Cardiac Cycle and JVP

• A cardiac cycle refers to the mechanical and electrical events that occur with each heartbeat
• The heart consists of two upper chambers (atria) and two lower chambers (ventricles)
• The left atrium receives oxygenated blood from the lungs via the pulmonary veins
• The right atrium receives deoxygenated blood from organs/tissues via the superior and inferior vena cava
• The left ventricle pumps oxygenated blood to organs/tissues through the aorta
• The right ventricle pumps deoxygenated blood to the lungs via the pulmonary arteries
• Each heartbeat includes systole (heart contracts, pumps blood) and diastole (heart relaxes, ventricles fill)
• A cardiac cycle graph expresses events during one cycle

Graph Axes and JVP

• The y-axis of the cardiac cycle graph includes aortic pressure, left atrial pressure, left ventricular pressure, heart sounds, ventricular volume, right atrial pressure curve, and ECG
• The x-axis of the cardiac cycle graph represents time
• Jugular venous pulse (JVP) mirrors pressure changes in the right atrium
• Increased atrial pressure results in increased JVP, and vice versa

Cardiac Cycle Phases

• There are seven phases of the cardiac cycle
• Atrial contraction lasts about 0.1 seconds
• Isovolumetric ventricular contraction, rapid ventricular ejection, and reduced ventricular ejection are phases of ventricular systole and last about 0.3 seconds
• Isovolumetric ventricular relaxation, rapid ventricular filling, and reduced ventricular filling (diastasis) are phases of ventricular diastole and last about 0.4 seconds

Atrial Contraction

• The sinoatrial (SA) node fires, sending an electrical signal to depolarize the atria
• This corresponds to the P wave on the ECG
• Atrial depolarization leads to the contraction of the right and left atria
• Pressure within the left atrium increases during contraction
• Ventricular volume and pressure slightly increase as the atria pump blood into the ventricle
• An increase in right atrial pressure is noted as the a wave on the right atrial pressure curve
• The fourth heart sound (S4) may be heard due to vibration of a stiffened ventricular wall

Isovolumetric Contraction

• Follows atrial contraction
• Begins with the QRS complex on the ECG, representing ventricular depolarization
• Ventricular depolarization is followed by ventricular contraction
• Atrioventricular valves close when ventricular pressure exceeds atrial pressure, producing the first heart sound (S1)
• Both atrioventricular and aortic/pulmonary valves are closed, so blood volume in the ventricles remains constant ("isovolumetric")
• Ventricular pressure increases rapidly due to contraction
• Left ventricular pressure is less than aortic pressure, so the aortic valve remains closed
• Slight increase in left and right atrial pressure occurs due to the atrioventricular valves bulging into the atria
• This increase in atrial pressure is registered as the C wave on both left and right atrial pressure curves
• The pressure within the ventricles becomes higher than the pressure within the aorta and pulmonary arteries

Rapid Ventricular Ejection

• Follows isovolumetric contraction
• Aortic and pulmonary valves open at the start
• Rapid ejection of a large amount of blood from the ventricles
• Pressure from the left ventricle transmits equally to the aorta
• Ventricular and aortic pressures reach their maximum
• The volume of blood within the left ventricle decreases sharply
• This phase matches the ST segment on the ECG (period between ventricular depolarization and repolarization)

Reduced Ventricular Ejection

• Follows rapid ventricular ejection
• Marked by the beginning of the T wave on the ECG (ventricular repolarization)
• Blood outflow occurs due to inertial energy rather than ventricular contraction
• Ventricular pressure starts to decrease
• Ventricular volume gradually continues to fall
• Aortic pressure also starts to fall since less blood is leaving the ventricle
• Atrial pressure starts to increase as atria collect blood

Isovolumetric Ventricular Relaxation

• Begins ventricular diastole
• Marked by the end of the T wave on the ECG
• Ventricular pressure continues to fall as ventricles relax
• Aortic pressure is higher than left ventricular pressure, causing blood to flow backward toward the heart
• Backflow results in the dicrotic notch on the aortic pressure graph
• Aortic valve closes to prevent backflow, producing the second heart sound (S2)
• Aortic valve closes before the pulmonic valve
• Atrioventricular, aortic, and pulmonary valves are closed
• No blood enters or leaves the ventricles, so the volume within the ventricles remains the same (isovolumetric)
• The “v” wave is still going up, since atria are still filling with blood

Rapid Ventricular Filling

• The volume of blood within the ventricles remains the same, until ventricular pressure is lower than the atrial pressure
• Atrial pressure exceeds ventricular pressure, the two atrioventricular valves open
• Ventricles start to fill rapidly with blood from the atria
• Opening of atrioventricular valves leads to a rapid fall in atrial pressure, which is noted as the “y” descent
• Rapid filling of the ventricle may produce the third heart sound (normal in children, indicates volume overload/ventricular dilation in adults)

Reduced Ventricular Filling (Diastasis)

• Last and longest phase of the cardiac cycle
• Ventricles receive about 90% of blood during this and the previous phase
• The other 10% is received during atrial contraction
• 90% of ventricular filling occurs before atrial contraction (passive ventricular filling)
• Aortic pressure continues to fall

Cardiac Cycle Recap

• The cardiac cycle refers to the sequence of events that repeat during each heartbeat, a cardiac cycle graph expresses them
• The phases are atrial contraction, isovolumetric ventricular contraction, rapid ventricular ejection, reduced ventricular ejection, isovolumetric ventricular relaxation, rapid ventricular filling, and reduced ventricular filling (diastasis)
• Isovolumetric ventricular contraction, rapid ventricular ejection, and reduced ventricular ejection are phases of ventricular systole (heart contracts, pumps blood)
• Isovolumetric ventricular relaxation, rapid ventricular filling, and reduced ventricular filling are phases of ventricular diastole (heart relaxes, ventricles fill with blood)

Explanation of the cardiac cycle, including systole and diastole. Description of what a cardiac cycle graph expresses. The y-axis includes aortic pressure, left atrial pressure, left ventricular pressure, heart sounds, ventricular volume, right atrial pressure curve, and ECG, while the x-axis represents time.