Cardiac Output and Cardiac Cycle

Cardiac output is the amount of blood the heart pumps each minute. The stroke volume is multiplied by the heart rate to compute cardiac output. The rate of blood flow through the heart and its accompanying blood arteries is measured by cardiac output. Blood can move through the cardiac cycle because of pressure changes. The cardiac conduction system is in charge of this, and the medulla regulates it via the autonomic nervous system. 

The cardiac cycle involves all heart functions in one complete beating, including the atria and ventricles’ contraction and relaxation. Systole is the contraction event, while diastole is the relaxation event. At rest, the heart pumps around 5 litres of blood per minute across the body, although this may rise dramatically during activities and exercises. Every pulse follows a precisely regulated sequence to attain this high output properly.

What is Cardiac Output?

The amount of blood pumped by the heart in one minute is called the cardiac output. It is calculated by multiplying the heart rate (number of beats per minute) by the stroke volume (amount pumped per pulse).

Alternatively, it may be described as the amount of blood pumped per minute by the left (or right) ventricle. The pulmonary blood flow is the cardiac output pumped by the RV. 

The stroke volume is the amount of blood pumped per beat (SV). HR stands for heart rate. Cardiac output is calculated by multiplying the heart rate by the stroke volume.

CO = HR x SV in [L/min] 

The value of CO will rise as  HR, SV, or both rise. Overall, SV does not change significantly, with only a minor rise in response to activity. On the other hand, HR rises rapidly and is the most critical factor in raising CO.

Exercise-induced increases in CO are critical because the Cardiovascular system is attempting to fulfil the body’s needs to deliver oxygen-rich blood and eliminate waste. Doctors use the test to discover if you have heart disease or assess how effective therapy or medication is. 

Preload, contractility, and afterload all influence stroke volume. The usual range for cardiac output is 4 to 8 L/min. The importance of cardiac output is that it forecasts oxygen supply to cells.

The degree to which cardiac muscle cells are stretched by blood entering the heart chambers is preloaded. The degree to which cardiac muscle cells contract to respond to external forces is contractility. Afterload is the pressure created by the ventricles to open the semilunar valves. When the heart rate rises, the cardiac output rises as well. Additionally, as stroke volume rises, cardiac output also rises.

What does a low Cardiac Output mean?

While we have read the cardiac output definition, let us understand what a low cardiac output means. A low cardiac output means that the heart pumps insufficient blood to fulfil the body’s metabolic demands. The most evident causal reasons are pathologic alterations in the heart’s muscle or electrical conduction system, congenital cardiac malformations, and electrolyte imbalances.

Low cardiac output syndrome (LCOS) is a medical disorder induced by a temporary reduction in peripheral perfusion cardiac failure. Physical activity of a sort of intensity that the patient cannot endure due to lower oxygen supply might lead to alterations in a patient’s functional capabilities due to decreased cardiac output.

This results in an imbalance between oxygen supply and oxygen consumption at the cellular level, resulting in metabolic acidosis. Although LCOS is most typically seen in individuals after cardiac surgery, it can also be found in various diseases that cause cardiac dysfunction.

Cardiac Output Cycle

The cardiac cycle definition can be written as a time-dependent description of pressure, volume, and flow processes in the ventricles. Both the left and right ventricles go through the same cycle.

Diastole and systole are the two stages of the Cardiac cycle. During diastole, blood from the superior and inner vena cava returns to the heart and passes into the right atrium. As blood rushes into the right atrium, the pressure inside it rises. The tricuspid valve opens passively when the pressure in the right atrium exceeds the pressure in the right ventricle, enabling blood to flow into the right ventricle. The oxygenated blood returning from the lungs flows into the left atrium simultaneously. The mitral valve opens when the left atrial pressure rises, allowing blood to flow into the left ventricle. 

There are four phases to the cardiac cycle:

• During diastole and atrial systole, the ventricles fill with blood.
• Isovolumetric contraction — the ventricles contract, increasing pressure in preparation for blood pumping into the aorta/pulmonary trunk.
• The ventricles contract, forcing blood into the aorta and pulmonary trunk throughout the outflow phase. Systole is another name for this.
• Isovolumetric relaxation — the ventricles relax, preparing for the next filling phase.

How do measure cardiac output?

Everyone’s normal cardiac output varies based on their size. An adult heart pumps around 5 litres of blood each minute at rest. When you run or exercise, your heart may beat triple or quadruple times to ensure that your body receives sufficient oxygen and food. The cardiac output can be measured using a pulmonary artery catheter, arterial pulse waveform analysis, or echocardiography.

Conclusion

The effective amount of blood released by either ventricle of the heart per unit of time (typically per minute) is known as cardiac output. It usually refers to left ventricular output. It is calculated by multiplying the stroke volume by the heart rate. A quantity of 4 to 8 litres per minute is normal.

There comes the point when the blood pumped by the heart is insufficient to support the body’s metabolic demands. Tissue perfusion difficulties emerge when cardiac output is reduced.

The cardiac cycle refers to all events that occur from the start of one heartbeat to the next. It is split into two parts: diastole, which is a period of relaxation, and systole, which is a period of contraction. During the cardiac cycle, pressure and shifts occur.