site.btaJuly 23, 1973: Bulgaria Enters Medical History with Sixth Heart Sound Discovery

In 1973, Bulgarian pediatrician and scientist Prof. Ivan Mitev identified a previously unknown component of the human cardiac cycle, the sixth heart sound, a discovery with potential diagnostic value that continues to draw international recognition. Officially registered in 1979, the finding remains a rare and significant contribution from Bulgaria to global cardiology.

The sixth heart sound is a low-frequency tone occurring just before the first heart sound (S1), during the early diastolic phase of the heartbeat. Detected primarily in children and young adults, it reflects subtle changes in ventricular filling and myocardial compliance, which are critical factors for understanding heart performance.

Although not commonly captured by a standard stethoscope, the sixth heart sound can be identified through phonocardiography and advanced acoustic analysis. Specialists consider it an important non-invasive marker for assessing early or latent cardiac dysfunction, especially in children.

Prof. Ivan Mitev (1924-2007) was a pioneering figure in pediatric medicine and cardiology in Bulgaria. In recognition of his achievements, the Faculty of Pediatrics at the Medical University Sofia bears his name, and Bulgaria’s future National Pediatric Hospital is also set to be named in his honour.

Half a century after its discovery, the sixth heart sound continues to be referenced in specialized cardiology literature and is recognized as a unique scientific achievement from Bulgaria on the international stage.