COMMENT:
Gauging the degree of clinical change is the primary goal of this grading scale. The use of an ABI change of 0.10 here is not intended as indirect evidence of patency but as the least acceptable evidence of hemodynamic improvement, to guard against the fallibility of basing success on symptomatic improvement alone. It will be noted that, in attempting to provide an objective basis for claiming “improvement” here, and later for defining “hemodynamic success” or “failure” and for supporting a claim of patency, an ABI change of 0.10 has been chosen. In an earlier study, Carter²¹ recommended 0.15 as the minimum requirement for significant change. This was widely accepted, and many vascular surgeons still prefer this. However, the ad hoc committee that originally developed these standards believed that a difference of 0.10 was sufficient to signify true change, if combined with categorical clinical improvement, as required. It was thought that 0.15 was too strict and might unfairly exclude patients who truly benefitted. For example, using a 0.15 increase as a requirement would categorize as a failure a patient with a blood pressure of 120, whose ankle pressure increased after treatment from 24 to 41 mm Hg (from 0.20 to 0.34 ABI), even though such a patient would likely be relieved of rest pain. Similarly, a patient relieved of claudication by iliac percutaneous transluminal angioplasty (PTA) with an ABI increased from 0.86 to 1.00 would be considered a failure if a 0.15 ABI increase were required. Thus the original recommendation has been retained.

Hemodynamic success or failure.
The term “hemodynamic failure” has been used to indicate a lack of significant hemodynamic improvement (i.e., an increase in ABI) in spite of a patent revascularization. The common setting for this is multilevel disease where a proximal reconstruction is performed in the face of residual distal disease or “poor runoff.” It can also be seen after PTA where dissection or elastic recoil of an unyielding plaque may result in incomplete restoration of luminal diameter (although this can also be considered a form of technical failure). Again, for the sake of uniformity, a specific degree of change must be recommended, and for reasons given above an increase of less than 0.10 in the distal pressure index constitutes a hemodynamic failure. Thus in the specific circumstance of a proximal or inflow procedure (e.g., femorofemoral bypass or iliac PTA) being performed in the face of outflow disease or poor runoff (e.g., superficial femoral artery occlusion), failure to increase the ABI by at least 0.10 is considered a hemodynamic failure. Conversely, increasing the ABI by more than 0.10 can be considered a “hemodynamic success,” but it would not be considered a “clinical success” without categorical clinical improvement, as described earlier.

Criteria for patency.
Articles in scientific journals should only accept patency rates that are based on objective findings. A bypass graft or otherwise reconstructed arterial segment may be considered patent when one of the following five criteria is met. Beyond the last date of such proof of patency, they must be considered lost to follow-up.

1. Demonstrably patent graft by an accepted vascular imaging technique, such as arteriography, Duplex ultrasound color-flow scan, or magnetic resonance imaging.
2. The presence of a palpable pulse, or the recording of a biphasic or triphasic Doppler wave form at two points directly over a superficially placed graft.
3. Maintenance of the achieved improvement in the appropriate segmental limb pressure index, that is, not more than 0.10 below the highest postoperative index. Although a greater reduction in pressure index may occur and the graft or reopened segment may still be patent, imaging proof is required in these instances or any other doubtful or marginal circumstances covered under criteria 2, 3, or 4. To avoid the confusing effects of distal runoff disease, the most appropriate pressure index for this purpose is at the next level beyond the revascularized segment or distal anastomosis (see comment below).
4. Maintenance of a plethysmographic tracing distal to the reconstruction that is at least 50% or 5 mm greater in magnitude than the preoperative value and close to the postoperative value. (This is the weakest criterion and acceptable only when accurate pressures cannot be measured, as with calcific arteritis in a diabetic patient. However, even in such cases, stronger evidence of patency, in the form of imaging, is clearly preferred.)
5. Direct observation of patency at operation or postmortem examination.

COMMENT:
Although palpable pedal pulses that are readily felt by an experienced observer are adequate for routine clinical assessment, such observations (and particularly comments to this effect in the patient’s record by nurses, residents, or fellows) cannot be accepted as proof of patency for reports in scientific journals. Accurate patency data are so crucial to comparisons of arterial reconstructive techniques that reliable objective methods must be used. Duplex or color-flow Doppler scanning is now an accepted and commonly used method of graft surveillance and should be available in most centers. Failing this, Doppler measurement of ankle and brachial pressures can be used. At the time of the original Standards, before color-flow duplex scanning was widely available, the use of Doppler-derived pressure measurements was a reasonable expediency because angiographic follow-up was impractical. It allowed investigators to claim patency in the absence of other documentation, using retrospective vascular laboratory data. Now, the debate over the significance of a 0.10 versus 0.15 change in ABI has been tempered by the duplex scan, which can and should be used to settle the issue of patency in equivocal cases.
   At this point, it is worth reemphasizing that the designation of “clinically improved” is based on categorical clinical improvement plus objective evidence of hemodynamic improvement (i.e., the ABI). Hemodynamic success, or conversely hemodynamic failure, also applies to the entire limb and therefore also uses a distal monitoring site (i.e., the ABI). In contrast, patency applies to the revascularized or bypassed segment only, and if imaging studies or direct observation are not available, one should use the pressure index from the next level beyond that segment (e.g., the thigh-brachial index rather than the ABI for a proximal bypass graft or PTA). This is recommended to avoid the confusing effects of new or progressing occlusive disease between the revascularized segment and the point of pressure monitoring.

Failed and failing grafts.
A graft that has lost its patency—that is, has thrombosed—is considered a “failed” graft. This is in contrast to a “failing” graft, a graft that is still demonstrably patent but that has developed one or more stenoses that, if unrelieved, may lead to thrombosis. Such lesions may or may not produce symptoms or a significant drop in the resting ABI but can be detected by duplex surveillance of the graft and its anastomoses. Furthermore, their correction significantly improves assisted primary and secondary patency rates,²² as defined below. Diagnostic criteria include a peak systolic velocity in the stenosis that is greater than a certain level (e.g., 150 cm/sec) or is significantly greater than (e.g., at least 2.5 times) that of an adjacent “normal” segment.²³ With greater degrees of stenosis, the end diastolic velocity in the stenosis usually becomes similarly elevated and the ratio of accelerated to background velocity climbs.²⁴ Failure has also been predicted by an overall reduction in graft flow velocity below 45 cm/sec,²⁴ but this criterion applies mainly to femoropopliteal vein grafts, not tibial bypass grafts or those that using a prosthetic bypass graft. The specific criteria used are not yet standardized and differ somewhat from center to center. Their accuracy in predicting the actual degree of stenosis (e.g., >50% versus >75%) is also not well established, nor is the degree of stenosis beyond which failure is inevitable. Therefore, confirmatory arteriography is usually deemed necessary before intervening. Nevertheless, the concept and definition of a failing graft has gained wide acceptance and deserves inclusion in these reporting standards. Until standard diagnostic criteria are accepted, reports on this aspect should include the duplex criteria used, angiographic confirmation, or both.

Patency status: primary vs secondary patency.
With the help of graft thrombectomy or thrombolysis, revision or “redo,” it may be claimed that the original graft is still patent. It is important in this regard to separate “primary” from “secondary” patency. A graft is considered to have “primary” patency if it has had uninterrupted patency with either no procedure performed on it or a procedure (e.g., transluminal dilation or a proximal or distal extension to the graft) to deal with disease progression in the adjacent native vessel. Thus the only exceptions that do not disqualify the graft for primary patency are procedures performed for disease beyond the graft and its two anastomoses. Dilations or minor revisions performed for stenoses, dilations, or other structural defects, or closing missed arteriovenous fistulas in an in situ vein bypass graft before occlusion do not constitute exceptions, as they are intended to prevent eventual graft failure.
   When originally proposed,¹ considerable objections were raised against this last rule governing primary patency,²⁵ understandably, because bypass grafts that never occluded but underwent minor procedures to protect patency were considered the same as those that had actually thrombosed, that is, they were all listed as secondary patency data. Ultimately, the additional designation of “assisted primary patency” was suggested to apply to this situation, in which patency was never lost but maintained by prophylactic intervention.²⁶ This has proven useful and is included in this revised version.
   If graft patency is restored after occlusion by thrombectomy, thrombolysis, or transluminal angioplasty, and/or any problems with the graft itself or one of its anastomoses require revision or reconstruction, all must be listed under “secondary” patency. A “redo” or secondary reconstruction, as defined later, does not contribute to secondary patency unless most of the original graft and at least one anastomosis are retained in continuity.
   It should be understood that both primary and secondary patency rates are important. The former is important in judging the natural history of a graft or reconstructive procedure, and the latter is important to indicate how long function can be preserved with the aid of close surveillance and the use of secondary or adjunctive procedures. Both provide valuable information, but when only one or the other patency rate is presented and one is not identified, comparison between different reports on the same type of reconstructive procedure is difficult, if not impossible. Therefore, it is recommended that in each report, both primary and secondary patency rates be presented, and the patency rate under discussion is identified as primary or secondary. The same applies to the assisted primary patency rate, should it be used. Thus it is appropriate, in analyzing an experience with extremity bypass or in comparing two such procedures where a program of graft surveillance and intervention for preservation of patency is used, to present all three patency rates to demonstrate the intrinsic durability of the primary procedure, the impact of graft surveillance and prophylactic intervention, and the ability to restore function to a failed graft.
   It has been suggested that secondary reconstructions that do not qualify under the definition of secondary patency be allowed to contribute to “tertiary patency,” that is, patency across the same limb segment achieved by one or more additional procedures that do not preserve, in continuity, most of the original graft and one anastomosis. Although this adds some perspective regarding the ultimate status of the limb, and such procedures do contribute to limb salvage and function, this is not a recommended reporting standard. It only gauges the overall success of surgical management and not the merits of the original bypass or revascularization procedure, which is the primary focus of patency analysis.

Estimating patency rates.
Although subject to some artifact, so that projected and actual patency rates are not necessarily the same, the life table (LT) method is one of the best and most commonly used ways of presenting patency data on patients who undergo a revascularization procedure at different points in time and are followed-up for different lengths of time. Only the LT method was recommended for this purpose in our original standards.¹ It is still an accepted method if its rules are followed, but its limitations must be appreciated. The Kaplan-Meier survival estimate is an equally acceptable method under most circumstances. Both of these methods are described and compared in the following paragraphs.
   The LT method was best characterized by Peto et al.^27,28 in 1976 and 1977 in two articles in the British Journal of Cancer, but such methods were earlier described by Berkson and Gage in 1950²⁹ and also by Cutler and Ederer in 1958.³⁰ It was originally applied to the follow-up data of patients with different forms of cancer and cancer therapy. The LT method has two features that characterize the technique. The first is that events on the survival curve—for example, graft failures—are grouped into intervals. Survival rates are then calculated for each of these intervals and are used to generate cumulative patency rates that describe the survival curve. The second important feature is the assumption that any individuals lost to follow-up during an interval (also called censored data) are treated as withdrawn at the midpoint of the interval. It is this assumption that leads to the characteristic correction to the calculated failure rate for a given interval: Failure rate =
   This correction considers the individuals who were withdrawn to contribute to the risk pool for only half of the interval. However, this correction is mathematically equivalent to increasing the interval failure rate by the number of expected failures in half of the withdrawal group: + Failure rate × ½ Number of Withdrawals
   The further consequence of this correction for censored data or withdrawals is that the failure rate is assumed to be uniform over the interval. With this in mind, the use of the stair-step graphical presentation of the LT plot is not strictly necessary because the cumulative patency rate is the resulting conditional probability at the end of the interval based on the failure rate over the entire interval. The LT graph can thus be represented by straight line connections between the patency estimates located at the end of each interval. In this presentation, the only intervals with level lines are those with no failures.
   LT analysis should include the following columns in the table (alphabetically listed as in the example presented in Table IV): (A) intervals in months; (B) number of grafts at risk at the start of the interval; (C) number failed during the interval; (D) number of patients withdrawn with patent grafts during the interval, due to death, loss to follow-up, or with follow-up that ends during that time interval (these three may be tabulated in separate columns, then combined); (E) interval failure rate; (F) cumulative patency rate; and (G) standard error.