Cardiac Cycle Overview

Cardiac Cycle Duration

• The cardiac cycle is the sequence of events that occur in the heart during one heartbeat, consisting of one systole (contraction) and one diastole (relaxation).
• A typical cardiac cycle lasts 0.8 seconds at a heart rate of 70-75 beats per minute.
• Atrial systole lasts 0.1 seconds, ventricular systole lasts 0.3 seconds, atrial diastole lasts 0.7 seconds, and ventricular diastole lasts 0.5 seconds.
• The duration of the cardiac cycle is inversely proportional to the heart rate, meaning a faster heart rate leads to a shorter cycle duration.
• The decrease in cardiac cycle duration primarily occurs due to a shortening of diastolic time.

Cardiac Cycle Events and Phases

• The cardiac cycle consists of three main events: atrial systole, ventricular systole, and ventricular diastole.
• These events are further divided into five phases: atrial systole phase, isometric contraction phase, ventricular ejection phase (includes maximum and reduced ejection), isometric relaxation phase, and ventricular filling phase (includes maximum and reduced filling).

Ventricular Pressure Changes During Cardiac Cycle

• Atrial systole phase: Atrial contraction leads to increased ventricular pressure as blood flows from the atria to the relaxed ventricles through open atrioventricular (AV) valves. Ventricular pressure then decreases as the ventricles relax to accommodate the incoming blood.
• Isometric contraction phase: AV valves close, semilunar valves remain closed, and ventricles contract isometrically (without changing volume) leading to a rise in ventricular pressure (to 80 mmHg in the left ventricle and 10 mmHg in the right ventricle). This pressure eventually opens the semilunar valves.
• Maximum ejection phase: Ventricular contraction continues, forcefully ejecting blood into the aorta and pulmonary artery through open semilunar valves. This leads to a further increase in ventricular pressure to 120 mmHg in the left ventricle and 25 mmHg in the right ventricle, as ventricular contraction pressure exceeds the pressure drop due to blood flow.
• Reduced ejection phase: Blood ejection continues but at a slower rate, resulting in a decrease in ventricular pressure as the pressure generated by contraction becomes less than the pressure drop due to blood flow.
• Isometric relaxation phase: Semilunar valves close, AV valves remain closed, and ventricles relax isometrically. Ventricular pressure decreases. When ventricular pressure falls below atrial pressure, the AV valves open.
• Maximum filling phase: Ventricles relax, leading to a rapid influx of blood from the atria due to gravity and pressure difference. Ventricular pressure decreases as relaxation pressure exceeds the pressure increase due to filling.
• Reduced filling phase: Ventricular filling continues, but at a slower rate, leading to a slight increase in ventricular pressure as relaxation pressure becomes less than the rise due to filling.

Ventricular Volume Changes During Cardiac Cycle

• Atrial systole: Increase
• Isometric contraction: Constant
• Maximum ejection: Decrease
• Reduced ejection: Decrease
• Isometric relaxation: Constant
• Maximum filling: Increase
• Reduced filling: Increase

Valve States During Cardiac Cycle

• Atrioventricular (AV) valves: Open during maximum filling, reduced filling, and atrial systole phases.
• Semilunar valves: Open during maximum ejection and reduced ejection phases.

Atrial Pressure Changes During Cardiac Cycle

• Atrial pressure: Shows three distinct waves (a, c, and v), each with a positive (increasing pressure) and negative (decreasing pressure) phase.
• a wave: Coincides with atrial systole phase. The positive "a" is due to atrial contraction, and the negative "a" is due to blood flow from the atria to the ventricles.
• c wave: Coincides with isometric contraction. The positive "c" is due to the bulging of AV valve cusps into the atrial cavity. The negative "c" is due to the descent of the AV valve cusps.
• v wave: Coincides with reduced ejection and isometric relaxation phases. The positive "v" is due to blood accumulation in the atria. The negative "v" coincides with ventricular filling phases. The initial decrease is due to blood flow from the atria to the ventricles, followed by a gradual increase due to venous return to the atria.

Aortic Pressure Changes During Cardiac Cycle

• Ascending (anacrotic) limb: Coincides with maximum ejection phase. Aortic pressure increases to 120 mmHg due to a greater amount of blood entering the aorta than leaving it.
• Descending (catacrotic) limb:
  • Reduced ejection phase: Aortic pressure decreases as less blood enters the aorta than leaves it.
  • Dicrotic notch: Sudden drop in aortic pressure at the end of ejection due to aortic valve closure.
  • Dicrotic wave: Slight increase in aortic pressure due to elastic recoil of the aortic wall during isometric relaxation.
  • Gradual decrease: Aortic pressure gradually decreases as blood flows from the aorta to peripheral vessels during the filling phases (atrial systole and isometric contraction phases).

Heart Sounds

• First heart sound: Caused by the sudden closure of AV valves at the beginning of isometric contraction.
• Second heart sound: Caused by the sudden closure of semilunar valves at the beginning of isometric relaxation.

ECG During Cardiac Cycle

• ECG reflects the summed electrical activity of all cardiac muscle fibers, preceding mechanical activity.
• ECG shows P wave, QRS complex, and T wave.
• The P wave reflects atrial depolarization and precedes atrial contraction.
• The QRS complex represents ventricular depolarization, initiating ventricular contraction.
• The P wave starts 0.02 seconds before atrial systole.
• The QRS complex starts 0.02 seconds before isometric contraction.

Remarks on Cardiac Cycle

• Ventricular diastole constitutes about 2/3 of the cardiac cycle duration, while ventricular systole constitutes about 1/3.
• This is important because ventricular diastole allows for:
  • Coronary blood flow
  • Ventricular filling, mainly in early diastole
  • Ventricular rest
• Ventricular diastole is further divided into three stages:
  • Early Diastole: Includes isometric relaxation and maximum filling phases
  • Mid Diastole: Includes reduced filling phase
  • Late Diastole: Includes atrial systole
• Ventricular filling occurs during three phases: maximum filling, reduced filling, and atrial systole. 70-80% of blood passively enters the ventricles during the filling phases, particularly during the rapid filling phase. The remaining 20-30% is propelled by atrial systole, making the atria function primarily as blood reservoirs rather than significant pumps.

Pressure Changes Summary

• The systolic pressure in each ventricle matches its corresponding vessel, while diastolic pressure in both ventricles is equal to 0 mmHg.

Blood Flow During Cardiac Cycle

• Blood flows to tissues during both systole and diastole.
• During systole, blood flow is driven by ventricular contraction and ejection.
• During diastole, blood flow is maintained by elastic recoil of the aorta, generating sufficient diastolic pressure to sustain tissue perfusion.

This quiz covers the duration and phases of the cardiac cycle, detailing events such as atrial and ventricular systole and diastole. It explains how the cardiac cycle duration is affected by heart rate and provides insights into the sequence of events during one heartbeat.