Hemodynamic consequences of carotid-carotid bypass for innominate artery stenosis Steven W. Oweida, M D , David N. Ku, M D , PhD, Alexander G. Justicz, M D , Brenda Burnson, BS, and A t e f A. Salam, M D , Atlanta, Ga. The carotid-carotid cervical bypass is one surgical option for symptomatic atherosclerotic lesions of the innominate artery. Controversy exists regarding the necessity of surgically excluding the innominate plaque from the cerebral circuit. A canine study was instituted to characterize the hemodynamic alterations that occur in the right common carotid artery proximal to the bypass graft, termed the critical segment. The direction of flow in the critical segment determines whether emboli originating in the innominate may be propelled cranially despite a patent bypass graft. Six mongrel dogs underwent placement of an autogenous arterial crossover graft as a carotid-carotid bypass. A stenosis of the innominate artery was quantitatively altered, and an electromagnetic flowmeter measured the magnitude and direction of flow in the critical segment at three levels of diameter reduction in the innominate artery. For low-grade stenoses, flow in the critical segment was always prograde. For high-grade stenoses, the flow was always reversed. Stenoses between 57% and 67% yielded flow values of 10 + 24 ml/min, and it was in this range that mean flow reversal was found to occur. Even when the mean flow was near zero in the critical segment, flow was not stagnant but oscillated in antegrade and retrograde directions throughout the cardiac cycle. These data indicate that a carotid-carotid bypass causes complete flow reversal in the critical segment when there is high-grade stenosis in the innominate artery. Theoretical analysis of the hemodynamic circuit indicated that arm exercise would augment retrograde flow in the critical segment. Thus, for innominate stenoses greater than 75 %, division and oversewing of the proximal right common carotid artery should not be necessary. With low- and intermediate-grade stenoses, permanent surgical interruption of the innominate artery should be performed to prevent cerebral embolization. (J VAsc SURG 1991;13:416-22.)
Atherosclerotic occlusive lesions of the brachiocephalic vessels are infrequently encountered as compared with disease in the carotid bifurcation. It is estimated that fewer than 10% of all extracranial vascular reconstructions are performed on the primary branches of the aortic arch. 1 Nonetheless, stenoses of the innominate artery can cause cerebral dysfimction by flow reduction to the ipsilateral hemisphere or by distal atheroembolization. The use of arch aortography in the work-up of patients with extracranial cerebrovascular disease increases the likeFrom the Department of Surgery, Emory University School of Medicine, Atlanta, the Veterans Administration Medical Center, Surgical Service, Decatur, and the School of Mechanical Engineering, Georgia Institute of Technology, Atlanta. Supported in part by the Emory Department of Surgery and NSF EET-8657691. Reprint requests: David N. Ku, MD, PhD, George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0405. 24/1/25838
416
lihood that innominate artery lesions will be detected. Surgical treatment of disease confined to this area remains an important but controversial issue. Primary therapy for occlusive disease of the innominate artery involves a direct transthoracic repair of the offending lesion. 2 With the recent advances in cardiac anesthesia, improved perioperative care, and clear evidence that median sternotomy is very well tolerated, transthoracic repair of the great vessels has become the generally accepted procedurc of choice. 1-~ Unfortunately, many patients (e.g., the elderly and the infirm) are poorly suited to tolerate the thoracic approach. Compromised pulmonary function and previous sternotomy for treatment of coronary discase make this route less appealing for the vascular reconstruction. Initial reports indicating excessivc morbidity and mortality with transthoracic repair of arch lesions led to the development of a wide array of extraanatomic bypass procedures. 68 Such extrathoracic operations
Volume i3 Number 3 March 1991
Hemodynamics of carotid-carotid bypass 417
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Fig. 1. Schematicdiagram of arch anatomy of dog. The location of the stenosis is marked with an arrow.
have been performed with very low morbidity and with acceptable long-term results. As a resuk, !ess invasive extraanatomic bypass procedures, such as the carotid-carotid cervical bypass, are often performed. The extraanatomic bypass is a relatively simple and effective solution to a complex problem. A major concern with the carotid-carotid bypass is the embolic potential from the remaining innominate artery plaque. Some authors claim that this operation should be performed only in conjunction with ligation of the proximal recipient carotid artery. 7 Hemodynamically, distal embolization should occur only if the direction of flow through the right proximal carotid is antegrade to the brain. The present study was established to define the hemodynamics in this particular segment of the vasculature in the presence of a varying stenosis of the innominate artery in a canine model of the carotid-carotid bypass. The importance of physiologic variations in arm resistance is addressed. METHODS
Six mongrel dogs, each weighing 50 to 75 pounds, were evaluated in the acute experiments. All experiments were performed in accordance 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" (NIH Publication No. 80-23, revised 1985). Anesthesia was induced with pentobarbital sodium and maintained with endotracheal inhalation of halothane. The electrocardiogram was continuously mon-
itored through limb leads, and the arterial blood pressure was measured through a catheter placed in the right femoral artery by cutdown. Surgical exposure was afforded by a long midline neck incision that was extended down to a full median sternommy. Arterial blood pressure, heart rate, and cardiac output were kept reasonably constant throughout the experiment by judicious volume replacement and adjustments in the percentage of inspired anesthetic gases. A long segment of the left femoral artery was harvested and placed in heparinized saline solution to be used later to fashion the crossover graft. The arch anatomy of the dog differs somewhat from that of the human being (Fig. 1). In the dog, the left common carotid artery originates from a common brachiocephalic trunk just proximal to the true innominate artery. The proximal segment of the right common carotid artery just distal to the renominate stenosis and proximal to the recipient limb of the crossover graft is defined as the "critical segment." The animals were systemically heparinized and the carotid-carotid bypass was constructed in an endm-side fashion with a 6-0 polypropylene running suture under magnification. The outer diameters of the innominate, common carotid, and femoral arteries used as the crossover were carefully measured and recorded. The pericardium was incised and an appropriately sized electromagnetic flow probe (Carolina Medical, King, N.C.) was placed about the aortic root to give a continuous readout of the cardiac output. Baseline electromagnetic flow measurements were then made in the critical segment.
418
}'ournal of VASCULAR SURGERY
Oweida et aI.
300
Atherosclerotic occlusive lesions of the brachiocephalic vessels are infrequently encountered as compared with disease in the carotid bifurcation. It is estimated that fewer than 10% of all extracranial vascular reconstructions are performed on the primary branches of the aortic arch. 1 Nonetheless, stenoses of the innominate artery can cause cerebral dysfimction by flow reduction to the ipsilateral hemisphere or by distal atheroembolization. The use of arch aortography in the work-up of patients with extracranial cerebrovascular disease increases the likeFrom the Department of Surgery, Emory University School of Medicine, Atlanta, the Veterans Administration Medical Center, Surgical Service, Decatur, and the School of Mechanical Engineering, Georgia Institute of Technology, Atlanta. Supported in part by the Emory Department of Surgery and NSF EET-8657691. Reprint requests: David N. Ku, MD, PhD, George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0405. 24/1/25838
416
lihood that innominate artery lesions will be detected. Surgical treatment of disease confined to this area remains an important but controversial issue. Primary therapy for occlusive disease of the innominate artery involves a direct transthoracic repair of the offending lesion. 2 With the recent advances in cardiac anesthesia, improved perioperative care, and clear evidence that median sternotomy is very well tolerated, transthoracic repair of the great vessels has become the generally accepted procedurc of choice. 1-~ Unfortunately, many patients (e.g., the elderly and the infirm) are poorly suited to tolerate the thoracic approach. Compromised pulmonary function and previous sternotomy for treatment of coronary discase make this route less appealing for the vascular reconstruction. Initial reports indicating excessivc morbidity and mortality with transthoracic repair of arch lesions led to the development of a wide array of extraanatomic bypass procedures. 68 Such extrathoracic operations
Volume i3 Number 3 March 1991
Hemodynamics of carotid-carotid bypass 417
~ . Co}E;czo~ . . . . .
~a,'of icf - -.L. c o ~ o , z
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va/- -
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.
.
.
.
-/_ s
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Fig. 1. Schematicdiagram of arch anatomy of dog. The location of the stenosis is marked with an arrow.
have been performed with very low morbidity and with acceptable long-term results. As a resuk, !ess invasive extraanatomic bypass procedures, such as the carotid-carotid cervical bypass, are often performed. The extraanatomic bypass is a relatively simple and effective solution to a complex problem. A major concern with the carotid-carotid bypass is the embolic potential from the remaining innominate artery plaque. Some authors claim that this operation should be performed only in conjunction with ligation of the proximal recipient carotid artery. 7 Hemodynamically, distal embolization should occur only if the direction of flow through the right proximal carotid is antegrade to the brain. The present study was established to define the hemodynamics in this particular segment of the vasculature in the presence of a varying stenosis of the innominate artery in a canine model of the carotid-carotid bypass. The importance of physiologic variations in arm resistance is addressed. METHODS
Six mongrel dogs, each weighing 50 to 75 pounds, were evaluated in the acute experiments. All experiments were performed in accordance 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" (NIH Publication No. 80-23, revised 1985). Anesthesia was induced with pentobarbital sodium and maintained with endotracheal inhalation of halothane. The electrocardiogram was continuously mon-
itored through limb leads, and the arterial blood pressure was measured through a catheter placed in the right femoral artery by cutdown. Surgical exposure was afforded by a long midline neck incision that was extended down to a full median sternommy. Arterial blood pressure, heart rate, and cardiac output were kept reasonably constant throughout the experiment by judicious volume replacement and adjustments in the percentage of inspired anesthetic gases. A long segment of the left femoral artery was harvested and placed in heparinized saline solution to be used later to fashion the crossover graft. The arch anatomy of the dog differs somewhat from that of the human being (Fig. 1). In the dog, the left common carotid artery originates from a common brachiocephalic trunk just proximal to the true innominate artery. The proximal segment of the right common carotid artery just distal to the renominate stenosis and proximal to the recipient limb of the crossover graft is defined as the "critical segment." The animals were systemically heparinized and the carotid-carotid bypass was constructed in an endm-side fashion with a 6-0 polypropylene running suture under magnification. The outer diameters of the innominate, common carotid, and femoral arteries used as the crossover were carefully measured and recorded. The pericardium was incised and an appropriately sized electromagnetic flow probe (Carolina Medical, King, N.C.) was placed about the aortic root to give a continuous readout of the cardiac output. Baseline electromagnetic flow measurements were then made in the critical segment.
418
}'ournal of VASCULAR SURGERY
Oweida et aI.
300
