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This is the only stamp we have found with a phonocardiogram. The recording shows a late systolic click. It was issued by Bulgaria in 1994 to commemorate the work of Dr Ivan Mitev who described the abnormal sound as the “6th tone”.
Systolic clicks have been recognised since 1887 when Cuffer and Barbillon reported a “bruit de galop mesosytolique”. J N Hall in 1903 and Paul D White in 1931 ascribed late systolic murmurs or clicks as caused by mitral valve dysfunction in opposition to the view of Louis Gallavardin in 1913 that clicks were caused by pleuropericardial adhesions. John Reid in 1961 was the first to state that clicks arose from the mitral chordae—chordal snap. The extensive work by John Barlow in Johannesburg identified many features of non-ejection systolic clicks and late mitral systolic murmurs and his contributions are recognised in the term “Barlow’s syndrome”.