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Eur J Cardiothorac Surg 2000;17:169-174
© 2000 Elsevier Science NL

Micropumps to support the heart during CABG

Department of Cardiac Surgery, Gasthuisberg University Hospital, Herestraat 49, KULeuven, 3000Leuven, Belgium

Corresponding author. Tel.: +32-16-344-260; fax: +32-16-344-616
e-mail: bart.meyns{at}uz.kuleuven.ac.be

	Abstract

Top Abstract 1. Introduction 2. Methods 3. Results 4. Discussion Appendix A. Conference... References

 
Objective: To show the effect of myocardial support by micropumps during beating heart CABG for triple vessel disease. Methods: In 12 sheep, three coronary arteries (LAD, intermediate branch and circumflex) were consecutively occluded for 10 min. The animals were divided in two groups: group 1 without support (n=6) and group 2 with biventricular support of intravascular micropumps. The pumps (diameter 6.4 mm) were placed through peripheral access (femoral artery and jugular vein) and advanced under fluoroscopic guidance. The hemodynamic evolution was analyzed during the procedure and 2 h of reperfusion. Myocardial flow was assessed by colored microspheres. Differences between groups were analyzed by ANOVA for repeated measurements and post-hoc testing in case of significance. Results: All of the pump-supported animals survived the procedure, 1 of the control animals died of resistant ventricular fibrillation. At the end of the reperfusion period, the hemodynamic performance and myocardial contractility was significantly better in the pump-supported group (cardiac output: 2.4±0.9 vs. 3.3±0.9 l/min, P=0.0192; mean arterial blood pressure: 51±23 vs. 73±9 mmHg, P=0.036; first derivative of the left ventricular pressure: 561±271 vs. 947±316 mmHg/s, P=0.0074). After the procedure, subendocardial blood flow was significantly better in all areas of the left ventricle in group 2 (0.935±0.427 ml/min per g vs. 0.409±0.183 ml/min per g in group 1; P=0.0366). Conclusion: The supported heart is more resistant to repetitive local ischemia. Support by microaxial pumps can make beating heart surgery safer and applicable for more complex cases.

Key Words: Micropumps • Triple vessel disease • Myocardial support

	1. Introduction

Top Abstract 1. Introduction 2. Methods 3. Results 4. Discussion Appendix A. Conference... References

 
Coronary artery bypass grafting (CABG) on the beating heart has been shown to be feasible in selective cases. Several studies have shown satisfactory patency rates of anastomoses performed in beating heart surgery [1–4]. Others have indicated a better preservation of myocardial function and a reduced inflammatory response in off-pump CABG [5]. However, most of this experience is based on single and double vessel coronary disease. The majority of patients present with triple vessel disease not suitable for beating heart procedures. Microaxial blood pumps were developed for the right as well as the left ventricle [6]. They can be used to support the patient during the revascularization and can widen the indications of beating heart surgery. We investigated the effects of these pumps on the hemodynamic state, myocardial performance and myocardial blood flow during repetitive coronary occlusions.

	2. Methods

Top Abstract 1. Introduction 2. Methods 3. Results 4. Discussion Appendix A. Conference... References

 
2.1. The microaxial pumps
The blood pumps are rotary blood pumps of the axial flow type (Impella, Aachen, Germany). The left ventricular pump has an outer diameter of 6.4 mm (Fig. 1) The driving motor is miniaturized and incorporated in the housing. A differential pressure sensor measures the pressure difference between in- and out-flow and detects the exact positioning of the pump. Together with the rotational speed this pressure difference allows the immediate calculation of the produced pump flow. The catheter, on which the pump is mounted, contains the electrical supply and allows peripheral access. The pumps produce 4.5 l/min at a physiological pressure head of 70 mmHg and a rotational speed of 32 000 rotations/min.

View larger version (140K): [in this window] [in a new window]   Fig. 1. The left and right ventricular pump with their inflow cannula. A differential pressure sensor allows the online calculation of pump flow and reflects the position of the pump in the left ventricle. The right ventricular pump has an outflow cannula with an inflatable balloon on its tip and a tip pressure signal. It is placed in a manner similar to the Swan–Ganz method. A reverse flow cap stabilizes the pump position during flow production.

2.2. Anesthesia and preparation
Twelve adult sheep were selected. All animals received human care in compliance with the ‘Guidelines for the Care of and Use of Laboratory Animals’ published by the US National Institutes of Health (NIH publication no. 85-23, revised 1985). Premedication existed of 10 mg/kg of ketamine hydrochloride. They were intubated and mechanically ventilated with 6 l/min of O₂ and 3 l/min of N₂O through an Engström II ventilator with a tidal volume of 650 ml. Anesthesia was maintained with 0.5–2% halothane. Ventilation was adjusted to maintain pH and oxygen levels in satisfactory levels as indicated by arterial blood gas measurements.

A gastric tube was inserted and the animals were installed on their right side to allow left thoracotomy through the fifth intercostal space.

2.3. Animal instrumentation
The pericardium was opened and the heart suspended in a pericardial cradle. On opening of the pericardium, 100 mg of lidocaine was administered to avoid ventricular arrhythmia during surgical manipulation. Heparin, at a dose of 300 U/kg, was administered and further maintained to keep the activated clotting time above 400 s. The left anterior descending artery (LAD), intermediate branch (IM) and left circumflex coronary artery (CX) were identified and encircled. Pressure lines were inserted in the pulmonary artery, the left atrium, the carotid artery and the left ventricle and connected to a pressure module (Triton Technology Inc., San Diego, CA). A catheter was inserted in the coronary sinus to allow blood sampling of venous cardiac blood. A flow probe was placed around the pulmonary artery (Transonic System Inc., Ithaca, NY) for continuous cardiac output measurement. A 7 Fr. conductance catheter (CardioDynamics BV, Leiden, The Netherlands) was inserted in the left ventricle and connected to a signal generator-processor (Sigma 5, Leycom, Oegstageest, The Netherlands).

In the animals with pump support the left ventricular pump was advanced via the femoral artery, and the right ventricular pump via the jugular vein. The pump position was confirmed by fluoroscopy.

2.4. Colored microspheres
Myocardial flow was analyzed with the colored microspheres technique. At baseline and during the occlusions a set of nine million of 15 mm colored polystyrene microspheres (Triton technology Inc., San Diego, CA) were injected through a catheter in the left atrium. Arterial reference blood was withdrawn over 90 s from the aorta at a flow rate of 10 ml/min. On termination of the experiment 1-g tissue samples were isolated from the kidneys and the different regions of the myocardium. The microspheres were recovered from the tissue samples by digestion of the tissue by KOH. Subsequently the samples were filtered, dye-extracted and examined by spectrophotometry [7].

For analysis of regional myocardial perfusion, biopsies were taken from each zone as well from the subepicardium as from the subendocardium. Myocardial blood flow expressed as ml/min flow per 100 g of tissue was obtained in each of the identified regions and this in subendocardium and subepicardium.

2.5. Protocol
After baseline measurements the three major coronaries (LAD, IM and CX) were consecutively occluded over 10 min with 10 min of reperfusion between each occlusion. Hemodynamic measurements and coronary sinus blood sampling were performed each 5 min up to 2 h after reperfusion. Colored microspheres were injected at baseline, during LAD occlusion, CX occlusion and 10 min of reperfusion.

The 12 animals were divided in two groups: six animals underwent the procedure without support; and six animals were supported with the microaxial pumps. In the pump supported group the pumps were retrieved 10 min after the last occlusion.

2.6. Measurement and analysis of left ventricular contractility
Conductance catheter volumes were calibrated by aortic flow rate. The parallel conductance volume was calculated by transiently altering blood conductivity by the injection of hypertonic saline solution (10 ml of 10% NaCl).

Several parameters including stroke work (SW), left ventricular end-systolic pressure and volume, maximum and minimum first derivative of the left ventricular pressure, left ventricular end-diastolic pressure and volume were calculated automatically by PC-Conduct software (CardioDynamics BV, Leiden, The Netherlands).

Multiple left ventricular pressure-volume loops were obtained during transient preload reduction by occluding the inferior vena cava. This reduction in preload was only done at baseline and during the reperfuison period in order to obtain stable and reliable measurements. Left ventricular contractility was assessed by the end-systolic pressure-volume relationship (Ees) and the stroke work – end-diastolic relationship (preload recruitable stroke work, PRSW).

2.7. Statistical analysis
Data are presented as mean±SD. Repeated measures analysis of variance (ANOVA) was used to assess the evolution in time of the two groups. Where appropriate (P<0.05) post-hoc testing was performed with the unpaired Student's t-test.

	3. Results

Top Abstract 1. Introduction 2. Methods 3. Results 4. Discussion Appendix A. Conference... References

 
3.1. Hemodynamic performance
All of the pump-supported animals survived the procedure. One of the control animals died of resistant ventricular fibrillation. The hemodynamic performance was significantly better in the pump supported animals. Heart rate was not different for both groups but cardiac output and mean aortic blood pressure were significantly better preserved throughout the procedure (Fig. 2). In the control animals the hemodynamic deterioration is obvious at the occlusion of the third vessel (Cx) with a more depressed state during reperfusion (mean arterial blood pressure: 51±23 vs. 73±9 mmHg, P=0.036). The pump-supported animals show a nonpulsatile pressure curve during the procedure but maintain a stable mean blood pressure.

View larger version (36K): [in this window] [in a new window]   Fig. 2. Cardiac output (upper pannel), mean blood pressure (middle pannel) and left atrial pressure (lower pannel) evolution during occlusions and at reperfusion. , Pump supported group; , control group.

3.2. Myocardial contractility
The first derivative of the left ventricular pressure (dP/dt) shows significantly better minimal and maximal values during reperfusion for the supported group (Table 1). During pump support the maximal and minimal values of the dP/dt are significantly reduced indicating again the unloading effect of the left ventricular micropump. Stroke work follows the same evolution: a significant reduction for the supported group during unloading and a significant better performance at reperfusion (Table 1).

3.3. Myocardial blood flow
Myocardial blood flow is reduced during the occlusions in as well the control as the supported animals (Fig. 3). During reperfusion however, myocardial blood flow is significantly better in the pump supported group. This evolution is present in all biopsied areas of the left ventricle (anterior, septal, lateral and inferior). The improved myocardial flow at reperfusion is as well present in the subepicardium (1.006±0.39 vs. 0.694±0.38 ml/min per g; P=0.009) as in the subendocardium (1.071±0.462 vs. 0.517±0.218 ml/min per g; P=0.00005).

View larger version (27K): [in this window] [in a new window]   Fig. 3. Subepicardial (upper pannel) and subendocardial (lower pannel) myocardial blood flow at baseline, during ischemia and at reperfusion. The pump supported group shows signifcantly higher blood flows at reperfusion.

	4. Discussion

Top Abstract 1. Introduction 2. Methods 3. Results 4. Discussion Appendix A. Conference... References

Grundeman and colleagues showed in a pig model that mechanical support of the right heart is essential in maintaining a stable hemodynamic situation when lifting the heart [12]. The development of a practical, non-invasive right heart support system can lead to expand the indications of beating heart surgery.

The micropumps used in this study were designed to support as well the left as the right heart. They are based on the same principle as the Hemopump. Their size is smaller (6.4 mm) with a comparable power (4.2 l/min at physiological pressure head). This allows clinical applications with peripheral access.

In this experimental protocol we showed that support with the micro pumps during consecutive coronary occlusions leads to a superior hemodynamic state with better myocardial flow and contractility in the reperfusion phase. The reason for this dramatic difference in performance of the supported animals versus the control animals is dual. For one, there is an improved hemodynamic status during the coronary occlusions. Definitely the depression of myocardial function by snaring the coronaries leads to a severe reduction in output and perfusion pressure in the control animals. This depression of myocardial function can spiral down to clear cardiogenic shock leading to the death of one of the control animals. Secondly, the presence of the left side pump causes a significant unloading of the left ventricle during ischemia. The unloading effect of the left ventricular pump is shown in our experiments by the reduced stroke work during the pump run. The protective effect of unloading by an axial flow pump during ischemia has already been shown before with the Hemopump [13–15]. Mehrige et al. reported a reduction of infarct size, an increased myocardial blood flow and a superior myocardial contractility when the heart was unloaded with the Hemopump [15]. The clinical experience with the Hemopump has provided further evidence of the beneficial effect of unloading on myocardial function [16].

We observed a superior myocardial blood flow during reperfusion in the pump supported animals as compared with the control animals. This improved blood flow was present in all regions of the left ventricle and as well in the subendocardium as the subepicardium. It is possible that reduction of the end-diastolic pressure by the left assist during the coronary occlusions caused this improvement in flow. Previous studies have shown that intacavitary unloading with the Hemopump leads to improved myocardial perfusion during ischemia [15,17]. However, the flow data during coronary occlusion show a similar reduction in myocardial flow in both groups in all regions. The superior hemodynamic state in the pump supported group definitely contributes to a greater extend to this superior myocardial perfusion at reperfusion.

The consecutive coronary occlusions in our animal model did lead to severe cardiogenic shock in the control animals. It can be argued that, in clinical practice, beating heart surgery leads to less dramatic hemodynamic disturbances. However, beating heart surgery is still limited to one or two regions of the heart in almost all cases. Expanding the indications of beating heart surgery can be made possible with micropumps who sustain hemodynamic stability and protect the heart from warm ischemia. The benefit, in comparison with the classical cardiopulmonary bypass, in terms of reduced inflammatory response and possible effects on myocardial function, needs to be shown in clinical prospective randomized trials.

	Footnotes

 
Presented at the 13th Annual Meeting of the European Association for Cardio-thoracic Surgery, Glasgow, Scotland, UK, September 5–8, 1999.

	Appendix A. Conference discussion

Top Abstract 1. Introduction 2. Methods 3. Results 4. Discussion Appendix A. Conference... References

 
Dr L. Lima (Brasilia, Brazil): I'm doing about 85% of my cases on beating heart without any assistance. And the 15% of cases that I can't do without pump are cases that when I tilt my heart I have my pressure going down and I need drugs, I need to go on pump for that, because I can't tilt my heart, the heart can't stand this manipulation. And I used in the last eight cases that I've done with the OMs, the marginal cases, the Ahmed minipump circuit served as a right assistance device. And what I had is once I tilt my heart and I get my pressure going down, I just turn on the pump and get the pressure restored, the mean arterial pressure was restored, and also the cardiac output restored by the right assistance. So in my opinion the basic problem with beating heart is the right ventricle and not both ventricles. What do you think about that? And if you do think you need both assistance, how can you prove that both side assistance is important? Did you make any study looking for only right assistance?

Dr Meyns: I think your point is correct, and there has been also experimental data showing that indeed the kinking of the right ventricle is the cause of the hemodynamic collapse when you try to reach the lateral wall of the heart. It's definitely possible that only the right ventricular assistance will help us through this problem. However, the myocardial protective effects we have seen, the unloading, is definitely the work of the left ventricular assistance. So I think that we have to split up a little bit. The right ventricular assistance is probably necessary to maintain a stable hemodynamic situation. If you want, however, an additional protective effect to the warm ischemia, then you need the left ventricular pump.

Dr V. Subramanian (New York): The two prerequisites for a posterior vessel bypass in the beating heart are: first, to preserve hemodynamic situation by helping the right heart. The second most important issue, which may have a significant influence on how we perform the anastomosis on a somewhat dislocated heart to a parallel plane, is that the left ventricle has to be relieved of a lot of the load. With the right heart assist, although you improve the right ventricle hemodynamics, you still have a fully loaded left ventricle. Tell me how the fully loaded left ventricle is going to be retracted to give you an optimal angle at which you then perform anastomosis. We have to measure the amount of dislocation of the fully loaded left ventricle. Therefore the problem with the right ventricle assist is that you keep the load on the left ventricle still. Can you explain that?

Dr Meyns: Well, as we illustrated, you do not keep exactly the same load. I mean there is a significant unloading, the enddiastolic volume is reduced.

Dr Subramanian: The left ventricular end-diastolic?

Dr Meyns: The enddiastolic volume of the left ventricle is reduced by the microaxial blood pump which is inside and sucking it down. But, of course, you've still got a beating heart.

Dr Subramanian: I don't understand how you can keep and maintain the cardiac index if the left ventricle is not fully loaded. All you do is right heart support, so you are fully loaded on the left ventricle. How do you get the fully loaded left ventricle presented for an optimum condition for anastomosis?

Dr Meyns: I'm sorry, I thought you were talking about the biventricular support. You're talking now about the right ventricular support?

Dr Subramanian: I'm talking about the right ventricular support.

Dr Meyns: Okay. I'm sorry. Then you are completely correct, through the right ventricular support you will, but this was not the scope of this study. In this study we went for biventricular support. With the right ventricular support only, you can overcome kinking of the right ventricle, but you still have to compromise on the left ventricle, I agree completely.

	References

Top Abstract 1. Introduction 2. Methods 3. Results 4. Discussion Appendix A. Conference... References

 

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