In animal models CO can be measured directly by cannulating the aorta, pulmonary artery or any of the great veins and then using an electromagnetic or ultrasonic flowmeter. However, this is not appropriate in a clinical situation and CO is usually measured by indirect methods.
Thermodilution is at present the most commonly used method to measure CO at the bedside. A pulmonary artery catheter (PAC) is inserted, cold saline injected into the RA, and the change in blood temperature is measured by the PAC thermistor in the PA. The PAC is connected to an analogue computer, which calculates CO by using the modified Steward-Hamilton equation:
Volume of injectate tb ti K, K2
initial blood temperature (°C)
initial injectate temperature (°C)
density constant computation constant and f TK(t) dt = integral of Hood tcrnpcriture ch*ngi
CO is inversely proportional to the area under the temperature-time curve. This technique is popular because multiple CO estimations can be made at frequent intervals without blood sampling. The accuracy of the technique is influenced by several factors, which include intracardiac shunts, tricuspid regurgitation and positive pressure ventilation.
A modification of this principle is used in the 'continuous' CO monitor. A pulse of electrical current heats up a proximal part of the PAC creating a bolus of warmed blood. The temperature rise is sensed when the warmed blood passes a thermistor in the PA. A computer then calculates the 'area under the curve' and, hence, CO.
This was the most popular technique prior to thermodilution. Indocyanine green is injected into a central vein, while blood is continuously sampled from an arterial cannula. The change in indicator concentration over time is measured, a computer calculates the area under the dye concentration curve, and CO is computed. Unfortunately re-circulation and build up of the indicator results in a high background concentration, which limits the total number of measurements that can be taken. The dye is non toxic and rapidly removed from circulation by the liver.
The Fick principle states that the amount of a substance taken up by an organ (or the whole body) per unit time, is equal to the arterial concentration of the substance minus the venous concentration (a-v difference), times the blood flow. This can be applied to the oxygen content of blood to determine CO.
First, the steady state oxygen content of venous (CvO2), and arterial blood (CaO2), are measured. Then oxygen uptake in the lungs is measured over 1 min (VO2). Finally, the Fick principle is applied to calculate the blood flowing in 1 min:
Errors in sampling, and the inability to maintain steady state conditions limit this technique. Doppler Techniques
Ultrasonic Doppler transducers have been incorporated into pulmonary artery catheters, endotracheal tubes, suprasternal probes and oesophageal probes. These probes can then be used to measure mean blood flow velocity through the aorta or any valve orifice. Using an estimation for the cross sectional area of flow, the flow velocity-time integral, heart rate and a constant, CO can be calculated.
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