Innovations: Technology & Techniques in Cardiothoracic & Vascular Surgery:
The Accuracy of Transit Time Flow Measurement in Predicting Graft Patency After Coronary Artery Bypass Grafting
Walker, Patrick F. BS*; Daniel, William T. MD*; Moss, Emmanuel MD*; Thourani, Vinod H. MD*; Kilgo, Patrick MS*; Liberman, Henry A. MD†; Devireddy, Chandan MD†; Guyton, Robert A. MD*; Puskas, John D. MD*; Halkos, Michael E. MD*
From the Divisions of *Cardiothoracic Surgery, and †Cardiology, Emory University School of Medicine, Atlanta, GA USA.
Accepted for publication September 19, 2013.
Presented the Robert Emery Young Investigator Award at the Annual Scientific Meeting of the International Society for Minimally Invasive Cardiothoracic Surgery, June 12–15, 2013, Prague, Czech Republic.
Disclosures: Vinod H. Thourani, MD, serves on the Advisory Boards of St. Jude Medical, St. Paul, MN USA, and Boston Scientific, Corp., Natick, MA USA, has received research grants from Edwards LifeSciences, Corp, Irvine, CA USA, Sorin Group, Milan, Italy, and Maquet, Wayne, NJ USA, and is a co-founder of Apica Cardiovascular, Galway, Ireland. Michael E. Halkos, MD, is a consultant for Intuitive Surgical, Inc, Sunnyvale, CA USA, and has served on the hybrid revascularization advisory board for Medtronic, Inc, Minneapolis, MN USA. Patrick F. Walker, BS; William T. Daniel, MD; Emmanuel Moss, MD; Patrick Kilgo, MS; Henry A. Liberman, MD; Chandan Devireddy, MD; Robert A. Guyton, MD; and John D. Puskas, MD, declare no conflicts of interest.
Address correspondence and reprint requests to Michael E. Halkos, MD, Division of Cardiothoracic Surgery, Emory University School of Medicine, 550 Peachtree St, NE, 6th Floor, MOT, Atlanta, GA 30308 USA. E-mail: email@example.com.
Objective: Transit time flow measurement (TTFM) is a method used to assess intraoperative blood flow after vascular anastomoses. Angiography represents the criterion standard for the assessment of graft patency after coronary artery bypass grafting (CABG). The purpose of this study was to compare flow measurements from TTFM to diagnostic angiography.
Methods: From October 9, 2009, to April 30, 2012, a total of 259 patients underwent robotic-assisted CABG procedures at a single institution. Of these, 160 patients had both TTFM and either intraoperative or postoperative angiography of the left internal mammary artery to the left anterior descending coronary artery graft. Transit time flow measurements were obtained after completion of the anastomosis and after administration of protamine before chest closure. Transit time flow measurement assessment included pulsatility index, diastolic fraction, and flow (milliliters per minute). Angiograms were graded according to the Fitzgibbon criteria. The patients were grouped according to angiographic findings, with perfect grafts defined as FitzGibbon A and problematic grafts defined as either Fitzgibbon B or O.
Results: Overall, there were 152 (95%) of 160 angiographically perfect grafts (FitzGibbon A). Of the eight problematic grafts, five were occluded (Fitzgibbon O) and three had significant flow-limiting lesions (FitzGibbon B). Two patients had intraoperative graft revision after completion angiography, one had redo CABG during the same hospitalization, and five were treated with percutaneous coronary intervention. A significant difference was seen in mean ± SD flow (34.3 ± 16.8 mL/min vs 23.9 ± 12.5 mL/min, P = 0.033) between patent and nonpatent grafts but not in pulsatility index (1.98 ± 0.76 vs 1.65 ± 0.48, P = 0.16) or diastolic fraction (73.5% ± 8.45% vs 70.9% ± 6.15%, P = 0.13).
Conclusions: Although TTFM can be a useful tool for graft assessment after CABG, false negatives can occur. Angiography remains the criterion standard to assess graft patency and quality of the anastomosis after CABG.
Transit time flow measurement (TTFM) is a method used to assess intraoperative blood flow after vascular anastomoses, including coronary anastomoses after coronary artery bypass grafting (CABG). Angiography remains the criterion standard for the assessment of graft quality and flows despite the innovation of TTFM. Intraoperative or postoperative angiography, however, is not routinely performed after CABG. Therefore, most coronary artery bypass grafts are assessed only clinically and through palpation by the surgeon after their completion. The incidence of perioperative graft failure is estimated to be approximately 5% to 11%.1 When early postoperative graft failure occurs, 30-day mortality is reportedly as high as 9%.2 Graft patency, therefore, is an essential variable in determining the outcome of CABG. Transit time flow measurement has been used as a relatively simple and quick method to verify patency before the sternum is closed. Using an ultrasound probe, TTFM is able to specifically measure mean flow, pulsatility index (PI), and diastolic fraction (DF). Various cutoffs have been proposed and cited for these values to suspect when a graft may be at risk for failure.3–6 The purposes of this study were to compare TTFM parameters in patients undergoing minimally invasive left internal mammary artery (LIMA)–left anterior descending artery (LAD) grafting and to determine whether any differences were present between the patent and nonpatent grafts as defined by subsequent conventional diagnostic angiography.
From October 9, 2009, to April 30, 2012, a total of 259 patients underwent robotic-assisted CABG procedures at a single institution for LIMA-LAD bypass. The technique of robotic-assisted CABG has been previously described.7 Briefly, all patients underwent robotic LIMA harvest using one camera port and two operating ports. The camera port is placed in the middle of the sternum (fourth or fifth interspace), two fingerbreadths lateral to the midclavicular line. Two operating ports are placed two interspaces above and below the camera port under endoscopic guidance. After LIMA harvest, a full longitudinal pericardiotomy is performed. After identification of the target site on the LAD, the LIMA is divided distally between clips after systemic heparinization. A 3- to 4-cm anterolateral thoracotomy incision is then made after precise localization of the planned site of anastomosis with a long spinal needle. The LAD is stabilized with a NUVO stabilizer (Medtronic, Inc, Minneapolis, MN USA), and a manual off-pump LIMA-LAD anastomosis is then performed with 8-0 polypropylene suture. One-hundred sixty patients had both TTFM and either intraoperative or postoperative angiography of the LIMA-LAD graft. After administration of protamine, the Doppler probe was placed 2 to 3 cm proximal to the anastomosis. This was performed without occluding the artery proximally because of the space limitations associated with assessment through the 3- to 4-cm incision. The decision to perform intraoperative angiograms instead of postoperative angiograms (postoperative day 1 or 2) was ultimately according to the discretion of the operating surgeon and the limited availability of the hybrid operating room, which was not available until late 2010. Transit time flow measurements were obtained after completion of the anastomosis and administration of protamine before chest closure using MediStim probes and the VeriQ flowmeter system (MediStim ASA, Oslo, Norway). Transit time flow measurement assessment included mean flow (Qmean, in milliliters per minute), PI (PI = [Qmax − Qmin]/Qmean), and DF (DF = Qdiastole/[Qsystole + Qdiastole]).6 Angiograms were graded according to the Fitzgibbon criteria as Fitzgibbon A, B, or O.8 Graft occlusion was defined as either of the following: no flow in the LIMA or flow in the LIMA to the proximal LAD but with distal LAD occlusion. The patients were grouped according to angiographic findings, with patent grafts described as FitzGibbon A and nonpatent grafts described as either Fitzgibbon B or O. The patients who were found to have nonpatent grafts were treated with graft revision, redo CABG, or percutaneous coronary intervention depending on coronary anatomy, quality of target vessel and conduit, clinical status, and whether the angiogram was performed intraoperatively or postoperatively. Statistical analysis comparing the TTFM values of patent grafts and nonpatent grafts was performed using the Statistical Package for the Social Sciences software (IBM, Armonk, NY USA) to determine whether a significant difference between TTFM variables existed. Group differences were compared using the Wilcoxon rank sum test. This test was chosen because of the small sample size in the nonpatent group (n = 8) and the apparent variability in the parameters being compared between the two groups.
Of the 160 patients who received a LIMA-LAD bypass that was studied angiographically, the mean ± SD age was 62.6 ± 11.5 years and 119 patients (74.4%) were men. Overall, there were 152 (95.0%) of 160 angiographically patent grafts (FitzGibbon A). Of the eight nonpatent grafts, three had significant flow-limiting lesions (FitzGibbon B) and five were occluded (Fitzgibbon O) (Figs. 1,2). Two patients had graft revision after intraoperative angiography, one had redo CABG during the same hospitalization, and five were treated with percutaneous coronary intervention. On TTFM, the mean flow in problematic grafts ranged from 15 to 53 mL/min; the PI, from 1.1 to 2.5; and the DF, from 60% to 80%. In comparison, the mean flow in perfect grafts ranged from 15 to 80 mL/min; the PI, from 0.9 to 3.6; and the DF, from 34% to 90%. Figure 3 depicts a scatterplot of the flows for Fitzgibbon A grafts versus Fitzgibbon B or O grafts. The two problematic grafts that underwent immediate revision in the operating room were the only two that were found on intraoperative angiography. One was a Fitzgibbon O graft with no antegrade flow through the LAD. It had a flow of 25 mL/min, a PI of 1.8, and a DF of 70%. The other was a Fitzgibbon B graft with poor flows distally. It had a flow of 22 mL/min, a PI of 1.7, and a DF of 75%. The other six nonpatent grafts were discovered on postoperative angiography. Transit time flow measurement revealed a significant difference in mean ± SD flow between patent and nonpatent grafts (34.3 ± 16.8 mL/min vs 23.9 ± 12.5 mL/min, P = 0.033) but not in PI (1.98 ± 0.76 vs 1.65 ± 0.48, P = 0.16) or DF (73.5% ± 8.45% vs 70.9% ± 6.15, P = 0.13). These results are summarized in Table 1.
Transit time flow measurement is a useful tool to assess graft patency after CABG because of its ease of use and ability to assess various flow characteristics. Despite these benefits, however, TTFM is not as sensitive as angiography in detecting graft defects that can lead to graft failure. Of the three parameters measured with TTFM, only mean flow was significantly different. The mean flows for both sets of data (34.3 ± 16.8 mL/min for Fitzgibbon A and 23.9 ± 12.5 mL/min for Fitzgibbon B or O) were both reasonable, however, and a flow that was even 1 SD lower than the mean for the problematic grafts would be unlikely to cause concern in the absence of other abnormal TTFM parameters. One of the limitations of TTFM found in this study was the inability to detect technical errors affecting LAD flow distal to the anastomosis. This was the case for the patients in this study who had good LIMA flow that perfused the LAD retrograde from the anastomosis but provided limited or no antegrade flow beyond the LIMA-LAD anastomosis.
Previous cutoff values based on receiver operating curves have been proposed for TTFM. In terms of the parameters looked at in this study, recommended values5,6 include a DF of greater than 50%, a PI of less than 5, and a mean flow of at least 15 mL/min. If these values were to be applied to the data in this study, two false positives would be found in terms of DF (DF <50% with patent LIMA), one false positive would be found in terms of PI (PI >5 with patent LIMA), and six false positives would be found in terms of mean flow (flow <15 with patent LIMA). A false positive in this case refers to a graft that had at least one questionable parameter on TTFM but was found to be patent with unobstructed antegrade and retrograde LAD flow on angiography. Of the problematic grafts in this study, none would have been identified on the basis of the TTFM properties listed above. Thus, although high flows and low PI on TTFM are generally thought to be reassuring, their presence does not guarantee against graft failure. Our policy has been to revise grafts when TTFM results reveal one or more substantially suboptimal parameters or when there is clinical evidence of ischemia. None of the grafts in this study underwent revision on the basis of TTFM. Takami and Ina9 showed significant differences in mean flow and diastolic filling between patent and nonpatent grafts. They assessed 82 coronary artery bypass grafts with intraoperative TTFM and compared their results with postoperative angiography, defining nonpatent grafts as having a stenosis of at least 25%. Our study investigated a larger patient cohort than that of Takami and Ina and found similar differences in mean flow but not in PI between perfect and problematic grafts. It should be noted that our study defined a graft as problematic if a stenosis of at least 50% was present.
Other methods of intraoperatively evaluating the patency of grafts include intraoperative fluorescence imaging (IFI) using indocyanine green and thermal coronary angiography.10 A recent study by Kuroyanagi et al6 found that TTFM followed by IFI, with graft revisions performed on the basis of poor TTFM scores or delayed IFI contrast enhancement, resulted in an excellent patency rate when followed up by postoperative angiography. Although it does not provide quantifiable data, IFI may be valuable because it allows direct visualization of contrast enhancement through the graft and the distal coronary vasculature. The optimal solution may be hybrid operating rooms that enable intraoperative angiography while the patient is still on the operating room table. Zhao et al11 showed that routine intraoperative completion angiography identified significant defects in 12% of grafts. Identification of any issues intraoperatively allows for immediate graft revision without having to transfer the patient or readminister anesthesia if any problems are discovered on follow-up studies. Angiography is undoubtedly invasive, however, and the risk of administering radiographic contrast needs to be considered, especially in patients with renal insufficiency or significant vascular disease. Thus, the need exists for a less invasive and less potentially harmful test to confirm CABG graft patency.
One of the limitations of this study is its small sample size and, accordingly, low number of graft failures. Another limitation is that, on the basis of our results, we are unable to define threshold values of TTFM for which revision is warranted. Furthermore, one of the limitations with angiography is its sensitivity because some of the defects identified in the patent grafts may have been related to spasm or edema. It is also possible that our results would have been different if TTFM was performed with transient proximal occlusion of the LAD. However, this was not performed because of space constraints.
One of the conclusions of this study is that quality control for innovative cardiac procedures is imperative before widespread adoption can be recommended. For minimally invasive CABG, this study suggests that TTFM cannot replace angiography to ensure graft patency for these less invasive but more technically challenging procedures. With increasing experience and documented angiographic results, TTFM can be used as a less invasive alternative to angiography for the experienced surgeon. Because only 20% of cardiac surgeons use TTFM to assess graft patency,12 more study is needed about its utility after CABG. It can be combined with other means of assessment including electrocardiographic changes, hemodynamic status, echocardiography, and the surgeon’s judgment about the quality the anastomosis. The role of TTFM will be to give the surgeon an additional tool to determine the best course of action to optimize both graft patency and patient outcome.
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This is an interesting analysis of the utility of transit time flow measurement (TTFM) in 160 patients undergoing robotic-assisted coronary artery bypass grafting (CABG) at Emory University. The unique aspect of this study was that all patients had both TTFM and either intraoperative or postoperative angiography. The findings of this study were somewhat surprising. Although there was a significant difference in the mean flow between patent and nonpatent grafts, there was no difference in pulsatility index or diastolic fraction. Moreover, there was no cutoff value for flow that was specific to a nonpatent graft.
This study points out the shortcomings of TTFM and emphasizes the point that angiography remains the criterion standard for coronary artery bypass graft assessment. The main weakness of this study was its small sample size and the small number of graft failures. The power of this study to detect threshold values of TTFM for graft revision was therefore quite limited. However, this is an important contribution to the literature. The role of TTFM continues to be debated, and further investigation is needed to find its place in the armamentarium of quality control after CABG. Until then, angiography remains the best tool to optimize both graft patency and patient outcome. Future work from this group and others hopefully will better define this issue.
Transit time flow measurement; Coronary artery bypass grafting; Angiography
©2013 by the International Society for Minimally Invasive Cardiothoracic Surgery
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