Neovascularization after transmyocardial laser revascularization in a model of chronic ischemia

Induction of angiogenesis after TMR: a comparison of holmium: YAG, CO2, and excimer lasers

BACKGROUND

Transmyocardial laser revascularization (TMR) is an emerging treatment for end-stage coronary artery disease. A variety of lasers are currently available to perform the procedure, although their relative efficacy is unknown. The purpose of this study was to compare changes in myocardial blood flow and function 6 months after TMR with holmium:yttrium-aluminum-garnet (holmium:YAG), carbon dioxide (CO2), and xenon chloride excimer lasers in a model of chronic ischemia.

METHODS

Miniswine underwent subtotal (90%) left circumflex coronary stenosis. Baseline positron emission tomography and dobutamine stress echocardiography were performed to document hibernating myocardium in the left circumflex coronary artery distribution. Animals were then randomized to sham redo-thoracotomy (n = 5) or TMR using a holmium:YAG (n = 5), CO2 (n = 5) or excimer (n = 5) laser. Six months postoperatively, the positron emission tomography and dobutamine stress echocardiography studies were repeated and the animals sacrificed.

RESULTS

In animals undergoing TMR with holmium: YAG and CO2 lasers, a significant improvement in myocardial blood flow to the lased left circumflex regions was seen. No significant change in myocardial blood flow was seen in sham- or excimer-lased animals. There was a significant improvement in regional stress function of the lased segments 6 months postoperatively in animals undergoing holmium:YAG and CO2 laser TMR that was consistent with a reduction in ischemia. There was no change in wall motion in sham- or excimer-lased animals. Significantly greater neovascularization was observed in the holmium:YAG and CO2 lased regions than with either the sham procedure or excimer TMR.

CONCLUSIONS

Transmyocardial laser revascularization with either holmium:YAG or CO2 laser improves myocardial blood flow and contractile reserve in lased regions 6 months postoperatively. These changes were not seen following excimer TMR or sham thoracotomy, suggesting that differences in laser energy or wavelength or both may be important in the induction of angiogenesis.

Focal angiogenic therapy for myocardial ischemia

BACKGROUND

Therapeutic angiogenesis, which involves the administration of angiogenic cytokines to stimulate collateral formation and improve myocardial perfusion, is being tested as an alternative treatment for patients with medically intractable angina who are no longer candidates for conventional revascularization techniques.

METHODS

Administration of recombinant protein formulations for these growth factors and transfection of the DNA itself have proven effective in animal models. At present, transfection of the DNA (gene therapy) appears preferable in that a single administration achieves a prolonged but transient, localized exposure of the ischemic tissues to the angiogen. The important technical aspects of surgical gene therapy via a mini thoracotomy incision are described.

RESULTS

Four Phase I clinical trials of angiogenic therapy recently have been reported, two involving administration of the recombinant protein combined with coronary bypass and two involving gene therapy alone. These studies have shown that this approach is relatively safe; the early evidence of efficacy is encouraging. The largest study reported six-month follow-up on 30 patients who received plasmid DNA for vascular endothelial growth factor-165 (VEGF165) and documented improvement in angina, treadmill exercise tolerance, and myocardial perfusion in the majority of patients studied.

CONCLUSIONS

These preliminary results will need to be supported by randomized clinical trials in which angiogenic therapy alone is compared to standard medical therapy or a placebo. Catheter-based intramyocardial gene transfer and combined coronary arterial bypass grafting or transmyocardial laser revascularization plus gene therapy trials are also underway. If proven effective, angiogenic therapy may provide a relatively noninvasive modality for revascularization of ischemic myocardium not amenable to current techniques.

Early postoperative changes in regional systolic and diastolic left ventricular function after transmyocardial laser revascularization: a comparison of holmium:YAG and CO2 lasers

OBJECTIVES

The purpose of this study was to determine the short-term effects of transmyocardial laser revascularization (TMR) on regional left ventricular systolic and diastolic function, myocardial blood flow (MBF) and myocardial water content (MWC).

BACKGROUND

Clinical studies of TMR have noted a significant incidence of cardiac complications in the early postoperative period. However, the early post-treatment effects of laser therapy on the myocardium and their potential contribution to postoperative cardiac morbidity are unknown.

METHODS

Swine underwent holmium:yttrium-aluminum-garnet (holmium:YAG) (n = 12) or carbon dioxide (CO2) (n = 12) laser TMR. Regional systolic function for the lased and nonlased regions was quantitated using preload recruitable work area (PRWA) and regional diastolic function with the ventricular stiffness constant alpha.

RESULTS

Preload recruitable work area was significantly decreased in the lased regions both 1 (59.8+/-13.0% of baseline, p = 0.02) and 6 h (64.2+/-9.4% of baseline, p = 0.02) after holmium:YAG TMR. This decreased PRWA was associated with a significant reduction in MBF to the lased regions (13.2% reduction at 1 h, p = 0.02; 18.4% decrease at 6 h post-TMR, p = 0.01). These changes were not seen after CO2 laser TMR. A significant increase in MWC (1.4+/-0.3% increase with holmium:YAG, p = 0.004; 1+/-0.2% increase with CO2, p = 0.002) and alpha (217.4+/-44.2% of baseline 6 h post-holmium:YAG TMR, p = 0.05; 206+/-36.7% of baseline 6 h post-CO2 TMR, p = 0.03) was seen after TMR with both lasers.

CONCLUSIONS

In the early postoperative setting, impaired regional systolic function in association with regional ischemia is seen after TMR with a holmium:YAG laser. Both holmium:YAG and CO2 lasers are associated with increased MWC and impaired diastolic relaxation in the lased regions. These changes may explain the significant incidence of early postoperative cardiac morbidity. The impact of these findings on anginal relief and long-term outcome are not known.

Lasers in the treatment of ischaemic heart disease

Mounting evidence showing that transmyocardial laser revascularization (TMR) is a safe and effective treatment for angina pectoris arrives just as an increasing number of patients who have undergone angioplasty and coronary artery bypass grafting experience failure with time. TMR, nevertheless, remains controversial. It appears to relieve the symptoms without treating the underlying atherosclerotic disease, and its method of action is unproven. Like angioplasty and coronary bypass, TMR in fact offers palliation rather than a cure for atherosclerotic heart disease. The most sensible current formulations of the therapeutic mechanism of TMR posit a reconfiguration of the microcirculation, with blood shunted from epicardial to endocardial areas. These unresolved issues notwithstanding, TMR benefits patients with end-stage coronary disease and represents a pioneering effort to remodel the microcirculation of patients with arteriosclerotic occlusive disease.

Myocardial revascularisation by laser

Myocardial revascularisation by laser is an emerging treatment for refractory angina in patients with coronary artery disease that is not amenable to conventional revascularisation. With the original technique, laser energy was applied to epicardial surface of the heart through a lateral thoracotomy; so-called transmyocardial laser revascularisation (TMR). It is now possible, using catheter-based percutaneous myocardial revascularisation (PMR) to deliver laser energy to the myocardium via the endocardial surface. In this article we discuss the possible mechanism of action of laser revascularisation, and summarise the results of recent randomised controlled trials of TMR, PMR systems are described, and the growing evidence for their efficacy is reviewed.

Epicardial gene therapy and laser revascularization

Despite the proven effectiveness of coronary bypass surgery and percutaneous angioplasty techniques, an increasing number of patients are presenting with severe, medically intractable angina who are not candidates for either procedure. Two alternative strategies, transmyocardial laser revascularization and exogeneous administration of angiogenic growth factors (therapeutic angiogenesis) are currently being evaluated in such patients. This review focuses on the current status of these two procedures, emphasizing their similarities and differences in order to provide insight into what role each may ultimately play in the management of patients with otherwise unrevascularizable myocardial ischemia.

Up-regulation of vascular endothelial growth factor mRNA and angiogenesis after transmyocardial laser revascularization

BACKGROUND

Angiogenesis has been proposed as a potential mechanism whereby transmyocardial laser revascularization (TMLR) has provided clinical relief of angina. Experimental work has found histologic evidence supporting this, as well as an improved response when angiogenic growth factors have been added to TMLR. The purpose of this study was to demonstrate that the molecular response to TMLR was an increase in the production of endogenous vascular endothelial growth factor to promote angiogenesis.

METHODS

Ameroid constrictors were placed on the proximal circumflex artery in 12 domestic pigs. After a chronic ischemic zone was established the animals were randomly divided into two groups. In the TMLR group the ischemic zone was treated with carbon dioxide laser. In the control group the ischemic zone was untreated. Six weeks later the animals were sacrificed, and sections from the ischemic zone and the nonischemic zone were submitted for immunohistochemical, histologic, and molecular analysis. Messenger RNA was obtained from northern blot analysis after being probed with vascular endothelial growth factor.

RESULTS

There was a twofold increase in the vascular endothelial growth factor messenger RNA in the ischemic zone of the TMLR group compared with the control group. Additionally, there was a threefold increase in the number of new blood vessels in the ischemic zone of the TMLR group compared with the control group.

CONCLUSIONS

Transmyocardial laser revascularization promotes angiogenesis by upregulation of vascular endothelial growth factor. The resulting angiogenesis could be the principle mechanism for the clinical efficacy of TMLR.

Diagnosis, incidence, and clinical significance of early postoperative ischemia after transmyocardial laser revascularization

BACKGROUND

Clinical improvement after transmyocardial laser revascularization (TMR) is typically delayed, and patients therefore remain at risk for ischemic events after the procedure. The purpose of this study was to define the range of creatine phosphokinase (CPK) and CPK-MB enzyme elevation after TMR and to assess the incidence of early postoperative ischemic events.

METHODS

Twenty-one patients undergoing isolated TMR were evaluated for 48 hours after surgery with serial CPK and CPK-MB enzymes and 12-lead electrocardiograms for evidence of myocardial ischemia or injury. Clinically evident postoperative ischemic events including angina pectoris, myocardial infarction (MI), and cardiac death were recorded as well.

RESULTS

Eleven patients (52.4%) had ischemic electrocardiographic changes in the first 48 hours after TMR. Ischemia was clinically silent in 7 (63.6%) of these 11 patients. Cardiac death occurred in 1 patient (4.8%) as a result of acute MI. Nonfatal MI occurred in an additional 4 patients (19.0%). Of the 5 patients with MI, 4 had angina pectoris versus no angina in the 16 patients without MI (P =.02). All patients had elevated CPK and CPK-MB levels after TMR: however, peak CPK (P =.02) and CPK-MB (P =. 005) levels were significantly higher for patients suffering postoperative MI compared with those without MI.

CONCLUSIONS

Transient ischemia occurs frequently after TMR and is clinically silent in the majority of patients. Patients with postoperative MI are more likely to have symptomatic ischemia as well as significant cardiac enzyme elevation. The combination of 12-lead electrocardiogram and cardiac enzymes appears to have significant merit for the diagnosis of myocardial ischemia and infarction after TMR. These studies should be obtained in all patients undergoing TMR for the first 48 hours after surgery.

Improved perfusion and contractile reserve after transmyocardial laser revascularization in a model of hibernating myocardium

BACKGROUND

Transmyocardial laser revascularization (TMR) has been demonstrated effective for relieving angina, although prior studies have yielded inconsistent results regarding postoperative myocardial perfusion and function. This study evaluated long-term changes in myocardial perfusion and contractile reserve after TMR in a model of hibernating myocardium.

METHODS

Miniswine had subtotal left circumflex coronary artery occlusion to reduce resting blood flow to 10% of baseline. After 2 weeks in the low-flow state, positron emission tomography and dobutamine stress echocardiography were performed to document ischemic, viable (hibernating) myocardium in the left circumflex distribution. Animals then had sham redo thoracotomy (n = 4) or TMR (n = 6). Six months later the positron emission tomography and dobutamine stress echocardiography studies were repeated.

RESULTS

Myocardial blood flow in the left circumflex distribution as measured by positron emission tomography was significantly reduced in all animals after 2 weeks in the low-flow state. In animals that had TMR, there was significant improvement in myocardial blood flow to the lased regions 6 months postoperatively. No significant change in myocardial blood flow was seen in sham animals at 6 months. Dobutamine stress echocardiography after 2 weeks of low-flow demonstrated severe hypocontractility at rest in the left circumflex region of all animals, with a biphasic response to dobutamine consistent with hibernating myocardium. In animals that had TMR, there was a trend toward improved resting function and significantly improved regional stress function in the lased segments 6 months postoperatively, consistent with a reduction in ischemia. Global left ventricular wall motion at peak stress improved significantly as well. There was no change in wall motion 6 months postoperatively in sham-operated animals.

CONCLUSIONS

This study found improvements in myocardial perfusion and regional and global contractile reserve 6 months after TMR in a porcine model of hibernating myocardium. This improved perfusion and function likely accounts for the clinical benefits of the procedure.

Perioperative morbidity and mortality after transmyocardial laser revascularization: incidence and risk factors for adverse events

OBJECTIVES

The purpose of this study was to describe the incidence and spectrum of perioperative cardiac and noncardiac morbidity and mortality after transmyocardial laser revascularization (TMR) and to identify predictors of these adverse clinical events.

BACKGROUND

Clinical studies have demonstrated the efficacy of TMR for relieving angina pectoris, although no study to date has specifically addressed the associated perioperative morbidity and mortality.

METHODS

Between October 1995 and August 1997, 34 consecutive patients with end-stage coronary artery disease (CAD) underwent isolated TMR. The majority of patients (94%) had class III or IV angina pectoris, and two patients (6%) had unstable symptoms preoperatively. Patient records were reviewed for fatal and nonfatal adverse cardiac and noncardiac events.

RESULTS

Perioperative death occurred in two patients (5.9%) due to cardiogenic shock complicating acute myocardial infarction. Perioperative cardiac morbidity occurred in 16 patients (47.1%); noncardiac morbidity was seen in 12 patients (35.3%). Preoperative unstable angina was the only variable predictive of perioperative death (p = 0.005). Cardiac (p = 0.005) and noncardiac (p < 0.001) morbidity rates were significantly higher for the initial 15 patients undergoing the procedure. Other predictors of perioperative complications included lack of postoperative treatment with a furosemide infusion (p < or = 0.04) and preoperative unstable angina (p = 0.05).

CONCLUSIONS

Perioperative mortality in patients undergoing isolated TMR is low. Transmyocardial laser revascularization patients are at higher risk for adverse perioperative cardiac and noncardiac events, likely reflecting the lack of immediate benefit from the procedure in the setting of severe CAD. These patients merit vigilant surveillance for adverse events and aggressive medical management in the perioperative period.