Percutaneous transluminal angioplasty has been shown to be both feasible and efficacious for the treatment of aortic coarctation. Recent reports, however, have indicated that the development of aortic aneurysms at or near the coarctation segment may complicate attempts to treat this lesion by catheter-based intervention. Accordingly, we examined the light microscopic features of coarctation segments excised at surgery (n = 31) or obtained at autopsy (n = 2) in 33 patients with coarctation of the aorta. Cystic medial necrosis, defined as depletion and disarray of elastic tissue, was observed in each of the 33 specimens. In the majority of coarctation specimens (22 of 33 or 67%) the extent of cystic medial necrosis, graded semiquantitatively on a scale of 0 (normal aorta) to 3 +, was severe (3 + ). The finding that cystic medial necrosis represents a consistent histologic feature of coarctation of the aorta provides a pathologic basis for the formation of aneurysms observed after balloon angioplasty of coarctation sites.Circulation 75, No. 4, [689][690][691][692][693][694][695] 1987. SINCE Gruintzig's successful development of percutaneous transluminal coronary angioplasty,l catheterbased interventional techniques have been applied to an increasingly wider assortment of cardiovascular disorders. Preliminary trials of pulmonary,2 mitral,3 and aortic4 balloon valvuloplasty have yielded encouraging results. While the initial experience with balloon dilation for coarctation of the aorta was reported to be similarly favorable,5-9 recent postangioplasty followup studies in such patients have indicated that aneurysm formation at or near the coarctation segment may be a consequence of this intervention.'0' 11 Several lines of evidence have strongly suggested that aneurysm formation after coarctation angioplasty might be related to underlying aortic histopathology. First, cystic medial necrosis has been observed as a predictable consequence of aortic banding in a canine preparation of cardiac hypertrophy.'2 Second, before

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The carbon dioxide (CO2) laser has been utilized for preliminary intraoperative cardiovascular applications, including coronary endarterectomy and ventricular endocardiectomy. CO2 lasers used for these applications have been operated in the continuous wave, chopped or pulsed mode at low peak powers. To evaluate the extent of boundary tissue injury, continuous, chopped and pulsed energy delivery of CO2 laser emission was used to bore through 192 5 mm thick myocardial slices in air. Continuous, chopped and pulsed delivery at a peak power of 500 W or less failed to eliminate light microscopic or ultrastructural signs of thermal injury. Only when a high energy CO2 laser (pulse energy 80 to 300 mJ, pulse duration 1 microseconds) was used at a peak power greater than 80 kW were all signs of thermal injury eliminated; furthermore, high peak power prevented thermal injury only when the beam was focused to achieve a peak power density greater than 60 kW/mm2. Under these conditions, pathologic findings were identical to those observed using excimer wavelengths. The results of these experiments indicate that: conventional CO2 lasers fail to minimize boundary tissue injury, elimination of thermal injury during intraoperative laser ablation requires that CO2 laser energy be focused to achieve a peak power density greater than 60 kW/mm2, and elimination of thermal injury can be achieved at a variety of wavelengths, provided that an appropriate energy profile is employed.