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Ann Thorac Surg 2001;72:1270-1274
© 2001 The Society of Thoracic Surgeons

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Original article: cardiovascular

Relation of intraoperative flow measurement with postoperative quantitative angiographic assessment of coronary artery bypass grafting

^a Division of Cardiovascular Surgery, Kasugai Municipal Hospital, Kasugai, Aichi, Japan

Accepted for publication May 29, 2001.

Address reprint requests to Dr Takami, Division of Cardiovascular Surgery, Kasugai Municipal Hospital, 1-1-1 Takagi-cho, Kasugai City 486-8510, Japan
e-mail: cvs{at}hospital.kasugai.aichi.jp

	Abstract

Top Abstract Introduction Material and methods Results Comment References

 
Background. It is critical to evaluate the anastomotic quality of coronary artery bypass grafting (CABG) in the operating room. The aim of this study is to determine the validity of intraoperative flow measurement for predicting the quality of CABG by comparison with the postoperative quantitative angiographic evaluation of the grafts.

Methods. Eighty-two grafts, including 37 internal thoracic arteries, were examined intraoperatively with a transit-time flowmeter. Coronary angiograms were performed 14 ± 5 days after CABG to quantify the diameters at the toe, heel, and anastomosis proper of the grafts.

Results. There were significant differences between patent and nonpatent grafts in all intraoperative flow parameters. However, the only cut-off value to distinguish patent from nonpatent was a fast Fourier transformation (FFT) ratio of 1.0. FFT is the ratio of powers of the fundamental frequency and its first harmonic. Postoperative quantitative angiography indicated that the stenosis was greatest at the heel of the anastomosis. The degree of stenosis at the heel of the anastomosis alone correlated significantly with intraoperative mean flow values.

Conclusions. Fast Fourier transformation analysis of flow measurement may be useful to differentiate patent grafts intraoperatively. Intraoperative flow measurement may predict the most stenotic part of the anastomosis.

	Introduction

Top Abstract Introduction Material and methods Results Comment References

 
Coronary artery bypass grafting (CABG) has contributed to treatment of patients with ischemic heart disease to increase their survival and reduce ischemic complications [1]. Anastomotic quality of CABG is directly associated with both perioperative and long-term clinical results [2]. Therefore, it is critical for surgeons to evaluate the quality of the anastomoses of CABG in the operating room. It was traditionally common for a surgeon to determine the adequacy of the anastomosis based upon palpation of the graft pulsation, hemodynamic stability, and electrocardiographic changes, which are all unreliable and indirect. To increase reliability, several methods have been advocated for intraoperative assessment of the anastomotic quality in CABG [3–7]. Among these, transit-time flow measurement is considered to be more convenient, less invasive, more reproducible, and less time consuming [7]. The present study aimed to determine the feasibility and validity of the intraoperative transit-time flow measurement of grafts in CABG by comparing with postoperative quantitative angiographic evaluation, which has been used as the gold standard for assessing the results of coronary intervention and CABG [8–10].

	Material and methods

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Study patients and CABG
The present study included 35 consecutive patients (28 males and 7 females; mean age, 66.4 ± 7.4 years) who underwent CABG either with cardiopulmonary bypass (n = 28) or without (n = 7). Combined procedures included mitral valve replacement (n = 4), aortic valve replacement (n = 2), abdominal aneurysmectomy (n = 2), and femoro-femoral arterial bypass (n = 1). The patients received 82 grafts, including 37 right or left internal thoracic arteries, 21 radial arteries, 18 saphenous veins, and six right gastroepiploic arteries. All anastomoses were performed by one surgeon (Y.T.) in the same fashion. Distal anastomosis was constructed with 8/0 polypropylene for arterial and 7/0 for vein grafts. Five stitches were taken around the "heel" of the graft, with two stitches to one side of the apex of the graft, one stitch through the apex, and two stitches on the opposite side. The graft was held away from the coronary artery until then. The suture loops were pulled up to approximate the graft to the coronary artery. The anastomosis was completed by placing stitches around the toe of the graft in counter-clockwise direction on the coronary artery.

Intraoperative flow measurement
Graft flow tracing was obtained intraoperatively using a transit-time flowmeter (BF 2000; Medi-Stim AS, Oslo, Norway). A flow probe of 3 mm or 4 mm was placed around the graft when the hemodynamic condition became stable after the cardiopulmonary bypass was weaned in a standard CABG, or when an anastomotic procedure was completed in off-pump cases. Based upon the obtained flow profile, the following variables were calculated: mean graft flow (Qm, mL/min); pulsatility index (PI = [maximal flow - minimal flow]/Qm); percent insufficiency (% Insuf = volume of backward flow/volume of forward flow); and fast Fourier transformation (FFT) of the flow curve. FFT analysis is based upon the principal that all periodic waveforms can be broken down into a series of pure sine waves or harmonics [11, 12]. Harmonics exist at frequencies that are multiplies of the frequency of the original waveform ("the fundamental frequency") and are described in terms of an amplitude and phase. The pulsatile waveforms of graft flow in CABG can be considered to be periodic with a fundamental frequency (ie, the heart rate of the patient). As a parameter representing gradual decrease in power of the harmonics of the fundamental frequency, a FFT ratio (=F₀/H₁, where F₀ is a power of the fundamental frequency and H₁ is a power of the first harmonic) was calculated in the present study.

Postoperative quantitative angiographic evaluation
Every patient underwent a postoperative cardiac catheterization 14 ± 5 days after CABG with a standard technique through the femoral or brachial route. A dose of 2 mg isosorbide dinitrate was injected selectively in each bypass graft. All grafts were examined from at least three different views. Each anastomotic site was analyzed quantitatively with a computer-assisted analyzing software (CCIP-310/W; Cathex Co, Tokyo, Japan). After optical magnification (2:1), an automatic edge-detection program determined the graft and coronary artery contours by assessing brightness along scan lines perpendicular to the centerlines of the vessel [8–10]. The quantitative evaluation was focused on three anastomotic sites; proximal portion of the anastomosis (heel), distal portion of the anastomosis (toe), and anastomosis proper. In addition to their diameters, the degrees of stenosis of the heel, toe, and anastomosis proper were analyzed by using the adjacent coronary or graft segments as references, as illustrated in Figure 1. A graft with stenosis more than 25% at either heel, anastomosis proper, or toe of the anastomosis was considered to be "nonpatent." The term "nonpatent" included not only occluded but also severely and moderately stenotic grafts. In contrast, a graft with stenosis less than 25% at all three portions of the anastomosis was considered to be "patent."

View larger version (12K): [in this window] [in a new window]   Fig 1. Parameters in the postoperative quantitative angiographic evaluation. The diameters of the native coronary artery of the proximal portion of the anastomosis (the heel, Ah), the anastomosis proper (Ga), and the coronary artery at the distal portion of the anastomosis (the toe, At) were measured. The adjacent proximal and distal native coronary (Cp and Cd) and graft segments (Gp) were also measured as references to calculate the stenosis of each portion: Ah/Cp for the heel, Ga/Gp for the anastomosis proper, and At/Cd for the toe.

	Results

Top Abstract Introduction Material and methods Results Comment References

 
Intraoperative flow measurement and patency of grafts
Of 82 grafts, flows of five right gastroepiploic arteries could not be measured because they were too thick for the probes to be placed. Typical recordings of a left internal thoracic artery (LITA), which was revealed to be patent well in the postoperative angiograms, was demonstrated in Figure 2. The patent graft flow waveform, whether in situ or aortocoronary, showed two phases of antegrade systolic and diastolic flow. As for graft flow and derived variables, there were significant differences in Qm, PI, %Insuf, and FFT ratio between patent and nonpatent grafts (Qm: 51.2 ± 30.7 vs 13.7 ± 13.8 mL/min, p = 0.0004; PI: 2.74 ± 1.91 vs 21.8 ± 25.4, p = 0.021; %Insuf: 2.82% ± 6.98% vs 28.3% ± 33.6%, p = 0.027; FFT ratio: 3.20 ± 2.11 vs 0.65 ± 0.26, p = 0.0003), as demonstrated in Figure 3. However, it was impossible to define precisely a cut-off value to distinguish patent from nonpatent grafts in Qm, PI, or %Insuf. Only the FFT ratio appeared to have a cut-off value. While all the patent grafts had a ratio of greater than 1.0, as shown in Figure 2, all the nonpatent grafts yielded a FFT ratio of less than 1.0, as shown in Figure 4.

View larger version (52K): [in this window] [in a new window]   Fig 2. Data from a 73-year-old male patient who underwent an in situ grafting with a left internal thoracic artery (LITA) to left anterior descending artery (LAD). Shown are LITA flow tracing (I), fast Fourier transformation (FFT) of the flow curve (II), and postoperative angiogram of the LITA graft (III), which was patent well. (F0 = a power of the fundamental frequency; H1 = a power of the first harmonic.) The mean flow was 21 mL/min, the pulsatility index was 2.4, the percent insufficiency was 0 %, and the FFT ratio was 2.89.

View larger version (21K): [in this window] [in a new window]   Fig 3. Results of the intraoperative flow measurement (Qm, PI, %Insuf, and FFT ratio). There were significant differences in all of these parameters between the patent and nonpatent grafts. (FFT = fast Fourier transformation; %Insuf = percent insufficiency; PI = pulsatility index; Qm = mean graft flow.)

View larger version (51K): [in this window] [in a new window]   Fig 4. Data from a 62-year-old female patient who underwent in situ grafting with the left internal thoracic artery (LITA) to the left anterior descending artery. Shown are LITA graft flow tracing (I), FFT of the flow curve (II), and a postoperative angiogram of the LITA (III). Note that in spite of the intraoperative flow of 15 mL/min, the LITA graft was revealed to be occluded postoperatively. The FFT ratio was 0.12. (F0 = a power of the fundamental frequency; H1 = a power of the first harmonic; FFT = fast Fourier transformation.)

View larger version (13K): [in this window] [in a new window]   Fig 5. Results of the postoperative quantitative angiographic evaluation. The heel portion of the graft anastomosis was the most stenotic, while the anastomosis proper was the least stenotic.

View larger version (25K): [in this window] [in a new window]   Fig 6. Correlation of the intraoperative mean flow values and the degree of stenosis on the postoperative quantitative angiograms at each portion of the graft anastomosis. Significant correlation was demonstrated for only the heel (abbreviations are the same as in Figure 1).

	Comment

Top Abstract Introduction Material and methods Results Comment References

Quantitative coronary angiography with an edge detection algorithm plays an established role in coronary intervention [8, 9]. Although most surgeons have described the quality of the CABG anastomosis as only "patent," "stenotic," or "occluded," they should analyze the postoperative coronary angiograms quantitatively to increase the accuracy of the angiographic findings. In this study, we quantitatively examined the heel, toe, and anastomosis proper in the angiogram according to our definition. Our results, that the heel portion was the most stenotic, may have resulted from the surgical techniques of the surgeon who performed CABG for all patients enrolled in this study. The suturing bites of the stitches around the "heel" of the graft might result in these findings. In this way, quantitative angiographic evaluation may point out technical problems of the surgeons.

Quantitative angiographic evaluation also indicated in the present study that intraoperative Qm is closely related to the degree of the stenosis at the most stenotic portion of the CABG anastomosis. This finding suggests that intraoperative flow measurement reflects precisely the anastomotic quality of CABG. Therefore, surgeons can rely on the transit-time flow measurement in the operating room to evaluate their surgical techniques.

One limitation of this study is that we focused on only the distal anastomosis of CABG. Significant stenosis can be present proximally in cases with aortocoronary bypass. However, the proximal anastomosis is so much larger in diameter that its stenotic effect on the graft flow may be less than the distal anastomosis. The second limitation was the difficulty in obtaining intraoperative flow profiles of a right gastroepiploic artery graft by using a flow probe of 3 or 4 mm. To generalize our findings, we must collect the data of gastroepiploic artery grafts by using a larger probe or by skeletonizing the graft. The third limitation is that we did not report the parameters with which the graft flow varies. These include the perfusion area, blood pressure, heart rate, coronary resistance, and graft diameter. It is not practical to equalize these parameters strictly in the intraoperative flow measurement. These parameters might be mostly equal in this study, because flow measurement was performed in the hemodynamically stable condition.

In conclusion, FFT analysis of flow measurement might be useful to differentiate patent grafts intraoperatively. And the intraoperative flow measurement may predict the degree of the anastomosis portion that is the most stenotic.

	References

Top Abstract Introduction Material and methods Results Comment References

 

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