Extrathoracic carotid reconstruction: The subclavian-carotid artery bypass

Extrathoracic carotid reconstruction: The subclavian-carotid artery bypass Wtlliam R. Fry, MD, John D. Martin, MD, G. Patrick Clagett, MD, and Wtlliam J. Fry, MD, Dallas) Texas Although the predominant location of symptomatic carotid artery occlusive disease is the carotid b~cation, proximal common carotid artery lesions cause similar symptoms. Common carotid artery lesions occur as isolated disease or in tandem with carotid bulb disease. Restoration ofcarotid artery inflow from subclavian based extraanatomic bypasses should provide adequate reconstruction of these lesions. To evaluate subclavian-carotid artery bypass, a retrospective revieW' of all patients Wldergoing this procedure from Jan. 1, 1977, to Feb. 20, 1989, was performed. Twenty patients (14 men, 6 women) with a mean age of 60 years were treated. Fifteen patients (75%) were admitted with transient ischemic attacks. Five (25%) had nonfocal symptoms (e.g., dizziness, syncope). Arteriographic evaluation demonstrated severe proximal occlusive disease ofthe common carotid artery in all cases. Reconstruction bypasses were performed to the carotid bulb (45%), internal carotid artery (30%), and external carotid artery (25%). Four patients Wlderwent endarterectomy ofthe internal carotid artery in conjunction with subclavian-carotid artery bypass. Bypass conduits included saphenous vein (75%) and prosthetic grafts (25%). Asymptomatic phrenic nerve neuropraxia was identified by postoperative chest radiography in four cases, with no resultant respiratory disease. No perioperative strokes occurred.. One postoperative death (5%) resulted from a myocardial infarction. Long-term results were available for 18 patients (90%), with 'a mean follow-up of50 months (range, 1 to 122 months). Four patients have died of causes tmrelated to carotid vascular disease. Serial . duplex scans have documented graft patency in all 18 patients. A single patient returned with focal neurologic symptoms as a result of a posterior circulation infarct. This experience documents that subclavian-to-carotid artery bypass appears to be a safe, durable, and well-tolerated procedure for the reconstruction of symptomatic proximal carotid artery stenosis when the subclavian artery is an appropriate inflow source. For this procedure saphenous vein appears to be a suitable conduit. Long-term follow-up demonstrates excellent patency and protection against further anterior circulation neurologic events. (J VASe SURG 1992;15:83-9.)

The effect of extracranial carotid artery occlusive disease on the production of cerebral ischemia has undergone extensive experimental and clinical investigation. Common carotid artery (CCA) bifurcation lesions dominate clinical practice; thus most studies have centered around occlusive lesions at this loca~on. Less frequently, CCA occlusive disease alone or m combination with bifurcation involvement can cause the same spectrum of cerebral symptoms. From the Depamnent of Surgery, University ofTexas Southwestern Medical Center and Dallas Veterans Administration Hospital.

P~nted at the Thirty-ninth Scientific Meeting of the Interna-

bonal Society for Cardiovascular Surgery, North American Ch.apter, Boston, Mass., June 3-4, 1991. 1lepn~lt re.quests: William R. Fry, MD, Department of Surgery, Ul1lverslty of California - Davis East Bay, 1411 E. 31st St., ')~~ak1and) Ca 94602. ~6/33297

Presumably, carotid bifurcation and CCA occlusive lesions produce ischemic symptoms by· analogous mechanisms (e.g., embolization of atherosclerotic plaque or platelet aggregates, hypopeffusion). If the mechanisms causing cerebral ischemia are similar for both lesions, then restoration of CCA inflow should alleviate ischemic symptoms. Procedures performed on the CCA from art extrathoracic approach include endarterectomy, retrograde thrombectomy, carotocarotid bypass, axillary-carotid bypass, and subclavian-carotid bypass. . The purpose of this study is to determine the utility and durability of subclavian-carotid bypass in the treatment of CCA occlusive disease.

PATIENTS AND METHODS To evaluate subclavian-carotid bypass for the treatment of CCA occlusive disease, a retrospective 83

Journal '-~f VASCULAR SURGERY

84 Fry et al.

A

Occluded common carotid a.

Stenotic common carotid a.

Dacron grafts

Fig. 1. A, Patterns of stenosis and occlusion of the CCA with a patent nondiseased carotid bifurcation. B, Reconstruction of the lesions demonstrated with Dacron conduits.

Table I. Preoperative symptoms as related to carotid artery reconstruction Transient ischemic attack Amaurosis fugax Dizziness/syncope Cerebral infarct Asymptomatic

CCA

lCA

4 2 2 2

o

ECA.

Total

4

3

11

3

2 1 1 1

7 3 3 1

o o o

Some patients had more than one symptom.

study was undertaken of all patients undergoing su~lavian-carotid bypass for the treatment of symptomatic CCA occlusive disease at Parkland Memorial Hospital and the Dallas Veterans Hospital from Jan. 1, 1977, to Feb. 2, 1989. Risk factor analysis, symptoms, arteriographic findings, and details of the operative procedure'were reviewed. Operative results and 30-day mortality rates were recorded. This review also included long-term follow-up including graft patency, symptom relief, and long-term survival. Bypass patency was documented by duplex examination.

RESULTS Twenty patients Wlderwent subclavian-carotid artery bypass during the study period. Fourteen

men and six women with a mean age of 60 years (range, 32 to 82 years) were studied. Preoperative risk factors included hypertension in patients un.. dergoing six CCA bypasses, three internal carotid artery bypasses (ICA), and one external carotid artery (ECA) bypass. All patients had a history of .tobacco use. Preoperative symptoms are tabulated in Table I by type of carotid artery reconstruction. Many patients suffered from multiple symptoms, most frequendy transient ischemic attacks and amaurosis fugax. Eleven patients had reconstructions of the right carotid artery, with nine patients undergoing left carotid artery reconstruction. Arteriographic eval.. uation revealed four types of occlusive patterns (Figs. 1, A and 2, A): (1) Significant CCA stenosis alone; (2) Common carotid artery occlusion with a patent bifurcation; (3) Common carotid artery oc.. elusion with tandem stenotic internal carotid artery or small diameter leA, and (4) Common carotid artery and ICA occlusion with a patent external carotid artery. OPERATIVE TECHNIQUE Exposure of the subclavian artery was accom" plished through a transverse cervical incision approx" imately 1 cm above the clavicle over the clavicular

'Vol\l11'le 15 1 January 1992

Number

Subclavian-carotid bypass 85

B

A Stenotic internal carotid a. Occluded .common carotid 8.

Occluded internal and common carotid aa.

Vein grafts

Fig. 2. A, Patterns of occlusion ofthe CCA with-leA stenosis or occlusion. B, Reconstruction of the lesions demonstrated with greater saphenous vein conduits.

the

head of sternocleidomastoid muscle. After division of the platysma and clavicular head of the sternocleidomastoid muscle, the anterior scalene fat pad was carefully dissected from the anterior scalene muscle. The vasculature ofthis fat pad can be avoided by en bloc mobilization from a lateral to medial direction. This exposes the phrenic nerve, which is mobilized with a rim of the fat pad and anterior scalene muscle to avoid nerve devascularization. By dividing the anterior scalene muscle, the subclavian artery can be identified and isolated distal to the vertebral artery origin. Exposure of the carotid bifurcation was accomplished through either a transverse cervical incision or an incision on the anterior border of the sternocleidomastoid muscle. The usual precautions to avoid cranial nerve injuries and minimize embolization by careful dissection were observed. Once both vessels are isolated the bypass graft is first anastomosed to the subclavian artery end to side. Care should be taken to assure the optimum angle from the subclavian artery to the area of intended carotid artery anastomosis. The graft is passed under the sternocleidomastoid muscle to the carotid artery. Transection of the carotid artery at the level of anastomosis is performed next. If endarterectomy of the leA is planned, the posterior aspect of the ICA

is spatulated to facilitate this step. Reconstructions for the various types of lesions encountered are included in Figs. 1, B and 2, B. Carotid artery shunting was not used. Intraoperative evaluation of the reconstruction was performed by use of a continuous-wave Doppler. Operative intervention included subclaviancarotid artery bypass to the CCA below or at the carotid bulb in nine patients (45%), to the carotid bulb with an ICA patch angioplasty in six (30%), and the external carotid artery in five patients (25%). Four patients· underwent simultaneous leA endarterectomy with the bypass fashioned to create a patch angioplasty at the site of endarterectomy. The conduit used included the greater saphenous vein in 15 patients (75%) and prosthetic grafts in 5 (25%) (three Dacron and two polytetraB.uoroethylene). In the immediate postoperative period, four asymptomatic phrenic nerve neuropraxias were identified by chest radiography. No respiratoryabnormalities resulted, with all patients ·recovering dia... phragmatic function in the perioperative period. No postoperative strokes occurred. One postoperative death (5%) resulted from myocardial infarction complicated by respiratory failure. The duration of follow·up ranged from 1 to 122 months, with a mean of 50 months. One patient was

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86 Fry et al.

Table ll.. Retrograde thromboendarterectomy Author 3

Wylie

Robbs6 Ehrenfeld1 Rushton5

Moore2 VOgt4

Carotid IJnny bypflSSed Common Common Common Common Common

+ + + + +

bifurcation bifurcation bifurcation bifurcation bifurcation

Common

No. of'lISes

Months follow-up

15 5

NS 3-36 mo up to 38 mo 1-43 mo

8

3-129 mo

1

NS

2

11

Complictltions None Procedure not selected out 1 stroke, 1 death None 1 operative MI, death 1 patient with stroke and death 1 thrombosis-asymptomatic NS

NS, Not specified; MI, myocardial infarction.

lost to follow-up leaving 18 patients (90%) available for long-term follow-up. Duplex s~anning was performed on all patients. No occlusions or hemodynamically significant stenoses were detected. One patient returned with posterior circulation infarct at 103 months after procedure; however, the graft was found to be patent without stenosis on duplex examination. During the sntdy period, four additional patients died (21%), all ofcauses unrelated to their carotid vascular disease. These deaths occurred between 22 and 104 months after procedure, and were caused .by cardiac disease. in two, and malignancy in two.

DISCUSSION Although most patients with atherosclerotic carotid artery occlusive disease have the primary involvement in the carotid bulb, a small subset of patients remains who have more significant proximal carotid artery involvement. Multiple surgical procedures have been described for those patienFS with CCA occlusive disease. Retr9grade thromboendarterectomy involves blind disoblite(ation of the CCA. This procedure is usually ~ted to the right carotid artery, since surgical control of this artery can be safely gained through a cervical incision. Control of the carotid origin must be obtained to qllnimize embolization to the subclavian or vertebral artery. The published cases reviewed are summarized in Table II. I -7 The combined morbidity and mortality rates ranged from 0%4-6 to 37.5%.2 This high complication rate may be significandy decreased iit current practice because of improvements in patient selection and perioperative care. Carotid-subclavian artery transposition combines distal CCA thromboendartereetomy with transposition to the subclavian artery. This procedure is· most commonly performed on the left carotid artery because of the inability to obtain control of the left carotid origin from a· transcervical incision. In

reviewing carotid artery transposition,1,3? no signIficant morbidity or mortality rates were reported. One wound seroma did occur,7 which emphasizes the possibility of thoracic duct injury with this procedure, similar to subclavian-to-carotid artery transposition complications. , Carotid-carotid artery bypass has received little documentation in the literature. 2,4,8 In the five cases reviewed, no short-term or long-term complications were observed. This procedure may have a more important role in extrathoracic revascularization of the occluded innominate artery. Axillary-carotid artery bypass is re~rted by some authors t~ be technically less demanding than subclavian-carotid artery bypass. Common, internal, and external carotid arteries have been revascularized by this procedure. 9 -11 One possible advantage of this technique over others may occur in those patients with symptomatic ·carotid artery disease who have undergone radical neck dissection or radiation therapy for head and neck neoplasms or both. Routing a bypass graft from the axillary to carotid artery allows carotid artery reconstruction while minimizing disturbance of significandy altered tissue planes caused by previous therapy. In one reported case a Dacron prosthesis eroded through the skin. 9 Therefore autogenous tissue should be used in this procedure. Experience with subclavian-carotid artery bypasses has been overshadowed by reporting these procedures together with carotid-subclavian artery bypass for subclavian artery occlusive disease. Em.. phasis in these reports has been placed on subclavian artery reconstruction. Separation of data on subclavian-carotid artery bypasses from carotid.. subclavian artery bypasses by use ofpublished reportS becomes difficult because of complications not reported in a standard fashion to the recipient vessel of a bypass. Before this report, 81 cases of subclavian~carotid artery bypass with or without bifurcation endarterectomy have been reported. 2,4,5.12-21 A ~ummary of

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Subclavian-carotid bypass 87

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Table Ill. Subclavian-carotid artery bypass No. ofcases

Follow-up in months

Common

25

NS

Brockenbrough13 l4 Berguer Moore 2

External External Common

3 1

4

29 NS 3-119 mo

Kozol 15 Zarins l6 VOgt4

Common External Common

3 2 11

27mo up to 24 2-189 mo

Schuler l7

External

3

15

Riles l8 Hans 19 Rushton5 Ziomek2O

Internal, external External Common, external Common

9 3 3 5

NS NS 25 mo 6 mo-9 year

McGuiness21

External

9

4-28 mo

Current study

Common, internal, external

20

50

Author

----Diethrich

l2

Carotid artery bypassed

Cumplications 1 operative MI 1 with recurrent symptoms None None NS ? 2 late occlusion 1 CVA caused by hypotension 2 patients wJcontinued AF up to 4 occlusions 8-132 mo None

1 late occlusion (asymptomatic) None Subclavian-subclavian bypass graft erosion NS 1 graft failure - successfully reopened

IMI 1 posterior circulation CVA

Graft Dacron GSV GSV GSV Dacron NS GSV GSV PTFE 1 GSV 2PTFE GSV GSV Dacron GSV Dacron PTFE 4PTFE 5 GSV GSV Dacron PTFE

NS, Not specified; GSV, greater saphenous vein; PTFE, polytetrafluoroethylene; CVA, cerebrovascular accident.

these reports.is presented in Table ill. These bypasses have been to the CCA, ICl\ or ECA (Table Ill). Most cases were reconstructions for atherosclerotic occlusive disease, whereas the remainder were reconstructions of the carotid artery after radical neck dissection with radiation. All patients in the current study underwent reconstruction for atherosclerotic CCA occlusive disease. The small number ofreported cases with variable long-term follow-up creates difficulty in providing strong conclusions about the durability of this procedure, the conduit of choice, and its success in preventing further neurologic symptoms. In addition, frequendy patients are included who are treated with carotid-subclavian artery bypasses for subclavian artery disease without distinct separation of groups regarding the reconstructed vessel, complications, and postoperative symptoms. We suggest that carotid-subclavian artery bypasses and subclavian-carotid artery bypasses are in fact different procedures in terms of the symptoms, complications, graft orientation, and choice of graft conduit. An interesting example is in the choice of conduit. In the literature about carotid-ta-subclavian artery bypasses Ziomek et al. 20 found that the prosthetic graft offered better results for carotid.. subclavian artery bypass. Yet, this was a mixed procedure group including both carotid-subclavian artery and subclavian-carotid artery bypasses. The

superiority of prosthetic grafts was explained by the stiffness of the graft resisting lOnking better than saphenous vein. In reviewing our experience with carotid-subclavian artery bypass, we support the concept that prosthetic grafts have greater durability than venous grafts (unpublished data). When subclavian-carotid artery bypass is analyzed, the benefits of prosthetic over venous conduits is not upheld. Our review of the published data and our own experience fail to show increased rates of occlusions with saphenous vein conduits in subclavian-carotid artery bypass. This may be because the distal anastomosis in this operation is typically placed more distally on the common carotid artery, or on the carotid bulb or ECA. The subclavian-carotid artery bypass does not take an acute angular course across the base of the neck as in the typical carotid-subclavian artery bypass for proximal subclavian occlusion. The more vertically placed subclavian-to-carotid artery bypass runs closely perpendicular to the forces exerted on the graft with neck rotation. Increased graft stiffness is not needed to resist kinking and graft occlusion. Thus one can take advantage of- the possible benefits of an autogenous tissue recoDStruc.. tion. Carotid-subclavian artery bypasses typically run almost parallel to the applied forces of neck rotation. Thus rotational forces try to compact or kink as well as elongate a bypass conduit in this position. A

Journal of VASCULAR SURGERY

88 Fry et al.

conduit that resists kinking in this position should have the advantage of longer patency rates. Although statistically significant conclusions regarding long-term protection from neurologic events cannot be made on the basis of this report, it is noteworthy that no patients suffered ipsilateral anterior circulation ischemic symptoms during this follow-up period. In addition, duplex examination documented sustained graft patency, substantiating the durability of this procedure. A high percentage ofperioperative and long-term deaths occur from cardiac causes with all types of carotid reconstructions. In our series myocardial ischemia accounted for a 15% overall mortality rate. Based on this, an in depth cardiac evaluation is warranted for patients with CCA occlusive disease. Based on our review, subclavian-carotid artery bypass with either saphenous vein or prosthetic conduits is an excellent surgical option for symptomatic CCA stenosis or occlusion. The procedure is associated with low morbidity and mortality rates and provides long-term reduction of significant ipsilateral neurologic events. REFERENCES 1. Ehrenfeld WK, Chapman RD, Wylie ES. Management of

2.

3. 4. 5. 6.

7.

occlusive lesions of the branches of the aortic arch. Am J Surg 1969;"118:236..43. Moore WS, Malone JM, Goldstone J. Extrathoracic repair of branch occlusions of the aortic arch. Am J Surg 1976;132: 249-57. Wylie EJ, Effeney DJ. Surgery ofthe aortic arch branches and vertebral arteries. Surg Clin North Am 1979;59:669-80. Vogt DP, Hertzer NR, O'Hara PJ, Beven EG. Brachiocephalic arterial reconstruction. Ann Surg 1982;196:541-52~ Rushton FW Jr, Kukora JS. Surgical management of the occluded carotid artery. Surgery 1984;96:845-53. Robbs IV, Human RR, Rajamthnam P. Extracranial arterial reconstruction for chronic cerebral ischemia. S Afr Med J 1986;70:653·7. Gee W, Oiler DW, Schwartz JE. An alternative approach to

DISCUSSION Dr. Wesley Moore (Los Angeles, Calif.). The authors have reviewed experience with this operation in over a 12.. year interval, described their experience with 20 patients undergoing 11 right and nine left bypass procedures. The distal anastomosis was to the carotid bulb in nine, to the ICA in six, and to the ECA in five. The greater saphenous vein was used as the· conduit in 15, and prosthetic grafts were used in five. One postoperative death occurred for a mortality rate of 5%. No postoperative strokes occurred, and there were four phrenic nerve palsies, which are oflittle

8.

9.

10.

11. 12.

13. 14. IS.

16.

17.

18.

19. 20.

21.

lesions in the proximal segments ofthe brachiocephalic arterial system. Surg Gynecol Obstet 1977;144:339-42. Zelenock GB, Cronenwett JL, Graham LM, et al. Brachiocephalic arterial occlusions and stenoses. Arch Surg 1985;120: 370-6. ' Thompson BW, Read RC, Carnpbell GS. Operative correc· tion of proximal blocks of the subclavian or innominate arteries. J Cardiovasc Surg 1980;21:125-30. Carabasi RA Ill, DeLaurentis DA. Axilla-internal carotid artery bypass in the treatment of neck metastases. J V ASC SURG 1985;2:578·80. Archie JP Jr. Cerebral revascularization by axillary-carotid bypass. J Cardiovasc Surg 1989;30: 158-60. Diethrich EB, Garrett HE, Ameriso J, Crawford ES, EI·Bayar M, DeBakey ME. Occlusive disease of the common carotid and subclavian arteries treated by carotid-subclavian bypass. Am J Surg 1967;114:800-8. Brockenbrough EC. Subclavian - external carotid bypass graft for cerebrovascular insufficiency. Am J Surg 1972;124: 190-3. Berguer R, Bauer RH. Subclavian artery to external carotid artery bypass graft. Arch Surg 1976;111:893-6. Kokzol RA, Bredenberg CB. Alternatives in the management of atherosclerotic occlusive disease of aortic arch branches. Arch Surg 1981;116:1457-60. Zarins CK, DelBeccoro EJ, Johns L, Turcotte JK, Dohrmann GJ. Increased cerebral blood flow after external carotid artery revascuIarization. Surgery 1981;89:730-4. Schuler IT, Flanigan DP, DeBord JR, Ryan TJ, Castronuovo JJ, Lim LT. The treatment of cerebral ischemia by external carotid artery revascularization. Arch Surg 1983;118:567-72. Riles TS, Imparato AM, Posner MP, Eikelboom BC. Common carotid occluSion: assessment of the distal vessels. Ann Surg 1984;199:363·6. Hans SS. Subclavian - external carotid bypass. J Cardiovasc Surg 1984;25:404-7. Ziomek S, Quinones-Baldrich W}, Busuttil RW, Baker ID, Machleder Ill, Moore WS. The superiority of synthetic arterial grafts over autologous veins in carotid-subclavian bypass. J VASC SURG 1986;3:140-5. McGuiness CL, Short DH, Kersrein MD. Subclavian-external carotid bypass for symptomatic severe cerebral ischemia from common and internal carotid artery occlusion. Am.J Surg 1988;155:546-50.

Submitted June 10, 1991; accepted Aug. 23, 1991.

consequence. Therefore the authors have demonstrated that this is indeed a safe procedure. Furthermore, they have demonstrated excellent efficacy in that during follow-up that ranged from I to 122 months with a mean of 50 months, 18 of their original patients undergoing duplex scanning had patent grafts at the time of follow-up. Furthermore, no patient experienced an anterior circulation stroke during that follow-up interval. Therefore, it is certainly not possible to fault the choice of operation in this group of patients with an apparent 100%

Volume 15 Number 1 January 1992

late patency rate. However, it should be kept in mind that patients with this condition and operation on supraaortic trunks are relatively few in number, and therefore any individual series, including this one, is going to be relatively small, and therefore statistical analysis is somewhat limited. We have had experience with 36 patients undergoing carotid-subclavian artery bypass grafting for lesions of the subclavian artery. In a 5-year follow-up we have demonstrated a marked superiority of prosthetic grafts over the saphenous vein. We had a remarkably high thrombosis rate, with saphenous vein grafts occurring between the first year and 18 months. We postulated that a combination oftissue compression and neck motion may compress the relatively small vein graft inducing a larger thrombosis rate in contrast to the larger diameter of prosthetic graft, which is stiffer and more likely to resist these extrinsic forces. Indeed, review of an additional eight series in the literature demonstrated an average patency of 75% for vein grafts and 97% for prostheses. The authors in their discussion point out that the behavior and patency ofgrafts for these operations may be different when the indication is a bypass to the CCA rather than to the subclavian artery. They point out that the obliquity ofthe anastomosis, when a vein graft is taken from the subclavian to an area of the carotid bifurcation, is less than when it is taken the short distance between the subclavian and CCA in the base ofthe neck. This may well be true; however, I would like to propose a simpler and more direct alternative for the management of the proximal CCA lesion; one, in fact, that the authors mentioned in their list. This is a simple transposition of the CCA to the subclavian artery after transection low in the neck. This accomplishes a direct revascularization with use of autogenous artery with one anastomosis as a opposed to translocating the saphenous vein with two anastomoses and perhaps a questionable furore. Even in the instance of CCA thrombosis, retrograde thrombectomy or thromboendarterectomy with a transeeted artery can be carried out before its transposition. Should bifurcation endarterectomy be required, this is done in the usual fashion.

Subclavian-carotid bypass 89

I ask the authors to comment on this particular alternative surgical approach with respect to its applicability in their patient series and ask them what operation they would use when faced with a patient admitted with a CCA lesion next week. Dr. John Martin. We agree with many of the conclusions that you obviously have summarized for us. We too enjoy using the carotid-subclavian artery transposition, but use it in selective cases. We find it particularly suitable. for patients with proximal CCA stenosis alone with a patent common carotid artery. Although it can be used in combination with endarterectomy for the totally occluded artery, we have not been as satisfied with the results and the ease in endarterectomy in these chronically occluded arteries. We have had some difficulty in establishing a good plane and it sometimes can be more difficult than a simple direct bypass alone. In our hands we have found the bypass has worked better. It also avoids the occasional occurrence of early thrombosis in these extensive endarterectomies, but we would agree wholeheartedly with you that there are some patients in which this is an excellent option, particularly those with proximal stenosis alone. Dr.llobert Rutherford (Denver, Colo.). I would like to ask .Dr. Martin, in his consideration of the differences between the subclavian-to-carotid and the carotid-tosubclavian artery bypasses, whether possibly a more important difference is the fact that the subclavian-to~ carotid artery bypass has a much lower· resistance and approximately three times the flow rate of ~e carotid-tosubclavian bypass. I think this may be much mote important than kinking and graft choice in determining the demonstrated difference in patency. . Dr. Martin. I think these suggestions are important. Obviously flow dynamics are very important when assessing why one graft stays open and the other. does not. Obviously, the ·resistant beds are different and, as we suggested before in our additional answers, I think the outflow bed has a lot to do with the fact that saphenous vein grafts did stay open in this study. I would agree wholeheartedly.