Cardiac Output

Cardiac output definition

Cardiac output, also known as heart output, refers to the volume of blood pumped by the heart, which is done by the left and right ventricles per one compression or unit time.

Cardiac output is the amount of blood pumped by the heart per minute and is an excellent example of the human heart’s efficiency. At rest, a normal cardiac output results in pumping about 5-6 litres of blood per minute; this is known as the normal cardiac output. With the help of the calculation described later in the article, we will arrive at this figure.

The cardiac output is made up of two essential components:

• The number of heartbeats per minute is referred to as heart rate (bpm)
• The amount of blood pumped out of each ventricle with each heartbeat is stroke volume

Factors driving cardiac output

The cardiac output classification is done based on the factors that determine it. Listed below are some of the factors:

Venous Return

This is the amount of blood that flows through the veins into the heart each minute. The venous return equals the cardiac output after a specific time interval.

Force of Contraction

As the force of contraction increases, so does the volume of the stroke and the cardiac output. It refers to how hard the myocardium contracts for a specific preload. Increased contractility equals higher cardiac output and vice versa.

Positive inotropic factors such as sympathetic stimulation, caffeine, and hypercalcemia increase contractility. Parasympathetic stimulation, hypocalcaemia, potassium increase and myocardial hypoxia are negative inotropic factors that reduce contractility.

Heart Rate

The cardiac output increases as the heart rate rises. It is the number of beats per minute, and positive chronotropic factors influence the heart rate in a positive way (to raise the heart rate). Tachycardia is a fast heart rate of more than 100 beats per minute. Sympathetic stimulation, which increases heart rate by secreting adrenaline/noradrenaline or epinephrine/norepinephrine, is an example of a positive chronotropic factor. Hypercapnia, which causes increased CO2, and drugs like atropine are two other examples.

Negative chronotropic factors are to blame for bradycardia, defined as a heart rate of fewer than 60 beats per minute. Parasympathetic stimulation, whose main neurotransmitter is acetylcholine, hypoxia, which lowers oxygen and then lowers heart rate, and hypercalcemia, which raises calcium and lowers heart rate, are all examples.

Preload

It refers to the amount of blood that enters the ventricles. The increased blood volume entering the heart causes the ventricle to stretch and the preload to rise. It affects stroke volume, which then leads to an increase in cardiac output.

According to the Frank-Starling mechanism, the volume of blood that enters the ventricles equals the amount of blood ejected from the ventricles to the body. As a result, the end-diastolic volume (the volume of blood in the ventricles before ejection) and the stroke volume will be proportional.

Afterload

It’s the struggle ventricles have to overcome to circulate blood throughout the body. As resistance rises, the afterload rises as well. As a result, as afterload rises, stroke volume falls, and vice versa. All of this has an impact on cardiac output.

Calculating Cardiac Output

Two variables can be used to calculate cardiac output:

• The number of beats per minute is used to calculate heart rate (HR). The heart rate can also influence cardiac output

• The volume of blood pumped by each ventricle with each beat is known as stroke volume (SV). It is measured in millilitres per beat

So, here’s how the Cardiac Output formula works:

CO = HR * SV 

(ml/min) (beats/min) (ml/beat)

• The average heart rate is 75 beats per minute, and the average stroke volume is 70 ml per beat; this gives us Cardiac Output of 5,250 ml/min or 5 L/min.

• So, a person’s normal cardiac output, or the amount of blood passing through the heart each minute, is between 5 and 6 litres.

Cardiac Output Measurement

Cardiac output can be measured in a variety of ways, both invasive and non-invasive. However, there are advantages and disadvantages to measuring cardiac output.

Doppler ultrasound, echocardiography, transcutaneous and trans oesophageal echocardiography, partial CO2 rebreathing, and other invasive methods are available but may not be accurate or effective.

Cardiac Index

It’s a haemodynamic parameter that relates an individual’s heart performance to their size. It connects the left ventricle’s cardiac output (CO) in one minute to the BSA, or body surface area. L/min/m2 is the unit of measurement for litres per minute per square metre.

The cardiac index (CI) should be 2.5 and 4 L/min/m2.

A lower cardiac index of less than 2.5 L/min/m2 indicates a disturbance in cardiovascular performance.