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Even if the estimated VO2 had a better theoretical underpinning, the variations in direct continuously measured VO2 occasionally seen during catheterizations would make a single static estimation
of VO2 inaccurate. The large variability in repeated calculated CO using a static value suggests that changes in actual VO2 during the procedure is a contributing factor. And, of course, any difference in the CO measurement will have a direct continuous linear effect on the valve area calculation, i.e., a 25 percent difference
in CO measurement will alter the valve area by 25 percent. At least a 25 percent variation in CO was seen in 56 percent of measurements with both assumed VO2 and TD when compared to direct continuous VO2.11,13 Recent studies have shown that the correlation of direct continuous VO2 and assumed VO2 derived cardiac output was not impressive even in children.
In both of these studies the direct VO2 was assessed utilizing devices while children underwent general anesthesia, techniques not readily available to adult cardiologists.17,18
Comparisons of CO determination using estimated and direct measured values have previously differed in patients undergoing cardiac catheterization.9-13,17-22 In a recent study, Narang et al compared direct VO2 CO calculated as an average over time using Douglas bags to different formulas of estimated VO2 CO. The direct and estimated VO2 CO values differed by greater than 25 percent in 17-25 percent of patients depending on the formula used.11 Gertz et al, using a facemask device, recently showed that estimates of VO2 using of the LaFarge and hence underestimated AVA, resulting severity.3 In their study the difference
in calculated valve area in patients with aortic stenosis between using Lafarge table and measured VO2 was as much as
0.5cm2.3 In our study we found instances where the CO was both under- and over- estimated by the assumed Fick technique.
The other commonly used CO method
is thermodilution. TD Technique can show variability in measurement and therefore is not reliable in the setting of many underlying cardiac abnormalities especially tricuspid regurgitation and 23,24 Despite these limitations, it is still probably more reliable than estimated Fick in those 3 We found this technique to be reproducible but CO values did not correlate well with direct continuous VO2.
We used a device that allows an easy reproducible way to measure direct continuous VO2. The patient wears a full-face mask that is connected via
tubing to the actual metabolic cart for collection of data. The patient does not need to be intubated or heavily sedated while undergoing the procedure. VO2 assessed using metabolic cart has been validated versus those assessed by Douglas bag and was found to give extremely
high correlating values (R=0.99).14 Both
the American Thoracic Society (ATS)
and European Respiratory Society
(ERS) approve the use of the device,
and it is already used routinely during cardiopulmonary testing in many hospitals, as was ours. In our experience, the device is easy to use and the measurements
the procedural time, since the VO2 measurements may be taken very quickly once all catheters are in place. The fact
that all measurements of the CO equation (VO2 and blood saturations) can be done
at exactly the same time may be important, as both of these parameters can change during a procedure. The accuracy and ease of the use of this device, the availability of this device in most hospitals and the lack of
need for general anesthesia and intubation make it practical to use especially when making decisions about advanced heart failure therapies or valvular surgery.
This study has several limitations.
First, it included only a small number
of patients. Second, we often used mild sedation, which may affect VO2 or
depress cardiac output. Third, although regurgitation, which may be responsible for underestimation of CO by TD.23,24 Finally, some patients may be unable to carry out this test if they develop discomfort from wearing the mask or develop anxiety from a sense of claustrophobia, (although all of our patients were able to tolerate the mask for the duration of the procedure).
CONCLUSION
Cardiac output and AVA calculated from direct continuously measured VO2 varies substantially from those based on assumed VO2 values and thermodilution, which are the default methods used in most CCL. Direct continuously measured VO2-derived CO gives highly reproducible valve area measurements and may be more accurate.
ACKNOWLEDGEMENTS
This 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).
CONFLICT OF INTEREST
disclose.
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Del Med J | September 2016 | Vol. 88 | No. 9
