Mechanism of laser ablation in an absorbing fluid field.

Selection of a laser source for intravascular applications has frequently been predicated upon assumptions involving transmissibility in blood of the wavelength of light emitted by a given laser. Standard absorption curves for ultraviolet radiation in blood and infrared radiation in water would suggest that transmission of ultraviolet radiation through a blood field and infrared radiation through any aqueous fluid field would be insufficient for tissue ablation. The present series of experiments was undertaken to determine whether these theoretical predictions would in fact obviate the use of these wavelengths in a blood field. Specimens of normal human myocardium and/or polyvinylchloride were submerged under blood and water and irradiated with ultraviolet radiation (351 nm) delivered as a focused beam and via an optical fiber and infrared radiation (10,600 nm) delivered as a focused beam. Ablation of myocardium was successfully accomplished with a focused beam of both ultraviolet and infrared radiation under as much as 5 mm of blood and with ultraviolet radiation via an optical fiber with the fiber tip up to 3 mm distant from the tissue specimen. High-speed cine recordings of ablation carried out using a focused beam of laser radiation demonstrated that formation of a dynamic optical cavity is the basis for successful pulsed ultraviolet and infrared laser transanguineous tissue ablation. These results thus demonstrate that prediction of wavelength transmission through fluid media based on optical properties of a static fluid does not predict ability to accomplish ablation under dynamic circumstances of laser irradiation.

Cystic medial necrosis in coarctation of the aorta: a potential factor contributing to adverse consequences observed after percutaneous balloon angioplasty of coarctation sites.

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.

Gas chromatographic-light microscopic correlative analysis of excimer laser photoablation of cardiovascular tissues: evidence for a thermal mechanism.

The present series of experiments used gas chromatography to identify vapor-phase photoproducts liberated during excimer laser irradiation of cardiovascular tissues in air and blood. In air, laser beams produced from ArF (193 nm) and XeF (351 nm) excimer laser gas mixtures were delivered to samples of myocardium and atherosclerotic coronary arterial segments through the wall of a quartz cell, using 8-40 mJ/pulse. In blood, 351 nm were delivered via an optical fiber, using 14 mJ/pulse. When the experiments were performed using an air-tissue interface, the dominant photoproducts identified in order of elution from the gas chromatographic column were methane, acetylene, ethylene, ethane, propyne, allene, propylene, propane, and butene. When a fiberoptic was used to accomplish 351-nm excimer laser tissue ablation in a blood field, a similar gas chromatographic spectral distribution was observed. These vapor-phase photoproducts are indistinguishable from those observed following continuous wave laser irradiation or flame torching of cardiovascular tissues. Thus, despite the fact that excimer laser ablation of cardiovascular tissues is characterized by the absence of signs of thermal injury, the results of these experiments suggest that the predominant mechanism of excimer ablation is, like continuous-wave laser irradiation, a thermal process.

Attenuation of the media of coronary arteries in advanced atherosclerosis.

Human coronary artery wall architecture was analyzed in detail in 127 histologic sections with varying degrees of narrowing due to atherosclerotic plaque. A planimetry-microscope system was used to morphometrically determine percent luminal cross-sectional area narrowing due to atherosclerotic plaque, absolute area of the coronary artery media and total cross-sectional area of the coronary artery section. In 65 sections in which the native coronary artery lumen was narrowed less than 75%, the area of the coronary artery media corrected for total coronary cross-sectional area (Mc) was 0.244 +/- 0.055 mm2. In contrast, among 62 sections in which the coronary artery lumen was narrowed more than 75% in cross-sectional area, Mc measured 0.180 +/- 0.078 (p less than 0.001). Thus, in coronary artery segments with advanced atherosclerosis, there is substantial attenuation of the media, normally the principal component of the coronary artery wall.

Use of pulsed energy delivery to minimize tissue injury resulting from carbon dioxide laser irradiation of cardiovascular tissues.

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.

The excimer laser: gross, light microscopic and ultrastructural analysis of potential advantages for use in laser therapy of cardiovascular disease.

Excimer lasers are pulsed gas lasers that use a mixture of a rare gas and halogen as the active medium to generate pulses of short wavelength, high energy ultraviolet light. A krypton-fluoride gas mixture was used to achieve an excimer emission at a wavelength of 248 nm. A total of 30 atherosclerotic coronary artery segments were irradiated over a range of pulse energies (250 to 750 mJ), repetition rates (2 to 25 Hz), average powers (1.9 to 18.8 watts) and cumulative exposures (3 to 12 seconds). In no case was there gross, light microscopic or ultrastructural evidence of the pathologic injury typically associated with continuous wave laser irradiation of coronary artery segments. Similar results were achieved after excimer laser irradiation of 30 samples of myocardium. Excimer irradiation of calcified aortic valve leaflets accomplished focal debridement without pathologic tissue injury; when total debridement was attempted, however, gross charring was observed. The paucity of pathologic alterations observed after excimer irradiation of cardiovascular tissue may prove beneficial in precisely controlling laser ablation of pathologic tissue without injury to the surrounding normal tissue. Clinical application of excimer laser irradiation requires resolution of several issues, including the development of suitable fiber optics and laser coupling, evaluation of potential ultraviolet toxicity, and demonstration that ultraviolet light can be transmitted through a blood-filled system.

Reduction of laser-induced pathologic tissue injury using pulsed energy delivery.

Continuous-wave (CW) laser irradiation of cardiovascular tissues is characterized by 2 distinctive histologic findings: a superficial zone of coagulation necrosis and a subjacent zone of polymorphous lacunae. The present investigation was designed to determine whether such injury could be eliminated by altering the temporal profile of laser energy delivery. One hundred forty-five myocardial slices were irradiated with an air-tissue interface using CW laser irradiation at wavelengths of 488 to 515 nm (argon), 1,064 nm (Nd-YAG) and 10,600 nm (CO2). Pulsed laser irradiation included 248 nm (excimer); 355, 532 and 1,064 nm (Nd-YAG); and 515 nm (mode-locked argon). Energy profiles in the pulsed mode included a range of repetition rates (1 Hz to 256 MHz), pulse duration (0.2 to 358 ns) and pulse energies (2 nJ to 370 mJ). Resultant average powers were 0.1 to 38 W. Grossly visible charring of myocardial tissue was observed at all laser wavelengths when the laser energy profile was CW or pulsed at high repetition rates (more than 2 KHz) and low pulse energies (less than 3 mJ) independent of the wavelengths used. In contrast, when laser energy was pulsed at low repetition rates (less than 200 Hz) and large pulse energies (more than 10 mJ), neither gross nor histologic signs of thermal injury were observed. Pathologic injury associated with laser-induced tissue ablation may thus be substantially reduced by use of pulsed energy delivery at low repetition rates. Potential advantages of pulsed laser energy include a more benign healing process, a less thrombogenic surface, and improved preservation of structural tissue integrity.

Factors contributing to perforations resulting from laser coronary angioplasty: observations in an intact human postmortem preparation of intraoperative laser coronary angioplasty.

This investigation was designed to assess the potential use of laser coronary angioplasty as an intraoperative adjunct in the surgical treatment of ischemic heart disease. Among 17 postmortem hearts, simulated laser coronary angioplasty was performed at 53 sites with a No. 4F guiding catheter and 240 micron (200 micron core) quartz optical fiber. Perforation complicated laser coronary angioplasty in 33 (62%) of the 53 attempts. Most (n = 29) perforations were thermal; four were purely mechanical. Perforation sites were characterized by extensive calcific deposits (21 of 33 cases [64%] ) and the origin of a side branch (13 of 33 [39%] ). Excessive tortuosity of the extramural coronary artery contributed to arterial perforation in four cases, and precluded attempts to perform laser coronary angioplasty in two other cases. In 19 of the 53 attempts to perform laser coronary angioplasty, a high-frequency two-dimensional echocardiographic probe was used to image the coronary artery during antegrade manipulation of the optical fiber/guiding catheter and laser irradiation of the target arterial stenosis. Although perforation nevertheless occurred in 11 (58%) of 19 sites, only one mechanical perforation resulted; the remaining 10 were thermal. Characteristics of the perforation sites in this group were similar to those noted for the group as a whole. Experience with this model of laser coronary angioplasty indicates that even when access problems associated with percutaneous laser coronary angioplasty are obviated by a simulated intraoperative approach, perforation of the underlying coronary arterial wall continues to represent the "rate-limiting" complication of laser coronary angioplasty. Most perforations occurred in relation to calcific deposits, branch points, and tortuous coronary segments.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of photoproducts liberated by in vitro argon laser irradiation of atherosclerotic plaque, calcified cardiac valves and myocardium.

To determine how laser light effects alterations in cardiovascular tissue, photoproducts liberated as the result of argon laser irradiation of atherosclerotic plaque, myocardium and calcified aortic valve leaflets were analyzed by gas chromatography, gas chromatography-mass spectrometry and absorbance spectroscopy. The products formed in gas phase are those expected when proteins and porphyrins are pyrolyzed--light hydrocarbon fragments, carbon monoxide and water vapor. The laser-generated products dissolved in solution are those expected when a protein chain or porphyrin ring is degraded in a thermal reaction, namely protein fragments and nitrogen heterocyclic ring fragments. These photoproducts are those typical of combustion or thermal degradation, and indicate that the fundamental nature of laser irradiation of coronary plaque, myocardium and calcified valve leaflets is thermal rather than photochemical. Thermal degradation of myocardium is more extensive than thermal degradation of atherosclerotic arteries or calcified valves because the red hue of myoglobin-containing myocardium enhances the absorption of the blue-green argon laser light. In contrast, the yellow-white hue of both atherosclerotic plaque and calcified aortic valve leaflets allows less complete absorbance of the argon laser light, leading to a lesser amount of converted heat and, therefore, less complete thermal degradation.

Laser-assisted debridement of aortic valve calcium.

Experimental debridement of aortic valve calcium by means of laser phototherapy was investigated in vitro. Near-total debridement of calcific deposits observed on pretreatment x-ray films was accomplished using carbon-dioxide laser phototherapy. Analysis of liberated photoproducts suggests that debridement is effected by thermal degradation of the surrounding connective tissue envelope and thermal expansion of the calcific nodules. These results suggest that in selected patients with calcific aortic stenosis, it may be possible to perform intraoperative laser-assisted debridement of aortic valve calcium in order to preserve the native aortic valve and thus avoid prosthetic valve replacement.

Laser photoablation of pathological endocardium: in vitro findings suggesting a new approach to the surgical treatment of refractory arrhythmias and restrictive cardiomyopathy.

In selected patients, malignant ventricular tachyarrhythmias have been successfully abolished by excision of subendocardial arrhythmogenic foci. Likewise, in certain patients in whom restrictive cardiomyopathy is due to endocardial thickening, endocardial resection has resulted in hemodynamic improvement. The present study was designed to explore the utility, in vitro, of laser photoablation of pathologically thickened endocardium. Endocardial photoablation was easily accomplished regardless of etiological or anatomical variations using either the focused beam of a carbon dioxide laser or argon laser light delivered through a 200-microns optical fiber. Photoablation of areas as large as 3.9 X 1.3 cm was performed within 40 seconds. The extent or depth of endocardial photoablation could be limited to 2 mm2 in area or 1 mm in depth using either form of laser therapy. These in vitro results suggest that either carbon dioxide or argon laser phototherapy can be successfully applied to the surgical treatment of refractory arrhythmias and restrictive cardiomyopathy. Advantages of laser photoablation include speed and precision. Furthermore, laser photoablation obviates the difficulty associated with conventional techniques in establishing tissue planes.

Coronary angiographic and morphologic correlation.

Necropsy and, more recently, physiologic studies have demonstrated the limitations of coronary angiography in estimating the percentage of luminal narrowing due to atherosclerotic plaque. Necropsy studies have also confirmed that coronary arterial spasm is typically associated with fixed atherosclerotic stenoses. Finally, necropsy studies of individual patients have clarified certain points regarding thrombolytic therapy, coronary "collateral" blood flow, myocardial bridges, and coronary arterial dissection.

Laser myoplasty for hypertrophic cardiomyopathy. In vitro experience in human postmortem hearts and in vivo experience in a canine model (transarterial) and human patient (intraoperative).

The feasibility of performing a myotomy/myectomy for hypertrophic cardiomyopathy (HC) by means of laser phototherapy was evaluated experimentally in vitro and in vivo, and the procedure then applied to a patient intraoperatively. In vitro experience revealed that the beam of an argon laser, delivered directly or via an optical fiber, could both cut and vaporize myocardium, producing a myotomy/myectomy morphologically similar to that produced by the conventional blade technique. In vivo experiments, in which the beam of an argon laser was delivered via an optical fiber to the ventricular septum of a canine heart, confirmed that a laser myoplasty could be achieved in 4 of 5 dogs by a transarterial approach. Finally, laser myoplasty was performed intraoperatively in a patient with HC, using a 200-mu fiber interfaced with an argon laser. Measured laser power was 1.5 W; cumulative exposure was less than 4 minutes; the myoplasty was 4 X 1 X 0.5 cm. These investigations establish the feasibility of using laser therapy to create a myoplasty trough that is similar in appearance to that typically achieved by the conventional blade technique. Illumination of the intraventricular operative field and precise modeling of the myoplasty trough constitute the principal advantages of laser myoplasty for HC.