Postoperative duplex scan surveillance of axillofemoral bypass grafts☆☆☆★★★

Patients and methods 

Between January 1, 1996, and January 1, 2001, 120 patients underwent AxFBG procedures with externally supported 8-mm polytetrafluoroethylene at Oregon Health & Science University and the Portland Veteran's Affairs Medical Center. Patients were included in the analysis if they had undergone at least one postoperative duplex scan graft flow study. Patients with grafts that failed from infection were excluded.

All operations were performed in a standardized fashion, with the proximal graft anastomosed to the axillary artery and tunneled deep and lateral to the pectoralis minor muscle. A small amount of redundancy was left in the proximal graft to decrease the risk of proximal graft disruption.4, 16 If an axillobifemoral bypass was performed, the distal limb of the axillofemoral graft was anastomosed to the anastomotic hood of one side of the femoral-femoral graft. Preoperative arteriograms were reviewed to determine outflow patency. Outflow patency was defined as either a patent superficial femoral artery or a patent infrainguinal bypass graft.

DS was carried out every 3 months for the first postoperative year and every 6 months thereafter. The DS protocol consisted of measurement of peak systolic flow velocities (PSV) at the native inflow and outflow arteries, at all proximal and distal anastomoses, and at the midportion of both the axillofemoral and femoral-femoral components. Bilateral arm blood pressures and ankle-brachial indices (ABI) were also recorded at each follow-up visit. All duplex scan examinations were performed by registered vascular technologists with either Hewlett-Packard (HP 4500, 5500, Hewlett-Packard Co, Palo Alto, Calif) or Acuson (Acuson 128 XP Computed Sonography, Mountain View, Calif) duplex scanners.

Statistical analyses were performed with comparison of the last recorded study obtained in primarily patent grafts (patent group) with the last recorded study obtained before occlusion (occluded group). Data were analyzed with Student t tests, χ2 analysis, and multivariate logistic regression analyses with SPSS (version 10.1, SPSS, Inc, Chicago, Ill). A P value of less than .05 was considered to be significant.

Results 

One hundred twenty patients underwent AxFBG procedures during the study period. Thirteen patients were excluded from analysis because of infection as the cause of graft failure. Fifteen patients died before a DS examination was obtained, and 14 patients were lost to follow-up. Seventy-eight patients, therefore, qualified for inclusion in this study. Twenty-three patients (29%) had graft occlusion (occluded group), and 55 (71%) had patent grafts throughout the follow-up period (patent group). The mean age was 67 years (range, 45 to 87 years). Follow-up ranged from 1 to 64 months (mean follow-up, 25 months). Patient characteristics and risk factors for both groups are listed in Table I.

Table I. Patient characteristics and comorbidities among 78 patients undergoing axillofemoral bypass grafting between January 1996 and January 2001

		Patent* (n = 55)	Occluded† (n = 23	P value‡
	Mean age (y; range)	68 (50-84)	65 (45-87)
	Male gender	37 (67%)	14 (61%)	1.0
	Female gender	18 (33%)	9 (39%)
	Axillobifemoral bypass	45 (82%)	21 (91%)	.30
	Axillounifemoral bypass	10 (18%)	2 (9%)
	Tobacco use	45 (82%	20 (87%)	.58
	Hypertension	43 (78%)	12 (52%)	.02
	Coronary artery disease	29 (53%)	13 (56%)	.76
	Hyperlipidemia	15 (27%)	6 (26%)	.92
	Warfarin sodium use	11 (20%)	7 (30%)	.32
	Diabetes mellitus	13 (24%)	4 (17%)	.45
	Hypercoagulable state	3 (6%)	2 (9%)	.60
	Chronic renal insufficiency	3 (6%)	3 (13%)	.26
	*Patent group: patients with grafts remaining patent throughout follow-up. †Occluded group: patients with grafts occluding during follow-up. ‡Student t test.

Limb salvage was the indication for AxFBG in 62% of patients (Table II).

Table II. Operative indications of all 78 patients undergoing axillofemoral grafting during study period

		Patent* (n = 55)	Occluded† (n = 23	P value‡
	Rest pain	30 (55%)	11 (48%)	.59
	Claudication	9 (16%)	6 (26%)	.33
	Ulcer/gangrene	13 (24%)	3 (13%)	.23
	Aortic graft infection	7 (13%)	2 (9%)	.62
	Inflow occlusion§	8 (14%)	3 (13%)	.86
	*Patent group: patients with grafts remaining patent throughout follow-up. †Occluded group: patients with grafts occluding during follow-up. ‡Student t test. §Native aortic occlusion (n = 3), aortofemoral graft thrombosis (n = 3), axillofemoral graft thrombosis (n = 2), femoral-femoral graft thrombosis (n = 3).

Table III. Percentage of patients with patent native superficial femoral artery or infrainguinal bypass as assessed with preoperative arteriography if available

		Patent* (n = 55)	Occluded† (n = 23)
	Patent SFA or IIB bilaterally	20 (37%)	11 (48%)
	Single limb with patent SFA or IIB	21 (38%)	5 (22%)
	Patent profunda femoral artery only	11 (20%)	5 (22%)
	No outflow data before surgery	3 (5%)	2 (8%)
	*Patent group: patients with grafts remaining patent throughout follow-up. †Occluded group: patients with grafts occluding during follow-up.

SFA, Superficial femoral artery; IIB, in frainguinal bypass.

The mean of the PSVs in the individual grafts at sequential locations along the axillofemoral grafts are listed in Table IV for the first 12 postoperative months.

Table IV. Mean PSVs (cm/s ± standard error) for three sites along axillofemoral graft components during first 12 postoperative months

	Site	3 mo	6 mo	9 mo	12 mo
	Proximal axillary anastomosis	153.5 ± 11	157.3 ± 18	194.1 ± 24	154.9 ± 22
		(n = 24)	(n = 16)	(n = 16)	(n = 17)
	Midgraft axillofemoral component	107.2 ± 9	108.1 ± 13	125.1 ± 9	100.7 ± 9
		(n = 26)	(n = 21)	(n = 20)	(n = 22)
	Distal axillofemoral anastomosis	97.9 ± 8	108.5 ± 13	129.5 ± 14	93.6 ± 10
		(n = 26)	(n = 19)	(n = 20)	(n = 18)

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Fig. 1. Comparison of mean PSV at proximal axillary anastomosis for both grafts remaining continuously patent and grafts with thrombosis during observation during first 12 postoperative months.

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Fig. 2. Comparison of mean PSV at midportion of axillofemoral graft for both grafts remaining continuously patent and grafts with thrombosis during observation during first 12 postoperative months.

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Fig. 3. Comparison of mean PSV at distal axillofemoral anastomosis for both grafts remaining continuously patent and grafts with thrombosis during observation during first 12 postoperative months.

We compared the last recorded graft flow study in the primarily patent grafts with the last study obtained before occlusion in the occluded group. Univariate analysis results revealed that the mean PSV at the midportion of the axillofemoral graft was significantly lower in the occluded group compared with the patent group (mean PSV, 84 versus 112 cm/s, respectively; P = .015; Fig 4).

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Fig. 4. Univariate analysis of mean PSV at midportion of axillofemoral component comparing last recorded study in patent grafts with last study obtained before occlusion in subsequently occluded grafts.

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Fig. 5. Univariate analysis of mean PSV at distal axillofemoral anastomosis comparing last recorded study in patent grafts with last study obtained before occlusion in subsequently occluded grafts.

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Fig. 6. Univariate analysis of mean PSV at proximal axillary anastomosis comparing last recorded study in patent grafts with last study obtained before occlusion in subsequently occluded grafts.

After examination of these results, we determined the risk of graft occlusion and patency for every 10 cm/s difference between 60 and 150 cm/s. We found that a midaxillofemoral PSV greater than 140 cm/s resulted in a 94% patency rate. We also found that 48% of all grafts with a midaxillofemoral PSV less than 80 cm/s occluded, and 82% of all grafts with a midaxillofemoral PSV greater than 80 cm/s remained patent (χ2 = 6.03; P = .014; Fig 7).

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Fig. 7. Percent risk of subsequent graft occlusion with midaxillofemoral PSV less than or greater than 80 cm/s.

The results of the multivariate logistic regression analysis showed that a mean PSV less than 80 cm/s at the midportion of the axillofemoral grafts was the only independent factor associated with an increased risk of graft failure (odds ratio, 7.09; 95% CI, 2.0 to 24.0; P = .002; Fig 8).

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Fig. 8. Multivariate odds ratio of graft failure with midaxillofemoral PSV less than 80 cm/s, outflow patency, blood pressure, ABI, smoking, presence of femoral-femoral bypass, and warfarin use, with 95 percent CIs.

Discussion 

In this study, we characterized the PSV seen at specific locations through AxFBG. We compared the mean PSVs of patent grafts with those of occluded grafts and found no trend toward increasing or decreasing velocities over time in grafts that went on to occlude. Although we were unable to show that DS of AxFBG could be used to predict graft failure, we did find that a midaxillofemoral PSV less than 80 cm/s is an independent factor associated with an increased risk of graft occlusion. In addition, we showed that a midgraft PSV greater than 140 cm/s is a significant predictor of graft patency, as 94% of grafts with velocities above this level remained patent (Fig 4).

That the identification of failing infrainguinal vein bypass grafts with duplex ultrasound scan is effective in prolonging patency is well accepted.5, 7, 17 Before this report, however, most studies evaluating the efficacy of DS for improving the patency of infrainguinal prosthetic bypass grafts have shown no potential benefit.7, 14, 15 Lundell and colleagues7 performed a prospective randomized trial evaluating the efficacy of an intensive DS protocol for improving patency and showed that there was a significant improvement in assisted primary patency and secondary patency with intensive surveillance compared with routine surveillance for evaluating vein grafts but that there was no significant improvement in either primary assisted or secondary patency with prosthetic or composite grafts.

In contrast, Sanchez et al8 retrospectively reviewed 85 patients with failing polytetrafluoroethylene grafts, seven with AxFBG, and showed that the predominant lesions responsible for graft failure were inflow and outflow lesions. With treatment of these lesions, they were able to achieve a 5-year primary assisted patency rate of 71% and a limb salvage rate of 73%. This led the authors to conclude that DS for prosthetic grafts is useful in prolonging patency. The number of AxFBGs in that study was low, and they did not look specifically at the differences between various types of prosthetic grafts.

Calligaro et al9 were able to show that DS was more sensitive than clinical measures, such as ABI, pulse examination, or pulse volume recordings, for detection of failing prosthetic grafts, including AxFBG, but they did not show improvement in patency of these grafts. The number of AxFBGs in their study was also small (n = 13). Because of the improved sensitivity of DS compared with clinical measures (81% versus 24%), they concluded that DS of prosthetic grafts is worthwhile, although this study also did not stratify the results on the basis of location of the grafts.

Examination of the flow velocities at various sites along the axillofemoral grafts in this study shows that the velocities vary widely within, and among, the grafts. The variability in the flow velocities among grafts was independent of outflow patency, which is consistent with our previous observation that outflow patency does not influence graft patency3 but inconsistent with other authors who have shown that outflow does affect patency.18, 19 Our data indicate that a patent superficial femoral artery is not necessary to provide the low resistance state needed to improve patency. A patent profunda femoral artery may be a sufficient outflow source, as the primary determinant of blood flow into an artery is arteriolar resistance.20 Alternatively, the degree of retrograde flow into the pelvis may also help to provide decreased resistance to maintain patency.

Understandably, the number of grafts in this study decreases over time as grafts occlude. This may affect the evaluation of velocity changes over time. The number of grafts that had complete data at each time point was low. However, this does not affect the evaluation of the last recorded study obtained before occlusion or the last study obtained in grafts that are still patent. This is the more important evaluation to make because if a decision must be made regarding a threatened graft, the most recent study is the most vital.

It should be noted that the number of graft failures observed in this study exceeds what we may have expected on the basis of our previously published results.3 The patients in this analysis are obviously a different and smaller group from a different period of time. We are aggressive in performance of AxFBG procedures and offer them to patients at high risk with advanced disease. Our data will eventually need to be reanalyzed to determine whether this aggressive posture is justified, perhaps with comparison of patients from two time periods.

We expected to find less variability in the flow velocities within grafts because all grafts have a uniform diameter and are constructed in a uniform fashion. One plausible explanation is that these grafts do not have laminar flow through them either because of the pulsatile nature of blood pressure or the viscosity of blood. This is speculative, and a close examination of the duplex color flow characteristics might answer this question. We did find a positive correlation between blood pressure and midgraft flow velocity; however, blood pressure was not an independent factor influencing graft failure in the multivariate analysis.

It is interesting to note that the presence of a femoral-femoral graft was not an independent factor associated with graft patency in the multivariate analysis. Other authors have found similar results.18, 21 This is in contradiction to the findings of LoGerfo et al22 who showed that axillobifemoral grafts had superior patency when compared with axillounifemoral grafts. The number of axillounifemoral grafts in our study was small (12 grafts total: 10 patent, two occluded), and it is possible that had an equal number of each configuration been used that a true difference might have been observed. Whether an axillobifemoral graft portends improved patency over axillounifemoral grafts remains an unanswered question.

The results of this study show that AxFBGs have a greater likelihood of remaining patent if the flow velocities through the midportion of the graft remain above a certain level (140 cm/s in our study). The implication is that low flow velocities are detrimental and may lead to thrombosis. Other studies have shown that low flow velocities through bypass grafts predict failure.11, 23 Bandyk, Cato, and Towne11 evaluated infrainguinal in situ vein bypasses and found that postoperative PSV less than 45 cm/s identified grafts at risk for failure and recommended angiographic evaluation to identify correctable lesions.

Understandably, most clinicians become concerned about the development of high flow velocities within bypass grafts that may imply the presence of a stenosis. In consideration of autologous vein bypass grafts, this fact is true. However, in the case of prosthetic grafts, it appears a high PSV does not necessarily identify a graft at risk for failure. Studies evaluating flow through prosthetic grafts have determined that many grafts will fail without an identifiable area of stenosis.12, 13 This is particularly true for smaller diameter grafts.24, 25

It has been suggested that patients with low postoperative PSVs and no identifiable areas of stenosis should undergo systemic anticoagulation therapy to improve prosthetic graft patency.17, 26 However, a recent multicenter, prospective, randomized trial has been performed comparing warfarin and aspirin, in combination, with aspirin alone to determine whether improved patency was achieved with systemic anticoagulation.27 The results of this study showed that only prosthetic, above-knee bypasses, which were constructed with 6-mm grafts, benefited from anticoagulation therapy. No difference was found in patients with AxFBG, although the number was small, likely because 6-mm grafts were rarely used for this operation. Patients treated with warfarin and aspirin had significantly more hemorrhagic events and significantly higher mortality than patients treated with aspirin alone.27

The results of this study show that a midaxillofemoral peak systolic flow velocity less than 80 cm/s is associated with an increased risk of graft occlusion and that a PVS greater than 140 cm/s through the same area is associated with continued graft patency. Patency of AxFBG was independent of outflow patency, blood pressure, operative indication, smoking, warfarin use, or the presence of a femoral-femoral bypass. Variability in flow velocities over time did not influence the likelihood of graft failure. Although we did not identify an independent factor that absolutely predicts graft failure, it does appear low flow velocities are a risk factor for thrombosis and higher velocities portend continued patency.