Lasers in Surgery and Medicine 830-39 (1988)

Laser Balloon Angioplasty: Effect of Tissue Temperature on Weld Strength

of Human Postmortem Intima-Media Separations

Ronald D. Jenkins, MD, 1. Nigel Sinclair, ME, FRACP, Raj Anand, MD, FACC, Arthur G. Kalil, Jr., us, Frederick J. Schoen, MD, PhD,

and J. Richard Spears, MD, FACC

Charles A. Dana Research Institute and the Harvard-Thorndike Laboratory of Beth Israel Hospital, Department of Medicine, Cardiovascular Division, and the Radiology Research

Laboratory of the Department of Radiology, Beth Israel Hospital (R.D.J., I.N.S., R.A., A.G. K., J. R.S.) and Department of Pathology, Brigham and Women's Hospital (F. J.S.),

Harvard Medical School. Boston

Dehiscence of portions of atheromatous plaques fractured during per- cutaneous transluminal coronary angioplasty may contribute to both abrupt reclosure and gradual restenosis. Laser balloon angioplasty has been shown to be effective in welding human plaque-arterial wall s e p arations in vitro by heating tissues with a Nd:YAG laser during balloon inflation. To define the potentially useful therapeutic range of tissue temperature required to achieve thermal welds, 220 l-cm diameter discs of human postmortem atheromatous aortic tissue, the intimal plaque of which had been separated from the media, were exposed to 3-25 watts of Nd:YAG laser radiation delivered over a l2-mm2 nominal spot size for 20 seconds via a 400-fim core optical fiber. As measured with a thermistor, adventitial temperature reflected the temperature at the plaque-media junction to within 10°C. The degree of tissue temperature elevation was related to delivered energy, while effective tissue penetra- tion increased to maximum depth of 3 mm at the highest power density. Strength of tissue welds was defined as the force required to shear opposing layers of welded segments. Adventitial tissue temperatures below 80°C were not associated with appreciable welds, while equilib- rium temperatures between 95°C and 140°C were consistently associ- ated with effective mean weld strengths, which increased linearly from 25 to 110 g, respectively. Temperatures greater than 150°C were associ- ated with rapid tissue dehydration and charring. These data suggest that the therapeutic range of tissue temperature that provides effective thermal fusion of intima-media separations is broad and that the depth and degree of thermal coagulation can be controlled by manipulation of laser energy delivery.

Key words: laser, transluminal angioplasty, PTCA, photocoagulation

INTRODUCTION

Well-recognized limitations of percutaneous Accepted for publication September 30, 1987. transluminal coronary angioplasty are abrupt re- This work was performed during the tenure of a Physician- closure ( 3 4 % ) immediately after the procedure Investigator Award from the American Heart Association, [cowley et al, 19841 and gradual restenosis (17- Massachusetts affiliate, Needham, MA, and was presented in

part at the 36th Annual Scientific Session of the American 34%) in the ensuing few months lHolmes et College of Cardiology, New Orleans, March 1987 by Ronald 19841. Coronary angioplasty often produces lumen D. Jenkins, MD.

by plaque fracture, when ex- Address reprint requests to J. Richard Spears, MD, Division tensive or when associated with excess intramural of Cardiology, Harper HospitaVWayne State University, 3990 shear stresses, may lead to abrupt luminal C O ~ - John R. Street, Detroit, MI 48201.

0 1988 Alan R. Liss, Inc.

Temperature Effect on Laser Thermal Welding 31

lapse from plaque dehiscence or propagation of a dissection [Block, 19841. Restenosis may also re- sult, at least in part, from increased thrombogen- icity of the disrupted, irregular luminal geometry, which may favor persistent deposition of micro- thrombi followed by organization.

In the recently proposed technique of laser balloon angioplasty, the coagulative effects of neo- dymium:yttrium aluminum garnet (NdYAG) laser irradiation might be used to improve luminal ge- ometry in part by thermal fusion of fragmented and separated tissues during the final balloon in- flation of coronary angioplasty [Hiehle et al, 1985; Spears, 19871. Although the feasibility of laser balloon angioplasty has been demonstrated in vi- tro [Hiehle et al, 19851 and experimentally in vivo [Sanborn et al, 1985; Serur et al, 19851, the poten- tially useful therapeutic range of tissue tempera- ture required to achieve laser thermal tissue fusion needs to be defined. Therefore, as described in this report, we examined the relationship be- tween tissue temperature during Nd:YAG laser irradiation and the resultant weld strength of sep- arated layers of atherosclerotic human postmor- tem aortic sections.

MATERIALS AND METHODS Nd:YAG Laser

A 50-watt continuous-wave Nd:YAG laser (Lasermetrics, Inc.) delivering 1.06 pm radiation was coupled to a 400-pm core silica optical fiber for all studies. Prior to each experimental session, the fiberoptic was cleaved and recoupled to the laser, and power output from the fiberoptic was determined with an integrating sphere (Lab- sphere, Inc.) calibrated against a Coherent power meter.

Aortic Segments Postmortem human atheromatous aortic seg-

ments were utilized either fresh on the day of postmortem examination or after up to 2 weeks of freezer storage. Freezing for this period of time was found to have no measurable effect on subse- quent laser thermal weld strength. All samples had moderate fibrofatty atherosclerotic changes. Segments were excised with a l-cm inner diame- ter cork borer, manually separated at the intima- media junction, and repositioned in their original orientation. Intimal and medial thicknesses were measured with a micrometer caliper under condi- tions of moderate tissue compression.

Temperature Measurement Tissue temperature was measured with an

electronic thermometer and a 5-mm small surface temperature thermistor with a 0.3-second time constant (YSI series 400, Yellow Springs Instru- ment Co.), previously calibrated against a mer- cury thermometer.

Concerns about possible overestimation of the adventitial temperature by direct effects of laser radiation penetrating the tissue and impacting on the thermistor, said to be the major source of mea- surement error [Welch and Motamedi, 19861, prompted studies to validate this method of tem- perature measurement. First, attempts to quanti- tate the temperature rise without direct laser effects, using reflective agents on the surface of the thermistor such as magnesium oxide or gold foil, suggested that the overestimation was less than 10°C for the effective welding range. Second, laser radiation was measured directly through aortic discs with each disc positioned flush against the port of the integrating sphere. The diameter of the port was made to be identical with that of the thermistor. Laser irradiation was performed via the 400-pm core fiberoptic with the same nom- inal spot size and range of power used to produce adequate tissue welds. For tissue thickness in the range of the laser welding study (approximately 1.0 mm), 10% of the laser power was detectable at the adventitial surface at all levels of power. When the thermistor was directly subjected to this range of power in air for 20 seconds, i.e., laser energy equivalent to 10% of welding doses, peak temper- ature elevations were always more than 20°C less than the adventitial temperature plateaus of discs subjected to full doses of energy. This observation suggests that the thermistor measurements more likely represented the higher temperatures of the tissue as a result of thermal diffusion.

Third, the validity and accuracy of the ther- mistor method of measuring plateau temperatures was confirmed with the observation of equivalent (+4"C) temperature recordings at the adventitial surface from both the thermistor and a Hughes Probeye thermographic camera which filtered out 1.06-pm radiation, focused on the thermistor dur- ing tissue lasing. Thermographic camera mea- surements of adventitial temperature were unchanged by the presence of the thermistor when lasing tissue of 2 mm thickness. The presence of the thermistor raised adventitial temperature, either by direct absorption or local concentration of heat, by up to 10°C when lasing tissues of 1 mm

32 Jenkins et al

Micromanipulator I Nd:YAG 1 r-

Fiberoptic holder

Plaque He:Ne reference bean

Glass plate I

/ I 1 \

Telethermorneter

/I 1 1, ;Underlying arterial w

Thermistor

Fig. 1. Testing apparatus for delivering Nd:YAG laser energy to atheromatous aortic in- tima-media separations maintaining constant tissue pressure while monitoring adventitial temperature.

thickness, and up to 20°C for tissues of 0.3 mm thickness. Of necessity, these measurements were done with the thermistor in air while in apposition with the adventitia and not embedded in saline- soaked gauze, as performed with the usual lasing apparatus. It is known that the temperature re- sponse to direct laser energy depends on the me- dium surrounding the sensor, and that the temperature rise in air may be ten times greater than the steady-state temperature reached in water [Welch and Motamedi, 19861. That observa- tion suggests that the temperature increment from direct thermistor exposure when the thermistor is in contact with saline-soaked gauze, is probably considerably less than that noted above.

Testing Apparatus The testing apparatus (Fig. 1) consisted of a

gravimetric scale upon which the 5-mm-diameter thermistor was embedded flush with the surface of saline-soaked gauze. Aortic segments were po- sitioned squarely over the thermistor with the su- periorly oriented intimal surface covered by a glass slide pressed onto the segment with a pressure of 28 psi. The flat cleaved optical fiber was oriented perpendicularly 8 mm above the slide, producing a 12-mm2 nominal spot size.

Study Design One hundred aortic samples were initially

studied, with ten samples for each 10°C increment in temperature (50-59°C through 140-149°C). Multiple levels of laser power, ranging from 3 to 25 watts, were delivered in a decremental fashion over 20 seconds such that the highest wattage was delivered for the first 5 seconds, followed by a lower wattage for the second 5 seconds, and sub- sequently by a still lower power for the final 10 seconds. The purpose of this method of laser en- ergy delivery was to achieve rapidly and there- after to maintain a constant target tissue temperature [Anand et al, 19871.

Empiric stepwise decremental-power-setting regimens were evaluated in preliminary studies to find those that yielded steady tissue tempera- tures in the 50-150°C range, and 20 different reg- imens were utilized. Each regimen was employed until ten samples in each temperature range were lased. Laser power output, plateau tissue temper- ature, and intima and media thickness were re- corded for each sample.

Shear Stress Weld Evaluation In order to quantitate the strength of intima-

media pair thermal welds, a graduated shear

33

Fig. 2. Apparatus for shear strength testing of thermal welds of intima-media pair separations.

stress was applied to apposing tissue layers as shown in Figure 2. The lased tissue segments were suspended by alligator clips applied to opposite sides of the weld. A Styrofoam cup, suspended from the lower clip, was filled with water via a Harvard infusion pump at a steady rate yielding the weight in grams required to split the thermal weld.

Weld surface area was determined by expos- ing three lased intima-media pairs to methylene blue and measuring the unstained area after weld separation.

Temperature Vs. Tissue Thickness In order to further the understanding of the

plateau tissue temperature achieved as a function of tissue thickness, a second study was performed. One hundred twenty aortic segments, ranging from 0.4 to 3.0 mm in thickness, were divided into five groups of different laser dose regimens. In each group, duplicate segments of at least 11 dif- ferent thicknesses were subjected to decremental stepped-doses of laser energy designed to yield tis- sue temperatures in one of the five 10°C tempera- ture ranges between 80°C and 130°C had the combined intima-media thickness been 1.0 mm.

Over the 20-second laser exposure, the energy de- livered for the five groups was 115, 141, 160, 179, and 280 joules, respectively. Plateau adventitial surface temperature was measured using the pre- viously detailed apparatus and procedure.

Histologic Examination Histologic examination was performed on

paired aortic segments from each 10°C level of plateau tissue temperature. Samples were fixed in 10% formalin, embedded in paraffin, and sectioned and stained with standard techniques using he- matoxylin and eosin and elastin stains for light microscopic analysis.

Data Analysis Data organization and statistical analysis

were performed on the PROPHET system, a na- tional computer resource sponsored by the Divi- sion of Research Resources, National Institute of Health. Tissue temperature was related to intima- media weld strength, laser energy, and tissue thickness using regression curves to obtain corre- lation coefficients and equations defining the relations.

34 Jenkins et al

150-

140.

130.

120. PLATEAU 110.

TEMPERATURE 'O0. ADVENTITIAL

("1 90.

80.

70.

60-

TABLE 1. Relationships Between Laser Energy and Tissue Temperature, Thickness, and Weld Strength in Nd:YAG Thermal Welding of Postmortem Atheromatous Intima-Media Pair Separations

Mean Mean Mean Mean Mean Temperature laser Mean weld intima media tissue

("CY (joules) SDb ("C)' (g) f SDd (mm) (mm) (mm)

50-59 67 f 14 54 0 f 1.6 .53 .58 1.11 60-69 81 f 24 65 o * o .38 .52 .90 70-79 96 f 42 74 1.5 & 2.4 .51 .45 .96 80-89 136 f 53 84 18.3 15.0 .52 .48 1.00 90-99 158 f 26 95 25.4 f 18.0 .61 .49 1.10 100- 109 195 f 55 104 35.2 & 30.8 .58 .41 .99 110-119 198 f 69 115 56.8 f 20.3 .54 .35 .89 120-129 256 f 71 124 76.0 f 31.2 .54 .44 .98 130-139 323 +_ 53 133 77.0 f 32.3 .53 .47 1 .oo 140-149 367 f 90 143 110.0 & 15.0 .64 .50 1.14

aTen samples per each 10°C range subjected to 20-second stepped-dose Nd:YAG lasing. bMean energy required to yield temperatures in this 10°C range. 'Mean adventitial tissue temperature at conclusion of lasing period. dGrams required for transverse shear of thermal intima-media welds.

range energy temperature strength thickness thickness thickness

RESULTS Tissue Thickness

The data summarizing the relationships be- tween laser energy, tissue temperature, thickness, and intima-media pair weld strength are pre- sented in Table I. The mean intimal thickness for each 10°C temperature range varied between .4 and .6 mm with a maximum individual intimal thickness of 1.1 mm. Mean intima-media pair thickness varied between .9 and 1.1 mm, with a maximum individual pair thickness of 1.7 mm. Thus, tissue thickness was relatively homogene- ous, and no significant correlation between tissue temperature and tissue thickness was therefore found in this aspect of the study.

Temperature Vs. Weld Strength As shown in Figure 3, a linear relationship

was found between tissue temperature and weld strength (r = 0.82, P < .0001) with no measurable welds obtained when adventitial tissue tempera- ture was below 80°C. A mean temperature of 95°C yielded a mean weld strength of 25.4 g or 66.0 g/ cm2 weld surface area. Temperatures above 95 "C were always associated with at least a nominal tissue weld. A maximum mean weld strength of 110 g or 286 g/cm2 was found at a mean plateau tissue temperature of 143°C. While the welds were invariably strong within the 140-149 "C tempera- ture range, rapid surface desiccation and mild tis- sue charring were observed in five of the ten

140

120]

MEDIA 8o INTIMA-

WELD STRENGTH

(grams) 601

20 401

b b Y

y=1.5X-108 r=.82

n= 100 p<.OOOl b b b . *.

8

OL 1 ,L- t . . . , I

50 60 70 80 90 100 110 120 130 140 150 PLATEAU ADVENTITIAL TEMPERATURE ( 'C)

Fig. 3. Relation between Nd:YAG laser-induced atheroma- tous aortic tissue temperature plateau and thermal weld shear strength.

b i y= -.0005X'+.45X+38.8 r'=.78 * *

I I

c Lb b

50 4 Y I I I I

400 0 100 200 300 Nd:YAG LASER ENERGY (joules)

Fig. 4. Relation between Nd:YAG laser 20-second stepped- dose energy output and atheromatous aortic tissue tempera- -

samples in this range. ture plateau.

Temperature Effect on Laser Thermal Welding 35

NdYAG

LASER

OUTPUT

(watts)

20

15

10

5

c 0 5 10 15 20

TIME (seconds)

Fig. 5. Range of ramped laser power output regimens for effective thermal welding.

1401 o

PLATEAU loo- ADVENTITIAL 90.

TEMPERATURE 8o ~

70 * ('C)

50- ~ 1 6 0 louler, r'=.94* p<.OOOl

do, A141 louler, r'=.92. p<.OOOl " 0115 joules. r'=.95, p<.OOOI

30 0 1 2 3

TISSUE THICKNESS (mm)

Fig. 6. Relation between Nd:YAG laser-induced atheroma- tous aortic tissue adventitial temperature plateau and mean tissue thickness, as a measure of the depth of thermal penetration.

Energy Vs. Temperature As shown in Figure 4, the relationship be-

tween energy output and mean tissue tempera- ture was parabolic 0. = -.0005x2 + .45x + 38.8, r2 = .78, P < .0001). Assuming the tissue temper- ature range to be between 95°C and 143°C for effective thermal welding, the corresponding laser energy ranged between 158 and 367 joules, deliv- ered in a stepped-dose fashion, or 1,317 and 3,058 joules/cm2, respectively, in terms of energy den- sity. These two temperatures were achieved with a mean decremental stepped laser dose of 10.8 W x 5 seconds, 7.4 W x 5 seconds, and 6.6 W x 10 seconds (95°C) and 22.2 W x 5 seconds, 18.2 W x 5 seconds, and 16.5 W x 10 seconds (143"C), as represented in Figure 5. Between these extremes was the range of ramped power regimens for effec- tive tissue welding.

Temperature Thickness Relationship In the study of adventitial tissue tempera-

ture as a function of tissue thickness, the results of which are shown in Figure 6, the depth of heat- ing and the degree of temperature elevation were found to be dependent on delivered laser energy. At the highest level of energy delivery (280 joules), there was a zone of nearly homogeneous tempera- ture elevation of 125"C, 0.4 to 2.2 mm below the intimal surface. The zone of homogeneous heating decreased to a depth of about 1.5 mm at a temper- ature of 115"C, when 179 joules were delivered. Although lasing with a lower energy of 160 joules was associated with a more rapid decrement in temperature with increasing tissue thickness, the adventitial temperature remained 95 "C or above to a thickness of 1.5 mm, which should be ade- quate for consistent thermal welding. When 141 joules were delivered, adventitial tissue tempera- ture was 90°C or above to a thickness of 1.0 mm, suggesting the capability of at least minimal ther- mal welding. At 115 joules, adventitial tempera- ture approached 95°C at a thickness of 0.6 mm (average intima-media interface depth) and 80°C at a thickness of 1.0 mm, suggesting negligible welding capability at this depth. In all samples, a marked decrement in temperature occurred be- tween 2 and 3 mm with temperatures below 60°C at 3 mm for those exposed to 160 joules or less. These curves allow estimation of the true intima- media interface temperature. For example, at the lowest level of laser energy delivery, adventitial temperature underestimates the intima-media in- terface temperature by approximately 15°C for a mediaUadventitia1 thickness of 0.5 mm (the mean value of the previous study).

Histologic Results The histologic appearance of thermal welds

is summarized in Figure 7. A marginally adherent weld was obtained at temperatures of 92-94°C. At 108 "C, weld adherence was more complete. Cellu- lar and matrix detail and elastic tissue integrity were preserved at welding temperatures to llO"C, although there was slight disorganization of elas- tic elements at the weld interface. Sections heated to 118°C showed a loss of tissue structural detail and increased rigidity as evidenced by occasional splitting during sectioning. Loss of histological structure definition became progressively more extensive at higher treatment temperatures. Sur- face coagulation and intimal vacuolization from local steam formation were prominent at temper- atures greater than or equal to 125"C, with

36 Jenkins et a1

Fig. 7. Photomicrographs of the zone of adherence of vessels welded at various temperatures. A Temperature 100-109°C. Vessel structure is maintained. Open arrow indicates region highlighted in B. B: Close-up of welded interface demonstrat- ing adherence with slight elastic fiber disorganization. c: Temperature 110-119°C. Diffuse coagulation is evidenced by mild structural homogenization. I): Temperature 130-139°C.

There is marked tissue coagulation with increased tissue rigidity, demonstrated by fracturing and also artifactual sep- aration of the weld zone during sectioning. Each section stained for elastin; A, C, and D have arrowheads demonstrat- ing plane of original tissue separations. Magnifications: A, C, and D, ~ 6 0 ; B, ~150.

Temperature Effect on Laser Thermal Welding 37

marked homogenization of structural detail and elastic elements. Intimal thinning and eosino- philic staining suggesting tissue desiccation were present at these higher temperatures.

DISCUSSION

Medical applications of laser photocoagula- tion technology are quickly expanding and include the treatment of diabetic retinopathy [Coscas and Chaine, 19791, port wine stains [Noe et al, 19801, telangiectasia [Shapshay and Oliver, 1984 1, gas- trointestinal hemorrhage [Keifhaber et al, 1977; Yellin, 19811, and control of intraoperative bleed- ing [Dixon, 19831. Jain and Gorisch first reported the use of a Nd:YAG laser for repair of small vascular tears in 1979 [Jain and Gorisch, 19791. Others subsequently documented the effective- ness of similar laser techniques for creating vas- cular anastomoses [Gomes et al, 19831 and for welding together a variety of tissues [Dew, 19831.

Effective Tissue Welding Temperature

In the proposed system of laser balloon angio- plasty, similar thermal effects of the NdYAG laser are desired for welding fractured tissues, for smoothing uneven, potentially thrombogenic sur- faces, and for reducing elastic recoil. Continuous- wave NdYAG laser radiation is suitable for achieving these effects because of the relatively low absorption coefficient and high scattering coefficient at 1.06 pm. Our data demonstrate that Nd:YAG thermal welding of atheromatous aortic separations is readily achievable within a fairly well-defined temperature range. We suggest that under the conditions of our study the minimum predictably effective temperature for thermal welding is approximately 95°C when measured at the adventitial surface 0.5 mm from the plane of tissue separation, or approximately 105°C at the intima-media interface.

With the 12-mm2 nominal spot size used in this study, the minimum energy required for cre- ating a measurable thermal fusion was 136 joules, with reproducible welds requiring at least 158 joules. The highest energy delivered (367 joules) was associated with significant intimal desicca- tion and occasional charring. Between these two extremes, however, a broad range of tissue tem- perature and power output is available for provid- ing effective thermal fusion of intima-media separations.

Depth of Effective Thermal Penetration The study of temperature verus tissue thick-

ness demonstrated that the depth to which effec- tive welding temperatures can be achieved was related to energy delivery in a parabolic fashion and was limited to 3 mm irrespective of the laser dose. Of interest was the disparity in slope of the temperature-thickness curves with increasing de- livered energy, such that higher energy was asso- ciated with a homogeneous temperature elevation to a depth of 2.2 mm. Possible explanations for this observation include a nonlinear reduction in thermal diffusion with increasing levels of deliv- ered energy and the natural tendency of the tem- perature to plateau at higher laser doses because of rapid evaporation of water [Welch et al, 19851. Other possible explanations include the decrease in tissue thickness associated with surface dehy- dration, thereby reducing the supposed incre- ments in thickness, and an alteration in optical properties of plaque with lasing. The latter hy- pothesis is unlikely, as repeat laser application after cooling has demonstrated identical tempera- ture patterns (unpublished observation). Finally, an artifact of the thermistor method of tempera- ture measurement is unlikely to be responsible, as thermographic analysis demonstrated that the thermistor was accurate and that adventitial tem- perature measurement was minimally affected by its presence.

Few data are available from the literature regarding the therapeutically useful range of tem- perature for Nd:YAG thermal tissue welding. Us- ing a thermographic camera while lasing gastric mucosa with a Nd:YAG laser, Marchesini et a1 studied the heat distribution during tissue coagu- lation [Marchesini et al, 19851. They found that the coagulation depth was related to radiant ex- posure both in the exposure duration and, more importantly, in the power delivered. Their data suggested that a temperature rise above 60"C, which they regarded as a coagulative tempera- ture, cannot extend beyond 3 mm, and that a cra- ter would occur for exposures of greater than 2,500 joules/cm2. Their procedure did not employ tissue pressure as in the present study and called for a higher power delivery over a shorter period of exposure. These differences may explain why our data, although agreeing with the marked decre- ment in temperature rise in soft tissue at a depth between 2 and 3 mm, suggest that temperatures above 60°C at a depth beyond 3 mm are achiev- able without vaporization of tissue with energy densities approaching 3,000 joules/cm2.

38 Jenkins et a1

Clinical Implications While the laser power output required to

raise the tissue temperature to the therapeutic range may vary with a different delivery system such as with cylindrical diffusion to an arterial wall as required for laser balloon angioplasty, the temperature required for thermal welding should be the same; the present data, therefore, provide a useful guide in the dosimetry required for success- ful application of laser balloon angioplasty. We suggest that temperatures in the low therapeutic range may be the most desirable. First, experience with coronary angioplasty has shown that merely tacking back arterial flaps by the use of an ex- tended balloon inflation period of 60-90 seconds often allows for restoration of flow and avoidance of abrupt reclosure [Kaltenback et al, 19841. Thus, a thermal weld of even a trivial degree may be satisfactory, as normal tissue mechanisms of re- pair should quickly strengthen the weld. Second, because coronary stenoses may be associated with hemorrhagic plaques or thrombi, the potential for producing higher tissue temperatures, which may be associated with lasing more highly absorbing tissues, increases the risk of tissue charring. Fi- nally, a lower power density might be advisable to reduce any potential risk of postprocedure perivas- cular fibrosis in the event that the depth of laser penetration greatly exceeds the thickness of a thin- walled artery. However, this potential problem must be balanced against the need to deliver effec- tive energy to the intima-media interface through plaques that may approach 2 mm in thickness in human coronaries. Certainly, the ability to manip- ulate the depth of temperature penetration will be important in tailoring laser delivery for different clinical situations.

CONCLUSIONS

In conclusion, effective Nd:YAG laser weld- ing of intima-media separations occurs over a broad temperature range from 95 to 140°C. Under the conditions of this study, a range of 1,000 to 3,000 joules per cm2 at the intimal surface was required to obtain effective welds. The depth to which potentially useful temperature levels can be achieved can be varied between 1 and 3 mm by manipulation of laser power density. Further ex- periments will be needed to evaluate power re- quirements necessary in a balloon catheter system to achieve therapeutically useful intima-media in- terface temperatures. With such data, in vivo in- vestigations of laser ballon angioplasty, such as

its effect on restenosis, can be properly accom p 1 ished .

ACKNOWLEDGMENTS

We express our appreciation for the technical assistance of Leslie James and for preparation of the manuscript by Carol Anne Centauro.

This study was supported in part by National Institutes of Health Grant HL37349, awarded by the National Heart, Lung, and Blood Institute, Bethesda, MD.

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