Abstract
In regard to the article ‘Endovenous laser treatment: a morphological study in an animal model’ by Vuylsteke et al., 1 several points must be addressed since this is an extremely important issue. First, let me commend the authors on the experimental design; however, some of the conclusions they make need clarification.
The authors state that no study exists that delineates histological changes occurring over time. As described by us in two earlier publications, 2,3 histological changes were documented acutely and up to four months post-injury using both a 940 and 1319 nm wavelength laser. These studies show a similar histological sequence of events. However, two factors that occur must be addressed and discussed further.
First, the authors fail to mention the loss of endothelium in the acute phase. This is always circumferential even distant to the point of laser contact. This phenomenon will be explained later in this letter. Although difficult to visualize from a black and white photograph, this seems to be present in their study although little mention is made of it. This was a consistent finding in over 50 excised veins used as the basis of our early in vivo studies.
Regarding perforations, the following can be said. Perforation only occurs when there is tissue contact with the fibre tip for greater than one second. Based on our studies, no perforations occurred using 15 W for one second pulse duration. Four pulses per centimetre would be equivalent to 60 J/cm. Perforations are unnecessary and due to prolonged contact or excessive heat accrual which may occur with continuous laser energy application.
Based on our extensive studies, we are in agreement with many of the histological findings with a few differences. First, there is a complete loss of endothelium with resultant thrombus formation. The next step is thrombus organization with fibroblast infiltration. Cellular injury occurs because of heat damage (the mechanism explained shortly). With no further inflow, gradual scar formation occurs with the secretion by fibroblasts of the extracellular matrix, collagen.
The authors touch on the controversy of mechanism of injury. This is very important and needs to be clarified. There are three types of energy transmission: (a) convection (steam bubbles); (b) conductive (tissue absorption); and (c) radiant (laser light energy). At some point convective and radiant must become conductive. Min et al. 4 stated that there must be laser contact with resultant conductive damage causing the subsequent findings.
The conduction theory (requiring laser tip contact with the vein wall) can be easily disproved by a mathematical model. This was first reported at the IVC 2009 by me. 5 Using the two-dimensional Green's function for the heat conduction equation and integrating the product of Green function and the initial condition, tissue temperatures at varying intervals can be determined. Based on the model, heat conduction at the point of laser contact (4 mm tip) affects only 15–20% of the circumference and this is for temperatures in the 45°C range. At 50°C, the zone of destruction is less.
Thus, convective energy (steam bubbles) that occurs with the heating of blood and/or tissue must be the main factor in producing eventual cellular damage. This steam bubble phenomenon is present in both the 940 and 1320 nm lasers. Radiant energy will be absorbed quickly by haeme proteins. From our studies, these bubbles are 100°C and simultaneously contact all points on the vein wall. There is immediate cell death of the endothelium and each of these hundreds of foci became the point of heat conduction to the inner wall. This accumulative effect transmits enough heat damage to initiate the resultant cellular sequence of events. At all points of contact, the zone of heat conduction by one focus of energy (steam bubbles) is minute. However, cumulative effects are magnified a thousand fold. This mathematical model shows that energy absorption by laser contact plays a minor role in the overall damage required to initiate the necessary histological sequence.