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Ann Thorac Surg 1998;66:477-481
© 1998 The Society of Thoracic Surgeons

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Original articles: cardiovascular

Coronary bypass flow during use of intraaortic balloon pumping and left ventricular assist device

^a Department of Surgery (1), Kanazawa University School of Medicine, Kanazawa, Japan

Accepted for publication March 26, 1998.

Address reprint requests to Dr Kawasuji, Department of Surgery (1), Kanazawa University School of Medicine, Takaramachi 13-1, Kanazawa 920, Japan

	Abstract

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
Background. Intraaortic balloon pumping (IABP) and left ventricular assist device (LVAD) are used for left ventricular support when low cardiac output occurs after a coronary bypass operation for serious coronary artery disease. There are hemodynamic differences in blood flow in various kinds of coronary artery bypass grafts, caused by their inherent physiologic characteristics. The hemodynamic effects of left ventricular assistance with IABP and LVAD on blood flow through various coronary artery bypass grafts were investigated.

Methods. An ascending aorta-coronary bypass graft (ACB), an internal thoracic artery, and a descending aorta-coronary bypass graft were anastomosed to the left anterior descending coronary artery in a canine model. In this experimental model, the blood flow to the same coronary bed in the three types of grafts could be evaluated. Blood flow in the left anterior descending coronary artery through the three types of coronary bypass grafts was studied in this model during or in the absence of ventricular assistance.

Results. In the control study, the systolic blood flow did not differ among the three types of grafts, but the diastolic flow decreased in the following order: with the ACB, the internal thoracic artery, and the descending aorta-coronary bypass graft. The systolic flow during IABP and LVAD was similar to the control flows. Use of IABP increased the diastolic flow by 75.3% ± 12.4% of the control value in the ACB, 37.9% ± 25.0% in the internal thoracic artery, and 21.2% ± 11.4% in the descending aorta-coronary bypass graft. The LVAD increased the diastolic flow by 97.7% ± 18.7% of the control value in the ACB, 64.5% ± 25.7% in the internal thoracic artery, and 63.0% ± 27.9% in the descending aorta-coronary bypass graft. The diastolic blood flows in the left anterior descending coronary artery and the three types of grafts were significantly greater with IABP than the control values, and significantly greater with LVAD than with IABP and the control values. The degrees of increase of diastolic flows in the left anterior descending coronary artery and the ACB with IABP and LVAD were significantly greater than in the arterial grafts (p < 0.01).

Conclusions. The diastolic flows in the internal thoracic artery and descending aorta-coronary bypass graft increased less than in the native left anterior descending coronary artery and ACB during left ventricular assistance, particularly with IABP. It is important for the selection of tactics for the management of catastrophic status after coronary bypass grafting to consider the hemodynamic characteristics of the graft.

	Introduction

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Surgical treatment for coronary artery disease has developed rapidly in this decade, and coronary intervention also has been utilized for severe coronary artery disease [1]. Coronary artery bypass grafting (CABG) is being performed in patients with severe coronary artery disease with poor left ventricular function [2]. Intraaortic balloon pumping (IABP) is widely used as the left ventricular assistance when low cardiac output occurs after CABG [3, 4]. Left ventricular mechanical assist device (LVAD) is most commonly used as a means of systemic circulatory support while awaiting cardiac transplantation [5, 6]. However, LVAD is sometimes also used in serious situations after CABG. In these circumstances, it is common to employ LVAD synchronously with the cardiac cycle by counterpulsation [6]. The diastolic augmentation caused by these left ventricular supports is expected to increase blood flows in coronary bypass conduits as well as in coronary arteries. The internal thoracic artery (ITA) has been the preferred conduit, because of its superior rate of long-term patency [7, 8]. The right gastroepiploic artery has been used as a third arterial graft [9, 10]. On the other hand, clinical conditions suggesting inadequate perfusion by the ITA graft have been recognized during and immediately after CABG [11–13]. Patients with disastrous intraoperative recovery courses suggesting ITA hypoperfusion could be weaned from cardiopulmonary bypass only after insertion of another saphenous vein graft distal to the ITA graft or removal of the ligature from the previous stenotic saphenous vein graft [11, 12].

We previously reported that arterial grafts may possibly have some disadvantages in blood flow supplying capacity because of their systolic-dominant hemodynamic characteristics, compared with the diastolic-dominant coronary arteries and aortocoronary bypass grafts [14, 15]. It is likely that blood flows in arterial grafts do not increase with left ventricular assistance, compared with native coronary artery or aortocoronary saphenous vein grafts. In this study, we report on the hemodynamic changes in blood flow through various coronary bypass grafts during left ventricular support with IABP and LVAD.

	Material and methods

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
Fourteen adult mongrel dogs weighing 24 to 30 kg were used. The animals were examined in the same fashion as in our previous canine bypass model [15]. The animals were anesthetized with intravenous ketamine hydrochloride (20 mg/kg) and were mechanically ventilated with a volume respirator. The left ITA was dissected from the thoracic wall to prepare as an ITA graft. The mean diameter of the distal ends of the ITA was 2.5 ± 0.4 mm. A femoral artery was excised in 2-cm lengths and prepared for use as a bypass conduit. The mean diameter of the femoral artery grafts was 3.0 ± 0.5 mm. Cardiopulmonary bypass was instituted and cardiac arrest was induced and obtained using cold crystalloid potassium cardioplegic solution. The femoral artery graft was anastomosed to the left anterior descending coronary artery (LAD) with a continuous 7-0 polypropylene suture. The mean diameter of the LAD was 2.0 ± 0.3 mm at the anastomosis. After releasing the aortic clamp, a 3-mm–diameter polytetrafluoroethylene graft was anastomosed to the ascending aorta and to the descending aorta at the level of the first lumbar vertebra. In this bypass model, a graft from the descending aorta was anastomosed in lieu of an in situ gastroepiploic artery graft.

In most dogs of this series, the gastroepiploic artery was about 1 mm in diameter and very spastic, and as such, not adequate for grafting. A Y-shaped graft was constructed from the two polytetrafluoroethylene grafts of sufficient length to reach the LAD. The ITA was anastomosed to this Y-shaped graft. Finally, the mean length of the ascending aorta-coronary bypass graft (ACB), ITA, and descending aorta-coronary bypass graft (DCB) were 75 ± 8, 136 ± 13, and 202 ± 10 mm, respectively. Cardiopulmonary bypass was discontinued after rewarming and stable hemodynamics had been achieved. Then, the composite graft was connected to the proximal portion of the femoral artery graft, which was already anastomosed to the LAD. A 3-0 polypropylene suture was settled around the LAD just proximal to the anastomosis with a tourniquet, by which the LAD was easily occluded and opened. Each graft could be occluded by a soft vascular clamp. Blood flow was measured by a transit-time ultrasonic blood flow meter (model T101; Transonic System Inc, Ithaca, NY). The transducer probe was fixed around the LAD adjacent to the distal anastomosis. This experimental model allowed evaluation of the blood flow in the three types of grafts to the same coronary bed through the same distal anastomosis.

An IABP balloon catheter with a 20-mL balloon volume (Kontron Instruments Inc, Everett, MA) was inserted through the right femoral artery. With a pneumatic blood driver (PBP model 20; Kontron), a balloon was inflated synchronously with the diastolic phase in a simultaneously monitored electrocardiograph. The LVAD system, which had a 20-mL pneumatically driven diaphragm pump (Toyobo, Osaka, Japan), was instituted with an ascending aortic inlet and left atrial drainage. The same pneumatic blood driver inflated the diaphragm pump of the LVAD simultaneously with the diastolic phase, as seen in electrocardiography. Counterpulsation on IABP and LVAD was achieved by inflating simultaneously at the end of the T wave and deflating at the end of the P wave, with a determining trigger point at 50% amplitude of the peak QRS. All animals were studied in the same sequence. Blood flows in the three types of coronary artery bypass grafts were measured in this CABG model during and in the absence of 1:1 assistance with IABP and LVAD. Systolic and diastolic blood flows were calculated using the peak of the R wave and the end of the T wave on the electrocardiogram as the references for systole.

The dogs received humane care in compliance with the "Principles of Laboratory Animal Care" formulated by the National Society for Medical Research and the "Guide for the Care and Use of Laboratory Animals" published by the National Academy of Sciences (NIH publication 85-23, revised 1985).

Mean values were calculated from pressures and flows measured during five consecutive cardiac cycles. Values are expressed as the mean ± standard deviation. Statistical analysis was performed with Student’s t test and analysis of variance to detect significant (p < 0.05) differences between measured variables.

	Results

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
There were no changes in systemic blood pressure, heart rate (which was 122 ± 16 on average), ST segments on the electrocardiogram, or ventricular wall motion during the measurement of blood flow in the three types of grafts, with or without left ventricular assistance.

In the control study, the blood flow in the LAD had systolic and diastolic components. The blood flow pattern in the ACB had a prominent diastolic component, similar to that in the native LAD. However, the blood flow pattern in the ITA and the DCB had decreased diastolic components (Fig 1). The systolic waveform changed to a spiny shape, which had deep negative components during ventricular assistance with IABP. The diastolic components of the blood flow wave in the LAD and ACB increased with IABP assistance. In the ITA and DCB, the diastolic components also increased with use of IABP; however, the degrees of increase were less than in the LAD and ACB (Fig 1). Use of an LVAD increased the diastolic components of blood flow in the LAD and ACB, more than IABP support did. Although the same tendency as in the LAD and ACB was shown also in the ITA and DCB, the degrees of increase were not marked in comparison with the LAD and ACB (Fig 1).

View larger version (19K): [in this window] [in a new window]   Fig 1. Waveform of the blood flow in the (A) left anterior descending coronary artery (LAD, (B) ascending aorta-coronary bypass graft (ACB), (C) internal thoracic artery (ITA), and (D) descending aorta-coronary bypass graft (DCB). (IABP = intraaortic balloon pumping; LVAD = left ventricular assist device.)

View larger version (34K): [in this window] [in a new window]   Fig 2. Summary of blood flows in the left anterior descending coronary artery (LAD) and the three types of grafts. (ACB = ascending aorta-coronary bypass graft; DCB = descending aorta-coronary bypass graft; IABP = intraaortic balloon pumping; ITA = internal thoracic artery; LAD = left anterior descending coronary artery; LVAD = left ventricular assist device.)

The increase ratio of each blood flow to each control value was calculated. Increase ratios of total flows in the LAD and ACB with IABP and LVAD were significantly greater than in the ITA and DCB (p < 0.05). The systolic flow during both IABP and LVAD was similar to that during the control periods. Support with IABP and LVAD increased the diastolic flow by 72.8% ± 7.0% and 92.2% ± 9.8% in the native LAD, by 75.3% ± 12.4% and 97.7% ± 18.7% in the ACB, by 37.9% ± 25.0% and 64.5% ± 25.7% in the ITA, and by 21.2% ± 11.4% and 63.0% ± 27.9% in the DCB, respectively. The increase ratios of diastolic flows with IABP and LVAD in the LAD and the ACB were significantly greater than in the ITA and DCB (p < 0.01) (Table 1).

	Comment

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

It is known that coronary blood flow distal to the stenotic lesion is reduced mainly in diastole, and that bypass flow increases the distal coronary flow mainly in diastole after CABG as well [16]. The diastolic augmentation of IABP increases driving pressure to the coronary circulation, and counterpulsatile LVAD has the ability to eject blood directly during diastole. Therefore, both could increase the blood flow in a stenotic coronary artery to improve the subendocardial flow for the ischemic myocardium. Similarly, the counterpulsation effects provided by IABP and LVAD are expected to be effective for blood flow in coronary bypass conduits, including arterial grafts.

Simon and associates [11] reported that after cardiopulmonary bypass, the left ventricle showed significant deterioration in ITA-revascularized myocardial regions, more so than in saphenous vein-revascularized regions. In our previous experimental study [14], the blood flow wave in the ACB had a prominent diastolic component, similar to that of the native coronary artery. However, blood flow in the ITA and the DCB had decreased diastolic components. Although there was no difference in the systolic flow, the diastolic flows in the ITA and DCB were significantly lower than in the ACB. From the same previous study, the inferior capacity of blood flow through arterial grafts was attributed mainly to reduced diastolic pressure in the arterial grafts, which may be caused by the anatomical characteristics of the arterial grafts. Thus, there may be circumstances in which an arterial graft originating from a systolic-dominant circulation distant from the heart, such as the ITA and the gastroepiploic artery, has limitations in the ability to supply blood to a diastolic-dominant coronary circulation.

Jones and co-workers [12] reported that some patients had disastrous intraoperative recovery courses, suggesting ITA hypoperfusion. For such a catastrophic ITA-revascularized myocardium, IABP support was not effective enough to discontinue cardiopulmonary bypass, and patients could be weaned from cardiopulmonary bypass only after the insertion of another saphenous vein graft distal to the ITA graft or removal of the ligature from the saphenous vein grafts. It was therefore necessary to evaluate the influence on each type of coronary artery bypass graft of diastolic augmentation during left ventricular assistance with IABP and LVAD.

Although the blood flow pattern in the ACB had a prominent diastolic component, similar to the native LAD, those in the ITA and DCB had decreased diastolic components. The systolic waveform changed to a spiny shape, which had deep negative components, during use of both ventricular assistance methods. The diastolic components in the LAD and ACB became greater and wider with IABP. In the ITA and DCB, the diastolic components also increased with the use of IABP; however, the degrees of increases in the ITA and DCB were less than in the LAD and ACB. Support with LVAD increased the diastolic components in the LAD and the three types of grafts, more than the use of IABP did. However, the degrees of increase in the ITA and DCB were not marked in comparison with those in the LAD and ACB. Systolic flows in the LAD and the three types of grafts during IABP and LVAD were similar to each control value. There were also no significant differences in systolic blood flow among the three types of grafts. Diastolic flows in the LAD and the three types of grafts increased more during LVAD support than during IABP. However, the increased rate of diastolic flows in ACB was higher than in the ITA and DCB. Moreover, the diastolic blood flow in the ACB, even in the absence of left ventricular assistance, was greater than those in the ITA and the DCB during support with IABP and LVAD. These results were compatible with the clinical experiences of the catastrophic situation with hypoperfusion of the ITA, when only an additional saphenous vein graft distal to the ITA graft enabled discontinuation of cardiopulmonary bypass.

When the coronary circulation is mentioned, many physiologic factors, such as left ventricular function and coronary artery resistance, have to be considered. In the clinical situation, patients can have left ventricular dysfunction, and therefore the systolic unloading of left ventricular assistance might help to improve left ventricular function, and consequently effect the improvement of coronary artery circulation. The systolic unloading effect of each left ventricular assist device was not reflected in the results of this study, because this canine model had no cardiac hypofunction. In that sense this study could indicate only one side of the hemodynamics, among many physiologic factors. However, we believe that diastolic augmentation by left ventricular assistance can exert a greater influence on the coronary circulation than can the systolic effect. That is why we specifically designed this experimental model for the purpose of the assessment of the effect on coronary circulation, in particular of the diastolic pressure.

In the canine model, the diameter of the IABP balloon and the volume of the LVAD have become a subject for discussion. In this canine model, a 20-mL balloon and a 20-mL LVAD system were used. It was difficult to evaluate whether the size of each instrument was adequate for the canine. However, the arterial pressure waveform during each type of assistance indicated a proper effect of IABP and LVAD on diastolic pressure augmentation. Therefore, the sizes of the instruments were acceptable for the purposes of this study. Because the ITA and DCB in this experimental model were longer distances from the heart than the ITA and gastroepiploic artery in clinical cases, the results of this study seemed to emphasize the anatomical influence on the hemodynamic characteristics. The compliance of the aortic wall in this model was also relatively higher than in clinical situations, judging from the presystolic negative flow. However, the tendency shown in this study should be recognized as an important characteristic of arterial grafts in the coronary circulation. Further studies of the coronary circulation under various kinds of hemodynamic conditions—not only in an experimental model but in clinical cases as well—remain to be done. Moreover, a detailed analysis of the coronary circulation during the use of various kinds of assist devices will be the subject of further research.

Arterial grafts have a superior rate of long-term patency and are currently the preferred conduit for CABG. However, arterial grafts originating from a systolic-dominant circulation distant from the heart, compared with the ascending aorta-originated grafts, have some limitations in their ability to supply blood to the diastolic-dominant coronary circulation. This tendency was also recognized even under left ventricular assistance with IABP and LVAD. Multiple revascularizations using only in situ arterial grafts may cause myocardial hypoperfusion even with left ventricular support, especially in the presence of left ventricular dysfunction. In the case of severe left ventricular dysfunction caused by myocardial ischemia, which indicates a high left ventricular end-diastolic pressure or very low left ventricular ejection fraction, aortocoronary bypass grafting should be recommended. For a catastrophic situation caused by arterial graft hypoperfusion, additional aortocoronary bypass grafting may be beneficial. This is because of the increased blood supply to the coronary circulation, as the ascending aorta-originating bypass is to be expected to experience the same diastolic augmentation effect as under counterpulsatile LVAD support.

	Acknowledgments

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
This study was supported by the Alexander von Humbouldt Foundation.

	References

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 

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