In Vivo Measurement of Regional Large Artery Compliance by Intravascular Ultrasound Under Pentobarbital Anesthesia

Abstract

Background: The presence of smooth muscle fibers on the wall of large arteries would suggest that arterial compliance might change in response to vasoactive substances. The purpose of this study is to determine the basal level of vasomotor tone in these arteries in a commonly used animal preparation and to learn whether the compliance of large conductance arteries can be altered in vivo by vasoactive agents. Methods: Proximal iliac arterial compliance was measured in 7 pentobarbital-anes thetized pigs, before and during local infusions of adenosine and norepinephrine. Luminal area was measured every forty milliseconds by means of a 30 MHz intravas cular ultrasound catheter and an automatic edge detection program. Simultaneous high- fidelity pressure measurements were obtained by means of a catheter-tipped pressure microtransducer positioned 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 norepinephrine infusions. The slope of the area/pressure regression line was defined as an index of arterial compliance. Measurements after three minutes of infusions of adenosine (5-5000 μg/minute) and norepinephrine (0.001-10 μg/minute) were compared with the control measurements. Results: Even at the highest infusion rate, adenosine did not significantly increase arterial compliance as compared with baseline (25 ± 7 vs 19 ±4 mm2/mmHg x 10-3, respectively, P = ns). In contrast, norepinephrine decreased arterial compliance as compared with the second baseline control (13 ±3 vs 20 ±3 mm2/mmHg x 10-3, respectively, P < 0.01). Conclusions: In this animal model with pentobarbital anesthesia, arterial compliance may be modified more by the acute infusion of norepinephrine than by adenosine in large conductance arteries such as the proximal iliac. Thus, in this preparation, smooth muscle tone tends to be minimal and arterial compliance near maximal (ie, 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 monitoring 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.