Percutaneous delivery of low-level laser energy reverses histamine-induced spasm in atherosclerotic Yucatan microswine.

Background: Previous in vitro experiments performed in our laboratory have shown that low-level laser energy may produce prompt reduction in isometric tension of vascular smooth muscle. The present study was designed to extend these previous in vitro findings to an in vivo model and thereby investigate the hypothesis that laser light delivered percutaneously in vivo could successfully reverse arterial spasm.

Methods And Results: Spasm defined as greater than 50% reversible reduction in luminal diameter persisting for greater than or equal to 5 minutes was successfully provoked by injection of histamine (100-400 micrograms/kg) in 13 arteries among 10 atherosclerotic Yucatan microswine; the magnitude of histamine-induced vasoconstriction was then documented angiographically by repeated injections of contrast media for as long as 30 minutes (controls). After return of angiographic luminal diameter to baseline, spasm was reproduced with a second injection of histamine into the same artery. Representative wavelengths generated by ultraviolet (UV), visible, and infrared lasers were then delivered percutaneously via conventional fiberoptics to the site of spasm, and angiographic assessment was repeated for as long as 30 minutes (treatment trial). In three arteries treated with UV (351 nm) light from an excimer laser, angiographic luminal diameter narrowing decreased from 100% to 23.9%, 50.0% to 9.3%, and 76.0% to 42.3%, respectively. The magnitude of laser-induced increase in luminal diameter was 50.2 +/- 22.7%, which was significantly greater than the magnitude of relaxation observed spontaneously during the control trials (10.9 +/- 9.8%, p = 0.02). Visible light from a helium-neon (632 nm) laser accomplished complete reversal of histamine-induced spasm in two of four arteries; in the remaining two arteries, luminal diameter narrowing percentages were reduced from 57.0% to 20.0% and from 76.5% to 30.8%, respectively. The magnitude of helium-neon laser-induced relaxation (55.8 +/- 17.9%) was again significantly greater than that observed during the control trials (0.9 +/- 1.9%, p = 0.01). Finally, infrared irradiation from a diode-pumped neodymium:yttrium aluminum garnet (1,064 nm) laser decreased histamine-induced luminal diameter narrowing in three arteries from 100% to 21.4%, 56.0% to 8.7%, and 68.3% to 35.3%, respectively. The magnitude of infrared laser-induced improvement in luminal diameter narrowing was 53.0 +/- 23.3%, which was significantly greater than that observed during the control trials (12.9 +/- 10.7%, p = 0.01). In three additional arteries, fiberoptic sham trials (without laser irradiation) failed to produce relaxation of histamine-induced spasm.

Conclusions: These findings document for the first time that light-induced relaxation of vascular smooth muscle, previously documented in vitro, may be reproduced in vivo.