Cardiac output is the volume of blood pumped out by both heart ventricles per minute, between 70-72 per minute. Cardiac output or heart output is denoted by the letter Q or Qc. The volume of blood is also called stroke volume. The measure of cardiac output holds excellent clinical significance in medical science to diagnose cardiac abnormalities.

  • Several methods study and measure cardiac output or heart output. The most common is Fick’s Cardiac output measurement. 
  • Other methods include ultrasound,  indicator dilution technique, colour Doppler and Arterial pulse contour analysis. 
  • To be accepted in science, these methods must be accurate, precise, non-invasive, easy to use, operator-independent and fast responding. 
  • It should be cheap and readily available to the consumers.

Measurement of Cardiac Output

Fick’s Cardiac Output Measurement

  • Fick first described direct Fick’s measurement for oxygen in July 1870.
  • His proposal was known as Fick’s contribution to measuring cardiac output.
  • He measured the cardiac output based on oxygen balance.

Direct Method

  • He proposed a theory in which the organ utilises oxygen measured by the arteries and the veins of the given concentration of the given substance O2.
  • We can calculate the cardiac output using the given fixed principle based on the concentrations.
  • He believed that the oxygen uptake in the lungs, O2 consumption in ml per minute, is entirely transferred to the bloodstream through the lungs.
  • After a minute’s breath, we can assist the amount of oxygen present in the veins and arteries. 
  • The artery indicates the pure blood entering the body after using the O2.

The Mathematical Equation for Calculating Cardiac Output

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Determination Of Cardiac Output By Indicator (Dye) Dilution

Method

  • The administrator or the known amount of radioactive substance or dye is injected into the veins. 
  • Once injected into the vein, the substance will go to the heart and return to the arteries.
  • The substance used to assist cardiac output should have the following characteristics. 
  • It should be non-metabolite, non-degradable, and easily measurable in the arterial sample.
  • The sample is then collected from the brachial artery to assess substance concentration in each circulation. 
  • The dye concentration changes in every circulation.

Mathematical Formula

  • The equation for calculating is CO = Average of the substance/dye fused into the branchial artery / Integral of average concentration from 0 to know the amount of concentration in a given time.
  • During exercise, we can measure cardiac output by obtaining these similar samples as the cycle during exercise gets completed in 10 sec.

Doppler Ultrasound

Method

  • Doppler ultrasound is a non-invasive method of studying the blood velocity of the heart.
  • The stroke volume (SV) is calculated by the product of aortic blood velocity and the cross-sectional area of the aorta.
  • There is a shift in the Doppler frequency of the returning ultrasound waves due to the blood velocity in the heart.

Mathematical Equation

  • Q = SV × HR
  • CSA = πr² 

Here CSA is the valve orifice cross-sectional area, and r is the valve radius.

Partial Carbon Dioxide Rebreathing

Based on Fick’s Principle

  • All gases obey Henry’s law of gaseous exchange. Similarly, we can apply Fick’s principle to CO2 using intermittent partial rebreathing. 
  • An infrared light absorption sensor and a differential pressure transducer measure the cardiac output by placing a pulse oximeter.

Mathematical Equation

e2

Conclusion

Development in science has made human lives better with increased quality. People who suffer mild cardiac arrest or heart disease are provided with pacemakers that increase their lifespan. Before any mishappenings, one can prevent it through different methods. ECGs, ultrasound, oximeters, and X-rays are some common ways to detect any abnormality in the heart. One focuses on its characteristics for any measuring device: it should be non-invasive, precise, accurate, easy to use, readily available, and affordable. Measuring cardiac output is an aid in the prognosis and diagnosis of any heart abnormalities. It is also used to monitor the accuracy of a therapy. There are different techniques to measure cardiac output, but each method has its strength and limitations. Electrocardiography and Doppler ultrasound are non-invasive techniques, whereas Fick’s cardiac output is invasive and not very accurate.