Single-pulse 30-J holmium laser for myocardial revascularization - a study on ablation dynamics in comparison to CO ₂ laser-TMR

Endocardial laser revascularization (ELR) is a new technique to treat coronary heart disease in a percutaneous, minimally invasive approach. A holmium laser (λ = 2.12 μm) was developed to emit pulse energies of up to 30 J in order to ablate the desired channels in a single laser pulse. The energy was transmitted by multimode flexible optical waveguides as required for ELR. Ablation dynamics were investigated in two model systems, water serving as blood model and polyacrylamide (PAA) as a transparent tissue phantom. Measurements were undertaken using pulse energies of 12 J at pulse durations of 2.2 and 8 ms with a beam diameter of 1 mm. For comparison with the clinically established method of transmyocardial revascularization (TMR), ablations were also investigated with a standard 800 W TMR CO ₂ laser. The dynamics were recorded with a drum camera and stroboscope illumination providing a high framing rate of a single ablation process. Tissue ablation was quantified with the holmium laser in vitro on porcine heart tissue using pulse energies of up to 20 J. Tissue morphology was evaluated using polarization light microscopy to determine thermal and mechanical collateral damage zones. Oscillating vapor bubble channels were found in water and PAA with all laser systems and parameters used. Quasi-static vapor bubbles are observed in water in the millisecond time range using the holmium laser. CO ₂ laser radiation performed deeper channels in PAA than holmium laser pulses using the same radiant exposure. Channel depths of up to 10 mm were achieved with the holmium laser in myocardial tissue with pulse energies of 17 J. Thermal damage zones of about 150 μm for the CO ₂ and 500 μm for the holmium laser were found. The orientation of myocardial fibrils significantly influences the shape of the ablated cavities and the thermo-mechanical collateral damage zones. In conclusion, the results are very encouraging and demonstrate the potential of a catheter-based minimal invasive procedure for heart reperfusion using single high energy laser pulses.