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Ann Thorac Surg 1998;66:1045-1049
© 1998 The Society of Thoracic Surgeons
a Cardiovascular Division, Department of Medicine, UCSF/Stanford Health Care, Stanford, California, USA
Presented at "Facts and Myths of Minimally Invasive Cardiac Surgery: Current Trends in Thoracic Surgery IV," New Orleans, LA, Jan 24, 1998.
Abstract
Despite extraordinary growth in percutaneous transluminal coronary angioplasty (>400,000 cases in United States in 1997) patients are still routinely referred for bypass grafting in large numbers. Why? Second-generation devices (directional coronary atherectomy, high-speed rotational atherectomy [Rotablator], and stents) have expanded the application of percutaneous catheter treatment of coronary disease. Specifically, highly eccentric lesions in large vessels, heavily calcified lesions, and coronary dissections can be effectively treated with these devices. Stents have substantially reduced the incidence of restenosis, but this benefit is largely confined to vessels more than 3 mm in diameter and stenoses less than 20 mm in length. A third generation of coronary devices has evolved in the late 1990s in response to continuing failures of conventional balloon angioplasty, atherectomy, and stenting. The failures of the 1990s were (1) restenosis, including in-stent restenosis, (2) chronic total occlusions, (3) diffuse small-vessel disease, and (4) aged vein graft disease. In response to these challenges novel devices are being developed: (1) for restenosis, intracoronary radiation therapy (brachytherapy); (2) for chronic total occlusions, Prima Laser wire; (3) for diffuse small-vessel disease, percutaneous myocardial laser revascularization; and (4) for aged vein grafts, antiembolization devices. Each of these new catheter technologies will need to be economically and clinically reconciled with the multitude of minimally invasive surgical revascularization techniques that are rapidly evolving.
Percutaneous coronary angioplasty was performed on more than 400,000 Americans in 1997. Despite this broad application of catheter-based coronary interventional procedures, an equal number of patients were referred for surgical revascularization. Why have cardiologists been unable to further apply their percutaneous strategies into the surgical population of patients? The failures of conventional balloon angioplasty are largely derived from the naive concept that concentric balloon expansion can adequately treat the markedly inhomogeneous character of human coronary atherosclerosis.
Failures of the 1980s
Although concentric soft plaque can be easily dilated by percutaneous transluminal coronary angioplasty (PTCA), many plaque morphologies have unpredictable and frequently inadequate results from balloon angioplasty. Coronary lesions that are unfavorable for simple balloon angioplasty include those with high degrees of eccentricity, heavy calcification, ostial locations (particularly right coronary artery and left main coronary artery), complete chronic occlusion, and locations in an angulated segment. Balloon angioplasty of such lesions is not associated with the 95% success rates seen with simple stenoses.
The early years of PTCA were associated with mandatory surgical stand-by. Abrupt closure after routine PTCA occurred in 5% to 6% of patients and was directly related to vessel wall disruption, spasm, and platelet aggregation. Before the advent of stents, emergency surgical revascularization was frequently the only remedy for this somewhat unpredictable event. Although abrupt closure was a serious limitation of PTCA, the greatest failing of conventional balloon angioplasty was and remains restenosis; a common response to vessel wall trauma, this complex sequence of elastic recoil, shrinkage, and smooth muscle cell proliferation leads to symptomatic recurrence in 30% to 50% of patients.
After the first 5 years of balloon angioplasty, a second generation of coronary devices were developed in response to the manifest failure of conventional balloon angioplasty. The concept of "coronary atherectomy" was introduced by John Simpson in the mid-1980s as a more logical approach to treating coronary plaque. The Simpson Directional Coronary Atherectomy Device (DCA; Guidant Corp, Santa Clara, CA) was promoted as a method to actually shave and remove plaque from the artery, potentially avoiding the barotrauma associated with balloon angioplasty. It was Simpson’s hope that atherectomy would not only relieve the coronary obstruction but would be associated with less restenosis. The configuration of the eccentric cutting mechanism made this device particularly attractive for proximal eccentric stenoses. A multicenter, randomized trial of DCA versus PTCA (CAVEAT I) [1] failed to demonstrate a mitigating effect of DCA on restenosis; however, subsequent trials in highly motivated DCA centers have shown that restenoses rates can be as low as 10% to 15% in the hands of a few diligent operators [2, 3].
High-speed rotational atherectomy was introduced in the early 1980s by David Auth as a percutaneous device that was particularly suited to heavily calcified or inelastic lesions. Although considered an atherectomy device, Rotablator (Boston Scientific Corp, Natick, MA) does not actually remove atheroma. The metallic burr is coated with industrial diamond chips. Rotating at speeds of 140,000 to 190,000 rpm, the burr pulverizes plaque into particulate debris. Most of the particles are <10 µm, and pass through the myocardial capillaries without obstructing flow. Now used in approximately 5% to 10% of patients, it is the only device that can effectively treat a heavily calcified stenosis. Restenosis rates have been unaffected by high-speed rotational atherectomy and are routinely in the range of 40% to 50% [4].
Excimer laser angioplasty (Spectranetics, Colorado Springs, CO) was initiated in the mid-1980s as an alternative "atherectomy" device. Delivered over a guidewire, a concentric bundle of optical fibers would render pulsed high-density laser energy to the plaque in an attempt to vaporize the obstructive lesion. Although particularly attractive for debulking total occlusions, the attendant acoustic and thermal injury associated with excimer angioplasty led to frequent restenosis. Compared with simple balloon angioplasty, excimer laser angioplasty has little demonstrable advantage [5].
The advent of coronary stents in 1987 led to the first demonstration of a nonballoon device with an antirestenosis effect [6]. Not only did stents have the immediate benefit of stabilizing dissections and obviating the need for intensive surgical standby, but they were shown to reduce restenosis rates to the range of 20% to 25% in two large-scale randomized trials completed in 1994 [7, 8]. The ability of stents to reduce restenosis has been demonstrated in de novo coronary lesions and vein grafts; however, the beneficial effect appears to be limited to larger vessels (>3 mm) and short lesions (<25 mm). Stents are now routinely implanted in 50% to 75% of interventional procedures. The majority of stent implantation is "off-label"; use of stents in long lesions, small vessels, and chronic total occlusions has had the validation of large randomized clinical trials.
Failures of the late 1990s
The first stent decade concluded with considerable optimism. A variety of stents are now available with enhanced characteristics; however, the need for a third generation of coronary devices is clear. These investigational devices are being developed in a continuing quest for a truly "minimally invasive" approach to coronary disease. Their development is focused on the major unresolved failures of balloon angioplasty, atherectomy, and stenting: (1) restenosis, including in-stent restenosis; (2) chronic total occlusions; (3) diffuse small-vessel coronary disease; and (4) aged vein graft disease.
These pathologies are represented in the large group of patients who either have failure of catheter-based interventions or are generally referred directly for surgical revascularization. In response to these challenges, investigational devices are being evaluated at limited number of centers in the United States and Europe.
Restenosis
Although stents have had a substantial effect on minimizing the incidence of restenosis, this benefit has been largely confined to short stents in large vessels. Restenosis after angioplasty, atherectomy and stenting remains the major failure mode for catheter intervention and one of the primary indications for secondary surgical referral.
Brachytherapy has been touted as a potential solution to the restenosis problem. Intracoronary radiation therapy can be easily delivered with catheter technology [9]. Using both gamma and beta sources, catheter systems have been devised that deliver and retrieve radioactive pellets or wires at the site of coronary intervention. High-dose local irradiation can be delivered with dwell times of less than 5 minutes. Doses in the range of 3,000 to 5,000 cGy are easily accomplished. Animal studies have demonstrated significant reduction in the proliferative component of restenosis after intracoronary brachytherapy [10, 11]. Limited human data support the thesis that these radioactive devices have the potential to significantly retard restenosis with in-stent restenosis [12]. Large-scale randomized trials using beta sources (phosphorus 32 and strontium 90) are underway at several centers in the United States. Follow-up data should be available by late 1998. It is anticipated that brachytherapy may bring restenosis rates to less than 10%.
Chronic total occlusions
The inability to cross a total occlusion with a guidewire precludes application of most catheter-based interventions: balloon catheters, atherectomy devices, and stents are all delivered into the treatment site over a guidewire. Total occlusions that have a chronicity beyond 3 to 6 months are exceedingly difficult to cross with conventional guidewires. Success rates are routinely less than 50% [13, 14]. The Prima Laser Guidewire (Spectranetics) was introduced as an investigational wire for crossing chronic total occlusions. As illustrated in Figure 1 , the device consists of an 0.018-inch hypotube containing a bundle of 45-µm optical fibers coupled to a pulsed excimer laser operating a tip fluence of 60 mJ/mm2. The laser wire can be torqued and redirected as it ablates its way through a densely occluded artery. This investigational device has been extensively used in Europe and the United States and has facilitated successful crossing of more than 50% of lesions that could not be crossed with conventional angioplasty wires [15, 16]. Success has been related to length (<20 mm) but not chronicity of the total occlusion. Once the laser wire has passed into the distal segment of the occluded artery, the total occlusion can then be addressed with conventional "over the wire" devices including PTCA, atherectomy, and stenting. The inability to treat a chronically occluded major epicardial artery (in the absence of infarct) represents a major indication for elective surgical referral. The promise of a reliable device for crossing chronic total occlusion gives hope that a substantial number of patients will eventually realize total revascularization with catheter-based technology.
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Aged vein graft disease
Aged vein grafts have been associated with catastrophic complications after angioplasty and stenting [22]. Distal embolization with "no-reflow," myocardial infarction and occasional death are frequent and unpredictable complications of catheter interventions within vein grafts. Unlike native coronary disease, graft disease is frequently dislodged leading to catastrophe. Patients with old vein grafts and nondiscrete disease are commonly referred directly for operation by cardiologists hesitant to encounter the risk of embolization.
A percutaneous system for preventing distal embolization during vein graft intervention has been reported recently [23]. Figure 3 illustrates the strategy of this antiembolization scheme. A 0.014-inch GuardWire (PercuSurge Inc, Sunnyvale, CA) is advanced across the stenosis and positioned in the distal graft. The distal portion of the GuardWire incorporates an elastomeric balloon that is inflated under low pressure, occluding the distal vessel and effectively blocking subsequent debris from downstream embolization. Standard catheter interventions (angioplasty, stenting) can be carried out over this GuardWire. After removal of the interventional device, an aspiration catheter is passed, and debris aspirated before deflation of the GuardWire balloon. Pilot studies in Canada have demonstrated the feasibility of using this system and the ability to retrieve significant debris that would otherwise embolize [24]. A randomized trial of this protection system will be initiated in late 1998.
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Although promising, these third-generation coronary devices are still investigational; their clinical benefits remain to be confirmed. Their ultimate threat to surgeons needs to be reconciled, both clinically and economically, in context with the rapidly evolving techniques for minimally invasive surgical revascularization. These catheter-based devices are derived from high technology and carry a potentially high price tag. Surgical revascularization will always have the attraction of potentially low cost—venous and arterial conduit are free. If the cost of enabling technology for minimally invasive direct coronary artery bypass grafting can be diminished, interventional cardiologists may find themselves and their new generation of devices lost in the eclipse of economic imperatives.
References
This article has been cited by other articles:
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  M. H. Rubenstein, L. C. Harrell, B. V. Sheynberg, H. Schunkert, H. Bazari, and I. F. Palacios Are Patients With Renal Failure Good Candidates for Percutaneous Coronary Revascularization in the New Device Era? Circulation, December 12, 2000; 102(24): 2966 - 2972. [Abstract] [Full Text] [PDF]
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 M. J. Mack Is there a future for minimally invasive cardiac surgery? Eur. J. Cardiothorac. Surg., November 1, 1999; 16(suppl_2): S119 - S125. [Abstract] [Full Text] [PDF]
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