There are many ways to classify murmurs that can aid in determining whether any given one is pathologic. Systolic murmurs should be classified according to their timing within systole as being either ejection or regurgitant. Systolic murmurs that begin immediately after the first heart sound (S₁) are called regurgitant, whereas those that have a delay in onset timing after S₁ are called ejection. The S₁ sound is created by closure of the mitral and tricuspid valves that occurs at the beginning of ventricular contraction. Systolic regurgitant murmurs begin immediately after S₁. Blood in a higher pressure ventricle can immediately begin to flow into a lower pressure area as soon as the ventricle contracts. The delay between an S₁ and a systolic ejection murmur occurs because the ventricular pressure must first rise higher than the pressure in another area before flow can begin. It is important to determine the onset timing of systolic murmurs, as this can help to differentiate the cause of the murmur.

During systole, blood leaving a ventricle can flow either into the corresponding atrium or great vessel or into the other ventricle. When one thinks of normal cardiac physiology, the left ventricle is typically at higher pressure than the right, and both ventricles are at higher pressure than their corresponding atria. Thus, systolic regurgitant murmurs are created by atrioventricular valve regurgitation (tricuspid or mitral valve regurgitation) or by a ventricular septal defect. Regurgitant murmurs are also typically described as “harsh” in quality or “pansystolic” in timing. Although it is important to make use of other murmur characteristics such as location, quality, intensity, and radiation to determine the cause of a murmur, any murmur that can be defined as regurgitant is by its nature pathologic and should not be ignored.