Computational Simulations for Infrared Laser Sealing and Cutting of Blood Vessels

Abstract

Blood vessel burst pressures were simulated and predicted for sealing and cutting of vessels in a two-step process, using low (<25 W), medium (~100 W), and high (200 W) power lasers at a wavelength of 1470 nm. Monte Carlo optical transport, heat transfer, Arrhenius integral tissue damage simulations, and vessel pressure equations were utilized. The purpose of these studies was to first validate the numerical model by comparison with experimental results (for low and medium power) and then to use the model to simulate parameters that could not be experimentally tested (for high power). The goal was to reduce the large range of parameters (power, irradiation time, and linear beam dimensions) to be tested in future experiments, for achieving short vessel sealing/cutting times, minimal bifurcated seal zones (BSZ), and high vessel burst pressures. Blood vessels were compressed to 400 μm thickness. A wide range of linear beam profiles (1-5 mm widths and 8-9.5 mm lengths), incident powers (20-200 W) and clinically relevant irradiation times (0.5-5.0 s) were simulated and peak seal and cut temperatures as well as thermal seal zones, ablation zones, and BSZ computed. A simplistic mathematical expression was used to estimate vessel burst pressures based on seal width. Optimal low-power parameters were: 24W/5s/8×2mm (sealing) and 24W/5s/8×1mm (cutting), yielding a BSZ of 0.4 mm, corresponding to experimental burst pressures of ~450 mmHg. Optimal medium-power parameters were: 90W/1s/9.5×3mm (sealing) and 90W/1s/9.5×1mm (cutting), yielding a BSZ of 0.9 mm for burst pressures of ~1300 mmHg. Simulated only optimal high-power parameters were: 200W/0.5s/9×3 mm (sealing) and 200W/0.5s/9×1mm (cutting), yielding a BSZ of 0.9 mm and extrapolated to predict a seal strength of ~1300 mmHg. All lasers produced seal zones between 0.4-1.5 mm, corresponding to high vessel burst pressures of 300-1300 mmHg (well above normal systolic blood pressure of 120 mmHg). Higher laser powers enable shorter sealing/cutting times and higher vessel strengths.

Keywords: ablation; blood vessel sealing; coagulation; laser.

Fig. 1.

(A) Representation of a compressed normal blood vessel; (B) Infrared laser vessel sealing/cutting method utilizing a laparoscopic device with jaws for clamping down on vascular tissues; (C) A compressed blood vessel which experienced a sequential optical process of first sealing (coagulation) and then cutting (vaporization) for bisection, with seal denoted by lighter zones.

Fig. 2.

Representation of a sealed and cut vessel, showing how (A) thermal seal zone, (B) bifurcated seal zone, and (C) ablation zone were measured.

Fig. 3.

Two and three dimensional images of optimal low power beam shapes (A) 8 × 2 mm for sealing; (B) 8 × 1 mm for cutting, yielding a BSZ of 0.4 mm and corresponding to experimental vessel burst pressures of about 450 mmHg.

Fig. 4.

A center slice of the Monte Carlo plots of the optical absorption of three million photons in the vessel for the laser parameters of 9 × 5 mm. (A) 2D plot; (B) 1D plot. This distribution of absorbed photons correlates with temperature, where temperature on the backside of the vessel is the lowest.

Fig. 5.

(A) Simulated BSZ (Blue) and gross measured experimental BSZ (Red) corresponding to optimum parameters in Table IV. (B) Prediction of burst pressures by using only simulated bifurcated seal zones (BSZ), where BSZ = (thermal seal zone − ablation zone) / 2 and Equation 2 up to one-quarter of the vessel width. These BSZ values were compared to experimental burst pressure measurements. These measurements show a trend where larger BSZ resulted in larger burst pressures. There is also a cap to this increase of burst pressures, which levels out at a simulated BSZ of about 0.9 mm.

Fig. 6.

Photographs of experimental seals. Ruler lines are in millimeter scale. (A) 24 W (low power), 5 s seal / 5 s cut times, 8 × 2 mm seal beam, 8 × 1 mm cut beam dimensions, with BSZ of 0.47 mm; (B) 90 W (medium power, 1 s seal / 1 s cut times, 9.5 × 3 seal beam, 9.5 × 1 mm cut beam dimensions, with BSZ of 0.93 mm; (C) 95 W (medium power, 2 s seal / 2 s cut times, 9 × 5 mm seal beam, 6.5 × 1.5 cut beam dimensions, with BSZ of 1.6 mm.