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

Relative changes in cerebral blood flow during cardiac operations using xenon-133 clearance versus transcranial Doppler sonography

BACKGROUND

Changes in cerebral blood flow (CBF) during cardiac operations have implications in terms of postoperative neurologic and neuropsychological dysfunction. Current techniques of CBF measurement are cumbersome and invasive. Transcranial Doppler sonography offers a noninvasive means of assessing changes in CBF. The aim of this study was validation of this technique with existing methods of CBF measurement during cardiac operations.

METHODS

We compared the changes in CBF using xenon-133 clearance with changes in middle cerebral artery velocity by transcranial Doppler sonography (VMCA) using pH-stat and alpha-stat acid-base management during cardiopulmonary bypass. Measurements were taken (1) before bypass, (2) at 28 degrees C on bypass, (3) at 37 degrees C on bypass, and (4) after bypass. Relative changes in CBF and VMCA, calculated as the percent change from the prebypass baseline value normalized to 100%, were used in this analysis.

RESULTS

During the hypothermic phase of cardiopulmonary bypass, CBF and VMCA increased by 45.9% and 51.8%, respectively (p < 0.001), during pH-stat acid-base management but decreased by only 26.4% and 22.4%, respectively (p < 0.0001), during alpha-stat acid-base management. Linear regression analysis of the absolute changes in CBF (mL . 100 g-1 . min-1) and VMCA (cm/s) showed a significant correlation (r = 0.60; r2 = 0.36; p < 0.0001), but a better correlation was obtained when relative changes in CBF and VMCA were compared (r = 0.89; r2 = 0.79; p < 0.0001).

CONCLUSIONS

Measurements of VMCA, expressed as relative changes of a pre-cardiopulmonary bypass level (using the noninvasive transcranial Doppler sonographic technique), can be used to examine CBF changes during cardiopulmonary bypass.

Visually evoked blood flow response assessed by simultaneous two-channel transcranial Doppler using flow velocity averaging

BACKGROUND AND PURPOSE

We assessed the influence of different visual stimuli and the reproducibility and habituation of evoked flow responses using simultaneous two-channel transcranial Doppler monitoring and flow velocity averaging.

METHODS

We measured stimulus-related percentage changes in posterior cerebral, basilar, and middle cerebral artery blood flow velocities in 14 normal volunteers using stimulus-triggered velocity averaging. With a two-channel transcranial Doppler system, simultaneous measurements in two arteries (both posterior cerebral arteries and the basilar and middle cerebral artery) were taken using multiple-array light-emitting diodes applying flash stimuli. Both posterior cerebral arteries were monitored to assess reproducibility and habituation of the evoked response with repetitive measurements under unchanged conditions and to analyze the influence of different features of the visual stimulus.

RESULTS

There was a distinctive increase in velocities resulting from visual stimuli in both posterior cerebral and the basilar arteries but not in the middle cerebral artery. The responses in both posterior cerebral arteries were larger than in the basilar artery (P = .0001). Brightness (P < .0001), as well as complexity (P < .0001), of the visual stimulus had a significant influence on the response amplitude. There was a trend toward a greater right-sided activation. Amplitudes of the evoked response were very stable during repetitive testing (coefficient of variation of the difference was 0.6). There was a trend toward habituation with monotonous (flash) but not with complex visual stimuli. A "zero" stimulus produced no responses.

CONCLUSIONS

The use of flow velocity averaging and two-channel simultaneous recording increases the sensitivity of transcranial Doppler monitoring to detect and correlate selective flow changes in the posterior cerebral arteries resulting from cerebral activation produced by visual stimulation.

Estimation of cerebral blood flow at bedside by continuous jugular thermodilution

The Kety-Schmidt technique can be regarded as the reference method for the measurement of cerebral blood flow (CBF). However, the method is somewhat cumbersome for routine use in the intensive care unit (ICU) at the beside. The continuous thermodilution technique developed many years ago for the measurement of coronary sinus blood flow can be applied for the measurement of jugular blood flow (JBF). However, the measurement of JBF by thermodilution has never been validated using the Kety-Schmidt reference method. We first validate the continuous thermodilution in vitro by comparison with a volumetric flow. The thermodilution method is accurate for flows between 50 and 900 ml min-1 with a mean difference volumetric-thermodilution flow of -1 +/- 18 ml min-1 (mean +/- SD), and precise with a coefficient of variability ranging between 1.21% and 2.50%. In vivo accuracy was assessed by comparing in 15 comatose patients CBF measured using the Kety-Schmidt (CBFKS) method and estimated from JBF measured by thermodilution (CBFTH) at four levels of arterial PaCO2 (25, 30, 35, and 40 mm Hg). The mean difference CBFKS-CBFTH is -0.9 +/- 3.6 ml min-1 100 g-1. In vivo precision of the method was good, with a coefficient of variability of 4.1% in mean. We conclude that jugular continuous thermodilution technique is a reliable method for estimating CBF at the bedside. This technique allows repeated measurements jugular bulb blood sampling for brain metabolic studies.

Cerebral vasoconstriction in comatose patients resuscitated from a cardiac arrest?

OBJECTIVE

To determine the role of cerebral vasoconstriction in the delayed hypoperfusion phase in comatose patients after cardiac arrest.

DESIGN

Prospective study.

SETTING

Medical intensive care unit in a university hospital.

PATIENTS

10 comatose patients (Glasgow Coma Score +/- 6)successfully resuscitated from a cardiac arrest occurring outside the hospital.

MEASUREMENTS

We measured the pulsatility index (PI) and mean blood flow velocity (MFV) of the middle cerebral artery, the cerebral oxygen extraction ratio and jugular bulb levels of endothelin, nitrate, and cGMP during the first 24 h after cardiac arrest.

RESULTS

The PI decreased significantly from 1.86 +/- 1.02 to 1.05 +/- 0.22 (p = 0.03). The MFV increased significantly from 29 +/- 10 to 62 +/- 25 cm/s (p = 0.003). Cerebral oxygen extraction ratio decreased also from 0.39 +/- 0.13 to 0.24 +/- 0.11 (p = 0.015). Endothelin levels were high but did not change during the study period. Nitrate levels varied widely and showed a slight but significant decrease from 37.1 mumol/l (median; 25th-75th percentiles: 26.8-61.6) to 31.3 mumol/l (22.1-39.6) (p = 0.04). Cyclic guanosine monophosphate levels increased significantly from 2.95 mumol/l (median; 25th-75th percentiles: 2.48-5.43) to 7.5 mumol/l (6.20-14.0) (p = 0.02).

CONCLUSIONS

We found evidence of increased cerebrovascular resistance during the first 24 h after cardiac arrest with persistent high endothelin levels, gradually decreasing nitrate levels, and gradually increasing cGMP levels, This suggests that active cerebral vasoconstriction due to an imbalance between local vasodilators and vasoconstrictors plays a role in the delayed hypoperfusion phase.

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.

Effects of defibrination on hemorheology, cerebral blood flow velocity, and CO2 reactivity during hypocapnia in normal subjects

BACKGROUND AND PURPOSE

Plasma fibrinogen is reported to be an independent risk factor for stroke and cardiovascular diseases. The effects of defibrination on hemorheology, middle cerebral artery (MCA) blood flow velocity, and CO2 reactivity during hypocapnia were evaluated in normal subjects.

METHODS

Twenty-five healthy subjects (mean age, 31.8 +/- 5.7 years) were included in the study. Measurements were done at rest and repeated 24 hours after administration of 10 batroxobin units. Plasma fibrinogen, plasma viscosity, and whole blood viscosity were measured as hemorheological factors. MCA blood flow velocity was measured with a transcranial Doppler flowmeter. Blood flow velocity was corrected to 40 mm Hg of end-tidal CO2 partial pressure (PETCO2), and expressed as CV40. CO2 reactivity was measured as percent change in mean blood flow velocity per millimeter of mercury PETCO2.

RESULTS

Plasma fibrinogen (from 7.04 to 2.29 mumol/L; P < .001), whole blood viscosity, and plasma viscosity decreased after administration of batroxobin. Mean MCA blood flow velocity at rest, CV40. and CO2 reactivity during hypocapnia increased significantly (from 67.4 to 73.6 cm/s, from 71.7 to 77.7 cm/s, and from 2.9%/mm Hg to 3.2%/mm Hg, respectively; P < .01) after defibrination. Mean arterial blood pressure and PETCO2 at rest were constant before and 24 hours after administration of batroxobin. There was a significant positive correlation between CV40 and CO2 reactivity (r = .623, P < .0001).

CONCLUSIONS

The increase in MCA blood flow velocity was associated with improved CO2 reactivity and reduced blood viscosity after defibrination. The data may suggest that defibrination increases cerebral blood flow by reducing blood viscosity.

A transcranial doppler ultrasonography study of cerebrovascular CO2 reactivity in mitochondrial encephalomyopathy

BACKGROUND AND PURPOSE

To elucidate the pathogenic role of vascular involvement such as mitochondrial angiopathy in patients with mitochondrial encephalomyopathy (MEM). we used the transcranial Doppler sonography (TCD) method to detect impairment of cerebrovascular CO2 reactivity.

METHODS

The cerebral perfusion reserve in 13 MEM patients, including 6 with MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes) was studied by TCD for different CO2 partial pressures. For the parameter of mean flow velocity, the mean spatial Doppler frequency (fm) was obtained from the right and left middle cerebral arteries and basilar artery under conditions of normocapnia, hypercapnia, and hypocapnia in cases in which hyperventilation was possible. By fitting the obtained fm and the end-tidal CO2 partial pressure (PETCO2) to the exponential formula fm = a x e(K < PETCO2), where a is the theoretical fm at a PETCO2 of 0 mm Hg, the parameter K, an index of CO2 reactivity, was calculated.

RESULTS

The K value was lower than control values at at least one site of the middle cerebral arteries and basilar artery of all patients with MELAS as well as the other MEM patients except for one patient with myoclonic epilepsy with ragged-red fiber and one with Kearns-Sayer syndrome.

CONCLUSIONS

Our results suggest that there is a high incidence of impairment of cerebrovascular CO2 reactivity in MEM patients. Moreover, the noninvasive TCD method was found useful for evaluation of cerebral hemodynamics in MEM patients.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

The relationship between cerebral blood flow and transcranial Doppler blood flow velocity during hypothermic cardiopulmonary bypass in adults

A noninvasive, simple, and continuous method to assess cerebral perfusion during cardiopulmonary bypass (CPB) could help prevent cerebral ischemia. Transcranial Doppler sonography (TCD) allows a noninvasive, on-line measurement of blood flow velocity in cerebral arteries. The correlation of TCD-estimated and actual cerebral blood flow (CBF) has not been well studied during CPB. We determined the correlation of middle cerebral artery (MCA) mean velocity and CBF determined by the Kety-Schmidt method during nonbypass and two hypothermic bypass flow conditions. Sixteen patients undergoing hypothermic (27 degrees C) CPB for coronary artery bypass grafting and/or valve replacement surgery were enrolled in the study. We were able to determine MCA velocity in only 12 patients. We determined CBF and MCA velocity in each patient during four 15-min study periods: 1) prebypass after sternotomy before aortic cannulation; 2) hypothermic (27 degrees C) CPB with 1.2 L.min-1.m-2 pump flow; 3) hypothermic CPB with 2.4 L.min-1.m-2 pump flow, and 4) 30 min after weaning from CPB. There was no difference in the mean arterial pressure between the two CPB pump blood flows. The pooled change in MCA velocity and CBF as percentage of baseline (prebypass) for all patients and at all time points had a correlation of 0.33 (r). A decrease or increase in MCA velocity did not necessarily indicate a corresponding decrease or increase in CBF. This technology may be of limited usefulness during the circulatory condition of hypothermic, nonpulsatile CPB.

Brain tissue pO2 in relation to cerebral perfusion pressure, TCD findings and TCD-CO2-reactivity after severe head injury

As a reliable continuous monitoring of cerebral blood flow and/or cerebral oxygen metabolism is necessary to prevent secondary ischaemic events after severe head injury (SHI) the authors introduced brain tissue pO2 (ptiO2) monitoring and compared this new parameter with TCD-findings, cerebral perfusion pressure (CPP) and CO2-reactivity over time on 17 patients with a SHI. PtiO2 reflects the balance between the oxygen offered by the cerebral blood flow and the oxygen consumption by the brain tissue. According to TCD-CO2-reactivity PtiO2-CO2-reactivity was introduced. After initially (day 0) low mean values (ptiO2 7.7 +/- 2.6 mmHg, TCD 60.5 +/- 32.0 cm/sec and CPP 64.5 +/- 16.0 mmHg/, ptiO2 increased together with an increase in blood flow velocity of the middle cerebral artery and CPP. The relative hyperaemic phase on days 3 and 4 was followed by a decrease of all three parameters. Although TCD-CO2-reactivity was except for day 0 (1.4 +/- 1.5%), sufficient, ptiO2-CO2-reactivity sometimes showed so-called paradox reactions from day 0 till day 3, meaning an increase of ptiO2 on hyperventilation. Thereafter ptiO2-CO2-reactivity increased, increasing the risk of inducing ischaemia by hyperventilation. The authors concluded that ptiO2-monitoring might become an important tool in our treatment regime for patients requiring haemodynamic monitoring.

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.

Cerebral blood flow velocity response induced by a 70-hPa Valsalva manoeuvre associated with normo- and hypergravity in humans

Anti-G straining manoeuvres, derived from the Valsalva manoeuvre (VM), are physiological methods for protecting fighter pilots against positive accelerations (+Gz). The aim of this study was to investigate the effects of a standard VM on cerebral haemodynamics, in normo- and hypergravity. In six healthy male volunteers, we investigated the cerebral blood flow velocity response induced by a 10-s, 70-hPa (52.5 mmHg) VM, under normogravity, +2, +3 and +4 Gz acceleration plateaus. Mean blood flow velocity [formula: see text] in middle cerebral artery was monitored by transcranial Doppler velocimetry. In normogravity, no significant variation in [formula: see text] was observed at the onset of VM. After a maximal period of 1.2 s, while VM was sustained, [formula: see text] decreased significantly (P < 0.05). Following the end of the manoeuvre [formula: see text] did not change significantly. When the expiratory pressure had returned to the control value, [formula: see text] was transiently increased (P < 0.05) before returning to control values. During hypergravity, [formula: see text] was significantly decreased at +3 and +4 Gz (P < 0.05) before the onset of VM. While performing VM under +Gz, the main difference compared to the normogravity condition was a significant increase of [formula: see text] (P < 0.05) at the onset of the manoeuvre. Our findings would suggest that when performed under +Gz stress, a 70-hPa VM can transiently improve cerebral haemodynamics. However, when VM is sustained for more than 1.2 s it results in a lasting decrease of cerebral perfusion which may lower +Gz tolerance.

Low pulsatility signals through the orbits

BACKGROUND AND PURPOSE

Low pulsatility signals (LPS) on transcranial Doppler ultrasonography are detected (1) with arteriovenous malformations, (2) distal to hemodynamically significant stenosis, and (3) in venous structures. We describe focal LPS in the territory of the internal carotid artery siphon that do not represent any of the above conditions.

METHODS

We performed retrospective and prospective reviews of transcranial Doppler studies on 3225 patients over 5 years. Clinical and radiological data of all patients with focal LPS were extracted. LPS was defined as a focal signal identified through the orbital windows with a low pulsatility index (< 0.6).

RESULTS

Sixteen LPS (mean flow velocity [mean +/- SD], 62 +/- 11 cm/s; pulsatility index [mean +/- SD], 0.41 +/- 0.08; depth range, 46 to 72 mm) from 15 patients (mean +/- SD age, 45 +/- 15 years; 4 men, 11 women) were identified. LPS flow direction was away from the probe in 13 cases and toward it in 3. Presenting symptoms included headache, focal neurological deficits, dizziness, and pulsatile tinnitus. All patients had cranial MRI (MRI and MR angiography in 11). Three patients underwent conventional cerebral angiography. Arteriovenous malformations or significant arterial stenoses were not detected on any study.

CONCLUSIONS

A focal signal from the internal carotid artery siphon region with low pulsatility index and normal mean flow velocity, identified in the absence of other transcranial Doppler abnormalities, is not related to an arteriovenous malformation or proximal arterial stenosis. LPS, as defined, are not of venous origin since mean flow velocity was in the arterial range. LPS are likely related to prominent venous flow in the cavernous sinus secondary to unusually strong pulsation of the intracavernous internal carotid artery.

An impedance index in normal subjects and in subarachnoid hemorrhage

The impedance of a hemodynamic system is defined as the ratio of each harmonic component of blood pressure to that of flow. Calculation of impedance cures has been extensively performed in the systemic circulation, leading to the recognition of reflected pressure and flow waves and clarifying the shape of ultrasound waveforms. Impedance in the human cerebral circulation has not been measured primarily because of the relative inaccessibility of simultaneous flow and pressure data in the human cerebral circulation. We defined an impedance index using the transcranial Doppler waveform for that of flow and a noninvasive applanation measure of the carotid artery pressure waveform. Middle cerebral artery velocities and carotid artery pressure waveforms were simultaneously recorded in 16 vessels from 10 normal volunteers, 42 vessels in 14 patients with aneurysmal subarachnoid hemorrhage, and 14 vessels in 7 subjects during conditions of hypocapnia, normocapnia and hypercapnia. Impedance was calculated by dividing the harmonic associated with pressure divided by that of flow, and averaging 10 to 20 such calculations. Relative impedance curves were calculated by dividing by the impedance at the first harmonic. Impedance was also studied in an electrical model consisting of a Windkessel element containing inductance in series with a second Windkessel to model the large vessel and vascular bed, respectively. Model parameters were taken from the literature for these calculations. For the normal subjects, the shape of the impedance index curve was similar to those found in the systemic circulation. The impedance index curves for patients in vasospasm (middle cerebral velocity was greater than 180) showed a peak at the second or third harmonic, which appeared more frequently than the nonspasm group (p < 0.01). Furthermore, the ratio of the second harmonic to the first harmonic was significantly > 1.0 in the spasm group but significantly < 1.0 in the normal group (p < 0.05). Calculations from the electrical model replicated the appearance of these peaks at the second or third harmonic for vasospasm parameters. A statistically significant peak appeared at the second or third harmonic in the impedance index curves for patients in vasospasm, which was replicated quantitatively by our electrical model. Although such peaks can be explained in the systemic circulation by the presence of reflected waves, the distance to the reflection site is larger than expected for the cerebral circulation. This suggests the importance of the inertia of blood as a stenosis worsens and as the origin for the observed changes in the impedance index curves.

Relationship between transcranial Doppler-determined pulsatility index and cerebrovascular resistance: an experimental study

Clinical studies with transcranial Doppler suggest that the pulsatility of the flow velocity (FV) waveform increases when the distal cerebrovascular resistance (CVR) increases. To clarify this relationship, the authors studied animal models in which the resistance may be decreased in a controlled manner by an increase in arterial CO2 tension, or by a decrease in cerebral perfusion pressure (CPP) in autoregulating animals. Twelve New Zealand white rabbits were anesthetized, paralyzed, and ventilated. Transcranial Doppler basilar artery FV, laser Doppler cortical blood flow, arterial pressure, intracranial pressure, and end-tidal CO2 concentration were measured continuously. Cerebrovascular resistance (CPP divided by laser Doppler cortical flux) and Gosling Pulsatility Index (PI, defined as an FV pulse amplitude divided by a timed average FV) were calculated as time-dependent variables for each animal. Four groups of animals undergoing controlled manipulations of CVR were analyzed. In Group I, arterial CO2 concentration was changed gradually from hypocapnia to hypercapnia. In Group II, gradual hemorrhagic hypotension was used to reduce CPP. In Group III, the short-acting ganglion blocking drug trimetaphan was injected intravenously to induce transient hypotension. Intracranial hypertension was produced by subarachnoid saline infusion in Group IV. During the hypercapnic challenge the correlation between the cortical resistance and Doppler flow pulsatility was positive (r = 0.77, p<0.001). In all three groups in which cerebral perfusion pressure was reduced a negative correlation between pulsatility index and cerebrovascular resistance was found (r = -0.84, p<0.001). The authors conclude that PI cannot be interpreted simply as an index of CVR in all circumstances.

Cerebral haemodynamic considerations in obstructive carotid artery disease

46 subjects with obstructive carotid artery disease were investigated with transcranial Doppler ultrasonography. Their baseline blood velocities (V) in the middle, anterior and posterior cerebral artery (MCA, ACA and PCA) and in the extracranial internal carotid artery (ICA) were measured and the pulsatility index (PI) calculated for each vessel. Thereafter the vasomotor reserve in both MCAs was tested. Typical patterns of V, PI and vasomotor reactivity are presented. Arterial collaterals were recognized by their relatively increased velocities. We demonstrated a close association of the baseline variables V and PI and the total vasomotor reactivity (hypocapnic plus no, hypercapnic response) by calculating an index of Uhem related to the cerebrovascular tone. The Uhem index is expressed by: Uhem index = VMCA.PIMCA/VPCA.PIPCA The relationship between Uhem index and the total vasomotor reactivity seemed to correspond to a hyperbolic curve. The hyperbolic tangent of Uhem index and total vasomotor reactivity correlated highly significantly, r = 0.8203, p < 0.0001, n = 49, the best fit for the regression line was Y = -0.005 + Uhem index 51.3. On the 99% significance level an Uhem index > or = 0.94 indicated normal total cerebral vasomotor reactivity in contrast to an impaired reactivity when < or = 0.81. Findings in 20 patients investigated post hoc supported the validity of our concept.

Continuous monitoring of partial pressure of brain tissue oxygen in patients with severe head injury

Ischemia is one of the major factors causing secondary brain damage after severe head injury. We have investigated the value of continuous partial pressure of brain tissue oxygen (PbrO2) monitoring as a parameter for cerebral oxygenation in 22 patients with severe head injury (Glasgow Coma Scale score, < or = 8). Jugular bulb oxygenation, intracranial pressure, and cerebral perfusion pressure were simultaneously recorded. O2 and CO2 reactivity tests were performed daily to evaluate oxygen autoregulatory mechanisms. PbrO2 monitoring was started an average of 7.0 hours after trauma with a mean duration of 74.3 hours. No complications were seen, and the calibration of the catheters after measurement showed a zero drift of 1.2 +/- 0.8 mm Hg and a sensitivity drift of 9.7 +/- 5.3%. In 86% of patients, PbrO2 was < 20 mm Hg in the acute phase. Mean PbrO2 significantly increased during the first 24 hours after injury. Two distinct patterns of change of PbrO2 over time were noted. The first pattern was characterized by normal stable levels after 24 hours, and the second was characterized by transiently elevated levels of PbrO2 during the second and third days. PbrO2 values < or = 5 mm Hg within 24 hours after trauma negatively correlated with outcome. O2 reactivity was significantly lower in patients with good outcomes. CO2 reactivity showed no constant pattern of change over time and was not correlated with outcome. Increased hyperventilation was shown to decrease PbrO2 in some patients. Accurate detection of the moment of cerebral death was possible on the basis of the PbrO2 measurements. The correlation between PbrO2 and other parameters, such as intracranial pressure and cerebral perfusion pressure, was weak. We conclude that PbrO2 monitoring is a safe and clinically applicable method in patients with severe head injury. The early occurrence of ischemia after head injury can be monitored on a continuous basis. Deficiency of oxygen autoregulatory mechanisms can be demonstrated, and their occurrence is inversely related to outcome. For practical clinical use, the method seemed to be superior to jugular oximetry.

Transcranial Doppler ultrasound assessment of intracranial hemodynamics in children with diabetic ketoacidosis

The pathophysiology of acute neurological complications of diabetic ketoacidosis (DKA) in children and adolescents is not completely understood. We sought to establish whether transcranial Doppler (TCD) was able to monitor the changes of cerebral blood flow regulatory mechanisms, as measured by cerebral blood velocities (CBF-V), Gosling's pulsatility index (PI), and cerebral vascular reactivity (VR), prior to and during treatment of DKA. The increased values of PI suggested an increase of intracranial pressure (ICP) due to the existence of cerebral vasoparalysis, based on the low values of VR prior to treatment and 6 hours after initiation of treatment. At 24 hours, the correction of hematocrit and pH was associated with a significant decrease of PI, suggesting a decrease of ICP, likely due to a return of vascular tone in response to the low PaCO2. This was further supported by an increase of VR in all patients. At 48 hours, when PaCO2 returned to normal, the PI remained low and the VR increased further, suggesting a complete reversal of vasoparalysis and a return of cerebral blood flow regulatory mechanisms.

Transcranial Doppler analysis of cerebral hemodynamics in primary craniosynostosis: study in progress

BACKGROUND

Regional elevations in intracranial pressure (ICP) have been monitored adjacent to areas of cranial stenosis by some authors [9,19,20]; however, noninvasive techniques such as transcranial Doppler (TCD) would be preferable if increased ICP could be detected just as reliably.

METHODS

TCD examinations of basal cerebral arteries were performed in 31 children with skull deformities. Skull X rays and computed tomography (CT) scans or magnetic resonance imaging (MRI) were analyzed for degree of cranial vault abnormality, as well as for presence or absence of intracranial pathology. Neurodevelopmental examination and clinical signs of increased ICP, if any, were recorded.

RESULTS

The overall incidence of increased resistivity indices (RI) was 70%. Patients with asymmetric skull deformities (plagiocephaly) or sagittal synostosis (scaphocephaly) were statistically more prone to exhibit increased RI compared to previously reported prevalences.

CONCLUSIONS

Because TCD is a convenient, rapid, accurate, and noninvasive method by which to gauge ICP, it should be considered in the preliminary work-up of a patient with craniosynostosis.

Cerebrovascular CO2 reactivity in migraine with aura and without aura. A transcranial Doppler study

INTRODUCTION

We studied by means of Transcranial Doppler (TCD) recordings the CO2 cerebrovascular reactivity in migraine patients during the headache-free period.

MATERIAL & METHODS

In three groups of subjects (15 controls, 15 suffering from migraine with aura and 15 from migraine without aura) the middle cerebral artery (MCA) mean flow velocity (MFV) was recorded under basal condition and hypocapnia induced by hyperventilation. Relative MFV, PI (Pulsatility Index) changes and Reactivity Index (RI) were calculated.

RESULTS

Reactivity Index values were: 0.019 +/- 0.007 (mean +/- SD) in control subjects: 0.029 +/- 0.008 in migraine with aura; 0.022 +/- 0.008 in migraine without aura. Statistical analysis showed a significantly (P < 0.05) increased RI in migraine with aura group.

CONCLUSION

Cerebrosvascular CO2 reactivity is increased during the interictal period in migraine with aura patients.

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.

Increased cerebrovascular pCO2 reactivity in migraine with aura--a transcranial Doppler study during hyperventilation

Cerebrovascular reactivity during hypocapnia was tested in 20 migraineurs (8 with aura, 12 without aura) and 30 sex- and age-matched healthy subjects, and during nitroglycerin-induced headache in 12 healthy subjects. Before and during hyperventilation, mean blood-flow velocity (Vmean) in the middle cerebral artery was measured with transcranial Doppler. In each subject a pCO2 reactivity index (RI) was calculated as (delta Vmean/baseline Vmean)/delta pCO2. Interictally, patients with migraine with aura showed higher RI (p < 0.05 ANOVA and multiple range test) than controls, whereas migraineurs without aura did not differ from healthy subjects. Ictal and interictal RIs were similar in 9 patients suffering from migraine without aura. No side-to-side differences were detected in RI. During nitroglycerin-induced headache, the RIs were no different from those recorded during migraine attacks and in non-nitroglycerin-provoked healthy controls (p > 0.05, ANOVA and multiple range test). The exaggerated response in migraine with aura might predispose for the characteristic changes in rCBF seen during attacks.

The effect of acute hypocapnia on middle cerebral artery transcranial Doppler velocity during orthotopic liver transplantation: changes at reperfusion

This study examines the effects of acute hypocapnia, instituted prior to reperfusion of the graft liver, on the middle cerebral artery (MCA) Doppler blood flow velocity response to reperfusion during orthotopic liver transplantation in humans. Seventeen patients with chronic liver disease underwent continuous, noninvasive Doppler imaging of the MCA. Hyperventilation to an end-tidal Pco2 of 25 +/- 1 mm Hg was associated with a decrease in mean MCA flow velocity (FVm) from 51.6 +/- 5.7 to 37.0 +/- 3.3 cm/s (P < 0.05). After reperfusion, the Paco2 increased from 32 +/- 1 to 40 +/- 1 mm Hg (P < 0.05), mean arterial pressure (MAP) decreased from 76 +/- 3 to 60 +/- 2 mm Hg, and the FVm increased from 37.0 +/- 3.3 to 54.0 +/- 4.7 cm/s (P < 0.05). FVm increased postreperfusion despite prior hyperventilation, decreased MAP, and abrupt increases in central venous and pulmonary artery pressure, but FVm did not exceed the prereperfusion level. In 10 of the 17 patients, the baseline FVm versus Paco2 response slopes and Paco2 measured postreperfusion were used to predict the FVm response to Paco2 after reperfusion. The slopes were similar to those reported for anesthetized patients without liver disease. Predicted FVm exceeded measured FVm in 9 of the 10 patients. We conclude that mild hyperventilation prior to reperfusion of the graft liver prevents FVm increases above prereperfusion baseline level.

Cerebral blood flow velocities are reduced during attacks of unilateral migraine without aura

It has been disputed whether or not large intracranial arteries are dilated during migraine attacks. In order to answer this question the present transcranial Doppler study focused on side-to-side differences of middle cerebral artery blood velocity during unilateral attacks of migraine without aura in 25 patients. Blood velocity in the middle cerebral artery was lower on the headache side (59 cm/s) than on the non-headache side (65 cm/s) during the migraine attack. No such difference was found outside of attack (65 cm/s both sides). The difference (headache side minus non-headache side) was on average -6.1 cm/s during attack compared to -0.4 cm/s outside of attack (p = 0.01). Assuming that rCBF is unchanged during attacks of migraine without aura, our results suggest a 9% increase in middle cerebral artery lumen (cross-sectional area) on the affected side during unilateral attacks of migraine without aura. The findings, however, do not necessarily mean that arterial dilatation is the only or even the most significant cause of pain.

Carotid diameter and blood flow velocities in cerebral circulation in hypertensive patients

BACKGROUND AND PURPOSE

The recent development of noninvasive techniques for the evaluation of the carotid arteries has focused attention on the study of arterial wall thickness to identify early lesions of vessels in patients at high risk for atherosclerosis, such as those with hypercholesterolemia, diabetes mellitus, and hypertension.

METHODS

In a sample of 70 hypertensive patients without clinical evidence of target organ damage, we showed a thickening of the intimal plus medial layers compared with age- and sex-matched normotensive control subjects. In this sample we also studied the diameter of the carotid arteries by ultrasound imaging, and we studied flow velocities in common carotid, internal carotid, and middle cerebral arteries by Doppler technique. Pulsatility and resistance indexes were calculated.

RESULTS

Absolute values of the carotid diameter were similar in the two groups (6.3 +/- 0.7 versus 6.0 +/- 0.8 mm); however, the ratio of diameter to blood pressure was significantly reduced in hypertensive compared with normotensive subjects (5.3 +/- 0.7 versus 6.5 +/- 0.8; P < .001 for mean blood pressure). Parietal stress was increased in the hypertensive subgroup and significantly correlated with arterial diameter in the normotensive group but not in the hypertensive group. No significant differences between the two groups were observed in blood flow velocities, with the exception of a slight significant increase of mean velocity in the internal carotid artery in hypertensive patients (37.5 +/- 9.1 versus 32.7 +/- 3.0 cm/s; P < .02).

CONCLUSIONS

These results indicate that in addition to the degenerative changes of the common carotid wall, the diameter of the carotid artery and the relation to parietal stress show an early impairment in patients with uncomplicated hypertension.

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.

Cerebral perfusion during human liver transplantation

During transplantation of the liver cerebral perfusion was monitored by transcranial Doppler determined middle cerebral artery mean flow velocity (Vmean) and pulsatility index (PI) in six fulminant hepatic failure patients and 11 patients with chronic liver disease. In both groups of patients Vmean, PI and central haemodynamic variables were recorded during (1) the last preanhepatic hour; (2) the anhepatic phase; (3) the first 15 min of reperfusion; and (4) for the following 45 min of reperfusion. No significant differences were detected between the two groups of patients with respect to changes of variables with time. The Vmean (40 +/- 13 cm s-1 [mean +/- SD]), thoracic electrical impedance (TI) (30 +/- 7 Ohm), heart rate (97 +/- 19 beats min-1), mean arterial pressure (84 +/- 9 mmHg) and arterial carbon dioxide tension (PaCO2, 4.5 +/- 0.4 kPa) remained stable in the anhepatic phase, while cardiac output (CO, 7.6 +/- 2.7 to 5.4 +/- 1.41 min-1), stroke volume (SV, 79 +/- 26 to 56 +/- 15 ml) and PI (1.2 +/- 0.3 to 0.9 +/- 0.2) decreased (P < 0.05). During reperfusion, CO (9.9 +/- 4.01 min-1), SV (105 +/- 40 ml), PaCO2 (5.5 +/- 0.6 kPa), Vmean (57 +/- 17 cm s-1) and PI (1.2 +/- 0.2) became elevated. Taken together, during the anhepatic phase of the liver transplantation a maintained central blood volume as indicated by the constant TI served for a stable blood pressure and in turn cerebral perfusion, whereas revascularization of the graft increased cerebral perfusion concomitant with an elevated carbon dioxide tension.

Cerebrovascular carbon dioxide reactivity assessed by intracranial pressure dynamics in severely head injured patients

Appropriate management of intracranial pressure (ICP) in severely head injured patients depends in part on the cerebral vessel reactivity to PCO2; loss of CO2 reactivity has been associated with poor outcome. This study describes a new method for evaluating vascular reactivity in head-injured patients by determining the sensitivity of ICP change to alterations in PCO2. This method was combined with measurements of the pressure volume index (PVI), which allowed calculation of blood volume change necessary to alter ICP. The objective of this study was to investigate the ICP response and the blood volume change corresponding to alterations in PCO2 and to examine the correlation of responsivity and outcome as measured on the Glasgow Outcome Scale. The PVI and ICP at different end-tidal PCO2 levels produced by mild hypo- and hyperventilation were obtained in 49 patients with Glasgow Coma Scale scores of less than 8 and over a wide range of PCO2 (25 to 40 mm Hg) in eight patients. Given the assumption that the PVI remained constant during alteration of PaCO2, the estimated blood volume change per torr change of PCO2 was calculated by the following equation: BVR = PVI x delta log(ICP)/delta PCO2, where BVR = blood volume reactivity. The data in this study showed that PVI remained stable with changes in PCO2, thus validating the assumption used in the blood volume estimates. Moreover, the response of ICP to PCO2 alterations followed an exponential curve that could be described in terms of the responsivity indices to capnic stimuli. It was found that responsivity to hypocapnia was reduced by 50% compared to responsivity to hypercapnia measured within 24 hours of injury (p < 0.01). The sensitivity of ICP to estimated blood volume changes in patients with a PVI of less than 15 ml was extremely high with only 4 ml of blood required to raise ICP by 10 mm Hg. The authors conclude from these data that, following traumatic injury, the resistance vessels are in a state of persistent vasoconstriction, possibly due to vasospasm or compression. Furthermore, BVR correlates with outcome on the Glasgow Coma Scale, indicating that assessment of cerebrovascular response within the first 24 hours of injury may be of prognostic value.

Monitoring intracranial dynamics by transcranial Doppler--a new Doppler index: trans systolic time

Since the introduction of transcranial Doppler sonography (TCD) several investigators have described the relationship between raised intracranial pressure (ICP) and Doppler waveform. This waveform has been expressed by several indices, such as the pulsatility index (PI) and the resistance index (RI). These indices are used to demonstrate the presence of raised ICP. In childhood hydrocephalus this information can be used to indicate the need for shunt implantation. However, PI and RI do prove to have certain disadvantages as both are strongly influenced by the heart rate. Moreover, both indices have a broad range of reference values, especially in children. Therefore, they are not very reliable for detecting insidious changes in the ICP. These drawbacks are due to the fact that these indices are composed of blood flow velocity measurements and do not embody the slope of the TCD waveform itself. An ideal TCD waveform analysis should be performed concerning the time-related changes of the velocities. We present a hydrodynamic model, with its electrical analogue, which shows the effects of raised ICP on the intracranial hemodynamic system. Based on these physical findings we define a new Doppler index, the Trans Systolic Time, reflecting specific changes in the TCD waveform induced by changes in the mean ICP. The applicability of this index, compared with PI and RI, is illustrated by consecutive simultaneous TCD and AFP measurements in three children with hydrocephalus.

Measurement of blood flow velocity in the basal cerebral circulation: advantages of transcranial color-coded sonography over conventional transcranial Doppler

Unlike conventional transcranial Doppler (TCD), transcranial color-coded sonography (TCCS) enables imaging of the basal cerebral arteries using color-flow ultrasonography and correction for the angle of insonation when determining blood flow velocities. We present hemodynamic data from 20 normal subjects, each studied with TCD and TCCS. Velocities derived using TCCS with angle correction were significantly greater than those derived using TCD in all vessels (mean velocities [cm/sec; mean with 95% confidence intervals)--anterior cerebral artery: TCD 48 (45-50), TCCS 62 (58-66), p < .0001; middle cerebral artery: TCD 61 (58-64), TCCS 70 (66-74), p < .0001; posterior cerebral artery: TCD 43 (41-46), TCCS 54 (50-57), p < .0001; basilar artery: TCD 40 (34-45), TCCS 45 (38-52), p < .01. Pulsatility index values were significantly greater in all arteries when determined by TCCS, and resistance index values were significantly greater except in the basilar artery. Correcting for the angle of insonation using TCCS may enable estimation of blood flow velocities closer to the "true" values than those derived using conventional TCD.

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.

Middle cerebral artery velocity changes during transfusion in sickle cell anemia

BACKGROUND AND PURPOSE

Sickle cell disease is associated with cerebral hyperemia, which is therapeutically reduced by transfusion; however, the process of transfusion-induced cerebral perfusion changes has heretofore not been observed.

METHODS

We document the acute changes of intracranial arterial velocity in 10 patients (7 with strokes, 3 without) undergoing transfusion therapy using transcranial Doppler ultrasonography. Middle cerebral artery velocities were bilaterally measured every 30 minutes for the duration of transfusion (4 to 5 hours). Regional cerebral blood flow was quantified in 5 of these patients before the transfusion and 24 hours later by the 133Xe technique.

RESULTS

Velocities in stroke-associated vessels (64.33 +/- 18.65 cm/s; n = 6) were significantly lower than in uninfarcted territories (99.54 +/- 27.39 cm/s; n = 13), and both types of vessels showed a robust reduction of blood flow velocities during transfusion. The rates of reduction were not significantly different as a function of prior stroke but did correlate with pretransfusion velocities and with the rise in hematocrit (multiple r = .887, P < .001). These reductions occurred rapidly within the first 3 hours of transfusion. Velocities attained at the end of transfusion were maintained in the hour after transfusion and the next day.

CONCLUSIONS

We conclude that transfusion induces rapid changes in cerebral hemodynamics that are related to pretransfusion velocities and a rise in hematocrit. Transcranial Doppler provides a safe, simple, and noninvasive technique of monitoring these changes and may provide a means of making therapeutic decisions regarding transfusion therapy in patients with sickle cell anemia.

Cerebral autoregulation in orthostatic hypotension. A transcranial Doppler study

BACKGROUND AND PURPOSE

Transcranial Doppler measurements of blood flow velocity permit an assessment of variations in intracranial hemodynamics in response to acute arterial pressure variations. The purpose of this study was to scan healthy volunteers and patients with autonomic failure for differences in cerebral hemodynamic patterns under an acute hypotensive stimulus.

METHODS

We used transcranial Doppler monitoring of blood flow velocity in the middle cerebral artery and noninvasive monitoring of arterial blood pressure and heart rate before, during, and after acute arterial hypotension induced by reactive hyperemia of the lower limbs.

RESULTS

After maximum hypotension, the mean blood flow velocity was higher in the healthy volunteers than in the patients. In the healthy subjects mean velocity rose significantly (P < .01) higher than arterial blood pressure after 30 seconds and 60 seconds; in the patients mean velocity and arterial pressure moved in parallel fashion. The diastolic blood flow velocity increased more in the control group than in the patients during the early stages of the test; furthermore, only in the healthy volunteers did it increase significantly more than arterial pressure after 30 seconds and 60 seconds. Regarding the pulsatility index, the differences between the two groups were similar to the diastolic velocity results.

CONCLUSIONS

(1) Monitoring of mean blood flow velocity showed the ability to maintain an adequate cerebral blood flow in healthy subjects; this mechanism was not efficient in the patients with autonomic failure. (2) Diastolic velocity and pulsatility index values clearly showed that only in healthy subjects were cerebral hemodynamics relatively independent of pressure values.

Evaluation of cerebral vasoreactivity by three-dimensional time-of-flight magnetic resonance angiography

BACKGROUND AND PURPOSE

Cerebral vasoreactivity is an important indicator of the reserve capacity of the cerebral circulation. To make a quantitative analysis of cerebral vasoreactivity in individual major arterial territories, we evaluated the response to acetazolamide using three-dimensional time-of-flight magnetic resonance angiography.

METHODS

We examined 10 healthy volunteers and 6 patients with unilateral stenosis of the middle cerebral artery by a 1.5-T superconducting magnetic resonance imaging system. After a baseline vascular image was obtained, each subject received 17 mg/kg IV of acetazolamide; a second scan was performed 20 minutes later. Using a generally available personal computer and image analysis software, we measured the areas of the individual major arteries on collapsed axial vascular images and then calculated the vasoreactivity.

RESULTS

The average vasoreactivity of individual major cerebral arterial territories in the healthy volunteers was as follows: anterior cerebral artery complex, 33%; right middle cerebral artery, 71%; left middle cerebral artery, 74%; right posterior cerebral artery, 68%; and left posterior cerebral artery, 68%. In the patient group, the vasoreactivity of the stenotic middle cerebral arteries was significantly smaller than that of the nonstenotic arteries (P < .05). In addition, the nonstenotic middle cerebral arteries showed significantly less vasoreactivity than the right arteries of the healthy volunteers (P < .01).

CONCLUSIONS

Three-dimensional time-of-flight magnetic resonance angiography can be used to quantitatively evaluate acetazolamide-induced vasoreactivity in individual major cerebral arterial territories.

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.

CO2 measurements during transcranial Doppler examinations in headache patients: methodological considerations

Transcranial Doppler (TCD) examinations are increasingly being used in studies of headache pathophysiology. Because blood velocity is highly dependent on PCO2, these parameters should be measured simultaneously. The most common way of performing measurements during TCD examinations is as end-tidal pCO2 with a capnograph. When patients are nauseated and vomit, as in migraine, the mask or mouthpiece connected to the capnograph represents a problem. We therefore evaluated whether a transcutaneous pCO2 electrode was as useful as the capnograph for pCO2 measurements in TCD examinations. We conclude that this is not the case, and recommend capnographic end-tidal pCO2 measurements during TCD examinations. However, transcutaneous pCO2 measurements may represent a supplement to spot measurements of end-tidal pCO2 in stable conditions when long-term monitoring is needed, and the mask or mouthpiece of the capnograph has to be taken on and off between recordings.

Temporal heterogeneity of the blood flow velocity at the middle cerebral artery in the normal human characterized by fractal analysis

The objective of this study is to characterize the temporal fluctuation of the axial blood flow velocity (BFV) at the middle cerebral artery (MCA). Biological observables such as BFV present complex oscillations. The irregularity of physiological systems may be assessed by fractal analysis by computing the fractal dimension (D gamma) and the corresponding temporal correlation (r gamma). The BFV at the MCA was registered with transcranial Doppler ultrasonography (TCD) in four adult volunteers. As fractal processes are assumed to have no absolute time scale, two time scales were compared. The digitized signal was averaged respectively at 1-s intervals and for each heart beat. D gamma and r gamma were determined using relative dispersion analysis. The results were D gamma = 1.24 +/- 0.09 and r gamma = 0.45 +/- 0.19 (mean +/- SD) for the 1-s based time scale and D(r) = 1.17 +/- 0.09 and r gamma = 0.57 +/- 0.20 for the heart-beat scale. We conclude that the temporal heterogeneity of the BFV at the MCA in the normal human has fractal properties. Fractal analysis of TCD data may become useful in clinical diagnosis because loss of complexity in physiological systems has been linked to senescence or disease conditions. Wide variations of the so called normal values of BFV measured by TCD have been reported. The physiological BFV fluctuations may explain, in part, the variability of values recorded during routine TCD diagnostic examinations. Our observations may also be of value for understanding the interaction of the vascular endothelium and the blood flow stream (shear stress).

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).

CO2 reactivity in arteriovenous malformations of the brain: a transcranial Doppler ultrasound study

Arteriovenous malformations (AVM's) are congenital tangles of vessels that have a high blood flow through a low-resistance nidus. The vessels in the nidus may lack normal vasoreactivity in response to changes in PaCO2 or perfusion pressure (autoregulation). Arteriovenous malformation hemodynamics have been assessed based on the response of AVM feeding arteries to hypocapnia. Twenty-five AVM patients, aged 34 +/- 11 years (mean +/- standard deviation), were admitted to the Massachusetts General Hospital for proton-beam radiation therapy. Fourteen healthy volunteers aged 30 +/- 7 years served as control subjects. Angiograms with calibrated markers permitting magnification correction were available for all patients. The limits of the middle cerebral artery, as determined by transcranial Doppler ultrasonography, were compared to measurements made on the angiograms. Hyperventilation was induced at a rate set by a metronome. Fixed bilateral Doppler probes allowed almost simultaneous sampling of two vessels. Volunteer control subjects were hyperventilated in two steps. The two PaCO2 step decreases were significant (mean resting PaCO2 40.6 +/- 3.5 mm Hg, Step 1 level 29.4 +/- 3.5 mm Hg and Step 2 level 23.8 +/- 3.5 mm Hg; p < 0.01). These decreases induced a significant decrease in mean flow velocity (Vm) and an increase in the pulsatility index (p < 0.001). Mean carbon dioxide reactivity (% delta Vm/delta PaCO2) was 2.74 +/- 1.0 for Step 1 and 1.44 +/- 1.8 for Step 2 (p < 0.003). The mean PaCO2 decrease in patients was from 39.5 +/- 4.0 mm Hg to 27.0 +/- 3.5 mm Hg. Carbon dioxide reactivity was 0.92 +/- 1.12 for feeding vessels and 2.59 +/- 1.78 for nonfeeding vessels (p < 0.001). Transcranial Doppler ultrasound and angiographic depth measurements correlated well. Hyperventilation induced significantly more hemodynamic changes in control and nonfeeding middle cerebral arteries than in feeding vessels. Impaired CO2 reactivity may help to identify AVM feeding vessels as well as the relative magnitude of the flow provided to the malformation.

Awake intubation increases intracranial pressure without affecting cerebral blood flow velocity in infants

Tracheal intubation is frequently required in neonatal anaesthetic practice. Awake intubation is one method of securing the airway and in certain circumstances, for many anaesthetists, can be preferable to intubation following induction of anaesthesia. Previous studies have inferred that the elevation in anterior fontanelle pressure observed during tracheal intubation in neonates was caused by an increase in cerebral blood flow although it was never measured. In this study, direct methods were used to observe changes in the cerebral circulation. Thirteen neonates, ASA I to III (E), aged from 1 to 34 days of age were studied. Patients were randomized to receive either tracheal intubation awake or following induction of anaesthesia with thiopentone 5 mg.kg-1 and succinylcholine 2 mg.kg-1. Heart rate, systolic arterial blood pressure, anterior fontanelle pressure, cerebral blood flow velocity (using transcranial Doppler sonography) and oxygen saturation were recorded at the following intervals: baseline (not crying), after intravenous atropine 0.02 mg.kg-1, during laryngoscopy, immediately after insertion of the endotracheal tube, one and five minutes later. The use of atropine masked the cardiovascular responses to intubation. Whereas the change in anterior fontanelle pressure from baseline was different between the groups (P < 0.05), the cerebral blood flow velocity variables were not. The rise in anterior fontanelle pressure seen in the awake group may be attributed to a reduction of the venous outflow from the cranium thereby increasing cerebral blood volume and subsequently the intracranial pressure.

Changes in blood flow velocity in the middle cerebral artery during nonpulsatile hypothermic cardiopulmonary bypass

BACKGROUND AND PURPOSE

We evaluated the utility of blood flow velocity measurements by transcranial Doppler ultrasonography as a tool to indirectly measure cerebral perfusion during cardiopulmonary bypass.

METHODS

We simultaneously measured blood flow velocity in the middle cerebral artery and physiological variables in 18 patients undergoing cardiac surgery under hypothermic cardiopulmonary bypass in which pH and PaCO2 were managed with the alpha-stat acid-base strategy. We expressed blood flow velocity as a relative value of control obtained under normothermia and normocarbia before bypass. We also developed an original index, modified cerebral metabolic rate for oxygen, to estimate cerebral metabolic rate for oxygen.

RESULTS

Relative velocity was significantly (P < .01) reduced during stable aortic cross-clamp compared with before bypass and was significantly (P < .01) increased during rewarming compared with at aortic cross-clamp. Modified cerebral metabolic rate for oxygen significantly correlated with nasopharyngeal temperature during cooling, aortic cross-clamp, and rewarming (r = .756, P < .0001; r = .4, P < .01; r = .725, P < .0005, respectively). Calculated temperature coefficient for modified cerebral metabolic rate of oxygen was 2.7 +/- 1.4 (mean +/- SD, n = 10) during cooling. Only nasopharyngeal temperature and PaCO2 were significant determinants of relative velocity during aortic cross-clamp.

CONCLUSIONS

We can monitor cerebral perfusion and metabolism by measurements of relative velocity and modified cerebral metabolic rate for oxygen during hypothermic cardiopulmonary bypass.

Effects of hemodialysis on cerebral circulation evaluated by transcranial Doppler ultrasonography

BACKGROUND AND PURPOSE

The effects of hemodialysis on the cerebral circulation of humans and the correlation between changes in blood flow velocity in the basal cerebral arteries and those of several physiological variables influenced by hemodialysis have been inadequately studied.

METHODS

Blood flow velocities were obtained from the middle cerebral artery and the basilar artery by transcranial Doppler ultrasonography in 27 patients receiving chronic maintenance hemodialysis immediately before and after the procedure. Changes in body weight, hematocrit, blood pressure, and arterial blood gases were recorded simultaneously.

RESULTS

There was a significant reduction in mean flow velocity in the middle cerebral artery (P < .01) and the basilar artery (P < .01) after hemodialysis. We observed a significant negative correlation between the relative change in mean flow velocity and the loss of weight after hemodialysis, the amount of fluid removed, and the increase in hematocrit in the middle cerebral artery and the basilar artery.

CONCLUSIONS

Hemodialysis and the associated physiological changes can significantly affect the cerebral circulation. Blood flow velocities in the middle cerebral artery and the basilar artery decrease significantly with hemodialysis. The loss of body weight, the amount of fluid removed, and the change in hematocrit significantly correlate with the change in mean flow velocity. The transcranial Doppler method can effectively monitor rapid changes in the cerebral circulation during potentially harmful procedures.

Factors influencing the hyperaemic response after carotid endarterectomy

Transcranial Doppler ultrasonography was used to evaluate serially the changes in middle cerebral artery blood flow velocity (MCAV) in 37 consecutive patients during the first 72 h after carotid endarterectomy to identify factors that may predispose towards postoperative hyperaemia. Within 6 h of endarterectomy, median MCAV in the operated hemisphere was 48 per cent (95 per cent confidence interval 37-60 per cent) above that on admission and remained 27 per cent (95 per cent confidence interval 19-37 per cent) higher at 72 h. There was a similar, but less marked, increase in MCAV in the contralateral middle cerebral artery during the same time period. There was no association between the postoperative increase in MCAV and clinical presentation, admission MCAV, the presence or absence of a residual neurological deficit or infarction on computed tomography before operation, carotid clamp time, shunt usage, internal carotid artery stump pressure or MCAV during clamping. The greatest increase in MCAV was observed in patients with internal carotid artery stenosis > or = 50 per cent and, more particularly, in those with preoperative evidence of impaired cerebrovascular reserve. In the latter patients, MCAV was 100 per cent above the admission level within 12 h of operation and was still 50 per cent raised at 72 h.