Noninvasive assessment of CO2-induced cerebral vasomotor response in normal individuals and patients with internal carotid artery occlusions

Role of collateral flow on cerebral hemodynamics in patients with unilateral internal carotid artery occlusion

The objective of this study was to evaluate the role of collateral blood flow via the anterior and posterior communicating arteries (ACoA and PCoA) and via the ophthalmic artery (OphA) on cerebral hemodynamics, metabolism, and border zone infarcts in 57 patients with unilateral symptomatic occlusions of the internal carotid artery. Collateral flow via the ACoA and PCoA was determined with magnetic resonance angiography (MRA) and collateral flow via the OphA with transcranial Doppler (TCD). Volume flow was studied with MRA, metabolism with 1H MR spectroscopy, CO2 reactivity with TCD, and the incidence of border zone infarcts with MRI. Compared with controls, patients had deteriorated volume flow, metabolism, and CO2 reactivity. No differences were found between patients with and patients without collateral flow through the ACoA and/or PCoA, or between patients with or without collateral flow via the OphA. Patients without collateral flow via any of these collaterals had decreased volume flow in the middle cerebral artery, decreased N-acetylaspartate/choline, and increased lactate/N-acetylaspartate, compared with the other patients. Patients with symptomatic internal carotid artery occlusion have deteriorated cerebral hemodynamics and metabolism. Different collateral flow patterns via the ACoA, PCoA, or OphA have no effect on the hemodynamic and metabolic parameters, as long as one of these pathways is present.

Echo-enhanced ultrasound for diagnosis and management in stroke patients

Early diagnosis of stroke and recognition of preceding transient ischemic attacks (TIAs) is important for clinical outcome. Better education of the signs and symptoms of TIA and stroke and improved diagnostic procedures will help to reduce the impact of stroke. Magnetic resonance imaging (MRI), computed tomography (CT) and single photon emission computed tomography (SPECT) are used to diagnose cerebral infarctions. MRI is the most effective diagnostic modality, however, MRI equipment is expensive and not always available. CT is therefore usually used but it has limitations, in detecting small infarctions, particularly in the posterior fossa. Ultrasound is a safe and inexpensive bedside diagnostic procedure. Ultrasound is often used in patients after TIA or stroke, however, it may be inadequate due to poor insonation conditions through the skull, or because the relevant vessel lies deep within the brain. Arteries with slow flowing blood are also difficult to image. The application of ultrasound has expanded with the development of two-dimensional transcranial color-coded sonography (TCCS) which provides both anatomical and functional information about the major cerebral vessels. In addition, the use of echo-enhancing agents now provides better images of vessels within the skull and enables ultrasound examinations to be performed even in patients with a poor transtemporal window. Echo-enhanced ultrasound can also provide images of vessels deep within the brain, which were previously non-evaluable. The echo-enhancing agent Levovist(R) has been extensively studied in patients undergoing ultrasound examination to detect cerebrovascular disease. Levovist improves diagnostic confidence without compromising safety. The improvement in images obtained of the cerebral vessels, including both arteries and veins, using echo-enhanced ultrasound may make this technique the future method of choice for patients with cerebrovascular disease.

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.

Hand gripping and acetazolamide effect in normal persons and patients with carotid artery disease

Transcranial Doppler measurement (TCD) of cerebrovascular reserve (CVR) is usually performed by the CO2 test, the acetazolamide test, or the breath-holding test. Since these tests are time-consuming and labor-intensive, alternative methods such as the hand-gripping test are of interest. Twenty-one normal persons and 25 patients with unilateral carotid artery disease were studied. Flow velocity changes in both middle cerebral arteries (MCAs) during bilateral hand gripping were measured by TCD and compared with acetazolamide test results. The increase in MCA mean flow velocity (FVmean) during hand gripping was 18.0 +/- 6.3% in normal persons; the increases in the post-stenotic MCA were 15.8 +/- 9.7% in all patients and 9.4 +/- 5.4% in patients with impaired CVR as determined by the acetazolamide test. Only in the group with impaired acetazolamide reactivity was the increase in the poststenotic MCA significantly lower compared to that in controls (p < 0.01) and to the contralateral, nonstenotic side (p < 0.01). Nevertheless the FVmean increases in both tests showed a weak, but significant correlation (r = 0.59, p < 0.01). All FVmean increases during hand gripping were significantly (p < 0.01) lower than those during the acetazolamide test. The test appears as a weaker stimulus for MCA blood flow velocity increase than the acetazolamide test. Thus, only a substantial reduction of acetazolamide reactivity leads to a reduced MCA FVmean increase using hand gripping. Although it is highly specific but less sensitive, hand gripping does not appear to be suitable as a screening measure of CVR, but might be useful in addition to standard tests.

Transcranial Doppler in 178 patients before, during, and after carotid endarterectomy

From July 1991 to March 1995, 178 patients who underwent 198 carotid surgical repairs were investigated preoperatively, intraoperatively, and postoperatively by transcranial Doppler sonography (TCD). Preoperative TCD evaluation showed stenosis of the middle cerebral artery (MCA) in 4 patients (2.2%), siphon stenosis in 3 (1.6%), incomplete circle of Willis in 23 (12.9%), a decrease of mean blood flow velocity more than 70% of the basal value during digital common carotid compression in 31 (17.9%), and a critical reduction of vasomotor reactivity (no significant increase of mean blood flow velocity in the MCA during breath-holding test) in 34 (19.1%). Nine patients (5%) had surgery without preoperative angiography. In those patients the indication for surgery was based on color Doppler imaging and TCD investigations. Ninety surgical procedures were carried out under general anesthesia and 188 under locoregional anesthesia. In 37 surgeries (31.7%) a shunt was inserted. The use of a shunt was based on a decrease of mean blood flow velocity in the MCA below 50% of the basal value under general anesthesia or loss of consciousness combined with a decrease of mean blood flow velocity in the MCA higher than 70% of the basal value when locoregional anesthesia was employed. Intraoperative TCD monitoring showed a decrease of mean blood flow velocity in the MCA due to shunt malfunction in (8.3%) of 36 surgeries, turbulence of blood flow during declamping in 79 procedures (39.8%), and microembolic events in 10 patients (5%) that were related to one transient and one permanent neurological deficit. Another permanent deficit occurred in a patient without TCD signs. After surgery, TCD reliably detected an early asymptomatic occlusion of the carotid artery, hyperperfusion syndrome in 12 (6.0%), and an increase of vasomotor reactivity in 10 (29.4%) of 34 surgeries.

A computing system for the clinical and experimental investigation of cerebrovascular reactivity

We present a computing system for the recording and on-line analysis of analogue signals derived from bedside cerebrovascular monitors in different pathophysiological conditions. These include arterial blood pressure and oxygen saturation, end-tidal carbon dioxide concentration, cerebral blood flow velocities using transcranial Doppler ultrasonography, and concentration changes in cerebral oxy- and deoxyhaemoglobin from near infrared spectroscopy. Configuration and analysis adopts arithmetic expressions of different signal processing functions, various statistical properties for each signal, frequency spectrum analysis using fast Fourier transformation, and correlation/cross-correlation. The software offers off-line analysis of non-invasive tests of cerebrovascular reactivity. Several examples of clinical assessment of cerebrovascular reactivity are presented, including cerebral haemodynamic stress tests which employ carbon dioxide, acetazolamide, the breath holding test, leg cuff inflation and deflation, and transient carotid artery compression. Application within the experimental setting with induced haemorrhagic hypotension can also be used.

Vasodilatory responses and magnetic resonance angiography. Extracranial and intracranial intravascular flow data

This study measured the responses of both extracranial (internal carotid arteries) and intracranial (middle cerebral/angular, basilar arteries) intravascular arterial volume flow rates to acetazolamide using phase-contrast magnetic resonance angiography. Twenty-eight newly studied patients were subdivided into four groups: Group I--Nonocclusive, asymptomatic (n = 7, or 14 carotid and middle cerebral/angular artery sides); Group II--unilateral carotid transient ischemic attacks, nonhemodynamic (embolic), varying stenoses (n = 11); Group III--unilateral carotid transient ischemic attacks, hemodynamic, varying stenoses (n = 5); and Group IV--unilateral carotid occlusion, asymptomatic (n = 5). The data were separated into nonischemic and ischemic sides so as to illustrate group differences based on vasodilatory responses to acetazolamide. For example, the percent change in volume flow rates over baseline values for the ischemic-side middle cerebral arteries of Group III was significantly the lowest of all of the vasodilatory responses (-25 +/- 11% vs 40 +/- 14% for group II ischemic middle cerebral/angular artery sides, p = 0.008). Group III patients also had significantly lower standing blood pressures (p = 0.012), higher number of transient ischemic attacks (p = 0.008), and shorter duration of events (p = 0.013). Determinations of volume flow rate continue to assist in determining the degree of hemodynamic compromise of a particular vascular territory.

Cerebral autoregulation in patients with cirrhosis and ascites. A transcranial Doppler study

BACKGROUND/AIMS

Patients with cirrhosis and ascites usually show alterations of systemic hemodynamics and are thus prone to develop arterial hypotension, which might result in cerebral hypoperfusion if cerebral autoregulation is impaired.

METHODS

We evaluated cerebral autoregulation in 15 patients with cirrhosis and ascites and 15 healthy subjects by monitoring mean blood flow velocity in the middle cerebral artery and arterial pressure during supine rest and passive tilting.

RESULTS

Tilt provoked a drop of arterial pressure in both groups. Control subjects had a prompt recovery of mean flow velocity and a progressive recovery of arterial pressure, so that, after 120 s, both parameters had returned to baseline: at 20 s the recovery of flow velocity was faster (p<0.01) than that of blood pressure. By contrast, patients with cirrhosis had a delayed and incomplete recovery of both parameters (p<0.01 vs healthy subjects). In eight patients, the recovery of mean flow velocity paralleled that of arterial pressure, indicating an impaired cerebral autoregulation. These patients had a worse liver function, a higher cardiac index and lower peripheral resistance.

CONCLUSIONS

Cerebral autoregulation is often impaired in patients with cirrhosis and ascites. These patients can develop cerebral hypoperfusion if arterial pressure falls abruptly.

Vascular reactivity in middle cerebral artery and basilar artery by transcranial Doppler in normals subjects during hypoxia

The anatomical and physiological differences between the carotid and vertebrobasilar circulations suggest the possibility of a different response to variations in systemic pO2. We evaluated cerebrovascular response (CR) in these two systems by monitoring variations in the blood flow velocities in the middle cerebral and basilar arteries during hypoxia. Eighteen healthy, non-smoking volunteers underwent transcranial Doppler study during a state of hypoxia obtained by means of the rebreathing method. Oxyhaemoglobin saturation (SaO2) was monitored using a pulsoxymeter in the 88-94% range. The cerebral blood flow velocity (BFV) was measured in the right middle cerebral artery (MCA) and the basilar artery (BA). Our findings indicate that the mean blood flow velocity (MFV) in the BA changes at a lower rate than that in the MCA during hypoxia.

Preoperative risk assessment for carotid occlusion by transcranial Doppler ultrasound

OBJECTIVES

An endovascular carotid balloon occlusion test with continuous intracranial monitoring by transcranial Doppler sonography was performed in 55 patients for prediction of tolerance of a required permanent occlusion of the carotid artery.

METHODS

Blood flow velocities of the ipsilateral middle cerebral artery during occlusion were recorded and compared with clinical tolerance during an occlusion test as well as with postoperative outcome after an eventual permanent occlusion. To stress the capacity of the cerebral circulation to tolerate the occlusion acetazolamide was injected before occlusion in all patients.

RESULTS

The onset of neurological symptoms during temporary occlusion was dependent on the percentage fall of mean blood flow velocity relative to baseline rather than on absolute flow velocities during the time of occlusion. Patients with a fall of mean flow velocity of less than 30% tolerated temporary and permanent occlusion, with the exception of two patients who developed an infarction due to thromboembolism after iatrogenic sacrifice of the carotid artery. Patients with a major decrease developed neurological symptoms during occlusion in 55% and, in cases of carotid ligation, a haemodynamic infarction occurred.

CONCLUSION

The results show that transcranial Doppler monitoring as a part of an endovascular balloon occlusion test may be a reliable technique for preoperative risk assessment for permanent occlusion of the carotid artery.

A new method for fixation of probes for transcranial Doppler ultrasound

Although monitoring of blood velocity in the cerebral arteries with transcranial Doppler ultrasound is standard practice, methods for fixation of the ultrasound probe to the skull continue to impose a technical challenge. This report describes a novel method of probe fixation in which an inexpensive polymer block is custom-made for each patient to hold the probe at a fixed angle. In addition to comfort and durability, its major advantage is that the polymer block prevents dislodgment of the probe from the intended angle by forces of gravity or by patient movement. Thirty-one temporal windows were monitored with this technique, with stable signals obtained in all subjects despite aggressive movements. Disadvantages include the inability to insonate more than one vessel and the cost for each block. However, the method is quick, is relatively inexpensive, and provides significant advantages of probe stability.

Inter-examiner variability of transcranial Doppler procedure and reports: a multicenter survey. Italian Transcranial Doppler Group

OBJECTIVE

To assess the clinical use of Transcranial Doppler (TCD) and the reproducibility of TCD reports.

METHODS

A multicenter survey involving 45 Italian laboratories (the Italian Transcranial Doppler Group-ITDG) was carried out by investigating the examination procedure, the parameters and normative data, and the interpretation criteria applied to reach diagnostic conclusions. The inter-examiner agreement on 10 TCD reports was computed using Kappa statistics. Results. Investigators consider both qualitative (flow direction, signal detectability) and quantitative measurements (mean flow velocity and pulsatility index values), applying their own normative criteria reference which may differ slightly between centers. TCD reports appear to be highly reproducible when based upon the interpretation of qualitative data (Kappa index: 0.95-1.00), whereas a moderate agreement is obtained when considering alterations in quantitative parameters (Kappa: 0.44-0.81).

CONCLUSIONS

The application of the same criteria in different laboratories may facilitate the standardization of TCD examinations and support the reproducibility of clinical reports based on TCD parameters.

Improvement of oxygen metabolic reserve after extracranial-intracranial bypass surgery in patients with severe haemodynamic insufficiency

The effectiveness of extracranial-intracranial arterial bypass (EC-IC bypass) surgery on impaired haemodynamic status was studied in 12 patients with reduced regional cerebral perfusion pressure (rCPP) and elevated regional oxygen extraction fraction (rOEF) in the area distal to the symptomatic arterial lesion. Postoperative positron emission tomography (PET) study demonstrated a statistically significant decrease of rOEF in the operated hemispheres with disappearance of the pre-operative interhemispheric rOEF difference. Regional cerebral blood flow (rCBF) and regional cerebral oxygen metabolism (rCMRO2) were also increased in the operated hemispheres with disappearance of the pre-operative interhemispheric differences. Regional CBF/regional cerebral blood volume (rCBV) ratios of the symptomatic hemispheres were increased after surgery, but were still lower than in the contralateral hemispheres. We conclude that EC-IC bypass surgery improves impaired cerebral oxygen metabolic reserve.

Sleep apnea syndrome and cerebral hemodynamics

The dynamics of cerebral blood flow velocity (CBFV) during sleep were investigated in the right middle cerebral artery of 10 patients with sleep apnea syndrome (SAS) (mean age, 37 years) and 10 healthy control subjects (mean age, 32 years) throughout the entire sleep period. A computer-assisted pulsed (2 MHz) transcranial Doppler ultrasonography system was modified for continuous long-term and on-line recording of cerebral hemodynamics. Concurrently, simultaneous polysomnography, continuous BP recordings, and measurement of the end-expiratory carbon dioxide were undertaken. CBFV showed comparable nocturnal profiles in both groups with decreases during non-rapid eye movement (NREM) sleep and increases during rapid eye movement (REM) sleep, indicating that the general pattern of brain perfusion during normal sleep is maintained in SAS. Sleep stage changes were not regularly accompanied by corresponding changes in CBFV. This reflected a quantitative uncoupling between cerebral electrical activity and cerebral perfusion during sleep and indicated a dissociation in the activity of central regulatory mechanisms. Sleep stage-related analysis showed slightly reduced CBFV in patients with SAS compared with healthy control subjects during wakefulness and the first NREM sleep period, suggesting depressed brain activity in the patient group. The higher CBFV values observed in patients with SAS compared with control subjects during REM sleep and sleep stage 2, both preceding and following REM sleep, underline the influence of dynamically changing sleep patterns on cerebral perfusion in these patients. Reproducible rapid decreases in CBFV were related to EEG arousals. Since apneas are terminated by arousals, these results showed that direct neuronal influences on brain perfusion during apnea are evident.

Haemodynamic classification of symptomatic obstructive carotid artery disease

63 subjects with symptomatic obstructive carotid artery disease were investigated with transcranial Doppler ultrasonography. Their blood velocities at rest (V) in the middle and posterior cerebral artery (MCA and PCA) and in the extracranial internal carotid artery were measured and the pulsatility index (PI) and Uhem index (VMCA.PIMCA/VPCA.PIPCA) calculated. The vasomotor responses in both MCAs were also tested. The subjects were divided into groups based on the findings on physical examination and cerebral computed tomography. In the patient group with lacunar/territorial infarction we found in the stroke hemisphere: VMCA > VPCA, PIMCA = PIPCA and normal values for the Uhem index and total vasomotor reactivity. In the patient group with watershed infarction this hemisphere was characterized by: VMCA < VPCA, PIMCA < PIPCA and subnormal scores for the Uhem index and total vasomotor reactivity. Displaying features from both stroke groups, we obtained in the hemisphere of interest in patients with transient ischaemic attacks: VMCA = VPCA, PIMCA < PIPCA and normal values for the Uhem index and total vasomotor reactivity. Five patients with clinical evidence of stroke but with negative cerebral computed tomography findings had scores similar to those of the watershed group of patients. For the stroke patients, individual measurements of V, PI and total vasomotor reactivity failed to clearly identify to which stroke group a subject might belong. However, such an identification was achieved in all subjects when using the Uhem index. The Uhem index data in patients with transient ischaemic attacks suggest two subgroups with different pathogenesis underlying, the ischaemic events.

Gonadotropin-releasing hormone agonist-induced hypoestrogenism and blood flows in cerebral arteries

OBJECTIVE

To evaluate the effect of the hypoestrogenism induced by GnRH agonist (GNRH-a) therapy on cerebral vessel blood flow.

DESIGN

Open, controlled study.

SETTING

Tertiary care units of the University of Milan, Italy.

PATIENTS

Young women scheduled to undergo 6 months of therapy with a GnRH-a; a control group was also enrolled.

INTERVENTIONS

In both groups, the pulsatility index of both the internal carotid artery (ICA) and middle cerebral artery (MCA) was measured by means of Doppler ultrasound over a period of 6 months.

MAIN OUTCOME MEASURE

The ICA and MCA pulsatility index.

RESULTS

No variation in the pulsatility index of either artery was found in either group.

CONCLUSIONS

A 6-month period of GnRH-a-induced hypoestrogenism in young women does not lead to any variation in the blood flow of cerebral vessels. This provides some reassurance as to the safety of these drugs in relation to the role that the reactivity of peripheral arteries may play in determining risk of cardiovascular disease. Furthermore, our results show that blood flow in the cerebral vessels of young subjects is under extraestrogenic control and that this may counterbalance estrogen deprivation through mechanisms that probably are no longer active in the perimenopausal years.

Long-term evaluation of EC-IC bypass patency

The EC-IC Bypass Study Group could not detect any benefit from surgery compared to medical management in the prevention of stroke in 1985 [15]. During the past years surgical revascularization was re-evaluated and considered as an appropriate treatment for a small subgroup of patients with recurrent focal cerebral ischaemia and impaired haemodynamics. This retrospective study examines the long-term benefit and patency rate of bypass. We present a follow-up of 5.6 years of 47 patients, all of whom underwent bypass surgery after 1985. Forty patients suffered recurring transient ischaemic attacks due to uni- or bilateral internal carotid artery occlusion. Examination included neurologic status, TCD with CO2 or Diamox challenge, angiography, CT and SPECT scans. Neurological improvement was seen in 23% of patients with better results after early surgery, a worsening in 22% suffering further ischaemic events on a postoperative average of 2.8 years. Patency rate for vein graft material was 50%, for the STA-MCA procedure 91%. Occlusion of the vein graft occurred on an average after 1.4 years, other anastomosis after 2.7 years. We conclude that only few patients derived long-term benefit from EC-IC bypasses. Functioning of the bypass worsens over time, suggesting a role for surgery predominantly in the first year of ischaemic events due to insufficient collateral supply. Actual indications for bypass surgery may be patients with failure of maximal medical therapy and progressive ischaemia and haemodynamic compromise.

Phase-contrast magnetic resonance angiography for the determination of cerebrovascular reserve

Cerebrovascular reserve (CVR) is the potential for cerebral arteriolar dilatation to occur, in response to decreased cerebral perfusion pressure, in order to maintain constant cerebral blood flow. Diminution or absence of CVR is considered a risk factor for stroke. Current methods for determining CVR include single-photon emission computed tomography, positron emission tomography and transcranial Doppler (TCD) ultrasonography. However, significant advantages could derive from the utilization of magnetic resonance angiography (MRA) based on the concurrent acquisition of hemodynamic information (CVR and collateral flow) with phase-contrast (PC) techniques and vascular morphology with three-dimensional, time-of-flight methods. With a 1.5-T scanner and acetazolamide (AZM), an arteriolar dilator, CVR was determined in 7 normal subjects. Mean flow velocity in the middle cerebral arteries was determined by PC MRA before and after AZM administration. For comparative purposes, mean flow velocities in the same middle cerebral arteries were determined by TCD before and after AZM administration. The mean flow velocities were as follows (mean +/- standard deviation, n = 7): 40 +/- 8 (PC MRA) versus 61 +/- 10 cm/sec (TCD) before AZM treatment and 58 +/- 11 (PC MRA) versus 85 +/- 15 cm/sec (TCD) after AZM administration. The increase in mean flow velocity (before vs after AZM), that is, the CVR, was 45 +/- 11% as shown by PC MRA and 39 +/- 14% as shown by TCD. Although significant differences were present between the mean flow velocities measured before and those after AZM administration, as determined by PC MRA and TCD, the CVR was not significantly different (45 vs 39%, respectively). These preliminary results suggest that PC MRA may be a method for determining CVR.

The size of territorial brain infarction on CT relates to the degree of internal carotid artery obstruction

We studied the relationship between infarct size and degree of internal carotid artery stenosis in 227 stroke patients without a source of embolism in the heart, who had a CT-verified territorial brain infarct. We used logistic regression analysis adjusting for differences between groups in co-associated variables such as age, sex, hypertension, diabetes mellitus, and a history of ischaemic heart disease. Ipsilateral carotid stenosis greater than 50% was more strongly associated with large than small infarcts; adjusted odds ration [(a)OR]: 4.56; 95% confidence interval (CI): 1.21-17.2; P = 0.02. For ipsilateral carotid occlusion the association was even stronger--(a)OR: 36.80; 95% CI: 2.54-533; P = 0.007. When large infarcts were compared with infarcts of small and moderate size together the ORs were 2.29, 95% CI 1.14-4.58 and 2.57, 95% CI 1.17-5.67 for carotid stenosis or occlusion, respectively. Our data show a relationship between greater than 50% carotid stenosis or occlusion and large brain infarcts. We suggest that haemodynamic impairment may contribute to infarct size in territorial infarcts of non-cardiac origin.

CO2 response for the brain stem artery blood flow velocity in man

We examined changes in the blood flow velocity of brain stem artery (BSA) and middle cerebral artery (MCA) in response to hypercapnic, normocapnic and hypocapnic hyperventilation in seven awake subjects with a transcranial Doppler to determine if there are differences in blood flow control in regional brain perfused by these respective arteries, and to separate the effects of CO2 and ventilation itself on blood flow velocity during CO2 loading. During hypercapnic hyperventilation, BSA flow velocity increased linearly with an increase in end-tidal partial pressure of CO2 (PETCO2). During hypocapnic hyperventilation, BSA flow velocity decreased linearly with decrease in PETCO2, but did not change during normocapnic hyperventilation. The mean CO2 reactivity of BSA was 2.8%/mmHg. The responses of MCA to these hyperventilations and CO2 reactivity were similar to those of BSA. These findings suggest that CO2 rather than ventilation per se is the important stimulus to changes in brain blood flow velocity and that the CO2 responses of brain arteries are not affected by differences in vascular beds.

Use of transcranial Doppler sonography and acetazolamide test to demonstrate changes in cerebrovascular reserve capacity following carotid endarterectomy

OBJECTIVES

To assess the effect of carotid endarterectomy on cerebrovascular reserve capacity.

METHODS

Cerebral blood flow velocity (CBFV) and cerebrovascular reserve capacity (CVRC) were measured by transcranial Doppler sonography (TCD) and acetazolamide test in 40 patients who underwent uncomplicated unilateral carotid endarterectomy (CEA). Indication for operation was limited to stenoses > 70% as documented by angiography and/or Duplex scanning. The TCD studies were carried out 6 days (range 1-14 days) before and 8 days (range 5-12 days) after endarterectomy.

RESULTS

Before endarterectomy, resting CBFV values and CVRC in the 40 patients were significantly different between the operated (51 +/- 19 cm/s; 20 +/- 16%) and the non-operated (60 +/- 19 cm/s; 34 +/- 24%) hemisphere (p < 0.05;p < 0.01). After CEA the overall increase of resting CBFV of the operated side was highly significant with preoperative CBFV values of 51 +/- 19cm/s and postoperative values of 62 +/- 15 cm/s (p < 0.01). Cerebrovascular reserve capacity after operation was increased on both sides significantly (non-operated side: from 34 +/- 24% to 43 +/- 19%, p < 0.05; operated side: from 20 +/- 16% to 51 +/- 18%, p < 0.001), and the preoperative asymmetry was no longer present.

CONCLUSIONS

CEA has a beneficial effect on the cerebral circulation in most patients, even those who presented with asymptomatic carotid artery stenosis. Since CVRC has been assessed in the early postoperative period, our findings also suggest that cerebral vascular adaption occurs within 2 weeks after CEA.

Cerebrovascular assessment of the high-risk patient: the role of transcranial Doppler ultrasound

With increased attention to the causes and effects of neurologic injury related to cardiopulmonary bypass anesthesia and surgery, multiple modality examination and monitoring of cerebral function and perfusion in the perioperative period may prove to be advantageous. Transcranial Doppler examination and monitoring is inexpensive, noninvasive, safe, provides unique information about the functional status of the intracranial circulation, and complements the duplex Doppler study of the extracranial carotid vessels of the neck for preoperative evaluation of the surgical patient. The transcranial Doppler examination permits quantitation of blood flow velocity of the intracranial vessels, evaluation of autoregulatory capacity and vasomotor reserve, determination of symmetry of flow velocity in the circle of Willis, assessment of collateral circulatory capacity, examination of vessels not accessible to the duplex Doppler and serves as a baseline for intraoperative monitoring and the postoperative examination. Noninvasive, unilateral or bilateral, continuous monitoring of brain blood flow velocity intraoperatively or postoperatively with trending, storage, and correlation with other physiologic variables provides evidence of cerebral perfusion, occurrence and rate of cerebral embolism, and continuous monitoring of therapeutic interventions. A review of the incidence of stroke and neuropsychologic deficit after bypass surgery is focused on parameters amenable to diagnosis using transcranial Doppler. Patient-specific risk factors for neurologic injury derived from previous studies are discussed as well as risk factors that are related to anesthetic and surgical management and equipment. A description of Doppler technology and the correlation of transcranial Doppler findings with angiography and radionucleotide scans establishes the accuracy of the Doppler examination. The preoperative examination, provocative tests of vasomotor reserve, the evaluation of cerebral collateral circulation, and examples of Doppler applications are discussed.

Clinical cerebrovascular applications of arterial ultrasound volume flow rate estimates

A variety of disorders affect cerebral hemodynamics. Volume flow rate (VFR) estimates now allow accurate quantification of the effect of cerebrovascular lesions on the conduit vessels, with excellent in vivo and in vitro correlation. Four selected cases with VFR data and angiographic correlation are presented to illustrate potential clinical uses of this method. The VFR estimates were obtained with a color M-mode-based velocity imaging technique, which uses time-domain processing (P-700 Color Velocity Imaging System, Philips Ultrasound International, Irvine, CA). In a patient awaiting coronary artery surgery, with unilateral internal carotid artery occlusion and contralateral angiographic stenosis (50-80%, reader variation), the baseline and acetazolamide-challenged common carotid artery VFRs showed excellent conduit function ipsilateral to this stenosis. Thus, the angiographic stenosis did not have significant hemodynamic effects and endarterectomy was avoided. In a patient with an arteriovenous malformation fed by the left vertebral and left external carotid arteries, high in the left cervical region, VFR estimates of two to three times normal predicted the feeding vessels, influenced management, and proved helpful in follow-up. In a patient with subclavian steal syndrome, VFR estimates quantified the steal after brachial hyperemia. Finally, in a patient with delayed vasoconstriction after subarachnoid hemorrhage, very low VFR estimates preceded clinical deterioration. Quantification of hemodynamic changes with VFR estimates was useful for the diagnosis, management, and follow-up of these patients with four types of cerebrovascular disease, and should be applicable in many others.

Changes in cerebral blood flow as monitored by transcranial Doppler during voluntary hyperventilation and their effect on the electroencephalogram

Hyperventilation results in a fall in carbon dioxide concentration, a fall in cerebral blood flow, and slowing of activity on the electroencephalogram. The temporal relationship and duration of these responses are uncertain, and were investigated using simultaneous monitoring of cerebral blood flow velocity and of the electroencephalograph, with end-tidal carbon dioxide monitoring. Sixteen patients and 9 normal volunteers were studied. Cerebral blood flow velocity in the middle cerebral artery was measured using transcranial Doppler sonography during 3 minutes of hyperventilation and during a 3-minute recovery period. Electroencephalographic recordings were rated by both visual score and measurement of the dominant posterior frequency. End-tidal expired carbon dioxide tension was monitored during the same hyperventilation protocol in the volunteers. Flow velocity fell rapidly during active hyperventilation. Electroencephalographic slowing closely correlated with the decrease in flow velocity (r = 0.86), but lagged behind it. In healthy volunteers capnographic records showed a very tight coupling between end-tidal carbon dioxide concentration and flow velocity (r = 0.94). Three minutes after hyperventilation, carbon dioxide concentration, cerebral blood flow velocity, and electroencephalographic activity were still not back to the resting state. The fall in both cerebral blood flow velocity and carbon dioxide concentration are related to but precede electroencephalographic slowing. The abnormalities persist for at least 3 minutes after hyperventilation and this must be taken into account in clinical electroencephalography. Transcranial Doppler sonography is well suited to monitoring short-term changes in the cerebral circulation.

Doppler study of the ophthalmic artery in patients with carotid occlusive disease

Assessing the adequacy of collateral circulatory pathways has become increasingly important in the investigation of cerebral circulation. Using transorbital Doppler ultrasonography (TOD), we examined the ophthalmic artery (OA) in patients with hemodynamic significant internal carotid artery (ICA) occlusive disease. The velocity and the direction of flow in the OA were studied in 45 patients (occlusion n = 27, stenosis > 75% n = 18), and in 30 age matched controls, under baseline conditions and within 30 minutes after the i.v. administration of 1 g acetazolamide as a vasodilatory stimulus. Based on the direction of flow before and after acetazolamide, the 45 patients could be separated into four groups with increasing degree of ICA lesions. In group I the OA flow was anterograde, but the velocity decreased after acetazolamide. In group II the OA direction became retrograde after acetazolamide, indicating collateral flow to the brain. In patients with retrograde OA flow following acetazolamide injection (group II-IV), the vessel reacted similarly to an intracranial artery, with marked increase in velocity when vasoreactivity was tested. TOD and the acetazolamide test provide useful information about potential collateral OA flow to the brain in patients with ICA occlusive disease.

Brain infarction developed in hypertensive and normotensive patients during hospitalization--hemodynamic factors

To determine the effects of changes in blood pressure (BP) on the development of ischemic stroke, 17 patients who developed acute brain infarction during hospitalization were analyzed in relation to the vascular risk factors such as hypertension, previous stroke, diabetes mellitus, and hyperlipidemia. Their BPs were retrospectively referred to the onset of stroke. The average values of mean arterial blood pressure (MABP) on admission were 137 mmHg in hypertensive patients (HT; n = 9) and 101 mmHg in normotensives (NT; n = 8). HT patients received antihypertensive treatment after admission and MABP decreased by 22 mmHg (14%) prior to the ictus (three to sixty-five days after admission). MABP in NT patients remained constant before the attack (-1.4 mmHg, four to one hundred ten days after admission). Similarly, MABP decreased by 17 mmHg (four to ninety-three days) before the ictus in patients with previous stroke (n = 9), whereas it changed only a little in patients without history of stroke (n = 8). BP in each group elevated immediately after the stroke and gradually decreased to 90% of the resting level after two weeks. An excessive reduction of BP may induce brain infarction in HT or chronic stroke patients or cause it to recur.

Cerebral perfusion during sleep-disordered breathing

Snoring, a leading symptom of the sleep apnoea syndrome (SAS), has been reported to be one of the risk factors for sleep-related cerebral strokes. Episodes of apnoea are accompanied by hypoxaemia as well as hypercapnia. As CO2 constitute a major regulatory factor controlling cerebral blood flow, it is likely that changes in cerebral perfusion are to be found in patients with SAS, which may be related to nocturnal stroke. A computer-assisted pulsed (2 mHz) Doppler ultrasonography system has been modified for continuous long-term and on-line recording of cerebral haemodynamics together with simultaneous polysomnography, continuous blood pressure recordings, and measurement of the end-expiratory CO2. The dynamics of cerebral blood flow velocity (CBFV) during sleep were measured in the right middle cerebral artery in 10 SAS patients. CBFV showed a characteristic nocturnal pattern with decreases during non-rapid eye movement (NREM) sleep and increases during REM sleep. Changes in sleep stage patterns as well as awakenings from NREM sleep were not regularly accompanied by corresponding changes in CBFV. Dramatic increases in CBFV could be observed during apnoeic episodes, with maximum increases during REM sleep. CO2 reactivity and changes in CBFV related to apnoea duration were markedly increased during sleep compared with the waking state in SAS patients. The dynamic feature of CBFV in relation to sleep patterns reflects quantitative uncoupling between cerebral electrical activity and cerebral perfusion during sleep in SAS patients as has been previously reported for normal subjects (Hajak et al. 1994). It supports a dissociation in the activity of central regulatory mechanisms during human sleep which might cause abnormal cerebral perfusion under certain circumstances. The increased CO2 reactivity during sleep in SAS suggests a 'hypersensitivity' of intracranial vasoactive receptors and/or disturbances in the central autonomic control of cerebrovascular functions. It may be concluded that, under certain conditions, the interaction of decreased cerebral perfusion in SAS patients with sleep-related cerebral perfusion patterns and haemodynamic changes during apnoeic episodes might lead to a critical reduction in cerebral perfusion.

Effects of intravenous anesthetic agents on middle cerebral artery blood flow velocity during induction of general anesthesia

OBJECTIVE

Our objective was to quantify the effects of intravenous anesthetics on values measured by or derived from transcranial Doppler sonography (TCD) during induction of general anesthesia.

METHODS

We recorded blood flow velocity in the middle cerebral artery (V-MCA) before, during, and after induction of general anesthesia in six groups of young patients without intracranial pathology (n = 10 each) using TCD. Patients were randomized to receive either 2 mg/kg propofol, 1.5 mg/kg methohexital, 5 mg/kg thiopental, 0.3 mg/kg etomidate, 2 micrograms/kg fentanyl and 0.15 mg/kg midazolam, or 1.5 mg/kg ketamine and 0.15 mg/kg midazolam intravenously. At 2 min after injection, each patient was intubated and given isoflurane 0.8% and nitrous oxide 66% in oxygen. Ventilation was set to achieve an end-tidal PCO2 of 40 mm Hg. V-MCA, arterial blood pressure, heart rate, hematocrit, and PCO2 (venous samples) were measured before and 1, 3, 5, 10, and 30 min after induction of anesthesia.

RESULTS

The preinduction data were not different between groups. At 1 min after injection, propofol, thiopental, methohexital, and etomidate significantly decreased V-MCA. TCD values were only slightly affected following fentanyl/midazolam. Ketamine/midazolam induced a modest rise in V-MCA. After endotracheal intubation, V-MCA increased in all groups, and slowly declined thereafter.

CONCLUSIONS

Under the circumstances of our study, values derived from TCD measurements responded differently to the agents used to induce general anesthesia in nonneurosurgical patients.

Compromised cerebral blood flow reactivity is a predictor of stroke in patients with symptomatic carotid artery occlusive disease

PURPOSE

The purpose of this study was to determine whether the hemodynamic consequences of extracranial carotid disease correlate with the risk of subsequent cerebral infarction.

METHODS

In 95 patients with symptoms who had greater than or equal to 70% stenosis (31 patients) or who had occlusion (64 patients) of the ipsilateral carotid artery, cerebral blood flow was measured by the stable xenon/computed tomography technique both at baseline and after vasodilatory challenge with intravenous acetazolamide. Patients were stratified into group 1, 43 patients with no more than a 5% decrease in flow in any vascular territory, and group 2, 52 patients with greater than a 5% decrease in one or more vascular territories after an acetazolamide challenge.

RESULTS

In group 2, 15 (28.9%) of 52 patients had a new stroke, but only one (2.3%) of 43 patients in group 1 did (p = 0.0005). Of patients with total carotid occlusion 10 (26%) of 38 in group 2 and none (0%) of 26 in group 1 had a new stroke (p = 0.003). Of patients with greater than or equal to 70% stenosis, five (36%) of 14 in group 2 and only one (6%) of 17 in group 1 had a stroke (p = 0.067).

CONCLUSION

The loss of cerebral reactivity in patients with symptoms who had greater than or equal to 70% carotid stenosis or occlusion is an important predictor of impending cerebral infarction.

Computerised transient hyperaemic response test--a method for the assessment of cerebral autoregulation

A simple bedside test has been developed to assess the state of autoregulation in subarachnoid haemorrhage patients. Transcranial Doppler was used to measure blood flow velocity in the middle cerebral artery after a brief common carotid compression. Acceleration of blood flow postcompression was interpreted as evidence of intact cerebral autoregulation. A program using the Windows environment was designed for signal analysis of the transient hyperaemic response test (THRT). The flow velocity signal from the TCD was recorded, carotid compression and release automatically detected and the test results immediately displayed and stored in a database. The program was verified in 614 tests; 552 of them were analysed off-line using previously recorded data and 62 on-line during the examination. A significant correlation was found between the results of computerised testing and the patient's neurological state.

Compressions of carotid and vertebral arteries in assessment of intracranial collateral flow: correlation between angiography and transcranial Doppler ultrasonography

The authors examined 61 subjects with carotid angiography and 50 with vertebral angiography. Angiograms were evaluated for collateral flow through the ophthalmic, anterior communicating, and posterior communicating arteries. The authors evaluated the patency of collateral vessels directly using transcranial Doppler ultrasonography; they made indirect detection after the compression of carotid and vertebral arteries while monitoring flow velocities in the middle cerebral artery. They established criteria for the hemodynamic significance of tested collateral vessels. A combination of carotid compressions and transcranial Doppler ultrasonography detected the patency of the ophthalmic and anterior communicating arteries with a specificity and sensitivity of 1.00. Examination of the posterior communicating artery had a sensitivity of 0.97 and specificity of 0.98. Indirect evaluation of collateral vessels can not only detect their presence but also establish their hemodynamic significance with high accuracy.

Vascular considerations and complications in cranial base surgery

The technical evolution of cranial base surgery has resulted in approaches that allow more radical surgical extirpation of complex cranial base lesions. Our service has extensively applied these cranial base approaches for lesions of the cranial base. A subgroup of 100 patients who had cranial base tumors involving potential manipulation or sacrifice of carotid arteries underwent 20-minute balloon test occlusions coordinated with vascular assessments consisting of a combination of the following: 1) four-vessel cerebral angiogram with compression studies; 2) occlusion transcranial Doppler ultrasonography; 3) occlusion single-photon emission computed tomography perfusion studies; and 4) xenon-133 cerebral blood flow studies. Transient neurological deficits associated with balloon test occlusion occurred in 7 of 100 patients (7%). Subsequently, 18 patients underwent permanent carotid occlusion by endovascular detachable balloons. Delayed ischemic complications (> 72 h) occurred in 4 of 18 (22%) patients. Additionally, a number of vascular complications not predicted by the balloon occlusion tests and vascular assessments were experienced. Repeat vascular assessments defined the causes and guided treatment of ischemic patients. Ischemic complications were caused by hemodynamic insufficiency, embolization, vasospasm, radiation vasculopathy, and venous anomaly. Our experience leads us to believe that no vascular assessment exists today that can predict the occurrence of vascular complications accurately. The current enthusiasm for cranial base surgery must be tempered with the sober reality that management of cerebrovascular anatomy and physiology remain significant limitations. Consideration of potential cerebrovascular complications is paramount to successful outcome and implementation of cranial base surgery.

The effect of acetazolamide on regional cerebral blood oxygenation at rest and under stimulation as assessed by MRI

The sensitivity of gradient echo magnetic resonance imaging (MRI) to changes in cerebral blood oxygenation (CBO) has been introduced for mapping functional brain activation. Here, we report that this approach allows monitoring autoregulation in the human brain under vasodilatory stress. Following the administration of acetazolamide, signal intensities of deoxyhemoglobin-sensitive images increased in cortical and subcortical gray matter and to a lesser extent in white matter. This result reflects a venous hyperoxygenation stemming from an increase in cerebral perfusion with oxygen consumption remaining constant. In addition, pharmacologic induction of vasodilation attenuated activity-related MRI signal changes in the visual cortex under photic stimulation. Although intersubject variability was high, this finding indicates individually persisting autoregulatory responsiveness to functional challenge despite an "exhausted" reserve capacity. It is suggested that recording CBO by MRI will foster our understanding of modulation of vasomotor tone and cerebral perfusion. Furthermore, this technique may prove valuable for assessing the cerebrovascular reserve capacity in patients with carotid artery occlusive disease.

Improvement of cerebrovascular reserve capacity by EC-IC arterial bypass surgery in patients with ICA occlusion and hemodynamic cerebral ischemia

Since the negative results of the international Bypass Study, extracranial-intracranial (EC-IC) bypass surgery is infrequently employed in the treatment of patients with cerebral ischemia. Newly acquired evidence concerning the pathophysiology of cerebral ischemia, however, has facilitated the identification of a small subgroup of patients with "hemodynamic" cerebral ischemia. Characteristically, these patients demonstrate severely impaired cerebrovascular reserve capacity due to occlusive disease and insufficient collateral blood supply. Over an 8-year period, 28 patients were defined by clinical and laboratory criteria as suffering from hemodynamic cerebral ischemia. All patients had recurring episodes of focal cerebral ischemia due to unilateral internal carotid artery occlusion. Computerized tomography (CT) scans either were normal or showed evidence of border zone infarction. The cerebrovascular reserve capacity was studied using 133Xe single-photon emission CT and acetazolamide challenge and was found to be significantly impaired in all patients. Based on these criteria, superficial temporal artery-middle cerebral artery anastomosis was performed to augment collateral flow to the ischemic hemispheres. Two patients died from myocardial infarction, one 4 days and the other 2 months postoperatively. One patient died from massive brain infarction and another suffered a postoperative stroke with incomplete recovery, resulting in a major morbidity and mortality rate of 14%. Minor morbidity included one patient with a subdural hematoma who subsequently recovered completely. The postoperative course was uneventful in 23 patients (82%). Over a mean follow-up period of almost 3 years, no patient had another episode of brain ischemia. Bypass patency was confirmed by postoperative angiography in 26 patients. Follow-up studies of cerebral blood flow (CBF) and cerebrovascular reserve capacity showed significant improvement of the latter while the resting CBF was essentially unchanged. In view of these findings, the authors conclude that EC-IC bypass surgery constitutes appropriate therapy for a subgroup of patients with recurrent focal cerebral ischemia, defined using the strict selection criteria employed in this study.

Control for carbon dioxide-related changes in flow velocity by transcranial Doppler monitoring

Transcranial Doppler ultrasonography can monitor changes in intracranial blood flow velocity over time in a variety of experimental and clinical settings with excellent temporal resolution. Alterations in arterial carbon dioxide pressure exert a profound influence on blood flow velocity. Such changes exhibit important individual fluctuation depending on respiratory status. This limits the ability of transcranial Doppler to accurately study subtle changes in blood flow velocity, independent of the respiratory state of the subject. Suggested here is a method to control for the respiration artifact on blood flow velocity. The middle cerebral artery of 7 healthy male volunteers was studied with transcranial Doppler under resting conditions, monitoring end-tidal carbon dioxide concentration and blood flow velocity. Hyperventilation was performed both voluntarily and with pharmacological induction by human corticotropin-releasing hormone. These studies were carried out both with and without the use of counterregulation of the end-tidal carbon dioxide concentration via a respiration unit, with an adjustable carbon dioxide-oxygen gas supply preventing significant changes in end-tidal carbon dioxide. The blood flow velocity in the middle cerebral artery during maximal voluntary hyperventilation decreased from baseline values of 100% to 44.4 +/- 4.3% (a 55.6% decrease), and with human corticotropin-releasing hormone-induced involuntary hyperventilation, to 65.1 +/- 5.3% (a 34.9% decrease). With the control method, blood flow velocities during voluntary and pharmacological hyperventilation were 100 +/- 1.6% and 100 +/- 2.8%, respectively. This method allows for control of respiration-induced artifacts during transcranial Doppler monitoring, and can be used to assess the effect of direct or indirect blood flow velocity stimuli independent of respiratory status.

Role of impaired CO2 reactivity in the diagnosis of cerebral low flow infarcts

Previous studies on CO2 reactivity in cerebral low flow infarcts (LFIs) included patients with lesions in the frontoparasagittal area, supraganglionic white matter, and temporoparieto-occipital zone. Supraganglionic white matter LFIs are, however, difficult to separate from non-low flow induced infarcts of the lacunar type, and temporoparieto-occipital LFIs from infarcts in the territory of the inferior stem of the middle cerebral artery. The CO2 reactivity of the middle cerebral artery was studied in 56 patients with high grade stenoses and occlusions of the internal carotid artery and LFIs (n = 9) in the frontoparasagittal border zone, territorial infarcts (n = 26), no infarcts (n = 21), and normal subjects (n = 25) by means of transcranial Doppler sonography. The aim was to investigate whether patients with LFIs have significantly lower CO2 reactivity than patients with territorial infarcts, no infarcts, and normal subjects. Patients with LFIs had the most severely reduced CO2 reactivity on the symptomatic side and CO2 reactivity was significantly lower than on the asymptomatic side. It was also lower than in patients with unilateral and bilateral internal carotid artery obstructions and territorial infarcts, asymptomatic patients, and healthy volunteers. It is concluded that LFIs are associated with significantly reduced CO2 reactivity.

Intra- and extracerebral blood flow changes and flushing after intravenous injection of human corticotropin-releasing hormone

To study facial flush after systemic administration of human corticotropin-releasing hormone (hCRH) we injected 100 micrograms hCRH intravenously to ten healthy young men. The increase in facial temperature was measured by infrared camera. A significant increase in facial temperature of 1.39 degrees C +/- 0.3 was found within 7 min in all patients, which lasted up to 60 min, although facial flushing was visible in only 50% (5/10) of the probands. In a second experiment 100 micrograms hCRH was then administered to seven other healthy young men. Intra- and extracerebral blood flow velocity changes in the medial cerebral artery (MCA) and external carotid artery (ECA) were measured after hCRH administration by use of Doppler sonography. We found a decrease of intracerebral blood flow which was caused by hyperventilation and was reversible following 6% CO2 hyperventilation during a second injection of 100 micrograms hCRH. Blood flow velocity in the ECA increased by 111.5 +/- 32.9% (compared to baseline level), lasted up to 60 min after hCRH injection, and was not reversible by 6% end-tidal CO2 ventilation. We thus demonstrated that the direct vasodilatory effect of hCRH involves the ECA-supplied vascular territory only. The intracerebral vasoconstrictory effect represents the result of hyperventilation following hCRH injection. The data thus clearly suggest an interaction of hCRH and the vascular endothelium of the ECA, causing a marked blood flow velocity increase and facial flushing.

Comparison of measurement of stump pressure and transcranial measurement of flow velocity in the middle cerebral artery in carotid surgery

Transcranial Doppler ultrasound measurement of the velocity of blood flow in the middle cerebral artery (MCA) was performed in 24 consecutive patients undergoing carotid endarterectomy. Measurements were performed preoperatively at rest, following common carotid artery compression, and continuously during surgery. In addition, internal carotid artery stump pressures were measured and a subjective assessment of back flow was made. No relationship between MCA flow velocity and stump pressure following carotid clamping was demonstrated. Peak and mean MCA flow velocity was significantly lower in patients with stump pressures < 30 mm Hg (p < 0.03) and those with poor back flow (p < 0.02).

Angle correction in transcranial Doppler sonography

Since the introduction of transcranial Doppler sonography in the early 1980s, flow velocity estimates have assumed a 0- to 30-degree angle of insonation. Based on limited radiological and anatomical studies, such as assumption appeared justified, and seemed to confer only minimal potential for error due to the cosine function in the Doppler formula. The introduction of transcranial color duplex sonography allows the direct evaluation of this assumption and the effect on flow velocities. Fifteen healthy volunteers were studied bilaterally using a unilateral transtemporal approach from the right. Velocity measurements were taken from the middle, anterior, and posterior cerebral arteries. Flow velocities were obtained with and without angle correction (0 degree). After completion of the color duplex study, velocities were obtained with a conventional, "blind" Doppler transducer at corresponding depths. For all insonated vessels the average angle of insonation was around 30 degrees. However, there was a wide variability of individual angles of insonation (0-70 degrees) in specific vessels. In 74.5% of all vessels, the angle-corrected flow velocity did not exceed the uncorrected velocity by more than 25%. In 14.5% the angle-corrected velocity was 25 to 50% higher and in 10.8% it was more than 50% higher as compared to the uncorrected velocity. Thus, the angle of insonation was unpredictable and often higher than originally expected. Angle-corrected velocities were higher than uncorrected values, and were more than 25% higher in about one-fourth of the vessels studied. Understanding of the clinical importance of such differences requires further study.

Unilateral supraclinoid internal carotid artery stenosis with moyamoya-like vasculopathy. Noninvasive assessments

Presented are the results of an extensive noninvasive assessment of supraclinoid internal carotid artery (ICA) stenosis with moyamoya-like vasculopathy in 3 patients with a history of stroke. Five noninvasive criteria for the diagnosis based on magnetic resonance imaging of the brain, and hemodynamic testing using ocular pneumoplethysmography, duplex carotid ultrasound, and transcranial Doppler sonography were established: (1) normal ocular pneumoplethysmography demonstrating no pressure significant stenosis to the level of the ophthalmic artery; (2) abnormal Doppler spectral waveforms showing either no flow or a high-resistance flow pattern for the ipsilateral cervical internal carotid artery; (3) paradoxically low flow velocities for the ipsilateral intracranial (ICA) and middle cerebral artery (MCA), and markedly abnormal high velocities for the contralateral ICA and MCA; (4) decreased ipsilateral MCA vasomotor reactivity; and (5) deep MCA territory ipsilateral subcortical watershed infarction evidenced by magnetic resonance imaging. This report demonstrates that a noninvasive battery of tests may be useful in the early diagnosis and treatment of these patients.

Cerebral vasomotor reactivity is significantly reduced in low-flow as compared to thromboembolic infarctions: the key role of the circle of Willis

To test the hypothesis that cerebral vasomotor reactivity (CVMR) is significantly more reduced in patients with hemispheric low-flow infarctions than in brain infarctions due to arterio-arterial embolism, a series of 64 consecutive patients with internal carotid artery occlusions were studied. CVMR was calculated from relative changes of blood flow velocity within the middle cerebral artery (MCA) measured by transcranial Doppler ultrasonography (TCD) during hypo- and hypercapnia. The configuration of the circle of Willis (COW) was also determined by TCD using common carotid artery compression tests. Anterior, posterior or ophthalmic artery collateral flow, and absence or combinations of these, were differentiated. CT scans were categorized as showing either no infarction (group I; n = 20) or territorial (group II; n = 28), or low-flow infarctions (group III; n = 16). As compared to normal, CVMR was significantly reduced but equal in groups I and II, however, even more reduced in group III. CVMR was lowest, and low-flow infarctions were most frequent in patients whose collateral hemispheric blood supply was from the ophthalmic artery as opposed to patients with a complete or nearly complete COW. Our findings indicate that low-flow infarctions in extracranial ICA occlusions represent brain damage due to a critical reduction in cerebral perfusion pressure, as opposed to thromboembolically induced lesions. The configuration of the COW seems to play the key role. Our findings also support the view that the pattern of hemispheric infarction seen on CT indicates the pathogenesis of stroke.

Prevalence of impaired cerebrovascular reserve in patients with symptomatic carotid artery disease

Cerebrovascular reserve (CVR) was studied in 104 consecutive patients with symptomatic carotid territory disease and ipsilateral internal carotid artery stenosis. Overall, 30 of 104 patients (29 per cent) had impaired CVR. The frequency of CVR impairment increased with the severity of internal carotid artery stenosis: impairment was present in none of 11 patients with stenosis of less than 50 per cent, four of 24 with stenosis of 50-69 per cent, 14 of 41 with stenosis of 70-89 per cent and 12 of 28 with stenosis of 90-99 per cent. Patients presenting with a stroke were significantly more likely to have impaired CVR than those with transient ischaemic attacks and/or amaurosis fugax (odds ratio 3.7 (95 per cent confidence interval (c.i.) 1.5-9.0)), as were those with a residual neurological deficit (odds ratio 4.3 (95 per cent c.i. 1.6-11.5)) and evidence of infarction from computed tomography (odds ratio 3.8 (95 per cent c.i. 1.6-9.4)).

Blood flow variations in internal carotid and middle cerebral arteries induced by postmenopausal hormone replacement therapy

OBJECTIVE

Our purpose was to clarify the mechanisms by which postmenopausal estrogen replacement therapy exerts its protective effect on cardiovascular risk.

STUDY DESIGN

By means of a bidirectional Doppler ultrasonographic system we measured pulsatility index variations the internal carotid artery and middle cerebral artery in 25 early postmenopausal women during a 6-month period of hormone replacement therapy. Transdermal estradiol (50 micrograms/day) was continuously administered. A 12-day course of medroxyprogesterone acetate (10 mg/day) was added every second month.

RESULTS

The pulsatility index showed a significant (p = 0.0001) reduction in both arteries after 6 weeks. At 22 weeks a 25% reduction was measured. No variation of the estrogen-induced pulsatility index reduction was observed at the end of every cyclic progestogen supplementation.

CONCLUSIONS

In early postmenopausal women hormone replacement therapy causes a rapid reduction of pulsatility index in brain arteries. Cyclical progestational supplementation does not modify this positive effect on reactivity of the blood vessels.