Lasers Med Sci DOI 10.1007/s10103-013-1505-0
ORIGINAL ARTICLE
Open-irrigated laser catheter ablation: influence of catheter irrigation and of contact and noncontact mode of laser application on lesion formation in bovine myocardium Helmut P. Weber & Michaela Sagerer-Gerhardt
Received: 3 June 2013 / Accepted: 25 November 2013 # Springer-Verlag London 2013
Abstract Lesions achieved by radiofrequency application increase with catheter irrigation and with catheter pressure on the endocardial surface. Purpose of this study was to test the influence of catheter irrigation and of contact vs. noncontact mode of laser application on lesion formation in bovine myocardium. By applying continuous wave 1,064 nm laser light via an open-irrigated catheter lesions were produced at 15 W (9.5 W/mm 2 )/30 s (285 J/mm 2 ), in stagnant blood (activated clotting time>350 s) at 18 °C, on bovine myocardium. During flow rates of 15, 30, and 50 ml/min radiation was applied with the catheter end hole in contact (n =10, each) or 2 mm away from the endocardial surface (n = 5, each). Lesions were evaluated morphometrically, and groups of lesions were compared by using the unpaired t test. By augmentation of irrigation flow from 15 to 30 ml/min, contact lesions increased significantly (p =0.0001). A further increase of flow from 30 to 50 ml/min increased lesions significantly in depth (p =0.0011) but not in width (p = 0.639) and volume (p =0.218). Noncontact lesions were significantly smaller than contact lesions (p >0.05). Lesions of homogenous coagulation necrosis were clear-cut and sharply demarcated from the surrounding normal myocardium. There was no occurrence of steam-pop with intramural cavitation or with tissue vaporization with crater or thrombus formation. It is suggested that by using an open-irrigated laser catheter as described in this study, catheter
H. P. Weber (*) CCEP Center Taufkirchen, Section Research and Development, 4 Schlesierst, 82024 Taufkirchen, Germany e-mail: [email protected] M. Sagerer-Gerhardt Department of Anesthesiology, Hospital Neuperlach, Teaching Hospital of the Ludwig-Maximilian-University of Munich, Munich 81737, Germany
irrigation at flow rates of 30 to 40 ml/min are optimal for myocardial coagulation, and catheter pressure on the endocardial surface is not needed for lesion formation. Laser lesions can be achieved also without intimate endocardial catheter contact. Keywords Catheter ablation . Laser ablation . Catheter irrigation . Noncontact laser application . Contact pressure
Introduction Laser catheter ablation damages the tissue by heating. Increase of tissue temperature occurs by absorption of photons. Absorption of 1,064 nm laser light in water and in transparent or translucent tissue such as the endocardium, epicardium, and scarred tissue is low and photons passes through these tissues without heat generation [1, 2]. In this study, the optical fiber tip itself is protected within the catheter hose at a given distance from the end hole and is never in contact with the illuminated substrate. The catheter and the irrigation liquid are not heated up. On the contrary, saline flow at room temperature effectively cools the endocardium and creates a transparent path for the laser light. In contrast to that, laser application with the optical fiber in intimate contact with the endocardium would result in tissue vaporization creating transmyocardial channels. This is the case when transmyocardial laser revascularization (TMLR) is the goal. The aim of this study, however, was to avoid tissue vaporization with crater formation and to achieve lesions of homogenous coagulation necrosis devoid of electrical activity, with the aim of arrhythmia ablation without tissue vaporization with crater or channel formation (Fig. 1). Photons are frequently scattered penetrating deep into the myocardium and are eventually absorbed by the chromophores like myoglobin [3]. The absorbed photons excite
Lasers Med Sci
Fig. 1 Laser lesions produced in stagnant blood by applying the same amount of energy settings with optical fibers fed into open-irrigated catheters. Left: with the plane polished optical fiber tip in contact, and right: with a conically shaped fiber tip at a distance of 1–2 mm from the epicardial surface of a bovine specimen. In the contact mode of radiation,
there is a deep myocardial channel with carbonization surrounded by a large zone of myocardial coagulation necrosis (horizontal arrows). In the noncontact mode of radiation, a large clear-cut (horizontal arrows) ovalshaped homogenous coagulation necrosis without tissue vaporization is achieved
molecules, and heat is generated by friction of photons with the molecules of the tissue. By heating up myocardial tissue deep intramurally irreversible damage to myocytes will occur when tissue temperatures exceed 50 °C [4]. By using the open-irrigated laser catheter, maximum temperature is achieved intramurally [5]. For larger laser lesions, an increase of power is not needed. On the contrary, too high power may overheat the tissue with its attendant risks, such as tissue vaporization with crater formation, cardiac perforation, and malignant arrhythmias. If larger lesions are desired, longer radiation times at lower power levels have to be used. Lesions will grow gradually; the coagulated myocardium from dark turns pale and will absorb less photon, which will be scattered more peripherally. In addition, lesions will further grow also by heat conduction until a steady state is reached between the energy deposited and the cooling effect of the blood flow and irrigation fluid. Radiofrequency (RF) lesions are limited in sizes, especially in depth, so that intramural ventricular foci deeper than 8 mm cannot be reached and surgical interventions are needed [6, 7]. Our experiments suggest that 1,064-nm laser light produces significantly larger lesions than RF current [8], and, that sizes of lesions produced are dependent on catheter flow rate. In addition, for lesion formation, catheter pressure is not needed. Lesions can be produced also without intimate contact of the catheter with the endocardium [9]. A higher irrigation flow could more effectively wash away the blood, creating a larger clear path for the laser light and a better heat induction that may result in larger lesions. However, a too high irrigation flow may destabilize the catheter tip and volume overload may endanger patients especially those with congestive heart failure, hypertension, and the elderly. The optimal irrigation flow for ablation by using an open-irrigated laser catheter system has not yet been determined. Purpose of this study was to evaluate the effects of various irrigation flows on lesion formation in bovine myocardium with the catheter in contact and with its tip 1–2 mm away.
Materials and methods Laser light was applied via an 8 F open-irrigated electrodelaser mapping and ablation (ELMA) heart catheter (LasCor GmbH) with a 400-μm optical fiber fed into its central lumen. The tip of the fiber was mounted at a given distance from the end hole. Thus, it was protected from the blood stream and from contact with the endocardial surface. The space between the lumen of the catheter and the optical fiber was used for flushing with saline. The catheter was connected to the laser via a SubMiniature version A (SMA) connector and to the peristaltic pump via a standard Luer connector. Power source was a continuous wave 1,064 nm laser (4060 N, Dornier MedTech, GmbH) with a foot switch that automatically started the peristaltic pump simultaneously with the laser. Laser applications at 15 W (9.5 W/mm2)/30 s (285 J/mm2) were aimed at the endocardial sites of the left ventricular free wall of bovine myocardium freshly harvested from a healthy 10-month-old bull at the local slaughter house. The endocardium appeared smooth and translucent without mechanical damages or mural thrombi. The thickness of the myocardial wall ranged from 2.1 to 3.2 cm. Visually, the myocardial structure was homogenous without fibrous or fatty degeneration or calcifications. Experiments were performed in stagnant heparinized blood, with activated clotting time (ACT) >350 s, at room temperature (18 °C). Saline irrigation was performed by means of a peristaltic pump. With the start of the laser continuous irrigation flow of 15 ml/min either remained unchanged or was augmented automatically to 30 or 50 ml/min. By means of a support the catheter was maintained in a stable perpendicular position with the catheter tip in contact with but without pressure on the endocardial surface (n =10, each), or at a distance of 1–2 mm away (n =5, each). After each application, the blood was removed from the recipient, and lesion and catheter tip were inspected. Immediately following the tests, the specimen was dissected and lesions were measured for depth, width, and surface diameter. Volumes were calculated. To compare the
Lasers Med Sci
Fig. 2 Laser lesions produced in stagnant blood with the catheter mounted in perpendicular position with its end hole at a distance of 1– 2 mm (fiber tip to tissue distance is 3–4 mm) upon the targeted endocardial surface of bovine myocardium. Top: endocardial view of the specimen displaying three pale circular spots (arrows) consistent with the sites where laser applications at 15 W/30 s were aimed at by using irrigation flows of 15, 30, and 50 ml/min (from right to the left). There is no tissue vaporization with crater or thrombus formation conspicuous on the spots of acute coagulation necrosis. Note: There is no contact mark of the rim of the end hole of the catheter tip as usually seen after catheter contact. Bottom: the same specimen after dissection showing shapes and depths of lesions
groups of lesions achieved during various irrigation flows of 15 vs. 30 and of 30 vs. 50 ml/min the unpaired t test was used.
Results After laser application, the endocardial surface showed clearcut circular spots of acute coagulation necrosis at sizes
Fig. 3 Diagram showing depth of laser lesions
according to the flow rate applied. The coagulated myocardium was sharply demarcated from the surrounding healthy myocardium and was covered by layers of undamaged translucent endocardium. There was no tissue vaporization with crater or thrombus formation. The endocardial surface of the noncontact lesions showed also clear-cut circular spots of subendocardial coagulation necrosis but without the typical traces present after laser application with endocardial catheter contact (Fig. 2). Regardless of the contact or of the noncontact mode of laser application, sizes of lesions increased with the irrigation flow applied. Contact lesions showed a minimum depth of 5.1 mm after an irrigation flow of 15 ml/min whereas maximum depth achieved after an irrigation flow of 50 ml/min was 11.3 mm (Fig. 3). Similarly, minimum surface diameter of lesions was 3.8 mm vs. a maximum of 9.2 mm (Fig. 4), and minimum volume was 35 mm3 vs. a maximum of 385 mm3 (Fig. 5). There was a significant increase of depth, diameter, and volumes of lesions achieved by augmentation of the irrigation flow from 15 to 30 ml/min. After augmentation of the irrigation flow from 30 to 50 ml/min, contact lesions increased significantly in depth, but not in diameter or volume. Noncontact lesions achieved with the catheter tip 1–2 mm away from the endocardial surface were smaller as compared to those achieved with endocardial contact. Minimum depth of noncontact lesions was 1.9 mm after irrigation flow of 15 ml/min, and maximum depth was 3.8 mm after a flow of 50 ml/min; minimum diameter was 3.2 vs. 5.1 mm and volumes were 20 vs. 80 mm3. The mean maximum depth of the noncontact lesions was 33.3 %, mean maximum diameter was 63.3 %, and mean maximum volumes were 21.9 % of that of the contact lesions.
Lasers Med Sci Fig. 4 Diagram showing width of laser lesions
Discussion Recently, we have demonstrated that levels of energy >800 J may cause unwanted thermal effects [10]. Thus, a further increase of laser lesions by the augmentation of power is limited. However, by maintaining a constant power of 15 W, a significant increase of lesions is achieved when the catheter irrigation flow is raised from 15 to 30 ml/min. This is best explained by the fact that a higher flow does better wash away the blood and will create a larger clear path for the laser light. More photons can hit the illuminated field and a faster and higher increase of temperature is induced. This is of importance when deep or even transmural lesions are needed for ablation of subepicardial foci Fig. 5 Diagram showing volume of laser lesions
or for arrhythmogenic substrates located deep intramurally, not amenable for ablation by RF or by other techniques [6, 7]. High energy lasers are not suitable for myocardial coagulation. As compared to the Nd:YAG laser, Excimer lasers are pulsed gas lasers producing high-energy ultraviolet light for tissue vaporization with channel formation [11]. Excimer lasers as well as other high energy lasers including Q-switched YAG lasers are suitable rather for removal of tissue, for TMLR, coronary, and peripheral revascularization, and for removal of fractured, abandoned, and malfunctioning leads of cardiac pacemakers. Partial dissipation of laser energy into blood may be the major reason for the smaller sizes of lesions achieved with the
Lasers Med Sci
noncontact as compared to the contact mode of radiation. This is supported also by the fact that depth of noncontact lesions still increases significantly by the augmentation of catheter flow from 30 to 50 ml/min. However, maximum noncontact lesion depth of 4.2 mm may suffice for transmural coagulation of atrial walls, and may help produce lesions in regions where stable catheter contact with the endocardial surface is difficult to achieve. The total amount of the given saline depends on procedure duration, flow rate, number of lesions, and duration per lesion. Power titration and fluid management for optimal safety and efficacy of irrigated RF ablation was the topic of a series of publications [4, 12–14]. In general, during RF mapping, catheter irrigation flow is 2 ml/min whereas during ablation it is increased to 30 ml/min, plus 5 s before and after each lesion. For an assumed ablation procedure time of 90 min and a total of 30 lesions of 90 s, volume of irrigation fluid totalizes 1,580 ml. As compared to that, irrigation flow is 15 ml/min during laser mapping and 35 ml/min during ablation. Applying the same amount of lesions of 30 s each during the same procedure time of 90 min, irrigation fluid totalizes a volume of about 1,575 ml. When considering that laser induces larger lesions and lesion formation is faster, fewer lesions are required and procedure times are shorter. Thus, total volume of irrigation flow during laser ablation is rather less as compared to that of the RF method.
Conclusions With irrigation flow of 30 ml/min, cw1,064 nm laser catheter application at 15 W/30 s, in a contact and noncontact mode of radiation, on bovine myocardium, can achieve lesions at depth of up to 10.2 and 4.0 mm, respectively. Substantially, larger laser lesions can be expected after longer radiation times. Pressure with the catheter on the radiation field is not needed for laser lesion formation. This special claim of the laser method contributes substantially to the safety and efficacy of the method. Optimal irrigation flow for laser catheter ablation by using the open-irrigated ELMA catheter is suggested to be about 35 ml/min.
Study limitations Study limitations are the in vitro test on bovine myocardium at room temperature. These are discrete conditions as compared to those in the in vivo tests. Lesions achieved at body temperature are substantially larger. Volumes of coagulated myocardium achieved at 15 W/30 s with an irrigation flow of 30 ml/ min in this study were 295±43 mm3. As compared to that,
in vivo lesions achieved in a dog thigh muscle model with the same energy setting and irrigation flow, and by using the same laser catheter were 1,170±114 mm3, +75 % [9], and those achieved in dog hearts were >1,400 mm3, always transmural, and were increasing by over 79 % [10]. Acknowledgments This study was supported in part by the LasCor GmbH, Laser Medical Devices Taufkirchen, Taufkirchen, Germany.
References 1. Boulnois J-L (1986) Photophysical processes in recent medical laser developments: a review. Laser Med Sci 1:47–66 2. Weber H, Heinze A, Enders S, Ruprecht L, Unseold E (1998) Laser catheter coagulation of normal and scarred ventricular myocardium. Laser Surg Med 22:109–119 3. Bruneval P, Mesnildrey P, Camilleri P (1987) Nd:YAG laser induced injury in dog myocardium: optical and ultrastructural study of early lesions. Eur Heart J 8:785–792 4. Stevenson WG, Cooper J, Sapp J (2004) Optimizing RF output for cooled RF ablation. J Cardiovasc Electrophysiol 15:24–27 5. Weber H, Heinze A, Enders S, Hauptmann G, Ruprecht L, Unsoeld E (1997) In vivo temperature measurement during transcatheter endomyocardial Nd:YAG laser irradiation. Laser Med Sci 12:352– 356 6. Haines D (2004) Biophysics of ablation: application to technology. J Cardiovasc Electrophysiol 15:S2–S11 7. Vergara P, Maisano F, Maccabelli G, Della Bella P (2013) Radiofrequency and cryoenergy endo-epicardial catheter and surgical approach for a case of incessant ventricular tachycardia ablation. Europace 15:540 8. Weber H, Heinze A, Enders S, Ruprecht L, Unsoeld E (1997) Laser vs. radiofrequency catheter ablation of ventricular myocardium in dogs: a comparative test. Cardiology 88:346–352 9. Weber H, Sagerer-Gerhardt M (2013) Open-irrigated laser catheter ablation produces flow dependent sizes of lesions. PACE 36:1132– 1137 10. Weber H, Sagerer-Gerhardt M (2013) Open-irrigated laser catheter ablation: relationship between the level of energy, myocardial thickness, and collateral damages in a dog model. Europace. doi:10.1093/ europace/eut150 11. Isner JM, Donaldson RF, Deckelbaum LI, Clarke RH, Laliberte SM, Icci AA, Salem DN, Konstam MA (1985) The excimer laser: gross, light microscopic and ultrastructural analysis of potential advantages for use in laser therapy of cardiovascular disease. Am J Cardiol 6: 1102–1109 12. Nakagawa H, Wittkampf FH, Yamanashi WS, Pitha JV, Imia S, Campbell B, Arruda M, Lazzara R, Jackman WM (1998) Inverse relationship between electrode size and lesion size during RF ablation with active electrode cooling. Circulation 98:458–465 13. Matsudaira K, Nakagawa H, Wittkampf FH, Yamanashi W, Imia S, Pitha JV, Lazzara R, Jackman WM (2003) High incidence of thrombus formation without impedance rise during RF ablation using electrode temperature control. PACE 26:1227–1237 14. Scavee C, Jais P, Hsu LF, Sanders P, Hocini M, Weerasooriya R, Macle L, Raybaud F, Clementy J, Haissaguerre M (2004) Prospective randomized comparison of irrigated-tip and largetip catheter ablation of cavotricuspid istmus-dependent atrial flutter. Eur Heart J 25:963–969
ORIGINAL ARTICLE
Open-irrigated laser catheter ablation: influence of catheter irrigation and of contact and noncontact mode of laser application on lesion formation in bovine myocardium Helmut P. Weber & Michaela Sagerer-Gerhardt
Received: 3 June 2013 / Accepted: 25 November 2013 # Springer-Verlag London 2013
Abstract Lesions achieved by radiofrequency application increase with catheter irrigation and with catheter pressure on the endocardial surface. Purpose of this study was to test the influence of catheter irrigation and of contact vs. noncontact mode of laser application on lesion formation in bovine myocardium. By applying continuous wave 1,064 nm laser light via an open-irrigated catheter lesions were produced at 15 W (9.5 W/mm 2 )/30 s (285 J/mm 2 ), in stagnant blood (activated clotting time>350 s) at 18 °C, on bovine myocardium. During flow rates of 15, 30, and 50 ml/min radiation was applied with the catheter end hole in contact (n =10, each) or 2 mm away from the endocardial surface (n = 5, each). Lesions were evaluated morphometrically, and groups of lesions were compared by using the unpaired t test. By augmentation of irrigation flow from 15 to 30 ml/min, contact lesions increased significantly (p =0.0001). A further increase of flow from 30 to 50 ml/min increased lesions significantly in depth (p =0.0011) but not in width (p = 0.639) and volume (p =0.218). Noncontact lesions were significantly smaller than contact lesions (p >0.05). Lesions of homogenous coagulation necrosis were clear-cut and sharply demarcated from the surrounding normal myocardium. There was no occurrence of steam-pop with intramural cavitation or with tissue vaporization with crater or thrombus formation. It is suggested that by using an open-irrigated laser catheter as described in this study, catheter
H. P. Weber (*) CCEP Center Taufkirchen, Section Research and Development, 4 Schlesierst, 82024 Taufkirchen, Germany e-mail: [email protected] M. Sagerer-Gerhardt Department of Anesthesiology, Hospital Neuperlach, Teaching Hospital of the Ludwig-Maximilian-University of Munich, Munich 81737, Germany
irrigation at flow rates of 30 to 40 ml/min are optimal for myocardial coagulation, and catheter pressure on the endocardial surface is not needed for lesion formation. Laser lesions can be achieved also without intimate endocardial catheter contact. Keywords Catheter ablation . Laser ablation . Catheter irrigation . Noncontact laser application . Contact pressure
Introduction Laser catheter ablation damages the tissue by heating. Increase of tissue temperature occurs by absorption of photons. Absorption of 1,064 nm laser light in water and in transparent or translucent tissue such as the endocardium, epicardium, and scarred tissue is low and photons passes through these tissues without heat generation [1, 2]. In this study, the optical fiber tip itself is protected within the catheter hose at a given distance from the end hole and is never in contact with the illuminated substrate. The catheter and the irrigation liquid are not heated up. On the contrary, saline flow at room temperature effectively cools the endocardium and creates a transparent path for the laser light. In contrast to that, laser application with the optical fiber in intimate contact with the endocardium would result in tissue vaporization creating transmyocardial channels. This is the case when transmyocardial laser revascularization (TMLR) is the goal. The aim of this study, however, was to avoid tissue vaporization with crater formation and to achieve lesions of homogenous coagulation necrosis devoid of electrical activity, with the aim of arrhythmia ablation without tissue vaporization with crater or channel formation (Fig. 1). Photons are frequently scattered penetrating deep into the myocardium and are eventually absorbed by the chromophores like myoglobin [3]. The absorbed photons excite
Lasers Med Sci
Fig. 1 Laser lesions produced in stagnant blood by applying the same amount of energy settings with optical fibers fed into open-irrigated catheters. Left: with the plane polished optical fiber tip in contact, and right: with a conically shaped fiber tip at a distance of 1–2 mm from the epicardial surface of a bovine specimen. In the contact mode of radiation,
there is a deep myocardial channel with carbonization surrounded by a large zone of myocardial coagulation necrosis (horizontal arrows). In the noncontact mode of radiation, a large clear-cut (horizontal arrows) ovalshaped homogenous coagulation necrosis without tissue vaporization is achieved
molecules, and heat is generated by friction of photons with the molecules of the tissue. By heating up myocardial tissue deep intramurally irreversible damage to myocytes will occur when tissue temperatures exceed 50 °C [4]. By using the open-irrigated laser catheter, maximum temperature is achieved intramurally [5]. For larger laser lesions, an increase of power is not needed. On the contrary, too high power may overheat the tissue with its attendant risks, such as tissue vaporization with crater formation, cardiac perforation, and malignant arrhythmias. If larger lesions are desired, longer radiation times at lower power levels have to be used. Lesions will grow gradually; the coagulated myocardium from dark turns pale and will absorb less photon, which will be scattered more peripherally. In addition, lesions will further grow also by heat conduction until a steady state is reached between the energy deposited and the cooling effect of the blood flow and irrigation fluid. Radiofrequency (RF) lesions are limited in sizes, especially in depth, so that intramural ventricular foci deeper than 8 mm cannot be reached and surgical interventions are needed [6, 7]. Our experiments suggest that 1,064-nm laser light produces significantly larger lesions than RF current [8], and, that sizes of lesions produced are dependent on catheter flow rate. In addition, for lesion formation, catheter pressure is not needed. Lesions can be produced also without intimate contact of the catheter with the endocardium [9]. A higher irrigation flow could more effectively wash away the blood, creating a larger clear path for the laser light and a better heat induction that may result in larger lesions. However, a too high irrigation flow may destabilize the catheter tip and volume overload may endanger patients especially those with congestive heart failure, hypertension, and the elderly. The optimal irrigation flow for ablation by using an open-irrigated laser catheter system has not yet been determined. Purpose of this study was to evaluate the effects of various irrigation flows on lesion formation in bovine myocardium with the catheter in contact and with its tip 1–2 mm away.
Materials and methods Laser light was applied via an 8 F open-irrigated electrodelaser mapping and ablation (ELMA) heart catheter (LasCor GmbH) with a 400-μm optical fiber fed into its central lumen. The tip of the fiber was mounted at a given distance from the end hole. Thus, it was protected from the blood stream and from contact with the endocardial surface. The space between the lumen of the catheter and the optical fiber was used for flushing with saline. The catheter was connected to the laser via a SubMiniature version A (SMA) connector and to the peristaltic pump via a standard Luer connector. Power source was a continuous wave 1,064 nm laser (4060 N, Dornier MedTech, GmbH) with a foot switch that automatically started the peristaltic pump simultaneously with the laser. Laser applications at 15 W (9.5 W/mm2)/30 s (285 J/mm2) were aimed at the endocardial sites of the left ventricular free wall of bovine myocardium freshly harvested from a healthy 10-month-old bull at the local slaughter house. The endocardium appeared smooth and translucent without mechanical damages or mural thrombi. The thickness of the myocardial wall ranged from 2.1 to 3.2 cm. Visually, the myocardial structure was homogenous without fibrous or fatty degeneration or calcifications. Experiments were performed in stagnant heparinized blood, with activated clotting time (ACT) >350 s, at room temperature (18 °C). Saline irrigation was performed by means of a peristaltic pump. With the start of the laser continuous irrigation flow of 15 ml/min either remained unchanged or was augmented automatically to 30 or 50 ml/min. By means of a support the catheter was maintained in a stable perpendicular position with the catheter tip in contact with but without pressure on the endocardial surface (n =10, each), or at a distance of 1–2 mm away (n =5, each). After each application, the blood was removed from the recipient, and lesion and catheter tip were inspected. Immediately following the tests, the specimen was dissected and lesions were measured for depth, width, and surface diameter. Volumes were calculated. To compare the
Lasers Med Sci
Fig. 2 Laser lesions produced in stagnant blood with the catheter mounted in perpendicular position with its end hole at a distance of 1– 2 mm (fiber tip to tissue distance is 3–4 mm) upon the targeted endocardial surface of bovine myocardium. Top: endocardial view of the specimen displaying three pale circular spots (arrows) consistent with the sites where laser applications at 15 W/30 s were aimed at by using irrigation flows of 15, 30, and 50 ml/min (from right to the left). There is no tissue vaporization with crater or thrombus formation conspicuous on the spots of acute coagulation necrosis. Note: There is no contact mark of the rim of the end hole of the catheter tip as usually seen after catheter contact. Bottom: the same specimen after dissection showing shapes and depths of lesions
groups of lesions achieved during various irrigation flows of 15 vs. 30 and of 30 vs. 50 ml/min the unpaired t test was used.
Results After laser application, the endocardial surface showed clearcut circular spots of acute coagulation necrosis at sizes
Fig. 3 Diagram showing depth of laser lesions
according to the flow rate applied. The coagulated myocardium was sharply demarcated from the surrounding healthy myocardium and was covered by layers of undamaged translucent endocardium. There was no tissue vaporization with crater or thrombus formation. The endocardial surface of the noncontact lesions showed also clear-cut circular spots of subendocardial coagulation necrosis but without the typical traces present after laser application with endocardial catheter contact (Fig. 2). Regardless of the contact or of the noncontact mode of laser application, sizes of lesions increased with the irrigation flow applied. Contact lesions showed a minimum depth of 5.1 mm after an irrigation flow of 15 ml/min whereas maximum depth achieved after an irrigation flow of 50 ml/min was 11.3 mm (Fig. 3). Similarly, minimum surface diameter of lesions was 3.8 mm vs. a maximum of 9.2 mm (Fig. 4), and minimum volume was 35 mm3 vs. a maximum of 385 mm3 (Fig. 5). There was a significant increase of depth, diameter, and volumes of lesions achieved by augmentation of the irrigation flow from 15 to 30 ml/min. After augmentation of the irrigation flow from 30 to 50 ml/min, contact lesions increased significantly in depth, but not in diameter or volume. Noncontact lesions achieved with the catheter tip 1–2 mm away from the endocardial surface were smaller as compared to those achieved with endocardial contact. Minimum depth of noncontact lesions was 1.9 mm after irrigation flow of 15 ml/min, and maximum depth was 3.8 mm after a flow of 50 ml/min; minimum diameter was 3.2 vs. 5.1 mm and volumes were 20 vs. 80 mm3. The mean maximum depth of the noncontact lesions was 33.3 %, mean maximum diameter was 63.3 %, and mean maximum volumes were 21.9 % of that of the contact lesions.
Lasers Med Sci Fig. 4 Diagram showing width of laser lesions
Discussion Recently, we have demonstrated that levels of energy >800 J may cause unwanted thermal effects [10]. Thus, a further increase of laser lesions by the augmentation of power is limited. However, by maintaining a constant power of 15 W, a significant increase of lesions is achieved when the catheter irrigation flow is raised from 15 to 30 ml/min. This is best explained by the fact that a higher flow does better wash away the blood and will create a larger clear path for the laser light. More photons can hit the illuminated field and a faster and higher increase of temperature is induced. This is of importance when deep or even transmural lesions are needed for ablation of subepicardial foci Fig. 5 Diagram showing volume of laser lesions
or for arrhythmogenic substrates located deep intramurally, not amenable for ablation by RF or by other techniques [6, 7]. High energy lasers are not suitable for myocardial coagulation. As compared to the Nd:YAG laser, Excimer lasers are pulsed gas lasers producing high-energy ultraviolet light for tissue vaporization with channel formation [11]. Excimer lasers as well as other high energy lasers including Q-switched YAG lasers are suitable rather for removal of tissue, for TMLR, coronary, and peripheral revascularization, and for removal of fractured, abandoned, and malfunctioning leads of cardiac pacemakers. Partial dissipation of laser energy into blood may be the major reason for the smaller sizes of lesions achieved with the
Lasers Med Sci
noncontact as compared to the contact mode of radiation. This is supported also by the fact that depth of noncontact lesions still increases significantly by the augmentation of catheter flow from 30 to 50 ml/min. However, maximum noncontact lesion depth of 4.2 mm may suffice for transmural coagulation of atrial walls, and may help produce lesions in regions where stable catheter contact with the endocardial surface is difficult to achieve. The total amount of the given saline depends on procedure duration, flow rate, number of lesions, and duration per lesion. Power titration and fluid management for optimal safety and efficacy of irrigated RF ablation was the topic of a series of publications [4, 12–14]. In general, during RF mapping, catheter irrigation flow is 2 ml/min whereas during ablation it is increased to 30 ml/min, plus 5 s before and after each lesion. For an assumed ablation procedure time of 90 min and a total of 30 lesions of 90 s, volume of irrigation fluid totalizes 1,580 ml. As compared to that, irrigation flow is 15 ml/min during laser mapping and 35 ml/min during ablation. Applying the same amount of lesions of 30 s each during the same procedure time of 90 min, irrigation fluid totalizes a volume of about 1,575 ml. When considering that laser induces larger lesions and lesion formation is faster, fewer lesions are required and procedure times are shorter. Thus, total volume of irrigation flow during laser ablation is rather less as compared to that of the RF method.
Conclusions With irrigation flow of 30 ml/min, cw1,064 nm laser catheter application at 15 W/30 s, in a contact and noncontact mode of radiation, on bovine myocardium, can achieve lesions at depth of up to 10.2 and 4.0 mm, respectively. Substantially, larger laser lesions can be expected after longer radiation times. Pressure with the catheter on the radiation field is not needed for laser lesion formation. This special claim of the laser method contributes substantially to the safety and efficacy of the method. Optimal irrigation flow for laser catheter ablation by using the open-irrigated ELMA catheter is suggested to be about 35 ml/min.
Study limitations Study limitations are the in vitro test on bovine myocardium at room temperature. These are discrete conditions as compared to those in the in vivo tests. Lesions achieved at body temperature are substantially larger. Volumes of coagulated myocardium achieved at 15 W/30 s with an irrigation flow of 30 ml/ min in this study were 295±43 mm3. As compared to that,
in vivo lesions achieved in a dog thigh muscle model with the same energy setting and irrigation flow, and by using the same laser catheter were 1,170±114 mm3, +75 % [9], and those achieved in dog hearts were >1,400 mm3, always transmural, and were increasing by over 79 % [10]. Acknowledgments This study was supported in part by the LasCor GmbH, Laser Medical Devices Taufkirchen, Taufkirchen, Germany.
References 1. Boulnois J-L (1986) Photophysical processes in recent medical laser developments: a review. Laser Med Sci 1:47–66 2. Weber H, Heinze A, Enders S, Ruprecht L, Unseold E (1998) Laser catheter coagulation of normal and scarred ventricular myocardium. Laser Surg Med 22:109–119 3. Bruneval P, Mesnildrey P, Camilleri P (1987) Nd:YAG laser induced injury in dog myocardium: optical and ultrastructural study of early lesions. Eur Heart J 8:785–792 4. Stevenson WG, Cooper J, Sapp J (2004) Optimizing RF output for cooled RF ablation. J Cardiovasc Electrophysiol 15:24–27 5. Weber H, Heinze A, Enders S, Hauptmann G, Ruprecht L, Unsoeld E (1997) In vivo temperature measurement during transcatheter endomyocardial Nd:YAG laser irradiation. Laser Med Sci 12:352– 356 6. Haines D (2004) Biophysics of ablation: application to technology. J Cardiovasc Electrophysiol 15:S2–S11 7. Vergara P, Maisano F, Maccabelli G, Della Bella P (2013) Radiofrequency and cryoenergy endo-epicardial catheter and surgical approach for a case of incessant ventricular tachycardia ablation. Europace 15:540 8. Weber H, Heinze A, Enders S, Ruprecht L, Unsoeld E (1997) Laser vs. radiofrequency catheter ablation of ventricular myocardium in dogs: a comparative test. Cardiology 88:346–352 9. Weber H, Sagerer-Gerhardt M (2013) Open-irrigated laser catheter ablation produces flow dependent sizes of lesions. PACE 36:1132– 1137 10. Weber H, Sagerer-Gerhardt M (2013) Open-irrigated laser catheter ablation: relationship between the level of energy, myocardial thickness, and collateral damages in a dog model. Europace. doi:10.1093/ europace/eut150 11. Isner JM, Donaldson RF, Deckelbaum LI, Clarke RH, Laliberte SM, Icci AA, Salem DN, Konstam MA (1985) The excimer laser: gross, light microscopic and ultrastructural analysis of potential advantages for use in laser therapy of cardiovascular disease. Am J Cardiol 6: 1102–1109 12. Nakagawa H, Wittkampf FH, Yamanashi WS, Pitha JV, Imia S, Campbell B, Arruda M, Lazzara R, Jackman WM (1998) Inverse relationship between electrode size and lesion size during RF ablation with active electrode cooling. Circulation 98:458–465 13. Matsudaira K, Nakagawa H, Wittkampf FH, Yamanashi W, Imia S, Pitha JV, Lazzara R, Jackman WM (2003) High incidence of thrombus formation without impedance rise during RF ablation using electrode temperature control. PACE 26:1227–1237 14. Scavee C, Jais P, Hsu LF, Sanders P, Hocini M, Weerasooriya R, Macle L, Raybaud F, Clementy J, Haissaguerre M (2004) Prospective randomized comparison of irrigated-tip and largetip catheter ablation of cavotricuspid istmus-dependent atrial flutter. Eur Heart J 25:963–969
