IN-VIVO MEASUREMENT OF REGIONAL LARGE ARTERY COMPLIANCE BY INTRAVASCULAR ULTRASOUND UNDER PENTOBARBITAL-ANESTHESIA

Background: The presence of smooth muscle fibers on the wall of large arteries would suggest that arterial compliance might change in respon se to vasoactive substances. The purpose of this study is to determine the basal level of vasomotor tone in these arteries in a commonly use d animal preparation and to learn whether the compliance of large cond uctance arteries can be altered in vivo by vasoactive agents. Methods: Proximal iliac arterial compliance was measured in 7 pentobarital-ane sthetized pigs, before and during local infusions of adenosine and nor epinephrine. Luminal area was measured every forty milliseconds by mea ns of a 30 MHz intravascular ultrasound catheter and an automatic edge detection program. Simultaneous high-fidelity pressure measurements w ere obtained by means of a catheter-tipped pressure microtransducer po sitioned at the origin of the iliac artery. Linear regression analysis of the area/pressure relationship in two consecutive cardiac cycles ( systolic phase only) was performed before and during adenosine and nor epinephrine infusions. The slope of the area/pressure regression line was defined as an index of arterial compliance. Measurements after thr ee minutes of infusions of adenosine (5-5000 mu g/minute) and norepine phrine (0.001-10 mu g/minute) were compared with the control measureme nts. Results: Even at the highest infusion rate, adenosine did not sig nificantly increase arterial compliance as compared with baseline (25 +/- 7 vs 19 +/- 4 mm(2)/mmHg x 10(-3), respectively, P = ns). In contr ast, norepinephrine decreased arterial compliance as compared with the second baseline control (13 +/- 3 vs 20 +/- 3 mm(2)/mmHg x 10(-3), re spectively, P<0.01). Conclusions: In this animal model with pentobarbi tal anesthesia, arterial compliance may be modified more by the acute infusion of norepinephrine than by adenosine in large conductance arte ries such as the proximal iliac. Thus, in this preparation, smooth mus cle tone tends to be minimal and arterial compliance near maximal tie, mostly a passive phenomenon). However, in response to norepinephrine, arterial compliance can decrease significantly as smooth muscle tone increases. Intravascular ultrasound allows continuous and accurate mon itoring of these changes of arterial dimensions, suggesting that this technique may be useful in the evaluation of pharmacologically induced changes in the compliance of large arteries by vasoactive agents.