In order to determine the safety limit of endoscopic Nd-YAG laser irradiation, 28 regions in the mucous membrane of the normal trachea of 13 dogs were subjected to the irradiation, and chronological changes in the trachea were studied from the endoscopic and pathological aspects. The irradiation was applied on half of the circle of 2 to 5 rings in the direction of major axis by way of continuous wave at 40W×3 seconds until the mucous membrane turned white.
The chronological changes of the trachea were classified as ulceration stage (up to the 14th day), granulation stage (up to the 28th day) and scar stage (after the 29th day). At the granulation stage, stenosis was pathologically attributable to the decreased supporting power of the tracheal lumen and hyperplasia of granulation by irradiation at the tracheal ring.
The degree of changes after irradiation was in proportion to the irradiated range of the major axis, if the minor axis was fixed. The cause which brought early death to 2 dogs was the perforation between the tracheal rings. The maximum dose was 1, 000 joule per ring.
There was no exacerbation of stenosis due to reirradiation on the granulation tissue by 20W×3 seconds or 60W×1 second on the 21st day.
From the above, it is surmised that the decreased supporting power of the trachea is concerned with the stenosis of the airway by the laser irradiation. The Nd-YAG laser irradiation should be applied in a small and shallow range to minimize irradiating the whole tracheal ring layer to prevent re-stenosis.