced a markedly different temporal pattern of activation than reading aloud. There were individually reproducible significant side to side differences. Right-handed persons (n = 15) almost without exception showed a significantly higher increase in flow velocity on the left hemisphere (e.g., reading silently 8.7% versus 5.3%; P < 0.0001). Three out of ten left-handed individuals, however, exhibited no significant side to side difference or exhibited lateralization to the right during one or more of the tasks. These findings suggest that reading induces task-specific temporal patterns of regional neuronal activity, which show habituation with longer duration of activation. Additionally, the observed side to side differences could be useful to predict language dominance.

Cerebral hemodynamic patterns during stimuli tasks in multi-infarct and Alzheimer types of dementia

OBJECTIVES

To examine differences in cerebrovascular reactivity between multi-infarct and Alzheimer types of dementia.

PATIENTS AND METHODS

Using transcranial Doppler, measurements of flow velocity in the middle cerebral arteries during apnea, thumb-to-finger opposition and verbal and design discrimination tasks were recorded in patients with multi-infarct (n=10; mean age 68+/-7.5 years) and Alzheimer types of dementia (n=10; mean age 62+/-9.1 years). Controls were 20 healthy subjects matched for age and sex.

RESULTS

Cerebral reactivity to apnea was significantly lower (P<0.0001) in the multi-infarct group compared to the other 2 groups. During the motor task, a nearly selective increase of mean flow velocity in the middle cerebral artery contralateral to the hand performing the task occurred in both controls and Alzheimer patients. A bilateral increase, without side-to-side differences, of flow velocity during movement of each hand was observed in multi-infarct patients. With respect to baseline values, the cognitive tasks produced significant and distinct effects on the left and right side in the controls but not in the patients. Controls showed a significant increase of left middle cerebral artery mean flow velocity during a verbal task (P<0.0001), and of the right middle cerebral artery mean flow velocity during a design discrimination task (P<0.001) when side-to-side comparisons were done. Differently, a bilateral and comparable increase of flow velocity was observed in all patients during performance of the same cognitive tasks.

CONCLUSION

These data suggest that cerebrovascular reactivity to apnea could be an additional criterion for discriminating between MID and DAT patients. Transcranial Doppler assessment during cognitive and motor tasks could provide useful complementary information for comprehension changes in cerebral activity in patients with dementia.

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

OBJECTIVE

An important limitation of transcranial Doppler (TCD) ultrasonography is its inability to directly measure blood flow or vessel diameter. To extend the ability of TCD ultrasonography, indices were derived from an intensity-weighted mean of the entire Doppler spectrum. The objective of this article is to test the behavior of these indices under conditions of diameter constancy (hyper- and hypoventilation) and when vessel diameter decreases (vasospasm).

METHODS

A flow index (FI) was calculated by averaging several heartbeats of spectral data and calculating the first spectral moment. An area index (AI) was defined as the FI divided by the mean velocity, motivated by the knowledge that vessel flow is the product of vessel diameter and mean velocity. To test the FI and the AI under conditions of diameter constancy, middle cerebral artery Doppler signals were obtained from 20 patients during conditions of hypercarbia, hypocarbia, and normocarbia. To test the ability of these indices to evaluate a decrease in vessel diameter, signals from 41 sites on 23 arteries were obtained from patients who underwent both TCD and angiographic studies on two separate occasions after the occurrence of subarachnoid hemorrhage. The changes in the AI were compared with the arterial diameters measured from angiograms.

RESULTS

The FI was proportional to the mean velocity in the cohort of healthy patients (r=0.97). The AI changed by less than 3% in the same cohort. The AI predicted the direction of the diameter change in all vessels showing angiographic changes in area. Changes in the AI and the measured angiographic changes in cross-sectional areas were correlated (overall, r=0.90; with two outlines removed, r=0.86).

CONCLUSION

This variant of the intensity-weighted mean predicts changes in vessel cross-sectional area under conditions of changes in CO2 and cerebral vasospasm. This preliminary study suggests that careful use of this tool may provide accurate evaluation of cerebral blood flow through the large vessels and quantitative changes in diameter, which occur frequently after subarachnoid hemorrhage.

Frequency domain analysis of cerebral blood flow velocity and its correlation with arterial blood pressure

We applied frequency domain analysis to detect and quantify spontaneous fluctuations in the blood flow velocity of the middle cerebral artery (MCAFV). Instantaneous MCAFV of normal volunteers was detected using transcranial Doppler sonography. Spectral and transfer function analyses of MCAFV and arterial blood pressure (ABP) were performed by fast Fourier transform. We found the fluctuations in MCAFV, like ABP, could be diffracted into three components at specific frequency ranges, designated as high-frequency (HF, 0.15 to 0.4 Hz), low-frequency (LF, 0.04 to 0.15 Hz), and very low-frequency (VLF, 0.016 to 0.04 Hz) components. The HF and LF components of MCAFV exhibited high coherence with those of ABP, indicating great similarity of MCAFV and ABP fluctuations within the two frequency ranges. However, it was not the case for the VLF component. Transfer function analysis revealed that the ABP-MCAFV phase angle was frequency-dependent in the LF range (r = -0.79, P < 0.001) but not in the HF range. The time delay between LF fluctuations of ABP and those of MCAFV was evaluated as 2.1 seconds. We conclude that in addition to traditional B-wave equivalents, there are at least two different mechanisms for MCAFV fluctuations: the HF and LF fluctuations of MCAFV are basically secondary to those of ABP, and cerebral autoregulation may operate efficiently in LF rather than HF range. Frequency domain analysis offers an opportunity to explore the nature and underlying mechanism of dynamic regulation in cerebral circulation.

Hyperventilation restores cerebral blood flow autoregulation in patients with acute liver failure

BACKGROUND/AIMS

In patients with acute liver failure loss of cerebral blood flow autoregulation may result from cerebral vasodilatation. Since arterial hypocapnia induces cerebral vasoconstriction, we investigated whether cerebral blood flow autoregulation could be reestablished by mechanical hyperventilation.

METHODS

Seven consecutive patients (median age 45, range 30-50 years) with acute liver failure and hepatic encephalopathy stage IV entered the study. They were all maintained on mechanical ventilation. Cerebral blood flow autoregulation was evaluated by using transcranial Doppler sonography to assess mean flow velocity (Vmean) in the middle cerebral artery, during a rise in mean arterial pressure by norepinephrine infusion (0.5-10 microg/h). The patients were subsequently hyperventilated for 15 min before cerebral blood flow autoregulation was re-evaluated in the same mean arterial pressure range.

RESULTS

At baseline PaCO2 (4.0 (3.5-4.9)kPa), all patients had impaired cerebral blood flow autoregulation as Vmean increased from 47 (30-78) to 68 (49-107) cm x s(-1) (p<0.05), as MAP was raised from 82 (60-88) to 106 (89-123) mmHg. During hyperventilation, five of seven patients restored cerebral autoregulation as Vmean remained unchanged at 51 (45-70) cm x s(-1) during a rise in MAP from 84 (65-94) to 110 (89-130) mmHg. Cerebral blood flow autoregulation was not restored in two patients, but hyperventilation reduced the slope of the mean arterial pressure-Vmean correlation. These two patients had renal failure and were treated with intermittent hemodialysis.

CONCLUSIONS

Cerebral blood flow autoregulation was restored by hyperventilation in five of seven patients with acute liver failure, indicating that cerebral vasodilatation is of pathophysiological importance in dysregulation of cerebral circulation in acute liver failure.

Changes in cerebral blood flow velocities during childhood absence seizures

A simultaneous recording of mean flow velocity in the right middle cerebral artery by transcranial Doppler ultrasonography and electroencephalographic activity was performed in 5 children with multiple daily typical absence seizures. Twenty-eight absence episodes were recorded. Mean flow velocity increased gradually a few seconds before the clinical and electroencephalographic manifestations of each seizure and reached the maximum values (range of increase: 25.5-42.8% with respect to baseline) within 2-3 seconds from their onset. This increase was then followed by a fast reduction in flow velocity, with the lowest levels (range of decrease: 30.8-44.0% with respect to baseline) recorded within 4-6 seconds from the end of each absence seizure. These findings suggest that changes in cerebral blood flow and activity are quite complex during absence seizures and that they do not fully correlate with clinical and electroencephalographic manifestations.

Effect of head-upright tilt on the dynamic of cerebral autoregulation

The effect of head-upright tilting on the rate of cerebral autoregulation was studied in 12 healthy volunteers (nine men and three women; age range 20-36 years). The dynamics of cerebral autoregulation was determined from the rate of change in cerebral resistance (RoR) during a drop in arterial blood pressure induced by rapid deflation of a 3-min ischaemic thigh cuff and from the ratio of changes in cerebral blood flow and arterial blood pressure (CAI) during the recovery period after the drop in arterial blood pressure. The test was performed supine and with 40 degrees head-up tilt (40 degrees HUT). Middle cerebral artery mean blood flow velocity was measured by transcranial Doppler simultaneously with peripheral arterial blood pressure using Finapres. The thigh cuff deflation induced a larger drop in arterial pressure during 40 degrees HUT [median -28% (25 percentile -36, 75 percentile -19)] than in the supine position [-16% (-23, -15)] (P < 0.01) and in cerebral resistance [supine: -12% (-15, -6); 40 degrees HUT: -15% (-20, -12); P < 0.05]. There was no significant change in RoR [15% s-1 (12, 15)] and CAI [1.9 (1.5, 3.1)] measured supine and during 40 degrees HUT [RoR: 13% s-1 (12, 15); CAI: 1.3 (0.99, 1.9)]. During the drop in arterial pressure, the relationship between arterial blood pressure and systolic peak-to-peak interval exhibited an hysteresis loop, indicating a cardiopulmonary and/or baroreflex activation that was not observed with cerebral resistance. The rate of autoregulation is an intrinsic property of the cerebral vascular bed and is not affected by the vasodilator state in the range of arterial blood pressure changes induced by the tight cuff method.

Transient autobiographic amnesia: EEG and single-photon emission CT evidence of an organic etiology

We describe a 44-year-old patient who had a transient attack of autobiographic amnesia. When assessed during the attack, her learning abilities were normal, with no sign of anterograde amnesia. In the remote memory domain, she showed a striking dissociation between a detailed knowledge of public events and famous people and a complete loss of autobiographic information. During the attack, EEG recorded bilateral frontotemporal slow waves and single-photon emission CT (SPECT) showed hypoperfusion in the right temporal and parietal lobes; no abnormalities were detected when both EEG and SPECT were repeated 1 week later. This case provides evidence for an organic etiology for the episode and supports the hypothesis that autobiographic memory is independent of other forms of retrograde memory.

Role of NO in the O2 and CO2 responsiveness of cerebral and ocular circulation in humans

It is well known that changes in PCO2 or PO2 strongly influence cerebral and ocular blood flow. However, the mediators of these changes have not yet been completely identified. There is evidence from animal studies that NO may play a role in hypercapnia-induced vasodilation and that NO synthase inhibition modulates the response to hyperoxia in the choroid. Hence we have studied the effect of NO synthase inhibition by NG-monomethyl-L-arginine (L-NMMA, 3 mg/kg over 5 min as a bolus followed by a continuous infusion of 30 micrograms.kg-1.min-1) on the changes of cerebral and ocular hemodynamic parameters elicited by hypercapnia and hyperoxia in healthy young subjects. Mean flow velocities in the middle cerebral artery and the ophthalmic artery were measured with Doppler ultrasound, and ocular fundus pulsation amplitude, which estimates pulsatile choroidal blood flow, was measured with laser interferometry Administration of L-NMMA reduced ocular fundus pulsation. (-19%, P < 0.005) but only slightly reduced mean flow velocities in the larger arteries. Hypercapnia (PCO2 = 48 mmHg) significantly increased mean flow velocities in the middle cerebral artery (+26%, P < 0.01) and fundus pulsation amplitude (+16%, P < 0.005) but did not change mean flow velocity in the ophthalmic artery. The response to hypercapnia in the middle cerebral artery (P < 0.05) and in the choroid (P < 0.05) was significantly blunted by L-NMMA. On the contrary, L-NMMA did not affect hyperoxia-induced (PO2 = 530 mmHg) hemodynamic changes. The hemodynamic effects of L-NMMA (at baseline and during hypercapnia) were reversed by coadministration of L-arginine. The present study supports the concept that NO has a role in hypercapnia induced vasodilation in humans.

Evaluation of hemodynamic responses in head injury patients with transcranial Doppler monitoring

Transcranial Doppler (TCD) can monitor middle cerebral artery (MCA) velocity which can be recorded simultaneously with other physiologic parameters such as end tidal (Et) CO2, arterial blood pressure and intracranial pressure (ICP), in head injured patients. Relative changes in MCA velocity can be used to reflect relative MCA blood flow changes during ICP waves, and also to evaluate cerebral autoregulation, CO2 reactivity and hemodynamic responses to mannitol and barbiturates. The utility and practicality of short intervals of TCD monitoring to evaluate hemodynamic responses, was evaluated in a group of 22 head injured patients (average Glasgow coma score 6). During ICP A waves, MCA velocity always decreased during the peak of the wave, and during ICP B waves, fluctuated synchronously with the ICP. Dynamic cerebral autoregulation, and reactivity to CO2, were reduced within 48 hours of admission. Impaired cerebral autoregulation within 48 hours of admission did not correlate with outcome at 1 month. Mannitol infusion caused an increase in MCA velocity (15.4 +/- 7.9%) which was significantly correlated to the impairment of dynamic autoregulation (r = 0.54, p < 0.0001). The MCA velocity response to a test dose of barbiturates was significantly correlated to the ICP (r = 0.61, p < 0.01) response as well as to the CO2 reactivity (r = 0.37, p < 0.05). Continuous MCA velocity monitoring using TCD may be useful in evaluating a variety of hemodynamic responses in head injury patients and may replace more cumbersome cerebral blood flow techniques which have been used in the past for these purposes.

Assessment of cerebrovascular reactivity by dynamic susceptibility contrast-enhanced MR imaging

In patients with cerebrovascular disease the acetazolamide (ACZ) test is performed to evaluate the decrease in cerebral perfusion pressure (CPP) through the investigation of the vasomotor reactivity (VMR). This latter is currently assessed with ACZ with several methods. Recently, magnetic resonance imaging (MRI) techniques have been developed that are sensitive to stimulus-induced changes in blood flow. Dynamic susceptibility contrast material-enhanced gradient-echo MRI techniques (DSC-MRI) might be an attractive tool to assess VMR. We aimed to test the ability of DSC-MRI in the assessment of VMR. Relative hemodynamic parameters rCBV, MTT, and rCBF were evaluated at baseline after the first injection of gadopentetate dimeglumine and 10 min after the intravenous administration of ACZ (1 g) with a second bolus of contrast agent. Assessment of hemodynamic parameters was performed over the whole hemisphere and also within regions of interest. The significances of the mean differences, before and after ACZ, were assessed with repeated-measures ANOVA with two within factors: laterality (right-left) and ACZ. DSC-MRI with ACZ test was performed in ten healthy controls (aged 51.4+/-16.2 years). The cerebral hemispheric ratio for the three parameters (cerebral blood volume (CBV), mean transit time (MTT), and cerebral blood flow (CBF)) ranged between 1.01 and 1.03. The mean gray matter-to-white matter ratio for CBV, CBF and MTT were 2.44, 2.41 and 1.05, respectively. As the laterality effect was not significant, left and right hemispheric values were averaged. A significant increase of all hemodynamic parameters was observed after ACZ (P<0.01-0.001). The same changes for CBV, CBF and MTT were observed after ACZ according to the regions of interest (P<0.006-0.015). DSC-MRI is a non-invasive method which enables the assessment of VMR. This technique may be added to any conventional MRI in order to detect a hemodynamic impact of an ICA stenosis. Therefore, it might be useful in determining the appropriate management when the indication for surgical versus medical therapy is in question.

Cerebral hemodynamics and metabolism during infant cardiac surgery. Mechanisms of injury and strategies for protection

There is an established link between congenital heart disease and acquired brain injury, which relates to the dependence of the nervous system on a consistent and responsive supply of oxygen and glucose. The advances in the field of infant cardiac surgery have presented new and different challenges to the arena of child neurology. This review provides an overview of the mechanisms of neurologic injury and cerebral hemodynamics and metabolism during cardiac surgery. This review discusses current and future strategies for the management of children with congenital heart disease.

Transcranial doppler assessment of cerebral flow velocity during perception and recognition of melodies

The role of each cerebral hemisphere in the perception and recognition of musical information is not yet well understood. We studied cerebral blood flow changes during a melody perception task and a melody recognition task. Blood flow velocity in the two middle cerebral arteries of twenty right-handed musically naif volunteers were simultaneously measured by means of bilateral transcranial Doppler ultrasonography during two minutes of passive melody listening and two minutes of a melody recognition task. With respect to baseline values, a bilateral increase of flow velocity occurred in the middle cerebral arteries with a non-significant trend for the right artery during the melody perception task. During the melody recognition task, a significant increase in flow velocity was recorded on the right side with respect to the left side, where a slight simultaneous decrease was found. Our data suggest that melody perception requires bilateral activation of hemispheres and melody recognition mainly an activation of the right hemisphere. This study confirms the ability of transcranial Doppler ultrasonography to correlate artery flow dynamics with selective cerebral activation.

Cerebrovascular reactivity in migraine during headache-free intervals

Alterations of intracranial vessel tone have been implicated in the pathophysiology of migraine. The cerebrovascular reactivity was measured by means of transcranial Doppler in 60 migraine patients with (n = 30) or without aura (n = 30) during the headache-free interval and in 30 healthy controls. The vasomotor response was evaluated during hypercapnia induced by inhalation of a mixture of CO2 5% and O2 95% and during hypocapnia obtained after voluntary hyperventilation. To improve the power of the study in detecting possible abnormalities of cerebrovascular reactivity, two different measures were performed at 1 week intervals in migraine patients and controls. Reactivity index values during CO2 inhalation were significantly different (p = 0.01) among the three groups during the first and second measurements; in particular, lower values were found in patients suffering from migraine without aura with respect to controls (p < 0.05, Scheffé's test). Values of reactivity index obtained following induction of hypocapnia did not differ between migraine patients and controls (all p values > 0.05). Our data suggest a reduced vasodilatory response to hypercapnia of cerebral arterioles in patients suffering from migraine without aura with respect to controls that might be related to baseline arteriolar vasodilation.

Brain temperature monitoring and modulation in patients with severe MCA infarction

BACKGROUND

Brain temperature has been measured only occasionally in humans. After head trauma, a temperature gradient in brain temperature compared with body temperature of up to 3 degrees C degrees higher in the brain has been reported. Elevated temperature facilitates neuronal injury after ischemia. At present, no information concerning changes in brain temperature after acute stroke is available.

METHODS

In 15 patients who had suffered severe ischemic stroke in the MCA territory, intracerebral temperature was recorded with use of two different thermocouples, with intraventricular, epidural, and parenchymatous measurements. Body-core temperature (Foley catheter temperature) and jugular bulb temperature (n = 5) were recorded simultaneously. Measures for reducing brain temperature were compared.

RESULTS

In all patients, brain temperature exceeded body-core temperature by at least up to 1 degrees C (range, 1.0 to 2.1 degrees C). Temperature in the ventricles exceeded epidural temperature by up to 2.0 degrees C. Brain temperature modulation was independent of single pharmacologic (paracetamol, metamizol) treatments. Only systemic cooling was effective and sustained hypothermic (33 to 34 degrees C) brain temperatures.

CONCLUSION

After MCA stroke, human intracerebral temperature is higher than central body-core temperature. There is also a temperature gradient within the brain, with the ventricles warmer than the surface. Mild hypothermia in the treatment of severe cerebral ischemia with use of cooling blankets is both easy to perform and effective in the therapy of severe hemispheric infarction.

Evaluation of cerebral arterial flow with transcranial Doppler ultrasound: theoretical development and phantom studies

Blood flow information available from transcranial Doppler ultrasound is usually derived from velocity alone because no knowledge of vessel caliber is available. In cases such as vasospasm, where vessel size changes, the inference of flow from velocity becomes questionable. A computational technique was used to calculate a flow index and 2 vessel area indices based on the first and zero moments of the Doppler power spectrum. These indices were tested in a steady and pulsatile flow phantom using 6 different diameter elastic tubes. Changes in the flow index showed good agreement with changes in timed volume flow for different flow rates. The vessel caliber indices correctly predicted changes in area when different diameter tubes were examined. These indices may prove useful in clinical settings where the constancy of flow or vessel diameter between studies are in question.

Long-term prognosis and the effect of carotid endarterectomy in patients with recurrent ipsilateral ischemic events. North American Symptomatic Carotid Endarterectomy Trial Group

BACKGROUND AND PURPOSE

The present study examines how the prognosis of patients who present with an ischemic event (TIA or nondisabling stroke) referable to a 70 to 99% carotid artery stenosis is modified by the pattern of their ischemic history. We also examined the benefits of performing carotid endarterectomy on the recently symptomatic artery.

METHODS

A total of 608 patients was divided into two groups. The recent group (N = 444) consisted of patients who became newly symptomatic within the previous 6 months of their presenting event. Prior to this, they were asymptomatic. The recurrent group (N = 164) consisted of patients who had one or more ischemic events within the previous 6 months of their presenting event, as well as one or more within the previous 7 to 12 months. All events were ipsilateral to the presenting event.

RESULTS

Kaplan-Meier risk estimates of ipsilateral stroke at 2 years for medically treated patients were 18.6 +/- 3.3% in the recent group and 41.2 +/- 6.9% in the recurrent group (p = 0.0002, logrank test). For patients who underwent carotid endarterectomy, the risks were 7.8 +/- 2.0% and 10.8 +/- 3.4% (p = 0.36, logrank test). Multivariate analyses did not identify any baseline patients characteristics as confounders nor any statistical interactions.

CONCLUSIONS

There is a need for urgency in considering carotid endarterectomy for patients with 70 to 99% carotid artery stenosis who have had recurrent ipsilateral ischemic events extending back more than 6 months. These patients are at more than twice the risk of stroke as those who are newly symptomatic.

Reduction in posterior cerebral artery blood flow velocity during caloric vestibular stimulation

Transcranial Doppler (TCD) sonography was performed to assess the differential effects of caloric vestibular stimulation on cerebral blood flow (CBF) to the vestibular [middle cerebral artery (MCA)] and the visual [posterior cerebral artery (PCA)] cortices in 15 healthy volunteers with eyes closed. Caloric irrigation caused a sensation of rotation and nystagmus and not only increased velocity in the MCA (2.7 +/- 3.3%) but also, simultaneously, decreased velocity in the PCA (4.2 +/- 6.2%, p < 0.001 vs. MCA). CBF changes indicated by changes in velocity through the MCA and PCA may reflect functional deactivation of the visual cortex together with activation of the vestibular cortex: this beneficial multisensory interaction suppresses distressing oscillopsia.

Sleep apnea in patients with transient ischemic attack and stroke: a prospective study of 59 patients

Although sleep apnea (SA) appears to be a cardiovascular risk factor, little is known about its frequency in patients with transient ischemic attack (TIA) and stroke. We prospectively studied 59 subjects (26 women and 33 men; mean age, 62 years) with stroke (n = 36) or TIA (n = 23) with the use of a standard protocol that included assessment of snoring and daytime sleepiness (Epworth Sleepiness Score [ESS]), a validated SA score (Sleep Disorders Questionnaire [SDQ-SA]), and a severity of stroke score (Scandinavian Stroke Scale [SSS]). SA was considered clinically probable (P-SA) when habitual snoring was associated with an ESS of > 10 or when SDQ-SA score was > or = 32 in women and > or = 36 in men. Polysomnography (PSG) was obtained in 36 subjects (group 1) a mean of 12 days after TIA or stroke. In 23 subjects (group 2), PSG was not available (n = 11), refused (n = 10), or inadequate (n = 2). Clinical and PSG data were compared with those obtained in 19 age- and gender-matched control subjects. Groups 1 and 2 were similar in mean age (61 versus 64 years), type of event (36% versus 44% TIA), reported habitual snoring (58% versus 52%), and P-SA (58% versus 50%). PSG showed SA (Apnea-Hypopnea Index [AHI], > or = 10) in 25 of 36 subjects (69%). The proportion of subjects with SA was similar in the TIA and stroke groups (69% versus 70%) and was well above the frequency found in our control group (15%). An AHI of > or = 20 and a minimal oxygen saturation of < 85% were each found in 20 of 36 subjects (55%). Gender and age did not correlate with severity of SA. Subjects with habitual snoring, P-SA, or severe stroke (SSS of < 30) had a significantly higher AHI (p < 0.05). The sensitivity of P-SA for SA was 64%, and the specificity was 67%. We conclude that SA has a high frequency in patients in the acute phase of TIA and stroke and SA cannot be predicted reliably on clinical grounds alone but is more likely in patients with habitual snoring, abnormal SDQ-SA, or severe stroke.

Transcranial Doppler assessment of cerebral blood flow velocity during visual spatial selective attention in humans

Blood flow velocities of the posterior cerebral arteries were obtained while healthy subjects were engaged in a visual spatial attention task. Experimental runs consisted of series of stimuli comprised of four elements (two left and two right of a central fixation point) presented briefly in blue against a purple screen. After a period of passive viewing a left or right pointing arrow indicated the visual half-field to be attended by the subjects in order to detect identical symbols on the attended side. Relative to the passive viewing condition a marked increase of flow was seen in both posterior cerebral arteries during the attention period. No differential increase of flow as a function of attended field was detected. These results are discussed in comparison with recent positron-emission tomography (PET) and electrophysiological data obtained with the same task.

Effect of transient moderate hyperventilation on dynamic cerebral autoregulation after severe head injury

OBJECTIVE

This study was undertaken to evaluate the effect of acute moderate hyperventilation on cerebral autoregulation in head-injured patients.

METHODS

Dynamic cerebral autoregulation was analyzed by use of transcranial doppler ultrasonography before and after hyperventilation in 10 patients with severe head injury. All of the patients were artificially ventilated and underwent continuous monitoring of arterial blood pressure, intracranial pressure, and end-tidal carbon dioxide. To test autoregulation, rapid transient decreases in systemic blood pressure were achieved by quickly releasing large blood pressure cuffs that were inflated around both thighs. This resulted in a drop of 24 +/- 6 mm Hg in mean systemic blood pressure, which lasted an average of 49 +/- 24 seconds. Cerebral blood flow velocity was monitored continuously in both middle cerebral arteries by use of transcranial doppler ultrasonography. The percentage change in middle cerebral artery velocity was used as an index of the change in cerebral blood flow during the autoregulatory response. The change in estimated cerebrovascular resistance, immediately after the blood pressure drop, or the rate of regulation was used to analyze the effectiveness of the cerebral autoregulation. This value was calculated by determining the rate of increase in middle cerebral artery velocity during the 1st 5 seconds after a blood pressure drop, relative to the rate of increase of the cerebral perfusion pressure.

RESULTS

The average rate of regulation during normocapnia at pCO2 of 37 mm Hg was 11.4 +/- 5% per second. After reduction of the pCO2 to 28 mm Hg, the average rate of regulation improved significantly (P < 0.001) to 17.7 +/- 6% per second. Autoregulation improved, despite no significant change in the cerebral perfusion pressure during hyperventilation. The degree of improvement in autoregulation was significantly correlated with the CO2 reactivity (r = 0.45, P < 0.05) but did not correlate (r = -0.23, P = 0.33) with the change in arterial pH value after hyperventilation.

CONCLUSION

These results confirm the finding that dynamic autoregulation is disturbed in severe head injury and that moderate transient hyperventilation can temporarily improve the efficiency of the autoregulatory response, probably as a result of a transient increase in vascular tone.