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J Thorac Cardiovasc Surg 2007;133:1396-1397
© 2007 The American Association for Thoracic Surgery
Letter to the Editor |
Division of Cardiac and Vascular Surgery, Sunnybrook Health Sciences Centre, University of Toronto, Toronto, Canada
Reply to the Editor:
We thank Dr. Kulik and colleagues for their interest in our prospective study comparing transit-time ultrasound flow (TTF) and indocyanine green angiography (ICG).1
We appreciate the opportunity to elaborate on the three issues they raise:
To improve visualization, we ensure that there is no pedicle covering the area of the anastomosis itself and we routinely expose the native vessel for approximately 1 cm beyond the anastomosis. We do not skeletonize the entire graft. Our evaluation of patency is based on both anatomic visualization of the anastomosis itself and the opacification characteristics (ie, TIMI flow) of the native circulation. Also, further experience with the technique has led us to perform selective angiograms directly into the grafts which allows for improved visualization of the distal anastomosis verses injections into the central venous line which can be confounded by native flow.2
Regarding the second issue, whereas criteria for pulsatility index (PI) and diastolic flow fraction (DFF) are standard, there is little agreement on cut-off values that distinguish between normal and abnormal mean flow values. Although most surgeons would agree that a flow of less than 5 ml/min is definitely abnormal and will often prompt revision, flows between 5 to 40 ml/min could potentially represent abnormal grafts. There is no clear consensus regarding cutoff values that determine graft problems and little prospective data on the subject. The study by Di Giammarco and colleagues, who recommend a 15 ml/min mean flow cutoff, was retrospective in design.3 Because patients who had follow-up angiograms were generally symptomatic, their study design could not accurately assess the false positive rate. The study by Walpoth and colleagues, who recommend a 20ml/min mean flow cutoff, had no angiographic controls to determine sensitivity or specificity.4 In our study, surgeons were permitted to intervene on any graft they deemed to be poorly functioning, even if it was "normal" by the prespecified TTF criteria (mean flow>10 ml/min), but this did not occur in any cases. Indeed, as clearly stated in the manuscript, the mean flow in abnormal (>50% occluded) grafts was 24.4 ± 8.6 ml/min and was 16.4 ± 23.0 ml/min in grafts deemed to be totally occluded by the reference standard. In secondary analyses, if we used a cutoff mean flow of 15 ml/min, we would have identified no additional true positives and 6 false positives. Using a mean flow cutoff of 20 ml/min, we would have identified only one additional true positive and 10 false positives. Our findings are in agreement with the only other prospective comparison of these two techniques by Taggart, who demonstrated that up to 10% of patients may receive erroneous graft revisions based on false positive TTF findings.5
Hence, our cutoff value of 10 ml/min minimizes false positives while identifying nearly all true positives and may be an ideal reference value when used in conjunction with PI and DFF. Higher mean flow cutoffs appear to have too many false positives to be reliable measures, and many surgeons are unlikely to revise grafts based on higher flow values for this reason.6 This is especially true if revision means reinstating cardiopulmonary bypass.
Finally, in regard to cost effectiveness, we have not performed such a comparison, because this was a clinical effectiveness study. It is unlikely that either technology will be highly cost effective in terms of Quality Adjusted Life Years (QALY) which is the generally accepted measure, but there may be cost benefits from non-fatal events.
Footnotes
Note: The following Reply to the Editor refers to a letter to the editor published in a previous issue of the Journal: Kulik A, Rubens FD, Ruel M. Intraoperative indocyanine green angiography: Ready for prime time? J Thorac Cardiovasc Surg 2007;133:592-3.
References
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  H. B. Ward, R. F. Kelly, and E. K. Weir Assessment of graft patency during coronary artery bypass graft surgery mitigating the risk. J. Am. Coll. Cardiol. Img., May 1, 2009; 2(5): 613 - 615. [Full Text] [PDF]
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