CARDIAC CYCLE

CARDIAC CYCLE

The cardiac cycle refers to the sequence of mechanical and electrical events that repeats with every heartbeat. It is essentially the process of the heart alternating between a filling phase and an emptying phase.

Duration of the Cardiac Cycle:

• The duration of the cardiac cycle is the reciprocal of the heart rate.

• For a heart rate of 75 beats per minute, the cardiac cycle lasts approximately 0.8 seconds (or 800 milliseconds).

• The cycle consists of two major periods:

    ◦ Systole: The phase of contraction, where blood is ejected from the heart. At a heart rate of 75 bpm, ventricular systole occupies approximately 300 milliseconds (0.3 seconds)

.

    ◦ Diastole: The phase of relaxation, where the heart chambers fill with blood. At 75 bpm, ventricular diastole occupies approximately 500 milliseconds (0.5 seconds).

• With increasing heart rate, diastole shortens relatively more than systole does.

Phases of the Cardiac Cycle

The cardiac cycle can be divided into distinct phases based on the actions of the ventricles and the positions of the cardiac valves. The events in the right and left sides of the heart are similar, though pressures on the right side are typically lower.

Ventricular Diastole

1. Protodiastole of ventricles

Ventricles start relaxation and the pressure in the ventricles starts decreasing.

Ventricular pressure is more than atrial pressure.

Ventricular pressure is less than aortic pressure.

Aortic pressure shows a characteristic dicrotic notch (or incisura), which is a small upward deflection caused by the elastic recoil of the aorta and the brief backflow of blood against the closed aortic valve.

• Heart Sounds: The second heart sound (S2), described as "DUB," occurs at the onset of this phase. It is caused by the simultaneous closure of the semilunar valves (aortic and pulmonary). S2 is typically shorter and higher-pitched than S1.

2. Isovolumetric Ventricular Relaxation (Diastole)

This is the initial phase of ventricular diastole where the ventricles begin to relax, and their volume remains constant because all valves are closed.

• Duration: Approximately 0.1 seconds. This phase, along with ventricular filling, constitutes the entire ventricular diastole.

• Valve Status: The semilunar valves close due to the backflow of blood as ventricular pressure falls below arterial pressure. The AV valves remain closed because ventricular pressure is still higher than atrial pressure.

• Pressure and Volume Changes:

    ◦ Ventricular pressure falls precipitously to near zero.

• Ventricular pressure is more than atrial pressure.

• Ventricular pressure is less than aortic pressure.

    ◦ Ventricular volume remains constant.

• ECG Correlation: S2 coincides with the end of the T wave on the ECG, marking the completion of ventricular repolarization. The period on the ECG from the end of the T wave to the beginning of the next P wave (TP segment) represents the time when all four cardiac chambers are relaxed

3. Ventricular Filling (Diastole)

This is the period when the ventricles are relaxed and filling with blood.

• Duration: Approximately 0.5 seconds at a normal heart rate of 75 bpm. Atrial systole contributes to the final part of ventricular filling, lasting about 0.1 seconds.

• Valve Status: The atrioventricular (AV) valves (mitral and tricuspid) are open, allowing blood to flow from the atria into the ventricles. The semilunar valves (aortic and pulmonary) are closed.

• Pressure and Volume Changes:

    ◦ Ventricular volume gradually increases.

• Ventricular pressure is less than atrial pressure.

• Ventricular pressure is less than aortic pressure.

    ◦ Pressures in both the atria and ventricles slowly rise.

    ◦ Atrial pressure is only slightly higher than ventricular pressure, reflecting low resistance across the open AV valves.

4. Rapid Ventricular Filling: Occurs immediately after the AV valves open, as blood rushes into the relaxed ventricles. The third heart sound (S3) may be heard during this rapid distension of the ventricles.

5. Diastasis (Reduced Ventricular Filling): A phase of slow filling where ventricular volume slowly rises and approaches a plateau, with minimal blood flow. This phase shortens significantly with increased heart rate.

6.  Atrial Systole: Occurs at the very end of ventricular diastole. It is a peristalsis-like contraction of the atria that completes ventricular filling by contributing about 20% to 30% of the ventricular end-diastolic volume (VEDV), which is typically 120-130 ml. The a wave is seen on the venous pulse curve during atrial systole. The fourth heart sound (S4) may be heard during forceful atrial systole, especially in conditions like ventricular hypertrophy.

• Ventricular pressure is less than atrial pressure.

• Ventricular pressure is less than aortic pressure.

• Heart Sounds:

S3 (Third Heart Sound): May be audible in early diastole during rapid ventricular filling.

S4 (Fourth Heart Sound): May be audible in late diastole, associated with forceful atrial contraction against a stiff ventricle.

• ECG Correlation: The P wave on the ECG, representing atrial depolarization (excitation), occurs at the end of diastasis and precedes atrial systole.

Ventricular Systole

1. Isovolumetric Ventricular Contraction (Systole)

This is the phase where the ventricles begin to contract, but their volume remains constant because all valves are closed.

• Duration: Approximately 0.05 seconds.

• Valve Status: The AV valves close due to the rapid rise in intraventricular pressure. The semilunar valves remain closed because the ventricular pressure has not yet exceeded the pressure in the great arteries.

• Pressure and Volume Changes:

    ◦ Ventricular pressure rises steeply and rapidly.

    ◦ Ventricular volume remains constant.

• Ventricular pressure is more than atrial pressure.

• Ventricular pressure is less than aortic pressure.

    ◦ Aortic pressure is around 80 mmHg (diastolic pressure) at the start of this phase.

• Heart Sounds: The first heart sound (S1), described as "LUB," occurs at the onset of this phase. It is caused by the simultaneous closure of the AV valves (mitral and tricuspid). S1 is typically prolonged, low-pitched, and booming.

• ECG Correlation: S1 coincides with the R-wave of the ECG, as the QRS complex represents ventricular depolarization and marks the electrical beginning of ventricular contraction.

Ventricular Ejection (Systole)

This is the phase during which the ventricles pump blood into the arteries.

• Duration: This phase itself lasts about 0.25 seconds

• Rapid ejection ~0.1s + 

• Reduced ejection ~0.15s. 

The total ventricular systole duration is about 0.3 seconds.

• Valve Status: The semilunar valves open when the intraventricular pressure exceeds the arterial pressure. The AV valves remain closed.

• Pressure and Volume Changes:

2. Rapid Ejection: Ventricular and aortic pressures rise sharply to a peak (systolic pressure, typically around 120 mmHg in the aorta). Ventricular volume decreases rapidly.

3. Reduced Ejection: Ejection of blood continues but at a slower pace. Ventricular and aortic pressures begin to decrease as blood flows from the large arteries to the periphery.

• Heart Sounds: No specific heart sounds are produced during this phase as the valves are open.

• ECG Correlation: This phase roughly corresponds to the ST segment and T wave on the ECG.

Significance in Medicine: Understanding the cardiac cycle is crucial for assessing cardiac function and diagnosing various heart conditions. 

Clinically, observing the cardiac cycle through methods like 

Auscultation (listening to heart sounds) and 

Electrocardiography (ECG) provides valuable insights into the heart's health.

For instance, abnormalities in heart sounds (murmurs) can indicate valve disorders, while ECG patterns help diagnose ischemic heart disease, arrhythmias, and heart blocks.