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Ann Thorac Surg 2004;78:2054-2056
© 2004 The Society of Thoracic Surgeons

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Original Article: Cardiovascular

Flow in Coronary Bypass Conduits On-Pump and Off-Pump

Department of Cardiothoracic Surgery, Gentofte Hospital, University of Copenhagen, Hellerup, Denmark

Accepted for publication June 2, 2004.

^* Address reprint requests to Dr Kjaergard, Department of Cardiothoracic Surgery, Gentofte Hospital, Niels Andersens Vej 65, DK-2900 Hellerup, Denmark (E-mail: henrik{at}dadlnet.dk).

	Abstract

Top Abstract Introduction Patients and Methods Results Comment References

 
BACKGROUND: The purpose of the study was to measure the blood flow in coronary artery bypass grafting conduits on-pump and off-pump and to estimate the total conduit flow.

METHODS: In a 3.5-year period, 120 patients having coronary artery bypass grafting on-pump and 97 patients having coronary artery bypass grafting off-pump with the left internal mammary artery anastomosed to the left anterior descending artery and saphenous vein grafts to the remaining diseased coronary arteries were included in the study. Flow in the bypass conduits was measured with the transit-time method.

RESULTS: In men the left internal mammary artery flow (mean ± standard error of the mean) was 33.7 ± 2.0 mL/min on-pump and 34.4 ± 2.9 off-pump (p > 0.05). In women the left internal mammary artery flow was 29.4 ± 3.0 mL/min on-pump and 22.8 ± 1.9 mL/min off-pump (p > 0.05). In men the vein graft flow per anastomosis was 30.4 ± 1.3 mL/min on-pump and 37.8 ± 5.4 mL/min off-pump (p > 0.05). In women the vein graft flow per anastomosis was 28.0 ± 2.9 mL/min on-pump and 23.2 ± 2.9 mL/min off-pump (p > 0.05). Consistently in women, the mean conduit flows were numerically lower than in men. In patients undergoing coronary artery bypass grafting on-pump the total conduit flows (left internal mammary artery plus vein grafts) were 131.4 ± 2.5 mL/min in men and 108.4 ± 3.2 mL/min in women.

CONCLUSIONS: There were no major differences in conduit flow on-pump versus off-pump. Conventional coronary artery bypass grafting on-pump may restore up to approximately half of the normal resting coronary artery blood flow (250 mL/min).

	Introduction

Top Abstract Introduction Patients and Methods Results Comment References

 
The immediate goal of any revascularization procedure is to increase myocardial blood flow. It is assumed that the outcome, eg, alleviation of symptoms, of a revascularizing procedure is caused by the increase in myocardial blood flow. However, in most cases it is not known exactly how much the coronary blood flow is increased by a revascularizing procedure whether it is performed by interventional cardiology (percutaneous coronary intervention) or by cardiac surgery on-pump (coronary artery bypass grafting [CABG]) or off-pump (off-pump coronary artery bypass grafting [OPCAB]).

The purpose of this study was to compare conduit flow in a standardized type of CABG and OPCAB using the left internal mammary artery (LIMA) and vein grafts and to quantitate the total graft flow.

	Patients and Methods

Top Abstract Introduction Patients and Methods Results Comment References

 
In a 3.5-year period from mid-2000 to 2004, 120 consecutive patients having a standardized first-time CABG and 97 patients having OPCAB were included (Table 1). The patients who underwent CABG all had three-vessel coronary artery disease, whereas patients selected for OPCAB had one-vessel, two-vessel, or three-vessel disease. In all cases the LIMA was harvested as a pedicled graft, and 2 mL of papaverine solution was injected into the lumen of the LIMA, using an olive-shaped needle and ensuring that the solution was distributed throughout the length of the LIMA [1]. The LIMA was anastomosed as a single graft end-to-side to the left anterior descending coronary artery (LAD). The greater saphenous vein was harvested from the lower leg and stored in a solution containing papaverine. In patients undergoing CABG, a single vein conduit was anastomosed sequentially to all the remaining diseased coronary arteries. In OPCAB procedures the LIMA was anastomosed to the LAD in all cases, and in 36 additional patients (26 men and 10 women) one or more vein grafts were anastomosed to stenotic coronary arteries. The CABG operations were performed with cardiopulmonary bypass in normothermia using cold crystalloid cardioplegia. Immediately before termination of cardiopulmonary bypass, the graft flow was measured. In OPCAB procedures, the graft flow was measured after the proximal grafts were completed. None of the patients received large doses of inotropic agents. Graft flow was measured with the transit-time method using the CardioMed flowmeter (model CM 1005; Medi-Stim A/S, Oslo, Norway). The probes measured 3 and 4 mm to fit the actual size of the vessels. In most instances 3-mm probes were used for the LIMA and 4-mm probes for the vein grafts. The transit-time probes were placed around the vessel giving the true volume flow in milliliters per minute. In case the graft flows were less than 10 mL/min, corrective measures were taken (use of papaverine to dilate the vessel or redoing the anastomosis). Therefore, no flow below the lower limit of 10 mL/min was included. All flow measurements were performed in 2 minutes.

	Results

Top Abstract Introduction Patients and Methods Results Comment References

 
The patients' characteristics and results of graft flow measurements are shown in Table 1. Comparing men and women in the CABG and OPCAB groups there were no significant differences with regard to age, surface area, or LIMA-to-LAD flow (p > 0.05). In the CABG group, total saphenous vein graft (SVG) flow appeared to be 19% (p < 0.01) higher in men than in women. In the OPCAB group, the same trend was observed; however, it did not reach the conventional level of statistical significance. Calculating the SVG flow per anastomosis, it appeared that there was no difference in flow between men and women either in the CABG group or in the OPCAB group (p > 0.05). Comparing men in the CABG group with men in the OPCAB group, there were no significant differences with regard to age, surface area, or LIMA-to-LAD flow (p > 0.05). Likewise comparing women in the CABG group with women in the OPCAB group, there were no significant differences with regard to age, surface area, or LIMA-to-LAD flow (p > 0.05). Men in the CABG group had a 45% (p < 0.001) higher total SVG flow than men in the OPCAB group, and women in the CABG group consistently had a 35% (p < 0.01) higher total SVG flow than women in the OPCAB group. However, calculating the SVG flow per anastomosis cancelled this effect: thus, there were no statistically significant differences in SVG flow per anastomosis either between men in the CABG and OPCAB groups or between women in the CABG and OPCAB groups (p > 0.05). Thus, it seems that there is a link between the total flow in SVGs and the number of anastomoses performed using a sequential vein graft technique.

	Comment

Top Abstract Introduction Patients and Methods Results Comment References

 
This study is the first attempt to quantitate the total flow in coronary artery bypass grafts after CABG and OPCAB. In our department we have routinely measured coronary bypass flow in all CABG and OPCAB procedures for the last 8 years as a quality-control measure. Transit-time flowmetry has shown good correlation with the directly measured blood flow [2] and with the Doppler ultrasound method, and it is more applicable for clinical measurements than the other methods are [3].

Our results showed that in both men and women the flow in the LIMA was no different whether the operation was performed on-pump or off-pump. Differences in SVG flow were also minor. In a recent retrospective study also using transit-time flow measurement in on-pump and off-pump coronary artery surgery, it was found that the flows in both LIMA and vein grafts were lower in off-pump surgery [4]. In our measurements mean conduit flows were numerically lower in women than in men. Because this observation appeared to be consistent it is likely that it does not reflect a mere play of chance, although it did not reach the conventional level of statistical significance. The obvious reason is that women on average have smaller coronary artery vessel calibers than men.

The average flow rates in CABG assessed in our series were in accordance with those of other published series [4, 5] and with data from the manufacturer (Medi-Stim) of the transit-time flowmeter: LIMA-to-LAD flow of 32 mL/min and SVG flow of 33 mL/min. In one Japanese study [6] the flow rates of internal mammary artery grafts (65 mL/min) were higher than in this and other published series [4, 5]. The authors used smaller probes (2 or 3 mm in size) than in this study, which could be one explanation for the measured difference in flow rates of internal mammary artery grafts [6].

The total graft flow in CABG was 131.4 ± 2.5 mL/min (n = 96) in men and 108.4 ± 3.2 mL/min (n = 26) in women. The resting coronary artery flow in humans is approximately 250 mL/min [7], meaning that nearly half of the resting coronary artery flow may be restored by CABG in patients with three-vessel disease. We believe this is higher than for any other type of revascularization procedure. Together with the flow in the native coronary arteries the cumulative flow may relieve angina pectoris in the majority of the patients. In OPCAB procedures the total flow was less than that in CABG procedures, reflecting that a number of the OPCAB patients did only have one-vessel or two-vessel coronary disease and therefore had fewer distal anastomoses performed. The surgeons selected these patients for OPCAB because they found it easier and safer, whereas the majority of patients with three-vessel disease were revascularized with CABG.

Early graft occlusion after CABG or OPCAB may have deleterious consequences as it is associated with a high risk of postoperative myocardial infarction, postoperative hemodynamic instability, and even sudden death. Intraoperative measurement of graft flow in internal mammary artery and vein grafts during CABG and OPCAB is a useful quality control to reveal technical errors. In at least 2% of the patients we found a graft flow of less than 10 mL/min, which was corrected (use of papaverine to dilate the LIMA or resuturing of the anastomosis) to improve graft flow before finishing the operation [8]. In a study of OPCAB patients, graft revision was performed in 8% of the patients, with improvement in graft flow in 92% of the revised grafts [9]. We have also been successful in improving graft flow in the majority of cases, except when a graft erroneously has been sewn to a large coronary artery with an insignificant stenosis. In such cases, because of a high competitive flow in the native coronary artery the graft flow remains low. In CABG patients with an internal mammary artery graft, this condition can be disclosed by applying the aortic cross-clamp, which is followed by a steep increase in internal mammary artery flow. Thus, we believe grafting large coronary arteries with insignificant stenoses should be avoided; however, we also believe the significance of this type of error is not as serious as kinking or obstruction of an anastomosis. We have previously shown that measuring graft flow is worthwhile because a patient may have significant ischemia as a result of graft occlusion at the time of termination of the procedure without apparent electrocardiographic changes or absence of pulse in the graft [5, 8]. We also believe that measuring graft flow has improved the knowledge of the physiologic aspects of revascularization surgery and the pitfalls of this type of surgery to both staff surgeons and residents.

Today, revascularization procedures include a variation of percutaneous coronary interventions and bypass grafting procedures using different arteries (mammary artery, radial artery, gastroepiploic artery) connected as Y-grafts and T-grafts or extended grafts. These efforts are done to replace vein grafts with arteries because the long-term patency of arteries is considered better than for SVGs. Approximately half of the vein grafts may become stenotic after 10 years [10]. The LIMA is well established as the graft of choice to the LAD because of excellent long-term patency [11]. However, neither the graft flow nor the long-term patency has been documented in many of the new types of percutaneous coronary intervention techniques or in bypass grafting procedures using other arteries or other anastomotic techniques [12]. Our measurements of baseline flow in coronary artery bypass conduits allows us to continue to improve the technical results of CABG and OPCAB by a critical evaluation of intraoperative graft patency in the new type of procedures.

In summary, we did not find a difference in coronary artery bypass conduit flow on-pump versus off-pump. It appears that there is a link between the total flow in SVGs and the number of anastomoses performed. Thus, sequential vein graft patency could prove to be better than in single-vein conduits. It was also found that in patients with three-vessel coronary artery disease a standard CABG with LIMA and vein grafts may restore nearly half of the normal resting coronary blood flow.

	References

Top Abstract Introduction Patients and Methods Results Comment References

 

1. Vilandt J, Kjaergard H, Aggestrup S, Andreasen JJ, Olesen A. Intraluminal papaverine with pH 3 doubles blood flow in the internal mammary artery Scand Cardiovasc J 1999;33:330-332.[Medline]
2. Laustsen J, Pedersen EM, Terp K, et al. Validation of a new transit time ultrasound flowmeter in man Eur J Vasc Endovasc Surg 1996;12:91-96.[Medline]
3. Matre K, Birkeland S, Hessevik I, et al. Comparison of transit-time and Doppler ultrasound methods for measurement of flow in aortocoronary bypass grafts during cardiac surgery Thorac Cardiovasc Surg 1994;42:170-174.[Medline]
4. Schmitz C, Ashraf O, Schiller W, et al. Transit time flow measurement in on-pump and off-pump coronary artery surgery J Thorac Cardiovasc Surg 2003;126:645-650.[Abstract/Free Full Text]
5. Walpoth BH, Bosshard A, Genyk I, et al. Transit-time flow measurement for detection of early graft failure during myocardial revascularization Ann Thorac Surg 1998;66:1097-1100.[Abstract/Free Full Text]
6. Hirotani T, Kameda T, Shirota S, et al. An evaluation of the intraoperative transit time measurements of coronary bypass flow Eur J Cardiothorac Surg 2001;19:848-852.[Abstract/Free Full Text]
7. Ganong G. Review of medical physiology, 6th edLos Altos, CA: Lange Medical Publications; 1973.
8. Jakobsen HL, Kjaergard HK. Severe impairment of graft flow without electrocardiographic changes during coronary artery bypass grafting Scand Cardiovasc J 1999;33:157-159.[Medline]
9. D'Ancona G, Karamanoukian HL, Ricci M, Schmid S, Bergsland J, Salerno TA. Graft revision after transit time flow measurement in off-pump coronary artery bypass grafting Eur J Cardiothorac Surg 2000;17:287-293.[Abstract/Free Full Text]
10. Suma H. Arterial grafts in coronary bypass surgery Ann Thorac Cardiovasc Surg 1999;5:141-145.[Medline]
11. Loop FD, Lytle BW, Cosgrove DM, et al. Influence of the internal-mammary-artery graft on 10-year survival and other cardiac events N Engl J Med 1986;314:1-6.[Abstract]
12. Sakaguchi G, Tadamura E, Ohnaka M, et al. Composite arterial Y graft has less coronary flow reserve than independent grafts Ann Thorac Surg 2002;74:493-496.[Abstract/Free Full Text]

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