Cardiac Output

Cardiac output is the amount of blood the heart pumps into the body in a minute. Heart rate and stroke volume measurements are used to obtain the cardiac output of a person. Various cardiac output measurement methods are used. Analyzing cardiac output is important because it allows physicians to ensure that the heart and its associated organs function properly.

What is Cardiac Output?

Cardiac output (CO) is the measurement of blood pumped by the left ventricle of the heart in a minute. The cardiac output is calculated by the measurement of heart rate (HR) and stroke volume (SV). The cardiac output value is measured in liters (L) per minute.

Cardiac Output Definition

Cardiac output is defined as the volume of blood pumped by the heart per minute.

Cardiac output Normal range

The normal CO of a healthy human is 5-6 L/minute at rest and 3 to 4 times more while working or exercising. Measurement of Cardiac output can be essential for an early diagnosis of cardiovascular diseases.

Cardiac Output Diagram

The diagram below shows the cardiac output:

Cardiac Output Formula

The formula of cardiac output is represented as :

Cardiac output (CO) = Heart Rate (HR) x Stroke Volume (SV)

• For Example,
  • If the heart rate is 64 bpm and stroke volume is 65 ml.
  • Cardiac output (CO) = 64 x 65 = 4160 ml/min or 4.16 liters per minute.

Cardiac Output Calculation

Cardiac output is calculated by multiplying stroke volume (SV) by the heart rate (HR).

1. Heart rate is the heart muscle contractions per minute (or beats per minute, bpm), the number of times the heart beats in a minute. Stroke volume is the amount of blood pumped by each ventricle in one compression or the amount of blood released to the body in one heartbeat (ml/heartbeat).
2. Your body sometimes needs more oxygen such as during physical activity. The more the need for oxygen, the more blood is pumped by the heart, subsequently changing the cardiac output. This shows that cardiac output is altered by the changes in the heart rate and stroke volume.

For example: If the heart rate is 70 bpm and stroke volume is 70 ml.

Using the formula: HR X SV

= 70 X 70

= 4900 ml/min or 4.9 liters per minute.

Factors Affecting Cardiac Output

There are several factors determining the efficiency of cardiac output regulation of a person. The body’s demand for blood changes during heavy workouts or any physical activity. Thus the demand for blood increases the heart beat and increases the cardiac output. That is the cardiac output can be altered by the change in heart rate and stroke volume.

Factors that affect cardiac output are:

• Physical activity: Intense physical activity generates the need for more energy and oxygen, consequently increasing the heartbeat.
• Age: The functioning of the heart and blood vessels gets less efficient in older people due to the decrease in elasticity of muscles or thickening of blood vessels.
• Contractability: It refers to the force exerted by the heart muscles to contract which contributes to stroke volume modulations. As the contractability increases, the heart can push more blood, increasing stroke volume
• Preload: It is proportional to end-diastolic volume (EDV), that is the amount of blood in the ventricles immediately before systole. The greater the EDV of blood returning to the heart, greater the passive stretching of heart muscles and thus causes more blood flow.
• Afterload: It can be represented as the systemic resistance the ventricles must overcome to eject the blood to the vasculature. It is inversely related to stroke volume. It largely depends on arterial blood pressure.

Cardiac Output Measurement Methods

The measurement of cardiac output is done by various invasive or non-invasive methods. Invasive methods are done by entering into the body part with the help of medical instruments while non-invasive means instruments used without breaking the skin or causing an injury. Some examples of such methods are given below:

Invasive methods

Oxygen Fick method

• This method uses the Fick equation that the total uptake of oxygen is equal to CO and arterial-venous oxygen. So to calculate the cardiac output, we need to use the formula: VO₂ (CaO₂ – CvO₂)
• The difference between inspired and expired oxygen is measured to determine the VO₂. From arterial blood gases, which are used to determine hemoglobin saturation and concentration, CaO₂ is calculated. Mixed venous blood gases, which must be drawn from a pulmonary artery, are used to determine CvO₂. The individual variables are taken from invasive testing.

Lithium dilution method

• It was primarily developed as a simple calibration for the PulseCO trademark (LiDCO, London, UK), a continuous arterial waveform monitor. A small dose of lithium chloride is injected as an intravenous bolus. The cardiac output is derived from the dilution curve generated by a lithium-sensitive electrode attached to the arterial line.

Non-invasive methods

• Doppler ultrasound: This uses an ultrasound machine with a specialized probe to measure the Doppler shift in the returning ultrasound waves. This deciphers the blood flow rate and volume of blood. A common instrument used for assessing Doppler ultrasonography is the echocardiogram.
• Echocardiogram : Echocardiography is the test to form live images of the heart using sound waves. It can form Doppler shift measurement in two-dimensional ultrasound. The image produced is called an echocardiogram. This can also elucidate blood flow rate and volume which helps in the assessment of CO.

Cardiac Index

Cardiac index (CI) is the evaluation of cardiac output based on a person’s body size. It is calculated by dividing cardiac output by a person’s body surface area (BSA).

• Cardiac index (CI) = Cardiac output (CO) / Body surface area (BSA)
• BSA is calculated generally by the Mosteller formula. That is BSA = [body weight (kg) x height (cm) / (3600)]. The average BSA for an adult male is 1.9 and for an adult female 1.6.
• The normal range for the Cardiac index of a person is 2.5 to 4 L/min/m².

What is the Significance of Cardiac Output?

The Cardiac output and Cardiac index of a person are important to know if the patient’s heart is pumping enough blood and delivering sufficient oxygen to cells.

Low Cardiac Output

The low level of cardiac output indicates that the heart doesn’t pump enough blood. If there is a low level of blood-carrying capacity of the heart, it may be a signal to heart failure or other cardiovascular diseases.

High Cardiac Output

• In a healthy person, the heart rate may increase to 150 bpm due to heavy exercise and SV can also increase to 70- 130ml. This can make cardiac output rise to 19.5 L/min approximately such as in athletes.
• But if the cardiac output is above this mark, it indicates certain heart-associated problems such as high blood pressure and heart failure.
• The cardiac output is higher during pregnancy.

Diseases Associated with Cardiac Output

• According to WHO, Cardiovascular disease (CVD) is among the leading causes of death worldwide. Measurement of Cardiac output can be essential for an early diagnosis of cardiovascular diseases.
• Cardiac output is very high in the body indicating that there is a lack of oxygen-carrying RBCs resulting in the condition called anaemia.
• The rise and fall of the cardiac output can also be because of some defects of the heart like atrial fibrillation (the abnormal rhythm of the heart) and aortic stenosis (narrow aortic valve)

Conclusion – Cardiac Output

The cardiac output gives the analysis of the functioning of the heart. There are various factors affecting cardiac output like age, physical activity, contractability, preload and afterload. Many techniques are used to measure the cardiac output of a person. The value cannot be excessively low or high; it should be steady. It is always better to have a healthy lifestyle with constant exercise and good dietary foods to ensure a healthy heart.

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FAQs on Cardiac Output

What is Cardiac Output Class 11?

Cardiac output is the amount of blood pumped by the heart in a minute. It is measured in litres (L) per minute.

What is the Cardiac Output Formula?

Cardiac output is calculated by multiplying stroke volume by the heart rate. Cardiac output (CO) = Heart rate (HR) * Stroke volume (SV).

What Factors Determine the Cardiac Output of a Human Heart?

The factors determining the cardiac output are changes in the heart rate and stroke volume. These can be altered by the intensity of physical activity, age of the person, contractability of the heart, preload, and afterload.

What is Normal Cardiac Output?

The normal CO of a healthy human is 5-6 L/minute at rest and 3 to 4 times more while working or exercising.

Why is Cardiac Output Important?

The Cardiac output of a person is important to ensure the proper functioning of the heart and related organs. Measurement of Cardiac Output can be essential for an early diagnosis of cardiovascular diseases.

What is the Function of Cardiac Output?

Cardiac output is the measure of blood volume ejected by the heart per minute, crucial for delivering oxygen and nutrients to tissues and removing waste products. It reflects the heart’s efficiency in maintaining adequate circulation throughout the body.

What is Low Cardiac Output?

Low cardiac output is when the heart pumps blood at a rate insufficient to meet the needs of the body leading to decreased circulation.

What is the Difference Between Stroke Volume and Cardiac Output?

Stroke volume is the amount of blood pumped by the heart in one beat, while cardiac output is the total volume of blood pumped per minute, calculated by multiplying stroke volume by heart rate.

What are the 4 Determinants of Cardiac Output?

The four determinants of cardiac output are heart rate, stroke volume, preload, and afterload.