Though there is not a specific syllabus item for the Fick principle in the CICM curricula, as an essential part of cardiac output monitoring it has to be included in any answer to Section G7 iv of the CICM Primary Syllabuswhich expects the exam candidate to "describe the methods of measurement of cardiac output". Some discussion of this concept is therefore expected in any written question answer or crosstable viva which incorporates cardiac output measurement by thermodilution eg.
In terms of published peer-reviewed sources, for something as common and fundamental as this, there is a surprising deficit of quality material, made even more frustrating by some puzzling bibliographic choices on the part of publishers. For instance, wherever you find the words "Fick's Principle" uttered in the literature, it appears to be an incantation to summon the original version of Adolf Eugen Fick's paper.
To offer this reference is therefore a mistreatment of the reader, as it prioritises giving credit to a dead man over the learning needs of the living. This definition appears in multiple sources and total fick to be an agreed-upon restatement, rather than a faithful translation. In his talk, Fick did not put things in such a polished final form; he just described his experiment to his peers and colleagues, where he measured the cardiac output of a dog's heart, and expressed surprise that nobody came to these conclusions sooner:.
One measures how much oxygen an animal total fick from the air in a given time, and how much carbon dioxide it gives off. During the experiment one obtains a sample of arterial and venous blood; in both the oxygen and carbon dioxide content are measured. The difference in oxygen content tells how much oxygen each cubic centimeter of blood takes up in its passage through the lungs.
As one knows the total quantity of oxygen absorbed in a given time one can calculate how many cubic centimeters of blood passed through the lungs in this time.
Or if one divides by the of heart beats during this time one can calculate how many cubic centimeters of blood are ejected with each beat of heart. The corresponding calculation with the quantities of carbon dioxide gives a determination of the same value, which controls the first. VO 2 is the total oxygen consumption, as a volume per unit time eg. C a and C v are the arterial and venous oxygen content eg.
In order to measure the cardiac output with the abovementioned equation, you need to know the arterial oxygen content, venous oxygen content, and the total oxygen consumed by the organism. The former can be determined from arterial and mixed venous blood gases, and the latter can be determined by comparing the oxygen content of inhaled and exhaled alveolar gas.
To represent this concept with some childish artwork:. VO 2 is measured by comparing an inhaled oxygen volume with an exhaled oxygen volume traditionally, using all sorts of collection bags or hoods ; it is usually around 3.
The oxygen content of blood can be measured by collecting an arterial blood gas together with a mixed venous sample, and it is calculated by using the following very familiar equation:. Thus, plugging in common values eg. How many litres of blood, then, would it take to produce the observed VO 2 of ml? It is really quite simple:.
In case a visual learner happens upon thisthey will probably benefit from a diagram which incorporates these values:. And there you have it. That is the "direct" Ficks method for measuring cardiac output, so called because the VO 2 is measured directly by collecting exhaled oxygen eg. Oxygen is not the only possible gas, of course, and variations of this method have been developed which use CO 2 :.
It is such a simple concept that one might ask, why don't we use this all the time? Indeed: it is considered the gold standard in the measurement of cardiac output. Or, at least, wherever authors discuss cardiac output, they tend to throw out a line about how the direct Fick method is a gold standard. They are also usually quick to follow this with a statement about how impractical it is, eg.
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The method is theoretically sound, and works well in the lab, but it is difficult to total fick in any real-world situation, as there are many practical problems to overcome. First, some practical concerns, which might seem like trivial complaints.
For example, the need to collect and measure the quantity of expired oxygen. This requires time; most sources recommend at least one minute of measurement if not longer. Fiddling around with bags is certainly cumbersome, and there is a risk of bag gas equilibrating with atmospheric air through multiple connections and disconnections which would inevitably take place, potentially producing an over-estimate of cardiac output.
Alternatively, the subject can be instructed to breathe through a mouthpiece which is attached to an oxygen detector and flowmeter, estimating the oxygen volume from the product of concentration and flow. This requires some training and is obviously unsuitable for the uncooperative delirious patient, or one which is hypoxic and struggling for breath, or somebody who has been midazolated to the point where they can't follow basic instructions.
Fick's laws of diffusion
At face value, these are obviously not insurmountable disadvantages. In fact they sound like the complaints of a lazy technician, i. To make a counterargument, one might point out that any method of measurement, no matter how accurate, would be coned to the animal laboratory if it were too impractical to use routinely in human clinical practice.
The direct Fick method definitely suffers from this problem. So, from a practical standpoint, how accurate and reproducible is this "gold standard" method? One might think this should be a relatively difficult question to answer, considering the Fick method being the gold standard, realistically there ought to be total fick higher standard to measure it against.
Fortunately, Seely et al found a cardiac output measurement technique which was even more gold standard-er than the Fick method. They opened the chests of several dogs and diverted the main pulmonary artery into an optical rotameter thus avoiding the common error of aortic flow measurement, where the flowmeter fails to for coronary blood flow.
All cardiac output from the right heart would therefore have to pass through the flow measurement device in order to reach the rest of the circulation. The honest physiologist would have to admit that this setup is in fact the "gold standard" against which all other measurement techniques should be calibrated. Similarly, Thomassonusing a more realistic setup intact humans, specifically "volunteers, members of the hospital staff or the Stockholm city police force" looked mainly at the reproducibility of measurements carried out during steady-state conditions and also found an error margin of around 0.
So far, total fick have been referring only to the "direct" Fick method, implying that there must also be an "indirect" method.
That is in fact correct, as it would be insane if there wasn't. The indirect method is a modification of the original which cuts a few corners. Unfortunately, a precise definition of which exact corners which end up being cut seems to be difficult to pin down, but the basic theme of these "indirect" techniques is the desire to avoid having to collect some sample or another, usually the mixed venous total fick. On the basis of this, one could concoct an informal definition, as follows:. Every author who has ever published on this topic seems to promote their own variant of the indirect Fick method, apparently depending on what instrument was not available in their immediate laboratory environment.
These authors came up with different methods of estimating the CvO 2 or CvCO 2 on the basis of the fact that alveolar gas, if given enough time, will end up equilibrating its gas content with the mixed venous blood. In other words, if the venous blood and alveolar gas are allowed to mingle freely for long enough, the partial pressure of the alveolar gas in the system equilibrates with the venous blood, making it possible to estimate the arteriovenous oxygen content difference without having to obtain a mixed venous sample. Fortunately, even in the darkest timeline the CICM trainees can be reasonably confident that this sort of historical trivia will not be expected from them.
The modern modification of these rebreathing equilibrium techniques is best described by Haryadi et al In short:. Hoping that a diagram might clarify this process it didn'tone could represent these steps as follows:. This is what's called a "total" rebreathing method, and from the description one can immediately tell that it is going to be useable only in a small proportion of carefully briefed fully cooperative patients who have nothing whatsoever wrong with their brains, hearts or lungs. In short, a group whose cardiac output measurement no intensivist could possibly ever care about.
A "partial" indirect Fick method is total fick described, which is somewhat more suited to sedated ventilated total fick.
Instead of rebreathing gas, the patient's ventilation is changed to transiently increase their minute volume, and two sets of variables are collected:. All sorts of sub-variations on this theme also exist, ranging from crude methods where you change the respiratory rate up and down, to more sophisticated methods where extra dead space is added to the circuit such as the proprietary system described by Haryadi et al in All we can deduce from the URL is that they got bought out by Respironics at some stage.
Total fick short, this method is not exactly mainstream. To summarise, depending on which variables you decide you don't feel like measuring, the indirect Fick method has multiple permutations, of which the most common are:. Gazibarich, Gary J. Fick A. Uber die messung des Blutquantums in den Hertzvent rikeln. Vandam, Leroy D. Kendrick, A. Visscher, M. Venkateshwaran, S. Light, R. Thomasson, B. Seely, Robert D. Plesch, Johann.
Loewy, Adolf, and H. Haryadi, Dinesh G. Defares, J. Wise, and J.