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Indirect Measurement of Cardiac Output

There are two basic ways to measure cardiac output:

    1. direct
    2. indirect

The latter is the most widely used, and several methods are available for the indirect measurement of cardiac output.

figure
    1. Electromagnetic flow probes use a transducer placed around a vessel (aorta), which measures the distortion of a magnetic field by blood flowing through the vessel.

    2. Echocardiography uses the reflection of sound waves back toward the source when they encounter tissues of various densities. The speed with which they return is determined by the depth in the thorax and density of the structure involved.

    3. Radionuclide ventriculography involves injection into the blood of radioactive material that can be quantified in the chambers of the heart by means of a detector placed on the chest wall. Estimates of chamber volume and computation of stroke volume can be derived.

    4. Dye dilution methods involve the bolus injection of a known amount of a dye (such as Cardiogreen), which remains in the vascular system and can be detected. The dye is injected into the right atrium, where it is diluted and evenly mixed. Blood samples are drawn from the brachial artery and the dye concentration is measured with a densitometer. The CO is calculated as the amount injected divided by average concentration per unit of time flowing through the densitometer (see top figure). For example, if the mixed arterial sample contains a dye concentration of 2 mg/L/min, and 10 mg was injected, CO=10mg divided by 2mg/L/min=5.0 L/min.

    figure 5. The Fick Principle uses the concept that the amount of oxygen entering the lung (flow x venous oxygen content) plus the amount of oxygen added to the blood by the lung (oxygen consumption) is equal to the amount of oxygen leaving the lung (flow x arterial oxygen content). The flow in both cases is the cardiac output. The equation derived from the principle as shown (CO=oxygen consumption/arterial-venous oxygen difference) permits calculation of the cardiac output averaged over a 3- to 5- minute time period (see bottom figure).

    6. Thermodilution methods use a single venous catheter with a temperature sensor at the tip and an injection port upstream from the sensor. Ice-cold saline is injected, and the degree of cooling of the blood caused by the volume and temperature of the saline injected is measured by the sensor. A computer then calculates the cardiac output based on the data.

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