An Analysis of the Prevalence of the Arg403Gln, Gly584Arg and Leu908Val Mutations in the Beta-Myosin Heavy Chain in Portuguese Patients Who Are Carriers of Familial Hypertrophic Cardiomyopathy [Análise da Prevalência das Mutações Arg403Gln, Gly584Arg e Leu908Val No Gene da Cadeia Pesada Beta da Miosina Em Doentes Portugueses Portadores de Cardiomiopatia Hipertrófica Familiar.]