Heartbeat Phases and Heart Sound Types
The cardiac cycle, commonly referred to as the heartbeat, is the complete sequence of hemodynamic and electrical events that occurs from the beginning of one heart contraction to the beginning of the next. This rhythmic, self-regulating process is essential for the continuous circulation of blood, propelling oxygen-depleted blood to the lungs and newly oxygenated blood to the systemic circulation. The efficiency of the heart is intrinsically tied to the coordinated, alternating states of contraction and relaxation. These mechanical actions, particularly the rapid opening and forceful closing of the four cardiac valves, generate vibrations that transmit through the chest wall, creating the characteristic heart sounds which are a primary tool for clinical diagnosis.
The Two Primary Phases of the Cardiac Cycle: Systole and Diastole
Each cardiac cycle is divided into two major, inverse periods: systole and diastole. Systole is the contraction phase, where the heart muscle, primarily the ventricles, contracts to pump or eject blood out of the chambers into the major arteries (the aorta and pulmonary trunk). Diastole is the relaxation phase, where the heart muscle relaxes and the chambers expand, allowing the ventricles to passively and actively fill with blood returning from the atria. In a healthy adult at a resting heart rate of about 75 beats per minute, the complete cycle lasts approximately 0.8 seconds. Crucially, diastole typically occupies a greater portion of the cycle time than systole, ensuring sufficient time for ventricular filling. As heart rate increases, the duration of diastole shortens significantly more than systole, which can compromise the heart’s ability to fully fill with blood.
Detailed Stages of Ventricular Function
To more accurately describe the mechanical events, the cardiac cycle is further divided into specific stages. Systole begins with the Isovolumetric Contraction Phase, immediately following the closure of the atrioventricular (AV) valves. During this brief period, the ventricular muscle contracts, causing a sharp rise in pressure, but because both the AV and semilunar (SL) valves are closed, no blood volume change occurs. The pressure build-up eventually exceeds that of the great arteries, leading to the Ejection Phase, where the SL valves (aortic and pulmonic) open and blood is rapidly expelled. Diastole begins when the ventricles start to relax and the arterial pressure overcomes the ventricular pressure, causing the SL valves to snap shut, initiating Isovolumetric Relaxation—a period of rapid pressure drop with constant volume. Finally, the lower ventricular pressure allows the AV valves to open, starting the Ventricular Filling Phase, which consists of rapid inflow, slow filling (diastasis), and concludes with Atrial Systole (the atrial kick), which actively contributes the final 20-30% of blood to maximize ventricular volume.
S1: The First Heart Sound (“Lub”)
The first heart sound, S1, generates the “lub” component of the classic “lub-dub” rhythm and signifies the beginning of ventricular systole. It is generated primarily by the closure of the two atrioventricular valves: the mitral valve (M1) on the left and the tricuspid valve (T1) on the right. The sound is caused by the sudden tension and vibration of the valve leaflets, the attached chordae tendineae, and the surrounding blood and ventricular walls as the valves slam shut to prevent regurgitation into the atria. While M1 and T1 are normally heard as a single sound, the closure of the mitral valve slightly precedes the tricuspid valve closure due to the earlier and higher pressure generated by the left ventricle. A split S1 is occasionally audible and can be a normal finding, though a widely split S1 may be associated with conditions that delay right ventricular contraction, such as a complete right bundle branch block.
S2: The Second Heart Sound (“Dub”)
The second heart sound, S2, which creates the “dub,” marks the end of ventricular systole and the beginning of ventricular diastole. S2 is produced by the simultaneous, high-pressure closure of the two semilunar valves: the aortic valve (A2) and the pulmonic valve (P2). Due to the higher systemic arterial pressure, the aortic valve typically closes marginally before the pulmonic valve. This temporal difference can be accentuated by respiratory mechanics, resulting in a phenomenon known as physiologic splitting. During inspiration, the negative intrathoracic pressure increases venous return to the right side of the heart, delaying the closure of the pulmonic valve (P2) and creating a wider, audible split between A2 and P2. During expiration, the increased intrathoracic pressure minimizes the delay, often resulting in S2 being heard as a single sound. Conversely, a fixed or wide split that does not vary with respiration is often a pathological finding, commonly associated with an atrial septal defect.
S3 and S4: Gallop Rhythms
In addition to the two normal sounds, two low-frequency sounds, S3 and S4, may be present, which often collectively form a “gallop” rhythm. The Third Heart Sound (S3) is an early diastolic sound that occurs during the rapid phase of ventricular filling. It is theorized to be caused by vibrations resulting from the rapid deceleration of blood against a stiff or already volume-filled ventricular wall. While an S3 is a normal, physiological finding in children and young adults, its presence in individuals over the age of 40 is usually pathological, strongly suggesting ventricular failure (increased ventricular volume) or volume overload. The Fourth Heart Sound (S4) is a late diastolic sound that occurs just before S1. It is produced by the forceful, vigorous contraction of the atria trying to push the final volume of blood into a non-compliant or stiff ventricle (decreased ventricular compliance). Since S4 directly indicates diastolic dysfunction, it is almost always a pathological sound in adults, often observed in patients with systemic hypertension, aortic stenosis, or hypertrophic cardiomyopathy. At very fast heart rates, S3 and S4 may merge to create a single, prominent sound called a summation gallop.
Heart Murmurs: Sounds of Turbulent Flow
Heart murmurs are distinct from the sharp, brief heart sounds; they are relatively prolonged sounds described as a whooshing or swishing, caused by turbulent blood flow. This turbulence results from either flow through a narrow or irregular opening (stenosis), backward flow across an incompetent valve (regurgitation or insufficiency), or flow through an abnormal pathway (shunt). Murmurs are classified by their timing within the cardiac cycle. Systolic Murmurs occur between S1 and S2 and include Ejection Murmurs, such as those from aortic or pulmonic stenosis, and Regurgitant Murmurs, such as mitral or tricuspid regurgitation. Diastolic Murmurs occur between S2 and the next S1 and are generally indicative of more serious conditions, most commonly valve stenosis (mitral or tricuspid) or aortic/pulmonic regurgitation. Murmurs are clinically graded by intensity on a scale of 1 (very faint) to 6 (audible with the stethoscope entirely off the chest, associated with a palpable vibration called a thrill), helping practitioners assess the severity of the underlying valvular disease.