Exercise-induced myocardial perfusion defects as assessed by thallium-201 single photon emission computed tomography (SPECT) develop in over 2/3 of patients with hypertrophic cardiomyopathy (HCM), and these abnormalities may be reduced (or prevented entirely) by verapamil therapy. The mechanism responsible for thallium defects during exercise in HCM has been assumed to be myocardial ischemia. An alternative explanation is disturbed cellular active cation uptake across the sarcolemma related to the cardiomyopathic process. Since uptake of technetium-99m methoxybutyl isonitrile (MIBI) is not dependent on the active Na-K ATPase transport system, as is TI-201, we performed exercise MIBI studies in 10 HCM patients with exercise-induced Tl-201 defects. Of the 50 myocardial regions analyzed, there was concordance of Tl-201 and MIBI uptake in 23/24 (96%) of abnormal and 26/26 (100%) of normal myocardial regions. There was also 100% concordance of exercise-induced cavity dilatation. Thus, these data support the concept that Tl-201 defects in HCM represent true perfusion abnormalities and not a fundamental disturbance of the active Na-K ATPase transport system. Evidence of myocardial ischemia in HCM has also been demonstrated using positron emission tomography (PET). We studied 12 symptomatic HCM patients with severe hypertrophy (septum >20 mm) by exercise TI-201 SPECT and resting PET, using 18F-deoxyglucose (FDG) to assess regional myocardial glucose utilization (rMGU) and (15)O-water to quantitate regional blood flow (rMBF). When assessed relative to rMBF, rMGU demonstrated regional heterogeneity in 7 patients, with a pattern of increased rMGU/rMBF compatible with regional myocardial ischemia. These data at rest were supported by the TL data: in all 7 patients, the same regions with increased rMGU/rMBF at rest developed reversible Tl-201 perfusion defects with exercise. Thus, elevated rMGU relative to rMBF in patients with HCM represents either metabolic evidence of MI at rest or the metabolic sequelae of prior ischemic episodes.
