The course of dynamic cerebral autoregulation during cervical internal carotid artery occlusion

Abstract

Objectives: The selection of patients with cervical internal carotid artery occlusion (ICAO) for extracranial-intracranial bypass surgery is based on exhausted cerebrovascular reactivity to vasodilatory stimuli. However, a spontaneous increase in this reactivity can occur with time, questioning the ideal time for bypass surgery. In contrast, the natural course of dynamic cerebral autoregulation is not known in these patients.

Methods: Patients with cervical ICAO were examined at baseline and after a mean interval of 15 months. Dynamic autoregulation was determined by transcranial Doppler sonography in both middle cerebral arteries via respiratory-induced 0·1-Hz oscillations (phase, available for n=47 patients) and correlation analysis between diastolic blood pressure and Doppler signal (index Dx, n=55 patients). Pre-defined cut-off values and repeatability measures of healthy controls were used to define significant individual changes in autoregulation.

Results: Group mean comparisons between studies were not significant for any autoregulation parameter. The intraclass correlation coefficient between studies was high for phase (ipsilateral: 0·83; contralateral: 0·74), and moderate for Dx (ipsilateral: 0·63; contralateral: 0·35). There was no clear trend for an improvement across cut-off values. A significant individual improvement/deterioration in autoregulation occurred in 6%/6% for phase and 13%/9% for Dx.

Discussion: Dynamic autoregulation only rarely improves during the course of ICAO. This finding should be considered when deciding for or against a policy of delaying extracranial-intracranial bypass surgery for reasons of a potentially improving hemodynamic status.     

Transcranial Doppler for evaluation of cerebral autoregulation

Transcranial Doppler ultrasound (TCD) can measure cerebral blood flow velocity in the main intracranial vessels non-invasively and with high accuracy. Combined with the availability of non-invasive devices for continuous measurement of arterial blood pressure, the relatively low cost, ease-of-use, and excellent temporal resolution of TCD have stimulated the development of new techniques to assess cerebral autoregulation in the laboratory or bedside using a dynamic approach, instead of the more classical ‘static’ method. Clinical applications have shown consistent results in certain conditions such as severe head injury and carotid artery disease. Studies in syncopal patients revealed a more complex pattern due to aetiological non-homogeneity and methodological limitations mainly due to inadequate sample-size. Different analytical models to quantify autoregulatory performance have also contributed to the diversity of results in the literature. The review concludes with specific recommendations for areas where further validation and research are needed to improve the reliability and usefulness of TCD in clinical practice.     

Use of middle cerebral velocity and blood pressure for the analysis of cerebral autoregulation at various frequencies: The coherence index

Abstract

A common component in many protocols for the evaluation ofcerebral autoregulation is the comparison of transcranial Doppler ultrasound (TCD) velocities with blood pressure recordings, in which correlations between these two signals correspond to impaired autoregulation. With long data sets and complicated paradigms, however, visual inspection alone cannot adequately distinguish random coincidence from consistent correlation in a statistically valid fashion. We suggest and illustrate the use. of the coherence index for this purpose. To illustrate this technique, long-term recordings of TCD velocity and blood pressure were obtained from 6 normal subjects and using 23 data segments from 8 patients following subarachnoid hemorrhage. Each signal was first normalized to its mean, and coherence calculated by dividing the data into qverlapping subintervals and computing an average. Coherence was specifically examined over time periods of 30 sec. Coherence calculations identified correlations between signals for which interpretation by visual inspection was unclear, and obvious correlations could be quantified. In 4 of the 6 normal subjects, the coherence was less than 0.60 but slightly greater than o. Five of the 8 patients showed segments with coherence of greater than 0.60. The coherence index provides a quantitative toot for the evaluation of comparisons between two complex signals. As this .task becomes more common in the evaluation of cerebral autoregulation, algorithms of this sort will become increasingly necessary. [Neural Res 1997; 19: 634-640]     

Red cell velocity and autoregulation in the cerebral cortex of the cat

This report deals with a comparison of total and regional cerebral blood flow with red cell velocity in the microcirculation of the ecto- and suprasylvian areas of the cerebral cortex of the cat and their response to changes in systemic blood pressure. Total and regional blood flows were measured during a steady-state with radioactive microspheres; a newly devised method, based on transillumination was employed for direct visualization of the microcirculation in the cerebral cortex. Craniotomy failed to influence either autoregulation or red cell velocity under the skull opening or of the total cerebral flow. Autoregulation of total and regional cerebral flow was present. In contrast marked pressure-dependent variations in microcirculatory red cell velocity occurred. A fall in systemic pressure was accompanied by a decrease in red cell velocity; return of pressure resulted in increased red cell velocity and in some instances, in an overshoot. Considerable inhomogeneity of red cell velocity in different microcirculatory vessels was encountered. In some cases a fall in systemic pressure led to complete disappearance of red cells.     

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

Summary 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°, 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.     

Assessment of the thigh cuff technique for measurement of dynamic cerebral autoregulation

BACKGROUND AND PURPOSE: Dynamic methods of measuring cerebral autoregulation have become an accepted alternative to static evaluation. This article aims to describe a set of data collected from healthy volunteers by a dynamic method, the purpose being to qualify and quantify expected results for those who may be designing a study using this technique. METHODS: Cerebral blood flow velocity (CBFV) (measured by transcranial Doppler) and arterial blood pressure (Finapres) were recorded in 16 normal subjects before, during, and after the induction of a blood pressure drop (release of bilateral thigh cuffs). This procedure was repeated 6 times for each subject. A mathematical model was applied to the data to generate an autoregulatory index (ARI) with values between 0 and 9. RESULTS:The ARI values for this sample population follow a normal distribution, with a mean+/-SD of 4.98+/-1.06 (n=15). Analysis of the cumulative mean ARI values of all subjects showed an exponential-type convergence of ARI toward the sample mean as the number of test iterations increased. The population average blood pressure drop on thigh cuff release was 26.4+/-7.1 mm Hg (n=16), occurring in 4.6+/-1. 7 seconds. The corresponding population average drop for CBFV was 15. 6+/-5.8 cm/s, taking 2.5+/-1.0 seconds. No significant trend was noted in the measurements as the number of test iterations increased. The correlation between the predicted and actual CBFV, having a mean value of 0.76+/-0.19, showed evidence of a nonlinear relationship to ARI values. Significant correlation was also found between ARI and (1) arterial blood pressure before cuff release and (2) the magnitude of the drop in CBFV on cuff release. CONCLUSIONS: The distribution of ARI values is not significantly different from normal. At least 3 iterations of the test procedure should be performed and averaged to obtain the mean ARI for each subject. There is no significant evidence of physiological accommodation as the number of test iterations increases. The effects of mean blood pressure and the magnitude of the change in CBFV should be considered as possible covariates when ARI data are analyzed.     

Impaired dynamic cerebral autoregulation is associated with the severity of neuroimaging features of cerebral small vessel disease

AimsCerebral small vessel disease (CSVD) is characterized by functional and structural changes in small vessels. We aimed to elucidate the relationship between dynamic cerebral autoregulation (dCA) and neuroimaging characteristics of CSVD.MethodsA case‐control study was performed. Cerebral blood flow velocity (CBFV) of bilateral middle cerebral arteries and spontaneous arterial blood pressure were simultaneously recorded. Transfer function analysis was used to calculate dCA parameters (phase, gain, and the rate of recovery of CBFV [RoRc]). Neuroimaging characteristics of CSVD patients were evaluated, including lacunes, white matter hyperintensities (WMH), cerebral microbleeds (CMBs), perivascular spaces (PVS), and the total CSVD burden.ResultsOverall, 113 patients and 83 controls were enrolled. Compared with the control group, the phase at low frequency and the RoRc in CSVD patients were lower, and the gain at very low and low frequencies were higher, indicating bilaterally impaired dCA. Total CSVD burden, WMH (total, periventricular and deep), severe PVS, and lobar CMBs were independently correlated with the phase at low frequency.ConclusionsOur findings suggested that dCA was compromised in CSVD patients, and some specific neuroimaging characteristics (the total CSVD burden, WMH, severe PVS and lobar CMBs) might indicate more severe dCA impairment in CSVD patients.     

Diagnosis of middle cerebral artery spasm by determination of flow velocity and the Lindegaard index with transcranial color Doppler sonography

BACKGROUND AND PURPOSE: Recent reports have demonstrated the high utility of transcranial color-coded duplex sonography (TCCS) in the diagnosis of advanced spasm of the middle cerebral artery, whereas its accuracy in the diagnosis of mild vasospasm is reported to be lower. Relation of blood flow velocity in the middle cerebral artery (MCA) to that in the extracranial internal carotid artery (VMCA/VICA index) is recommended as being helpful in the diagnosis of vasospasm (the so called Lindegaard Index). Nevertheless, the exact diagnostic value of this index using the TCCS method remains to be established. The purpose of this study is to estimate the accuracy of TCCS in the diagnosis of MCA vasospasm, as based on the VMCA/VICA index. MATERIAL AND METHODS: The VMCA/VICA index was calculated in 195 patients (285 middle cerebral arteries) who were scheduled for cerebral arteriography. The TCCS study and color-coded duplex sonography of the internal carotid arteries were performed immediately before arteriography. RESULTS: A mild grade of MCA spasm was diagnosed angiographically in 21, and moderate-to-severe spasm in 29 MCAs out of 285 successfully insonated arteries. Peak-systolic, mean and end-diastolic blood velocities were measured using transcranial color sonography in the MCA and related to the respective velocities in the ipsilateral extracranial internal carotid artery. Receiver-operating characteristic curves (ROC) were calculated for particular velocities and for the related VMCA/VICA indices. By comparison of the areas under the ROC curves it was shown that the use of the VMCA/VICA index does not improve the accuracy of TCCS in the diagnosis of advanced MCA spasm, whereas it improves accuracy in the diagnosis of mild vasospasm. In particular, accuracy can be improved by the use of the VMCA/VICA index based on the end-diastolic velocity. The optimal diagnostic threshold of the VMCA/VICA index was determined at 3.9. CONCLUSIONS: The use of VMCA/VICA index improves the accuracy of TCCS in the diagnosis of mild vasospasm of the middle cerebral artery.     

Measurement of cerebral circulation time by contrast-enhanced Doppler sonography

We present a new, non-invasive ultrasound method for the measurement of cerebral circulation time. After injection of a galactose-based echo-contrast agent (Levovist) into an antecubital vein, cerebral circulation time is measured as the interval between the beginning of a stable signal enhancement of at least 5 dB in the internal carotid artery and in the ipsilateral internal jugular vein. Both vessels are insonated simultaneously at the mandibular angle using a single 2-MHz range-gated transducer. For this study, 25 healthy volunteers ranging in age from 23 to 55 years (30.1 +/- 7.5 years; mean +/- SD) were examined. Cerebral circulation time was 5.6 +/- 1.7 s without significant side-to-side or sex-related differences. Measurement of circulation times offers a new possibility for the employment of echo-contrast agents in functional ultrasound.     

Transcranial Doppler sonographic studies of cerebral blood flow velocity in neonates

Using the transcranial Doppler technique to investigate the postnatal and developmental changes of cerebral blood flow in newborn infants, we found that the systolic and mean blood flow velocities increased after birth. A rapid increase in blood flow velocities was demonstrated during the first several days. Pulsatility index, which was initially high, gradually fell after birth and remained stable thereafter. The blood flow velocities of small-for-date infants were determined by their gestational ages. The systolic and mean blood flow velocities were influenced more by postnatal age than by gestational age, birth weight, or body weight. The increase in conceptional age was associated with a decrease in the pulsatility index and an increase in the systolic and mean flow velocities. The transcranial Doppler technique is a useful method and will provide more accurate information about the cerebral hemodynamics in neonates than the previously employed continuous wave Doppler technique.     

Assessment of cerebral autoregulation dynamics from simultaneous arterial and venous transcranial Doppler recordings in humans

We investigated the validity of transcranial Doppler recordings for the analysis of dynamic responses of cerebral autoregulation. We found no significant differences in percentage changes among maximal (centerline) blood flow velocity, cross-sectional mean blood flow velocity, and signal power-estimated blood flow during 24-mm Hg stepwise changes in arterial blood pressure. We investigated blood flow propagation delays in the cerebral circulation with simultaneous Doppler recordings from the middle cerebral artery and the straight sinus. The time for a stepwise decrease in blood flow to propagate through the cerebral circulation was only 200 msec. Brief (1.37-second) carotid artery compression tests also demonstrated that the volume compliance effects of the cerebral vascular bed were small, only about 2.2% of normal blood flow in 1 second. Furthermore, transients associated with inertial and volume compliance died out after 108 msec. We also investigated the hypothesis that autoregulatory responses are influenced by hyperventilation using the same brief carotid artery compressions. One second after release, the flow index increased by 17% during normocapnia and 36% during hypocapnia. After 5 seconds, the flow index demonstrated a clear oscillatory response during hypocapnia that was not seen during normocapnia. These results suggest that the intact human cerebral circulation in the absence of pharmacological influences does not function as predicted from pial vessel observations in animals.     

Transcranial Doppler assessment of cerebral flow velocity during cognitive tasks.

BACKGROUND AND PURPOSE: The purpose of this study was to assess the ability of transcranial Doppler ultrasonography to detect selective circulatory changes during cognitive activity. METHODS: We measured cerebral artery flow velocity in 21 normal volunteers by transcranial Doppler ultrasonography during rest followed by cerebral activation. Mean and peak systolic flow velocities of the anterior, middle, and posterior cerebral arteries were measured during the performance of a commercial video game. We also measured flow velocity of the anterior cerebral arteries in 18 subjects during a mental arithmetic task. Serial measurements of the right and left sides were made with a headband with two probes. RESULTS: We observed a global increase in the flow velocity above baseline measurements during task performance. During the video game, both middle cerebral arteries (t = 2.6, p = 0.02 for the left; t = 3.3, p = 0.004 for the right) and the left posterior cerebral artery (t = 2.2, p = 0.004) had selective increase in mean flow velocity compared with the ipsilateral anterior cerebral artery. This selective activation was most prominent in the right middle cerebral artery, which had a greater degree of activation than the right posterior cerebral artery (t = 2.8, p = 0.013). We did not observe a statistically significant difference between the right and left middle cerebral arteries, but there was a trend toward a greater activation on the right for both the mean velocity (t = 1.7, p = 0.098) and the peak velocity (t = 1.9, p = 0.079). CONCLUSIONS: Our preliminary investigation suggests that this noninvasive technique has the potential to correlate selective cerebral artery flow dynamics with cognitive activity.     

Longitudinal determination of cerebral blood flow velocity in neonates with the Doppler technique

Using the Doppler technique, we followed the postnatal changes of the cerebral blood flow velocity longitudinally. Pulsatility index (PI) was measured in 91 neonates. In 26 normal term infants who were supine, PIs were initially raised (PI = 0.91 +/- 0.10), gradually falling over the first 12 hours after birth and remaining stable thereafter (PI = 0.78 +/- 0.05). PIs tended to be lower in neonates born by caesarean section but remained grossly within the normal range. Low birthweight infants showed high PIs during the first 12 hours after birth. Infants with a prone position showed a narrow standard deviation in PI soon after birth. There was a negative correlation between PI and the conceptional age and the coefficient of the correlation was 0.97156. The stage of active sleep, especially the REM period, was found to be an important factor which decreased the PI. We concluded that neonatal cerebral circulation significantly changes during the perinatal period and is affected by various physiological factors, especially sleep stages, the position of the infant and the conceptional age.