Carbonized blood deposited on fibres during 810, 940 and 1,470 nm endovenous laser ablation: thickness and absorption by optical coherence tomography

Abstract

Endovenous laser ablation (EVLA) is commonly used to treat saphenous varicosities. Very high temperatures at the laser fibre tip have been reported during EVLA. We hypothesized that the laser irradiation deposits a layer of strongly absorbing carbonized blood of very high temperature on the fibre tip. We sought to prove the existence of these layers and study their properties by optical transmission, optical coherence tomography (OCT) and microscopy. We analysed 23 EVLA fibres, 8 used at 810 nm, 7 at 940 nm and 8 at 1,470 nm. We measured the transmission of these fibres in two wavelength bands (450–950 nm; 950–1,650 nm). We used 1,310 nm OCT to assess the thickness of the layers and the attenuation as a function of depth to determine the absorption coefficient. Microscopy was used to view the tip surface. All fibres showed a slightly increasing transmission with wavelength in the 450–950 nm band, and a virtually wavelength-independent transmission in the 950–1,650 nm band. OCT scans showed a thin layer deposited on all 13 fibres investigated, 6 used at 810 nm, 4 at 940 nm and 3 at 1,470 nm, some with inhomogeneities over the tip area. The average absorption coefficient of the 13 layers was 72 ± 16 mm−1. The average layer thickness estimated from the transmission and absorption measurements was 8.0 ± 2.7 µm. From the OCT data, the average maximal thickness was 26 ± 6 µm. Microscopy of three fibre tips, one for each EVLA wavelength, showed rough, cracked and sometimes seriously damaged tip surfaces. There was no clear correlation between the properties of the layers and the EVLA parameters such as wavelength, except for a positive correlation between layer thickness and total delivered energy. In conclusion, we found strong evidence that all EVLA procedures in blood filled veins deposit a heavily absorbing hot layer of carbonized blood on the fibre tip, with concomitant tip damage. This major EVLA mechanism is unlikely to have much wavelength dependence at similar delivered energies per centimetre of vein. Optical–thermal interaction between the vein wall and the transmitted laser light depends on wavelength.

Moving heat source in a confined channel: Heat transfer and boiling in endovenous laser ablation of varicose veins

“…may reach temperatures high enough to cause damage to the fiber [15], [18]. The main conclusions of this work are the following:…”

“…8a). The carbonized layer absorbs 55% of the energy provided at the fiber tip ( [15]). The extent of the channel volume directly heated by laser light emanating from the fiber tip is primarily determined by radiation absorption in the fluid; see Poluektova et al [28].…”

“…Also heat diffusion from the hot fiber tip was considered to be significant; see van den Bos et al [12]. Fiber tips can reach temperatures of over 800°C (see Verdaasdonk et al [13] and Weiss [14], for example), as a result of the absorbing thin layer of carbonized blood which is deposited on the fiber tip during EVLA treatment, Amzayyb et al [15]. Direct contact between the tissue and the fiber tip was found to contribute to vein wall perforations; see Vuylsteke et al [16].…”

“…When a fiber tip emits intense light when moving through blood, the tip becomes carbonized, Amzayyb et al [15] and Van den Bos et al [12]. The level of carbonization changes during the process and the final level of carbonization is a priori unknown.…”

Contribution of Laser-Induced Gas-Vapor-Liquid Dynamics to the Mechanism of Endovenous Laser Ablation

“…Later, lasers with the emission in the region of water absorption peaks (1.32-1.56 μm band) found their application. In connection with EVLA these lasers were named water absorbing [14][15][16]. Radiation of water absorbing lasers is considered to reach readily a venous wall without being absorbed by blood cells -erythrocytes, whereas hemoglobin absorbing lasers affect originally only erythrocytes, initiating thrombus formation.…”