Cardiac Cycle Phases

• In a normal heart, cardiac activity repeats in a regular cycle.
• With a resting heart rate of 75 beats/min, each cardiac cycle lasts 0.8 seconds.

Atrial and Ventricular Cycles

• The atrial cycle consists of 0.11 seconds in systole and 0.69 seconds in diastole.
• The ventricular cycle consists of 0.3 seconds in systole and 0.5 seconds in diastole.

Phases of the Cardiac Cycle

• There are 3 major phases of the cardiac cycle, subdivided into 7 phases.
• The sequence of events are atrial systole, ventricular systole, and diastole of the whole heart.
• Atrial and ventricular systoles do not occur at the same time, but their diastoles occur at the same time during the diastole of the whole heart.
• Cardiac valves open and close
• Heart sounds are recorded in a phonocardiogram.
• Changes occur on an electrocardiogram (ECG).
• Ventricular volume changes
• Intra-atrial pressure changes
• Intraventricular pressure changes
• Aortic blood pressure changes

Heart Valves

• Atrioventricular valves include the tricuspid and bicuspid (mitral) valves.
• Semilunar valves include the aortic and pulmonary valves.
• The purpose of heart valves is to prevent blood from flowing backward.

Atrial Systole

• Duration is 0.11 seconds.
• Ventricles are in their late diastole
• 25% of ventricular diastolic filling volume (DFV) pumps to ventricles during this time.
• It's not essential for ventricular filling.
• Semilunar valves are closed and atrioventricular valves are open.
• S4 heart sounds can occur as the atrium contracts against a stiffened ventricle.
• The P wave on the ECG precedes atrial systole, marking atrial depolarization.
• Left atrial pressure increases due to atrial contraction and manifests as the 'a' wave on the venous pulse record.
• A transient increase in left ventricular pressure occurs as blood ejects from the atrium.
• Ventricular volume increases from blood ejecting from the left atrium through the open mitral valve.
• Aortic pressure gradually decreases.

Isovolumetric Ventricular Contraction

• Ventricles contract, but there is no change in volume and blood ejection has not started.
• Semilunar and atrioventricular valves are closed.
• When left ventricular pressure exceeds left atrial pressure, the mitral valve closes.
• The tricuspid valve closes with the same mechanism in the right heart.
• S1 heart sounds occur as atrioventricular valves close, due to the mitral valve closing slightly before the tricuspid valve.
• This phase begins during the QRS complex, which represents the electrical activation of the ventricles.
• Left atrial pressure increase.
• Left ventricular pressure increases dramatically, from 0 to 80 mmHg.
• Ventricular volume remains constant.
• Aortic pressure gradually decreases to 80 mmHg.

Rapid Ventricular Ejection

• The aortic valve opens, and blood is ejected into the aorta at a high rate.
• The ventricle continues to contract and reaches its highest pressure value
• The aortic valve opens when ventricular pressure exceeds aortic pressure.
• Semilunar valves are open and atrioventricular valves are closed.
• First heart sounds continue.
• The T wave represents repolarization of the ventricles and marks the end of ventricular contraction and rapid ventricular ejection.
• Left atrial pressure increases as atrial filling begins and blood returns to the left heart from the pulmonary circulation.
• The C wave on the venous pulse curve occurs from the bulging of the tricuspid valve.
• Left ventricular pressure increases more slowly to a maximum of 120 mmHg.
• Ventricular volume decreases sharply as most of the stroke volume is ejected.
• Aortic pressure increases due to a large volume of blood being suddenly added to the aorta.

Reduced Ventricular Ejection

• Ejection of blood continues at a slow rate.
• Semilunar valves are open, and atrioventricular valves are closed.
• There are no heart sounds during this phase.
• Ventricles begin to repolarize, representing the beginning of the T wave on the ECG.
• Left atrial pressure continues to increase as blood returns from the lungs.
• The V wave represents blood flow from the right atrium into the right ventricle.
• Left ventricular pressure gradually decreases to 110 mmHg.
• Ventricular pressure falls because the ventricles stop contracting.
• Ventricular volume continues to decrease, but at a reduced rate.
• Aortic pressure falls.

Isovolumetric Ventricular Relaxation

• The ventricles relax but there is no change in volume.
• Ventricular filling has not started yet.
• Semilunar and atrioventricular valves are closed.
• Heart sound S2 is present.
• Inspiration delays closure of the pulmonic valve and causes splitting of the second heart sound.
• Splitting occurs during inspiration due to the decrease in intrathoracic pressure, which produces an increase in venous return to the right side of the heart.
• The increase in right ventricular end-diastolic volume leads to an increase in right ventricular stroke volume, due to Frank-Starling mechanism
• This prolongs right ventricular ejection time.
• The ECG marks repolarization of the ventricles by the end of the T wave.
• Left atrial pressure increases.
• Left ventricular pressure decreases sharply from 110 to 0 mmHg.
• The ventricle is relaxed, and when left ventricular pressure falls, the aortic valve closes.
• No change in ventricular volume since all valves are closed.
• A dicrotic wave appears: at aortic valve closure, the aortic pressure curve shows a "blip," called the dicrotic notch or incisura.

Rapid Ventricular Filling

• Atrioventricular (A-V) valves open, and blood flows from the atria to the ventricles at a high rate.
• 55% of diastolic filling volume (DFV) flows into the ventricle.
• The mitral valve opens when ventricular pressure falls to its lowest level slightly below left atrial pressure.
• Semilunar valves are closed, and atrioventricular valves are open during this phase.
• S3 heart sounds occur
• There is rapid flow of blood from the atria to the ventricles.
• An S3 is not heard in normal adults as it is normal in children, and in middle-aged or older adults, indicates volume overload, congestive heart failure, or advanced mitral or tricuspid regurgitation.
• The ECG shows a T-P segment.
• Left atrial pressure decreases.
• Left ventricular pressure remains low, as the ventricle is relaxed and compliant.
• Due to the high compliance of the ventricle, a large volume can be added without changing the pressure.
• Once the mitral valve opens, the ventricle begins to fill with blood from the left atrium, and ventricular volume increases rapidly.
• During this phase, and continuing for the cardiac cycle, aortic pressure decreases.
• This occurs as blood runs off from the aorta into the arterial tree, to the veins, and back to the heart.

Reduced Ventricular Filling (Diastasis)

• This is the longest phase of the cardiac cycle that includes the final portion of ventricular filling at a slower rate.
• Diastasis is the period of diastole between early and late filling.
• Assumed quiescent, with balance between left atrial and left ventricular pressures.
• Ventricular filling continues at a slow rate, with 20% of the ventricular diastolic filling volume (DFV) filling the ventricle
• It is the final portion of ventricular filling.
• Duration is 0.2 seconds.
• Semilunar valves are closed.
• Atrioventricular valves are open.
• There are no heart sounds.
• The ECG shows the beginning of the P wave.
• Left atrial and ventricular pressure increase.
• Ventricular volume increases, and atrial systole marks the end, as volume equals to end-diastolic volume.
• Aortic pressure decreases during this phase.
• Changes in heart rate alter the time available for diastasis.
• Increased heart rate reduces the time interval before the next cycle and the final portion of ventricular filling.
• Frank-Starling relationship says, if reduced diastasis is reduced by an increase in heart rate, ventricular filling will be compromised.
• End-diastolic volume and stroke volume will be reduced, aortic pressure decreases as blood runs off from the aorta into the arterial tree.

Intra-Atrial Pressure Curve

• The intra-atrial pressure curve is similar to the jugular venous pulse tracing.
• It has three positive waves (a, c, v) and three negative waves (x, x1, y).
• 'a' Wave: First positive wave occurring during atrial systole.
• Pressure rises sharply.
• ‘x’ Wave: First negative wave during the onset of atrial diastole.
• Atrioventricular valves close.
• ‘c’ Wave: Second positive wave during isometric contraction.
• Rise in pressure due to the closure of atrioventricular valves and the increased intraventricular pressure.
• ‘x1’ Wave: Second negative wave appearing during the ejection period.
• Contraction pulls atrioventricular rings, causing a fall in atrial pressure.
• ‘v’ Wave: Third positive wave observed during atrial diastole. It shows a gradual increase in atrial pressure due to venous return.
• ‘y’ Wave: Third negative wave that appears after the opening of AV valves, when blood rushes from atria into ventricles.

Summary Points

• In a normal heart, cardiac activity repeats in a regular cycle.
• There are 3 major phases of the cardiac cycle: atrial systole, ventricular systole, and diastole of the whole heart, subdivided into a total of 7 phases. The right side heart chambers have similar pressures, but quantitatively lower values.