Myocardial canalization. A new method of myocardial revascularization

35 years of experimental research in transmyocardial revascularization: what have we learned?

In the past 35 years many experimental studies have been performed to investigate the revascularization potential of transmyocardial revascularization and the possible working mechanisms underlying the observed clinical improvement in angina pectoris after this treatment. In this review of the experimental literature, the various methods that have been used to create transmyocardial channels and the most supported hypotheses on the working mechanism (channel patency, angiogenesis and myocardial denervation) are discussed and evaluated.

Transmyocardial and percutaneous myocardial revascularization: current and future role in the treatment of coronary artery disease

Transmyocardial revascularization (TMR) is a new treatment modality under evaluation in patients with severely symptomatic, diffuse coronary artery disease, in whom the potential for medical or interventional management has been exhausted. Preliminary clinical trials show improved ischemic symptoms within the first 3 months in about 70% of TMR-treated patients. The original proposed mechanism of surgical or catheter-based TMR (percutaneous myocardial revascularization [PMR]) was that channels mediate direct blood flow between the left ventricular cavity and ischemic myocardium. However, several alternative explanations for the clinical success of TMR have recently been suggested, including improved perfusion by angiogenesis, an anesthetic effect by nerve destruction, and a potential placebo effect. This article reviews the clinical role of TMR/PMR, its possible pathophysiologic mechanisms, and its controversies. It provides an overview of the actual scientific and clinical status of TMR and details future directions.

Clinical studies of TMR with the CO2 laser

OBJECTIVE

This paper reviews the current status of transmyocardial laser revascularization by a carbon dioxide laser.

SUMMARY

Since 1990 over 3000 patients worldwide have been treated with a carbon dioxide laser. A nonrandomized phase II trial was completed in 1995. A randomized controlled phase III trial has completed enrollment, and analysis of the follow-up is pending.

METHODS

In each trial 200 patients with endstage coronary artery disease and severe disabling angina that was not amenable to conventional revascularization were enrolled. Preoperative evaluation included confirmation of angina class and evidence of reversible ischemia based on myocardial perfusion scans. Repeat evaluations were done postoperatively at 3, 6 and 12 months.

RESULTS

80% of the patients showed a significant improvement in angina class status postoperatively and 30% had no angina at one year of follow-up. Concomitant with this there was significantly less ischemia noted on follow-up perfusion scans.

CONCLUSIONS

Early results from nonrandomized and randomized controlled trials of transmyocardial laser revascularization by carbon dioxide laser indicate that this technique provides angina relief and improved perfusion in patients with end-stage coronary artery disease.

Transmyocardial laser revascularization: results of a multicenter trial with transmyocardial laser revascularization used as sole therapy for end-stage coronary artery disease

BACKGROUND

Transmyocardial laser revascularization was used as the sole therapy for patients with ischemic heart disease not amenable to percutaneous transluminal coronary angioplasty or coronary artery bypass grafting. This technique uses a carbon dioxide laser to create transmyocardial channels for direct perfusion of the ischemic heart.

METHODS

Since 1992, 200 patients, at eight hospitals in the United States, have undergone transmyocardial laser revascularization. The patients have a combined 1560 months of follow-up for an average of 10 +/- 3 months per patient. Their age was 63 +/- 10 years and their ejection fraction was 47% +/- 12%. Eighty-two percent had at least one previous bypass graft operation and 38% had a prior angioplasty. Preoperatively, the patients underwent nuclear single photon emission computed tomography perfusion scans to identify the extent and severity of their ischemia. These scans were repeated at 3, 6, and 12 months. Angina class, admissions for angina, and medications were recorded.

RESULTS

The perioperative mortality was 9%. Angina class decreased significantly from before treatment to 3, 6, and 12 months (p < 0.001). Likewise, there was a significant decrease in the number of perfusion defects in the treated left ventricular free wall. Concomitantly, there was a significant decrease in the number of admissions for angina in the year after the procedure when compared with the year before treatment (2.5 vs 0.5 admissions per patient-year).

CONCLUSION

These combined results indicate that transmyocardial laser revascularization provides angina relief, decreases hospital admissions, and improves perfusion in patients with severe coronary artery disease.

Transmyocardial laser revascularization: operative techniques and clinical results at two years

OBJECTIVES

A new technique, transmyocardial laser revascularization, provides direct perfusion of ischemic myocardium via laser-created transmural channels. From 1993 to 1995, we have treated 20 patients (mean age 61 years, four women and 16 men) with transmyocardial laser revascularization. Preoperatively, the average angina class was 3.7. The patients were screened before the operation by a technetium sestamibi perfusion scan to identify the location and extent of their reversible ischemia.

METHODS

Operative exposure is gained via a left anterior thoracotomy. With the use of a 850-watt carbon dioxide laser, an average of 21 +/- 4 channels were created in 22 minutes with a total operative time of less than 2 hours.

RESULTS

The in-hospital mortality was two of 20 patients. Three additional patients died after discharge. After an accumulated 172 patient-months (mean follow-up 11 +/- 8 months, range 1 to 26 months), the mean angina class is I (p = 0.01). Postoperative sestamibi scans were obtained at 3, 6, and 12 months. Using the septum as a control and comparing the postoperative results with the preoperative baseline, we noted a significant improvement in perfusion particularly in the areas of reversible ischemia.

CONCLUSION

These early results indicate that transmyocardial laser revascularization is a simple operative technique that may improve myocardial perfusion and provide angina relief for patients in whom standard methods of revascularization is contraindicated.

Transmural channels can protect ischemic tissue. Assessment of long-term myocardial response to laser- and needle-made channels

BACKGROUND

We previously found that transmural laser channels failed to acutely increase myocardial blood flow. Nevertheless, this method is being used to treat patients with coronary artery disease who are unable to undergo angioplasty or bypass graft surgery and in cases in which previous surgery has failed. To reconcile the lack of an acute increase in blood flow with beneficial effects claimed in patients, our hypothesis was that the channel-making process might, over time, stimulate a protective effect, possibly by the growth of new vessels linking channels to the existing circulation. We tested this hypothesis in rat hearts, which have little native collateral circulation.

METHODS AND RESULTS

We made six transmural channels in the left ventricle of each heart using a 400-micrometer-diameter optic fiber coupled to a holmium:yttrium-aluminum-garnet laser or a 400-micrometer-diameter syringe needle. Two months after the channels were made, rats were randomized to either an infarct-size study or analysis of myocardial capillary density. We challenged any induced protective mechanism by acutely occluding the left coronary artery for 90 minutes, followed by 4.5 hours of reperfusion. The artery was then reoccluded, and pigment was injected into the circulation to delineate tissue perfused by the occluded vessel and to detect perfusion via the channels. We used triphenyltetrazolium staining to determine the amount of muscle necrosis and the location of muscle protection. Infarct size in needle-treated hearts was smaller than in controls (15 +/- 6% versus 40 +/- 3% of the left ventricle, P<.01). Infarct size in laser-treated hearts (27 +/- 5%) did not differ significantly from controls; however, all eight laser-treated hearts showed evidence of muscle protection in areas adjacent to channels. We found that the laser-made channels were associated with more fibrosis than the needle-made channels (mean width of fibrosis 430 +/- 50 versus 180 +/- 30 micrometer, P<.0001), and, in tissue remote from channels, fibrosis was increased more in laser-treated hearts (3.6 +/- 0.3%) versus both control (2.5 +/- 0.2%) and needle-treated (2.5 +/- 0.3%) hearts (P<.05). In addition, muscle disarray was seen adjacent to channel-associated fibrosis. We observed injected pigment within fibrosis associated with the channels and in surrounding myocardium. We also found vessels that appeared to be connected to channels; however, there was no overall increase in capillary density.

CONCLUSIONS

We were able to protect the heart against coronary artery occlusion by making transmural channels 2 months before occlusion. Channels created by a needle provided greater protection than channels created by a laser, probably because they caused less initial injury. Our results are consistent with the concept that the channels were able to provide blood flow to the tissue directly from the ventricular cavity; however, we cannot rule out the possibility that other mechanisms of protection may be involved.

Recovery and viability of an acute myocardial infarct after transmyocardial laser revascularization

OBJECTIVES

The short- and long-term effectiveness of transmyocardial laser revascularization was evaluated in the setting of an acute myocardial infarction.

BACKGROUND

Theoretically, transmyocardial laser revascularization allows direct perfusion of the ischemic area as ventricular blood flows through the channels to the myocardium.

METHODS

Infarcts were created by coronary occlusion in 30 sheep. Eighteen of these sheep were studied to assess short-term efficacy. The infarct was reperfused after 1 h by either removing the occlusion or by laser drilling using a high power carbon dioxide laser. The occlusions were left in place for the control group. To monitor regional recovery, percent systolic shortening was measured. To evaluate long-term effectiveness, 12 additional sheep underwent creation of an infarct. Six were treated with the laser, and six were untreated. The animals were restudied 30 days later.

RESULTS

In the short-term experiment, the control and reperfusion groups exhibited no recovery of regional contractility. The laser group demonstrated improvement throughout the recovery period. There was a significant difference in the area of necrosis within the same area at risk (reperfusion group 44 +/- 6% and control group 39 +/- 5% vs. laser group 6 +/- 2%). After 30 days, none of the control animals showed evidence of contraction in the infarct, whereas the laser-treated animals did. Histologic analysis of the laser-treated infarcts revealed patent channels surrounded by viable myocardium. The control-group infarcts were necrotic and scarred.

CONCLUSIONS

On the basis of both short- and long-term improved contractility, as well as diminished necrosis in the area at risk, these results indicate that transmyocardial laser revascularization may be an alternative method of treating ischemic heart disease.

Transventricular revascularization by laser

Transventricular acupuncture or revascularization by the method of producing channels between the ventricular cavity and the ischemic myocardium has challenged several investigators. The concept of producing these channels using high-energy CO2 laser was based on the finding that the laser beam could produce small channels devoid of debris, and fibrosis would not occur. Four groups of mongrel dogs with six animals in each group were studied. In three groups the left anterior descending (LAD) branch of the coronary arteries was ligated above the first diagonal, and channels were made in the myocardium according to the protocol for each group. The fourth group was the control, the LAD was ligated but the myocardium was not exposed to laser treatment. In the control group all animals died within 20 minutes of LAD ligation. Animals in the other groups survived ligation with laser, and were subsequently sacrificed at varying intervals. At autopsy the epicardial component of the channels was visible. Penetration through the myocardium could be demonstrated. Microscopically the channels were patent and endothelialized. Further investigations are in process, but we believe laser channels protect and nourish the ischemic myocardium.

Revascularization of the heart by laser

The possibility of revascularizing the myocardium with high-energy laser was investigated based on the finding that the laser beam could produce small channels in the tissue of the myocardium that were devoid of debris and scarring. The technique was investigated in 4 groups of 6 mongrel dogs each. In 3 of the groups, the left anterior descending (LAD) coronary artery was ligated above the first diagonal branch and the myocardium was treated with the laser energy according to the protocol of that group. In the fourth control group, the LAD artery was ligated but the myocardium was not subjected to laser treatment. In the first 3 groups, the myocardium was protected by the channels produced by the application of the laser energy. The animals were subsequently sacrificed at various intervals. In the control group, all of the animals died within 20 minutes of ligation of the artery. At autopsy, the epicardial sites of the channels created by the laser were clearly visible and a lack of charring in the channels was demonstrated. The penetration through the endocardial surface could be demonstrated. Microscopically, the channels were patent and endothelialized. It is concluded that the channels created in the myocardium effectively helped to protect the myocardium from acute coronary artery occlusion in this model in dogs.

Effects of transmyocardial punctures on left ventricular performance after coronary occlusion in the acute experiment

The results of multiple transmyocardial punctures were studied in 12 dogs in which ligation of the left descending coronary artery had been performed. Instead of improvement, deterioration in the electrocardiogram, paradoxical movement, left ventricular pressure, left ventricular dp/dt max, and mean aortic flow were observed. These results are in contrast to the acute effects observed by Sen et al. (1965, 1968) but do not exclude the possibility of beneficial long-term effects.