The cardiac cycle is the rhythmic sequence of heart muscle contraction and relaxation. It's a crucial process that pumps blood throughout the body, maintaining life. Understanding the cycle's phases and associated heart sounds is key to grasping how the heart functions.

This topic dives into systole and diastole, pressure and volume changes, and the distinct phases of the cardiac cycle. It also covers heart sounds, which provide valuable insights into heart health and function. These concepts are fundamental to understanding the heart's role in the cardiovascular system.

Systole vs Diastole

Defining Systole and Diastole

• Systole is the contraction phase of the cardiac cycle during which the heart muscle contracts and pumps blood out of the heart
• Diastole is the relaxation phase of the cardiac cycle during which the heart muscle relaxes and the chambers fill with blood
• The cardiac cycle consists of one complete sequence of systole and diastole which occurs with each heartbeat (60-100 beats per minute in adults)

Timing and Duration of Systole and Diastole

• Systole lasts for a shorter period than diastole, typically about one-third of the total cardiac cycle duration
• In a normal resting heart rate (60-100 bpm), systole lasts approximately 0.3-0.4 seconds, while diastole lasts about 0.4-0.7 seconds
• The relative durations of systole and diastole change with heart rate; as heart rate increases, diastole shortens more than systole

Pressure and Volume Changes in the Heart

Ventricular Pressure and Volume Changes

• During systole, ventricular pressure increases as the ventricles contract, causing the aortic and pulmonary valves to open and blood to be ejected from the heart
• During diastole, ventricular pressure decreases as the ventricles relax, allowing the mitral and tricuspid valves to open and blood to flow from the atria into the ventricles
• Ventricular volume decreases during systole as blood is ejected (stroke volume) and increases during diastole as the ventricles fill with blood (end-diastolic volume)

Atrial Pressure and Volume Changes

• Atrial pressure and volume changes are less pronounced than ventricular changes, with a slight increase in pressure during atrial systole and a decrease during atrial diastole
• During atrial systole, atria contract and pump blood into the ventricles, causing a small increase in atrial pressure (a-wave)
• During atrial diastole, atria relax and fill with blood from the venae cavae and pulmonary veins, leading to a gradual increase in atrial volume and a slight decrease in pressure

Phases of the Cardiac Cycle

Ventricular Phases

• Isovolumetric ventricular contraction: Ventricles begin to contract, all valves are closed, and ventricular pressure rises without a change in volume
• Ventricular ejection: Aortic and pulmonary valves open, and blood is ejected from the ventricles into the aorta and pulmonary arteries
• Isovolumetric ventricular relaxation: Ventricles begin to relax, all valves are closed, and ventricular pressure decreases without a change in volume
• Ventricular filling: Mitral and tricuspid valves open, allowing blood to flow from the atria into the ventricles (rapid filling phase followed by diastasis)

Atrial Phases

• Atrial systole: Atria contract, pumping blood into the ventricles (occurs at the end of ventricular diastole)
• Atrial diastole: Atria relax and fill with blood from the venae cavae and pulmonary veins (occurs during ventricular systole and most of ventricular diastole)

Heart Sounds: Origin and Significance

First and Second Heart Sounds

• The first heart sound (S1) is caused by the closure of the mitral and tricuspid valves at the beginning of ventricular systole (lub sound)
• The second heart sound (S2) is caused by the closure of the aortic and pulmonary valves at the end of ventricular systole (dub sound)
• S1 and S2 are normally clear, distinct sounds that can be heard using a stethoscope over the precordium

Clinical Significance of Heart Sounds

• Heart sounds can be used to assess the function of the heart valves and the timing of the cardiac cycle events
• Abnormal heart sounds, such as murmurs or extra sounds, may indicate underlying cardiac pathology, such as valve stenosis, regurgitation, or septal defects
• Splitting of S1 or S2 can occur in various conditions affecting the synchrony of valve closure (bundle branch blocks, atrial septal defect)
• Additional heart sounds, like S3 and S4, may be heard in certain pathological conditions (heart failure, hypertrophic cardiomyopathy)