Cardiac output refers to the volume of blood that is pumped by the heart over a span of one minute. Cardiac output is determined by the stroke volume and the heart rate. Considering the heart rate, this refers to beats per minute. On the other hand, the stroke volume refers to millimeters that are ejected per beat.
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Any of these parameters can bring influence cardiac output. Under normal circumstances, the stroke volume varies from 60 millimeters to a hundred millimeters for each beat. Considering the heart rate, under normal conditions, this ranges from 60 beats per minute to a hundred beats per minute.
The heart’s function is to pump blood so that it can effectively and efficiently reach the appropriate points. The blood supplies oxygen as well as nutrients and other important body chemicals to the cells to ensure that these cells survive and function in a proper manner. Blood movement in the body also facilitates the elimination of the waste products that may be harmful to the body by taking them to the elimination sites.
Cardiac output is of clinical use in that it gives an indication of how well the heart is carrying out its function. Basically, the regulation of cardiac output is carried out by the demand for oxygen by the body cells (Velmahos, et al, 2000).
When an individual carries out some exercise, the level of metabolism, as well as the requirement for oxygen, goes up. This gives an implication that there has to be an increase in the blood flow to the active tissues. In turn, the increase in the blood flow is brought about by an increase in cardiac output. At a time the tissue is not active, the demand for oxygen decreases, and in situations like these, it would be efficient for the body to decrease cardiac output. AS Premkumar (2003) points out, it is of great importance for the body to change cardiac output in accordance with the needs in order to realize the maintenance of homeostasis.
Cardiac output measurement is a very crucial issue and particularly in the care of ill patients who may be in critical conditions (Brown, Shoemaker, Wo and Chan, 2005). Such critical conditions may involve situations where the patients may be having multiple traumas, overwhelming sepsis, and those patients having a myocardial infarction.
Knowing the best method to carry out measurement of cardiac output can play a major role in taking appropriate care and giving effective treatment to patients who may be in critical conditions. More so, knowing the best method of cardiac output measurement can play a major part in avoiding inaccuracies in the measurements among patients and thus avoiding risks
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Overview of some of the methods used for measuring cardiac output
There exist several methods of measuring cardiac output that are clinically useful. These methods are either invasive or noninvasive methods. Some of the methods that have been used to measure cardiac output are briefly looked at below. This will be followed by a comprehensive literature review of the best method for measuring cardiac output.
Many people have referred to this method as “the gold standard for measuring cardiac output”. It is based on the notion that the quantity of a substance that is taken up or given out by a body organ is the product of the blood flow that takes place in that particular organ and the venous and arterial difference of the very substance. This method makes use of lungs as the organ and oxygen as the substance. The Fick equation is given as;
CO = Vo2 (in ml/min)/ (Cao2 – Cvo2) × 10
Where, 10 is the conversion factor per 100 ml6. Oxygen consumption measurement in most cases makes use of exhaled gas analysis.
The conditions under which determination of oxygen consumption is carried out are supposed to be consistent in carrying out the estimation of the cardiac output measurements that follow. The measurement of oxygen consumption in those patients that are critically ill may be labile because of the changing requirements for oxygen. If there is assuming of the oxygen consumption measurement, the cardiac output measurement that is eventually arrived at by using the Fick equation may turn out not to be accurate.
This is an invasive method for measuring cardiac output. This method was not used in a most consistent manner up to the time the Swan-Ganz catheter development was carried out in the course of the starting years of the 1970s.(Headley, 2000). In this method, there is injecting of a known quantity of a solution at a known temperature in to the proximal port of a thermo-dilution catheter.
This is followed by plotting on the graph (time-and-temperature curve) of the temperature variation or change that occurs between the time of injection and the time the mixed injectate reaches the thermostor at the end of the catheter. The area under this curve that is plotted is what determines the cardiac output. In this curve, there is inverse proportion. Therefore, this implies that a small area under this curve gives an indication of a higher cardiac output and vice versa (Smith, Levy and Ferrario, 2006).
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According to Headley (2000), the accuracy that is realized through employing this method is fairly good. However, this accuracy may not be there in the case where “there is a retrograde flow on the right side of the heart, as in tricuspid or pulmonic-valve regurgitation, and atrial or ventricular septal” (Headley 2000, page 36).
There should be stability in the injectate temperatures and the readings are supposed to be within between five to ten percent of each other (Kaukinen et al. 2003). In most cases, there is use of 10 ml for an injectate volume but there can still be the use of 5 ml in the case where a fluid restriction is something necessary.
Dye dilution test
Under this method, in order to carry out the calculation of cardiac output, there is entering of some determined values into a formula or plotted into a time and dilution-concentration curve. According to Lippincott and Wilkins (2007) “measurements of dye dilution are mostly of great help in carrying out detection of intracardiac shunts and vascular regurgitation” (page 213).
This method of measurement for cardiac output has some limitations. These limitations “have restricted its application at the bedside” (Wheeler, Wong and Shanley, 2007). The first limitation is that “the concentration-time curve is measured using a densitometer, which requires calibration with samples of the patient’s blood containing known concentration of the dye” (Wheeler, Wong and Shanley, 2007, page 606). This can always be very much time consuming.
The other limitation is that measurement normally takes place extracorporeally implying that more blood must be withdrawn with every measurement carried out. However, these limitations have been gotten over to a great extend through utilization of fiberoptic sensors making use of the pulse technique of pulse dye densitometry. But unlucky enough, as Imai et al (2001) observe, the accuracy that is realized by employing this technique is questionable, especially at the level where cardiac output is low and also among particular patients.
According to Wheeler, Wong and Shanley (2007), this principle is closely related to the principle of pulmonary thermodilutions. However, these two principles differ in a way that under this one, the measurement of temperature change is carried out in a large artery (femoral artery). As Von and Hoeft (2000) put it, “the journey from injection to measurement traverses the right atrium, right ventricle, pulmonary vascular bed, left atrium, left ventricle and aorta” (pg 226).
At the initial consideration, a person may see this to be a violation of one of the basic requirements of indicator measurement. However, according to Pauli et al (2002), there has been indications that so long as the injectate is cooled in a most efficient way, thus to a temperature of below 10 degrees Celsius, the heat loss is small and is constant in relative terms and does not have an effect on the accuracy of measurement of cardiac output.
More so, the heat loss has been used in carrying out the calculation of the extravascular lung water, which can make diseases to be more severe in particular cases (Davey-Quinn et al, 2000).
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More advantages are derived from transpulmonary thermodilution than from pulmonary dilution. According to Wheeler, Wong and Shanley (2007), these advantages include;
The need to access the pulmonary artery is obviated
A commercially available device combines this modality with pulse contour analysis and thus representing an application of this technique to the provision of continuous cardiac output.
Transthoracic electric bioimpedance (TEP) method
This is a noninvasive method that is used in measuring cardiac output.
Performance of bioimpedance is carried out by the application of a small electrical
current to the chest and this electrical current goes through electrodes that are put on the person’s neck and sides (Van et al, 2003). “The pulsatile flow of blood causes fluctuations in the current, and the device calculates cardiac output from the impedance waveform” Cotter et al (2004, page 435).
Peacock, Summers and Emerman (2006), Critchley (2000), Imhoff, Lehner and Lohlein (2000),Turner (2000), Van et al (2005) and (Suttner, 2006), point out that the studies that have been carried out have provided some evidence that in the case where there has been proper selection of patients, in the use of this technique, there can be provision of information in regard to change in cardiac output in a similar manner with provision of information that is obtained by use of invasive techniques, with no risks that are linked to cardiac catheterization.
However, there exist various factors that affect the accuracy that is obtained by using this technique and therefore the absolute figures that are obtained for cardiac output may not be all that accurate (Shoemaker, Wo, and Chien et al (2006).
Best method for measuring cardiac output
There exists no true accuracy standard in a large number of applications in medicine. According to Engoren and Barbee (2005),”the most commonly used technology becomes the gold standard against which newer technologies are compared to see if they are accurate enough” Engoren and Barbee (2005 Page 43).
They point out that initially there was comparison of the thermodilution method for measuring cardiac output with the Fick method. In the current day, the comparison has shifted to be carried out between thermodilution and bioimpedance to establish whether bioimpedance is accurate to a level of substituting the thermodilution method for clinical utilization at the bedside (Raaijmakers, et al, 2005).
In the study Engoren and Barbee (2005) carried out in which they made a comparison between various methods (Fick method, thermodilution and bioimpedance method) to establish the best method for measuring cardiac output, the results they obtained gave an indication that bioimpedance method is not accurate to a level of becoming a substitute for the thermodilution method.
According to them, even if agreement between cardiac outputs measured by thermodilution and by bioimpedance is superior to the common agreement between any of those two methods and cardiac outputs measured by making use of the Fick method, the 0.95 level of agreement of 2.6 L/Min is not accurate enough to make it possible to make use of bioimpedance instead of thermodilution for measuring cardiac output in those patients who are seriously ill (Engoren and Barbee, 2005).
Engoren and Barbee (2005) established in the study that, in patients in whom the three methods under investigation were employed to measure cardiac output, the values that they came up with by employing the Fick method were larger in comparison with those obtained by employing bioimpedance and thermodilution and the difference that was observed became higher and higher with the increase in cardiac output.
These results that they obtained did not go in line with those obtained by Drazner et al (2002) who came up with results in their study that cardiac outputs that are measured by making use of the Fick method were, in a systematic way, lower in comparison with the cardiac outputs that were measured using the thermodilution method as well as the bioimpedance method in a convenience population going through catherization for heart failure.
In a similar manner, Espersen et al (2000) in their study came up with results that gave an indication that cardiac output measured using the thermodilution method were higher to an unacceptable level as compared to those cardiac outputs measured by the Fick cardiac output measurement method in those patients who were healthy and exercising.
However, the results that were obtained by Engoren and Barbee (2005) went in line with those results that were obtained by Dhingra et al (2002) in which they carried out a study on eighteen patients who were critically ill in which it was found that cardiac outputs were gradually higher in using the Fick method as compared with using the thermodilution method carried out at the end-expiration.
Engoren and Barbee (2005) came to draw a conclusion that the measurement of cardiac output is not carried out in a direct manner in any of the three methods they considered. There is measurement of different parameters carried out using each method and thus this implies that each of these methods has unique and distinct error sources (Leslie et al, 2004).
In line with this idea, considering the thermodilution method, the measurement of temperature is carried out by the computer and the computer carries out the calculation of the cardiac output basing on the volume of the injectate. Following this the errors that results from the measurement of the temperature and the volume, “loss of thermal signal through the walls of the catheter, which is particularly a problem at lower cardiac outputs and with slower injections, tricuspid or pulmonic regurgitation, and septal defects all cause inaccurate determinations of cardiac outputs (Engoren and Barbee, 2005).
Determination of cardiac output carried out by the Fick method is seen to go in agreement with the determination of cardiac output using either bioimpedance method or the thermodilution method in those patients who are not severely ill [Hoeper et al (2000), Sherman et al (2001) and Koobi (2001)].
A probable reason for the absence of accuracy in the studies that have been carried out in determining the best method in the patients who are more sick is that, by carrying out the measurement of the total oxygen consumption by the body in the use of the Fick method, and then carrying out the division of this total with the difference between the mixed venous oxygen content and arterial content, larger values for pulmonary consumption of oxygen will bring about higher mathematical errors of in the cardiac outputs. The outputs are found to be higher (Sageman, 2002).
According to Albert, Hail, Li and Young (2004), indication is given by a review of literature that there are various factors that bring about differences or discrepancies in the measurements of cardiac output that are arrived at by employing two methods that are different. In the study that they carried out to determine the best method of cardiac output measurement, they made a comparison of Thermodilution and metabolic measurements in critically ill patients and they established that “the mean coefficient of variation between measurements was only 3.5 %, however, CIs for comparisons of thermodilution and metabolic measurements were wide” (Page 474).
This prompted them to draw a conclusion that utilizing estimated oxygen consumption for cardiac output measurements whose determination is carried out by employing the Fick Method were a poor replacement for utilization of calculated oxygen consumption.
In today’s practice, there is no measuring of oxygen consumption of many patients and thus there has been substitution of the Fick method by the thermodilution method and this has turned out to be a method that is used in most cases for measuring cardiac output (Albert, Hail, Li and Young, 2004).
However, as indicated through evidence provided by researchers such as Renner, Morton and Sakuma (2002) and Kalassian and Raffin (2001), those factors linked to the clinicians as well as to equipments and those that are inherent to patients may have an effect on the accuracy of those measurements carried out by thermodilution method and its reproducibility. In the same manner as observed by Albert, Hail, Li and Young (2004), “the accuracy of impedance cardiography may be influenced by factors related to clinicians such as sensor replacement, the digital signal processing system, and the algorithm and equations used to calculate cardiac output” (Page 476).
This gives an implication that several factors could bring an impact on the relationship of cardiac output measurements carried out with the clinical standards as well as those carried out by employing impedance cardiography. The impedance cardiography devices that were developed in the most recent times utilize equations that are more accurate to carry out the calculation of cardiac output.
In a review that was carried out in the recent times of the impedance cardiography algorithms utilized in the calculation of cardiac output, it was established by Van De Water et al (2003), that the calculation of cardiac output carried out by making use of the ZMARC equation gave an indication of the closest agreement to measurement of cardiac output using thermodilution.
In the clinical studies carried out by Drazner, Thompson B, Rosenberg PB, et al (2002) in which the ZMARC was used in patients having cardiac disease, there was indication of very close linear agreement and bias that was acceptable and accuracy between thermodilution cardiac out measurement and impedance cardiography measurement of cardiac output.
A comparison was carried out by Drazner et al (2002), in their study concerning patients having advance heart failure who went through pulmonary catheterization between impedance cardiography and invasive hemodynamic measurements. It was established in that study that “Bland-Altman agreement and correlation between cardiac output measurements using bioimpedance method and direct Fick method were the similar to those measurements of cardiac output that were determined by thermodilution and determined by the direct Fick method” (Drazner, 2002, Page, 994).
However, the final results presented by Albert, Hail and Young (2004) of their study showed that impedance cardiography method offers precise cardiac output measurements with minimum bias and smaller limits of agreement in comparison with the bolus thermodilution method. They drew a conclusion that it is very much easier to carry out measurement of cardiac output using the impedance cardiography methods as compared to using the thermodilution with a pulmonary artery catheter.
They point out that the application of impedance cardiography can be carried out without waste of time and it also does not expose the patient to the dangers of infection, loss of blood or any other danger or risk that is linked to arterial catheters. More so, they also state that “impedance cardiography allows continuous monitoring of cardiac output, unlike intermittent measurements with thermodilution, which involve injections of fluid boluses with the attendant risk of volume overload” (Albert, Hail and Young, 2004).
The best method of measurement is the one that can be easily administered, relatively less expensive and yields very accurate results and poses minimal risks to the patients. Basing on the to the literature, a general agreement by the studies that have been carried out concerning the best possible method of measuring cardiac
output has not yet been reached. Some of the studies tend to contradict with one another. However, the literature points out thermodilution method as being a method that is commonly used although it has its own complications. Studies have been carried out to determine whether there should be replacement of the thermodilution method by the bioimpedance cardiography method.
Some people like Engoren and Barbee (2005) through their studies found out that this replacement is not possible while on the other hand, researchers like Albert, Hail and Young (2004).suggest that impedance cardiography is the best method and is better as compared to thermodilution. Considering these contradictions that have been looked at in the literature, there is need to carry out further studies in order to come up with a precise method that can be used for measuring cardiac output.
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