Beta carotene uptake into atherosclerotic plaque: Enhanced staining and preferential ablation with the pulsed dye laser

The yellow color of atherosclerotic plaque is due to the presence of carotenoids, which absorb light between 430–530 nm and account for the preferential ablation of plaque by the pulsed dye laser operating at 480 nm. This study was designed to examine tissue uptake of β-carotene and the effect of uptake on arterial plaque ablation. Forty-two atherosclerotic NZW rabbits were given intravenous β-carotene at a dose of 40 mg/kg, twice weekly and killed between 1 hour and 28 days after the initial injection. β-carotene was not detected in control specimens but was significantly greater in plaque than in normal wall at all time points following β-carotene injection (P < 0.04 Mann Whitney U test). The ablation threshold was significantly lower in β-carotene treated plaque than in untreated plaque or normal arterial wall (P < 0.01, Fisher's exact test). In this model β-carotene is preferentially taken up into arterial plaque, resulting in increased absorption of laser radiation at 480 nm and enhanced tissue ablation. © 1993 Wiley-Liss, Inc.     

Acoustic and plasma-guided laser angioplasty

The feasibility of using acoustic and plasma-guided laser (APGL) for angioplasty was studied in vitro. A flashlamp-pumped tunable dye laser operating at a wavelength of 504 nm (coumarin green) was used as the laser source. Acoustic signals were recorded with a hydrophone, which has a useful frequency response of up to 350 kHz. Plasma optical emissions were transmitted retrograde along the laser fiber and reflected through a beam splitter to an optical detection system consisting of a series of spectral filters (to transmit plasma radiation from 380 nm to 440 nm and block any 504 nm laser light) and a photomultiplier tube. Measurements of the acoustic and the plasma optical signals were obtained from blood, atheromatous plaque, and normal arterial wall. Results of monitoring show that it is possible to know without direct vision whether the laser energy is being discharged in the lumen (blood), on the normal arterial wall, or on the atheromatous plaque. Blood produced strong acoustic signals but no plasma signals; plaque produced strong plasma and strong acoustic signals. Neither plasma nor significant acoustic signals were produced by the normal arterial wall. These distinctions may allow clinical laser ablation of plaque to be performed with fewer complications.     

Photocoagulation with a copper vapour laser in rabbit liver: A comparative study between the 510 nm and the 578 nm wavelengths

The coagulation effect, penetration depth and healing process of the 510.6 nm (green) and 578.2 nm (yellow) wavelengths of copper vapour laser (CVL) were compared in vivo in rabbit liver (n=15). A pulsed CVL, the Cu 15 from Oxford Laser—pulse repetition 10 kHz, peak-power 70 kW, pulse width 25 ns, and average maximal power 16W—was used connected to a dichroic system. The beam was transmitted through a 1000 μm quartz fibre and focused with a handpiece providing a 2 mm diameter spot size. By means of this delivery system 270 focused lesions are achieved at a power output of 2.65 W (power density 80 W cm⁻²) with irradiation times of 3, 5 and 10 s. The operative and microscopic verifications were achieved at 0 hour, and on days 3, 10, 20 and 30. Immediately after laser application, the lesions were triangular, well demarcated, and characterized by a central vaporization surrounded by four peripheral zones: carbonization; coagulation; oedema; and transition. The penetration depth was noticeably bigger in the yellow wavelength than with green wavelength, evidenced on day 10 by superior size of yellow wavelength photocoagulations and coagulation necrosis. Fibrosis appeared by day 3 and was gaining ground quickly and intensively after yellow wavelength while the fibrotic reaction was delayed on day 10 after green wavelength. The more penetrating effect of yellow wavelength advocates for its use in liver tumour destruction and photoradiation therapy while the green wavelength, inducing less aggressive effect on the surrounding tissue, seems more suitable for liver resection.     

Discrimination of normal and atherosclerotic aorta by laser-induced fluorescence

Precise targeting of laser energy to atherosclerotic plaque is crucial for the safe performance of laser angioplasty. The present study was designed to evaluate whether laser-induced fluorescence could distinguish atherosclerotic from normal aorta. Segments of human aorta obtained at necropsy were classified grossly and histologically as normal aorta (n = 7), thin yellow fatty plaque (n = 5), and thick white atheromatous plaque (n = 9), and analyzed by laser-induced fluorescence spectroscopy using a pulsed nitrogen laser. Fluorescence spectra were recorded over a wavelength range from 385 to 725 nm. Normal specimens had maximal fluorescence intensity at 514 nm. A prominent fluorescence peak at 448 nm was present in specimens characterized as white atheromatous plaque and at 538 nm in specimens characterized as yellow fatty plaque. The ratios of fluorescence intensity at 448 nm/514 nm and at 538 nm/514 nm correctly classified all specimens according to their gross and histologic type (p < .001). Thus, a “smart” laser angioplasty catheter system might incorporate low-power laser radiation for arterial fluorescence spectroscopy to guide delivery of high-power laser radiation for plaque ablation.     

Fluorescence spectroscopy guidance of laser ablation of atherosclerotic plague

Laser-induced fluorescence (LIF) spectroscopy can only be used for laser angioplasty guidance if high-power laser ablation does not significantly alter the pattern of tissue fluorescence. Although the spectra of normal and atherosclerotic arteries differ, the change in fluorescence spectra following laser angioplasty has not been well studied. Therefore, the purpose of this study was to assess whether laser-induced fluorescence spectroscopy could guide selective laser ablation of atherosclerotic plaque and, if so, to develop a quantitative LIF score that could be used to control a "smart" laser angioplasty system. Baseline LIF spectroscopy of 50 normal and 50 atherosclerotic human aortic specimens was performed using an optical fiber coupled to a He-Cd laser and optical multichannel analyzer. LIF was then serially recorded during erbium:YAG laser ablation of 27 atherosclerotic specimens. Laser ablation was terminated when the arterial LIF spectrum visually appeared normal. Histologic analysis revealed a mean initial plaque thickness of 1,228 +/- 54 microns and mean residual plaque thickness of 198 +/- 27 microns. Ablation of the media occurred in only three specimens. A discriminant function was derived to discriminate atherosclerotic from normal tissue for computer guidance of laser angioplasty. The LIF score, derived from stepwise multivariate linear regression analysis of the LIF spectra, correctly classified 93% of aortic specimens. The spectra obtained from the atherosclerotic specimens subjected to fluorescence-guided laser revealed a change in score from "atherosclerotic" to "normal" following plaque ablation. Seven atherosclerotic specimens were subjected to laser angioplasty with on-line computer control using the LIF score. Mean initial plaque thickness was 1,014 +/- 86 microns, and mean residual plaque thickness was 78 +/- 29 microns. There was no evidence of ablation of the media. Therefore, LIF guidance of laser ablation resulted in minimal residual plaque without arterial perforation. These findings support the feasibility of an LIF-guided laser angioplasty system for selective atherosclerotic plaque ablation.     

In vitro Alexandrite laser angioplasty: A study on fibre conduction and tissue effects

Pulsed ultra-violet excimer laser radiation is capable of tissue ablation with only minimal thermal injury of adjacent tissue structures. Since difficult fibre optic coupling of energy was observed, alternative Q-switched laser sources capable of ablation of atherosclerotic plaque are under current investigation. To evaluate tissue effects of Alexandrite laser radiation, 160 arterial segments with macroscopic evidence of atherosclerotic disease were treated. The laser light was transmitted via silica based quartz fibres with different diameters. Using the Q-switched Alexandrite laser at the fundamental wavelength (748 nm) with a pulse duration of 300 ns the energy density threshold for tissue ablation was found to be in the range of 63 to 126 J cm^–2 using a 300m fibre. On macroscopic examination only limited thermal and acoustic injury was found in crater adjacent tissue structures. Crater edges were even and did not reveal signs of crater charring or debris in the crater lumen. However, the histological cross-sections revealed thermal injury extending from 100 up to 200m lateral into adjacent tissue. The crater margins revealed fissuring as a result of shock wave injury. Thermal damage was most evident if irradiation of atherosclerotic tissue was performed in blood.     

Autofluorescence spectroscopy using a XeCl excimer laser system for simultaneous plaque ablation and fluorescence excitation

Laser–induced fluorescence may be used to guide laser ablation of atherosclerotic lesions. This study was performed to evaluate arterial autofluorescence spectroscopy in vitro using a single XeCl excimer laser (308 nm) for simultaneous tissue ablation and fluorescence excitation. The laser beam was coupled to a 600-μm silica fiber transmitting 40–50 mJ/mm² per pulse. The fluorescence radiation emanating retrogradely from the fiber was collected by a concave mirror for spectroscopic analysis over a range of 321–657 nm. The arterial media (n = 26), lipid plaques (n = 26), and calcified lesions (n = 27) of aortic specimens from ten human cadavers were investigated in air, saline, and blood. Whereas the spectrum of calcified lesions changed with the surrounding optical medium, the other spectra remained constant. In air and blood, the spectra of arterial media, lipid plaques, and calcified lesions could be differentiated qualitatively and quantitatively (P < 0.0001). In saline, there was no clearcut spectroscopic difference between lipid plaques and calcified lesions. However, normal arterial media and atherosclerotic lesions (lipid plaques plus calcified lesions) could still be discriminated. Thus spectroscopy and plaque ablation can be combined using a single XeCl excimer laser. These encouraging results should stimulate further studies to determine the potential use of this approach to guide laser angioplasty in humans. © 1994 Wiley-Liss, Inc.     

The effect of pulsed holmium-YAG laser on in vitro and in vivo atherosclerotic plaque

Laser application for atherosclerotic ablation is still limited. The pulsed Holmium-YAG (HO-YAG) laser has physical characteristics which may improve vascular recanalization. We therefore examined the effect of this laser on cadaver human atherosclerotic aortae, human amputated legs and atherosclerotic rabbits in vivo. The pulsed HO-YAG laser successfully ablated calcific and fibrotic aortic segments. Totally occluded arteries in amputated legs including calcified atherosclerotic lesions were successfully recanalized using 165–350 pulses of 0.35–0.4 J energy per pulse transmitted through commercially available fibre optics. Percutaneous delivery of laser energy to the descending aorta of atherosclerotic rabbits was not traumatic to the arterial wall. These results demonstrate the advantages of the pulsed HO-YAG laser to ablate fibrotic and calcific atheroma and to safely recanalize occluded arteries.     

Occurrence,extent, and implications of pressure waves during excimer laser ablation of normal arterial wall and atherosclerotic plaque

Ablation of atherosclerotic plaque and normal arterial wall was performed using a Xenon-Chloride Excimer laser with a wavelength of 308 nm and a pulse duration of 115 ns. The light was transmitted via a 600 μm bare fibre and adjusted to an energy density of 3.5J/cm². The acoustic signals generated by the laser pulse were measured with two types of hydrophones consisting of polyvinylidenefluoride with active diameters of 0.3 mm and 0.5 mm and recorded on a dual channel digital storage oscilloscope using either a 0.5 m coaxial cable or a broadband fibre-optic transmission system. Tissue was retrieved from nine cadaver human aortas and macroscopically classified as either normal or calcified atherosclerotic plaque. Histological analysis (Haematoxylin eosin, elastica van Gieson, and immunohistochemical staining) was carried out after the experiments to verify the macroscopic diagnosis and to correlate the acoustic responses with the tissue characteristics. For normal arterial wall, maximum peak pressure was 1.28 MPa ± 0.85 MPa, rise time 163 ns ± 43 ns, and pressure increase 8,2k Pa ± 5,4k Pa/ns. For calcified, atheromatous segments, a maximum peak pressure of 2,02 MPa ± 1,16 MPa, a rise time of 69,9 ns ± 25,8 ns, and a pressure increase of 32,3 kPa ± 21,3 kPa/ns was found. Statistical analysis showed a significant shorter rise time (P < 0.0001) and a higher pressure increase (P < 0.0001) for calcified tissue in comparison to normal arterial wall, whereas maximum pressures alone did not allow a differentiation of tissue characteristics. Several hundred kPa are generated during Excimer laser ablation. The results suggest that focal tissue fragmentation is one mechanism of plaque ablation. A differentiation of tissue characteristics is possible by analysis of rise time and pressure increase, potentially providing the possibility of acoustic ablation control. © 1993 Wiley-Liss, Inc.     

Effects of super-pulsed Nd-YAG laser on arterial tissue: comparison with continuous wave

The effects of a super-pulsed Nd-YAG laser at 1.32 μm wavelength on normal or atherosclerotic human arterial tissue were evaluated and compared with those obtained with continuous wave. One joule per pulse was delivered through a 0.2 mm optical fibre with a pulse width of 10 ms at 10 Hz (super-pulse), or 10 W (10 J) were delivered at continuous wave in saline or blood. Ten joules were delivered with super-pulse or continuous wave for each tissue specimen. The aortic specimens were lased either by continuous wave or super-pulse. At super-pulse mode, ablation efficiency (mm³ J⁻¹) was 0.0149±0.0044 for normal tissue in saline, 0.0148±0.0043 for atheroma in saline, 0.0138±0.0062 for normal tissue in blood, and 0.0146±0.0049 for atheroma in blood. There was no significant difference between the groups. At continuous wave mode, ablation efficiency was 0.0507±0.0299 for atheroma in blood (p<0.001 vs super-pulse). However, extensive charring was observed with continuous wave lasing (41% with continuous against 14% with pulsed mode,p<0.001). Heavily calcified plaques were also ablated at 1.5 J per pulse and 15 W (continuous wave), resulting in extensive charring with continuous wave (77% vs 18% with super-pulse,p<0.01). In conclusion, at super-pulse mode, 1.32 μm Nd-YAG laser has neither the selectivity for atheroma nor influence of blood, thermal injury induced by super-pulse is less than that induced by continuous wave (cw), calcified plaques can be ablated by super-pulse, and super-pulsed Nd-YAG laser angioplasty is safer to use than continuous wave.     

Pulsed ultraviolet lasers and the potential for safe laser angioplasty

Endoscopic laser ablation of atheroma using continuous wave lasers is limited by imprecise control of thermal ablation, resulting in a crater that expands in width and depth, with thermal damage to adjacent normal tissue. We compared the gross and histologic effects of pulsed 308 mm excimer irradiation to continuous-wave Nd:YAG and Argon Ion laser irradiation, and pulsed 1,060 nm, 532 nm, 355 nm, and 266 nm laser irradiation in 205 atherosclerotic aortic segments. In contrast to the continuous-wave Nd: YAG, Argon Ion, and pulsed 1,060 nm, 532 nm, and 355 nm laser irradiation, which produced gross and histologic evidence of uncontrolled ablation, the 308 nm and 266 nm pulsed lasers induced incisions that conformed precisely to the beam configuration without gross evidence of thermal injury. The incision edges from these two lasers were histologically smooth and comparable to a scalpel incision. Our histologic findings suggest that rapid, precise endoscopic ablation of vascular and nonvascular tissue can be performed at these shorter pulsed wavelengths with very high precision with relatively little damage or risk to adjacent tissue.     

Characteristics of 308 nm excimer laser activated arterial tissue photoemission under ablative and non-ablative conditions

The present study was designed to assess the characteristics of tissue photoemission obtained from normal and atherosclerotic segments of human postmortem femoral arteries by 308 nm excimer laser irradiation of 60 ns pulsewidth. Three ablative (20, 30, and 40 mJ/pulse) and three non-ablative (2.5, 5, and 10 mJ/pulse) energy fluences were employed. Both the activating laser pulses and the induced photoemission were guided simultaneously over one and the same 1,000 micron core optical fiber that was positioned in direct tissue contact perpendicular to the vascular surface. The spectral lineshape of normal arterial and noncalcified atherosclerotic structures was characterized by a broad-continuum, double-peak emission of relevant intensity between wavelengths of 360 and 500 nm, with the most prominent emission in the range of 400-415 (407 nm peak) and 430-445 nm (437 nm peak). Fibrous and lipid atherosclerotic lesions, however, exhibited a significantly reduced intensity at 437 nm compared to normal artery layers (P less than 0.001), expressed as a 407/437 nm ratio of 1.321 +/- 0.075 for fibrous and 1.392 +/- 0.104 for lipid lesions. Normal artery components presented with approximately equal intensity at both emission peaks (407/437 nm ratio: intima, 1.054 +/- 0.033; media, 1.024 +/- 0.019; adventitia, 0.976 +/- 0.021). Comparison of spectral lineshape obtained under various energy fluences within a group of noncalcified tissues disclosed no substantial difference using the 407/437 nm ratio (P greater than 0.05). In contrast, calcified lesions revealed high-intensity multiple-line (397, 442, 461, and 528 nm) emission spectra under ablative energy fluences, whereas a low-intensity broad-continuum, single-peak spectrum resulted from irradiation beyond the ablation threshold. Thus, these findings suggest fluorescence phenomena for broad-continuum spectra, and plasma emission for multiple-line spectra as an underlying photodynamic process. Regardless of the activating energy fluence, spectral analysis of 308 nm activated photoemission provides accurate information about the laser target under standardized in vitro conditions. It is demonstrated that direct contact ablation and simultaneous spectral imaging of the target tissue via the same optical fiber is feasible.(ABSTRACT TRUNCATED AT 400 WORDS)     

Quantitative optical coherence tomography of arterial wall components

Optical coherence tomography (OCT) can be used to visualize the arterial wall and atherosclerotic plaques with high resolution. In this study, we verified the application of OCT to the quantitative analysis of plaque structural dimensions and optical attenuation coefficients of the components. We assessed the effect of balloon dilation on the OCT signal from the medial layer of porcine carotid artery ex vivo. Imaging of human autopsy samples was performed from the luminal side with a high (3.5 m axial and 7 m lateral) resolution OCT system (~800 nm) or a regular (15–20 m axial and 20 m lateral resolution) OCT system (~1,300 nm). For each sample, dimensions were measured by histomorphometry and OCT, and the optical attenuation was measured. In a tissue culture set-up, porcine carotid arteries were dilated and the attenuation coefficients of the dilated segments were compared to a control segment for 4 h. Quantitative analysis showed a strong and significant correlation between OCT and histology cap thickness measurements for both OCT systems. For both systems, the measured attenuation coefficients for diffuse intimal thickening and lipid-rich regions differed significantly from that of calcified tissue. Balloon dilation induced a time-dependent increase in the attenuation coefficient, which may be attributed to the induction of apoptosis. In conclusion both the high and regular resolution OCT systems can image the atherosclerotic plaques precisely. Quantitative analysis of the OCT signals allowed in situ determination of the intrinsic optical attenuation coefficient for atherosclerotic tissue components within regions of interest, which can help to discriminate between plaque and arterial wall components.     

Cavernous haemangiomas in the oral cavity: Results of treatment with the 578 nm copper vapour laser

Three patients with symptomatic mucosal haemangiomas in the oral cavity were treated with 578 nm yellow light generated by a copper vapour laser. All have shown marked shrinkage after a single treatment utilizing an energy density of 20 J cm⁻². The patients have not required any further surgical or laser treatment. Conservative management of these haemangiomas using this laser is both useful and logical. The biological effects of the copper vapour laser are described as related to these haemangiomas resulting in highly satisfactory clinical responses.     

Sapphire probe laser ablation of human arteries with CO2 gas and saline perfusion: The effect of flow rate,lasing power and lasing time

To evaluate flow rate dependence of CO₂ gas and saline perfusion for sapphire probe ablation, all together 204 human arterial specimens of atheroma and normal vessel were irradiated with Nd-YAG laser, in an experimental circulation-occlusion model within 37°C flowing whole blood. During lasing procedures, various flow rates of CO₂ gas (0.2–2.01 min⁻¹) and saline (2.0–20.0 ml min⁻¹), and various lasing powers (7, 12 and 17 W) and lasing time (1–20 s) were used. Histological changes of all specimens irradiated were microscopically examined. The results showed that the laser ablation area enlarged with increasing CO₂ flow rates and decreasing saline flow rates. Relative ablation efficiency on atheromatous plaque, in comparison to those on normal vessel wall and surrounding tissue site, increased slightly with increasing lasing power and lasing time. In this experimental setting, the mode of action of the sapphire probe ablation on human arterial atheroma seems to be more satisfactory with CO₂ gas perfusion than with saline perfusion.     

Laser-induced fluorescence used in localizing atherosclerotic lesions

We have investigated laser-induced fluorescence frompost mortem human arteries in order to find spectroscopic properties allowing discrimination between normal and atherosclerotic vessel wall. A pulsed nitrogen laser emitting light at a wavelength of 337.1 nm was used as an excitation source. The fluorescence spectrum from 370 to 700 nm was captured and analysed by an optical multichannel analyser. Dimensionless contrast functions were formed by using characteristic spectral features at 390, 415, 480, 580 and 600 nm. All samples were investigated in scans across a region where normal as well as diseased vessel wall appeared. The types of plaque were histopathologically divided into four groups, of which three could be singled out using one or more of our spectroscopic criteria. We also investigated the different layers of the normal and diseased vessel wall in order to determine the various contributions to the fluorescence signal. Furthermore, plasma emission spectra were recorded while ablating the normal as well as the diseased vessel wall with an excimer laser, emitting radiation at 308 nm, thus detecting the change in spectral characteristics during the ablation process down into deeper layers.     

Excimer laser irradiation of non-calcified and calcified human atheroma (248 and 308nm wavelengths)

In order to develop a system of peripheral arterial angioplasty, we carried out an in vitro study to define the quantitative, thermal and morphological characteristics of human-atheroma ablation by excimer laser. A multigas ‘Sopra’ laser was used. The study was performed by using 248nm, krypton fluoride (KrF), then 308nm, xenon chloride (XeCl) wavelengths. The delivered energy was up to 150 mJ pulse⁻¹, pulse duration was 25ns, and the repetition rate could be adjusted to up to 20Hz. Irradiated tissue segments of the superficial femoral and external iliac arteries were obtained in man during surgical procedures and were both calcified and non-calcified atherosclerotic lesions. Quantitative measurements showed a linear increase of ablated tissue mass depending on the energy delivered. For the same energy, the loss of mass was greater with the 248nm wavelength than with the 308nm. The maximum temperature rise measured at the site of irradiation was 6°C at 248nm and 25°C at 308nm. Histological analysis of the irradiated segments revealed neat and precise ablation without thermal injury of adjacent tissue. At 248nm, this phenomenon was observed for calcified as well as non-calcified atheromas. It is concluded that quantitative, thermal and morphological characteristics of in vitro ablation of calcified and non-calcified human atheroma by excimer laser are compatible to clinical requirements. The results observed at 248nm were experimentally more satisfactory.     

Responses of atherosclerotic aorta to argon laser

Lasers have been advocated to resect atherosclerotic plaques in the cardiovascular system, yet little information is available regarding the effects of laser on the range of occlusive lesions seen in the peripheral arterial tree. This study was conducted to assess the risk of perforation in human cadaveric aorta involved with variable degrees of atherosclerosis. Ten fresh segments of atherosclerotic human aorta were graded for extent of atherosclerosis, then subjected to argon laser energy within 48 hours. Using air as the conduction medium and with the fiber tip 2 or 5 mm from the vessel wall, the argon laser was applied to matched calcified and non-calcified arteries at 3.0-7.0 W and 10.0–13.5 W with energy density identical for matched pairs. Results were compared among segments which were normal in appearance or had only fatty streaks grossly with those with gross regional wall calcification. The mean penetration time (T) for calcified and non-calcified lesions at low and high power outputs was compared. Mean time to perforation and range of time necessary to produce perforation were greater in calcified than non-calcified segments at all power levels employed. These data suggest that atherosclerotic lesions vary in their response to argon laser. The presence of calcium may preclude resection of some plaques and protect against wall perforation.     

Infrared absorption spectra ranging from 2.5 to 10 microns at various layers of human normal abdominal aorta and fibrofatty atheroma in vitro

The infrared absorption spectra ranging from 2.5 to 10 microns in wavelength at various layers of both the human normal abdominal aorta wall and the fibrofatty atheroma were measured by conventional transmission spectrophotometry. Optical specimens 7 microns in thickness were prepared using a cold microtome. Pathological examination was simultaneously performed to identify tissue type of the optical specimen. We found that the presence of characteristic absorption peaks at 5.75 and 3.4 microns was restricted to the atheromatous (fibrofatty) layer of the aorta wall. These peaks may be attributed to the accumulated cholesterol deposits. We also discovered that the normal media layer had strong absorption at 6.05 microns. The discrimination between normal media, fibrofatty atheroma, and aged intimal tissue was made possible by the normalized peak ratio which was based on the ratio of the 5.75- and 6.05-microns absorption peaks. These results may be significant for selective laser ablation of atheromatous tissue, as well as tissue diagnosis, to prevent artery perforation during laser angioplastic procedures.     

Arterial response to excimer and argon laser irradiation in the atherosclerotic swine

Injury associated with laser-induced tissue ablation may be reduced by using pulsed energy delivery at low repetition rates, as opposed to using continuous wave energy delivery. This study was designed to examine the similarities and differences between these two systems as regards the healing process, and to examine whether one is superior to the other. In order to test this postulate, the healing response of normal and atherosclerotic aorta were examined after exposure in vivo to argon and excimer (XeCl 308 nm) laser radiation in hypercholesterolemic swine. Swine were fed hyperlipidemic diets for eight months following balloon denudation of the descending aorta. Following general anaesthetic, the descending aorta was isolated and laser burns were made on both normal and atherosclerotic intima using a continuous wave argon laser delivered through a 50 diameter quartz fibre, and a XeCl excimer laser carried through a 1 mm diameter fibre. Energy levels of 3 to 5 J were applied with the argon laser. The pulse duration for the excimer laser was 30 ns and craters were produced using 10 to 60 pulses at a repetition rate of 20 Hz and an energy density of 2 J cm^–2.Forty-eight hours after laser application, craters created by both lasers were filled with thrombus material. Argon burns were surrounded by thermal and acoustic injury which was not seen with excimer burns. Three weeks after laser application all crater surfaces were reconstituted. Unlike the excimer burns, argon craters demonstrated necrosis well beyond the crater margins and were characterized by multinucleate giant-cell reaction surrounding char debris. By nine weeks both excimer and argon laser burns were covered by fibrous tissue but could be distinguished by the fact that char debris and subjacent tissue injury arose with the argon burns.The results suggest that both lasers can be used to remove focal atherosclerotic plaque from arteries without inducing excessive thrombogenicity. Rapid healing is observed with both; however, damage to surrounding tissue is significantly greater with a continuous energy delivery laser as opposed to pulsed energy delivery.Work supported in part by: Heart and Stroke Foundation of Ontario, Grant-in-Aid No. 5-17