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01 сентября 2001 00:00

Beating heart coronary surgery supported by an axial blood flow pump

Background . We have previously presented a method for performing coronary artery bypass graft operation on the beating heart without cardiopulmonary bypass (CPB). This method has now been explored.
Method . Thirty-two patients were prospectively randomized. The study group was operated on using an axial blood flow pump (Hemopump; HP) as circulatory support. Operations were performed on the beating heart. The control group was operated on using CPB, aortic cross-clamping, and cardioplegic arrest.
Results . All patients went through the procedure without major complications, and were discharged from the hospital. No statistical differences were observed between the groups for time on support (HP, 60.5 minutes; CPB, 70.5 minutes) or total operating time (HP, 178 minutes; CPB, 162 minutes). The number of grafts was greater in the CPB group (HP, 1.8; range, 1 to 3; CPB, 2.5; range, 1 to 4; p = 0.03). Statistical differences were found for intraoperative bleeding (HP mean, 312 mL; CPB mean, 582 mL; p = 0.0003) and myocardial trauma as measured by postoperative troponin-T values (HP, 0.23 g/L; CPB, 1.17 g/L; p = 0.004).
Conclusions . Hemopump-supported coronary artery bypass graft operation has been shown to be a safe and feasible procedure with the potential benefits of reduced operative bleeding and myocardial damage without prolonging intraoperative support or total operating time.
The use of coronary artery bypass grafting (CABG) supported by cardiopulmonary bypass (CPB) evolved into a practical technique during the late 1960s and has since then been the state of the art with low morbidity and mortality. There are, however, several effects related to CPB that may be of concern in certain patient groups carrying high risk [1] [2] .
Beating heart CABG has been reported earlier by Buffolo and coworkers [3] , Benetti and associates [4] , and Pfister and colleagues [5] as an alternative to the gold standard, ie, use of CPB, aortic cross-clamping, and cardioplegic arrest. This procedure is technically more demanding and concerns have been raised about the long-term results. Since the introduction of minimally invasive direct coronary artery bypass grafting (MIDCAB), this approach has become a widespread technique [6] . The most common procedure has been left internal mammary artery grafting to the left anterior descending coronary artery on the beating heart, through a limited anterior thoracotomy [7] . Long-term results of MIDCAB are not yet available.
Ventricular assist devices have been successfully used to support patients during CABG [8] . These techniques avoid the risk of CPB and an artificial oxygenator because the patient's own lungs are functioning. Sweeney and Frazier [9] reported good results when using ventricular assist device-supported CABG in high-risk patients. They used the <Рисунок: beta>-blocking agent esmolol as an adjunct to make the heart flaccid. The technique using both left- and right-sided assist with Bio-Medicus pumps is, however, cumbersome with the risk for device-related complications. In a few cases Sweeney and Frazier used an intracorporeal axial blood flow pump for left ventricular support with encouraging results [9] . This method may become an alternative to the MIDCAB procedure as it provides the surgeon with an approach by which he can reach more than one vessel, while avoiding the negative effects of CPB. It can also provide a less dramatic way of using the MIDCAB approach, operating on the beating heart but with the convenience of a sternotomy, giving better control over the surgical field. We recently reported our experience with this approach in an animal model followed by a pilot study in humans [10] [11] .
This report describes a prospective randomized study comparing Hemopump (HP)-supported CABG operations on the beating heart with the gold standard.
Material and methods
This study was approved by the Human Ethics Committee at the University Hospital, LinkoЁping, Sweden.
Thirty-two patients with stable angina scheduled for CABG were selected for this study. Previous experience has shown us that there may be problems reaching the middle branches of the circumflex artery when using the HP technique; patients needing a graft to this area were therefore excluded. The patients were prospectively randomized into two groups. The patients in the control group were operated on using CPB and aortic cross-clamping with cold ischemic arrest. Those in the study group were operated on with an «empty-beating» heart by means of the Hemopump (Medtronic Inc., Grand Rapids, MI) support. Both groups were treated according to our routines for preoperative and postoperative care. Demographic data are shown in . [Table 1] The patients were mechanically ventilated with a mixture of oxygen and air (Servo 900 D, Siemens-Elema, Stockholm, Sweden) and anesthetized as for open heart surgery, ie, fentanyl, isoflurane, and benzodiazepine. A Swan-Ganz catheter was placed (Baxter Healthcare, Irvine, CA) before anesthesia allowing hemodynamic monitoring as well as real-time measurements of mixed venous oxygen saturation. Muscle relaxation was achieved with pancuronium. A midline sternotomy was performed, and the pericardium was opened to visualize the areas of intervention.
Cardiopulmonary bypass group
            Patients randomized to CPB had standard cannulation of the right atrium and the ascending aorta. Cold crystalloid cardioplegia (Plegisol, Abbott Laboratories, Chicago, IL), together with aortic cross-clamping and mild hypothermia (32° to 34°C) were used. The CPB circuit involved a non-heparin-coated membrane oxygenator, and a hard shell venous reservoir, (Sorin Biomedica Cardio, Saluggea, Italy). Heparin was given to achieve an activated clotting time not less than 400 seconds. The operation was then performed in a standard fashion. Hemodynamic recordings were made before anesthesia, after induction of anesthesia, after termination of CPB, on arrival at the intensive care unit (ICU), 6 hours postoperatively, and at discharge from the ICU. Hemodynamic data were recorded in 14 of 16 patients . [Figure 1]
Hemopump group
Heparin was given in a dose of 5,000 to 10,000 U to achieve an activated clotting time of approximately 200 seconds. A 12−mm Hemashield graft (Meadox Medical, Oakland, NJ), 15 cm long and with a diameter of 12 mm, was cut at an angle of 45 degrees at one end. This end was sutured to the ascending aorta about 5 cm distal to the sinus of Valsalva with the edge of the graft pointing toward the aortic valve. A number 31, 24F Hemopump cannula was placed through the graft into the left ventricle . [Figure 2] Satisfactory position of the pump was ascertained by an on-off test of the pump and feeling the position of the pump-housing by a gentle compression of the aorta just distal to the aortic valve. Transesophageal echocardiography was in some instances used to confirm the position of the pump.
As soon as the pump was inserted through the graft it was set at the lowest speed while being positioned in the left ventricle. When in the right place the speed was increased to the highest speed within a minute. A bolus dose of 1 mg · kg -1 esmolol (Brevibloc, DuPont Pharmaceuticals, Wilmington, DE) was then given to the patients followed by a continuous infusion starting with 400 g · kg -1 · min -1 .
The esmolol infusion was titrated up to a dose at which the surgeon felt the heart was flaccid enough to perform bypass procedures without difficulty. Before each increase in infusion rate an additional bolus dose of 0.5 mg · kg -1 of esmolol was given. Proximal and distal to the site of anastomosis on the coronary artery, special air-cushioned vessel loops (Retract-o-tape, Quest Medical Inc., Allen, TX) were placed to prevent bleeding. In both groups the anastomoses were carried out with a running 6−0 or 7−0 Prolene (Ethicon, Somerville, NJ) suture.
The hemodynamic response was measured before anesthesia, after induction of anesthesia, at a steady-state level with maximal esmolol effect during the operation, after removal of the HP, on arrival at ICU, 6 hours postoperatively, and at discharge from the ICU. Hemodynamic data were recorded in 12 of 16 patients . [Figure 3] The esmolol infusion was stopped just before completion of the bypass operation, and at least 15 minutes of reperfusion was maintained with the HP at full speed before weaning the patient off circulatory support. After weaning, the HP was withdrawn into the graft and a sidebiter was placed just proximal to the insertion site, the graft was removed, and the arteriotomy was sutured with two layers of running 4−0 Prolene.
Two different techniques were used for the vein grafts. (1) The distal anastomosis was completed first and then connected to the aorta in a standard fashion. (2) The proximal anastomosis was sutured first at the same time as the graft for the HP was placed. With this approach flow to the native coronary artery could be established as soon as each distal anastomosis was done.
In both groups blood chemistry data were obtained the day before the operation and the morning after the operation at the ICU, except for plasma-free hemoglobin and troponin-T, which were only measured postoperatively. To determine heart muscle damage the enzymes aspartate aminotransferase was measured together with troponin-T. Renal function was assessed by means of serum creatinine, and platelet consumption was assumed to be the difference between preoperative and postoperative counts.
            Descriptive statistics were used to summarize the data in terms of mean, median, and range. Unpaired Student's t test was used to detect differences between the groups.
In the HP group there were 4 patients with complications. One patient had to be converted to CPB because of intolerance of the <Рисунок: beta>-blockade. The patient developed bradycardia and right heart failure and was not able to deliver blood to the left ventricle and the HP. The patient was placed on CPB and the operation was performed on the beating heart but without cardioplegic arrest. Because of this the patient became a hybrid between the groups, and for that reason was excluded from this analysis. One patient experienced a subendocardial infarction during the operation. One patient had a temporary rise in serum creatinine level that had been normal preoperatively. At discharge from the hospital the value had normalized. One patient had a superficial sternal wound infection.
In the CPB group there were 4 patients with complications. One patient experienced reversible neurologic symptoms involving the left arm. One patient had to be reoperated on because of postoperative bleeding. One patient had respiratory insufficiency requiring prolonged ventilator treatment (3 days), and one patient had respiratory insufficiency together with sternal wound infection and was transferred to the referring hospital for rehabilitation on the 12th postoperative day.
Intraoрerative data are shown in . [Table 2] Statistically, there were more grafts placed in the CPB group than in the HP group (2.5, range, 1 to 4 versus 1.8, range, 1 to 3; p = 0.03). Time on cardiac support or total operating time did not differ between the groups. Heparin doses and activated clotting times differed because of the different need for heparinization between the two methods. Intraoperative bleeding was statistically different between the groups (HP group mean, 312 mL; range, 150 to 500 mL; CPB group mean, 582 mL; range, 350 to 1150 mL; p = 0.0003). Total fluid balance, including all fluids given or lost (crystalloids, colloids, urine output, evaporation), did not differ between the groups. The average aortic cross-clamp time in the CPB group was 34.1 minutes (range, 15 to 75 minutes).
Postoperative data are shown in . [Table 3] There were no statistical differences in the time to extubation between the groups (HP mean, 6.5 hours; CPB, 29.8 hours). The median time was 6 hours compared with 10 hours. Mean time spent in the ICU was in the HP group, 1.1 days, and in the CPB group, 1.9 days; this did not differ statistically. Four patients needed inotropic support in the CPB group compared with 1 in the HP group. Length of total stay in the hospital was on average 8 days in both groups.
Blood chemistry data are shown in . [Table 4] There were no statistical differences in the levels of aspartate aminotransferase, serum creatinine, or platelets preoperatively and postoperatively between the groups. The level of troponin-T as a more sensitive marker of heart muscle ischemia during operation differed between the groups; the mean value in the HP group was 0.23 g/L compared with 1.17 g/L in the CPB group ( p = 0.004). There were 13 values recorded in the HP group and 12 in the CPB group.
Hemodynamic data are shown in . [Figure 4] There were no statistically significant differences between the two groups at the various points of measurements, apart from just after induction of anesthesia. The cardiac index at this time was on average 1.93 L · min -1 · m -2 in the HP group, compared with 2.43 L · min -1 · m -2 in the CPB group ( p = 0.03). Postoperative hemodynamic patterns did not differ between the groups.
Since the introduction of the MIDCAB procedure, operating on the beating heart has become an alternative choice in selected patients for many thoracic surgeons. It is, however, a more demanding task to perform than the gold standard, especially if done through a very small incision. We believe that the method described in this paper constitutes an intermediate step in the process of converting classic CABG operation to a closed-chest procedure. The question as to whether sternotomy or the CPB is the first thing to be done away with has not yet been answered. For us it has been a natural step to move into the field of less-invasive CABG operation by first excluding the CPB while retaining the convenience of full sternotomy. In this way the limitations of MIDCAB procedure with access to only one or two vessels per incision can be avoided and control of the operative field is optimal. In a small study like this some parameters are reported as not significant. One should be aware that there may be real differences in important parameters that this study is too small to detect.
Hemodynamic conditions were stable throughout the operation in our patients in the HP group, and there were no signs of ischemia on the electrocardiogram. The HP was easily inserted into the left ventricle through the graft sutured to the ascending aorta. When the HP was in the correct position it effectively unloaded the left ventricle as has been reported previously [12] [13] . A concomitant reduction in wall tension should decrease the oxygen demand and increase perfusion of the subendocardial tissue [14] , which is beneficial for impaired left ventricles with high filling pressures. An increase in myocardial perfusion pressure and decrease in myocardial oxygen consumption during HP treatment of experimental cardiogenic shock has been reported [15] . In our patients, the decompressed heart was manipulated and supported with bolsters of laparotomy pads without signs of ischemia or circulatory instability. Arrhythmias were temporary provoked by these maneuvers, probably because of changes in HP position. The HP was withdrawn within 15 minutes of completion of the operation in all patients. Should there have been any heart failure or signs of ischemia we could easily have continued with the HP support in the ICU as a left ventricular assist device.
Titration of the short-acting <Рисунок: beta>-blocker esmolol was very carefully done, as we previously have experienced adverse effects with severe circulatory failure when we used high doses in animals [10] . Careful monitoring of right ventricular function is essential when using this approach, especially in the beginning of one's learning curve. A right ventricular assist backup may be a good idea, especially in patients with severely impaired heart function. Administration of esmolol did not significantly decrease heart rate but the overall contractility of the heart was reduced. When the infusion rate was optimally titrated, CABG operation could be performed with the same technique and precision as during conventional operation. We had no device-related complications.
One benefit of HP-supported CABG as shown in this study is the reduction in intraoperative bleeding, which is most likely dependent on the lower doses of heparin in the HP group. Massive bleeding from the aorta is a threat that cannot be coped with as effectively when using CPB. Some form of blood-preserving system for intraoperative and postoperative autotransfusion is essential if we are to fully exploit the potential of HP-supported CABG. Two patients in the HP group received blood transfusions. One patient was an older woman with anemia before her operation. She received one unit of erythrocytes. An additional patient received one unit of blood postoperatively, for reasons that are unclear. In the CPB group 19 units of blood or blood products were given to 4 patients; 13 of these were given to the patient who had to be reoperated on because of bleeding in the early postoperative period. Even if there is a need for transfusion in 25% to 75% of patients when CPB is used, a simple but strict protocol can reduce this to less than 5% [16] .
Avoidance of full heparinization is an advantage with HP-supported CABG. Similar results can probably be achieved using heparin-coated CPB systems [17] . Temporary occlusion of the coronary vessels has not caused problems in previous series of CABG without CPB [3] [4] [5] . Occlusion of the coronary arteries is usually well tolerated during percutaneous transluminal coronary angioplasty without any cardiac decompression. The absence of ST-segment changes during HP-supported CABG might be explained by the combined use of ventricular unloading and high doses of <Рисунок: beta>-blocker, creating a global left ventricular preconditioning.
Coronary artery bypass grafting supported by HP may be an alternative way of handling patients with ongoing ischemia or with poor ventricular function. Besides providing good myocardial protection, an effective ventricular assist device is already in place if needed. In compromised hearts, improved patient outcome may be possible because intervention is early and the heart is maintained in an unloaded state postoperatively. There are also data indicating a reduction of the size of an acute infarction when mechanical ventricular decompression is applied [18] .
In both groups we followed our standard clinical protocol regarding anesthesia and postoperative follow-up. We did not therefore expect any differences for time on ventilator and time in hospital for these patients. Should our anesthesia protocol be modified using short-acting substances the patients could probably be extubated in the operating room or very soon after. A critical factor is the preservation of normothermia. A significant number of these patients would probably be sent home on the third or fourth postoperative day if included in a fast-track protocol, but because we have followed our routine protocol for all patients this was not evaluated. We hope to change that situation in future trials. The reason for the significant difference in the number of grafts placed in these groups is unclear; all patients received bypass grafts to the intended vessels based on preoperative angiograms.
The most suitable candidates for this procedure are presumably patients with impaired left ventricular function or acute ongoing ischemia, as myocardial ischemia is reduced using this technique and if the heart should fail, left ventricular assist device is already in place.
In this study, HP-supported CABG was shown to be a safe and feasible method compared with the gold standard. Potential benefits included less intraoperative bleeding and less ischemia of the heart muscle. The procedure takes no longer to perform, and time of cardiac support during the operation does not differ. No device-related complications were seen. Esmolol was an important adjuvant facilitating precise surgical procedures. It should be pointed out that the results from this study only apply narrowly to the short-term postoperative period, and the real test of this method can only be done in large long-term studies.
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2.Hammerschmidt D.E., Stroncek D.F., Bowers T.K.. Complement activation and neutropenia occurring during cardiopulmonary bypass. J Thorac Cardiovasc Surg 1981;81:370−377.
3.Buffolo E.A., Andrade J.C.S., Branco J.N.R., Aquiar L.F., Ribeiro E.E., Jatene A.D.. Myocardial revascularization without extracorporeal circulation. Eur J Cardiothorac Surg 1990;4:504−508.
4.Benetti F.J., Naselli G., Wood M., Geffner L.. Direct myocardial revascularization without extracorporeal circulation. Chest 1991;100:310−316.
5.Pfister A.J., Zaki M.S., Garcia J.M., Mispereta L.A., Corso P.J.. Coronary bypass without cardiopulmonary bypass. Ann Thorac Surg 1992;54:1085−1092.
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11.LoЁnn U., Peterzeґn B., Granfeldt H., Casimir-Ahn H.. Coronary artery surgery with support of the Hemopump cardiac assist system. Ann Thorac Surg 1994;58:519−523.
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13.Frazier O.H., Wampler R.K., Duncan J.M.. First human use of the Hemopump, a catheter mounted ventricular assist device. Ann Thorac Surg 1990;49:299−304.
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15.Scholtz K.H., Hering J.P., SchroЁder T.. Protective effects of the Hemopump left ventricular assist device in experimental cardiogenic shock. Eur J Cardiothorac Surg 1992;6:209−214.
16.Шverum E., Holen EA, Abdelnoor M., Шystese R.. Conventional blood conservation techniques in 500 consecutive coronary artery bypass operations. Ann Thorac Surg 1991;52:500−505.
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