Backward Flux Re-Deposition Patterns during Multi-Spot Laser Ablation of Stainless Steel with Picosecond and Femtosecond Pulses in Air

We report on novel observations of directed re-deposition of ablation debris during the ultrafast laser micro-structuring of stainless steel in the air with multi-beams in close proximity on the surface. This interesting phenomenon is observed with both 10 ps and 600 fs NIR laser pulses at 5 kHz repetition rate. Ablation spot geometries could be altered with the use of beam splitting optics or a phase-only Spatial Light modulator. At low fluence (F ~ 1.0 J cm⁻²) and pulse exposure of a few hundred pulses, the debris appears as concentrated narrow “filaments” connecting the ablation spots, while at higher fluence, (F ~ 5.0 J cm⁻²) energetic jets of material emanated symmetrically along the axes of symmetry, depositing debris well beyond the typical re-deposition radius with a single spot. Patterns of backward re-deposition of debris to the surface are likely connected with the colliding shock waves and plasma plumes with the ambient air causing stagnation when the spots are in close proximity. The 2D surface debris patterns are indicative of the complex 3D interactions involved over wide timescales during ablation from picoseconds to microseconds.     

Excimer laser-induced simultaneous ablation and spectral identification of normal and atherosclerotic arterial tissue layers

A krypton-fluorine excimer laser at a 248-nm wavelength was used to irradiate normal and severely atherosclerotic segments of human postmortem femoral arteries. Single pulses and multiple pulses required for penetration or perforation of the arterial wall were applied with 16 nsec pulse width and 5 J/cm2/pulse energy fluence. The total fluorescence of irradiated and ablated tissue was analyzed in real-time mode by means of spectroscopy. Each laser pulse produced one spectrum that was characteristic of the composition of the tissue layer, which was ablated. Fluorescence spectroscopy indicated a broad-continuum emission between 300 and 700 nm with peak fluorescence of equal intensity at wavelengths of 370 and 460 nm (ratio, 1.004 +/- 0.087) for normal media layers. Atheromas without calcification (lipid, fibrous, and mixed) were found with spectral maxima at the same wavelengths but with significantly reduced intensity at 460 nm (ratio, 1.765 +/- 0.263; p less than 0.001). In contrast to this broad-continuum fluorescence, calcified plaques displayed multiple-line emission with the most prominent peaks at wavelengths of 397, 442, 450, 461, 528, and 558 nm. These fluorescence criteria identified the histologically classified target tissue precisely. Histological examination of the corresponding arterial layers indicated sharply delineated and circumscribed tissue ablation. These results indicate that simultaneous tissue identification (diagnosis) and ablation (treatment) by excimer laser irradiation is feasible under strict laboratory conditions. We conclude that this principle demonstrates the potential for laser beam control by means of target-specific ablation.     

Laser-Tissue Interactions With the Pulsed Dye Laser

The laser-tissue interaction of the pulsed dye laser was assessed. The histological appearance of the craters showed precise margins with no evidence of collateral thermal tissue damage. The ablation of soft yellow atheroma was consistently about two- to threefold that of normal wall, but fibrous white atheroma was resistant to laser energy, and was ablated less than normal wall. The maximum probe tip temperature in air was 197°C, but there was relatively little heating of the coronary artery wall during lasing, and this was minimized by saline perfusion at low flow rates. Lasing produces up to 10¹² irregularly shaped debris particles per liter. Debris from thrombus and normal aorta caused significant platelet aggregation in vitro, but atheromatous debris did not. In conclusion, the characteristics of the pulsed dye laser are suitable for intravascular lasing, but selective ablation of atheroma was not achieved.     

Laser probe ablation of normal and atherosclerotic human aorta in vitro: a first thermographic and histologic analysis

The metal-tipped optical fiber or "laser probe" has been extensively studied in animal preparations in vivo and in human clinical trials of revascularization. The aim of this study was to evaluate the thermal characteristics of laser probe tissue ablation and to contrast the vascular tissue response to exposure to the laser probe and bare optical fiber. A 2 mm laser probe was heated with up to 4 W of argon-ion laser irradiation and applied to six postmortem strips of human nonatherosclerotic aorta as well as to five atherosclerotic aortic specimens. Surface temperature maps of the laser probe and of the vascular tissue in air were obtained via 8 to 12 micron thermographic imaging. Laser probe temperature was additionally monitored via thermocouples. Two strips each of normal and diseased aorta were irradiated directly with the bare optical fiber. Thus a total of 43 laser probe application sites and 19 bare fiberoptic laser irradiation sites on a total of 15 aortic strips were analyzed both thermographically and histologically. Based on measured temperature rises and histologic findings, the following observations were made: (1) The laser probe heats initially at its tip and attains a uniform surface temperature distribution within 5 sec. The steady-state temperature attained by the probe is inversely related to the thermal conductivity of the surrounding media. In all media studied, probe temperature increases linearly with applied laser energy. (2) Tissue ablation starts at temperatures greater than 100 degrees C, and ablation temperatures typically exceed 180 degrees C. Adventitial temperatures during laser probe application may reach 70 degrees C. Tissue ablation is enhanced both by greater laser energy deposition in the probe and by higher force at which the probe is applied to tissue. (3) Ablation of fibrofatty atheromata is more extensive than of nonatherosclerotic aortic tissue. This may be due to the lower thermal conductivity of atheromatous tissue. (4) In contrast to direct argon-ion laser ablation of aortic tissue, laser probe-mediated ablation occurs in a controlled fashion, is not associated with extensive subintimal dissections, and allows uniform conduction of heat to tissue as reflected by essentially "isothermal" injury lines.     

Some Laser-Tissue Interactions in 308 nm Excimer Laser Coronary Angioplasty

Some physical concepts of laser-tissue interactions that occur in 308-nm excimer laser angioplasty are addressed. Monte Carlo numerical computations were used to analyze the light fluence rate distributions resulting from finite diameter laser beams incident on tissue, as applied by fiber-optic light delivery catheters. The fluence rate at the inside part of the tissue surface from a 0.2-mm diameter fiber emitting 308-nm light, is increased more than twice relative to the incident power density. The light fluence rate distribution inside the tissue spreads very little outside the incident beam diameter. Therefore, the distributions from different fibers in multifiber catheters will not overlap unless the fibers are very close together. The maximum fluence rate decreases with decreasing beam diameters. Ablation of tissue by a 308-nm excimer laser delivery system in contact with the tissue resulted in a damage zone adjacent to the crater wall, due to expansion of the gaseous debris trapped under the tip of delivery system. In case of contact irradiation, the ablation was more efficient than in case of noncontact irradiation. Direct temperature measurements during excimer laser ablation by an infared (IR) camera showed that temperature accumulation will occur when a sequence of pulses is applied at frequencies of at least 5 Hz. The temperature rise above ambient under circumstances simulating clinical conditions is measured to be 66°± 7°C.     

The interaction between excimer laser energy and vascular tissue

The effects of XeF1 excimer laser on isolated normal and atherosclerotic aorta were studied. Experiments were performed in flowing water at constant temperature, flow rate, water depth, pulse width (10 nsec), wavelength (351 nm), beam size (1 mm2) and focal length (50 cm). The number of pulses, the pulse energy, and the pulse frequency were varied, and the vascular tissue was studied histologically. The following observations were made: tissue ablation required a minimum threshold pulse energy and was nonlinearly proportional to the number of pulses and the pulse energy delivered; precise tissue ablation occurred at low pulse frequencies, but changes resembling a thermal process were seen as pulse frequency increased; calcified plaque was more photoresistant than atheroma or normal vessel; excimer laser energy was markedly attenuated by blood; and the time interval between pulses and high peak power are related to the precision of ablation by pulsed excimer laser. It is concluded that excimer laser can rapidly and precisely ablate vascular tissue by a photothermal process.     

Pulsed ultraviolet laser irradiation produces endothelium-independent relaxation of vascular smooth muscle

Recent studies have shown that continuous wave laser irradiation induces contraction of vascular smooth muscle, except at powers far below the threshold for tissue ablation. To determine the corresponding effects of pulsed laser irradiation on vascular smooth muscle tone, vascular rings of rabbit thoracic aorta were mounted isometrically with 1 g tension in Krebs-bicarbonate buffer and irradiated with 308 or 351 nm from an excimer laser through a 400-microns optical fiber. A total of 250 exposures were performed with 1-6.5 mJ/pulse (fluence = 0.8-5.5 J/cm2), 10-50 Hz, and cumulative exposures of 10-120 seconds. Excimer laser irradiation in combinations of pulse energy (PE), repetition rate (RR), and cumulative exposure below, at, or above threshold for tissue ablation consistently produced relaxation unassociated with contraction in each of the 250 exposures. For the total 250 exposures, the magnitude of relaxation (reduction in recorded tension, Rmax) was 55 +/- 4% (mean +/- SEM) of maximum vasomotor reactivity recorded in the specimen in response to administration of serotonin. Rmax varied directly with both PE and RR. When PE was increased from 1 to 5 mJ/pulse (n = 13), Rmax increased from 57 +/- 19% to 80 +/- 19% (p less than 0.0001); when RR was increased from 10 to 50 Hz (n = 10), Rmax increased from 27 +/- 8 to 46 +/- 8 (p less than 0.0001). Rmax varied independently of endothelial integrity (assessed anatomically and pharmacologically) and wavelength (308 vs. 351 nm). Simultaneously recorded tissue-temperature profiles disclosed that during pulsed laser irradiation, tissue temperature rise did not exceed 5 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)     

Laser ablation of normal and diseased human ventricle

The feasibility and safety of laser photoablation in patients with ventricular tachycardia (VT) and accessory pathways are currently being examined. We studied the qualitative and quantitative effects of argon laser radiation on normal and diseased human ventricle to determine the relationship between the size of tissue lesion and delivered energy. Twenty-nine human ventricle segments (normal ventricle = 10; diseased ventricle = 19) were excised from patients during mapping-guided subendocardial resection for VT (seven patients), mitral valve replacement (five patients), or immediately at autopsy (three patients). Lasing was performed with a 15 W argon laser coupled to a 300 micron optical fiber. Incremental laser discharges from 10 to 1000 J were delivered in air and saline with the optical fiber 5 mm from the endocardial surface. Gross and microscopic damage was quantified and correlated with laser discharges at low (10 to 100 J), intermediate (101 to 300 J), and high (greater than 300 J) energies. Histologic examination of laser-induced lesions in both normal and diseased human ventricle in either medium showed focal thermal injury with crater formation, vacuolization, and coagulation necrosis of endocardium and myocardium. In normal ventricle, mean lesion diameter and depth in air increased with increasing energies up to 300 J. Over 300 J, tissue perforation was frequently observed. In saline, the mean lesion depth was significantly reduced (p less than 0.02) at comparable energies. In diseased ventricle, mean lesion diameter and depth in air and saline also increased with increasing laser discharge energies up to 300 J. Higher energy laser discharges did not increase mean lesion dimensions or result in tissue perforation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Intracardiac Doppler echocardiographic quantification of pulmonary vein flow velocity: an effective technique for monitoring pulmonary vein ostia narrowing during focal atrial fibrillation ablation

INTRODUCTION: Ablation at the pulmonary vein (PV) ostium to isolate triggers for atrial fibrillation (AF) may induce PV narrowing. The AcuNav ultrasound catheter can image PV flow and quantify peak velocity and may be useful in assessing the degree of narrowing of PV ostia. METHODS AND RESULTS: In 93 patients with AF undergoing PV ostial ablation (up to 40 W, 52 degrees C, 90 sec), the ultrasound catheter was placed in the right atrium and PV peak flow velocities were measured during systole and diastole before and after ablation. Ostial PV electrical isolation was achieved in 216 of the 219 targeted PVs. The ultrasound catheter provided flow imaging of all PVs. The ostial peak flow velocities measured 56 +/- 12 cm/sec before ablation and increased to 101 +/- 22 cm/sec after ablation (P < 0.001). Peak velocity >100 cm/sec was detected in 103 (47%) of 219 and > or = 158 cm/sec (estimated pressure gradient 10 mmHg) with turbulent flow features, in 7 (3.2%) of 219 PVs. The highest velocity detected in one PV was 211 cm/sec (17.7 mmHg). Follow-up ultrasound catheter measurements were obtained in 13 patients (30 previously ablated PVs) during repeat ablations. The ostial peak velocity had decreased by 22 +/- 14 cm/sec and in 25 (83%) of 30 PVs was within the baseline range (<100 cm/sec) at a mean follow-up of 4.9 +/- 2.2 months. Follow-up magnetic resonance imaging (MRI) or contrast-enhanced CT was obtained at 7.0 +/- 3.8 months in seven patients with PV velocity > 158 cm/sec after initial ablation. No significant stenosis (<30%) was identified, and no patient suffered clinical symptoms (follow-up 6-18 months) related to the described acute changes in PV flow after an initial ablation procedure. Of 13 patients with repeat ablation, two had PV velocities >100 cm/sec before repeat ablation, and three PVs in two patients had flow velocity >158 cm/sec after repeat ablation. One of these patients developed symptoms of exertional dyspnea; MRI at 4 months showed 50% to 60% ostial narrowing. CONCLUSION: Ostial ablation for PV isolation may induce a mild-to-moderate increase in PV flow velocity, which can be identified using an ultrasound catheter with Doppler color flow imaging. Increases in PV flow velocity (<158 cm/sec) after a primary ablation procedure appear to be well tolerated, and a return toward baseline flow characteristics should be anticipated by 3 months. A more cautious approach may be required for patients undergoing repeat PV isolation.     

Fluorescence-guided laser-assisted balloon angioplasty in patients with femoropopliteal occlusions

In 12 patients (aged 64 +/- 10 years) with femoropopliteal occlusions (1-27 cm; average, 8.4 cm length) that could not be recanalized by standard guidewire-balloon angioplasty techniques, percutaneous laser-assisted balloon angioplasty was performed by use of a new fluorescence-guided dual-laser system. Plaque detection by 325-nm laser-excited fluorescence spectroscopy provided real-time feedback control to a 480-nm pulsed dye laser (2-microseconds pulses) for atheroma ablation. By means of a common 200-microns optical fiber, after diagnostic fluorescence sensing, computer algorithms directed a fire or no-fire signal (5 Hz) to the treatment laser for selective plaque removal. Laser recanalization (15-50 mJ/pulse) was successful in 10 of 12 patients; this procedure was followed by definitive balloon angioplasty in seven of 12 patients with increased ankle/arm indexes (from 0.60 +/- 0.12 at baseline to 0.84 +/- 0.11 after treatment, p = 0.0043). In laser and balloon angioplasty failures, all femoropopliteal occlusions were heavily calcified, and there were two mechanical guidewire perforations without clinical sequelae. Ablation of calcified lesions required higher pulse energies and greater total energy per centimeter of recanalized tissue (1,837 +/- 1,251 mJ/cm vs. 90 +/- 39 mJ/cm, p = 0.0036). Fluorescence spectroscopy (n = 219 sites) was helpful in flush occlusions and correctly identified plaque, underlying media, and thrombus by changes in fluorescence intensity, shape, and peak position. Thus, when fluorescence-guided laser angioplasty was used in a subgroup of patients refractory to standard angioplasty techniques, primary recanalization and subsequent balloon angioplasty of femoropopliteal occlusions was successful in 83% and 58% of the patients, respectively. Importantly, treatment of heavily calcified lesions accounted for all of the failures and will require modified delivery systems to create larger primary channels and to increase catheter-tip control, which should improve clinical results in the future.     

Radiofrequency ablation in a porcine liver model： Effects of transcatheter arterial embolization with iodized oil on ablation time, maximum output, and coagulation diameter as well as angiographic characteristics

AIM:To evaluate the effects of combined radiofrequen-cy ablation and transcatheter arterial embolization with iodized oil on ablation time,maximum output,coagula-tion diameter,and portal angiography in a porcine liver model. METHODS: Radiofrequency ablation (RFA) was applied to in vivo livers of 10 normal pigs using a 17-gauge 3.0 cm expandable LeVeen RF needle electrode with or with-out transcatheter arterial embolization (TAE) with iodized oil (n = 5). In each animal,2 areas in the liver were ab-lated. Direct portography was performed before and af-ter RFA. Ablation was initiated at an output of 30 W,and continued with an increase of 10 W per minute until roll-off occurred. Ablation time and maximum output until roll-off,and coagulated tissue diameter were compared between the 2 groups. Angiographic changes on portog-raphy before and after ablation were also reviewed. RESULTS: For groups with and without TAE with iodized oil,the ablation times until roll-off were 320.6 ± 30.9 seconds and 445.1 ± 35.9 seconds,respectively,maxi-mum outputs were 69.0 ± 7.38 W and 87.0 ± 4.83 W and maximal diameters of coagulation were 41.7 ± 3.85 mm and 33.2 ± 2.28 mm. Significant reductions of abla-tion time and maximum output,and significantly larger coagulation diameter were obtained with RFA following TAE with iodized oil compared to RFA alone. Portography after RFA following TAE with iodized oil revealed more occlusion of the larger portal branches than with RFA alone. CONCLUSION: RFA following TAE with iodized oil can increase the volume of coagulation necrosis with lower output and shorter ablation time than RFA alone in nor-mal pig liver tissue.     

Selective absorption of ultraviolet laser energy by human atherosclerotic plaque treated with tetracycline

Tetracycline is an antibiotic that absorbs ultraviolet light at 355 nm and preferentially binds to atherosclerotic plaque both in vitro and in vivo. Tetracycline-treated human cadaveric aorta was compared with untreated aorta using several techniques: absorptive spectrophotometry, which demonstrated a distinct absorptive peak at 355 nm in tetracycline-treated plaque that was absent in treated normal vessel; ultraviolet microscopy, which showed that treated atheroma acquired the characteristic fluorescence of tetracycline under ultraviolet light; and tissue uptake of radiolabeled tetracycline, which showed 4-fold greater uptake by atheroma than by normal vessel. In addition, intravenous tetracycline administered to patients undergoing vascular surgery demonstrated characteristic fluorescence in surgically excised diseased arteries. Because of tetracycline's unique properties, we exposed tetracycline-treated and untreated aorta to ultraviolet laser radiation at a wavelength of 355 nm. We found enhanced ablation of tetracycline-treated atheroma compared with untreated atheroma. The plaque ablation caused by ultraviolet laser radiation was twice as extensive in tetracycline-treated vs nontreated plaque (2.2 +/- 0.25 mm vs 1.3 +/- 0.55 mm, p less than 0.017). This study demonstrates the potential of tetracycline plaque enhancement for the selective destruction of atheroma by ultraviolet laser radiation.     

Current Status of Lasers for Arrhythmia Ablation

Current Status of Lasers for Arrhythmia Ablation. Wavelength specific effects and mode of laser operation allow either photocoagulation or tissue removal as a means of approaching arrhythmia ablation. Successful intraoperative ablation of ventricular tachycardias has been performed with the Nd: YAG laser (photocoagulation) and argon laser (tissue vaporization). The argon laser has been used intraoperatively for transection of accessory pathways. Experimental studies indicate a strong theoretical potential for Nd:YAG laser catheter ablation of ventricular tachycardia. Laser energy has been used experimentally to evaluate the possibility of AV junctional ablation/modification and accessory pathway ablation. Adaption of laser energy to effective catheter systems for arrhythmia ablation requires solutions to problems inherent in all catheter systems and some unique to laser energy. (J Cardiovasc Electrophysiol, Vol. 3, pp. 345–353, August 1992)     

Endoluminal diode laser vein damage: preclinic study for vein insufficiency

BACKGROUND: The ex-vivo phase of this research aims to define fibre laser activity ranges with the intent of using a safety protocol on the operating table to ensure maximum results in occluding the vein lumen with minimum necessary energy parameters. METHODS: The study was performed on 20 human saphenous veins removed during phlebological surgery. A 1 mm diameter optical fibre was inserted in the vein to the proximal extremity where administration of laser energy began along the vein axis. The veins were subdivided into three groups based on the parameters used: Group 1: treated with 20 Watt power; Group 2, treated with 15 Watt power; Group 3, treated with 10 Watt power. On conclusion of the procedure the veins were immersed in formalin and sent for histological examination. RESULTS: The greatest endothelial damage was observed in the group of veins treated with fluence above 300 J/cm(2), in which carbonisation of the endothelium was observed; in those treated with lower fluence (200-100 J/cm(2)) damage consisted in coagulation necrosis and interstitial oedema; at fluence of 100 J/cm(2) endothelial damage was less uniform. CONCLUSIONS: Experimental evidence would appear to support the hypothesis that the optimal protocol is below 300 J/cm(2) fluence with 15 Watt power and pulse train 100-300 ms.     

Prognostic factors for success in catheter ablation of the right Kent bundle: report of 27 patients

In 27 patients with atrial fibrillation and/or reciprocating tachycardia, ablation of right-sided Kent bundles (23 in the right posterior paraseptal region and four of the right free wall) was performed. The anterograde refractory period of the accessory pathways was 253 +/- 70 msec and the shortest R-R interval during atrial fibrillation 211 +/- 47 msec. Cumulative energies of 589 +/- 396 J per patient were used, with 3 +/- 2 shocks and 195 +/- 312 J per shock. Accessory pathway ablation was effective in 16/27 patients (59%) during an 11 +/- 8 months of follow-up (in 15/16 pre-excitation disappeared; in nine of them the anterograde and retrograde conductions were abolished and in the other six the anterograde and retrograde conductions were severely altered; ablation was partially ineffective in 1/16 patients who remained asymptomatic on medical treatment and without inducible reciprocal rhythm). Ablation was ineffective in 11/27 patients. Three hours after ablation a patient died from electromechanical dissociation; during ablation a 200 J shock cancelled preexcitation, not being possible to produce reciprocal rhythm. The emergency echocardiography only showed a minimum posterior pericardial effusion. Ablation was effective in 15/23 patients (65%) right posterior paraseptal. Overall success in 1989 (10/27 patients) was 70% (7/10 patients). The success in 1989 was 70% (6/8 patients) right posterior paraseptal. The shortest RP' interval (was 82 +/- 19 msec) during the reciprocal rhythm, where the ablation was performed, was 82 +/- 19 msec (74 +/- 17 msec with success and 99 +/- 19 msec with failure).(ABSTRACT TRUNCATED AT 250 WORDS)     

In vitro study of a radiofrequency guidewire aimed at recanalization of totally occluded peripheral arteries

A novel radiofrequency ablative system (40 msec-train pulses with twenty 200 msecpulses at the carrier frequency of 750 KHz and 1 Hz repetition rate) aimed at recanalizing totally occluded peripheral arteries was investigated by means of in vitro tissue ablation from human postmorten arterial wall samples. The samples were submitted to irradiation with a guidewire 150 cm long, maximum diameter of ceramic tip 0.033 inch positioned perpendicular to the tissue surface in saline, contrast medium or blood using varying generator power. Ablation efficacy was determined as the depth of vaporization per pulse delivered. Electrical current for the train duration was measured as voltage at the 1 ohm-resistor. In saline, the ablation efficacy increased from 8 to 65 μm/pulse with generator power increasing from 11 W to 27.5 W. There was no significant difference in the ablation efficacy between saline and blood. In contrast medium, the ablation efficacy was significantly lower. For the same generator power, the electrical current varied during the ablation procedure from 1.3 ± 0.2 A at the beginning of the procedure to 1.1 ± 0.2 A after the first pulses and to 2.0 A before artery wall perforation occurred. Neither tissue ablation nor current variations were observed when radiofrequency energy was emitted on calcified tissue. The diameter of craters was 0.89 ± 0.1 mm (range: 0.85–0.96 mm). No major thermal injury such as carbonization or charring was observed. Thus. (1) radiofrequency energy emitted through a guidewire is reliable and effective for the ablation of arterial tissue; (2) the depth of penetration is related to the generator power; (3) the ablation efficacy is greater in saline and blood than in contrast medium; (4) the system is not effective in highly calcified tissue; and (5) the procedure may be monitored by electrical current measurements. © Wiley-Liss, Inc.     

Origin of arterial wall dissections induced by pulsed excimer and mid-infrared laser ablation in the pig.

To study adjacent tissue damage after delivery of holmium, thulium and excimer laser pulses, porcine thoracic aortas were irradiated in vivo. After 3 days, microscopic analysis of 67 craters produced by all three lasers demonstrated large dissections extending from the craters. The mean diameter of the dissections was smaller for excimer-induced craters (1.38 +/- 0.42 mm; n = 22) than for holmium-induced (2.7 +/- 0.87 mm; n = 22) and thulium-induced (2.37 +/- 0.42 mm; n = 14) craters (p less than 0.01 vs. mid-infrared dissections). In addition, microscopic analysis demonstrated necrosis adjacent to the crater. The lateral necrotic zones of the thulium-induced craters were smaller than the holmium- and excimer-induced necrotic zones (p less than 0.01). To identify the origin of the excessive tissue tearing, laser-saline and laser-tissue interaction were compared in vitro by time-resolved flash photography. In saline solution, the mid-infrared lasers showed bubble formation on a microsecond time scale. The excimer laser produced similar bubbles in the vicinity of tissue. For all three lasers, elevation of the tissue surface was shown during in vitro ablation. Dimension (diameter up to 4 mm) and time course (rise time of 100 to 300 microseconds) of bubble formation and tissue elevation were strikingly similar. Thus, tissue dissections are caused by the expansion of a vapor bubble within the target tissue. Coronary dissections after excimer and mid-infrared laser angioplasty might be related to the forceful bubble expansion.     

Ablation of rabbit liver, stomach, and colon with a pulsed holmium laser

A pulsed holmium laser (wavelength 2.1 microns, pulse duration 250 microseconds) was used to ablate rabbit liver, stomach, and colon in vivo. Microscopic examination of the tissues revealed zones of thermal damage extending 0.5-1.0 mm from ablation sites. In addition, ablation rates were measured using a mass loss technique and found to increase linearly with delivered radiant exposure. The threshold radiant exposure for ablation was calculated to be 50 J/cm2 with a heat of ablation of 7000 J/cm3. Because the holmium laser produces less thermal necrosis than current endoscopic laser systems, such as the continuous-wave neodymium:YAG laser, and because the ablation rate can be precisely controlled, the holmium laser shows promise as an alternative method for endoscopic removal of tissue.     

Fibrin formation, fibrinopeptide A release, and platelet thrombus dimensions on subendothelium exposed to flowing native blood: greater in factor XII and XI than in factor VIII and IX deficiency

Fibrin deposition and platelet thrombus dimensions on subendothelium were studied in four groups of patients with coagulation factor deficiencies. Five patients with factor VIII deficiency (APTT 120 +/- 8 sec) and three patients with factor IX deficiency (APTT 125 +/- 11 sec) were severe bleeders, whereas four patients with factor XII deficiency and seven with factor XI deficiency were either asymptomatic or only mild bleeders despite APTT values of 439 +/- 49 and 153 +/- 13 sec, respectively. Everted segments of deendothelialized rabbit aorta were exposed at a shear rate of 650 sec(-1) for 5 and 10 min to directly sampled venous blood in an annular chamber. Blood coagulation was evaluated by measuring fibrin deposition (percent surface coverage) on the subendothelium and post-chamber fibrinopeptide A levels; platelet thrombus dimensions on the subendothelium were evaluated by determining the total thrombus volume per surface area (using an optical scanning technique) and the average height of the three tallest thrombi. Consistent differences were observed among the patient groups for both the 5-min and 10-min exposure times. The larger of the 5- and 10-min exposure-time values was used to calculate group averages. Fibrin deposition in normal subjects was 81% +/- 5% surface coverage, and post- chamber fibrinopeptide A values were 712 +/- 64 ng/ml. Markedly decreased fibrin deposition and fibrinopeptide A levels were observed in factor VIII deficiency (2% +/- 1% and 102 +/- 19 ng/ml) and factor IX deficiency (11% +/- 7% and 69 +/- 11 ng/ml). In contrast, significantly higher values were obtained in patients deficient in factor XI (33% +/- 5% and 201 +/- 57 ng/ml) and factor XII (66% +/- 12% and 306 +/- 72 ng/ml). Differences in thrombus dimensions were also observed. In normal subjects, the value for thrombus volume and average height of the tallest thrombi were 8.3 +/- 1.3 cu micron/sq micron and 145 +/- 11 micron, respectively, and in patients were as follows: FVIII, 2.7 +/- 0.6 and 71 +/- 7; FIX, 4.5 +/- 1.8 and 88 +/- 14; FXI, 11.8 +/- 1.9 and 125 +/- 10; and FXII, 7.9 +/- 3.1 and 130 +/- 25. Platelet thrombus dimensions were normal in a patient with fibrinogen deficiency, indicating that the smaller thrombi in factor VIII and factor IX deficiencies were probably due to impaired evolution of thrombin rather than diminished fibrin formation.(ABSTRACT TRUNCATED AT 400 WORDS)     

Endocardial photoablation by excimer laser

A xenon-chlorine excimer laser was used to irradiate normal endocardium of fresh sheep and pig hearts as well as unfixed human endocardial scar. Forty pulses of 370 J and 35 ns each resulted in penetration of up to 12 mm in normal tissue and only 3.5 mm in scarred endocardium. Dosimetry indicated that the volume of vaporized scarred tissue was 1/10th that of normal endocardium (0.19 to 0.40 versus 1.35 to 3.22 mm3/J). Ultrastructurally, there was a sharp demarcation of only 10 mu between the region of injury and normal myocardium, with little evidence of heat injury. The high power and short duration of these lasers coupled with the lack of a boundary zone of injury suggest that excimers may be an ideal tool for arrhythmia ablation.