Michelle Thompson

Selective coagulation necrosis of canine adventitia and media induces extracellular matrix accumulation without neointima formation

Endothelial cell injury, the disruption of the internal elastic membrane and medial damage represent important stimuli for the development of a neointima. It is unclear whether selective adventitial and medial injury also induce neointima formation. Incremental argon laser energies (11.4-180 J/cm2) were applied to the external surface of dog femoral arteries to evaluate the vascular repair of acute adventitial or medial necrosis without injury of the intima. The animals were sacrificed either one hour after the initial procedure or after an 8 week follow up period for histologic examination. Acute, and mild to moderate necrosis of the arterial wall was found above 50 J/cm2. Ablation of the internal elastic membrane or mural thrombi was not detected. Eight weeks after photocoagulation with laser energies above 50 J/cm2, a significant increase in mean wall thickness of the media was observed. The medial thickening was characterised by an accumulation of extracellular matrix and a loss of smooth muscle cells. Necrosis of adventitia and media resulted in arterial wall thickening without neointima formation. It is concluded that, in dogs, an acute, selective injury of adventitia and media stimulates the production of extracellular matrix and not the proliferation of cells. Smooth muscle cell migration and subsequently neointima formation are induced by viable smooth muscle cells when blood-borne stimuli are available.

Computer simulated light distributions in myocardial tissues at the Nd-YAG wavelength of 1064 nm

Continuous wave Nd-YAG myocardial laser photocoagulation of ventricular tachycardia at the 1064 nm wavelength is a new investigational procedure in the treatment of arrhythmias. Computer simulated light diffusion patterns, generated by the Monte Carlo technique, are used to illustrate the calculated light distribution in myocardial tissues such as normal, and photocoagulated (canine) myocardium, epicardial fat on myocardium, and aneurysm for the optical properties at the Neodymium-YAG wavelength of 1064 nm.Calculated total reflection from myocardial tissue increased from 16% to 28% as a result of photocoagulation in the Monte Carlo light distribution simulations. The calculated total transmission dropped from 0.8% to 0.1% as a result of coagulation. Transmission reduced from 5% to 0.8% due to an increase in absorption from 0.04 mm−1 to 0.1 mm−1, based on capillary blood absorption. Adding an epicardial fat layer to normal myocardium total reflection and backscatter increased from 16% to 44%.Computer simulations for normal and pathological myocardium showed a wide diversity in light propagation characteristics, indicating the importance of including all myocardial tissues in light dosimetry analyses.

Computerized three-dimensional activation mapping study of spontaneous ventricular arrhythmias during acute myocardial ischemia in dogs: evidence against macroreentrant mechanism

This study was undertaken to investigate the activation patterns of spontaneous ventricular arrhythmias during acute myocardial ischemia in dogs. In 14 open-chest dogs, the left anterior descending coronary artery was occluded for 2 hours. Three-dimensional activation maps were derived from 240 bipolar sites by insertion of 60 plunge needle electrodes into both ventricles and the septum. Global ventricular activation sequences were displayed in five planes in 10 dogs, whereas the high density regional activation maps of the anterior wall were displayed in four layers in 4 dogs. Three-dimensional activation maps of 95 sinus beats, 82 premature ventricular complexes (PVCs), and 210 beats of ventricular tachycardia (VT) were analyzed. Sinus beats had a uniform activation pattern with total ventricular activation times measuring 42 +/- 4 ms and 67 +/- 8 ms during baseline and ischemia, respectively (P < .05). The PVCs and VTs originated from the subendocardial and intramural layers, and activation patterns invariably suggested focal excitation. Macroeentry was not operative because (1) the breakthrough sites were always remote from the latest activation areas; (2) there was no electrical activity bridging the gap between the termination of a beat and initiation of the subsequent beat; and (3) impulse conduction was not sufficiently delayed to reexcite the area of impulse origin even though functional conduction block was frequently present. In high-density regional activation maps, fragmented activity spanning the diastolic interval was never found. In conclusion, spontaneously occurring PVCs and VTs during acute myocardial ischemia in dogs display focal excitation with no evidence of macroreentry.

Myocardial ischemia and infarction can be detected by microwave spectroscopy. I. Experimental evidence

The two dimensional (2D) microwave tomographic system has been constructed. The cross sectional static and dynamic images of the canine explanted heart have been reconstructed. It is our hypothesis that microwave spectroscopy (multifrequency probing) is a sensitive indicator of myocardial ischemia and infarction. The specific aim of this investigation is to evaluate our hypothesis and quantify the changes in the dielectric properties of the myocardium as a function of the degree of ischemia and infarction. Various experiments were conducted using canine: 100% left anterior descending coronary artery (LAD) long time occlusion, 100% LAD short time occlusion/reperfusion sequence, partial LAD flow reduction, chronic myocardium infarction (MI) (5 days, 2 weeks, 5 months). It has been experimentally proven that microwave spectroscopy is a sensitive indicator of the myocardial ischemia and infarction.

Determination of optical properties in situ from radial backscattering profile using a CCD camera

Local tissue optical parameters are of principal importance in determining the ideal dosage of laser energy and choosing the most suitable therapeutic wavelength for a specific organ. We have developed a non-invasive method for the determination of optical parameters of whole tissue in-situ. Backscattered laser light from a pencil beam incident on the tissue surface was measured using a CCD camera interfaced with a personal computer. Backscatter measurements were performed on in-vitro canine myocardium and on phantoms, with optical properties predetermined by the integrating sphere method. Backscattering radiance proved to be a function of the distance to the incident beam. Regression analysis of the backscatter showed that the backscatter can be expressed as an exponential function of the distance to the beam with a regression coefficient ((mu) r) resembling the tissue diffusion coefficient ((mu) eff), for tissue thickness much greater than optical free path. The CCD derived backscatter radiance exponent showed excellent agreement with the diffusion coefficient, with a correlation coefficient of 0.91. The backscattering technique might prove to be a simple, non-invasive, accurate new technique in the determination of in-vivo optical properties.

Electrophysiologic characteristics of manifest and latent retrograde conduction in dogs

Atrioventricular (AV) nodal reentry requires intact retrograde ventriculoatrial (VA) conduction. The purpose of this study was to assess the contribution of various pacing and pharmacologic techniques to uncover VA conduction during apparent unidirectional VA block, and to evaluate the role of several biologic and electrophysiologic factors in concealment of retrograde conduction. Forty healthy dogs underwent catheter-electrophysiologic studies of AV and VA conduction. Group I (20 animals) had intact VA conduction. Group II (six animals) had VA dissociation with ventricular pacing initiated during sinus rhythm, but the presence of VA conduction was confirmed by isoproterenol infusion or by premature ventricular stimulation. In group III (14 animals), the above techniques failed to uncover VA conduction. Eight of 14 group III animals underwent thoracotomy and crushing or freezing of the sinoatrial (SA) node. Ventricular pacing initiated during sinus standstill was associated with 1:1 VA conduction in each experiment. VA conduction time and retrograde Wenckebach cycle length, both in the baseline state and during isoproterenol infusion, were significantly longer in the eight animals in group III than in those in group I. Age, gender, weight, breed, sinus cycle length, and anterograde AV conduction properties were not significantly different between groups I, II, and III. The data suggest that (1) in normal dogs, complete unidirectional VA block probably does not exist; (2) in the presence of anterograde input to the AV node, even sophisticated pacing and pharmacologic maneuvers may fail to uncover the presence of VA conduction; (3) although anterograde input is essential for concealment of VA conduction, the phenomenon is more closely associated with depressed retrograde than with anterograde AV nodal characteristics.(ABSTRACT TRUNCATED AT 250 WORDS)

Non-linear optical phenomena in contact fiber laser-photocoagulation of myocardium

The rise time of epicardial temperatures on the left ventricular free wall during endocardial transcatbeter delivery of Nd:YAG laser energy, recorded by thermography has sparked interest in anomalous optical phenomena. Transmission measurements on whole blocks of myocardium showed a decrease in the attenuation coefficient at high power densities. The definition of the attenuation coefficient is the natural log of the ratio of the incident and the transmitted beams, divided by the physical thickness of the sample. When Nd:YAG laser light at the 1064 um wavelength irradiated fresh myocardial tissue from the free left ventricular wall, the incident power density increased from 0.0026 W/mm2 to approximately 200 W/mm2, the attenuation coefficient dropped from 0.97 ± 0.11 mm−1 to 0.77 ± 0.07 mm−1. This would indicate a non-linearity in the optical properties of the tissue as a function of the laser irradiation power density.

Laser photoablation of experimental post-infarction ventricular tachycardia guided by three dimensional activation mapping

The purpose of this study was to evaluate the efficacy of epicardially delivered laser energy to ablate induced ventricular tachycardia in a post‐infarction canine model.

Exploring the optical window of myocardial tissue for localized laser ablation of ventricular tachycardia

Interest in lasa ablation of deep seated anthymic foci in myocardium has created the study for the optical properties of myocardiu~l] . With the usc of a Ti:Sapphire laser and intergrating sphere me&ods[2] we have investigated the spectral range of 690-10SOnm of myocardial tissue. Collimated 8s wet1 as diffuse light was transmitted hough bloodless tissue and measurements of the transmission and reflection were made. From these measuremeats the absorption, scattering and r e d u d scattering coefiiecients were determined. A minimum of these pprameters were found to be in the range of 800-84onm.

Broadband fiber optic interferometry: diagnostics by depth profiling of optical properties of biological media

Although non-invasive methods for in-situ analysis of laser-tissue interaction are not available yet, it continues to be an area of increased interest owing to its potential. This paper describes the development of an integrated diagnostic and therapeutic laser procedure through in-situ monitoring of the optical tissue characteristics in a non-invasive manner. Biological tissues are highly scattering media. The system under development uses a broadband (white-light) interferometer that derives the scattered signals obtained from different depths of the biological medium. The tissue light signature obtained in this manner will be curve fitted to the appropriate computer simulation resulting from the optical properties particular to well defined tissues, resulting from simulations within a range of optical parameters. The beam profile of the irradiation source appears to affect the ability to distinguish between certain tissues. A single mode fiber in tissue contact produces a statistically significant different light signature for at least three myocardial tissues. However, the theoretical resolution is limited to 4 to 8 mm depth. Identification of the local optical characteristics will provide discrimination between healthy and pathological conditions in addition to real time assessment of dosimetry.