1.
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What is transmyocardial
laser revascularization?
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| Transmyocardial laser revascularization (TMR) is a new technique that
attempts to improve the blood supply to ischemic myocardium by using a high-powered CO2
laser to create multiple transmyocardial channels. These new channels bring blood from the
ventricular cavity directly into the myocardium. The result is a heart that functions more
like a reptilian heart, in which approximately half of the myocardial blood supply comes
from sinusoidal perfusion by the left ventricular cavity.
TMR employs a high powered CO2 laser that puts a high energy pulse into the
myocardium and completely vaporizes a channel with a diameter of 1 mm transmurally. These
channels are placed in the ischemic myocardium approximately one cm apart, so that there
is multiple channel perfusion of ischemic areas. The CO2 laser is triggered to
the ECG to prevent onset of ventricular arrhythmias, including ventricular tachycardia.
For the procedure, the patient is off of the heart lung machine and heparin is not
administered. This procedure can be used to revascularize the free left ventricular wall,
but not the septum. |
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2.
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In whom should this
technique be employed?
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It is increasingly recognized that there are a large number of
patients who have had one or more of the following conditions:
- previous coronary bypass surgery
- previous cardiologic interventional techniques
- genetic metabolic disease which has produced extensive peripheral coronary disease
In all of these situations, there is poor runoff and no place to perform a conventional
revascularization either by coronary bypass or by interventional cardiologic techniques.
These patients have "end-stage coronary vascular disease" and can not be
reconstructed by the usual means. A radioactive perfusion scan should show evidence of
reversible ischemia. That is, there should be myocardium that are ischemic but not
infarcted or scarified producing little or no ischemia. The patients should have
reasonable ventricular function, with left ventricular ejection fractions above 0.20%.
Patients should have very severe disabling angina, class III or class IV, and should have
maximal anti-anginal therapy. |
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3.
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How does transmyocardial
laser revascularization work?
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| There is considerable controversy regarding the exact mechanism of
TMR. Theoretically, the new channels should allow blood supply from the left ventricular
cavity up the channels to form buds off of the main channels. That is, there should be a
revascularization outside of the normal coronary channels that have the ischemic
myocardium. It has been difficult to demonstrate that this revascularization has occurred
consistently in all patients. Some autopsy specimens have shown closed channels; some have
shown open channels. Animal studies have also been conflicting, but, angioneogensis has
definitely been shown to be a factor in both experimental and clinical studies. The
channels have been demonstrated to be patent in human autopsy material. Recently,
investigators in Hilemberg, Germany, using special angulated color doppler echo on human
beings, have seen patent channels thirty days after the creation of these channels. They
have never been demonstrated to be patent by conventional left. |
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4.
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What are the details of
operative techniques?
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| The patients are positioned for a left anterior thoracotomy that is
made usually through the fifth intercostal space. In some patients in Europe who have not
had previous operations, the thoracoscopic approach has been used. The pericardium is
opened, a cradle is made, and the dissection of the pericardium is carefully carried off
of the heart. No heparin is administered, nor is the patient cannulated for
cardiopulmonary bypass. A transesophageal echo is necessary to document the transmural
nature of the laser pulse by preservation of laser "steam" in the left
ventricle. Once the heart is in its pericardial cradle, the probes are placed directly on
the myocardium. The laser beam is synchronized with the ECG's and t hen a foot switch
allows for production of the laser pulse and vaporization of the transmural myocardial
channels. Digital pressure is usually sufficient to stop bleeding in almost all of these,
but occasionally, a simple prolene suture may be necessary. The channels are made one cm
apart in all of the ischemic areas and the borders in and around the ischemic areas noted
from the thallium sesta-mibi scans. After 25 to 40 channels are drilled, the pericardium
is loosely re-approximated and the patient is closed in the usual fashion for a small
thoracotomy and returned to the ICU. Postoperative care is extremely critical,
particularly in regards to the maintenance of the appropriate perfusion pressure in the
patients coronary arterial system. |
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5.
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What is the potential
morbidity of this procedure?
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| In over 1500 patients, intraoperative analysis has documented little
morbidity. In the postoperative period, we have noted occasional supraventricular
tachycardia, some pleural effusions, and the usual complications following a thoracotomy.
Because perfusion pressure drives what ever little blood goes through the badly
collateralized coronary arterial system, it is extremely important to maintain perfusion
pressures until the patient has recovered completely. Hypotension in any form is to be
avoided, and a myocardial support may be prophylactially sought by the use of intra-aortic
balloon pumping. The most lethal complication is obviously the development of a myocardial
infarction, which has been seen in the face of hypotension. Overall operative mortality
has been about 8% previously from AMI following prolonged hypotension. Mortality also
appears to be correlated with the left ventricular ejection fraction in all the national
studies, with highest early and late mortality in patients with worse left ventricular
function. |
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6.
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What are the long term
results, particularly in the FDA second and third phase trials?
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| The phase III trial consisted of 201 patients, operated on at eight
institutions across the United States. This was prospective non-randomized to document
efficacy. The overall operative mortality was approximately 7% and there was probably an
equal percentage in the last follow-up of the period of the study, which was concluded in
August of 1995. Mortality was higher in the patients with lower ejection fractions.
Approximately 75% of patients experienced meaningful relief of angina by one year. There
was concomitant reduction in anti-anginal medications and cardiac-related hospitalization.
Of importance was the fact that myocardial perfusion, which was determined by spect
scanning with computerized sesta-bibi left ventricular maps, was significantly improved in
those areas that were lasered in the reversibly ischemic areas of myocardium.
The phase III study is a prospective, randomized study comparing TMR to intensive
medical therapy for patients who do no t have recourse to other forms of
revascularization. As in the study of phase II, the patients are not candidates for
coronary bypass or PTCA. Seventy-seven patients were randomized to the TMR and
eighty-three patients were randomized to medical therapy. The results at six months
demonstrated a marked reduction in mortality comparing TMR to medical therapy, a marked
improvement in angina, and a decrease in post-procedure hospitalizations. Also noted was a
significant crossover of patients from medical therapy to the surgical arm because of
recurrent unstable angina, which required admission to the hospital. The prospective study
was completely enrolled and randomization was discontinued by the FDA on September 6,
1996. |
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7.
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Can any laser system be
used?
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| This is unknown. A number of different laser technologies are being
developed while these studies with the CO2 laser are finishing the course of
FDA approval. The entire experience described here has been gained only with the CO2
laser. Other laser technologies are not coordinated with the ECG and therefore may not
protect against the ventricular arrhythmias. Also, the other laser technologies are not
produced with high energy so they do not protect against perichannel "burns" and
other forms of tissue destruction around the channels. |
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