Richard H. Clarke

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.

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.

Optically detected zero field magnetic resonance studies of the photoexcited triplet states of chlorophyll a and b

The zero‐field EPR transitions for the triplet states of chlorophyll a and chlorophyll b in n‐octane solutions at 2°K have been observed by optical detection methods. Both triplet absorption detection of magnetic resonance and fluorescence‐microwave double resonance techniques were used. Sharp‐line (5–20 MHz) zero‐field EPR spectra were recorded for both molecules. The rates of depopulation for the individual triplet state spin sublevels were determined by microwave‐modulated fluorescence intensity measurements. These experiments show that the middle spin sublevel is the most active in triplet state intersystem crossing for both chlorophyll a and chlorophyll b.

Triplet state radiationless transitions in polycyclic hydrocarbons

The zero‐field EPR transitions and spin sublevel intersystem crossing rates for the lowest triplet states of tetracene, perdeuterotetracene, 1,2‐benzanthracene, and 1,2,3,4‐dibenzanthracene have been measured in n‐alkane solvents at 2°K by optical detection of magnetic resonance techniques. The rates of spin sublevel intersystem crossing are primarily nonradiative, and an analysis of these molecules, as well as the triplet state dynamics of several groups of polycyclic hydrocarbons, is presented with conclusions applicable to planar aromatic organic systems in general. Comparison of the triplet state intersystem crossing rates of the polycyclic hydrocarbons suggests that nonradiative transitions for the individual spin sublevels of the lowest triplet state may be qualitatively understood and predicted by a determination of the symmetry‐selected one‐center spin‐orbit coupling mechanism available to each triplet sublevel.

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.

Investigation of the structure of the reaction center in photosynthetic systems by optical detection of triplet state magnetic resonance

Abstract Properties expected of the triplet state of a pair of interacting pigment molecules in photosynthetic systems are described in terms of the triplet state properties of the monomer pigment. The triplet state zero-field splittings and intersystem crossing rate constants measured in isolated chlorophylls and the same properties measured in vivo are utilized to elucidate the geometry of the eraction center in photosynthetic systems.

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.

Investigation of the structure of the reaction center in photosynthetic bacteria by optical detection of triplet state magnetic resonance.

Measurements of the triplet state zero-field splitting and intersystem crossing rate constants for isolated bacteriochlorophyll a and for chemically reduced photosynthetic bacteria are utilized to investigate the geometry of the bacteriochlorophyll dimer in the reaction center.