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Interact CardioVasc Thorac Surg 2009;8:507-511. doi:10.1510/icvts.2008.200626 © 2009 European Association of Cardio-Thoracic Surgery
Intraoperative assessment of coronary bypass graft to posterior descending artery by means of transesophageal echocardiographyDepartment of Cardiovascular Surgery, Hiroshima University Hospital, Kasumi 1-2-3, Minami-ku, Hiroshima, 734-8551 Japan Received 12 December 2008; received in revised form 9 February 2009; accepted 9 February 2009
*Corresponding author. Tel.: +81-82-257-5216; fax: +81-82-257-5219.
Intraoperative transesophageal echocardiography (TEE) assessment of coronary artery graft anastomosed to posterior descending artery (PDA) was evaluated. Twenty-one patients with a saphenous vein (SV) graft (n=17) or right gastroepiploic artery (RGEA) graft (n=4) anastomosed to the PDA were examined. In the transgastric mid short-axis view, the graft was depicted as an echo-free zone between the right ventricle and diaphragm. The depth, diameter, angle for Doppler measurement, and angle-corrected blood flow velocity were determined. The graft was visualized in 20 cases (95.2%). The diameter of the SV graft was 3.0–6.5 mm (mean 4.0 mm), while that of RGEA graft was 2.2–2.9 mm (mean 2.5 mm), at the depth of 1.2–4.4 cm (mean 2.4 mm) with incident angle of 14–57° (mean 38.6°). Blood flow was detected in 17 cases but was difficult to detect in three cases (velocity <10 cm/s). Postoperative coronary angiography showed patent graft in 16 of former cases (one case of operative death excluded) but occluded graft in all of latter cases. Intraoperative TEE assessment was feasible nearly consistently. Diastolic blood flow velocity <10 cm/s suggests an early occlusion of the graft.
Key Words: Coronary artery bypass grafting; Echocardiography; Outcomes; Assessment
In coronary artery bypass grafting (CABG), anastomosis is commonly assessed by palpation of the graft or using a transit-time flow meter or other modalities before chest closure. However, these methods are not available when graft patency needs to be examined after chest closure in the operating room or postoperative intensive care unit. In such situations, a bed-side assessment of graft patency is desirable, if feasible, instead of coronary angiography (CAG). Although transthoracic echocardiography may be applied to evaluate the left internal thoracic artery (LITA) graft [1, 2], there is considerable interference with flow visualization in cases under positive pressure ventilation. It is not suitable for assessing other grafts such as the saphenous vein (SV) or right gastroepiploic artery (RGEA) graft. We previously reported an assessment of LITA graft using transesophageal echocardiography (TEE) [3]. The LITA flow could be measured at its origin in 90% of patients and a diastolic dominant flow pattern and interruption of the flow by temporary clamping of the graft indicated successful anastomosis without significant stenosis, while a systolic dominant flow pattern suggested stenotic or occluded anastomosis. However, it was not sufficient because free grafts could not be assessed. Although Niimi et al. reported intraoperative TEE evaluation of an SV graft by measuring blood flow velocity at the proximal anastomosis [4], it was successful in only two-thirds of patients. Recently, we have developed a novel method using TEE to assess a graft that is anastomosed to the posterior descending artery (PDA) as the second step. We report the technical aspect of this method and its preliminary results regarding correlation to the postoperative CAG.
2.1. Subjects and equipment Twenty-one consecutive patients in which the PDA was revascularized with an SV (17 cases) or RGEA graft (4 cases) were examined. They consisted of 14 males and 7 females with ages ranging from 55 to 82 years (mean, 68.8 years) including four emergency cases (Table 1). Anastomosis was performed under off-pump, on-pump beating, or cardiac arrest in 16, 3, and 2 cases, respectively. The total number of grafts was 2, 3, or 4 in 6, 12, and 3 cases, respectively. Concomitant surgery included mitral valve replacement and left ventriculoplasty with Dor's procedures each in one case. This study was approved by the Ethics Committee on Human Research in our institute and informed consent was obtained from every patient.
The echocardiographic system used in these cases included EUB550 (Hitachi Co, Tokyo, Japan), SSD-5500 (Aloka Co, Tokyo, Japan), or iE33 (Philips Co, Eindhoven, The Netherlands). TEE examinations of the grafts were performed by a single operator (the first author) after the patient was weaned from cardiopulmonary bypass and was hemodynamically stable but before the chest was closed. The TEE images were recorded on VHS videotapes for later analysis. For visualizing the graft, a transgastric mid short-axis view was employed because the PDA is located in the posterior interventricular sulcus and the graft courses along the right ventricle between the heart and diaphragm toward the ascending aorta or to the abdomen after penetrating the diaphragm (Fig. 1). The graft was depicted as an echo-free zone between the diaphragm and right ventricle in the 0° scanning plane (Fig. 1b). The echo-free zone was identified as the graft, rather than the fluid around the heart, because it became circular in the orthogonal scanning plane (90°).
The blood flow in the echo-free area was examined with color flow imaging. The velocity range was set at the commonly used level (50–80 cm/s) for intraoperative TEE monitoring. In the pulsed-wave Doppler mode, the sample volume was placed in the graft lumen. When the incident angle was large, the probe tip was bent leftward to rotate the image of ventricles counterclockwise (Fig. 2a), or the sample volume was moved to a more distant portion of the graft (Fig. 2b) to minimize the incident angle. When the blood flow in the graft was stable and could be recorded, the graft was temporarily clamped and changes in flow were recorded.
2.3. Postoperative assessment of grafts The graft patency was assessed by postoperative CAG in every case as part of a routine postoperative examination by cardiologists who were blinded to the intraoperative TEE results. The results of CAG were correlated with the intraoperative TEE findings on the SV or RGEA graft. 2.4. Data collection and analysis The TEE images recorded on VHS videotapes were captured on a personal computer in an mpeg2 format (720x480 pixels) and still-frame images were extracted using video-editing software (MediaStudio Pro version 7.01, Corel Co, Ottawa, Canada).On these TEE images, the following parameters were measured: 1) the distance from the transducer to the center of sample volume, 2) the internal diameter of the graft at the sample volume, 3) the incident angle at the point of Doppler measurements, and 4) the maximal velocity of the graft flow (Fig. 3). These measurements were performed using graphic software (CANVAS version 11, ACD Systems Co, Victoria, Canada). The measured data were calibrated with a scale of distance or velocity on the recorded TEE images. The maximal velocity of the graft flow was corrected by the incident angle. The data are expressed as mean±S.D.
There was one early death (case 17) due to cardiac failure, in which postoperative assessment of the coronary graft was not available. The grafts were successfully visualized with TEE in 20 cases (95.2%) within a few minutes. The measurement data are listed in Table 1. The distance of the sampling point from the transducer was as small as 2.4±0.8 cm (1.2–4.4 cm). The diameter of the SV graft in 16 patients was 4.0±0.8 mm (3.0–6.5 mm), while that of the RGEA graft in four patients was smaller, 2.5±0.3 mm (2.2–2.9 mm). The portion of RGEA graft that passed the diaphragm could be visualized in all four cases (Fig. 4). The blood flow signal was readily detected in 17 of 20 cases, but was difficult to detect in three cases (case 2, 7 and 15). In the latter cases, the surgeon could palpate adequate pulsation of the graft, and TEE showed that the graft lumen was echo-free, indicating that the graft was patent in the OR.
The incident angle for Doppler measurement was 38.6±14.0° (14–57°), equal or smaller than 45° in 14 cases. In the 17 cases with a flow signal detected, the blood flow was diastolic dominant (Fig. 5a,b), and the maximal flow velocity in the graft corrected by the incident angle ranged from 11.5 to 111.6 cm/s (Table 1). The instantaneous interruption of blood flow following temporary clamping of the graft by the surgeon was recorded (Fig. 5c). However, in the three cases where blood flow was difficult to detect, both forward and reverse flow was observed, and the maximal forward flow velocity in diastole was below 10 cm/s (Fig. 6).
Postoperative CAG was performed in all but one case of early postoperative death. A patent graft was found in the 16 cases that had blood flow velocity 10 cm/s, but an occluded graft was found in the remaining three cases that had blood flow velocity <10 cm/s. In the case of early death, post-mortem examination was not feasible.
This study showed that: 1) the SV or RGEA graft near the PDA can be visualized with TEE in nearly all patients; 2) the diastolic dominant color flow signal in the graft and an interruption of blood flow under temporary clamping of the graft is helpful for identifying the graft; 3) clearly detectable blood flow with maximal velocity 10 cm/s indicates successful anastomosis, while a difficult-to-detect flow signal with reduced blood flow velocity (<10 cm/s) suggests an early graft occlusion. The distal portion of an SV or RGEA graft is readily and clearly visualized because a graft is located in the vicinity of the transducer through the liver and diaphragm as excellent acoustic windows. However, in case 18, the transgastric mid short-axis view could not be visualized, probably due to inadequate space in the stomach for advancing the probe to an appropriate level. This is a limitation of this method. The graft is identified morphologically as a tubular echo-free zone along the right ventricular wall and its patency can be assessed in the hemodynamic aspect by diastolic dominant flow pattern that is instantaneously interrupted by clamp test. As in the TEE assessment of LITA graft, the current method minimizes an interruption of surgery during graft assessment. The above findings except clamp last are helpful for assessing the graft after chest closure. When a graft occlusion is potentially responsible for low cardiac output in the postoperative ICU, the TEE finding of patent graft with adequate blood flow enables us to concentrate more on other possible causes. While the commonly used flow meter provides blood flow rate, our method uniquely provides blood flow velocity in the graft. Several investigators examined the blood flow in SV grafts by means of an intravascular Doppler wire [5], pulsed-wave Doppler in the operative field [6], or intraoperative TEE [4] and reported that the blood flow velocity in the SV graft averaged 25–39 cm/s with the lowest velocity 10 cm/s. Bach et al. [5] reported that the blood flow velocity in a mammary artery graft was significantly higher than that in a vein graft and mentioned that high velocity in the arterial graft might be related to its long-term patency. In contrast, Kamp et al. reported that a low flow velocity in the left atrial appendage (<20 cm/s) was associated with a high incidence of thrombus formation [7]. Reduced blood flow velocity in a graft can be caused by various mechanisms including: 1) adequate blood flow in the native coronary artery (competition); 2) reduced systemic arterial pressure; 3) stenotic proximal anastomosis; 4) kinking or twisting of the graft; 5) stenotic distal anastomosis; and 6) inadequate coronary run-off. In the three cases with low graft flow velocity in the current series, mechanism of 5) or 6) was likely to be responsible because the surgeon tried to anastomose the graft even to the small PDAs and felt some resistance at injection of blood to the graft. However, we did not make a decision of redo anastomosis solely based on the TEE findings, because we could not be certain of accuracy of TEE for assessment of graft patency at that time. There are several limitations of this study. First, the number of cases in this study is not large and interobserver variability is not examined. We would like to call trials of this method in many institutions. Second, the blood flow velocity in the graft depends on the internal diameter of the graft portion where the sample volume is placed, although we measured the velocity at the portion of graft without apparent stenosis or dilatation. The cut-off value of 10 cm/s needs to be evaluated in further investigations. Unfortunately, TEE assessment of coronary graft to the anterior or lateral walls is not devised as of this moment. With development of these methodologies, the TEE assessment would be much more beneficial. In conclusion, intraoperative TEE assessment of the SV or RGEA graft anastomosed to the PDA is feasible in nearly all patients. The current results suggest that the diastolic dominant flow pattern with blood flow velocity over 10 cm/s indicates a patency of the graft, while reduced diastolic blood flow predicts an early occlusion of the graft. Further investigations with a large number of patients in multiple institutions with interactive discussions are expected.
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Abstract
1. Introduction
