Mitral valve prolapse as heard with the patient standing

Electrocardiogram and phonocardiogram of mitral valve prolapse as heard at the apex. S₁ (mitral and tricuspid valve closure) is followed by a midsystolic click resulting from the prolapse of the mitral valve. After the prolapse occurs, there is the midsystolic murmur of mitral regurgitation. With standing, there is a decrease in venous return, a decrease in left ventricular volume, and the mitral valve prolapses earlier in systole. Consequently, the mitral regurgitation murmur is lengthened.