Decreased transcranial Doppler carbon dioxide reactivity is associated with disordered cerebral metabolism in patients with internal carotid artery stenosis

Asymptomatic Carotid Artery Stenosis: Past, Present and Future

BACKGROUND

The role of carotid endarterectomy (CEA) for asymptomatic carotid artery stenosis (aCAS) remains a matter of debate. It seems that not only the degree of stenosis, but also other factors have to be taken in account to improve patient selection and increase the benefit of CEA for aCAS.

METHODS AND RESULTS

The literature pertaining aCAS was reviewed in order to describe the natural history, risk of stroke and benefit of CEA for patients with aCAS in regard to several factors.

CONCLUSION

The benefit of CEA for aCAS is low. Current factors influencing the indication for CEA are severity of stenosis, age, contralateral disease, stenosis progression to >80%, gender, concomitant operations and life expectancy. To improve patient selection investigations will concentrate on plaque characteristics and instability and cerebral hemodynamics and metabolism.

Local Haemodynamic Changes During Carotid Endarterectomy—The Influence on Cerebral Oxygenation

OBJECTIVES

To characterize carotid bifurcation haemodynamics and cerebral oxygenation during clamping and at reperfusion after carotid endarterectomy (CEA).

MATERIALS AND METHODS

Sixty-two patients with a symptomatic high-grade stenosis of the internal carotid artery (ICA), who underwent CEA under general anaesthesia, were studied prospectively. Measurements of stump-pressure, volume flow (transit time flowmetry) and changes in cerebral oxygenation (near-infrared spectroscopy (NIRS)) were performed. Selective shunting was based on stump pressure only.

RESULTS

Stump pressure correlated with both ICA flow before clamping (r=0.45; p=0.03) and changes in cerebral oxygenation (rSO2) during clamping (r=0.61; p=0.002), the latter was reversed by shunt placement. ICA flow before clamping also correlated with changes in rSO2 during clamping (r=0.41; p=0.01).

CONCLUSION

Measurements with transit time flowmetry and cerebral oximetry are technically easy and help to determine the need for selective shunting during CEA. High ICA flow before clamping in combination with a low stump pressure usually indicates the need for a shunt. Volume flow measurements may also be useful in the quality assessment of the CEA.

Heterogeneity of cerebral blood flow in symptomatic patients undergoing carotid endarterectomy

There is still controversy concerning which patients with asymptomatic carotid stenosis or symptomatic moderate stenosis are likely to benefit from carotid endarterectomy. The surgical candidates for carotid endarterectomy should have a high risk for stroke, but a low risk for operative complications. Therefore, new effective patient selection strategies, including haemodynamic testing, schemes of risk stratification and pre-operative cardiac testing, are under investigation. To improve haemodynamic assessment of patients with carotid artery stenosis, we evaluated a novel global cerebral blood flow (CBF) heterogeneity index at rest and after acetazolamide injection in patients undergoing carotid endarterectomy. CBF heterogeneity index was measured in 15 patients by using basal and acetazolamide enhanced 99mTc-HMPAO SPET both before and 1 month after surgery. CBF heterogeneity index was calculated as the coefficient of variation of a total of 44 cerebral regions representing mainly both ipsi- and contralateral grey matter. A high linear correlation was observed between CBF heterogeneity index and ipsilateral carotid stenosis degree (r=0.74, P=0.003). Before surgery, CBF heterogeneity index increased significantly after acetazolamide injection when compared to the basal condition (from 7.0+/-1.5 to 8.3+/-1.7%, P=0.008). This response disappeared after carotid endarterectomy. When compared to pure asymmetry of CBF (ipsi/contralateral CBF ratio), the CBF heterogeneity index seemed to reflect, more sensitively, the haemodynamic effects of carotid endarterectomy. The CBF heterogeneity index after acetazolamide injection is a sensitive marker of the haemodynamic consequences of carotid artery stenosis and its operative treatment.

Cerebral hemodynamics and metabolism in patients with symptomatic occlusion of the internal carotid artery

BACKGROUND AND PURPOSE

The goals of this study were to investigate (1) whether the concentrations of choline, creatine, and N-acetyl aspartate (NAA) in cerebral white matter are changed in patients with symptomatic occlusion of the internal carotid artery (ICA) and (2) whether possible changes in metabolite concentration are related to regional cerebral perfusion or cerebral vasoreactivity.

METHODS

In 19 patients (mean+/-SD age, 60+/-9 years), white matter metabolite concentrations were measured with proton MR spectroscopic imaging on average 4+/-2 months after symptoms occurred. In selected voxels, corresponding cerebral blood flow and volume, mean transit time, and time-to-bolus peak were determined with dynamic susceptibility contrast MRI. Cerebral CO2 reactivity was determined with transcranial Doppler sonography.

RESULTS

No significant changes in choline and creatine concentrations were observed. NAA concentration was significantly reduced in the hemisphere on the side of the symptomatic ICA (9.1+/-1.7 mmol/L) compared with the contralateral hemisphere (10.5+/-1.7 mmol/L, P<0.005) and control subjects (10.5+/-0.9 mmol/L, P<0.01). Although no significant interhemispheric difference in NAA concentration was found in patients who presented with retinal ischemia, patients with cerebral ischemia had a significantly lower NAA concentration in the symptomatic hemisphere (9.0+/-1.7 mmol/L) compared with the asymptomatic hemisphere (10.4+/-1.6 mmol/L, P<0.05). In all patients, NAA concentration was not significantly correlated with quantitative cerebral perfusion parameters or CO2 reactivity.

CONCLUSIONS

Patients with symptomatic ICA occlusion may show chronic neuronal damage in cerebral white matter as evidenced by reduced NAA concentration. This seems to be related to previous symptomatology rather than to the cerebral hemodynamic status in a chronic stage.

Cerebral haemodynamic aspects of severe carotid stenosis: asymptomatic vs symptomatic

OBJECTIVES

to compare cerebral haemodynamics in patients with asymptomatic and symptomatic severe (> or =70%) internal carotid artery (ICA) stenosis.

METHODS

we assessed 195 consecutive patients, 116 with asymptomatic carotid stenosis (ACS) and 79 with symptomatic carotid stenosis (SCS). Using transcranial Doppler we assessed cerebral vasoreactivity (CVR) following acetazolamide test, the middle cerebral artery flow velocity ratio after/before carotid clamping (mv-MCA ratio), and the carotid back pressure (CBP) during crossclamping.

RESULTS

no significant differences between the two groups were demonstrated regarding CVR (47 vs 39%), mv-MCA ratio (50 vs 52%), or CBP (36 vs 44 mmHg). However, in patients with contralateral ICA occlusion all three variables were significantly lower as compared to patients with patent contralateral ICA. Also patients who needed a shunt during surgery had significantly lower values of mv-MCA ratio and CBP. Patients who suffered peri-operative neurologic deficits (n=6; 3%) did not differ from patients who had an uneventful course.

CONCLUSIONS

clinical state of ICA stenosis is independent of cerebral haemodynamics. Occluded contralateral ICA is more important for predicting cerebral ischaemia caused by carotid clamping. Finally, none of the haemodynamic parameters showed predictive value for peri-operative neurologic morbidity.

The haemodynamic effect of carotid endarterectomy

OBJECTIVES

to assess the haemodynamic effect of carotid artery surgery, and to relate postoperative changes to the state of cerebral circulation before revascularisation.

MATERIALS AND METHODS

using transcranial Doppler we studied bilateral middle cerebral artery (MCA) flow velocities before and on 1st day, 2nd or 3rd day and 4th or 5th day and 3 months after carotid surgery in 61 patients. In addition, ipsilateral MCA flow velocity was monitored continuously during surgery. Data were related to the internal carotid artery (ICA) perfusion pressure (cerebral perfusion pressure index, CPPI), measured directly before ICA clamping.

RESULTS

postoperatively, MCA flow velocities increased significantly overall (p<0.01), mainly due to pronounced and longer lasting flow velocities in the group of 18 patients with CPPI<0.7 (p<0.05). Flow velocities peaked - absolute as well as relative - on the first postoperative day and then gradually levelled off to reach preoperative values after 4-5 days in patients with high CPPI, whereas MCA flow velocities remained increased in the group of patients with low CPPI. At 3 months flow velocities in both groups were normalised. New neurological symptoms occurred in four patients, who all had low CPPI preoperatively (22% (4/18) vs 0%; Fisher's exact test: p=0.006).

CONCLUSION

some degree of hyperperfusion was seen in most patients, but the changes were significantly more pronounced in patients with preoperative hypoperfusion, who also suffered significantly more neurological complications.

Magnetic resonance markers of ischaemia: their correlation with vasodilatory reserve in patients with carotid artery stenosis and occlusion

OBJECTIVES

Better methods of identifying patients with asymptomatic carotid artery stenosis who are at high risk of stroke are required. It has been suggested that proton magnetic resonance spectroscopy (MRS) may allow the identification of ongoing ischaemia in this patient group by the detection of a potentially reversible reduction of N-acetyl aspartate (NAA), a presumed marker of neuronal integrity, and the presence of lactate, a marker of anaerobic metabolism. Previous studies have reported metabolite ratios rather than absolute concentrations. This study was performed to determine if NAA was reduced ipsilateral to carotid stenosis or occlusion, and if its concentration was related to carbon dioxide reactivity, a marker of cerebrovascular reserve.

METHODS

Twenty one patients with unilateral carotid stenosis (>70%) or occlusion were studied. Single voxel proton MRS was performed in the ipsilateral and contralateral hemispheres, with the voxel positioned in the arterial borderzone region between the middle and anterior cerebral artery territories. Absolute quantification of metabolite concentrations was performed. Cerebrovascular reactivity to 6% carbon dioxide was determined in both middle cerebral artery territories using transcranial Doppler ultrasonography.

RESULTS

Mean (SD) cerebrovascular reactivity was significantly lower in the stenosed compared with the contralateral hemisphere (13.3 (7.7) v 19.2 (8.2)%/kPa, p=0.002). There were no significant differences in the absolute concentrations of NAA, choline, or creatine between the ipsilateral and contralateral hemispheres (for example, NAA 10.1 (1.1) v 10.5 (1.1) mmol/l, p=0.1). No lactate peak was seen in any spectra. For each metabolite measured, there was no correlation between the absolute concentration and cerebrovascular reactivity for either hemisphere.

CONCLUSIONS

In patients with carotid stenosis and occlusion we found no evidence that chronic hypoperfusion is associated with a reduction in NAA or the presence of lactate. Magnetic resonance spectroscopy is unlikely to help in the selection of patients with asymptomatic carotid stenosis for endarterectomy.

Cerebral ischaemic changes in association with the severity of ICA lesions and cerebropetal flow

OBJECTIVES

To study the effect of the severity of internal carotid artery (ICA) lesions on cerebral haemodynamics.

DESIGN

Cross-sectional study.

MATERIALS AND METHODS

Magnetic resonance (MR) imaging, angiography (MRA) and spectroscopy (MRS) were used to study the prevalence of (border-zone) infarctions, volume flow in the main cerebropetal and middle cerebral arteries (MCA) and metabolic changes in the MCA territories in 170 patients with symptomatic ICA stenoses or occlusions and 25 control subjects.

RESULTS

No significant correlation was found between severity of the carotid lesion and the prevalence of border-zone infarctions. Also, no significant correlation was found with changes in the N -acetyl-aspartate/choline ratio nor with the prevalence of cerebral lactate. In patients with at least one severe ICA lesion, flow in the basilar artery was increased. Flow in the MCA on the symptomatic and asymptomatic side was decreased when at least one ICA was occluded. Total cerebropetal flow (flow through the ICAs plus basilar artery) was decreased when at least one ICA was occluded. No significant correlation was found between changes in cerebropetal flow and the N -acetyl-aspartate/choline ratio nor with the prevalence of border-zone infarctions.

CONCLUSION

Border-zone infarctions and ischaemic metabolic changes are not directly the result of cerebral hypoperfusion caused by severe ICA lesions.