Eur J Cardiothorac Surg 1999;16:S83-S87
© 1999 Elsevier Science NL
Spectral analysis of graft flow for anastomotic error detection in off-pump CABG
Steven C. Koenig,
Daniel J. VanHimbergen,
Saad F. Jaber,
Dan L. Ewert,
Patricia Cerrito,
Paul A. Spence*
Jewish Hospital Cardiothoracic Surgical Research Institute at the University of Louisville, 500 South Floyd Street, Department of Surgery, University of Louisville School of Medicine, Louisville, KY 40202, USA
* Corresponding author. Tel.: +1-502-852-4838; fax: +1-502-852-4868
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Abstract
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Objective: Flow probes have been introduced as a non-invasive means of anastomotic quality assessment in off-pump coronary artery bypass graft (CABG). Flow waveform morphology cannot reliably be assessed visually unless severe anastomotic stenosis is present (>90%). We applied spectral analysis techniques to determine whether the frequency content of graft flow can improve the surgeon's ability to detect anastomotic errors. Methods: Forty-six mammary to left anterior descending artery (LAD) anastomoses were created in mongrel dogs during off-pump CABG surgery. Graft flow was measured using transit-time flow probes with the LAD closed, and the mammary graft patent and with varying degrees of stenosis. The degree of anastomotic stenosis was created by an artificial stitch and verified by random post-operative angiography. Spectral analysis of the graft flow waveforms was performed. Differences in the magnitude and phase components of the graft flow for the first five harmonics were determined for the varying anastomosis test conditions. Differences were determined using analysis of variance and least square means techniques. Results: The magnitude of the fundamental (zeroth) harmonic was statistically different in the internal mammary artery (IMA) with 0–25% stenosis compared to IMA with 50–75% stenosis (P<0.01
). Further, the magnitude of the first, second, and fourth harmonics were statistically different in IMA with 0–25% compared to IMA with 75% (P<0.01
). The phase of the first harmonic was statistically different in IMA with 25% stenosis than IMA with 50% stenosis (P<0.01
). No differences in interaction between the LAD and IMA for all ranges of stenosis were detected (P>0.50
). Conclusion: Spectral analysis of graft flow waveforms may be beneficial in detecting lesser degrees of anastomotic stenosis (i.e. <90%) compared to traditional visual assessment of mean graft flow and/or graft flow waveform morphology.
Key Words: Anastomotic quality, Off-pump CABG, Spectral analysis
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1. Introduction
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During the past decade, off-pump coronary artery bypass graft (CABG) procedures have become more widely accepted and practiced due to the deleterious effects of cardiopulmonary bypass (CPB) [1,2]. The introduction of cardiac stabilizers has significantly facilitated the procedure. However, off-pump CABG may not be universally accepted until the quality of the anastomosis is superior or equal to that of the conventional on-pump procedure [3].
In general, most surgeons would agree that the best time to revise a marginal anastomosis is during the initial surgery. Therefore, a variety of methods have been introduced in an effort to assess the quality of the anastomosis intraoperatively. Angiography is universally considered the ‘gold standard' technique in the assessment of graft patency. However, it can be costly, invasive, and oftentimes not readily available in the operating room.
Flow measurement techniques have been gaining increasing popularity for the assessment of anastomotic quality in off-pump CABG. These techniques readily provide the surgeon with a mean flow value and the corresponding flow waveform of the graft, which have been used to assess the patency of the graft. Our group has shown that grafts with anastomotic stenosis greater than 75% can be reliably detected using this approach [4,5]. Unfortunately, this technique can not reliably catch anastomotic errors in which the graft may still have 50–75% stenosis.
The objective of our study was to determine whether the frequency content (magnitude and phase components) of the graft flow waveforms could be used to reliably detect anastomotic errors with less than 75% stenosis.
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2. Materials and methods
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2.1 Surgical technique
Twenty-seven mongrel dogs (25–35 kg) were anesthetized with nembutal (30 mg/kg) and maintained on 2% isofluorane. Respiration was maintained with a volume control respirator on 100% oxygen. Continuous ECG was monitored throughout the procedure. Arterial blood gases were sampled every 30 min, and bicarbonate added as needed to maintain a physiologic pH between 7.35 and 7.45. Aortic blood pressure was monitored using a 5F-micromanometer-tipped catheter (Millar Instruments, Houston, TX) introduced through the left femoral artery. Through the left fifth interspace anterolateral thoracotomy, the left and right mammary arteries were dissected from their origin to their bifurcation and wrapped with gauze soaked in papaverine solution (1 ml of papaverine diluted in 10 ml of normal saline).
The pericardium was opened and the margins sutured to the edges of the wound. The left anterior descending artery (LAD) distal to the first diagonal was selected for grafting the left mammary artery followed by grafting the right mammary to the proximal LAD. The anastomotic region was controlled proximally and distally with 3.0 prolene snares. Heparin (1 mg/kg) was given intravenously followed by two periods of ischemic pre-conditioning (3 min each) before opening the selected site of the LAD. A cardiac stabilizer (Origin, Menlo Park, CA) was used to mechanically stabilize the grafting site. The mammary to LAD anastomosis was then constructed ‘off-pump' using 7.0 continuous prolene suture technique. Stenosis was created by placing a blind suture through the heel of the anastomosis (8.0 prolene), thereby reducing the cross-sectional area. Post-operative angiography was randomly used to determine the extent of the stenosis created. The degree of stenosis was categorized as patent (<15%), mild (<25%), moderate (<50%), moderately severe (<75%), or severe (>75%).
All animals received humane care in compliance with the Guide for the Care and use of Laboratory Animals published by the National Institutes of Health (NIH publication 85–23, revised 1985) and with approval by the University of Louisville Institutional Animal Care and Use Committee.
2.2 Experimental design
Forty-six patent (<15% stenosis) left and/or right internal mammary artery (IMA) grafts to LAD were constructed. Transit-time flow probes (Model 3SB, Transonic Systems Inc., Ithaca, NY) placed on the graft(s) were used to measure graft flow. Continuous beat-to-beat graft flow was recorded with the LAD occluded and the graft to LAD anastomosis under patent (<15%), mild (<25%), moderate (<50%), moderately severe (<75%), and severe (>75%) stenosis conditions. One-minute data sets containing graft flow (Q), arterial pressure (AoP), and ECG were analog-to-digital (A/D) converted, sampled at 100 Hz, and recorded using a MacLAB data acquisition system (MacLAB, Milford, MA) during each experimental condition. The degree of anastomotic stenosis was determined by random post-operative angiography.
2.3 Data analysis
Data files containing graft flow waveforms were analyzed using standard averaging and digital fourier analysis (DFT) techniques. The data were sampled at 100 Hz with each data file containing between 20–30 beats. Custom computer programs were written using MATLAB (Mathworks, Natick MA) to select beats and to find the systolic/diastolic intervals for each beat.
Additional programs were written to obtain the DFT for all beats, and the corresponding magnitudes and phase angles of the flow coefficients were calculated for the zeroth, first, second, third, fourth, and fifth harmonics. Means and standard errors of the magnitude and phase components for all harmonics were calculated and compared for the varying degrees of stenosis as described in the experimental design. A repeated measures analysis of variance (ANOVA) was used to detect differences. Where statistical significance were detected, a least squares means (LSM) was calculated to determine where the differences occurred.
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3. Results
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The LSM and standard errors for the magnitude and phase components for the first six harmonics are listed in Table 1
and shown in Figs. 1 and 2.
The magnitude of the fundamental (zeroth) harmonic was statistically different in IMA with 0–25% stenosis compared to IMA with 50–75% stenosis (P<0.01
). Further, the magnitude of the first, second, and fourth harmonics were statistically different in IMA with 0–25% compared to IMA with 75% (P<0.01
). The phase of the first harmonic was statistically different in IMA with 25% stenosis than IMA with 50% stenosis (P<0.01
). No differences in interaction between the LAD and IMA for all ranges of stenosis were detected.
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Table 1. Least squares mean (LSMEAN) and standard error of magnitude and phase components of graft flow waveforms from DFT analysis
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Fig. 1. Magnitude component of graft flow waveforms from DFT analysis for patent anastomoses (<15%), and those with mild (<25%), moderate (<50%), moderately severe (<75%), and severe (>75%) stenosis.
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Fig. 2. Phase component of graft flow waveforms from DFT analysis for patent anastomoses (<15%), and those with mild (<25%), moderate (<50%), moderately severe (<75%), and severe (>75%) stenosis.
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4. Discussion
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Minimally invasive CABG is an appealing approach to the management of coronary artery bypass disease. Although this procedure is being practiced more often in the operating room, the most critical issue remains the anastomotic quality. Generally, there is a consensus that the probability of creating an anastomotic error in off-pump CABG is higher than in conventional cases performed with CPB. However, this is unproven and based on the higher technical demands of performing an operation on the beating heart. Our group has shown that there is improved neurologic outcome as a result of minimally invasive CABG when compared to routine CABG [5]. For this reason, it is important that surgeons are able to find a reliable method of anastomotic assessment so off-pump CABG will become more universally accepted.
It is well known that the best time to revise a faulty anastomosis is during the initial procedure. Unfortunately, a proven reliable technique for intra-operative assessment of the quality of anastomoses does not exist. Surgeons traditionally relied on probing the anastomosis, absence of hemodynamic compromise, and/or the absence of abnormalities in ECG as methods for determining the adequacy of their anastomosis. These crude methods are unreliable and may only detect very poor anastomoses.
There has been very limited research involving spectral analysis and anastomotic quality in CABG surgery. However, a variety of flow measurement techniques have become more popular for intraoperative anastomotic quality assessment. Flow probes provide a continuous flow tracing and mean flows through the graft with a high degree of accuracy [6,7]. Due to the peculiar physiology of the coronary circulation, flow through the coronary grafts occurs during diastole with a short systolic peak. Very little diastolic flow in a graft is indicative of an occluded graft, which may warn the surgeon to the necessity of revision of the anastomosis.
Oftentimes, the surgeon may create an ‘imperfect' anastomosis that may be unoccluded, but still contain some degree of stenosis. In these grafts, the flow is also predominantly diastolic, but with taller systolic peaks. The decision whether or not to revise these anastomoses intra-operatively can be difficult, because it is difficult to distinguish the severity of the stenosis using visual assessment of the flow wave morphology alone. Therefore, the use of spectral analysis may help distinguish some of these intermediate degrees of stenosis.
The zeroth harmonic of the magnitude component from the DFT analysis can be used to distinguish between patent and moderately severe stenotic anastomoses. The first, second, and fourth harmonics of the magnitude component can be used to differentiate between patent and severely stenotic anastomoses. Differences between mild and moderately stenotic anastomoses can be detected using the phase component of the first harmonic. Collectively, these findings provide additional information for characterizing varying degrees of anastomotic stenosis that could not be detected using traditional time domain techniques, such as mean graft flow and/or visual assessment of graft flow waveforms. However, the spectral analysis technique could not definitively and reliably distinguish intermediate anastomotic stenoses. Therefore, we propose integrating the information derived using time and frequency domain techniques into a neural network approach to further classify varying degrees of anastomotic stenosis.
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Acknowledgments
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This work was supported by a grant from the Jewish Hospital Heart and Lung Institute, Louisville, KY, USA.
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Footnotes
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Presented at the 2nd MITSIG International Symposium: Controversies in Cardiothoracic Surgery, Hong Kong, November 20–21, 1998.
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References
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- Jaber SF, Koenig SC, BhaskerRao B, VanHimbergen DJ, Cerrrito P, Ewert DL, Gray LA, Spence PA. Role of graft flow measurement technique in anastomotic quality assessment in minimally invasive CABG. Ann Thorac Surg 1998;66:1087-1092.[Abstract/Free Full Text]
- Jaber SF, Koenig SC, BhaskerRao B, VanHimbergen DJ, Spence PA. Can visual assessment of flow waveform morphology detect anastomotic error in off-pump CABG?. Eur J Cardio-thorac Surg 1998;48:476-479.
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Abstract
1. Introduction
