901-95 Stress-induced Subendocardial Underperfusion: A Potential Mechanism of Ischemia in Hypertrophic Cardiomyopathy (HCM)

Abstract

Myocardial ischaemia is known to occur in patients with HCM, despite angiographically normal coronaries. However, the precise mechanism for this phenomenon is unknown. Preliminary work with positron emission tomography (PET) has demonstrated selective subendocardial underperfusion during stress in some patients with very thick interventricular septa. To investigate this further, we measured regional myocardial blood flow [MBF (ml/min/g)], at baseline and following infusion of dipyridamole (Dip, 0.56 mg/kg over 4 minutes), using H215O or 13NH3and PET in 22 patients with HCM, mean age 39 ± 9 years. The thickness of the interventricular septum and the left ventricular free wall were 26 ± 5 and 15 ± 5 mm respectively. Values of MBF were calculated in the subendocardial (endo), and subepicardial (epi) regions of the interventricular septum. Coronary vasodilator reserve (CVR) was calculated as Dip/baseline MBF. At baseline endo-MBF was 0.87 ± 0.36 and epi-MBF 0.80 ± 0.34 (p = NS), resulting in an endo/epi flow ratio of 1.09 ± 0.28. Following Dip, endo-MBF was 1.13 ± 0.43 and epi-MBF 1.44 ± 0.44, resulting in an endo/epi flow ratio of 0.80 ± 0.22 (p < 0.001 vs baseline). In 13/22 patients (59%), the endo/epi ratio decreased to less than 0.80 following Dip (animal data show that normally the endo/epi ratio is close to 1 and ranges between 0.8 and 1.2, both at baseline and during maximal coronary vasodilatation). The CVR was 1.33 ± 0.53 in the endo and 1.95 ± 0.70 in the epi (p < 0.01). A linear relation (R = 0,58, p < 0.01) could be demonstrated between endo-CVR and the endo/epi ratio following Dip. 1) Dipinduced subendocardial underperfusion occurs frequently in HCM; 2) This appears to be associated with a more impaired CVR in this layer; 3) Transmural maldistribution of MBF during stress may be a mechanism of ischaemia in these patients despite normal coronaries