CLINICAL STUDIES
Optimal balloon angioplasty versus elective stent implantation*
Antonio Colombo, MD, FACCa and
Carlo Briguori, MDa
a Centro Cuore Columbus, Milan, Italy
Reprint requests and correspondence: Antonio Colombo, Centro Cuore Columbus, Via M. Buonarroti 48, I-20145, Milan, Italy
columbus{at}micronet.it
It is a constant challenge to the physician performing percutaneous transluminal coronary angioplasty (PTCA) to judge when a PTCA result is adequate to relieve symptoms and provide a low likelihood of restenosis and when the result is inadequate and further dilations or devices are necessary. The ability of stents to limit dissections and restenosis is acting as the true catalyst for a continuous increase in elective stenting. However, several reports have highlighted the discrepancies between patients undergoing stent insertion in the "real world" and the select group of patients studied in a randomized trial (1). Furthermore, stent implantation has engendered a new and clinically frustrating entity—in-stent restenosis. Routine stent implantation is also more expensive than balloon angioplasty.
Powerful evidence for the improved efficacy of PTCA is apparent on examination of the clinical outcomes of angioplasty in the balloon arms of some recently completed trials (2). A strategy of more "aggressive" balloon dilation, with the limited use of "provisional" stenting for complicated or suboptimal balloon results, may be highly effective (3). At present, the critical question is, how do we prospectively identify which patients treated with successful angioplasty will benefit from adjunctive stent placement? In an attempt to answer this question, in this issue of JACC, van Liebergen et al. (4) give us important information. They demonstrated in a small and select sample that after angiography-guided PTCA, adjunctive intravascular ultrasound (IVUS)-guided balloon angioplasty induced an additional increase in hyperemic blood velocity related to a reduction of residual lumen obstruction. Subsequent elective stent implantation resulted in a further increase of coronary lumen dimensions, whereas the hyperemic blood flow velocity variables remained unchanged, indicating the absence of a functional residual lumen obstruction after IVUS-guided balloon angioplasty. The authors (4) suggest the presence of a plateau phase of the optimal hemodynamic response in relation to the residual lumen obstruction, which can be achieved using IVUS-guided balloon angioplasty. With IVUS-guided balloon angioplasty, the balloon-to-artery ratio increased from 1.07 ± 0.14 to 1.35 ± 0.21, a value close to that reported in the Core LaboratOry UlTrasound analysis (CLOUT) study (5). A 50% increase in early gain produced a 44% decrease in percent diameter stenosis. Of note, this "aggressive" balloon angioplasty strategy did not increase the incidence of major dissections. This study gives us the chance to make some consideration on how to address the problem of "provisional" stenting.
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IVUS-guided balloon angioplasty
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To achieve a larger postprocedural minimal lumen diameter (MLD), proper sizing of the balloon to the arterial segment to be dilated by the selective use of IVUS may be a strategy to enhance balloon angioplasty. Intravascular ultrasound guidance facilitates decision-making in terms of balloon sizing, especially in angiographically small vessels that might be large vessels with diffuse atherosclerosis (6). We recently found that the ratio between the angiographic and IVUS reference diameter is significantly lower in small vessels than in large vessels (0.7 vs. 0.8, p < 0.001), supporting the concept that angiographically small vessels might in fact be large vessels with diffuse atherosclerosis (7). As a consequence, balloons chosen for PTCA based on IVUS measurements appear to be oversized angiographically. This discrepancy between IVUS and angiographic vessel size is explained on the basis of a large plaque burden. The application of balloons sized larger than the true vessel size (external elastic membrane) would be expected to result in major arterial disruption and a higher risk of dissections. Even if this complication was not frequently seen in the IVUS-guided PTCA studies (5,6), the occurrence of dissections should not necessarily be seen as a significant complication owing to the easy availability of stents.
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Physiologic guidance of balloon angioplasty
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Physiologic guidance of an intervention with intracoronary Doppler (8–10) and pressure measurement (11) is a new concept to optimize the results. The coronary Doppler guide wire was the first practical method of physiologic assessment of coronary stenosis severity applicable in the catheterization laboratory, correlating to the results of noninvasive nuclear stress tests. The last 20 years have seen rapid changes in intracoronary Doppler technology, with progressive miniaturization of the transducers and refinement of signal analysis. The large piezoelectric crystals mounted at the tip of standard 8F Sones or Judkins catheters have been replaced by circular transducers attached to 3F intracoronary catheters and, more recently, by a miniaturized crystal at the tip of 0.018-in. and 0.014-in. guide wires. The real-time spectral analyzer using on-line fast Fourier transformation provides a gray-scale spectral display. The system software automatically tracks the instantaneous peak velocity and calculates on-line the average peak velocity over two consecutive beats. The Doppler wire can be detached from the rotary connection so that PTCA balloons or other interventional devices can be advanced over it. The wire can then be reconnected to monitor the flow velocity changes during and immediately after PTCA and to repeat the flow reserve assessment after treatment.
Measurements of coronary flow reserve (CFR) (i.e., the ratio of coronary flow under maximal coronary dilation to coronary flow under rest conditions) using the Doppler guide wire represent a potential end point on which to base the decision of whether to provisionally stent a lesion after balloon angioplasty has been completed.
In the study of van Liebergen et al. (4), CFR increased after standard balloon angioplasty from 1.5 ± 0.6 to 2.6 ± 0.7 toward the CFR measured in the normal adjacent coronary artery (3.2 ± 0.7, relative CFR 0.83 ± 0.22). Adjunctive IVUS-guided balloon angioplasty resulted in a further increase in hyperemic blood flow velocity, whereas adjunctive stent implantation did not yield a further gain in the hyperemic blood flow velocity, indicating the absence of a functional residual lumen obstruction after IVUS-guided balloon angioplasty. The lumen area after IVUS-guided balloon angioplasty was 10.24 ± 2.22 mm2. From these data, we can extrapolate that 5.80 mm2 (the lowest lumen area according to the standard deviation) was not flow limiting. Abizaid et al. (12) recently demonstrated that an IVUS minimal lumen area  4.0 mm2 had a diagnostic accuracy of 89% in identifying a CFR 2.0. Thus, a further increase in the lumen area by stent implantation may not necessarily produce an increase in CFR. However, it should be noted that in the study of van Liebergen et al. (4), the CFR after IVUS-guided balloon angioplasty was 2.69 ± 2.22; this means that the lowest CFR was 1.1. This stresses the Achilles’ heel of Doppler velocity measurements—that is, the large overlap between normal and abnormal values inherent to every method using absolute CFR as an end point. Reported threshold values of CFR for clinical decision-making range from as low as 1.8 to 3 (8,13). The wide scatter of threshold values created a relative uncertainty among groups currently using the technology and has deterred those who were about to implement physiologic measurements. To improve the diagnostic accuracy of the test and to identify false positive flow reserve measurements (normal scintigraphy with CFR <2), measurement of flow reserve in a second artery without significant stenosis has been advocated. The relative CFR is the ratio between CFR in the artery under evaluation and CFR in the normal artery (13). The disadvantage of the concept lies in the necessity to perform another measurement in a reference vessel.
Significant changes in baseline flow can be assumed to explain why the CFR may remain depressed after angioplasty or even after stent implantation. An increase in baseline flow may occur because of removal of the epicardial stenosis, repetitive administration of vasodilating drugs, endogenous vasodilatory mediators in response to the angioplasty procedure, suppression of vasoconstrictive substance, endothelium-dependent vasodilation and hemodilution (14). Finally, impairment of autoregulatory mechanisms may cause an increased rest flow. Prolonged vasodilation of the coronary vessels resulting from low flow perfusion may impair the autoregulatory adaptive process. Abrupt restoration of normal perfusion may then fail to induce appropriate regulatory adaptation, resulting in hyperemic rest flow. In contrast, the maximal increase in hyperemic blood flow might be impaired in response to repetitive episodes of ischemia during balloon angioplasty (14). In addition, the maximal vessel stretch produced by stenting could impair the effective dilation of the microvascular bed, explaining the lack of further improvement in CFR despite an increase in lumen gain (15). A different explanation for the reduction of flow reserve after PTCA or stenting could be the damage produced by particulate embolization occurring during the procedure. Of note is that CFR determined directly after coronary angioplasty and CFR determined after six months in the presence of an unchanged angioplasty result may dissociate, depending on the improvement or the worsening of the mocrovascular bed (14).
Recently, the value of intracoronary Doppler measurements for assessment of coronary stenosis severity has been challenged by the concept of the myocardial fraction flow reserve (FFR), which is based on intracoronary pressure measurement (11). Fraction flow reserve is defined as the ratio of the hyperemic flow in the stenotic coronary artery to the hyperemic flow in that same artery (or territory) in the hypothetical case in which the epicardial vessel were completely normal. In contrast to the CFR, the concept of FFR has presented a threshold value and does not seem to be affected by the status or the responsiveness of the distal vascular bed. An FFR value <0.75 indicates a hemodynamically significant stenosis with high accuracy and reproducible induction of myocardial ischemia on the basis of stress tests and thallium scintigraphy.
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Long-term outcome and restenosis rate
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Restenosis is a complex process involving vessel recoil, remodeling and intimal smooth muscle cell hyperplasia. The beneficial effects of stent implantation on restenosis have been attributed to: 1) the larger early lumen dimension achieved; and 2) the positive effect on chronic recoil and remodeling. Early recoil accounts for a 50% loss in early lumen gain during standard balloon angioplasty. At present there are only preliminary and indirect data on the relation between optimal (both anatomic and functional) balloon angioplasty and chronic elastic recoil and vessel remodeling (9–11). Preliminary results of the Doppler Endpoints Stent International Investigation (DESTINI) trial (9) indicate that when optimal angiographic and physiologic end points (e.g., final residual diameter stenosis <35%, final CFR >2.0) are met after PTCA, the early and late clinical outcomes are equivalent to the outcomes observed after elective stent implantation. It is interesting to note that in the PTCA group only 43% of patients could achieve the predetermined end points. This limitation may be related to the angiographic and non–IVUS-guided balloon sizing used in the DESTINI trial.
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Clinical implications
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A possible clinical implication of this emerging field of IVUS-guided PTCA and physiologic-guided interventions is that after optimal "guided" PTCA, further stenting may not be necessary. An alternative conclusion could be that because of the fact that elective stenting gives the same results of optimal "guided" PTCA, this last modality may not be necessary, provided elective stenting is applied routinely.
A few other important aspects need to be considered: 1) if it is realistic to assume that in lesions located on small vessels and bifurcational lesions coronary stenting may perform inferiorly to PTCA, the need to avoid elective stenting may be appropriate and alternative solutions such as IVUS-guided angioplasty will be welcome; 2) in long lesions the use of long stents may perform inferiorly as compared with spot stenting and adjunctive PTCA (16,17); IVUS guidance will therefore become an important tool for decision-making; and 3) a strategy of elective stenting needs to be evaluated, taking into account the changing cost of stenting, IVUS catheters and procedural time.
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Footnotes
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* Editorials published in the Journal of the American College of Cardiology reflect the views of the authors and do not necessarily represent the views of JACC or the American College of Cardiology. 
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References
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The meaning of suboptimal coronary flow reserve after coronary balloon angioplasty
Eur. Heart J.,
January 2, 2002;
23(2):
99 - 100.
[Full Text]
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IVUS-guided balloon angioplasty
Physiologic guidance of balloon...