First heart sound: caused by atrioventricular valves – Mitral (M) and Tricuspid (T).
Second heart sound caused by semilunar valves – Aortic (A) and Pulmonary/Pulmonic (P).
Heart sounds are the noises generated by the beating heart and the resultant flow of blood through it. Specifically, the sounds reflect the turbulence created when the heart valves snap shut. In cardiac auscultation, an examiner may use a stethoscope to listen for these unique and distinct sounds that provide important auditory data regarding the condition of the heart.
In healthy adults, there are two normal heart sounds, often described as a lub and a dub that occur in sequence with each heartbeat. These are the first heart sound (S1) and second heart sound (S2), produced by the closing of the atrioventricular valves and semilunar valves, respectively. In addition to these normal sounds, a variety of other sounds may be present including heart murmurs, adventitious sounds, and gallop rhythms S3 and S4.
Heart murmurs are generated by turbulent flow of blood and a murmur to be heard as turbulent flow must require pressure difference of at least 30 mm of Hg between the chambers and the pressure dominant chamber will outflow the blood to non-dominant chamber in diseased condition which leads to Left-to-right shunt or Right-to-left shunt based on the pressure dominance. Turbulence may occur inside or outside the heart; if it occurs outside the heart then the turbulence is called bruit or vascular murmur. Murmurs may be physiological (benign) or pathological (abnormal). Abnormal murmurs can be caused by stenosis restricting the opening of a heart valve, resulting in turbulence as blood flows through it. Abnormal murmurs may also occur with valvular insufficiency (regurgitation), which allows backflow of blood when the incompetent valve closes with only partial effectiveness. Different murmurs are audible in different parts of the cardiac cycle, depending on the cause of the murmur.