This chapter explores the relationship of Section G7(iv) of the 2017 CICM Primary Syllabus, which expects the exam candidate to "describe the methods of measurement of cardiac output including calibration, sources of errors and limitations ". It is also relevant to Section G7 (vi), "outline methods and principles used to measure regional blood flow", as the methods of measuring blood flow are the same irrespective of whether total or regional flow is being measured. This is a common feature of past CICM First Part Exams and should be a priority for the revising candidate, in spite of the fact that many of the techniques described here have suffered from a progressive loss of bedside popularity. Historical appearances have included:
The most frightening potential variation of these questions would probably have to be something where the trainees have to create a table which compares and contrasts the advantages and limitations of each method. Hopefully, this tabulated summary will of use, if we ever see this happen again:
Methods of Cardiac Output Measurement Method Advantages Limitations Direct Fick
Total uptake of oxygen by the body is equal to the product of the cardiac output and the arterial-venous oxygen content difference:
CO = VO2 / (Ca - Cv)
- "Gold standard"
- Good accuracy
- Necessary invasive devices are often already available in ICU patients
- Requires stable CO over some minutes
- Highly invasive (requires PAC and arterial line)
- Requires cumbersome VO2 measuring equipment
Indirect Fick
Measurement of cardiac output using the Fick equation, but substituting estimated values for some of the measured variables
- Less invasive than direct method
- Reasonably accurate
- Error is introduced by estimates
Indicator dilution
Cardiac output is calculated from the dose of indicator and the area under the concentration-time curve, measured by a downstream detector:
V̇ = m/Ct
- Does not require mixed venous blood
- Numerous indicator options (eg. thermodilution)
- Good accuracy
- Accuracy is highly technique-dependent
- Rendered inaccurate by intacardiac shunts and valve disease
- Accuracy is reduced by estimated coefficients in the equation
Pulse contour analysis
Stroke volume can be calculated from the area under the flow/time curve which is derived from the arterial pressure waveform using a calibration factor.
- Less invasive (only needs art line and CVC)
- Continuous
- Reasonably accurate
- The calibration factor needs to be measured
- Dependent on good arterial waveforms
- COnfused by AF and IABP
LVOT VTI
CO is calculated from the cross-sectional area (CSA) of the LV outflow tract, and from integrating the area under the veolicty/time curve (VTI) measured by Doppler from the aorta:
CO = HR × (VTI × CSA)
- Non-invasive
- Easily available
- In the right hands, quite accurate
- Poor reproduceability (interobserver variability)
- Limited by ultrasound window availability
- Accuracy dependent on beam angle
For a variety of reasons, not the least of which being their relevance to thermodilution measurements of cardiac output, the indicator dilution method and the Fick Principle are mainly discussed in the section concerning Swan-Ganz pulmonary artery catheters. Because these concepts got their own chapters, here they will only form a part of the dimly illuminated background.
As with every core topic, there is no shortage of quality peer-reviewed literature. Ehlers et al (1986) offer an excellent brief overview of the main techniques, which features a pragmatic "advantages/disadvantages" sort of breakdown. A freely available article by Lavdaniti (2008) is almost as good, just without the same structure. Jhanji, Dawson & Pearce (2008) are another free alternative.
Put in the simplest way, the Fick method of measuring cardiac output relies on the observation that the total uptake of oxygen by the body is equal to the product of the cardiac output and the arterial-venous oxygen content difference. Logically, that principle is called the Fick principle. Rearranging the equation:
CO = VO2 / (Ca - Cv)
where
- VO2 is the total oxygen consumption, as a volume per unit time (eg. L/min)
- CO is the cardiac output, also as volume per unit time (L/min)
- Ca and Cv are the arterial and venous oxygen content (eg. ml/L)
This whole thing is discussed in greater detail in a separate chapter dealing with the Fick Principle. It will suffice to say that the proper application of this method requires the cumbersome measurement of total inhaled and exhaled oxygen (usually using some sort of mask or collection bag), as well as the simultaneous measurements of arterial and mixed venous blood. Those would be the essential ingredients of the "direct" Fick method. "Indirect" options also exist, where one of the more inconvenient measurements is replaced by some sort of estimated value, eg. where you use an age/weight/sex-based nomogram to estimate the VO2. Obviously, the use of estimates introduces an element of error into a measurement which is already not particularly precise. The direct Fick method, even when performed under perfect laboratory conditions, has an error range of around ±8%, according to an interesting animal study by Seely et al (1950).
Method:
Sources of error:
Advantages:
Limitations:
For some reason, apparently completely unrelated to its exam importance, an entire chapter was dedicated to the indicator dilution method of cardiac output measurement. Fortunately, there is no point in reading it, as the basics are summarised here. In short, this methods rests on the premise that giving a known dose of a substance intravenously can be used to measure the cardiac output by measuring the rate of transit of that substance at some downstream detector. To be more precise, the area under the concentration/time curve can be used to determine the flow:
Cardiac output = indicator dose / area under the concentration-time curve
This is a simplification of the Stewart-Hamilton equation:
Method
Sources of error:
Advantages:
Limitations:
Cardiac output monitoring by pulse contour cardiac output monitor devices (PiCCO) is a method of continuously monitoring the cardiac output by using the shape of the arterial pressure waveform. It is also discussed in some detail by Jörn Grensemann (2018), if detail is what you are after. More than likely, it's not, in which case:
Method
Sources of error:
Advantages:
Limitations:
Again, for some reason LVOT Doppler measurement of cardiac output ended up getting its own (very brief) chapter, even though it has never been mentioned in any exam setting. A more detailed discussion of this technique and its limitations has been published by Huntsman et al ( 1983). In short, it rests on the assumption that the volume of blood, as it moves out of the heart during systole, can be represented mathematically as basically a cylindrical column. The flat dimension of this column (i.e. cross-sectional area of the LV outflow tract) is obviously not perfectly circular, but it is close enough for the accuracy standards of cardiac output monitoring, and we tend to approximate it from two echo measurements of the LVOT. The column with this circular LVOT-shaped base moves in the direction of the systemic circulation with some sort of velocity. That velocity is obviously not constant, as the cardiac output is pulsatile, but that doesn't matter as long as you measure it and plot it as velocity over time. This gives you the area under the velocity-time curve, which is otherwise referred to as the velocity-time integral. Thus, the cross-sectional area of the aorta, multiplied by the distance travelled by the column of blood, gives you the volume ejected per beat; and once you have stroke volume and heart rate, you have cardiac output; or:
CO = HR × (VTI × CSA)
where:
- CO is the cardiac output,
- HR is the heart rate,
- VTI is the velocity-time integral, i.e. the area under the velocity/time curve
- CSA is the cross-sectional area of the LVOT
- Thus, VTI × CSA is the stroke volume
Method
Sources of error:
Advantages:
Limitations:
Incidentally, there's a lot of different ways of measuring these parameters, and Doppler ultrasound is only one method, made more popular by its non-invasiveness. One could be a lot more intrusive. For example, Ehlers et al (1986) describe heated in-dwelling sensors which use heat transfer between a hot wire and the blood to determine the rate of flow, similar to the hot wire anemometers used to measure gas flow in mechanical ventilators.
The rest of these are, for lack of a better word, niche options. One may go through their entire career in critical care without ever encountering even one of these methods, and to include them in any sort of exam would be the very peak of rudeness. They are really included here only because they are occasionally mentioned in textbooks, listed in order of most invasive to least invasive:
Ehlers, Kevin C., et al. "Cardiac output measurements. A review of current techniques and research." Annals of biomedical engineering 14.3 (1986): 219-239.
Lavdaniti, M. "Invasive and non-invasive methods for cardiac output measurement." International Journal of Caring Sciences 1.3 (2008): 112.
Jhanji, S., J. Dawson, and R. M. Pearse. "Cardiac output monitoring: basic science and clinical application." Anaesthesia 63.2 (2008): 172-181.
Blanco, Pablo. "Left ventricular outflow tract velocity-time integral: A proper measurement technique is mandatory." Vascular Medicine (2020): 1358863X20907700.
Villavicencio, Christian, et al. "Basic critical care echocardiography training of intensivists allows reproducible and reliable measurements of cardiac output." The ultrasound journal 11.1 (2019): 5.
Huntsman, L. L., et al. "Noninvasive Doppler determination of cardiac output in man. Clinical validation." Circulation 67.3 (1983): 593-602.
Seely, Robert D., WILLIAM E. NERLICH, and Donald E. Gregg. "A comparison of cardiac output determined by the Fick procedure and a direct method using the rotameter." Circulation 1.6 (1950): 1261-1266.
Grensemann, Jörn. "Cardiac output monitoring by pulse contour analysis, the technical basics of less-invasive techniques." Frontiers in Medicine 5 (2018): 64.