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the second CO determinative. Calls to Phillips technical support did not provide help for this technique. Finally, our technicians used trial and error to derive at a series of 11 clicks to make this change empirically. It took several calls to technical support
to ascertain that the Lafarge calculation was operative for the
onsite documentation.
DISCUSSION
We were surprised that our widely used CCL software
Oxygen saturations are measured and entered into the CCL software in many cases before thermodilution is done, especially if intracardiac shunt is suspected. In these cases oxygen saturations from the superior vena cava and pulmonary artery are obtained early during the initial introduction of the pulmonary artery catheter to exclude left-to-right shunting.1 Once the saturations are entered to the CCL computer system, the estimated Fick will always be the default CO used in hemodynamic calculations to assess AVA. Although the second CO type could be selected on the computer as the determinative output and temporarily display the related AVA in the screen monitored by the technician, this value will never appear
a cumbersome non-intuitive 11-step process. Using the Fick calculation in patients older than 40 years of age, the age of nearly all AS patients, instead of TD in AVA assessment can assessment. Any difference in the CO measurement will have a 1:1 effect on the valve area calculation, i.e., a 25 percent difference in CO measurement will alter the valve area by
25 percent.
Many CCL computer systems, including ours, make an assumption of a patient’s VO2 based on an equation developed by LaFarge and Miettinen using regression analysis from a cohort of 879 patients who were studied between 1961 and 1966.9 This is potentially worrisome as the equation (Figure 1) to estimate the VO2 was done using patients from ages 3 to 40 years with congenital heart disease, with an average age of 12 years. This certainly is not the population that is encountered on a daily basis in most CCLs. This equation included measurements that were performed in various postures and since O2 consumption is altered by changes in position, it may be not applicable to the adult CCL patients who are almost always supine.17 LaFarge demonstrated that O2 consumption decreases with age during
continues in later decades is unknown. The LaFarge calculation was not recommended by the authors to be used in patients older than 40 years of age.9
Comparisons of CO determination using estimated and directly measured values have demonstrated considerable discrepancies in patients undergoing cardiac catheterization.13-21 Gertz et al. recently showed that estimates of VO2 using the Lafarge and of AS severity.3 In their study, the difference in calculated valve area in patients with aortic stenosis between using the Lafarge table and measured VO was as much as 0.5cm2.3 Thermodilution
measured Fick, and superior to the estimated Fick method.3 However TD Technique can show variability in measurement
and therefore is not reliable in the setting of many underlying cardiac abnormalities, especially tricuspid regurgitation and atrial 7,8 Despite these limitations, it is still probably more 3
In our study we found instances where the CO was both
under- and over-estimated by the Fick technique. Using Fick calculation to estimate AVA may subject patients to inappropriate care by either depriving them from needed surgery if it underestimates AS or directing them to undergo an unnecessary surgery early on and deprive them of the chance to get a possibly less invasive procedure in the future (especially in the light of development of Transcatheter Aortic Valve Replacement (TAVR) and the expected expansion of indication in the future to include moderate risk patients).22
CONCLUSION
The fact that we may base our clinical decisions regarding
congenital heart disease patients, aged 3-40 is worrisome. Cardiologists must be aware of the limitations of CCL computer systems when measuring CO, especially when using these values to calculate valve areas.
ACKNOWLEDGEMENT/FUNDING:
The study was funded in part by an Institutional Development Award (IDeA) from the National Institute of General Medical Sciences of the National Institutes of Health under grant number U54- GM104941 (PI: Binder-Macleod).
2
technique in that study was presumed to be as accurate as a
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Del Med J | July 2016 | Vol. 88 | No. 7
