QUALITATIVE VERSUS QUANTITATIVE ASSESSMENT OF CEREBROVASCULAR RESERVES

OBJECTIVE: Quantitative studies of cerebral blood flow (CBF) combined with a vasodilatory challenge have defined a subgroup of patients with symptomatic carotid occlusion who have an increased risk for stroke. These are patients whose CBF paradoxically decreases in response to a vasodilatory challenge. Recent reports suggest that qualitative CBF te chniques, such as single photon emission tomography with 99m-hexamethy lpropyleneamine oxime, can also define the same high-risk subgroup. To determine whether qualitative measures of CBF are sufficient for pred icting the risk of stroke, we converted our quantitative CBF data, obt ained with xenon-enhanced computed tomography (Xe/CT), to qualitative ratios in a manner similar to that used with single photon emission to mography data. METHODS: We analyzed CBF values within the territory of the middle cerebral artery for 94 patients with symptomatic carotid o cclusion. Values obtained using Xe/CT before and after the intravenous administration of 1 g of acetazolamide were used to derive an asymmet ry index: (C-occl - C-non)/C-avg x 100. The difference between the pos tacetazolamide asymmetry index and the baseline asymmetry index was us ed to classify the patients into groups according to CBF values. The t hreshold for abnormal qualitative CBF reactivity was defined as a perc ent change in the asymmetry index of less than -10%. Quantitative (Xe/ CT) CBF was considered abnormal (''steal'' response) when the response to acetazolamide (percent change) on the occluded side was a decrease of 5% or greater. RESULTS: Of 34 patients whose cerebrovascular reser ves were considered compromised based on qualitative criteria, 17 (50% ) did not have a steal response as defined by quantitative Xe/CT CBF ( i.e., false positive). Eleven of 62 (18%) who were not considered comp romised by qualitative criteria had a steal response (i.e., false nega tive). Our data indicate that a qualitative approach has a 61% sensiti vity and a 75% specificity for detecting patients with compromised res erves. Further, the positive predictive value of this method is only 5 0%. Therefore, the two methodologies do not predict the same patients as having compromised reserves. CONCLUSION: Previous studies have show n that patients at high risk for stroke can be identified with quantit ative CBF methods. This study shows that the important subgroup cannot be accurately defined with qualitative methodology. The implications of using the more reliable methodology are important for individual pa tient management and for designing clinical trials.