Four factors affect the intensity of the first heart sound. Since the M1 portion of S1 is much louder than T1, it is only important to discuss what affects the intensity of M1. The first factor is the thickness of the chest wall. Obese individuals will have a soft S1 while a thin person will have a more intense S1 heart sound. The greater the distance separating the leaflets of the mitral valve at the beginning of systole, the louder the S1. This is affected by the duration of the PR interval on the ECG. Remember that the PR interval represents part of diastole, so a longer PR interval would result in a longer diastolic filling time. As the left ventricle fills, the pressure gradually increases. This gradual increase in pressure causes the mitral valve leaflets to slowly drift together. Therefore, when ventricular systole occurs in the setting of a long PR interval, the leftist will be separated by a smaller distance and the S1 sound will be softer. The converse is also true. A short PR interval results in an accentuated S1 since the mitral valve leaflets will be further apart at the onset of ventricular systole.

The mobility of the valve leaflets in the second factor influencing the intensity of M1. In mild to moderate mitral stenosis, the increased left atrial pressure causes the mobile portions of the mitral valve leaflets to be more widely separated, thus resulting in an accentuated M1 sound. In severe to critical mitral stenosis, the valve leaflets are so calcified and immobile that the M1 sound is diminished or absent.

The rate of ventricular contraction also affects the intensity of S1. The faster the heart rate and the faster the rise in ventricular pressure, the louder the S1. Thus, high flow states such as anemia, thyrotoxicosis, or sepsis results in an accentuated S1. Also, during exercise or any other setting of tachycardia, the S1 will be accentuated.