"Hot tip": another method of laser vascular recanalization

Peripheral artery recanalization in humans using balloon and laser angioplasty

The treatment of patients with complex peripheral arterial disease and those who have had previous unsuccessful attempted revascularization procedures can be clinically challenging. Initial treatment was begun using therapy by percutaneous balloon and laser angioplasty, then proceeding to bypass surgery if severe ischemia persisted. Both percutaneous and cut-down approaches were used to access totally occluded arteries. An attempt was made to cross the occlusion mechanically with either a guide wire or an activated laser probe. If laser recanalization was not successful, the patient underwent bypass surgery or was managed with medication unless an amputation was necessary. Following initial screening of 381 patients, 115 procedures were performed on 103 patients. In 31 procedures (28 patients), only balloon angioplasty was performed. In 84 procedures (75 patients), laser recanalization was attempted: 55 percutaneously and 29 by cut-down. Overall technical success (crossing the obstruction without perforation) was 86/115 (75%). Technically successful procedures were characterized by shorter arterial occlusions than were technical failures (8.4 +/- 1 cm vs. 14.3 +/- 1.9 cm; p < 0.004). Clinical success (residual stenosis < 50%, symptom relief, improved ankle brachial index > or = 0.15, and no complications) was achieved in 22/31 (71%) of balloon angioplasty procedures alone. The stenoses decreased from 98 +/- 4% to 31 +/- 24%, p < 0.00001. Combined percutaneous laser and balloon angioplasty had a technical success of 36/55 (65%). Stenoses were reduced from 99 +/- 2% to 56 +/- 14% after laser angioplasty, to 30 +/- 15% after balloon angioplasty, p < 0.0001. Laser angioplasty performed via a cut-down had a clinical success of 9/29 (31%). However, major complications were rare. Device staging for treatment of peripheral vascular disease provides additional options for patients who are at high surgical risk and/or in whom standard therapy has failed.

Modeling of intraluminal heating of biological tissue: implications for treatment of benign prostatic hyperplasia

A computer model for predicting the thermal response of a biological tissue to different intraluminal heating modalities is presented. A practical application of the model is to calculate the temperature distributions during thermal coagulation of prostate by contact heating and radiative heating. The model uses a two-dimensional axisymmetric diffusion approximation method to calculate the light distribution during radiative heating. The traditional Pennes' bio-heat equation is used to calculate the temperatures in the presence of blood flow. An implicit finite difference scheme with nonuniform grid spacings is used to solve the diffusion equation for light distribution and the bio-heat equation. Model results indicate that the radiative heating of prostate by Nd:YAG (1064 mm) and diode (810 mm) lasers can be a more effective and efficient means of coagulating a large volume of prostate, as compared to contact heating of the tissue. Blood perfusion is shown to provide a considerable heat sink as the laser exposure time is increased. Surface cooling by irrigation during the laser irradiation of tissue is shown to be an effective method for delaying tissue explosion and obtaining a large volume of coagulated tissue. The model also shows that the volume of the coagulated tissue is appreciably altered by a change in the rate of energy deposition.

Technique of percutaneous laser-assisted valve dilatation for valvar atresia in congenital heart disease

OBJECTIVE

To investigate the efficacy and safety of transcatheter laser-assisted valve dilatation for atretic valves in children with congenital heart disease.

DESIGN

Prospective clinical study.

SETTING

Supraregional paediatric cardiology centre.

SUBJECTS

Eleven children (aged 1 day-11 years; weight 2.1-35.7 kg) with atresia of pulmonary (10) or tricuspid (one) valve underwent attempted laser-assisted valve dilatation as part of the staged treatment of their cyanotic heart disease.

INTERVENTION

After delineating the atretic valve by angiography and/or echocardiography a 0.018 inch "hot tip" laser wire was used to perforate the atretic valve. Subsequently the valve was dilated with conventional balloon dilatation catheters up to the valve annulus diameter.

RESULTS

Laser-assisted valve dilatation was successfully accomplished in nine children. In two neonates with pulmonary valve atresia, intact ventricular septum, and coexistent infundibular atresia the procedure resulted in cardiac tamponade: one died immediately and one later at surgery. During a follow up of 1-17 months (mean 11) two infants with pulmonary valve atresia and intact ventricular septum died (one with congestive cardiac failure). The remainder are either well palliated and do not require further procedures (three), or are awaiting further transcatheter or surgical procedures because of associated defects (four).

CONCLUSIONS

Laser-assisted valve dilatation is a promising adjunct to surgery in this high risk group of patients. It may avoid surgery in some patients, and may reduce the number of surgical procedures in those requiring staged operations.

Micromachining technology and biomedical engineering

Medical science and clinical medicine include many microscopic environments. Recent micromachining techniques fit the microscopic environments and are applied to microsurgery, fiberscopic operation, micromanipulation, artificial organs, and drug delivery systems. Microactuators, microsensors, and micro mechanical parts will be prepared for such medical devices and techniques. Virtual reality, stereovision, and fiber imaging support handling of cells and small targets of living body. The paper reports some perspectives of microtechnologies in biomedical engineering.

In vitro evaluation of ablation parameters of normal and fibrous aorta using smooth excimer laser coronary angioplasty

A modified excimer laser energy delivery system was used to irradiate 100 segments of normal and fibrous aorta in vitro. The laser beam was scanned into 8 fiber bundles consisting of 50 fibers each resulting in a reduction of the applied pulse energy. The total repetition rate was increased to 150 Hz in order to keep the repetition rate per fiber bundle close to 20 Hz and to minimize thermal injury. The results demonstrate that effective ablation (etch rate per 8 pulses > 2.0 microns) occurred at an energy fluency of 50 mJ/mm2 in both normal and fibrous aorta. Tissue damage (carbonization, tissue separation, fissures, cracks, and vacuolization) was in a range of 100 +/- 28 to 152 +/- 30 microns for normal aorta and in a range of 57 +/- 35 to 110 +/- 39 microns for fibrous aorta. We conclude that effective ablation of normal and fibrous human aorta can be achieved by the application of smooth excimer laser coronary angioplasty. This improvement of excimer laser technology may result in a reduction of shock wave- and cavitation-induced damage leading to a reduction of tissue injury. However, this awaits further in vitro and in vivo confirmation.

Excimer laser coronary angioplasty

Recent experience with excimer laser coronary angioplasty (ELCA) has shown the safety and efficacy of the technique in the treatment of coronary stenosis, with results similar to percutaneous transluminal coronary angioplasty (PTCA). Several advantages of the excimer laser for intravascular use have been identified by the ELCA Registry, a group of 15 institutions nationally that have pooled data on their cooperative experience with ELCA. Advantages include the ability of the laser to ablate atheroma with microscopic precision without thermal injury and the ability of vessel surfaces to heal without scarring. The initial results of the multicenter investigation of ELCA in 958 patients indicate that ELCA may be indicated particularly for long, diffuse lesions. With the advent of larger diameter catheters, ELCA is a viable alternative to PTCA. Restenosis was seen as a significant problem, and it remained problematic at 6-month follow-up. In comparison with conventional balloon angioplasty, ELCA may eliminate that component of restenosis due to inadequate dilation or elastic recoil, but it does not appear to affect restenosis caused by smooth muscle proliferation. Catheter design and energy delivery continue to evolve and may affect future results of laser coronary angioplasty.

A biological basis for re-stenosis after percutaneous transluminal angioplasty: possible underlying mechanisms

Intravascular catheterization is a commonly used diagnostic tool and percutaneous transluminal angioplasty is used to dilate stenosed blood vessels. Although these techniques are very successful diagnostically and therapeutically they may precipitate a number of acute and chronic complications. Chronic intimal changes following balloon angioplasty can result in re-stenosis. Intimal hyperplasia can be a long-term complication of both diagnostic and interventional vascular catheterization. This article details these long-term structural changes, specifically relating experimental in vivo and in vitro changes to those seen clinically.

Experimental treatment of thrombotic vascular occlusion

The role of laser energy in the treatment of thrombotic vascular occlusion was evaluated in two sets of experiments. First, 10 polytetrafluoroethylene grafts were used to replace segments of the superficial femoral arteries in dogs and were thrombosed by distal ligation. Occlusion was maintained for one hour, or for 7, 14, 21, and 28 days in each of two grafts. Patency was restored in all 10 grafts without perforation or anastomotic disruption using a 2 mm hot tip probe powered by an Argon laser. However, increased organization of thrombus related to the duration of occlusion lead to decreased laser channel diameters, and 75% of the 28 day thrombus remained in the graft after recanalization. The second experiments tested the added benefit of thrombolytic infusion following laser recanalization. Bilateral external iliac artery thrombosis was induced in dogs by operative vessel isolation, de-endothelialization, and thrombin injection. At 7 days the efficacy of laser-assisted thrombolysis (LAT) versus enzymatic thrombolysis (ET) alone was compared. Eight vessels underwent ET by urokinase (4000 I.U./min.); 14 vessels were laser recanalized prior to thrombolytic infusion. LAT was performed from a carotid artery approach in 8 vessels (antegrade) and from a femoral artery in 6 vessels (retrograde). In contrast to studies using the hot tip alone, both ET and LAT accomplished complete thrombus removal. However, LAT lead to significant iliac arterial flow in 9 +/- 8 min. (antegrade) and 25 +/- 8 min. (retrograde) while ET required 109 +/- 47 min (p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of probe motion on laser probe temperature in circulating blood

The purpose of this study was to evaluate the effect of probe motion on laser probe temperature in various blood flow conditions. Laser probe temperatures were measured in an in vitro blood circulation model consisting of 3.2 nm-diameter plastic tubes. A 2.0 mm-diameter metal probe attached to a 300 microns optical quartz fiber was coupled to an argon laser. Continuous wave 4 watts and 8 watts of laser power were delivered to the fiber tip corresponding to a 6.7 +/- 0.5 and 13.2 +/- 0.7 watts power setting at the laser generator. The laser probe was either moved with constant velocity or kept stationary. A thermocouple inserted in the lateral portion of the probe was used to record probe temperatures. Probe temperature changes were found with the variation of laser power, probe velocity, blood flow, and duration of laser exposure. Probe motion significantly reduced probe temperatures. After 10 seconds of 4 watts laser power the probe temperature in stagnant blood decreased from 303 +/- 18 degrees C to 113 +/- 17 degrees C (63%) by moving the probe with a velocity of 5 cm/sec. Blood flow rates of 170 ml/min further decreased the probe temperature from 113 +/- 17 degrees C to 50 +/- 8 degrees C (56%). At 8 watts of laser power a probe temperature reduction from 591 +/- 25 degrees C to 534 +/- 36 degrees C (10%) due to 5 cm/sec probe velocity was noted. Probe temperatures were reduced to 130 +/- 30 degrees C (78%) under the combined influence of 5 cm/sec probe velocity and 170 ml/min blood flow.(ABSTRACT TRUNCATED AT 250 WORDS)

Experimental studies on coronary laser angioplasty

To establish the method of coronary laser angioplasty using the argon laser, we investigated the optimal condition of laser ablation and the relationship between the size of laser probe and the diameter of the vessel. First, the relationship between laser energy and tissue reaction to the laser was examined using human cadaveric arteries. Subsequently, we recognized that repeated laser ablation is the optimal method with each shot being limited to 5 10 watts (W) in output and 2 seconds of ablation time when using a metal tip probe (MTP) of 1.5 mm in diameter. According to the experimental condition described above, we examined the response of normal coronary arteries to the laser in vivo. If the size of MTP was not suitable for the diameter of the vessel, thermal constriction of the coronary artery was observed; MTP/vessel diameter ratio should be 0.7 or less to avoid vessel constriction by thermal damage from the laser. Neither perforation nor spasm of the coronary artery was found in this experiment. In addition, the ablated area on the coronary artery was healed completely without thrombus formation and coronary stenosis, and it was covered with new endothelium 8 weeks after laser thermal ablation.

The effects of laser energy on the arterial wall

Laser energy has been proposed as a method of resecting atherosclerotic plaque since the mid 1960s. However, only over the past several years have we come to understand some of the unique interactions of the laser with cardiovascular tissue. In laser angioplasty a major challenge has been choosing the optimal laser and duration of laser exposure to achieve adequate resection of plaque, while minimizing such complications as thrombosis, perforation, embolization, aneurysm formation, and accelerated atherosclerosis. Ultimately we must develop a more selective laser that resects plaque while leaving adjacent arterial wall uninjured. This review describes the physics of laser energy, the different lasers available for use in the cardiovascular system, laser-arterial wall interactions, and some of the limitations of laser angioplasty.

Laser probe temperature control by measuring the returning infra-red radiation

The metal-tipped fibre or 'laser probe' developed for angioplasty comprises a metallic probe at the end of an optical fibre. The probe is heated by an argon or Nd:YAG laser and applied against the tissue to be vapourized. The heated probe generates infra-red radiation which is proportional to the temperature of the probe. The paper investigates the feasibility of a feedback control system that measures the temperature of the probe by detecting the infra-red radiation transmitted back through the fibre. The probe was initially heated by physical contact with a hot surface, and then by an argon laser via the optical fibre. The returning IR radiation was sensed by a lead sulphide detector, while probe temperature was simultaneously measured by a thermocouple. Temperatures as low as 200 degrees C were measured through a 5 m long fibre during the laser heating of the probe. The detector signal increased in an exponential fashion as the probe temperature increased. A resolution of 1 degree C was obtained at a probe temperature of 400 degrees C. It can be concluded that, for the laser probe, it is feasible to use a feedback control system which measures the infra-red radiation transmitted back through the same fibre that carries the heating laser light.

Laser thermal angioplasty probe ("hot tip") temperature: effects of flow

The temperature developed by the laser thermal ("hot tip") probe during arterial recanalisation is primarily dependent on the rate of energy delivery and the rate of dissipation to the surrounding medium. While higher probe tip temperatures enhance the efficacy of atheroma ablation, so too is the incidence of adverse effects increased. We studied the temperature developed in the probe tip in an artificial circulation using both saline and blood. In saline the peak probe temperatures were limited to 100 degrees C (boiling point), falling with each increment in flow. Small discrepancies in probes at different times and may be due to malalignment of the optical fibre-metal cap coupling, temperature measurement inaccuracy, tip insulation, or generator output instability. In blood, charring and clot formation insulated the tip raising the temperature (up to 700 degrees C within 5 seconds at 10 W) but also retarded dissipation of heat to the surroundings. The degree of clot and char formation was critical in determining subsequent thermal responses in any particular probe. The unknown rate and quantity of char buildup and changing blood flow during in vivo angioplasty are likely to be important obstacles to developing a reliable thermal feedback control system.

Effects of laser thermal angioplasty on arterial contractions and mechanics

In each of 8 rabbits, a fiberoptic with a 1.5 mm diameter metal tip was inserted into the abdominal aorta. After preheating with 8-10 watts of argon laser energy, the metal tip was passed through the external iliac artery. Thermal injury was evaluated by histology and by measuring arterial contractions and wall mechanics. The contralateral iliac artery was used as a control. Laser thermal angioplasty (LTA) produced foci of coagulation necrosis, medial thinning, and wall rupture. Concomitantly, maximal arterial contractile force was reduced by 80% (p less than 0.01) and arterial wall stiffness was decreased (p less than 0.05). These changes may have clinical implications with regard to the incidence and severity of vasospasm and restenosis following angioplasty.

Thermal effects of stationary "hot tip" laser coronary probes: an in vitro assessment

Percutaneous laser thermal probe angioplasty requires sufficient laser probe flexibility to access the coronary tree. This may entail a loss of axial strength and the resultant slow advancement may lead to unwanted heating of the normal coronary artery proximal to the lesion. To assess the lateral thermal effects of stationary coronary laser probes, laser thermal energy (50-150 J) was delivered to 25 coronary artery segments (diameter 1.9-4.0 mm) in a perfused cadaver heart preparation using a 1.7 mm tip probe. Adherence to the vessel wall occurred in 19 segments, endothelial charring in 8 segments, and perforation in 3 segments. Endothelial charring was seen in 8 of 13 nonperfused segments but in 0 of 12 segments perfused at 60 ml/minute (P less than 0.01). In all three perforations the vessel to probe diameter ratio was less than 1.6:1, perfusion was absent, and traction to dislodge the adherent probe was necessary. Lateral wall damage is a complication of stationary laser probes: smaller-tipped probes which are advanced rapidly at the time of energy delivery may enhance the safety margins of coronary laser thermal probe angioplasty.

Laser recanalisation of coronary arteries by metal-capped optical fibres: early clinical experience in patients with stable angina pectoris

The delivery of laser energy to the coronary circulation by bare optical fibres may cause perforation of the vessel. Experimental studies have shown that this complication can be avoided if the optical fibre is fitted with a metal cap to prevent the potentially dangerous forward projection of the laser beam. This study was performed to assess the feasibility and short term effects of percutaneous coronary laser recanalisation with these modified fibres. Recanalisation of a severe stenosis of the left anterior descending artery was attempted in six patients who were referred for coronary artery bypass grafting. Although the percutaneous technique was used, the laser procedure was performed during coronary bypass surgery before the start of cardiopulmonary bypass to minimise the effects of potential complications. A 1.5 mm diameter metal-capped fibre coupled to an argon laser was advanced percutaneously over a guide wire positioned across the stenosis. In the first patient the delivery of 152 J resulted in the gradual passage of the fibre through a 3 cm long stenosis. Repeat angiography showed a reduction in the severity of the stenosis. In the second patient the delivery of 112 J failed to allow fibre advancement; a further 80 J pulse caused perforation which was repaired. In the remaining four patients the delivery of laser energy in the attempt to traverse the stenosis was limited to less than 90 J. In two of the four patients the severity of stenosis was reduced. No further complications were seen. Percutaneous coronary laser recanalisation with metal-capped optical fibres is feasible but improvements of currently available technology are needed to increase the primary success rate.

Histopathology of human laser thermal angioplasty recanalization

Laserprobe thermal-assisted balloon, angioplasty (LTBA) has demonstrated promising initial clinical results in recanalizing stenotic or occluded superficial femoral and popliteal arteries. Over the past year we have obtained six specimens of laserprobe thermal (LT) and LTBA treated total occlusions (avg. length 12 cm) for histopathologic examination from patients who were treated for limb salvage. Three tissue specimens were obtained acutely, and one was obtained at 6, 8, and 13 days, respectively, after laser angioplasty at the time of revision for complications or failed procedures. Serial histologic sections of the treated LT segments demonstrated recanalization of atherosclerotic lesions to approximately 60-70% of the probe diameter. The LT channels were lined by a thin layer of carbonized or coagulated tissue and several layers of cell necrosis. The histology of the thermal injury was similar regardless of whether it was produced by the heated metal cap or by free argon laser energy. Stellate balloon angioplasty fractures were frequently filled with thrombus. Analysis of these human LT and LTBA specimens revealed that the thermal device produces a confined injury through the path of least resistance. Balloon dilatation produces fragmented cracks in the vessel wall, which appear to be more thrombogenic than the carbonized LT surface. With improved guidance methods, LTBA shows potential for continuing development.

Effects of blood flow on laser probe temperature in human arteries

Laser recanalization using metal capped fibers occurs by thermal vaporization of occluding plaque. However, little is known about the effects of blood and flow on the temperature of the laser probe or the arterial wall during lasing. To study this, probe and arterial wall temperatures were measured while a metal capped fiber, activated by an argon laser, was held stationary in a stenotic human peripheral artery. Arteries were perfused with saline and blood, and flow was varied from 0 to 140 cc/min. Probe temperatures were significantly higher in blood than in saline. However, the increased probe temperature achieved in blood was not transferred to the arterial wall. Increasing flow decreased probe temperature in both media, but again arterial wall temperatures were minimally affected. Thus, the presence of blood and flow may significantly affect heat generation and heat transfer during arterial recanalization using metal capped fibers.

Argon laser angioplasty with a laser probe

A preliminary basic study of argon laser angioplasty with the use of a specially designed probe is presented. Arterial specimens were harvested from 10 amputated lower extremities. The studies included the evaluation of coaxial laser angioplasty in 10 partially or totally occluded arteries; the observation of the effect of perpendicularly applied laser energy on 35 thrombi, 54 soft and 10 hard atherosclerotic, and 51 normal arterial walls; the comparison of laser energy requirements for coaxial vessel lumen enlargement or recanalization vs. perpendicular penetration in 10 occluded, hard atherosclerotic arteries; and the spectrographic analysis of calcium density gradient in two specimens. The results showed that the power required for vessel lumen enlargement was 10 +/- 6 J/mm of atherosclerotic vessel (mean +/- standard deviation). The probe followed the original arterial lumen, did not perforate the vessel wall, and created a smooth, enlarged path. The power required to penetrate perpendicularly to a similar depth for thrombi, soft and hard atherosclerotic plaques, and normal arterial walls was 15 +/- 4, 30 +/- 15, 65 +/- 32, and 246 +/- 123 J/mm, respectively. In the hard calcified specimens, laser energy required for coaxial lumen enlargement or recanalization was significantly less than that for perpendicular penetration (p less than 0.05), which correlated with the calcium density map indicating an increase from inside to outside.

Effects of simultaneous viewing and vaporization of plaques using the steerable, laser-heated metal cap in the atherosclerotic monkey model

A steerable, fiberoptic catheter coupled to a laser light guide tipped with a metal cap was used. Four monkeys fed an atherogenic diet for 7-8 years were angiographed and were found to have extensive mural plaque in the iliac arteries. Plaque sites in these monkeys were vaporized using the laser-heated metal cap. Energies of 1.5-9 Joules were employed. Application of the energy was tangential of perpendicular to the plaque. Lased sites were examined histologically at 24 hr or at 3 months after treatment. No effect was seen at 1.5 Joules. Three to six Joules tangentially produced a superficial lesion that extended into the tunica intima. Six Joules perpendicularly produced a burn into the tunica adventitia, with damage to the vasavasorum. Nine Joules tangentially produced a burn into the tunica media. Three months after treatment, this lased site showed no stenosis or aneurysm formation.

Percutaneous laser treatment of atherosclerosis: an overview of emerging techniques

In recent years, rapid progress in the application of lasers to the percutaneous treatment of atherosclerosis has been made. An overview of this progress is provided herein in terms of a description of promising laser approaches and problems to be solved. Direct vaporization of obstructing atheroma with fiberoptic delivery of laser energy has been associated with a high incidence of mural perforation, but each of a variety of techniques, including those for improved energy delivery, plaque recognition, alignment of the fiberoptic, and, perhaps, reduction of unnecessary thermal injury, has shown promise for reducing this complication. Nonablation applications of laser energy may also have a role in the treatment of atherosclerosis. During laser balloon angioplasty, the tissue coagulation effects of laser-thermal energy may be used during balloon inflation to eliminate arterial dissections and to reduce elastic recoil, thereby potentially eliminating abrupt reclosure and, perhaps, reducing the incidence of restenosis associated with conventional balloon angioplasty. Photochemical destruction of viable plaque tissue and vasa vasorum with porphyrins and intraarterial light represents a nonthermal, laser-based approach that could have a prophylactic role in slowing progression of diffuse atherosclerotic disease. The remarkable versatility of lasers is responsible for the multiplicity of approaches being investigated and for the current optimism that lasers will eventually play an important role in the percutaneous treatment of atherosclerosis.