Detection and assessment of laser-mediated injury in transmyocardial revascularization

Cellular Destruction Following Transmyocardial Laser Revascularization (TMR)

During transmyocardial revascularization, cellular destruction of cardiomyocytes occurs as a result of the high-energy laser. However, the features of myocardial cellular destruction are unclear. The present study was undertaken to examine the structural characteristics of cell death in the myocardium following transmyocardial revascularization. Myocardial specimens from 3 male patients who had died within 11 days following laser revascularization were collected within 1 h of death and were analyzed by immunohistochemistry and electron microscopy. For immunohistochemistry, antibodies to pro-apoptotic proteins CPP32 and BAX were used. Immunohistochemical examination demonstrated the presence of cells expressing both CPP32 and BAX along the laser channel. Electron microscopic analysis revealed that the lining surface of laser channels consisted of condensed acellular debris and dead cells. No endothelialization of channels was noted. The lumen of laser channels were surrounded by a rim of acellular debris with several outer concentric rims of cardiomyocytes showing features of cellular destruction. The present study identified features of both necrotic and apoptotic cellular death following laser revascularization.

Excimer laser (308 nm) based transmyocardial laser revascularization: effects of the lasing parameters on myocardial histology

BACKGROUND AND OBJECTIVE

The effect of the excimer laser (308 nm) parameters on transmyocardial revascularization (TMR) channels is not well defined. This study investigates the influence of the pulse repetition rate, the size of the delivery catheter and its advancement speed on the morphology of TMR channels in vivo.

STUDY DESIGN/MATERIALS AND METHODS

Myocardial ablation was performed in a porcine model (N = 27) using multifiber catheters of 1.0 and 1.4 mm in diameter. The catheters were advanced into the myocardium at different speeds (1.27 and 2.54 mm/sec) while ablating at various repetition rates (10-80 Hz). The radiant exposure was kept at 35 mJ/mm(2) throughout the experiments. The channel histology was quantified by digital microscopy.

RESULTS

The channel cross-sectional area and the extent of the thermal damage decrease as the catheter advancement speed exceeds the ablation speed and vice versa. Within the parameters tested, advancement speed of about 1.3 mm/sec and pulse repetition rates of 40 Hz produce channels of size comparable to the catheter's diameter with moderate thermomechanical damage.

CONCLUSIONS

The repetition rate, catheter size, and catheter advancement speed are closely intertwined and crucial to the histological outcome of excimer laser based TMR.

Transmyocardial laser revascularization

Low-powered lasers were first used in the early 1980s to produce transventricular channels as an adjunct to coronary artery bypass graft surgery (CABG). Early results were encouraging, but because of the combined procedure, could not be attributed directly to use of the laser [1]. High-powered lasers were introduced into clinical practice in 1990 [2]. These lasers are powerful enough to create a transmyocardial channel with minimal thermal damage to surrounding tissues [3]. Clinical studies, using transmyocardial laser revascularization (TMR) as the sole operative therapy for patients with severe and diffuse coronary artery disease (CAD) who have Class III or IV angina, and are on medical therapy, have been conducted since 1993. Based on the results of these studies, the FDA granted approval for the use of TMR as a sole therapy. Clinical studies are currently underway to assess the results of combined TMR and CABG [4]. Results of four controlled randomized studies have been published [5-8]. The data from two of these studies formed the basis for FDA approval of two different types of laser systems. The results of these studies have not provided any additional insights into the mechanism of action of TMR, which remains the Achilles' heel of this procedure. In this review, background information about the TMR procedure will be discussed along with an analysis of the recently published randomized studies.

Acute and chronic effects of transmyocardial laser revascularization in the nonischemic pig myocardium by using three laser systems

BACKGROUND AND OBJECTIVE

Transmyocardial laser revascularization (TMLR) improves symptoms in patients with coronary heart disease. It is based on the hypothesis of direct perfusion of ischemic myocardium by means of laser-created channels. Three different lasers were used to study alternative effects on myocardium.

STUDY DESIGN/MATERIALS AND METHODS

The present study was conducted to evaluate comprehensively and compare the short and long-term tissue effects and the basic interaction mechanisms of CO2, Ho:YAG, and Er:YAG laser radiation with myocardium. The dynamics of laser-induced impacts in gel used as tissue phantom was visualized by time-resolved flash photography. Pressure measurements performed during perforation of myocardium in vitro revealed the explosive character of the ablation process. Channels made into the left ventricle of normal pig hearts were examined immediately and 6 weeks after creation.

RESULTS

Regardless of laser source, all channels became occluded within 6 weeks by scar. Minimal acute thermal damage by Er:YAG laser corresponded to smaller scars. Pulsed Ho:YAG caused stronger tissue tearing than continuous wave CO2 irradiation. An increased volume density of intramyocardial vessels was found about the scars 6 weeks after treatment with all lasers.

CONCLUSION

The laser sources permitted to study outcome of pressure effects and thermal damage in vivo. There were only minor differences between the three laser systems used. Rapid channel occlusion suggests that rather than revascularization, subsidiary physiologic tissue effects elicited by the thermal, oxidative, or mechanical action of the laser impact may contribute to the beneficial clinical effects of TMLR.

Success of transmyocardial laser revascularization is determined by the amount and organization of scar tissue produced in response to initial injury: results of ultraviolet laser treatment

BACKGROUND AND OBJECTIVE

Previous studies of transmyocardial laser revascularization have reported open channels after ultraviolet laser treatment and closed channels with infrared lasers. We speculated that differences in long-term channel patency were determined by the healing response to injury.

METHODS

Channels were made in rat hearts with a frequency-tripled neodymium:YAG laser, at 5 and 10 mJ per pulse, by advancing an optic fiber through the myocardium, from the epicardium to the ventricular cavity. Several months later, we challenged the ability of the channel to supply blood by arterial occlusion and examined the channel structure with polarized light microscopy.

RESULTS

Low-pulse energy was associated with lower patency, more fibrosis, and larger infarcts than was the higher energy. Open channels were surrounded by collagen fibers aligned parallel to the channel; in closed channels, fibers were aligned perpendicular to the original channel direction.

CONCLUSION

The amount of initial injury and its repair determine channel patency and function.

Transmyocardial revascularization with CO2 laser in patients with refractory angina pectoris. Clinical results from the Norwegian randomized trial

OBJECTIVES

The purpose of the study was to evaluate clinical effects, exercise performance and effect on maximal oxygen consumption (MVO2) of transmyocardial revascularization with CO2-laser (TMR) in patients with refractory angina pectoris.

BACKGROUND

Transmyocardial laser revascularization is a new method to treat patients with refractory angina pectoris not eligible for conventional revascularization. Few randomized studies comparing TMR with conventional treatment have been published.

METHODS

One hundred patients with refractory angina not eligible for conventional revascularization were block-randomized in a 1:1 ratio to receive continued optimal medical treatment (MT) or TMR in addition to MT. The patients were evaluated at baseline and at three and 12 months with end points to symptoms, exercise capacity and MVO2.

RESULTS

Transmyocardial laser revascularization resulted in significant relief in angina symptoms after three and 12 months compared to baseline. Time to chest pain during exercise increased from baseline by 78 s after three months (p = NS) and 66 s (p < 0.01) after 12 months in the TMR group, whereas total exercise time and MVO2 were unchanged. No significant changes were observed in the MT group. Perioperative mortality was 4%. One year mortality was 12% in the TMR group and 8% in the MT group (p = NS.)

CONCLUSIONS

Transmyocardial laser revascularization was performed with low perioperative mortality and caused significant symptomatic improvement, but no improvement in exercise capacity.

Transmyocardial laser revascularization: current status

Transmyocardial laser revascularization (TMLR) has been widely evaluated for treatment of the ischemic myocardium either in conjunction with coronary artery bypass grafting or as sole therapy. Clinically, it has shown significant improvement for angina symptoms, but the mechanism by which this modality works is unknown at this time. The original premise on which transmyocardial revascularization was established depended on its ability to essentially generate channels that would directly carry blood from the ventricle into the ischemic myocardium. This theory, however, has not been substantiated, so other mechanisms have been postulated. This article gives a historical perspective on the advent of transmyocardial revascularization and the many animal and human studies that have paved the way for its clinical use. Current controversies are examined, along with the new advances in laser technology and where the future of TMLR is headed.

Transmyocardial revascularization: the fate of myocardial channels

Attempted cardiac revascularization through laser-made channels has gained considerable recent notoriety. Although the treatment reduces angina, its ability to enhance perfusion is unclear, and the mechanism of action unknown. The fate of the channels appears an obvious place to look for insight. Therefore, this review focuses on temporal and spatial changes in channel morphology. An appreciation of the natural history of the channels not only has potential to elucidate mechanisms, but also to provide the basis for optimization of channel-making.

Angiogenic response induced by mechanical transmyocardial revascularization

BACKGROUND

Angiogenesis is the proposed mechanism of transmyocardial revascularization. We evaluated mechanical transmyocardial revascularization in a chronically ischemic porcine model by measuring myocardial angiogenic response.

METHODS

Ameroid constrictors were implanted 6 weeks before mechanical transmyocardial revascularization. Group I (n = 5) and group II (n = 3) animals received 30 punctures with an 18-gauge needle and samples were harvested at 1 and 4 weeks, respectively, after the operation. Group III (n = 5) had sternotomy only and served as the control group. Myocardial samples were immunohistochemically stained for vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), and transforming growth factor beta (TGF-beta) using specific antibodies. Growth factor expression was quantified by means of computer-assisted morphometry. Vascular density was assessed by immunohistochemical stain for VEGF and factor VIII.

RESULTS

Compared with group III, increased angiogenic factor levels were found in group I (VEGF 0.47 +/- 0.03 mm(2) vs 0.05 +/- 0.05 mm(2), P =.000; bFGF 0.67 +/- 0.14 mm(2) vs 0.03 +/- 0.03 mm(2), P =. 000; TGF-beta 1.40 +/- 0.18 mm(2) vs 0.09 +/- 0.06 mm(2), P = 0.000), and in group II (VEGF 0.34 +/- 0.06 mm(2) vs 0.05 +/- 0.05 mm(2), P =.003; bFGF 0.06 +/- 0.02 mm(2) vs 0.03 +/- 0.03 mm(2), P =.135; TGF-beta 0.28 +/- 0.09 mm(2) vs 0.09 +/- 0.06 mm(2), P =.042). Vascular densities after mechanical transmyocardial revascularization were also increased (group I, VEGF stain 8.1 +/- 0. 6 vs 1.1 +/- 0.5, P =.000; factor VIII stain 5.1 +/- 2.7 vs 0.4 +/- 0.3, P =.018; group II, VEGF stain 1.9 +/- 0.5 vs 1.1 +/- 0.5, P = 0. 107; factor VIII stain 2.3 +/- 0.4 vs 0.4 +/- 0.3, P =.004).

CONCLUSIONS

Mechanical transmyocardial revascularization is associated with increased angiogenic factor expression and concomitant neovascularization at up to 4 weeks. These changes are indistinguishable from those of laser transmyocardial revascularization. Myocardial perfusion studies are needed to establish the functional significance of these angiogenic changes.

Laser-mediated reversal of cardiac expansion after myocardial infarction

BACKGROUND AND OBJECTIVES

A complication of transmural myocardial infarction is infarct expansion, which can lead to the development of heart failure. However, the necrotic muscle is replaced by collagen, a material known to shrink when heated. Thus, the hypothesis was that thermally-induced scar shrinkage could reverse infarct expansion.

STUDY DESIGN/MATERIALS AND METHODS

Four weeks after transmural infarcts were produced by coronary occlusion, rats were randomized to control or treatment with a neodymium:yttrium-aluminum-garnet laser. The epicardial scar surface was irradiated until shrinkage was observed. Thirty minutes later, hearts were excised and fixed at a distending pressure of 15 mm Hg, left ventricular cavity volume was measured, and histologic analysis was performed.

RESULTS

Cavity volume was reduced by laser treatment (0.72 +/- 0.07 ml vs. 0.54 +/- 0.05 ml; P= 0.044). In addition, treatment resulted in thicker scars, a leftward shift of the heart's electrical axis, and straightening of collagen fibers.

CONCLUSION

Laser treatment resulted in thermally-mediated scar shrinkage, which reversed infarct expansion and reduced cavity volume.

Thoracic Surgery Directors Association Award. Angiogenesis in transmyocardial revascularization: comparison of laser versus mechanical punctures

BACKGROUND

Transmyocardial laser revascularization (TMLR), which has been shown to reduce angina in clinical trials, was originally based on the belief that laser channels are unique and can remain patent. An increasing body of evidence indicates otherwise, and transmyocardial revascularization (TMR) angiogenesis is currently thought to be induced by nonspecific inflammatory response to tissue injuries. We tested the hypothesis that mechanical transmyocardial revascularization (TMMR) may induce angiogenic responses similar to that seen with lasers.

METHODS

Ameroid constrictors were implanted around proximal circumflex arteries of porcine hearts. Six weeks later, they were randomly assigned (n = 5 each) to receive 10 transmural channels in the ischemic zone by a carbon dioxide laser (group I) or by a needle (group II). A third group (group III) had 30 needle channels in the same area, while a control group (group IV) received no TMR. The hearts were harvested 1 week later, and, using immunohistochemistry, vascular endothelial growth factor (VEGF) expression was studied and quantified by computerized morphometric analysis. Densities of vascular structures positively stained for VEGF per high-power field (HPF) were also compared.

RESULTS

Virtually no TMR channels remained patent histologically. Group III had a significant higher level of total VEGF expression (14.18+/-0.78 mm2) compared with group I (7.07+/-2.06 mm2, p < 0.001) and group II (4.74+/-3.35 mm2, p < 0.001). Vascular density was significantly elevated in all treatment groups compared with the control (group I, 7.7+/-0.8/HPF vs group II, 4.5+/-2.3/HPF vs group III, 8.1+/-0.6/HPF vs group IV, 1.1+/-0.5/HPF).

CONCLUSIONS

In view of the significant cost implications, our findings that needle punctures may also induce angiogenic response comparable with that with laser suggest that it is justifiable and desirable to include a TMMR arm for comparison with TMLR in future clinical trials.

Clinical experience with the holmium:YAG laser for transmyocardial laser revascularization and myocardial denervation as a mechanism

Transmyocardial laser revascularization is emerging as a treatment option for patients with debilitating angina pectoris and no conventional treatment option. Results with the high powered CO2 laser have been reproducibly encouraging. As the mechanism of action remains uncertain, the importance of the type of laser used is unknown. Interest in developing less invasive approaches, ultimately including catheter-based technologies, has stimulated the evaluation of alternative laser energy sources with fiberoptic delivery systems. One such device is the holmium:YAG laser. Clinical experience with the holmium:YAG-based systems is still in its early stages. We review our experience with one such device and the existing literature on the subject. We also briefly describe experimental evidence that this device is capable of denervating the heart locally.

Transmyocardial laser revascularization--a technique in evolution

Transmyocardial laser revascularization (TMLR) is a rapidly evolving technique that represents the recent rediscovery of an old therapy. A growing clinical experience is being assimilated into clear indications and contraindications for and the appropriate performance of this procedure. This technique can be judged to improve patient quality of life and survival in appropriately selected patients. The therapy should still be applied cautiously, in that a full understanding of its indications and mechanism of action remains in a state of evolution.