Cardiac output measurement is widely used in clinics and hospitals. Every patient suffering from any disease or problems gets his cardiac output measurement done first. Cardiac output can also be called heart output, represented by the symbol Q. Cardiac output measurement measures the volume of blood pumped by the two ventricles of the heart in unit time. 

The formula for this measurement is based on the Fink principle, which is given by: 

CO = VO2/Ca-Cv

where, CO = Cardiac output, VO2 = consumption of gaseous oxygen in a minute in mL, Ca = content of oxygen in arterial blood, and Cv = content of oxygen in the venous blood, which is mixed with the arterial blood.

Factors that determine the cardiac output 

Many factors help to determine the cardiac output measurement of the human heart. The factors are given below:

• Venous return – The amount of blood that enters the heart through the veins in a minute is called venous return. There comes a time when the cardiac output is equal to the venous return.
• Force of contraction – The volume of stroke increases with an increase in the force of contraction. And the cardiac output also increases with the increase of contraction force. 
• Heart rate – Increase in heart rate results in an increase in cardiac output. 

Calculation of cardiac output 

Cardiac output can be measured based on the heart rate and stroke volume using the formula: CO = HR × SV.

Here, CO stands for Cardiac output 

HR = Heart rate 

SV = stroke volume 

For example, if a person’s heart rate is 75 bpm and the volume of stroke is 70 mL, then the cardiac output will be measured using the formula:

CO = HR × SV

CO = 75 × 70 

= 5250 mL/min or 5.25 L/min.

Cardiac output measurement 

There are two methods to measure cardiac output: the invasive and non-invasive methods. But these methods can have both merits and demerits in calculating cardiac output. There is no standard calculation system for cardiac output measurement which can be used as a reference. Generally, invasive methods are used to calculate the cardiac output, but some studies have shown that these methods are not accurate and effective to calculate these outputs. 

Some of these methods to calculate cardiac outputs are given below:

• Transcutaneous
• Doppler ultrasound
• Transoesophageal
• Echocardiography 

Cardiac output measurement importance

• Cardiac output measurement is important because, through this measurement, the blood flow into the human heart can be evaluated. 
• Maintaining the cardiac output is essential so that the blood pressure can be at its optimum level to supply oxygenated blood to the person’s brain and other organs. 

Cause of high cardiac output 

Many conditions cause high cardiac output, for example, 

• arteriovenous shunts caused by Paget’s bone disease, 
• chronic severe anaemia, 
• severe hepatic disease and its forms, renal disorders, septic shock, arteriovenous fistula, etc. 

Methods for cardiac output measurement 

There has been development in calculating cardiac output measurement over the years. The studies are limited to the methods for human pharmacology. One method proven to be accurate, independent of operators and precise is the ‘holy grail’. This method for calculating cardiac output is non-invasive, cheap, safe, readily available, and continuous. But this method is not present today. The Fick principle, arterial pulse contour analysis, ultrasound, indicator dilution, etc. methods are used in pharmacology.

• Fick principle for oxygen – Adolf Fick came up with an idea to calculate the cardiac output based on the balance of oxygen. He said that intaken oxygen (O2) in lungs in pure gaseous form (mL) per minute is transferred directly into the bloodstream by the lungs. Where there is no oxygen consumption by the lungs, then it is equal to the product of the flow of the blood (cardiac output) and arterial-venous oxygen difference. Therefore, the cardiac output can be represented by the following formula: CO = VO2/ (CaO2 – CvO2), Where VO2 is the uptake of oxygen, CaO2 is the arterial oxygen content, and CvO2 is the venous blood.
• Rebreathing partial carbon dioxide – The Fick Principle can be applied to those gasses which follow Henry’s law and diffuse in the lungs like carbon dioxide (CO2). Partial rebreathing of CO2 is done to calculate the cardiac output by the Cardiopulmonary Management System. It is done under a specific monitor with light absorption of infra-red rays for CO2, a pressure transducer for airflow measurement, and a pulse oximeter. 
• Dilution indicator techniques: The calculation of cardiac output is all based on the mass balance, mi = ⨑ q(t) * c(t), where c(t) is concentration in terms of time and q(t) is blood flow at an instant. Today’s common dilution indicator techniques are the PAV thermodilution method, pulmonary artery catheter, and transpulmonary lithium dilution method. These methods help clinicians measure cardiac output clinically. 

Conclusion 

Cardiac output measurement is done to check the blood flow in the heart of a human per minute. The simplest way to calculate the cardiac output is by calculating the heart rate. The faster the heartbeats, the more efficiently the blood will be pumped in a particular amount of time. As we have discussed above, there are many formulas for calculating and measuring cardiac output. The clinics have many cardiac output measurement diagrams explaining the cardiac outputs and their evolution with time. In 1870, Adolf Fick came up with this idea and principle of frick. Cardiac output can be increased by many disorders and diseases, which is not a sign of good health. Also, if a person is healthy, he should constantly visit clinics to measure cardiac output. A good cardiac output means a healthy heart rate.