Cardiac output is the quantity of blood pumped into the aorta each minute by the heart. This is also the quantity of blood that flows through the circulation.
METHODS OF MEASURING CARDIAC OUTPUT
Cardiac output is measured using direct and indirect methods;
Direct Methods
It is used to measure cardiac output only in animals. It includes;
- Using cardio-meter
- Using flowmeters like
- Mechanical flowmeter
- Electromagnetic flowmeter
- Ultrasonic Doppler flowmeter
Indirect Method (Invasive And Non-Invasive)
In humans, cardiac output is measured by indirect methods. It includes;
- Fick’s principle
- Indicator (dye) dilution technique
- Thermodilution technique
- Ballisticocardiography
- Doppler echocardiography
- Ultrasonic Doppler transducer technique
Fick’S Principle
The Fick’s principle states that the amount of a substance taken up by an organ in a given time is equal to the arteriovenous concentration difference of the substance multiplied by the volume of blood flow. This is an example of the Law of conservation of mass, a widely applied principle in physiology
Where;
- Q = Cardiac output
- Vo2 = O2 consumption (ml/min)
- Cao2 = arterial O2 content
- Cvo2 = mixed venous O2 content
The O2 consumption for the whole body can be determined using a spirometer to measure O2 uptake from the lungs. The arterial blood consumption is the same throughout the body. The arterial blood flow can be indicated by means of electromagnetic or Doppler flow probes. The blood sample for arterial oxygen content is estimated from any convenient artery, while venous blood is obtained from the pulmonary artery by cardiac catheterization. Mixed venous blood, which is representative in O2 content of the entire body, must be obtained from the right ventricle or pulmonary artery;
- Cardiac output is the volume in L/min flowing through the lungs;
- Oxygen consumption is the quantity of oxygen measured by spirometry in ml/min (which is taken up by the blood); and
- Arteriovenous oxygen difference is the difference in the concentration of oxygen between the incoming blood and the outflowing, measured as oxygen content in ml/L (it is not Po2 which is a measure of dissolved oxygen only); ‘oxygen content’ includes oxygen combined with Hb
As an illustration, using the following data from a normal adult;
- Oxygen consumption at rest = 250 ml O2/min
- Arterial O2 content = 200 ml/L
- Venous O2 content = 150 ml/L
The arteriovenous oxygen difference = 50 ml/L
Indicator (Dye) Dilution Technique
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Theory
a) Volume.
If a suitable indicator is injected into an unknown volume of distribution, the volume can be estimated from the resultant indicator concentration:Where;
- V = the volume of distribution
- A = the injected amount of indicator
- C = the resultant indicator concentration
After the peak concentration is reached, the indicator concentration follows an exponential time-course, so that the curve can be extrapolated to zero concentration, and the duration of the indicator passage (t) can be estimated
a) Indicators used for a cardiac output measurement include; Evans blue, Cardio-Green, hypertonic or hypotonic saline, ascorbate, and cold saline.
Cold saline, used in Thermodilution, is the most commonly used indicator in human cardiac output studies
b) Thermodilution uses a multi-lumened balloon catheter with a thermistor near the distal end (termed a Swan-Ganz catheter afterwards its inventors)
When this is applied to the oxygen taken up by the lungs per minute, the volume of blood flowing through the lungs per minute (which is equal to the cardiac output) can be determined using the equation
( )
Note the similarity of Fick's equation with the simple relationship above;
* Cardiac output is the volume in L/min flowing through the lungs;
* Oxygen consumption is the quantity of oxygen measured by spirometry in ml/min (which is taken up by the blood); and
* Arteriovenous oxygen difference is the difference in the concentration of oxygen between the incoming blood and the outflowing, measured as oxygen content in ml/L (it is not Po2 which is a measure of dissolved oxygen only); 'oxygen content' includes oxygen combined with Hb
