Introduction
Cardiac output is the amount of blood pumped by the heart through the circulatory system every 60 seconds. It represents the volume of blood distributed to the body and is a determining factor of how effective the heart will deliver blood to the entire body. In the past years, many methods for measuring cardiac output have been developed and although each of the methods has both strengths and weaknesses, it is generally agreed that the ideal method for monitoring cardiac output should be non-invasive, accurate, reliable, dependable, and easy to use.
Methods of Measuring Cardiac Output
Measuring cardiac output helps physicians to know the condition of the heart concerning the entire cardiovascular system. There are many clinically proven methods used to measure cardiac output, both invasive and non-invasive. Invasive methods include thermodilution and Fick method whereas non-invasive methods include oesophageal Doppler, lithium dilution, transesophageal echocardiography, and thoracic electrical bioimpedance[1].
Invasive Methods
Fick method
This method works by measuring the oxygen concentration of blood flowing out of any organ in the body. It involves the computation of oxygen that is used up over a duration and this is obtained from the measurement of oxygen levels in blood coming from the blood vessels after going through the organs[2]. Although obtaining accurate levels of oxygen that have been consumed can be difficult, the Fick method is considered one the most accurate methods. When used carefully, the Fick method can prove to be very efficient in detecting low cardiac output in patients.
Thermodilution Method
This method calculates cardiac output concerning how quickly the flowing blood can dilute a marker matter that is introduced in blood using a pulmonary artery catheter. The marker should be very dissimilar to the blood and it must be measured fast with a high degree of accuracy[3]. Even though thermodilution involves many risks that may include valve damage, it is widely accepted clinically and remains one of the most commonly used methods in measuring cardiac output today.
Non-Invasive Methods
Oesophageal Doppler
This method employs both ultrasound and the Doppler Effect to determine a person’s cardiac output. It measures blood velocity in the ascending aorta and this helps in estimating the length of blood flowing through the aorta every 60 seconds. This method is cheap and accurate and has been in clinical use since the 1960s. The precision stems from the high accuracy of the cross-section measurements. To increase precision, the ultrasound beam should be directed parallel to the blood flow and the beam direction should not experience major alterations amid measurements[4].
Transoesophageal Echocardiography
This method provides diagnosis and monitoring of a range of functional and structural heart abnormalities[5]. Maximum technical skills are required in this technique because heavy sedation is needed and patients require close and constant monitoring from a medical expert because it takes a very long time to perform. The equipment used in this process is very expensive and requires expert knowledge[6].
Thermodilution remains the most commonly used method despite controversies regarding its efficacy[7] because its accuracy has been established compared to other methods.
Transoesophageal Echocardiography is the best method that can be used for the measurement of cardiac output at a roadside in road accident scenes because it uses ultrasound and through the pictures taken, doctors can know how severe one has been hurt, particularly the heart. This method uses sound waves to create 3D pictures of the heart after and this makes it possible to ascertain the level of damage of the heart so that appropriate medical processes can be initiated.
- [1]LB Rowell, Human cardiovascular control, Oxford, Oxford University Press, 1993, pp. 54.
- [2]DP Bernstein, Impedance cardiography: Pulsatile blood flow and the biophysical and electrodynamic basis for the stroke volume equations, Journal of Electrical Bioimpedance, Vol1, No.1, 2010, pp. 10.
- [3]PR Freund, Transesophageal Doppler scanning versus thermodilution during general anesthesia, An initial comparison of cardiac output techniques, American Journal of Surgery, Vol.153, 1987, pp.492.
- [4]MN Levy, Matthew, and RM Berne, Cardiovascular physiology, 7th ed, St. Louis, Mosby, 1997, pp. 42.
- [5]RA Linton, DM Band, and KM Haire, A new method of measuring cardiac output in man using lithium dilution, British Journal of Anesthesia, Vol.71, 1993, pp.264.
- [6]AC Guyton, and E John, Textbook Of Medical Physiology,11th ed, Philadelphia, Elsevier Inc., 2006, pp. 76
- [7]E Braunwald, Heart disease: a textbook of cardiovascular medicine, 5th and, Philadelphia, Saunders, 1997, pp. 112.
Bibliography
Bernstein, DP, Impedance cardiography: Pulsatile blood flow and the biophysical and electrodynamic basis for the stroke volume equations, Journal of Electrical Bioimpedance, Vol.1, No.1, 2010, pp. 2-17.
Braunwald, E, Heart disease: a textbook of cardiovascular medicine, 5th edn, Philadelphia, Saunders, 1997.
Freund, PR, Transesophageal Doppler scanning versus thermodilution during general anesthesia. An initial comparison of cardiac output techniques. American Journal of Surgery, Vol. 153, 1987, pp.490–494.
Guyton, AC and John, E, Textbook Of Medical Physiology, 11th edn, Philadelphia, Elsevier Inc., 2006.
Levy, MN, and Berne, RM, Cardiovascular physiology, 7th ed, St. Louis, Mosby, 1997.
Linton, RA, Band, DM, and Haire, KM, A new method of measuring cardiac output in man using lithium dilution, British Journal of Anesthesia, Vol. 71, 1993, pp. 262–266.
Rowell, LB, Human cardiovascular control, Oxford, Oxford University Press, 1993.