Dependency of blood flow velocity in the middle cerebral artery on end-tidal carbon dioxide partial pressure--a transcranial ultrasound Doppler study

Comparison of carbon-dioxide-enriched, oxygen-enriched, and normal air in treatment of acute mountain sickness

Twenty mountaineers with acute mountain sickness (AMS) at an altitude of 4559 m were randomly allocated to treatment with oxygen-enriched (33% oxygen), carbon-dioxide-enriched (3% carbon dioxide), or normal compressed air. Treatment with oxygen significantly improved the arterial partial pressure of oxygen (PaO2), relieved symptoms of AMS, and reduced cerebral blood flow as estimated by transcranial doppler ultrasound examination of the median cerebral artery. The only significant effect of carbon dioxide was increased ventilation resulting in a slight rise in PaO2. Thus, in contrast to previous uncontrolled trials, this study does not support the usefulness of carbon dioxide treatment in AMS.

A simple test to assess cerebrovascular reserve capacity using transcranial Doppler sonography and acetazolamide

The goal of this study was the development of a simple bedside test to assess cerebrovascular reserve capacity using transcranial Doppler sonography. We studied 33 normal persons at rest and after stimulation of cerebral blood flow with 1 g acetazolamide. Their mean +/- SD increase in blood flow velocity in 54 middle cerebral arteries 10 minutes after stimulation was 24.4 +/- 9.2 cm/sec. We tried to validate the increase in blood flow velocity as cerebrovascular reserve capacity in 21 patients with obstructive carotid artery disease and symptoms of cerebral ischemia. The patients were studied using transcranial Doppler sonography and xenon-133 dynamic single-photon emission computed tomography after acetazolamide stimulation. Their increases in blood flow velocity (delta FV) and increases in cerebral blood flow (delta CBF) correlated significantly in both hemispheres (asymptomatic: Y = 0.32X + 10.65, r = 0.45, p = 0.04; symptomatic: Y = 0.36X + 2.28, r = 0.59, p = 0.004). There was no significant difference between the slopes of the regression lines. Blood flow velocity and cerebral blood flow at rest were not correlated. The increase in blood flow velocity after acetazolamide stimulation offers a simple and reliable method for assessing cerebrovascular reserve capacity.

Continuous recording of middle cerebral artery blood velocity in clinical neurosurgery

Intracranial pressure, arterial blood pressure, and middle cerebral artery blood velocity were monitored for periods from 1 to 10 days in 30 neurosurgical intensive care patients. The recordings revealed rapid changes in the cerebral perfusion and gave insight into individual cerebral haemodynamic states. Twenty patients consistently showed CO2 reactivity within normal limits, between 2.5 and 5% per mm Hg (19-38% per kPa). Severely impaired CO2 reactivity, considerably below 1% per mm Hg (7.5% per kPa) was observed in four patients. Three of these patients died, while the fourth patient survived in a persistent vegetative state. Seven patients demonstrated pressure-passive blood velocity changes throughout the observed CPP range. The four patients with severely impaired CO2 reactivity all belonged to this group. The recordings from three of the remaining 23 patients showed signs of MCA blood velocity autoregulation with a lower regulatory limit of about 40-45 mm Hg. This observation is in keeping with findings from electromagnetic flowmetry on brain arteries in the neurosurgical operating field, and supports blood velocity measurements as a relevant index of brain perfusion in clinical neurosurgery and neuro-intensive care settings.

An online technique for estimating cerebral carbon dioxide reactivity

A technique for measuring cerebral reactivity using transcranial pulsed Doppler ultrasound is described; the system includes a spectrum analyser and capnometer. Data acquisition and manipulation is under software control. Main stem middle cerebral artery blood velocity is monitored continuously using the transcranial Doppler technique, whilst the operator initiates data collection and controls the inspired gas composition. The calculation of cerebral CO2 reactivity is based upon linear regression analysis of normalized, time-averaged middle cerebral velocity on end-tidal pCO2 and is displayed graphically. Measurement of middle cerebral CO2 reactivities can be completed within 15 min. Results from two subjects, a healthy volunteer and a patient with occlusive disease, are shown to illustrate the technique.

Intracranial haemodynamics in diffuse and focal brain injuries. Evaluation with transcranial Doppler (TCD) ultrasound

Intracranial haemodynamics were studied in 20 patients with diffuse and focal brain injury and experimental animals with acute intracranial hypertension by the use of TCD ultrasound. The mean flow velocity in the middle cerebral artery (MCA) commonly decreased on the side of the haematoma depending on intracranial pressure (ICP) elevation and cerebral perfusion pressure (CPP) reduction in focal injury. The decrease of the MCA flow velocity returned to normal after treatment. The flow velocities decreased bilaterally and there was no difference between the right and left side in diffuse injury. But the velocities increased in spite of ICP elevation when diffuse cerebral swelling developed. Cerebrovascular CO2 reactivity was impaired in two groups of patients with low Glasgow Coma Scale (GCS) scores. The mean velocity of the MCA and blood flow in the internal carotid artery exhibited flow patterns which changed correlatively depending on CPP reduction in experimental animals. Noninvasive study by use of TCD ultrasound can provide valuable information on variant haemodynamic phenomena in patients with diffuse and focal brain injury.

Perfusion insufficiency in limb-shaking transient ischemic attacks

We describe a 63-year-old man with severe bilateral internal carotid artery disease who presented with repeated, brief attacks of left limb shaking precipitated by his standing up. Cerebral blood flow measured by xenon-133 inhalation showed reduced resting flows and a focal perfusion deficit in the right dorsofrontal and upper rolandic regions. Blood flow velocity and pulsatility index of the right middle cerebral artery measured by transcranial Doppler ultrasonography were also reduced. With hypercapnic challenge, both hemispheric tissue perfusion and blood flow velocity showed impaired reactivity. With induced hypotension, the focal perfusion deficit in the right dorsofrontal region was accentuated. Following right internal carotid endartectomy, resting cerebral blood flow and blood flow velocity improved, as did hypercapnic vasoreactivity. These reversible deficits in cerebral blood flow and vasoregulation, which were maximal in the dorsofrontal region, are consistent with low perfusion in the border zone territory or the distal fields and demonstrate that hemodynamic failure is the likely mechanism for limb-shaking transient ischemic attacks from severe carotid artery disease.

The hyperventilation-induced ischaemia model in human neuropharmacology: neurophysiological and psychometric studies of aniracetam and 3-OH aniracetam

Standardized hyperventilation in young subjects induces changes in the EEG, a decrease in the velocity of the cerebral blood flow and a decline in cognitive performance, which are comparable to those occurring in patients with cerebral ischaemia. The anti-ischaemic properties of aniracetam and 3-OH aniracetam were tested in this model. A single oral dose of 3-OH aniracetam 1500 mg appeared to have the most pronounced effect on hyperventilation-induced EEG changes and cognitive deterioration. The test drugs had no effect on the heart rate or blood flow velocity. The effects agree with those of other drugs classified as noötropics.

Effect of acetazolamide on cerebral artery blood velocity and regional cerebral blood flow in normal subjects

The effect of intravenous acetazolamide 1 g on cerebral artery blood velocity and regional blood flow (rCBF) was investigated in eight normal subjects. Blood velocity was measured with 2 MHz pulsed Doppler in the proximal segments of the middle, anterior and posterior cerebral artery (MCA, ACA, and PCA) and in the distal extracranial internal carotid artery (ICA). The rCBF in the regions of interest tentatively corresponding to the perfusion territories of these vessels was estimated using 133Xe inhalation and a rapidly rotating single photon emission computer tomograph. Both blood velocity and rCBF increased after acetazolamide. There was no significant difference between the percentage ICA blood velocity increase (22 +/- 12%) and the percentage rCBF increase in the ICA region of interest (25 +/- 9%). In the MCA, ACA, and PCA, however, blood velocity increased more (mean increase 36-42%) than the rCBF in the corresponding regions of interest (mean increase 24-26%). These differences were highly significant suggesting a direct and site specific effect of acetazolamide in narrowing the lumen of the proximal MCA, ACA, and PCA, but not of the extracranial ICA. We also propose that the effect of acetazolamide induces reciprocal changes in the extent of adjacent perfusion territories in individual brain hemispheres. Data compiled from all subjects investigated at two very different perfusion levels (before and after acetazolamide) revealed a significant positive correlation between blood velocity and rCBF.

CO2 reactivity of cerebral vasospasm after aneurysmal subarachnoid haemorrhage

CO2 reactivity of the brain vessels was investigated in 33 patients (Grade I-III after Hunt and Hess) with cerebral vasospasm after an aneurysmal subarachnoid haemorrhage (SAH) and after early operation within 72 hours. In all cases, transcranial Doppler sonography was used to measure flow velocities in the middle cerebral artery (MCA) and internal carotid artery (ICA) and vasomotor reactivity to CO2 changes. Vasospastic conditions lead to higher flow velocities through the narrow segment, lower peripheral stream resistance due to the post-stenotic pressure drop and lower vasodilating capacities of arterioles under hypercapnia. In severe vasospastic conditions, the peripheral stream bed is already maximally dilated and the hypercapnic response is weak. On the other hand, the peripheral vascular bed reacts normally to hypocapnia in all vasospastic situations. Our results point out two dangerous conditions of vasospastic disease: 1) exhaustion of peripheral vasodilating capacities, and 2) hyperventilatory therapy. Both of these situations can result in a reduction of CBF to brain tissue, mainly for two reasons: 1) In the former, a further increase in vasospasm cannot be compensated for anymore when the peripheral arterioles are maximally dilated, and 2) in the latter, hypocapnia produces a strong peripheral vasoconstrictor response with further reduction of CBF.

Transcranial Doppler sonographic studies of cerebral autoregulation in Shy-Drager syndrome

A study is reported of mean arterial blood pressure and heart rate in four patients suffering from Shy-Drager syndrome. Blood flow velocity in the middle cerebral artery (MCA) was recorded by transcranial Doppler sonography. Concomitant changes in cerebral blood flow and the effect of cerebral autoregulation were thus examined. During tilt (60 degrees, head up) mean arterial blood pressure decreased by 40 mm Hg or 35%, while MCA blood flow velocity dropped by 14 cm/s or 28% (mean values). The lower percentage reduction in flow velocity may indicate a preserved cerebral autoregulation in central autonomic insufficiency.

Blood velocity and regional blood flow in defined cerebral artery systems

Cerebral artery blood velocity and regional blood flow (rCBF) were investigated in 17 normal subjects. Blood velocity was measured with 2 MHz pulsed Doppler ultrasound in the proximal segments of the middle, anterior and posterior cerebral artery (MCA, ACA, and PCA) and in the distal extracranial internal carotid artery (ICA). The rCBF in the regions of interest tentatively corresponding to the perfusion territories of these vessels was estimated using 133Xe inhalation and a rapidly rotating single photon emission computer tomograph. Concomitant capnograph recordings showed that the end-expiratory pCO2 was higher during the rCBF than during the blood velocity examinations. This differences was highly significant. While there was no significant correlation between blood velocity and rCBF when these clear differences in pCO2 were disregarded, we did find significant positive correlations when the data were normalized to a standard pCO2 (5.3 kPa) using accepted formulas. The best correlation was found for the MCA (r = 0.630, p less than 0.001) and the PCA (r = 0.73, p less than 0.001), with a lower correlation in the ACA (r = 0.49, p less than 0.01) and the ICA (r = 0.41, p less than 0.05). The estimated blood velocity (V) given rCBF = 0 was not significantly different from 0. The results support the validity of expressing the relationship between blood velocity (V) and rCBF in defined cerebral artery systems as: V = 1/60 (rCBF) T (A)-1, where A represents the area of the lumen of the vessel segment where the velocity is being measured, and T denotes the size of the brain region being perfused from this artery.

Two transcranial Doppler studies on blood flow velocity in both middle cerebral arteries during rest and the performance of cognitive tasks

While the middle cerebral artery (MCA) blood flow velocity changes relative to rest were assessed with transcranial Doppler sonography (TCD), 28 right-handed subjects with no sign of cerebrovascular disease performed two series of 6 cognitive tasks (two series = right and left MCA). The tasks included "reading", "finding nouns with a given first letter", and "multiplication" in four comparable versions to be performed aloud and silently in each of the two series. All of the tasks increased the MCA blood flow velocity bilaterally (2.7-12.1%). A significant left-right difference was present during "noun finding aloud" (left greater than right by 4.7%). A statistically insignificant tendency in the same direction was also present for the five other tasks. During loud reading the increase was higher bilaterally than during silent reading. Blood flow velocity changes in the right MCA for the three tasks performed aloud were lower in older than in younger people. It is posited that older people perform the tasks more slowly and under less stress requiring less right-hemispheric participation. Averaging the results for each of the four subjects involved leads to the conclusion that future TCD-lateralization studies should be done bilaterally at the same time to assess lateralization in a single person.

Transcranial Doppler sonography in children

The blood flow velocities in the basal cerebral arteries can be recorded at any age by transcranial Doppler sonography. By a standardized examination technique, the vessel identification is reliable even without visual control of the site of the sample volume. A stable state of vigilance is necessary to obtain constant recordings. Age has a tremendous influence on the flow velocities. The velocities increase rapidly during the first weeks and reach their maximum around the sixth year of life. The influence of carbon dioxide partial pressure, gestational age, birth weight, hematocrit, and vigilance on the velocities has to be considered. Heart rate and arterial blood pressure are relevant only in cases of extreme values. The clinical applications of transcranial Doppler sonography cover diseases like patent ductus arteriosus, perinatal brain damage, increased intracranial pressure, cerebral malformations, brain death, and stenoses and occlusions of main cerebral arteries. The technique is helpful for control of certain therapies. A continuous recording technique has also been developed for this purpose. Care should be taken in deriving a prognosis from Doppler recordings.

Changes in quantitative EEG and blood flow velocity due to standardized hyperventilation; a model of transient ischaemia in young human subjects

A standardized hyperventilation (HV) procedure has been developed in which the end-tidal pCO2 was decreased to 2 kpa. In 24 young male subjects blood flow velocity and qEEG were studied before, during and after HV. This standardized hyperventilation procedure gave rise to a decrease in blood flow velocity to 40% of baseline value and highly significant qEEG changes in 3 derivations. Both relative and absolute band power estimates showed an increase in slow activity and a decrease in alpha and beta activity. The use of subtraction spectra led to a more precise and detailed presentation of these changes than the use of classical qEEG parameters. These changes were reproducible after 1 week. The effects found in the presented model of HV-induced ischaemia appeared to be twice as large as those found in a model of hypobaric hypoxia. The present model might be used to test the efficacy of anti-ischaemic drugs in young human subjects.

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

To evaluate the CO2-induced vasomotor reactivity of the cerebral vasculature, relative changes of blood flow velocity within the middle cerebral artery were measured by transcranial Doppler ultrasonography during normocapnia and various degrees of hypercapnia and hypocapnia. We studied 40 normal individuals and 40 patients with unilateral and 15 patients with bilateral internal carotid artery occlusions. When blood flow velocity changes as percent of normocapnic values were plotted against end-tidal CO2 volume percent, a biasymptotic curve (a tangent-hyperbolic function) gave the best fit of the scattergram. The distance between the upper and lower asymptotes was defined as cerebral vasomotor reactivity. In the normal individuals, mean +/- SD vasomotor reactivity was 85.63 +/- 15.96%. In patients with internal carotid artery occlusions, vasomotor reactivity was significantly lower than normal on both the occluded (mean 45.2%, median 50.4%; p less than 0.0001) and the nonoccluded (mean +/- SD 67.7 + 13.3%, p less than 0.01) sides in the unilateral group and on both sides (mean +/- SD 36.6 +/- 15.9% and 44.9 +/- 24.6%, p less than 0.0001) in the bilateral group. The difference between vasomotor reactivity for symptomatic and asymptomatic unilateral occlusions was also highly significant (mean 37.6% and 62.9%, p less than 0.006). Vasomotor reactivity was also significantly lower in patients with low-flow infarctions on computed tomography than in patients with normal scans (mean +/- SD 36.7 +/- 25% and 60.2 +/- 16.9%, p less than 0.008). A striking association of low-flow infarctions, ischemic ophthalmopathy, and hypostatic transient ischemic attacks was found with vasomotor reactivities of less than 34% or even paradoxical reactions.(ABSTRACT TRUNCATED AT 250 WORDS)

Transcranial Doppler

TCD recording of flow velocities in intracranial vessels was first described by Aaslid in 1982. The utility of this instrument becomes more apparent as it is used in different clinical settings and compared with angiographic findings (Figures 1 and 2). Its importance in early detection of vasospasm in subarachnoid hemorrhage is now clearly known; increased flow velocity can be documented prior to neurologic deterioration and thus allow early institution of therapy. In patients with stroke or transient ischemic attack of unclear etiology, especially in blacks, Orientals, or females, who have a higher incidence of intracranial arterial disease, TCD can be a very important noninvasive means for detecting stenosis of intracranial vessels. Its value for assessing collateral circulation, intraoperative monitoring, and measuring CBF is quite promising. Hopefully, through further work with TCD, we will be able to clarify the spectrum of its usages as well as its limitations, though the preliminary data indicate that it should be an important addition to present noninvasive evaluations.

Intracranial blood velocity in head injury. A transcranial ultrasound Doppler study

We report our experience monitoring head-injured patients by means of transcranial Doppler sonography. Cerebral velocity measurements and waveforms change in a consistent pattern with the existence of intracranial pressure, and it is possible to discriminate low versus high flow states. The technique, in contrast to cerebral blood flow measurement, is reliable, atraumatic, and repeatable so that diagnostic assessments can be made and the patient can be followed for therapeutic efficacy.

Evaluation of intracranial pressure from transcranial Doppler studies in cerebral disease

The extent to which estimations of intracranial pressure can be derived from intracranial flow patterns was studied. The blood flow velocity in the middle cerebral artery was recorded with the EME TC 2-64 transcranial Doppler (TCD) device in 26 patients suffering from various severe cerebral diseases. Simultaneously the mean intracranial pressure (ICP) was measured by means of an epidural device. Arterial carbon-dioxide tensions were monitored by blood gas analysis. In all patients it was observed that the middle cerebral artery flow patterns changed distinctly when the ICP increased; these changes were distinguished by a decrease of the mean flow velocity and an increase of the Pourcelot index. A good correlation between the ICP and the flow parameters (especially the product mean systemic arterial pressure x Pourcelot index/mean flow velocity) was found in a select group of 13 patients, in whom comparable initial conditions existed and in whom additional parameters influencing the TCD recordings could be kept constant (r = 0.873; P less than 0.001).

Cerebral hemodynamics in angioma patients: an intraoperative study

Local hemodynamics were investigated during 33 operations for cerebral arteriovenous malformation (AVM). In all cases, microvascular Doppler sonography was used to measure flow velocities and vasomotor reactivity to CO2 changes. Intravascular pressure recordings were performed in six patients. The AVM feeders had low intravascular pressure, high flow velocity, low peripheral stream resistance, and very poor vasomotor reactivity. Remote brain arteries showed no abnormalities. Doppler findings in arterial branches of AVM feeders that supplied normal brain indicated arteriolar dilation in their peripheral distribution. On removal of the angiomas, the arteries that formerly supplied them showed a return to normal intravascular pressure, whereas flow velocities dropped far below normal in these vessels. Remote arteries and branches of the former AVM feeders supplying the brain did not show any signs of impaired vasomotor reactivity following angioma removal. The results are in contrast to the normal perfusion pressure breakthrough theory.

Variations in middle cerebral artery blood flow investigated with noninvasive transcranial blood velocity measurements

Observations on blood velocity in the middle cerebral artery using transcranial Doppler ultrasound and on the ipsilateral internal carotid artery flow volume were obtained during periods of transient, rapid blood flow variations in 7 patients. Five patients were investigated after carotid endarterectomy. A further 2 patients having staged carotid endarterectomy and open heart surgery were investigated during nonpulsatile cardiopulmonary bypass. The patient selection permitted the assumption that middle cerebral artery flow remained proportional to internal carotid artery flow. The integrated time-mean values from consecutive 5-second periods were computed. The arithmetic mean internal carotid artery flow varied from 167 to 399 ml/min in individual patients, with individual ranges between +/- 15% and +/- 35% of the mean flow. The mean middle cerebral artery blood velocity varied from 32 to 78 cm/sec. The relation between flow volume and blood velocity was nearly linear under these conditions. Normalization of the data as percent of the individual arithmetic means permitted a composite analysis of data from all patients. Linear regression of normalized blood velocity (V') on normalized flow volume (Q') showed V' = 1.05 Q' - 5.08 (r2 = 0.898).

Evaluation of cerebrovascular disease by combined extracranial and transcranial Doppler sonography. Experience in 1,039 patients

Results from 1,039 combined cervical and transcranial Doppler examinations are reported. Satisfactory transcranial signals were not found in 2.7% of the cases. Compared with angiography, the accuracy of transcranial criteria in assessing collateral flow over the circle of Willis was 94 and 88% for anterior and posterior circulation, respectively. The method also appeared very promising for detection of lesions of the intracranial arteries although the number of such cases with angiographic verification was limited in the present series. Arterial narrowing due to cerebral vasospasm was diagnosed with a sensitivity of 80%. In patients with ruptured intracranial aneurysms, an incidence of 93% arterial narrowing in basal cerebral arteries was found. Patients with subarachnoid hemorrhage and no aneurysm on angiography also showed arterial narrowing with an incidence of 56%. It was possible to monitor the time course and severity of cerebral vasospasm. Arteriovenous malformations were characterized by Doppler findings of high velocities and low pulsatilities. These lesions were diagnosed with an accuracy of 95%.

The detection of microemboli in the middle cerebral artery during cardiopulmonary bypass: a transcranial Doppler ultrasound investigation using membrane and bubble oxygenators

Twenty-seven patients were examined who were undergoing cardiopulmonary bypass (CPB) surgery with either a bubble oxygenator or a capillary membrane oxygenator. The latter incorporated an arterial filter and bubble trap. A noninvasive Doppler ultrasound technique is described for monitoring irregularities in the Doppler flow signals attributable to gaseous microemboli detected in the middle cerebral artery during CPB. The ultrasound index for detecting gaseous microemboli (MEI) indicated the presence of such microemboli in 22 of the 27 patients during insertion of the aortic cannula. Measurements during CPB showed the MEI ranged from 4 to 39 in the 17 patients with a bubble oxygenator. However, all 10 patients with a membrane oxygenator had an MEI of 0. Varying the gas flow rates in 3 patients with bubble oxygenators showed a change in MEI from 4 +/- 4 (SD) at a flow rate of 2 L/min to 17 +/- 9 at a flow rate of 5 L/min. This observation supports the assumption that the MEI is providing quantitative information regarding the presence of gaseous emboli in the middle cerebral artery.

Intracranial flow patterns at increasing intracranial pressure

In the course of a pilot study, changes in intracranial pressure were compared with the transcranial Doppler findings of the middle cerebral artery. The cases of five patients were discussed who developed dissociated brain death in spite of intensive therapeutic measures. The studies showed that changes of the intracranial pressure influenced the flow patterns considerably: at increasing intracranial pressure (decreasing cerebral perfusion pressure) a progressive reduction of the systolic and above all diastolic flow velocities and finally a pendular flow occurred. These changes could be recorded quantitatively by means of the "Pourcelot index" and the mean flow velocity. Acute changes of the intracranial pressure can be detected at an early stage by noninvasive transcranial Doppler studies and can be followed by adequate intensive therapy.

Visually evoked dynamic blood flow response of the human cerebral circulation

The dynamics of the metabolic mechanism that regulates cerebral blood flow was studied in 10 normal human subjects using a noninvasive transcranial ultrasonic Doppler method. Flow volume in the posterior cerebral artery, supplying the visual cortex, increased 20.2% in response to light stimulation of the retina, while flow velocity in the same artery increased 16.4%. The regulation of blood flow was very rapid; only 2.3 seconds elapsed from application of the light stimulus to 50% of full response. Full regulation (90% of full response) took 4.6 seconds. The blood flow response adapted slightly after about 10 seconds. Flow velocity in the middle cerebral artery increased significantly, by 3.3%, while flow in the superior cerebellar artery showed no significant change in response to this stimulus. These findings suggest the mechanism of very fast metabolic regulation of cerebral blood flow in humans.

Transcranial Doppler assessment of cerebral collateral during carotid endarterectomy

Sixteen patients (mean age 59 +/- 9 years) who were to undergo carotid endarterectomy were examined pre- and peroperatively using pulsed Doppler-shifted ultrasound; blood velocities in the middle cerebral artery were measured before and during common carotid compression in the conscious patient, and before and during measurement of carotid stump pressures at surgery in the anaesthetized patient. Measurements at endarterectomy showed a significant difference in middle cerebral artery blood velocities from patients with stump pressures of greater and less than 50 mmHg (t = 4.0, P less than 0.005). A threshold of 10 cm s-1 distinguished between stump pressures of greater and less than 50 mmHg in 15 of the 16 patients. Pre-operative blood velocity measurements during carotid compression did not correlate with those taken peroperatively at carotid clamping.

Cerebral perfusion during major cardiac surgery in children

Six children undergoing major cardiac surgery had extensive cerebral monitoring during cardiopulmonary bypass (CPB). The monitoring included continuous recording of arterial blood pressure (BP), central venous pressure (CVP), cerebral electrical activity by a cerebral function monitor (CFM), and middle cerebral artery (MCA) flow velocity by the transcranial pulsed Doppler (TCD) technique. Introduction of the precooled blood containing priming solution resulted in rapid fall in BP as well as MCA velocities in these children at the start of CPB. During steady-state CPB at 20 degrees C, MCA flow velocities were reduced in five of six children, range 45%-105% of pre-bypass value. These flow velocity values were recorded at cerebral perfusion pressures (CPP = BP - CVP) in the range of 14-26 mmHg. This reduced cerebral perfusion during steady-state CPB appears to be more than sufficient to meet the cerebral metabolic demands at the particular temperature. The reduced cerebral perfusion is in contrast to the enhanced perfusion found in adults during moderately hypothermic (28 degrees-30 degrees C) low-flow, low-pressure CPB previously reported. It was presumably due to the reduced temperature, reduced perfusion pressure, and less hemodilution. During periods of constant temperature, hematocrit, and partial pressure of carbon dioxide (PaCO2), MCA flow velocities varied passively with changes in CPP, demonstrating that cerebral autoregulation was not operative. Transcranial Doppler appears to be a suitable tool for investigating CPB techniques optimal with respect to cerebral circulation.

Transcranial Doppler CO2 test for the detection of hemodynamically critical carotid artery stenoses and occlusions

Cerebral CO2-reactivity was tested by transcranial Doppler sonography (Doppler CO2 test) in 232 patients. Time averaged flow velocity in the middle cerebral artery at the 40 mm Hg blood pCO2 level was taken as a reference point, and the relative increase of flow in hypercapnia of 46.5 mm Hg pCO2 was defined as "Normalized Autoregulatory Response" (NAR). A total of 82 patients with no evidence of cerebrovascular disease gave "normal" values for NAR (23.2 +/- 5.2 SD). In 150 patients with 233 stenoses and occlusions of the internal carotid artery NAR was significantly decreased in higher-grade stenoses (P = 0.01 for 80% diameter reduction, P less than 10(-6) for 90% or more). In such stenoses, patients with NAR less than 14 had suffered more frequently (P less than 0.01) from ipsilateral transient ischemic attacks and/or stroke during the previous 6 months than patients with "normal" NAR. Preoperative NAR less than 14 always improved to "normal" values following carotid surgery, while preoperative NAR greater than 19 remained unchanged (60 cases). The transcranial Doppler CO2 test is thought to be a reliable noninvasive method to detect hemodynamically critical carotid stenoses and occlusions. This may be of interest in selecting patients for superficial temporal artery-middle cerebral artery bypass and carotid surgery. For practical use 4 categories of NAR are suggested.

Evaluation of cerebral AVM's using transcranial Doppler ultrasound

Blood flow velocities in basal cerebral arteries were recorded noninvasively in 28 patients with cerebral arteriovenous malformations (AVM's) and were correlated with the angiographic findings. In normal arteries remote from the AVM, flow velocities ranged from 44 to 94 cm/sec (median 65 cm/sec) with pulsatility indexes from 0.65 to 1.10 (median 0.87). This is consistent with findings in normal individuals. Arteries feeding the AVM's were identified by the high flow velocities (ranging from 75 to 237 cm/sec, median 124 cm/sec). The pulsatility index ranged from 0.22 to 0.74 (median 0.48). The difference of these results from findings in normal remote arteries was highly significant (p less than 0.001). Hyperventilation tests illustrated the hemodynamic difference between an AVM and normal cerebrovascular beds. Flow velocity measurements permitted noninvasive diagnosis of AVM's in 26 of the 28 patients. Furthermore, the identification of individual feeding arteries permitted good definition of the anatomical localization of individual AVM's. Flow velocity measurements combined with computerized tomography scans are useful in the diagnosis of AVM's. With the feeding artery's configuration identified on angiography, flow velocity measurements permit a new insight into the "hemodynamic dimension" of an AVM and its possible effects on adjacent normal brain-tissue perfusion in the individual patient.

Transcranial Doppler measurement of middle cerebral artery blood flow velocity: a validation study

Measurement of intracranial arterial blood flow velocity is a new technique with potentially a number of very useful applications. This study validates the technique by comparing it to cerebral blood flow (CBF) measured using intravenous Xenon133 and extracranial clearance recording. We have measured the middle cerebral artery (MCA) blood flow velocity in 17 symptomatic patients with the EME TC 264 transcranial Doppler velocimeter and compared these measurements to the ipsilateral hemispheric cerebral blood flow measured with an intravenous Xenon133 technique (Novo Cerebrograph 10A). Measurements were made at rest and during hypercapnia. The absolute measurement of MCA velocity and hemispheric CBF showed a poor correlation (r = 0.424, p less than 0.01) due to wide between-patient variations at rest but the blood flow response to hypercapnia, expressed as a reactivity index, showed a good correlation (r = 0.849, p less than 0.001). Thus changes in MCA velocity reliably correlate with changes in cerebral blood flow but the absolute velocity cannot be used as an indicator of CBF.

Anatomical validation of middle cerebral artery position as identified by transcranial pulsed Doppler ultrasound

The basal cerebral arteries were insonated using transcranial pulsed Doppler ultrasound (TPDU) at 2 MHz. The Doppler sample volume (SV) depths at which signals were obtained which could be attributed to the middle, anterior and posterior cerebral arteries (MCA, ACA and PCA) were compared with measurements in adult cadavers and with B-scan ultrasound studies in infants. The depth of the internal carotid artery (ICA) terminal division into ACA and MCA was closely correlated for both groups. In adults, it was found at 5.6 +/- 1.0 cm using TPDU while in cadavers it was found at 5.3 +/- 0.5 cm from the temporal bone. In infants, it was found at 3.2 +/- 0.3 cm for the right side, and 3.2 +/- 0.2 cm for the left side using TPDU, and at 3.4 +/- 0.4 cm and 3.4 +/- 0.5 cm for right and left sides respectively using B-scan ultrasound. The mean depth of the MCA mid-point in infants as defined by TPDU and B-scan was also closely correlated, with values of 2.8 +/- 0.3 cm and 2.7 +/- 0.3 cm for right and left sides respectively using TPDU and of 2.8 +/- 0.4 cm and 2.7 +/- 0.4 cm for right and left sides respectively using B-scan ultrasound. Values for the most lateral part of the MCA did not correlate. In adults, signals from the ACA and PCA were obtained at greater SV depth than the MCA, thus preventing confusion.

Cerebral carbon dioxide reactivity during nonpulsatile cardiopulmonary bypass

Five patients undergoing extensive cerebral monitoring during cardiopulmonary bypass (CPB) procedures were subjected to studies on cerebral CO2 reactivity during nonpulsatile CPB. The cerebral monitoring included recording of arterial blood pressure (BP), central venous pressure (CVP), epidural intracranial pressure (EDP), cerebral electrical activity by a cerebral function monitor (CFM), and middle cerebral artery (MCA) flow velocity by transcranial Doppler technique. The cerebral perfusion pressure (CPP) was thus continuously recorded (CPP = BP - EDP). During steady-state CPB with constant hematocrit, temperature, and arterial carbon dioxide tension (PaCO2), MCA flow velocity varied with changing CPP in a pressure-passive manner, indicating that the cerebral autoregulation was not operative. During moderately hypothermic (28 to 32 degrees C), nonpulsatile CPB, with steady-state hematocrit, temperature, and pump flow, we deliberately and rapidly changed PaCO2 for periods of 1 or 2 minutes by increasing gas flow to the membrane oxygenator, thereby testing the cerebral CO2 reactivity. Nineteen CO2 reactivity tests, performed at CPP levels ranging from 17 to 75 mm Hg, disclosed that the cerebral CO2 reactivity decreased with CPP, especially with CPP levels below 35 mm Hg. In these patients, concomitant changes in CPP during the CO2 reactivity test could be compensated for by adjusting the observed change in MCA flow velocity. The corrected CO2 reactivity values obtained in this way ranged from below 1.0 (observed at CPP levels below 20 mm Hg) to a 3.0 to 4.5% X mm Hg-1 change in PaCO2 (observed at CPP levels above 35 mm Hg).(ABSTRACT TRUNCATED AT 250 WORDS)

Transcranial measurement of blood velocities in the basal cerebral arteries using pulsed Doppler ultrasound: velocity as an index of flow

Blood velocities have been measured transcranially, at small Doppler angles, in the middle cerebral artery of normal volunteers. Cerebral blood flow was changed by varying carbon dioxide tension. In four volunteers, the relationships between arterial pCO2 and percentage change in intensity weighted mean, median, and maximum Doppler-shifted frequencies in the internal carotid and middle cerebral arteries were linear with slopes of 2.5 and 2.8% per mm Hg change in pCO2. In 38 volunteers, the relationship between end-expiratory pCO2 and time-averaged maximum Doppler frequency was linear over the range of pCO2 20-60 mm Hg with slopes of 2.5 and 2.9 percentage change per mm Hg, for internal carotid and middle cerebral, respectively. These results are very similar to those reported using direct methods of measuring cerebral blood flow. As the transcranial Doppler method is reproducible, this indicates that changes in middle cerebral blood velocity may be used to monitor changes in flow.

Dissociation between cerebral autoregulation and carbon dioxide reactivity during nonpulsatile cardiopulmonary bypass

Five patients undergoing cardiopulmonary bypass (CPB) procedures were extensively monitored because of anticipated high risk for neurological complications. Arterial blood pressure (BP), central venous pressure, and epidural intracranial pressure (EDP) were continuously recorded throughout CPB; thus, information on the cerebral perfusion pressure (CPP) was also continuously available (CPP = BP - EDP). Cerebral electrical activity was recorded by a cerebral function monitor. The flow velocity in the middle cerebral artery (MCA) was recorded using a transcranial Doppler technique. During steady-state CPB (constant hematocrit, constant temperature, and constant flow from the heart-lung machine) partial pressure of arterial carbon dioxide (PaCO2) was repeatedly changed to study the effect of changes in this variable on MCA flow velocity during nonpulsatile bypass. During CPB with constant temperature, hematocrit, and PaCO2, the effect of changes in CPP on MCA flow velocity was recorded and analyzed. During nonpulsatile, moderately hypothermic (28 degrees to 32 degrees C), low-flow (1.5 L/min/m2) CPB, there was no evidence of cerebral autoregulation, with CPP levels ranging from 20 to 60 mm Hg. The CO2 reactivity, however, was clearly present and in the range of 1.9 to 4.1%/mm Hg, indicating that there was a dissociation between cerebral autoregulation and CO2 reactivity under these circumstances.

Assessment of intracranial hemodynamics in carotid artery disease by transcranial Doppler ultrasound

Noninvasive transcranial Doppler recordings were correlated to the angiographic findings in 77 patients with carotid artery disease. Stenoses reducing the luminal area of the internal carotid artery by 75% or more also reduced the pulsatility transmission index (PTI) of the ipsilateral middle cerebral artery (MCA). The PTI is the pulsatility index of the artery under study expressed as a percent of the pulsatility index of another intracranial artery with presumed unimpeded inflow in the same individual. For stenoses in the 75% to 89% category. PTI reduction was significantly greater in patients with bilateral carotid stenosis, indicating an impaired potential for collateral flow in these patients. The PTI reduction probably reflects both the pressure drop across the stenosis and the cerebral autoregulatory response. Two criteria proved useful in demonstrating collateral MCA supply through the circle of Willis. On the recipient side, retrograde flow in the proximal anterior cerebral artery was demonstrated in 29 of the 31 patients when this flow pattern was disclosed angiographically. In 26 of these patients, the anterior cerebral artery on the supplying side also had clearly increased flow velocity. Increased flow velocities in the proximal posterior cerebral artery were present in 26 of the 30 vessels that were acting as a collateral channel to the ipsilateral MCA.

Cerebral perfusion during nonpulsatile cardiopulmonary bypass

The recording of middle cerebral artery (MCA) flow velocity by the transcranial Doppler method offers a new, noninvasive, continuous technique for studies of cerebral circulation. Comparative studies of electromagnetic internal carotid artery (ICA) flowmetry and MCA flow velocity by the transcranial Doppler technique have demonstrated that observed changes in MCA flow velocities reflect concomitant changes in cerebral circulation. Eleven high-risk patients undergoing cardiopulmonary bypass (CPB) procedures were included in a pilot study. Arterial blood pressure (BP), central venous pressure, and epidural intracranial pressure (EDP) were recorded during CPB. Cerebral electrical activity was recorded by a cerebral function monitor. Flow velocity in the MCA was increased during nonpulsatile CPB in 10 of the 11 patients. This increase was related to the degree of hemodilution, and the flow velocity during steady-state CPB was 80 to 300% of the prebypass value. The MCA flow velocity changed, however, in a pressure-passive manner with the cerebral perfusion pressure (CPP = BP - EDP) in the individual patient, which indicates that cerebral autoregulation was not operative. During the first 15 minutes after termination of bypass, the MCA flow velocity was reduced, but remained higher than the prebypass level, 110 to 210% of the level during the last 5 minutes preceding CPB.