RESUMEN
At the National Institute of Pediatrics DIF (formerly IMAN), a comparative study was performed in 157 healthy children. Ninety four (59.8%), had an innocent cardiac murmur, and 63 children (40.1%) had no heart murmur detectable. The presence of innocent murmurs was more frequent in pre-school and school age; murmurs of basal location were predominant. The murmurs were brief midsystolic and of the ejective type. All of them had the characteristics of a vibratory murmur of sinusoidal type, with diagonal radiation and low frequency. One more dynamic of pharmacological tests were performed in 60 children. In 88.5% of the cases, the murmur showed left behavior during the Valsalva monouver. Only in 15.7% during the Azoulay maneuver suggested right origin of the murmur. In 70% the murmur decreased with orthostatism and in the children who inhaled amyl nitrite, the murmur showed a behavior suggestive of aortic ejective origin. The comparison between the groups with and without murmurs showed that the heart rate was lower for those children with murmurs (P less than 0.05), the left ventricle ejection time was shorter in children with murmurs (P less than 0.01), but instead the preejection period was longer in children with murmurs (P less than 0.05). These differences let us point out that in children with murmurs the blood flow during the early systole is higher than in those without murmurs. This conditions probably a determinant in the origin of the innocent murmur.