Can TCD Monitoring During Carotid Endarterectomy Reduce the Risk of Perioperative Stroke?
abstract & commentary
Source: Ackerstaff RG, et al. Association of intraoperative transcranial Doppler monitoring variables with stroke from carotid endarterectomy. Stroke 2000;31:1817-1823.
Stroke is the most common major complication of carotid endarterectomy (CEA). Among experienced surgeons, acceptable stroke rates as documented by large multicenter trials (e.g., NASCET) range from 5-6%. Both embolism and/or hemodynamic compromise may account for these strokes.
Transcranial Doppler (TCD) monitoring of the middle cerebral artery (MCA) during CEA may provide useful information about both microembolic signals to the brain and significant flow limitations during carotid cross clamping. Ackerstaff and colleagues report on 1058 patients undergoing CEA in two centers in the Netherlands and the United States. There were 39 strokes (31 ischemic, 8 hemorrhagic). Diagnosis of stroke was made retrospectively based on chart review. TCD characteristics associated with a poor outcome were: emboli detected during the dissection, during surgical wound closure, accompanying a drop of 90% or more in MCA flow velocity at cross-clamping, and an increase of 100% or more in pulsatility index at clamp release. Emboli detected during shunting and during clamp release were not significant. These latter emboli are predominantly gaseous rather than particulate and are not typically associated with the development of neurological deficits.
Ackerstaff et al found a larger number of post-op strokes among patients with pre-existing cerebral ischemia. Stroke risk in symptomatic patients undergoing CEA has been previously shown to be double that of asymptomatic individuals (6% in NASCET vs 3% in ACAS). Ackerstaff et al’s data also indicate that stenoses of 70% or more were inversely related to outcome. They suggest that cross clamping of severely stenosed arteries had less of an influence on cerebral blood flow that did cross clamping of arteries with lesser stenoses.
Commentary
Because the benefit of CEA is so greatly dependent on low surgical morbidity and mortality, development of methods to make this procedure safe are of utmost importance. The reactions to changes in neuroprotective monitoring during CEA depend almost entirely upon the operating surgeon. Ipsilateral hemispheric slowing on EEG may indicate inadequate collateral circulation and a need for shunting during carotid cross clamping. Other surgeons depend on measurement of carotid stump pressures. As shown by Ackerstaff et al, a drop in MCA flow velocities is another useful indicator of a need for shunting.
Because embolism rather than hemodynamic change may explain the cause of a majority of CEA-associated strokes, maintenance of blood flow alone will not assure a safe procedure. As Ackerstaff et al showed, microembolic signals detected by TCD significantly relate to adverse outcomes. Upon hearing such signals in the operating room, the surgeon might modify his technique of carotid dissection or more quickly proceed to distal clamping of the carotid artery. As observed in the accompanying editorial (Stroke 2000;31:1799-1801), Ackerstaff et al’s study would be strengthened by better quantification of primary methods. Specific criteria should be provided to surgeons to differentiate between occasional benign signals and showers of potentially dangerous emboli. TCD monitoring could also be extended to the immediate 24 hours following surgery, a time when further emboli may occur. Such emboli might be prevented by anticoagulation with heparinoids or antiplatelet agents.
Most experts know that the majority of microembolic signals detected by TCD are asymptomatic. For instance, continuous monitoring by TCD during aortic manipulation in cardiac surgery has not consistently correlated with neurological outcome. (Barbut D, et al. Ann Thorac Surg 1997;63:1262-1267). Immediately following Ackerstaff et al’s report is another by Barth and colleagues (Stroke 2000;31:1824-1828) who report on 53 patients studied with diffusion-weighted imaging (DWI) before and after CEA. Two patients suffered small infarcts on DWI. Both were asymptomatic. Microemboli detected by TCD as well as plaque ulceration and a need for shunting were not shown to correlate with the rare occurrence of stroke as measured by DWI. These observations should be taken with caution, however, as they are based entirely on two patients who had positive DWI outcomes.
Finally, Ackerstaff et al present an intriguing finding that marked increases in flow on clamp release may be a marker for the postoperative hyperperfusion syndrome. It is not clear from the data whether any of the adverse post-procedure events occurred by this mechanism. —alan z. segal
All of the following statements regarding carotid endarterectomy (CEA) are true except:
a. Stroke may occur on a hemodynamic or embolic basis.
b. Microemboli detected by transcranial doppler (TCD) may be gaseous or particulate.
c. Emboli detected by TCD during shunting are more dangerous than those during initial dissection or wound closure.
d. Other methods such as EEG may be useful means of neurological monitoring during CEA.
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