Catheter Flexible fibersGuide wire

Fig. 4.58. Scheme of laser angioplasty for vessel recanalization

After moving the guide wire into the coronary artery, the catheter was guided into the correct position. The fibers were coupled to a XeCl excimer laser emitting at a wavelength of 308 nm and pulse durations of 60 ns. Typical energy densities of up to 5 J/cm2 were applied. The mean percentage of stenosis fell from 85 % initially to 41 % immediately after laser treatment, and the primary success rate was as high as 90 %. In twenty patients, subsequent balloon dilatation was additionally performed. Perforations of the vessel wall did not occur in any of the patients. However, it was only shortly after when Karsch et al. (1991) published a second report admitting that one patient suddenly died two months after laser treatment. Postmortem histologic examination proved that a severe restenosis had occurred which had led to an acute myocardial infarction.

In Figs. 4.59a-b, two photographs are shown demonstrating the removal of atherosclerotic plaque with a XeCl excimer laser. For this ablation, Hanke et al. (1991) have applied pulse durations of 60 ns at a repetition rate of 20 Hz. An enlargement of the plaque itself is captured in Fig. 4.59a. On the right half of the picture shown in Fig. 4.59b, parts of the plaque have already been removed without injuring the vessel wall.

Today, it is well accepted that restenoses are extremely pronounced following excimer laser angioplasty. Their occurrence can be attributed to an enhanced proliferation of smooth muscle cells as has been demonstrated by Hanke et al. (1991). Most of these cells undergo DNA synthesis during two weeks after laser treatment, resulting in intimal thickening within the first four weeks. Obviously, the mechanism of photoablation is more stimulating than only mechanical cracking or abrasion. Thus, even though photoablation is a rather gentle technique for removing plaques, its long-term effects forbid its use for the purpose of vessel recanalization. Therefore, excimer laser an-gioplasty is generally being rejected today, and it is rather doubtful whether it will ever gain clinical relevance.

Fig. 4.59. (a) Histologic section of atherosclerotic plaque inside a blood vessel (bar: 150 ^m). The vessel wall is located at the bottom of the picture. (b) Ablation of atherosclerotic plaque with a XeCl excimer laser (pulse duration: 60 ns, repetition rate: 20Hz, bar: 150 |im, plaque: left, ablation: right). Photographs kindly provided by Dr. Hanke (Tubingen)

Fig. 4.59. (a) Histologic section of atherosclerotic plaque inside a blood vessel (bar: 150 ^m). The vessel wall is located at the bottom of the picture. (b) Ablation of atherosclerotic plaque with a XeCl excimer laser (pulse duration: 60 ns, repetition rate: 20Hz, bar: 150 |im, plaque: left, ablation: right). Photographs kindly provided by Dr. Hanke (Tubingen)

Meanwhile, other laser types have also been investigated concerning their application in angioplasty and cardiology. One of them is the Ho:YAG laser which has been studied in detail by Hassenstein et al. (1992). However, an extensive increase in intimal thickening was observed within the first six weeks after laser treatment. And, again, the proliferation of smooth muscle cells seems to be responsible for this effect. Thus, the clinical use of holmium laser angioplasty appears to be extremely limited.

More promising are CO2 laser systems which can be used to create additional channels for the blood supply of the heart. These channels originate from the epicardium, i.e. the periphery of the heart, and remain open after laser treatment. This technique is called transmyocardial laser revascularization (TMLR) and was initially proposed by Mirohseini et al. (1982). Shortly after, first clinical experiences were reported by Mirohseini et al. (1986). Yano et al. (1993) have confirmed the effect of revascularization. Other investigators, though, could not verify their results, e.g. Whittaker et al. (1993). Recently, Horvath et al. (1995) were able to judge treatment effects by measuring the local contractility of the heart muscle. They concluded that acute infarcts treated by TMLR show improved contractility both in the short- and long-term. Moreover, they observed diminished areas of necrosis. However, further studies regarding blood flow and recovery need to be performed prior to the general acceptance of TMLR.

Even if therapeutic laser treatments of blood vessels should never become a safe procedure, laser diagnostics will always play a significant role in angioplasty and cardiology. Apart from X-ray and ultrasound angiogra-phy, Doppler angiography and laser endoscopy are very sensitive techniques. A typical example of a laser endoscope is shown in Fig. 4.60. Visible laser radiation is emitted from the distal end of a flexible fiber and illuminates the area of interest. Modern engineering science has meanwhile enabled the design of extremely miniaturized and highly sophisticated laser endoscopes.

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