Copyright © 1993 by the European Society of Cardiology.
© 1993 The Europen Society of Cardiology
Excimer laser-induced vasoreactivity
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*Hadassah Hospital Jerusalem, Isreal
Department of Medicine (Cardiology) and Biomedical Research, St Elizabeth's Hospital, Tufts University School of Medicine Boston, MA U.S.A.
Brandon Cardiologists Brandon, FL, U.S.A.
Received 16 December 1992; revised 14 April 1993; .
Correspondence: Jeffrey M. Isner. MD. St Elizabeth's Hospital, 736 Cambridge St., Boston, MA 02135, U.S.A.
Abstract
Earlier generations of excimer lasers, designed for industrial or non-cardiovascular medical applications, have been previously shown consistently to induce vasorelaxation of vascular smooth muscle in vitro. Such lasers were typically characterized by pulse durations of 
15 nanoseconds (ns). Excimer lasers currently employed for cardiovascular applications were designed with longer pulse durations (up to 220 ns) to facilitate fibreoptic transmission. Because arterial spasm has been observed in patients undergoing percutaneous revascularization with such lasers, we investigated the effect of so-called ‘stretched pulse’ excimer laser irradiation on vasomotor reactivity.
A total of 69 rings of aorta harvested from New Zealand white rabbits were mounted isometrically in Krebs buffer solution and exposed to 308 nm from an excimer laser with a pulse duration of 120 ns. Fifty rings were exposed without pharmacological pre-treatment. The remaining 19 rings were exposed after pharmacological pre-treatment: 11 were pre-contracted with norepinephrine (NE, 10–9–10–5 m), while eight were irradiated in Ca2$-free buffer after pre-relaxation with nitroglycerin (NTG, 7 x 10–5 m).
Without pharmacological pre-treatment, the vasomotor response to the excimer laser was variable: vasoconstriction was observed in 27 rings (16.1±0.8% (mean±SEM) of response to 5-HT), vasorelaxation in 21 rings (43.2±17.0% of response to 5-HT), and a heterogeneous response (vasoconstriction 4.9±1.0%, vasorelaxation 12.9±0.3% ) in two rings. The vector of vasomotor response in non-precontracted rings was not predicted by fluence, frequency or temperature rise.
A consistent vasomotor response was recorded only when pharmacological pre-treatment was employed. Among 11 rings pre-contracted with NE, the excimer laser produced vasorelaxation in 34/34 (100%) exposures; in contrast, among eight rings pre-relaxed with NTG in Ca2$-free buffer, the excimer laser produced vasoconstriction in 40/40 (100%) exposures. For all rings, including pre-contracted, pre-relaxed and those which were not pharmacologically pre-treated, the vector of vasomotor response was endothelium-independent. The magnitude of all vasomotor responses, including vasoconstriction in non-precontracted rings, could be diminished by limiting the duration of exposure.
Thus, in contrast to the earlier generation, short-pulse excimer lasers, long pulse-duration excimer lasers in current clinical trials produce an unpredictable, heterogeneous vasomotor response. This in-vitro finding is consistent with the unpredictable development of vascular spasm in patients undergoing excimer laster angioplasty. Furthermore, these findings support the concept of employing abridged pulse trains to diminish the likelihood of laser-induced vasoconstriction during excimer laser angioplasty. Finally, these observations suggest that the concept of using excimer laser radiation to reverse spasm encountered during attempts to ablate plaque may be problematic.
Key Words: Angioplasty • spasm • laser • pulse duration