What are S1 and S2 (first and second heart sounds) and how do they relate to systole and diastole in the cardiac cycle?

Understanding S1 and S2 Heart Sounds and Their Relationship to the Cardiac Cycle

Heart Sound Fundamentals

S1 (the first heart sound) marks the beginning of systole and occurs with closure of the atrioventricular valves (mitral and tricuspid), while S2 (the second heart sound) marks the beginning of diastole and occurs with closure of the semilunar valves (aortic and pulmonic). 1, 2

S1 Components and Timing

• S1 consists of two major components: The first component coincides with mitral valve closure in all patients and may also coincide with tricuspid valve closure in approximately 70% of patients 1
• The second major component of S1 coincides with aortic valve opening in all patients and may also coincide with tricuspid valve closure in approximately 30% of patients 1
• The average time from the Q wave on ECG to the mitral component (M1) is 0.06 ± 0.003 seconds, while the tricuspid component (T1) occurs at 0.09 ± 0.002 seconds 2
• In mitral stenosis, the Q-M1 interval is delayed to 0.10 ± 0.005 seconds, which can result in reversed splitting of S1 2

S2 Components and Timing

• S2 occurs when ventricular pressure drops sufficiently below aortic or pulmonary artery pressure, causing the semilunar valves to close 3
• S2 has two components: aortic closure (A2) and pulmonic closure (P2), with A2 typically occurring first 3
• Fixed splitting of S2 during both inspiration and expiration suggests an atrial septal defect, while reversed splitting may indicate severe aortic stenosis 3

Systole and Diastole Definitions

Systole

• Systole is the period of ventricular contraction that begins with S1 (AV valve closure) and ends with S2 (semilunar valve closure) 3
• During systole, ventricular pressure rises sufficiently to open the semilunar valves and eject blood into the aorta and pulmonary artery 3
• Systolic murmurs occur during this phase and include holosystolic, midsystolic (ejection), early systolic, and late systolic murmurs 3

Diastole

• Diastole is the period of ventricular filling that begins with S2 (semilunar valve closure) and ends with the next S1 (AV valve closure) 4
• During diastole, the ventricles relax and fill with blood from the atria 3
• Diastolic murmurs occur during this phase and include early diastolic, middiastolic, and presystolic murmurs 3

Clinical Significance of Timing

Murmur Classification by Cardiac Cycle Phase

• Midsystolic murmurs start shortly after S1 when ventricular pressure opens the semilunar valves, are often crescendo-decrescendo in configuration, and end before S2 3
• Holosystolic (pansystolic) murmurs begin with S1 and continue throughout systole until S2, occurring when there is flow between chambers with widely different pressures (e.g., mitral regurgitation, ventricular septal defect) 3
• Early diastolic murmurs begin with or shortly after S2, such as the high-pitched decrescendo murmur of aortic regurgitation 3
• Continuous murmurs begin in systole, peak near S2, and continue into diastole, arising from high-to-low pressure shunts 3

Dynamic Auscultation Maneuvers

• Right-sided murmurs (tricuspid and pulmonic) increase with inspiration, while left-sided murmurs are louder during expiration 5
• The Valsalva maneuver decreases most murmurs except those of hypertrophic cardiomyopathy and mitral valve prolapse, which become louder 5
• Standing decreases most murmurs except those of hypertrophic cardiomyopathy and mitral valve prolapse 5

Practical Clinical Application

Identifying Cardiac Cycle Phases

• Accurate localization of S1 and S2 is the prerequisite for proper heart sound segmentation and diagnosis, as different diseases generate murmurs at different stages of the cardiac cycle 6, 4
• S1 is typically louder at the apex (mitral area), while S2 is typically louder at the base (aortic and pulmonic areas) 1, 2
• The systolic interval (S1 to S2) is shorter than the diastolic interval (S2 to next S1) at normal heart rates 4

Pathological Findings

• A soft or absent A2 component of S2, or reversed splitting of S2, may indicate severe aortic stenosis 3
• Mechanical heart valves produce higher frequency components and greater energy in both S1 and S2 compared to native valves 7
• Diastolic murmurs virtually always represent pathological conditions and require further cardiac evaluation 3