Correlation of transcranial Doppler and noninvasive tests with angiography in the evaluation of extracranial carotid disease

Transcription

1 Correlation of transcranial Doppler and noninvasive tests with angiography in the evaluation of extracranial carotid disease Nancy L. Cantelmo, MD, Viken L. Babikian, MD, WiUard C. Johnson, MD, Rangi Samaraweera, MD, Charles Hyde, MD, and Val E. Pochay, Boston, Mass. To evaluate the usefulness of transcranial Doppler sonography in determining severity of extracranial carotid disease, we compared transcranial Doppler, ocular pneumoplethysmography, Doppler spectral analysis, and duplex scanning data to information derived from cerebral angiography. Fifty-one consecutive patients with unilateral extracranial internal carotid artery stenosis or occlusion were selected. Transcranial Doppler indexes included the peak systolic flow velocity in the middle cerebral artery ipsilateral to the stenosed internal carotid artery (imcafv), the difference between the peak systolic flow velocities in the middle cerebral artery ipsilateral and contralateral to the stenosed internal carotid artery (dmcafv), and the peak systolic flow velocity in the anterior cerebral artery contralateral to the stenosed internal carotid artery (cacafv). The minimal residual lumen determined angiographically was used as the index of internal carotid artery stenosis. Linear regression analysis with minimal residual lumen as the dependent variable and transcranial Doppler and noninvasive tests as independent variables showed the following correlation coefficients: (1) dmcafv and cacafv, R 2 = ; (2) ocular pneumoplethysmography, R 2 = ; (3) dmcafv, cacafv, A ocular pneumoplethysmography, duplex scanning, and spectral analysis R 2 = ; (4) ooalar pneumoplethysmography, duplex scanning, and spectral analysis, R2 = ; (5) imcafv, no association. These results were supported by sensitivity and specificity as well as bivariate analysis. We conclude that transcranial Doppler did not significantly add to the information obtained by our noninvasive battery of tests in the evaluation of unilateral extracranial carotid disease. (J VASe St~RG 1990;11: ) Noninvasive vascular laboratory studies play an important role in the evaluation of patients with extracranial vascular disease. Ocular pneumoplethysmography (OPG) detects abnormalities in the vascular physiology of the involved carotid artery, reflecting significant carotid stenosis in an indirect manner. Doppler spectral analysis (SA) is a direct physiologic study that evaluates the velocity of blood flow represented by the Doppler shift frequency. Real-time B-mode imaging provides direct detail of the vessel wall structure and is frequently combined with Doppler SA as the duplex scan. From the Depa~nents of Surgery, Neurology, and Radiology and the Vascular Laboratory, Boston Veterans Administration Medical Center. Presented at the Sixteenth Annual Meeting of the New England Society for Vascular Surgery, Bretton Woods, N.H., Sept , Reprint requests: Nancy L. Cantelrno, MD, Department of Surgery (112), Boston Veterans Administration Medical Center, 150 S. Huntington Ave., Boston, MA /6/19750 More recently the intracranial circulation has been evaluated by Doppler analysis, by use of a lower frequency (2 MHz) probe to obtain blood flow velocities through the cranial vault. Transcranial Doppler (TCD) studies, which can insonate the major [?- tracerebral vessels at the base of the brain, contribute valuable information about cerebral hemodynamics. Vasospasm and arteriovenous malformations are pathologic entities that lend themselves to study by TCD. Changes are noted in the intracranial vessels, which reflect extracranial carotid artery stenotic disease. The middle cerebral artery (MCA) velocity as well as the waveform pulsatility are reported to decrease ipsilatcral to severe internal carotid artery (ICA) stenosis or occlusion, 1-3 and it has been suggested that the ipsilateral MCA flow velocity might be used to evaluate ICA disease. There are in fact a number of factors that contribute to the velocity profile, and all patients with severe stenosis or occlusion do not demonstrate consistent changes in their TCD. This study compares the TCD and other non- 786

2 Volume 11 Number 6 June 1990 Transcranial Doppler, noninvasive tests and angiography 787 mvasive vascular tests, (OPG, SA, and duplex scanning) to angiography in patients with unilateral extracranial carotid artery stenotic disease. The purpose of this investigation is to evaluate the relative accuracy of these noninvasive modalities compared with angiography, and to further define the role of TCD in the routine vascular laboratory analysis of extracranial cerebrovascular disease. MATERIAL AND METHODS We reviewed the records of the Neurovascular Laboratory at the Boston Veterans Administration Medical Center and identified 89 consecutive patients with cerebrovascular disease referred for TCD testing between May 1987 and April All had received further evaluation with cerebral angiography and noninvasive studies within 1 month of TCD. The latter consisted of OPG, Doppler SA, and duplex scanning. Fifty-one patients with unilateral extra- :c"ial carotid stenosis or occlusion were selected for this review. The following groups were excluded: (1) patients with bilateral stenosing lesions of the cervical carotid arteries (N = 17), and those with angiographically demonstrated intracranial arterial stenoses (N -- 6) or aneurysms (N = 6); (2) patients with active signs and symptoms of vertebral or basilar artery distribution ischemia (N -- 5); and (3) patients with technically unsatisfactory TCD studies (N = 4). Ocular pneumoplethysmography was performed (OPG-Gee, Model OPG 4 Electro-diagnostic Instruments, Burbank, Calif.) in the vascular laboratory and interpreted by the method outlined by Gee. 4 A positive OPG finding, representing >-75% crosssectional area stenosis was identified by a difference ~ween bilateral ophthalmic systolic pressures of 5 mm Hg or greater. Spectral analysis was performed in the vascular laboratory with continuous-wave Doppler and the 8 mhz probe (Angioscan Flo Map, Unigon Industries, Mt. Vernon, N.Y.) Results were interpreted with regard to two indexes of altered carotid flow velocity, namely peak frequency and degree of spectral broadening during systole. Severe stenosis was characterized by a peak systolic frequency of at least 7000 mhz with spectral broadening or a peak systolic frequency of 8000 mhz with or without spectral broadening. When no ICA signal was detected, the artery was judged to be occluded. Duplex scanning was performed in the ultrasound section of the Radiology Department with a highresolution B-mode scanner and pulsed Doppler SA by use of an 8 MHz probe (Diasonics SPA 1000, Diasonics, Inc., Milpitas, Calif.). Scan quality was judged with criteria outlined by Comerota et al. s Severe stenoses were judged on B-mode scanning to contain plaque that produced a cross-sectional area reduction of 75% to 99% and met the above criteria for Doppler SA. Transcranial Doppler studies were performed on all patients as well as on 15 age-matched controls in the neurovascular laboratory, according to the technique described by Aaslid et al., 6 Arnold and Von Reuternf and Hennerici et al.8 A 2 MHz pulsed, range-gated Doppler instrument was used (TC2-64, EME, Uberlingen, Federal Republic of Germany). Focal depth of the signal varied in 5 mm increments from 45 to 85 mm. Bidirectional signals were recorded with a 9KHz low-pass filter and a 150 Hz high-pass filter. Spectral analysis was performed with 64-point fast Fourier transformation and displayed as velocity, assuming an angle of 0 degrees between the probe and the blood column. Depth, position of the probe, and direction of flow were used to identify the particular vessel studied. By use of the transtemporal window, Doppler signals were obtained from the MCA at a depth of 50 to 55 mm, and from the anterior cerebral artery at a depth of 70 to 75 ram. Peak systolic and end-diastolic velocities were recorded for each tracing, and spectral broadening was noted. Transcranial Doppler indexes included the peak systolic flow velocity in the MCA ipsilateral to the stenosed ICA (imcafv), the difference between peak systolic flow velocities in the MCAs ipsilateral and contralateral to the stenosed ICA (dmcafv), and the peak systolic flow velocity in the anterior cerebral artery contrallateral to the stenosed ICA (cacafv).9,10 The pulsatility index (PI), which represents the (imcafv systolic-imcafv distolic) + (imcafv mean), was also calculated. 1~ Angiography was performed in the angiography section of the Radiology Department by means of percutaneous technique, obtaining biplane studies. Forty-six of the 51 angiograms performed were done with selective common carotid arteriography, and five were done with arch arteriography. All studies were interpreted by one of us (R.S.) who was not aware of the noninvasive test results. The minimal residual lumen of the ICA was measured in two planes from the angiograms and was used as the index of ICA stenosis. Bivariate comparisons between the control group and the four TCD groups were performed with t tests. Reported p values were two tailed. Anglograms were statified into three groups: 0 or occluded, -1 ram, and > 1 mm minimal residual lumen. Cor-

3 788 Cantelmo et al. Journal of VASCULAR SURGERY Table I. Ipsilateral middle cerebral artery flow velocity Minimal residual Observations imcafv p value lumen (ram) (patients) (cm/sec) (vs control) Control > NS -< NS NS Table II. Change in middle cerebral artery flow velocity Minimal residual Observations dmcafv p value lumen (ram) (patients) (cm/ sec) (vs control) Control > < relation of minimal residual lumen with duplex scanning is as follows: 0 minimal residual lumen = occluded on duplex scanning -< 1 mm minimal residual lumen = severe stenosis of >80% on duplex scanning >1 mm minimal residual lumen = moderate or mild disease of <80% stenosis. Sensitivity, specificity, and accuracy were computed for the TCD groups and the OPG, SA, and duplex scanning, and compared with angiography. Sensitivitiy represents the probability of a patient having a positive test outcome when severe stenosis of >80% is present. Specificity is the probability of having a negative test outcome if a severe stenosis of >80% is absent. Accuracy is the proportion of correctly identified patients. Each test was compared with angiography, which was divided into two groups according to minimal residual lumen: >1 and -<1 mm. Each of the tests was also stratified into two groups: OPG, delta 0 to 4, and ->5; SA, <8000 mhz and ->7000 mhz with spectral broadening or ->8000 mhz; duplex scanning, mild to moderate disease, and severe-occlusive disease; imcafv, <52.1 cm/sec and ->52.1 cm/sec; dmcafv, <20.1 cm/sec and ->20.1 cm/sec; cacafv, <98.2 cm/sec and ->98.2 cm/sec. Multivariate analyses were performed with leastsquares multiple regression, with angiography as the dependent variable and TCD and other noninvasive tests as the independent variables. Results of the OPG were stratified in this calculation into two deka groups 0 to 4 (OPG negative) and ->5 (OPG, positive), since this is standard manner in which OPG results are interpreted. 4 All other values were entered in an unstratified manner. All analyses were calculated with the Statistical Analysis System (SAS) on the IBM mainframe computer at Boston University. RESULTS All patients in this group were men. Their mean age was 64.2 years. The mean age of the 15 controls was 61.2 years. Forty-eight of the 51 patients had symptoms of ocular or hemispheric ischemia refer- able to the involved carotid artery (94%), the other three were free of symptoms (6%). In all cases the imcafv was positive or toward the TCD probe, and the cacafv was negative or away from the probe. The TCD indexes (imcafv, dmcafv, and cacafv) and angiograms of 51 patients with extracranial carotid artery stenotic disease were compared to those of the control group. No statistical significance could be shown with imcafv (Table I), but there was an inverse relationship~e tween dmcafv and degree of carotid artery stenosis (Table II). The cacafv was significant for angiographic minimal residual lumen of 0 and -< 1 mm, but not for >1 mm (Table III). Pulsatility index showed no significance except for minimal residual lumen of > 1 mm (Table IV). The OPG detected unilateral carotid disease with a sensitivity of 87.9%, specificity of 93.3%, and an overall accuracy of 89.6%. Spectral analysis alone yielded a sensitivity of 85.3%, specificity of 81.3%, and accuracy of 84.0%. The duplex scan demonstrated a sensitivity of 94.3%, specificity of 92.9%, and accuracy of 93.9%. With regard to the TCD studies, the presence of severe carotid disease was reflected by imcafv with sensitivity of 97.1%, specificity of 18.8%, and "~ "- curacy of 72.6%. For dmcafv the sensitivity was 62.9%, specificity was 68.8%, and accuracy was 64.7%. The cacafv had a sensitivity of 71.4%, specificity of 87.5%, and overall accuracy of 76.5%. Regression analysis was performed with minimal residual lumen as the dependent variable and TCD and other noninvasive tests as the independent variables (Table V). With dmcafv in the model and considering the independent variables separately, results were dmcafv R 2= , cacafv R 2 = , SA R 2 = , OPG R 2 = , duplex scanning R 2 = To evaluate the effect of multiple tests, the independent variables were entered into the regression model in combination. Results were as follows: dmcafv and cacafv R 2= ;

4 Volume 11 Number 6 June 1990 Transcranial Doppler, noninvasive tests and angiography 789 Table III. Contralateral anterior cerebral artery flow velocity Minimal residual Observations cacafv p value lumen (mm) (patients) (on!see) (vs control) Control >i NS < Table IV. Pulsatility index Minimal residual Observations p value lumen (ram) (patients) PI (vs control) Control > < NS NS OPG and SA R 2 = OPG and duplex scanning R z = ; dmca, caca, OPG, SA, and duplex scanning R 2 = ; OPG, SA, and duplex scanning R 2 = dmcafv, imcafv, and PI could not be used in the same analysis since imcafv is used to calculate dmcafv and PI. When imcafv was used in the model no association was found, and it lowered the R 2 value when used in combination analyses. DISCUSSION The demonstration by Lindegaard et al.x in 1982 that the MCA and ACA could be insonated through the cranium led to the development of the noninvasive evaluation of the intracerebral circulation by the TCD. Although the Doppler had been widely used in the examination of the extracranial carotid artery, the intracranial circulation could be evaluated only by angiography. The use of the 2 MHz probe and the identification of the various cranial "windows" permit the localization of the major intracranial arteries. The changes that occur in the intracerebral circulation as the result of extracranial stenosis have been the subject of a number of studies. We evaluated ~;e ability of TCD to assess the intracranial hemodynamic effects of angiographically demonstrated unilateral cervical carotid artery stenosis or occlusion, and we tested whether its addition to a routine noninvasive test battery would improve the latter's accuracy. Our results suggest that although TCD can help identify changes in intracranial flow, it does not improve the ability of the noninvasive battery to detect carotid disease or to determine degree ofstenosis. Previous investigators have reported that the MCA velocity is often decreased ipsilateral to extracranial carotid artery severe stenosis or occlusion and have found a significant inverse correlation between imcafv and the degree of internal carotid artery stenosis. 13,12'13 Others have not, 9,x4 including the present study. When compared with controls, no statistical significance could be attained comparing Table V. Multivariate analysis of noninvasive tests compared with angiography Variables DS OPG SA cacafv dmcafv imcafv No association PI No association DS, SA, OPG DS, SA, OPG, cacafv, dmcafv OPG, DS OPG, SA cacafv, dmcafv DS, Duplex scanning; SA, spectral analysis; OPG, ocadar pneumoplethsmography; cacafv, contralateral anterior cerebral artery flow velocity; dmcafv, delta middle cerebral artery flow velocity; imcafv, ipsilateral middle cerebral artery flow velocity; PI, pulsatility index. imcafv to angiography in our study (Table I). It is important that age-matched controls be used since it is widely recognized that intracerebral flow velocity diminishes with age. 7 In addition to the poor correlation in bivariate analysis, no association could be established between imcafv and the angiographic minimal residual lumen in regression analysis. Analysis of sensitmty, specificity, and accuracy, the sensitivity of imcafv was 97.1%, but the specificity was only 18.8%, indicating that if the ICA disease were present it would be identified accurately by imcafv, but imcafv could not reliably identify those without disease. In spite of a 72.6% calculated accuracy, the low specificity of imcafv makes it an unreliable parameter, supporting the poor results obtained by the two other statistical studies. We have chosen to use minimal residual lumen, requiring one measurement, which we believe to be more accurate than estimated or calculated percent stenosis. In an effort to characterize the hemodynamic changes in the cerebrovascular bed, the PI has been developed, n An inverse correlation between PI and carotid disease has been shown by some authors, 12,13 r a

5 790 Cantelmo et al. lottrnal of VASCULAR SURGERY and the PI is felt to reflect intracranial collateralization and vasodilation. Our data do not support the PI as a useful test compared with standard noninvasive tests or angiography. The intracranial hemodynamic effects of lesions limiting carotid flow have also been studied with positron emission tomography, single photon emission computed tomography, and the xenon inhalation technique) s'~6 Although group analyses frequently show some reduction in flow distal to occluded or severely stenosed cervical carotid arteries when compared to controls, 16 most investigators report interindividual variations in perfusion pressure and blood flow. ~7'18 Powers ct al.~7 found a normal cerebral hemodynamic state distal to severely stenotic carotid lesions in 7 of 19 patients and suggested that the residual carotid lumen is not a reliable indicator of intracranial flow changes. Norrving et al.x~ noted the frequent occurrence ofinterhemispheric asymetry in blood flow. Bishop et al.16 used xenon cerebral blood flow measurements and compared them with imcafv. He found that absolute velocity did not correlate, but changes in imcafv with hypercapnia did reliably correlate to cerebral blood flow. We purposely chose an tmcomplicated pattern of disease, namely, no intracranial stenoses and tmilateral extracranial disease, to attempt a demonstration of the intracranial effects of severe extracranial occlusive disease. We looked at the dmcafv, which is the difference between the MCAFVs ipsilateral and contralateral to the carotid artery stenosis. Previous work by our group demonstrated this to be more significant than imcafv, 9'~ as does the present study. The other parameter we used, cacafv, reflects flow changes on the contralateral side, which presumably assist the affected hemispheric blood flow ipsilateral to the diseased internal carotid artery. In our present study comparison with agematched controls wc found significant differences between dmcafv and cacafv as compared with angiography (Tables II and III). Regression analysis did show some weak association between dmcafv and angiography (R = ) and cacafv and angiography (R = ). This was also borne out in the sensitivity, specificity, and accuracy data, with cacafv being the better of the two, and the result being only moderately good. The accuracy of our noninvasive battery is comparable to that of others centers. 19 Although duplex scanning is recognized as more accurate than SA alone, the duplex scanner is not available in all laboratories, which often only have spectral analysis. We therefore have included it as a separate test, performed on an instrument designated only for spectral analysis. Both of the statistical methods we used identiffed the noninvasive tests as more accurate than TCD in the evaluation ofextracranial disease. Duplex scanning was the best of all tests, OPG the next most accurate, and SA was third in both sensitivity, specificity, and accuracy as well as regression analysis. The combination of OPG, SA, and duplex scanning had the best association (R 2 = ) and the addition of TCD studies slightly lowers this association (R 2 = ). Both statistical analyses identified the standard vascular lab tests to be better than the TCD parameters. In the evaluation of the ICA, the OPG and the TCD arc indirect studies that might be viewed as evaluating similar territories, namely the circulation and collaterals distal to the extracranial ICA. Ocular pneumoplethysmography proved more accurate than TCD in reflecting severe ICA disease. A controversy exists about the role of the OPG in routine vascular lab StUdies, 19'20 with some in[~s~ tigators suggesting it does not add to the information provided by the duplex scan. 21 Our data support the addition of OPG to duplex scanning since the combination had a higher association (R 2 = ) than duplex scanning alone (R 2 = ). Our results lead us to conclude that in this subgroup of patients with unilateral extracranial carotid disease, TCD used as an indirect test does not contribute to the standard noninvasive tests in the evaluation of ECD. It is probable that its role as a direct reflector of the intracranial circulation is a more appropriate use. Many groups including our own are interested in the use of TCD for monitoring the intracerebral changes that occur during and after carotid endarterectomy. 3,21,23 Significant differences have been 6 "~ served during cross clamping, shunting, and in the postoperative period. It appears that comparing changes in the imcafv may be more significant than comparing the absolute resting values of groups of patients. 16 A number of investigators have used TCD to evaluate collateral circulation. 1'24 Transcranial Doppler may contribute to duplex scan information in the evaluation of patients for carotid endarterectomy who will not have angiograms. Further studies are necessary to clarify the role of TCD study for the vascular surgeon. The authors acknowledge Donna Baldwin, Janice Dion, Janice Hamilton, and Nancy Walker for their assistance in performing the noninvasive vascular tests, and Eligia Ratchell for her assistance in preparation of the manuscript. We also acknowledge the statistical analysis performed by Michael Winter, Research Data Analyst, Boston Univesity School of Public Health.

6 Volume 11 Number 6 June 1990 Transcranial Doppler, noninvasive tests and angiography 791 lo3ferences 1. Lindegaard K, Bakke SJ, Grolimtmd P, Aaslid R, Huber P, Normes H. Assessment of intracranial hemodynamics in carotid artery disease oftranscranial Doppler ultrasound. J Neurosurg 1985;63: Wechsler LR, Sekhar LN, Luyckx K, Kiok M. Transcranial Doppler and extracranial carotid stenosis. J Cardiovasc Ultrason 1987;6: Schneider PA, Rossman ME, Torem S, Otis SM, Dilley RB, Bernstein EF. Transcranial Doppler in the management of extracranial cerebrovascular disease: implications in diagnosis and monitoring. J VASC SURG 1988;7: Gee W. Carotid physiology with ocular pneumoplethysmography. Stroke 1982;13: Comerota AJ, Cranley JJ, Cook SE. Real time B-mode carotid imaging in the diagnosis of cerebro-vascular disease. Surgery 1981;89: Aaslid R, Markwalder TM, Nornes H. Noninvasive transcranial Doppler ultrasound recording of flow velocity in the basal cerebral arteries. J Neurosurg 1982;57: Arnolds JA, Von Reutern G. Transcranial Doppler sonography, examination technique and normal reference valves. Ultrasound Med Biol 1986;12: ~" Hermerici M, Rautenberg W, Sitzer G, Schwartz A. Transcranial Doppler ultrasound for the assessment of intracranial arterial flow velocity--part 1. Examination technique and normal valves. Surg Neurol 1987;27: Babikian VL, Araki C, Ahem G, Norrving B, Pochay V. The delta middle cerebral artery flow velocity: an index of extracranial carotid artery stenosis severity. J Cardiovasc Technol 1989;8: Babikian VL, Norrving B, Pochay V, Araki C, Ahem G. Intracranial hemodynamic effects of cervical carotid artery disease evaluated by transcanial Doppler. Neurology 1989; 39: Gosling RG, King DH. Arterial assessment by Doppler shift ultrasound. Proc R Soc Med 1974;67: Schneider PA, Rossman ME, Bernstein EF, Torero S, Ringelstein EB, Otis SM. Effect of internal carotid artery occlusion on intracranial hemodynamics. Stroke 1988;19: Ries F, Baier M, Ernst A, Solymosi L, Horn R. Is pulsatility a refiable index of intracranial hemodynamic changes in cvd? J Cardiovasc Technol 1989;8: Bishop CCR, Powell S, Insall M, Rutt D, Browse NL. Effect of internal carotid artery occlusion on middle cerebral artery blood flow at rest and in response to hypercapnia. Lancet 1986;1: Norrving B, Nilsson B, Risberg J. rcbf in patients with carotid occlusion. Resting and hypercapnic flow related to collateral pattern. Stroke 1982;13: Bishop CCR, Powell S, Rurt D, Browse NL. Transcranial Doppler measurements of middle cerebral artery blood flow velocity: a validation study. Stroke 1986;17: Powers WJ, Press GA, Grubb RL, Gado M, Raichle ME. The effect of hemodynamically significant carotid artery disease on the hemodynamic status of the cerebral circulation. Ann Int Med 1987;106: Lord RSA, Yeates M, Fernandes V, et al. Cerebral perfusion defects, dysantoregulation and carotid stenosis. J Cardiovasc Surg 1988;29: Castaldo JE, Nichols GG, Gee W, Reed JF. Duplex ultrasound and ocular pneumoplethysmography concordance in detecting severe carotid stenosis. Arch Neurol 1989;46: Gertler J, Cambria RP, Kesder P, et al. Carotid surgery without angiography- noninvasive selection of patients. [Submitted for publication]. 21. Martin KD, Kempczinski RF, Patterson RB, Fowl RJ. Is the continued use of ocular pneumoplethysmography necessary for the diagnosis of cerebrovascular disease? J VASC SURG 1990;11: Padayachee TS, Gosling RG, Bishop CC, Burnard K, Browse NL. Monitoring middle cerebral artery blood velocity during carotid endarterectomy. Br J Surg 1986;73: Babikian VL, Wechsler LR, Araki C, et al. Transcranial Doppler studies after carotid surgery: evidence for subgroups. Ann Neurol 1988;24: Schneider PA, Ringelstein EB, Rossman ME, et al. Importance of cerebral collateral pathways during carotid endarterectomy. Stroke 1988;19: DISCUSSION Dr. Alfred V. Persson (Burlington, Mass.). The TCD developed by Rune Assllp is the most recent development in the noninvasive vascular laboratory. It was hoped the TCD would allow us to determine who had intracranial disease so severe that carotid endarterectomy would be valueless. This hope has not been realized. Then it was hoped, as the present authors have stated, the TCD would improve the accuracy of the noninvasive carotid evaluation. That is, when used in conjunction with other tests, it would improve the overall accuracy. As we have heard today, this hope has not been realized. The question now being asked is will the TCD help in selecting those patients with an asymptomatic carotid stenosis who should undergo a carotid endarterectomy? Stated in another way, can the TCD be used to separate out those who have adequate from those who have inadequate collateral ciroalation? Several methods are being used to help with this determination. Measurements are taken at rest and after a challenge of either carotid compression or inhalation of CO2. One then compares the resting measurements with measurements after a challenge. My questions to the authors are (1) Have you made measurements before and after carotid compression or CO2 challenge? (2) If you have, do you feel that these tests are reproducible enough to be used in the longterm study to determine the prognosis of patients with asymptomatic carotid stenosis? (3) What is your personal opinion as to whether TCD would be helpfial in predicting the prognosis of patients with asymptomatic stenosis? What is your personal feeling about the outcome of such a 'study if it were to be done?

7 792 Cantelmo et al. Journal of VASCULAR SURGERY Dr. D. Eugene Strandness (Seattle, Wash.). Even though this method has been available for quite a period of time, I have been rather slow to pick it up. I can frankly tell you I do not know what to do with the technique. The neurosurgeons have found a place for it in the evaluation of subarachnoid hemorrhage. It is now routine to use TCD to monitor the cerebral vascular spasm that occurs in response to subarachnoid bleeding. It is possible to get an enormous amount of information out of this system, but it is going to take a great deal more work to find its true place. I suspect that you are going to have to conduct the studies with carotid compression or CO2 to obtain the maximum amount of information. The collateral supply can be evaluated as long as the studies include the orbit, the transtemporal, and the foramen magnum. Although important information is available, it will take a great deal of work. I agree totally with the conclusions of your paper. Even with regard to the role of shunts, it may simply create more certainty. If you see a change in the TCD when you cross clamp the carotid, what do you do? Most surgeons seeing the MCA velocity decrease would rather put in a shunt rather face the uncertainty of its meaning. I think it is going to take some very well planned studies to prove its application to many areas. I agree completely with your conclusions. Dr. Nancy L. Cantelmo (dosing). The message of our study was that in the evaluation of extracranial disease TCD is an indirect test and is not helpfial. I do not want to leave everyone with the opinion that it is a valueless test. Neurosurgeons and neurologists have found a use for it, and vascular surgeons are casting about looking for a use. Perhaps the changes in ipsilateral MCA may be more significant than the absolute value. In answer to Dr. Persson, we have not looked at vasomotor reactivity with the CO2 challenge or the elicited collaterals with carotid compression. Many people who feel that to do carotid compression in patients with extracranial carotid disease could actually be harmful. I really cannot answer any of your questions as to whether the evaluation of the TCD will be helpful in carotid disease. We do not really know whether demonstrating collaterals or vasomotor reactivity will be related to what happens with an embolic event subsequently. I feel we need further study before we can answer any of those questions.