Temperature measurements for dose-finding in steam ablation

We very recently demonstrated the safety and efficacy of steam ablation for varicose veins in sheep and humans in a pilot study.1 The effectiveness of endovenous thermal ablative treatments (using laser, radio frequency of steam) depends primarily on the amount of energy delivered to the venous wall.[2] and [3] Previously, it was estimated that one "puff of steam" in steam ablation equalled approximately 56 J, suggesting that 1 to 2 puffs/cm of vein would be sufficient to occlude the vein.1 However, in the first 20 patients treated, 1 pulse/cm was not optimal, and the pulse dose was subsequently doubled. The objective of our experiment was to better understand the heat profile induced by steam and to investigate the heat induction of 1, 2, and 3 puffs/cm using steam ablation. In a sealed glass beaker filled with demineralized water, analysis of the number of frames of a high-speed camera demonstrated that a steam pulse of the Steam Vein Sclerosis system (CERMA SA, Archamps, France) lasts 0.99 seconds. Also, using a balance with a precision of 0.01 grams, we estimated that the water mass of one steam pulse averaged 0.08 grams. In an experimental setup previously used to study thermal effects of endovenous laser ablation,4 we measured the temperature profiles induced by steam ablation using three thermocouples located in the wall of the tube, 1 cm apart, and intratubular thermocouples fixed at the top, bottom, and sides at 2-mm distance of the center of the catheter, which is moving in the center of the tube. The main parameter controlling the unsteady heat transfer process is the heat diffusivity, which is about 0.14 10–6 m2/s. The process of expansion and subsequent collapse and segregation of induced steam is because the effect of heat transfer through the bounding wall is minimal. The tube had an inner diameter of 4 mm and an outer diameter of 6 mm. Because of the fast spread of injected steam in the vein, conclusions about differences between the number of puffs given is likely to be comparable for other diameters. The circumferential locations were selected to assess the homogeneity of the temperature distribution caused by the steam delivered through a catheter with two opposite holes. Each of the measurements was repeated five times and presented as the mean with the standard deviation. The maximum temperature rise (¿Tmax) at the wall of the plastic tube was modest for one pulse and increased considerably with increasing number of pulses per centimeter (Table). One steam pulse/cm showed inhomogeneous ¿Tmax compared with 2 or 3 pulses/cm, for which the temperature rise was similar at the top, bottom, and sides at 2-mm radial distance from the steam catheter. At 1-mm distance outside the tube, ¿Tmax was 50°C, which induces denaturation of collagen (tden) and ¿Tmax was zero for 1 pulse/cm (Table). For 2 pulses/cm, the ¿tden varied between locations but was >10 seconds at the top and bottom, and for 3 pulses/cm, it was >20 seconds and more homogenous compared with 2 pulses. Compared with endovenous laser ablation (940 nm; 20 W; 2 mm/s), the ¿Tmax was considerably lower, but the tden was longer for steam ablation (data not shown).