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Classification of Coronary Lesions

Ronald Krone, MD

Department of Medicine (Cardiology), Barnes-Jewish Hospital,
Washington University in St. Louis, St. Louis, MO, USA

   Understanding the risk of a procedure is the key to intelligent decision making. From the inception of angioplasty in 1977, the importance of lesion characteristics in determining technical success and the importance of lesion characteristics and clinical issues in predicting complications were recognized.

   There are three possible outcomes of an intervention, not necessarily mutually exclusive: technical success, clinical success and an unsuccessful uncomplicated result.

   Technical success reflects a focus only on the lesion itself, whereas clinical success requires an improved overall clinical result.

Technical success requires:
1. The ability to deliver the balloon or the device to the lesion.
2. The ability to adequately dilate the vessel or otherwise improve the lumen. (In the days of limited expectations success was defined as improvement of stenosis by 20% or to a diameter of the stenosis of less than 50% of the "normal" vessel. Now success is defined as a minimum stenosis less than 20%).

   Clinical success requires technical success plus no peri-procedural complications leading to myocardial infarction, emergency coronary bypass and/or death.

   To predict clinical success or complications especially death, clinical factors are as important as lesion characteristics.1 Simple procedures on high-risk patients are basically high risk procedures, since any possible unsatisfactory result, even a temporary occlusion could lead to clinical disaster.

   In addition to identifying individual factors, which predict success, there is a need to combine and quantify this risk. Ideally, a simple score or lesion classification can be developed which reflects the difficulty of the procedure--the likelihood of success or complications. This permits normalization of the overall performance of the individual laboratory or operator according to the difficulty of the procedure. If a large database or registry is available, the lesion classification permits comparison of the individual operator and laboratory with the general experience. It also permits stratification of patients in large studies to account for or control for the complexity of the actual procedures. In practice, however, the goal of accurately predicting results has proven elusive. Unfortunately, success and complication rates present a moving target, moving so fast in some cases that before data can be published, the procedure has been improved. Thus the initial wide range of performance in lesions of differing complexity as predicted by the original 1988 classification (2) has been compressed at present due to improved technology and improved operator skills. In addition, developments in adjunctive pharmacology have further reduced the risks and improved success rates.

   The ability to successfully treat complex lesions is a measure of the skill of the operator. The role played by operator volume in angioplasty performance was evaluated in an analysis from the SCAI registry from 1990-1992. (3) We found that success rates were similar for all operators, about 95%, regardless of yearly volume. The complication rate in the more complex lesions varied according to operator case volume, with as predicted the highest volume operators having the lowest complication rate in the complex C lesions (Figure 1). As expected, the lowest volume operators (<25 procedures per year) had the highest rate of complications (8%) but surprisingly, the second highest complication rate (6%) was suffered by the patients revascularized by the highest volume operators (>200 procedures per year) (Figure 2).

Figure 1

Figure 2

    Analysis of the patient characteristics showed that the higher volume operators included a higher percentage of acute MIs in their procedures (11% vs. 5% for the < 100 per year operators). Of more interest, the percentage of C lesions tackled increased with increasing operator volume. Thus, the lesion classification system allowed normalization of the difficulty of the procedures and permitted an objective explanation for the higher complication rate in the higher volume operators. To a more limited extent, it provided an explanation for the relatively low complication rate in the lower volume operators as well. The more experienced the operator, the higher the percentage of the complicated (C) lesions.

   The less experienced operators did not tackle those more difficult lesions and thus were able to produce good results. (The very low volume operators not surprisingly did not choose difficult lesions but still did poorly). Thus, the individual operator seems to know his limitations, and select cases in which he would expect to be successful. The more experienced operator may choose to attempt more complex lesions, whereas the less experienced operator chooses more straightforward cases permitting him an acceptable success rate.

Challenges to developing a classification:
   The relationship of features of the lesions to success rates presents a moving target since characteristics of an individual lesion, which limit successful intervention, are targeted by device manufacturers as challenges to overcome. Thus, balloons have become smaller to aid in crossing the tighter lesions, more pushable to negotiate tortuous vessels and simplify crossing tight coronary lesions, and stronger to permit use of higher pressures. Devices to remove atheromas or burr through calcified lesions have been developed and, of course, stents have been developed to protect against vessel collapse, dissection and acute occlusion. Better understanding of the immediate response of the hemopoietic system to coronary interventions and the critical role played by platelets and thrombus in acute coronary syndrome has led to the widespread introduction of platelet glycoprotein IIbIIIa receptor blockers, further reducing the likelihood of sudden catastrophic closure after the procedure. In addition, training programs have been formalized and operators are certified. Operators who have performed over 1000 procedures performed are now common.

   To get a historical perspective on the process of codifying the obstacles to successful angioplasty we can look at the report from the National Heart, Lung and Blood Institute Registry published in 1984.(4) Faxon and coworkers found that a circumflex artery location, calcium in the lesion and an initial severe stenosis were associated with reduction in success (defined as greater than or equal to 20% improvement). For example, a 99.9% lesion had a 59% likelihood of success in that registry and success was only 37% for total occlusions.

   Other factors judged important in 1984 were the experience of an operator and the geometry of the lesion. In 1983, a balloon could traverse the lesion in 75% of cases where it was attempted. Using these and other data, the ACC and AHA derived a lesion classification which they published in 1988 and has just been renewed unchanged (Figure 3) (2). This system grouped the individual criteria into three large categories. Initially it was expected that type A lesions (low-risk) would have a success rate of 80%. The type B lesions predicted to have 60-85% success and type C lesions, less than 60% successful. The characteristics of the lesion class differed by greater amounts of tortuosity, angulation of the segment, the length of the vessel, the presence of occlusions, issues with side branches and potential for protection.

Figure 3

   Ellis, et. al. evaluated these criteria in over 1000 lesions from the Multivessel Angioplasty Prognostic study.(5) They found that a bend stenosis, high-grade stenosis, chronic total lesions, bifurcation stenosis and male gender were all associated with reduced success rates. It is worth noting that the overall success occurred in 82.6% of patients, reflecting the state of the art in 1986 and 1987. They confirmed the usefulness of the ACC/AHA classification for success that ranged from 92% for A lesions to 60% for C lesions. They also introduced a modification adding a class B2 for patients who had more than one B criterion. Myler et al. also reviewed their own results to evaluate the ACC/AHA lesion classification.(6) Occlusion greater than three months, increasing lesion length and any thrombus were indicators of lower success rates. Diffuse disease, angulated lesions, bifurcation lesions that were unprotected, calcification and lesion severity of greater than 95% were associated with a higher complication rate. In particular, angioplasties of grafts were predictably unpredictable. This reflected the unpredictability of the final result plus the risk of "no reflow" related to distal embolization. They introduced the C-I and C-II lesions, the C-II lesion having two C characteristics.

   The ACC/AHA Lesion Classification system was then evaluated by Tan et al. in a rigorous study using data from Guys Hospital in London.(7) They performed a careful prospective evaluation of 729 patients using two independent observers unaware of the outcome at the time of the lesion review. These procedures performed from 1990 to 1993, had an overall success rate of 91% with major complications in 3.3%. In a careful comparison of the ACC/AHA Lesion subclasses, they found no differences between the class A and class B-I but there were significant differences between B-I and B-II.

   There were no differences in the success or complication rates between C-I and C-II classes. They also evaluated the success and failure rate of individual criteria within a class. They found that the impact of individual criteria varied considerably. For example, within the B class, success for lesions with eccentricity, minor tortuosity, irregular contour, ostial location, and protected bifurcation was all greater than 90% (success rate for A lesions without any of these features was 96%). On the other hand lesions 10-20 mm long with moderate angulation had success rates between 80-90% and lesions with calcium or thrombus or occlusions older than three months had success rates ranging from 70-80%. Individual characteristics therefore within the B or C classification demonstrated success rates of 57% for chronically occluded arteries to 84% for longer lesions or with moderate angulation. Based on this careful study we can say that the B and C lesion categories as originally proposed were quite heterogeneous encompassing both prognostically irrelevant and prognostically very important criteria.

   A key feature of a classification system is reproducibility. This subject was addressed by Angiographic Core Lab of the Bypass and the Angioplasty Investigation (BARI).(8; 9) Seventy angiograms were read a second time by an experienced investigator blinded to his first reading. The reproducibility of the individual features of the lesions was evaluated. Most of the features evaluated were those used to classify lesions in the ACC/AHA system (Figure 4). The Kappa statistic was used to evaluate reproducibility. A k> .60 indicates good reproducibility whereas a k< .40 is fair to poor. There was an excellent ability to separate C from A and B lesions but moderate reproducibility separating A from B from C and moderate reproducibility identifying angulation. There was fair to poor correlation for the other lesion characteristics that were evaluated.

Figure 4

   Interobserver reproducibility of classifying lesions into the ACC/AHA ABC system was reported by Kleiman, Rodriguez and Raisner.(10) Two experienced operators each evaluated one hundred fifty lesions and compared their results. There was agreement in all three grades in only 61% of patients (k= .24). The observers agreed in differentiating C from non-C in 81%. They did not include the B-I and B-II breakdown, so one might expect that if these were used, the agreement of all classes would be worse.

   In spite of these issues, the ACC/AHA classification has been found useful. Zaachs et. al.(11; 12) evaluating patients angioplastied from 1994 to 1996 in a single center showed that success rates in the A, B-I and B-II classes were similar ranging from 96.3 to 95.1%, whereas the success rate with C lesions was 88.2%, (P < than .0001). They found that total occlusion and vessel tortuosity, both criteria for C classification, were predictive of procedure failure. The presence of thrombus, bifurcation lesions, inability to protect a major side branch and degenerated vein grafts were all associated with increased complications. The last three characteristics are used to define C lesions.

From the above one can make the following statements
1) The distinction between C and non-C lesions was reproducible at an acceptable level.
2) Patency was reproducible.
3) The reproducibility of many of the individual features of lesions, especially those that distinguish A from B lesions was fair to poor.

   The data from the Society of Coronary Angiography and Interventions (SCAI) collected in 41071 single vessel interventions performed from 1993 to 6/1996 was used to evaluate the predictive ability of the ACC/AHA lesion classification system.(13) We were especially interested in evaluating the value of making distinctions between the A and B classes. In addition we were anxious to simplify the classification, especially since the more detailed distinctions were poorly reproducible at best. Figure 5 shows the success rates by ACC/AHA lesion class. There is a decreasing success rate with increasing lesion complexity. However, when the classes B and C were divided into the patent and occluded lesions in the class, it is clear that patency has a profound influence on success rates, and that also both B and C classes are quite heterogeneous with regard to risk (Figure 6). There is no difference between the A and the patent B lesions while the success rate of the patent C lesions is actually better than the occluded B. We then restructured the classification into four groups distinguishing only the C or non-C classification and whether or not the lesion was patent (Figure 7).

Figure 5

Figure 6

Figure 7

    Thus while the ACC/AHA classification stratified success in lesions over a range of 97.2% to 84.1%, the SCAI classification graded patent success rate over the broader range from 96.8% to down to 75% for the occluded C lesions (Figure 8). In addition, the SCA&I system classified over 75% of patients into the low risk group which is consistent with expected results. With the AHA/ACC system only 31% of the patients were in the group A.

Figure 8

   The ability of lesion class to predict complications especially the major complications of myocardial infarction, urgent coronary artery bypass grafting or hospital death was more limited. While it may seem on first viewing that the lesion classification can predict complications (Figure 9), on closer analysis several facts become clear. The more complex the lesion, the more acutely ill the patient (Figure 10). Since complications are related to patient acuity, especially the presence of an acute infarction, the apparent predictive ability of lesion classification on complications is spurious. In multivariate analysis, after entering clinical data the lesion class added little to the model predicting complications.

Figure 9

Figure 10

   Dividing the patients into those with or without a myocardial infarction within 24 hours, one can see the different pattern of complications for each class of lesion for patients with or without an MI (Figure 11). In addition, segregation of lesions by vessel patency in the SCAI classification reveals a complicated interaction between patency and death and emergency bypass which is not seen with the ACC/AHA classification. In the non-infarcting patients, complications in class IV are less than with class II, In the ACC/AHA system these would both be lumped together as Class C lesions so this distinction would be lost. (Figure 12). This is due to the lower rate of bypass surgery in the Class IV patients and to a lesser extent, a lower rate of death. In fact if we looked at data for the unsuccessful procedures, grouped by SCAI vessel class, we can see that in the non-infarcting patient, SCAI class IV lesions have the lowest rate of complications, either death or emergency CABG (Figure 13).

Figure 11

Figure 12

Figure 13

   For patients with a myocardial infarction, death is more likely in those with initially occluded vessels, either non-C or C (SCAI class III or IV) although the emergency CABG rate is highest in patients with patent C lesions. These findings are similar to the results presented by Harrell, where clinical factors were preeminent in predicting complications.(14) They found that only lesion category C or non-C had any predictive value.

   Thus we must conclude that the simplified classification scheme, proposed by the Registry Committee of the SCAI13 (Figure 7) produces a better prediction of procedure success and as good a prediction of complications as the more complex ACC/AHA lesion classification system. In addition, the categories appear more homogeneous according to risk and complications.

   In the past 4 to 5 years, an explosion of technology has occurred as industry has met the challenges defined by interventionalists. Balloon technology has progressed dramatically with smaller, low profile balloons; more pushable to access the lesion, more powerful to dilate stents and arteries without bursting. Atherectomy devices, to remove atheromas or break-up calcium have improved success in bifurcation lesions, and in calcified lesions. Stents, which prevent vascular collapse or occlusion and reduce the likelihood of restenosis, have been made more flexible and more easily delivered. Understanding the critical role of platelet activation, and reducing the need for antithrombotic therapy has simplified the post procedure management reducing periprocedural acute and sub-acute occlusion, and nearly eliminated vascular access complications. Effective treatment with platelet receptor IIa/IIIb inhibitors, has essentially neutralized the effect of clot. Other vexing problems seem ripe for solution. Distal embolization is being addressed by several embolic entrapment systems.

   So the question must be asked: with all of this new technology, do lesion characteristics still have a role in risk stratification?

   Unfortunately, problems remain with stent delivery. Tortuous and or calcified proximal vessels impede the advancement of stents and other stiff devices, such as cutting balloons.(11) Small vessels, bifurcation lesions and complete occlusions still remain as challenges. Complication rates have fallen dramatically. Analysis of those features which pose difficulties for stent deployment show that most are features of Class C lesions.(11) With the continued problems treating occluded lesions, it would seem that the SCAI classification based on distinctions between C and non-C, patent and occluded would still have some predictive value.

   To evaluate the value of the lesion classification system in the more modern device era, these analyses were repeated in a more recent population of patients, in procedures performed from 06/96 to 06/99. Stents were placed in over 60% of these patients. Overall, success rates were higher, ranging from 97.9% to 83.2% for SCAI I to IV, and complications were also markedly reduced from, for example, a high of 4.8% for SCAI II in the earlier database to 1.3% for elective patients in SCAI Class II in the most recent database. Similar reductions were seen in all categories. Never the less, the lesion classification permitted a risk stratification, primarily for technical success.

    Ellis et al reviewed the relationship between lesion characteristics and complications of the intervention. They found, after evaluating 27 candidate variables, that 9 were useful to predict complications. They found non-chronic total occlusion, degenerated vein graft, vein graft older than 10 years, lesion length more than 10mm, severe calcium, lesion irregularity, large filling defect, angulations greater than 45 degrees and eccentricity were independently correlated with adverse outcome. However, they did not utilize clinical factors, which have been shown to strongly correlate with anatomic features, especially non-chronic total occlusion (SCAI II). A number of authors have pointed out the importance of clinical factors to determining the complication rate, reducing the impact of lesion classification.(1; 14; 15)

   In many respects, the lesion characteristics identified both by Ellis et al.(16) and Zaack et al.(11) are risk factors for stent placement. In fact, preliminary data from the ACC National Cardiovascular Data Registry (NCDR) suggest that lesion class, especially using the SCAI system, is an excellent predictor of stent usage. Ellis rightly points out the role of filling defects and non chronic occlusion. These features of the lesion are usually associated with acute coronary syndrome or infarction, and the risks of those procedures have been drastically reduced with the widespread use of IIb/IIIa platelet receptor blockers and now enoxaparin.

   The present ACC/AHA classification as simplified by the SCAI Registry committee maintains value to predict angiographic success. It continues to define aspects of a lesion which pose difficulty to the operator even with new techniques and pharmacology. As success rates improve and complications become less frequent, it would seem appropriate to simplify lesion classification rather than make it more complex.

   The use of lesion classification to predict complications is however limited by the overriding role played by the clinical situation. It is important therefore that some correction for the clinical state be used before using a lesion classification system to predict complications.

   In summary, features of the lesion, which are captured in the SCAI classification system (Figure 7), can be utilized to control for anticipated outcomes. The present ACC/AHA classification has within it the features that remain important in determining the outcome of percutaneous coronary intervention procedures. The simplification embodied in the SCAI lesion classification system requires distinguishing only between C and non-C lesions and whether or not the vessel is occluded. It reduces the number of lesion specific criteria to seven. When tested in a large database, such as the SCAI Registry, this simplification predicts intervention success as well or better than the more complex ACC/AHA classification. The classification also makes a small contribution to predicting complications, but here, clinical state is most important.


1. Block PB, Peterson EC, Krone R, Kesler K, Hannan E, O'Connor GT, Detre K: Identification of variables needed to risk adjust outcomes of coronary interventions: Evidence-based guidelines for efficient data collection. J Am Coll Cardiol 1998;32:275-282

2. Ryan TJ, Faxon DP, Gunnar RM, Kennedy JW, King SB, Loop FD, Peterson KL, Reeves TJ, Williams DO, Winters WLJ: Guidelines for percutaneous transluminal coronary angioplasty. A report of the American College of Cardiology/American Heart Association Task Force on Assessment of Diagnostic and Therapeutic Cardiovascular Procedures (Subcommittee on Percutaneous Transluminal Coronary Angioplasty). Circulation 1988;78:486-502

3. Krone RJ, Vetrovec GW, Noto TJ, Johnson LW, and the Registry Committee(SCA&I).: Procedure complexity and outcome from the Registry of the Society for Cardiac Angiography and Interventions. Circulation 1993;88:I-300(Abstract)

4. Faxon DP, Kelsey SF, Ryan TJ, McCabe CH, Detre K: Determinants of successful percutaneous transluminal coronary angioplasty: Report for the National Heart, Lung, and Blood Institute Registry. Am Heart J 1984;108:1019-1023

5. Ellis SG, Vandormael MG, Cowley MJ, and the POSCH Group.: Coronary morphologic and clinical determinates of procedural outcome with angioplasty for multivessel coronary disease: implications for patient selection. Circulation 1990;82:1193-1202

6. Myler RK, Shaw C, Stertzer SH, Hecht HS, Ryan C, Rosenblum J, Cumberland DC, Murphy MC, Hansell HN, Hidalgo B: Lesion morphology and coronary angioplasty: current experience and analysis. J.Am.Coll.Cardiol. 1992;19:1641-1652

7. Tan K, Sulke N, Taub N, Sowton E: Clinical and lesion morphological determinants of coronary angioplasty success and complications: Current experience. J.Am.Coll.Cardiol. 1995;25:855-865

8. Rosen AD, Detre KM, Alderman EL, Stadius M, Sopko G, and the Bypass Angioplasty Revascularization Investigation (BARI) Study Group.: How reliable is the assessment of coronary angiography? Circulation 1993;88 (Suppl I):I 653(Abstract)

9. Botas J, Stadius ML, Bourassa MG, Rosen A, Schaff HV, Sopko G, Williams DO, Alderman EL, and the BARI Investigators.: Angiographic correlates of lesion relevance and suitability for Percutaneous Transluminal Coronary Angioplasty and Coronary Artery Bypass Grafting in the Bypass Angioplasty Revascularization Investigation Study (BARI). Am J Cardiol 1996;77:805-814

10. Kleiman NS, Rodriguez AR, Raizner AE: Interobserver variability in grading of coronary arterial narrowing using the American College of Cardiology/American Heart Association grading criteria. Am J Cardiol 1992;69:413-415

11. Zaacks SM, Allen JE, Calvin JE, Schaer GL, Palvas BW, Parrillo JE, Klein LW: Value of the American College of Cardiology/American Heart Association stenosis morphology classification for coronary interventions in the late 1990s. Am.J.Cardiol. 1998;82:43-49

12. Zaacks SM, Klein LW: The AHA/ACC task force criteria: what is its value in the device era? American Heart Association/American College of Cardiology [editorial; comment]. Cathet.Cardiovasc.Diagn. 1998;43:9-10

13. Krone RJ, Laskey WK, Johnson C, Kimmel SE, Klein LW, Weiner BH, Cosentino JJA, Johnson SA, Babb JD, for the Registry Committee of the Society for Cardiac Angiography and Interventions.: A simplified lesion classification for predicting success and complications of coronary angioplasty. Am J Cardiol 2000;85:1179-1184

14. Harrell L, Schunkert EH, Palacios IF: Risk predictors in patients scheduled for percutaneous coronary revascularization. Cathet Cardiovas Intervent 1999;48:253-260

15. Block PB: Doing it-Where's the risk? Cathet Cardiovas Intervent 2000;48:261-261

16. Ellis SG, Guetta V, Miller D, Whitlow PL, Topol EJ: Relation Between Lesion Characteristics and Risk With Percutaneous Intervention in the Stent and Glycoprotein IIb/IIIa Era
An Analysis of Results From 10,907 Lesions and Proposal for New Classification Scheme . Circulation 1999;100:1971-1976


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