The Fick principle
Fick described the following relationship in the 19th century:
Q = M / (V - A)
Where Q is the volume of blood flowing through an organ in a minute, M the number of moles of a substance added to the blood by an organ in one minute, and V and A are the venous and arterial concentrations of that substance. This principle can be used to measure the blood flow through any organ that adds substances to, or removes substances from, the blood. The heart does not do either of these but the CO equals the pulmonary blood flow, and the lungs add oxygen to the blood and remove carbon dioxide from it.
The concentration of the oxygen in the blood in the pulmonary veins is 200 ml/L and in the pulmonary artery is 150 ml/L, so each litre of blood going through the lungs takes up 50 ml. At rest, the blood takes up 250 ml/min of oxygen from the lungs and this 250 ml must be carried away in 50 ml portions; therefore, the CO must be 250/50 or 5 L/min.
[Adolf Fick 1829-1901, German physiologist].
The original method described by Fick in 1870 is difficult to carry out. Oxygen consumption is derived by measuring the expired gas volume over a known time and the difference in oxygen concentration between this expired gas and inspired gas. Accurate collection of the gas is difficult unless the patient has an endotracheal tube, because of leaks around a facemask or mouthpiece. Analysis of the gas is straightforward if the inspired gas is air, but if it is oxygen-enriched air there are two problems, (a) the addition of oxygen may fluctuate and produce an error due to the non-constancy of the inspired oxygen concentration, and (b) it is difficult to measure small changes in oxygen concentration at the top end of the scale. The denominator of the equation, the arteriovenous oxygen content difference, presents a further problem, in that the mixed venous (i.e. pulmonary arterial) oxygen content has to be measured and therefore a pulmonary artery catheter is needed to obtain the sample. Complications may arise from these catheters. If carefully carried out, the Fick method is accurate, but it is not practicable in routine clinical practice. Several variants of the basic method have been devised, but usually their accuracy is less good.
A known amount of dye is injected into the pulmonary artery, and its concentration is measured peripherally. Indocyanine green is suitable due to its low toxicity and short half-life. A curve is achieved, which is replotted semi-logarithmically to correct for recirculation of the dye. CO is calculated from the injected dose, the area under the curve (AUC) and its duration. (Short duration indicates high CO).
Lithium has also been used as an alternative to indocyanine green. It is injected via a central venous catheter and measured by a lithium-sensitive electrode incorporated into the radial arterial cannula.
5-10 ml cold saline injected through the port of a pulmonary artery catheter. Temperature changes are measured by a distal thermistor. A plot of temperature change against time gives a similar curve to the dye curve (but without the second peak). Calculation of CO is achieved using the Stewart-Hamilton equation. Application of this equation assumes three major conditions; complete mixing of blood and indicator, no loss of indicator between place of injection and place of detection and constant blood flow. The errors made are primarily related to the violation of these conditions.
The amount of indicator (n) is related to its mean concentration (c), cardiac output (Q) and the time for which it is detected (t2 - t1).
[i] Measuring cardiac output. Allsager CM and Swanevelder J. Br J Anaesth CEPD Reviews 2003; 3 (1):15.