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• Definition of cardiac cycle
• The cardiac cycle refers to the sequence of mechanical and electrical events that occur in the heart during one complete heartbeat.
• Sequence of mechanical events during one heartbeat
• It includes alternating contraction and relaxation of the atria and ventricles, resulting in movement of blood through the heart and into the circulation.
• Includes systole and diastole of atria and ventricles
• Systole refers to the phase of contraction, while diastole refers to the phase of relaxation and filling of the heart chambers.
• Begins with atrial contraction and ends before next beat
• The cardiac cycle typically begins with atrial systole, followed by ventricular systole and ventricular diastole, and ends just before the next heartbeat begins.
• Importance in maintaining effective circulation
• The coordinated events of the cardiac cycle ensure efficient filling of the heart chambers and effective pumping of blood into the pulmonary and systemic circulation.
• Relationship with heart sounds and ECG
• Events of the cardiac cycle are closely related to electrical changes recorded in the electrocardiogram (ECG) and to heart sounds produced by valve closure.
• Clinical relevance
• Understanding the cardiac cycle is important for interpreting heart sounds, ECG changes, and diagnosing various cardiovascular disorders.
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• Duration of one cardiac cycle at normal heart rate
• At a normal resting heart rate of about 75 beats per minute, one cardiac cycle lasts approximately 0.8 seconds.
• Relationship with heart rate
• The duration of the cardiac cycle is inversely related to heart rate.
• When heart rate increases, the cardiac cycle becomes shorter.
• When heart rate decreases, the cardiac cycle becomes longer.
• Phases of cardiac cycle
• The cardiac cycle includes three main phases:
• Atrial systole
• Ventricular systole
• Ventricular diastole.
• Duration of atrial systole
• Atrial contraction lasts about 0.1 second and helps complete ventricular filling.
• Duration of ventricular systole
• Ventricular systole lasts about 0.3 seconds, during which blood is ejected into the pulmonary artery and aorta.
• Duration of ventricular diastole
• Ventricular diastole lasts about 0.5 seconds, allowing ventricular relaxation and filling with blood.
• Variation with changes in heart rate
• With increased heart rate, the duration of diastole shortens more than systole, which may reduce the time available for ventricular filling and coronary perfusion.
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• Relationship between atrial and ventricular events
• The cardiac cycle involves coordinated activity of atria and ventricles.
• Atrial contraction occurs first, pushing blood into the ventricles, followed by ventricular contraction to pump blood into the arteries.
• Pressure changes in atria and ventricles
• During the cardiac cycle, pressure in the heart chambers changes continuously.
• Atrial pressure rises during atrial systole, while ventricular pressure rises sharply during ventricular systole.
• Volume changes in ventricles
• Ventricular volume increases during ventricular filling in diastole.
• During ventricular systole, the volume decreases as blood is ejected into the pulmonary artery and aorta.
• Opening and closing of cardiac valves
• Cardiac valves open and close depending on pressure differences between chambers and vessels.
• Atrioventricular valves open during ventricular filling and close during ventricular contraction.
• Semilunar valves open during ventricular systole and close during ventricular diastole.
• Blood flow through chambers
• Blood flows sequentially from atria → ventricles → great arteries, ensuring efficient circulation.
• Correlation with heart sounds
• Valve closure produces heart sounds.
• First heart sound (S1) occurs due to closure of atrioventricular valves, and second heart sound (S2) occurs due to closure of semilunar valves.
• Coordination between electrical and mechanical events
• Electrical activity recorded on the electrocardiogram (ECG) precedes mechanical events.
• P wave corresponds to atrial depolarization and atrial contraction, while QRS complex precedes ventricular contraction, and T wave corresponds to ventricular repolarization.
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• Conceptual diagram representing cardiac cycle
• The cardiac cycle is sometimes illustrated using a circular diagram with two concentric rings to show the sequence of atrial and ventricular events.
• Inner ring showing atrial contraction and relaxation
• The inner ring represents atrial events, including atrial systole (contraction) followed by atrial diastole (relaxation).
• Outer ring showing ventricular contraction and relaxation
• The outer ring represents ventricular events, including ventricular systole and ventricular diastole.
• Relationship between atrial and ventricular phases
• The diagram shows that atrial systole occurs slightly before ventricular systole, allowing the atria to push blood into the ventricles before ventricular contraction begins.
• Visualization of synchronized cardiac activity
• This representation helps visualize the precise coordination between atrial and ventricular activities, ensuring efficient filling and pumping of blood during each heartbeat.
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• Beginning of ventricular systole
• Ventricular systole begins when the ventricular myocardium depolarizes following the QRS complex of the ECG.
• This electrical activation initiates contraction of the ventricular muscle fibres.
• Isovolumetric contraction phase
• At the start of ventricular contraction, all cardiac valves remain closed.
• During this phase, the ventricular muscle contracts but the volume of blood inside the ventricles does not change, hence the term isovolumetric contraction.
• Closure of atrioventricular valves
• As ventricular pressure begins to rise, it becomes greater than atrial pressure, causing the atrioventricular valves (mitral and tricuspid) to close.
• This valve closure produces the first heart sound (S1).
• Increase in ventricular pressure
• With continued contraction, ventricular pressure increases rapidly because the ventricles are contracting against closed valves.
• Opening of semilunar valves
• When ventricular pressure exceeds the pressure in the aorta and pulmonary artery, the semilunar valves (aortic and pulmonary valves) open.
• Beginning of ventricular ejection
• Once the semilunar valves open, blood is ejected from the ventricles into the pulmonary artery and aorta, marking the start of the ventricular ejection phase.
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• Rapid ejection phase
• After the semilunar valves open, blood is expelled quickly from the ventricles into the aorta and pulmonary artery.
• During this phase, most of the ventricular stroke volume is ejected.
• Reduced ejection phase
• As ventricular contraction begins to decline, the rate of blood ejection decreases.
• Blood continues to leave the ventricles but more slowly compared with the rapid ejection phase.
• Decrease in ventricular volume
• During the ejection phases, the volume of blood inside the ventricles decreases significantly as blood is pumped into the major arteries.
• Continued ventricular contraction
• Ventricular muscle fibres remain contracted throughout the ejection phases, maintaining the pressure required to push blood into the arteries.
• Changes in arterial pressure
• As blood enters the aorta and pulmonary artery, the pressure in these arteries rises, producing the systolic arterial pressure.
• Blood flow into aorta and pulmonary artery
• The left ventricle pumps oxygenated blood into the aorta, supplying the systemic circulation.
• The right ventricle pumps deoxygenated blood into the pulmonary artery, sending it to the lungs for oxygenation.
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• Beginning of ventricular relaxation
• Ventricular diastole begins when the ventricular muscle fibres start to relax after systole.
• This relaxation occurs following ventricular repolarization represented by the T wave in the ECG.
• Closure of semilunar valves
• As ventricular pressure falls below the pressure in the aorta and pulmonary artery, the semilunar valves close.
• This valve closure produces the second heart sound (S2).
• Isovolumetric relaxation phase
• Immediately after semilunar valve closure, the ventricles relax while all cardiac valves remain closed.
• During this phase, ventricular pressure decreases rapidly but ventricular volume remains constant, hence the term isovolumetric relaxation.
• Opening of atrioventricular valves
• When ventricular pressure becomes lower than atrial pressure, the atrioventricular valves (mitral and tricuspid) open.
• Rapid ventricular filling
• Blood accumulated in the atria during ventricular systole flows rapidly into the ventricles, producing the rapid filling phase.
• Reduced filling (diastasis)
• After rapid filling, the rate of ventricular filling gradually decreases, entering a phase called diastasis, where blood continues to flow slowly into the ventricles until the next atrial contraction begins.
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• Atrial systole
• The cardiac cycle begins with atrial contraction, during which the atria push the remaining blood into the ventricles, completing ventricular filling.
• Isovolumetric ventricular contraction
• Ventricular contraction begins with closure of the atrioventricular valves.
• All valves remain closed, so ventricular pressure rises rapidly without change in ventricular volume.
• Rapid ventricular ejection
• When ventricular pressure exceeds the pressure in the aorta and pulmonary artery, the semilunar valves open, allowing rapid ejection of blood from the ventricles.
• Reduced ventricular ejection
• As ventricular contraction begins to decline, blood continues to be ejected but at a slower rate.
• Isovolumetric ventricular relaxation
• Ventricular muscle begins to relax after ejection.
• Semilunar valves close and all valves remain closed while ventricular pressure falls rapidly.
• Rapid ventricular filling
• When ventricular pressure becomes lower than atrial pressure, the atrioventricular valves open, allowing blood to flow rapidly from atria into ventricles.
• Reduced ventricular filling (diastasis)
• Ventricular filling continues at a slower rate during diastasis, as blood gradually enters the ventricles.
• Preparation for next cardiac cycle
• At the end of diastasis, the heart prepares for the next atrial systole, and the cycle repeats with the next heartbeat.
• Relative duration of each phase
• The phases of the cardiac cycle occur in a precise sequence with specific time relationships, ensuring coordinated cardiac function.
• At a heart rate of about 75 beats per minute, the total cardiac cycle lasts approximately 0.8 seconds.
• Atrial systole timing
• Atrial systole lasts about 0.1 second.
• During this period, the atria contract and push the remaining blood into the ventricles, completing ventricular filling.
• Ventricular systole timing
• Ventricular systole lasts about 0.3 seconds.
• This phase includes isovolumetric contraction followed by rapid and reduced ventricular ejection.
• Ventricular diastole timing
• Ventricular diastole lasts about 0.5 seconds.
• During this time, the ventricles relax and fill with blood through rapid filling and diastasis.
• Changes during increased heart rate
• When the heart rate increases, the total duration of the cardiac cycle decreases.
• The diastolic period shortens more significantly than systole, reducing the time available for ventricular filling.
• Importance for ventricular filling
• Adequate diastolic duration is important for proper ventricular filling and optimal cardiac output, especially during conditions such as exercise or tachycardia.
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