Transmyocardial Laser Revascularization Channels Research Articles

Transmyocardial laser revascularization and phosphorylation of endothelial adherens junction and cytoskeletal elements

Transmyocardial laser revascularization (TMLR) is a technique whereby channels are made, using lasers or hypodermic needles, through ischemic tissue of the left ventricle. TMLR has been reported to be highly effective in relieving angina in patients who are unsuitable candidates for bypass surgery or angioplasty. However, the treatment remains somewhat controversial because the mechanisms by which TMLR lead to apparent benefits are unknown. Others have shown that within one to two weeks’ post‐TMLR treatment, areas of the infarcted myocardium exhibit significant increases in the levels of transforming growth factor‐β and basic‐fibroblast growth factor. In addition, the treated areas exhibited an increase in microvessel concentration, suggesting that the beneficial effects of TMLR may be connected with angiogenesis. Because angiogenesis is rooted in endothelial cell migration and growth, we wanted to determine any possible impact that TMLR might have on well‐known endothelial processes. We were able to determine that in rats, 30–120 min. following TMLR, the left ventricular tissue surrounding TMLR channels exhibited increases in the phosphorylation levels of both β‐catenin and myosin light chain. Phosphorylation of these junctional and cytoskeletal elements is known to occur concomitant with alterations in the barrier function of the endothelium, which also can lead to angiogenic responses.

Transgenic and transmural revascularization: regional myocardial tissue pressure during chronic ischemia*1

Channel patency and a cavito-myocardial pressure gradient are prerequisites for one potential mechanism of transmyocardial laser revascularization (TMLR), namely indirect (non-coronary) myocardial perfusion. We assessed the effect of TMLR combined with vascular endothelial growth factor (VEGF) on the myocardial tissue pressure (MTP) in chronic ischemia questioning firstly, whether transmural pressure allows perfusion of laser channels, and secondly, whether additional application of VEGF improves channel patency. One week after creation of an operative left anterior descending artery stenosis (2nd operation), pigs were designated to untreated ischemia (n=7), TMLR (n=8) or TMLR+VEGF-cDNA (2 mg intramyocardially, n=6). MTP and left ventricular pressure (LVP) were recorded simultaneously in the endo-, mid-, and epimyocardium before and after stenosis (1st operation), before and after therapy (2nd operation), and 12 weeks later (3rd operation). Myocardial samples were subjected to immunohistochemistry. Endo- and epimyocardial MTP exceeded LVP in all groups throughout the study, whereas midmyocardial MTP was constantly below LVP (P<0.05). Immediately after combined TMLR+VEGF, the endo-MTP decreased from 246.5+/-44.2 to 176.7+/-20.7 mmHg (P=0.043), remaining higher than LVP. After 12 weeks, it increased to 225.6+/-31.8 mmHg (P=0.04), but did not reach baseline values (P=0.04). Histological examination revealed occluded channels with surrounding vascular proliferation in both treatment groups. Additional VEGF-cDNA application in the vicinity of TMLR channels does not improve long-term patency. Direct blood flow from the cavity into the myocardium is impossible due to the high endomyocardial pressure. This limitation might be overcome by implantation of endomyocardial stents.

Survival after transmyocardial laser revascularization in relation to nonlasered perfused myocardial zones

Background. Transmyocardial laser revascularization for severe diffuse coronary artery disease reduces angina significantly. The effect on survival, however, is questionable, and risk factors are not adequately addressed. Considering that transmyocardial laser revascularization channels do not remain patent for improving direct myocardial blood supply, other variables such as perfusion through open native or grafted vessels in remote non–transmyocardial laser revascularization areas are probably more important for survival. This hypothesis is the subject of the study. Methods. Transmyocardial laser revascularization was performed with a CO 2 laser system in 63 patients between October 1995 and December 1997. Patients received transmyocardial laser revascularization alone or in combination with coronary artery bypass grafting. The heart was divided into three perfusion zones as determined by the three major coronary arteries. Patients were divided into three groups according to the number of zones that were perfused by either a native coronary artery or a patent bypass graft: group 1 (n = 9), none; group 2 (n = 24), one; and group 3 (n = 30), two. Follow-up was performed at 3, 6, 12, 24, and 36 months and was 100% complete. Mean latest follow-up was 26.2 months, minimal follow-up of survivors was at least 12 months. Results. Overall mortality was remarkably higher in group 1 (77.8%) compared with group 2 (20.8%, p = 0.005) and group 3 (13.3%, p = 0.001). Hospital mortality was 22.2% in group 1, 0% in group 2, and 3.3% in group 3. Late mortality was also higher in group 1 (55.5% versus 20.8%, and versus 9.9%, respectively). Cardiac deaths were more frequent in group 1 (55.5% versus 12.5% in group 2, p = 0.02, and versus 9.9% in group 3, p = 0.009). The number of perfused myocardial zones showed a significant influence for survival ( p = 0.002). Conclusions. These data give some directional evidence that survival seems to be beneficially affected by the number of nonlasered perfused myocardial zones through native vessels or grafts in patients undergoing transmyocardial laser revascularization.

Improved neoangiogenesis in transmyocardial laser revascularization combined with angiogenic adjunct in a pig model

Recent research has revealed neoangiogenesis as a basic phenomenon in transmyocardial laser revascularization (TMLR). Theoretically, myocardial neoangiogenesis could be further enhanced by the addition of angiogenic growth factors. Triads of TMLR channels were created in the lateral wall of the left ventricles of 12 pigs (mean body weight 73+/-5 kg), using a holmium:yttrium-aluminium garnet (YAG) laser. The animals were allocated randomly either to receive an injection of 100 microg of a bovine bone-derived growth factor mixture within the triads (n=6), or to a control group (n=6). Animals were killed 1 month later. Capillary and arteriolar densities were determined by computed morphometric analysis of histological sections of the triads. The capillary density of myocardial areas within the triads was significantly greater in the group receiving the bovine bone-derived growth factor mixture than in the control group (14.3+/-3.5/mm(2) and 5.7+/-1.4/mm(2) respectively; P<0.001). The difference was also significant when considering arteriolar density (0.7+/-0.4/mm(2) and 0.2+/-0.1/mm(2) respectively; P<0.001). For comparison, capillary and arteriolar densities of the TMLR channel scars were 48.7+/-9.7 and 1.9+/-0. 5/mm(2) respectively in the angiogenic group, and 46.3+/-13.7 and 2. 3+/-1.3/mm(2) respectively in the control group (no significant differences). These results demonstrate that the addition of angiogenic factors to TMLR stimulates neoangiogenesis significantly in the areas adjacent to the channels, but not within the channel scars. The latter are themselves strongly vascularized. Hence this combined approach, potentiating the effect of TMLR by establishing vascular connections between the neovessels of the channel scars, has the potential for improved clinical outcome.

Histologic evidence that basic fibroblast growth factor enhances the angiogenic effects of transmyocardial laser revascularization.

To determine whether addition of basic fibroblast growth factor (bFGF), an angiogenic growth factor, enhances the angiogenic effects of transmyocardial laser revascularization (TMR). TMR is an investigational therapy for treating patients with medically refractory angina not amenable to traditional therapies. Histologic and blood flow studies in animals have suggested that TMR enhances angiogenesis above that normally seen in ischemic myocardium. We tested the hypothesis that bFGF administered into TMR channels further enhance the angiogenic effects of TMR. Chronic ischemia was created in 3 groups of dogs using an ameroid constrictor on the proximal LAD. In the bFGF group (n = 5) non-transmyocardial channels were created in the LAD territory and bFGF, (100 ng/ml) dissolved in pluronic gel was injected into the each channel. In the TMR group (n = 7), transmyocardial channels were created without bFGF. A control group (n = 7) had ischemia without TMR of bFGF. 5-bromo-2'-deoxyuridine (BrdU) was administered to mark proliferating cells. After 8 weeks survival, colored microspheres were injected to assess the regional myocardial blood flow. TMR and TMR+bFGF increased total vascular density by approximately 40% over that observed in the control group. However, the number of large vessels (internal diameter > or = 50 microm) was doubled by the addition of bFGF, and this correlated with a 50% increase in the density of proliferating vascular cells and a tripling of the total estimated vascular cross sectional area. Blood flow to the LAD territory was increased by TMR compared to controls, with no further benefit observed in the bFGF group. On a histologic basis, basic fibroblast growth factor further enhances angiogenesis following TMR in ischemic myocardium mainly by increasing the size but not the total number of vessels.

Neovascularization after transmyocardial laser revascularization in a model of chronic ischemia

Background. The mechanism of clinical improvement after transmyocardial laser revascularization (TMR) is unknown. One hypothesis holds that TMR causes increased myocardial perfusion through neovascularization. This study sought to determine whether angiogenesis occurs after TMR in a porcine model of chronic myocardial ischemia.Methods. Six miniature pigs underwent subtotal left circumflex coronary artery occlusion to reduce resting blood flow to 10% of baseline. After 2 weeks in the low-flow state, dobutamine stress echocardiography and positron emission tomography were performed to document ischemic, viable myocardium. The animals then underwent TMR and were sacrificed 6 months later for histologic and immunohistochemical analysis.Results. Histologic analysis of the lased left circumflex region demonstrated many hypocellular areas filled with connective tissue representing remnant TMR channels. Histochemical staining demonstrated a highly disorganized pattern of neovascularization consistent with angiogenesis located predominantly at the periphery of the channels. Immunohistochemical analysis confirmed the presence of endothelial cells within neovessels. Vascular density analysis revealed a mean of 29.2 ± 3.6 neovessels per high-power field in lased ischemic myocardium versus 4.0 ± 0.3 (p < 0.001) in nonlased ischemic myocardium.Conclusions. This study provides evidence that neovascularization is present long term in regions of ischemic, viable myocardium after TMR. Angiogenesis may represent the mechanism of clinical improvement after TMR.

Evidence of Vascular Growth Associated With Laser Treatment of Normal Canine Myocardium

Background. Transmyocardial laser revascularization is a new therapy for patients with refractory angina. Although clinical studies suggest that transmyocardial laser revascularization decreases angina and may improve regional blood flow, the underlying mechanisms are not elucidated. We hypothesized that one mechanism may relate to stimulation of vascular growth in laser-treated regions. Methods. Transmyocardial laser revascularization channels were made with holmium:yttrium-aluminum garnet or carbon dioxide lasers in eight normal canine hearts; animals were sacrificed 2 to 3 weeks later and examined for vascular density and for evidence of smooth muscle proliferation. Results. The original channels were infiltrated by granulation tissue with associated vascularity. Vascular growth was stimulated immediately surrounding the channel remnant as evidenced by an increase in the number of vessels (approximately twice that of the control region) and an increase in the number of vascular cells staining positive for markers of cellular proliferation. Conclusions. Transmyocardial laser revascularization leads to local vascular growth as early as 2 weeks after treatment. It remains to be determined whether this mechanism contributes to increased regional blood flow or to clinical benefits associated with this novel form of therapy.

Angiogenesis: a possible mechanism underlying the clinical benefits of transmyocardial laser revascularization.

While clinical reports indicate that significant relief of angina is achieved with transmyocardial laser revascularization (TMLR), the mechanisms of benefit are still a matter of considerable controversy. Studies in our laboratory, as well as in the laboratories of other investigators, have challenged the classic hypothesis that benefits are derived from blood flow through chronically patent channels. While several alternatives have been proposed, our work has focused on investigating the possibility that TMLR stimulates vascular growth in the region around the TMLR channels. We have performed studies looking at histologic markers of vascular growth (including vessel counting and cellular proliferation assays) in order to test this hypothesis, the results of which are reviewed. In brief, we find that TMLR markedly enhances myocardial vascular growth above what is seen normally in ischemic myocardium. We hypothesize that the underlying mechanism relates to liberation of growth factors by inflammatory cells, which are recruited in response to the laser induced myocardial injury. Clarification of whether this mechanism contributes to observed clinical benefits is of fundamental importance, since such understanding may suggest means of enhancing the process.

Histological findings after transmyocardial laser revascularization.

In recent time, it has become more and more probable that patients with severe diffuse coronary artery disease, who are not candidates for aortocoronary bypass surgery or percutaneous transluminal coronary angioplasty procedures, can benefit from transmyocardial laser revascularization (TMR). But the underlying principle of TMR still remains unclear. This study reports on a histological analysis of eight patients, in whom a total of 250 channels had been created, who died after TMR. The TMR channels were created by a CO2 laser surrounded by a zone of necrosis with an extent of about 500 microns. In the hearts of patients who died in the early postoperative period (1 to 7 days postoperative), almost all channels were closed by fibrin clots, erythrocytes, and macrophages. There were no obvious connections between the channels and the ventricular cavity. In specimens from patients, who died 2 or more weeks after the procedure, a granular tissue with high macrophage and monocyte activity was observable. Within this tissue, we observed a developing network of capillaries. Otherwise, the tissue filling the channels did not substantially differ from scar tissue. We failed to observe connections between the ventricular cavity and the new capillaries. Whether these vessels within the closed channels have any impact on myocardial perfusion remains unclear, but it seems unlikely that the clinical effects of TMR are based on the principle of the amphibian heart.

901-10 Demonstration of Blood Flow Through Transmyocardial Laser Channels

Transmyocardial laser revascularization (TMR) is a new procedure under investigation for treatment of patients with myocardial ischemia who are unresponsive to other conventional therapies. In theory, TMR channels passing from the LV chamber directly into the myocardium allow oxygenated blood to directly perfuse the myocardium, bypassing the coronary vessels. However, experimental evidence that blood actually flows through the channels has not been available previously. Therefore, the purpose of this study was to test whether blood can reach the myocardium by passing through laser channels. An average of 28 channels/heart were made over the LAD distribution in 5 canine hearts using a pulsed holmium/YAG laser (20 pulses, 0.6J/pulse) with energy delivered from the endocardium through a 400 μ fiber optic cable introduced via the left atrial appendage. The heart was then excised while coronary perfusion pressure was maintained constant at 120 mmHg by retrograde aortic blood flow provided by a second dog (aortic valve always remains closed). The proximal LAD and epicardial collaterals were ligated. Collateral flow to the ischemic region, measured by colored microspheres injected into the coronary artery, was 20.5% of normal (0.42 ± 0.28 ml/g/min vs. 2.05 ± 0.68 ml/g/min in the non-ischemic region). Colored spheres were then injected into blood placed within the LV chamber to achieve peak LV pressure generation of 20 mmHg (low loading) and then 60 mmHg (high loading) (this blood is not ejected into the aorta). The number of spheres detected per gram of myocardium in the circumflex ILCX) region (control area without TMR channels) and in the LAD region with TMR channels are shown in the table as a function of LV loading: Region Low loading High loading LCX without channels 33.4 ± 64.5 5.84 ± 13.9 LAD with channels 2908 ± 249.8* 2205 ± 382.3 * * P &lt; 0.0001 vs. control The number of spheres in the channel region was significantly higher than in the control region but did not vary with loading conditions. These data provide direct evidence that blood from the LV can reach the myocardium through TMR channels in the acute setting. Quantification of the flow and whether this can be maintained in the chronic setting are important questions for future research.