A comparison of multiport and uniport epidural catheters in laboring patients.

The relative incidence of technical difficulties associated with multiport (three lateral ports) and uniport (single distal port) epidural catheters remains controversial. As part of a continuing institutional evaluation of epidural catheter insertion, 500 parturients were randomized to have either a multiport or a uniport epidural catheter inserted 6 cm into the epidural space. Multiport epidural catheters were associated with inadequate analgesia less often and required manipulation less often than uniport epidural catheters. The incidences of intravenous cannulation, subsequent catheter dislodgement, and catheter replacement were similar for each catheter type. No multiport epidural catheter was associated with multicompartment placement. We conclude that multiport epidural catheters are preferable for use in laboring patients since they reduce the incidence of inadequate epidural analgesia.

Liquid crystal domains and thixotropy of filamentous actin suspensions.

The thixotropic properties of filamentous actin suspensions were examined by a step-function shearing protocol. Samples of purified filamentous actin were sheared at 0.2 sec-1 in a cone and plate rheometer. We noted a sharp stress overshoot upon the initiation of shear, indicative of a gel state, and a nearly instantaneous drop to zero stress upon cessation of shear. Stress-overshoot recovery was almost complete after 5 min of "rest" before samples were again sheared at 0.2 sec-1. Overshoot recovery increased linearly with the square root of rest time, suggesting that gel-state recovery is diffusion limited. Actin suspensions subjected to oscillatory shearing at frequencies from 0.003 to 30 radians/sec confirmed the existence of a 5-min time scale in the gel, similar to that for stress-overshoot recovery. Flow of filamentous actin was visualized by polarized light observations. Actin from 6 mg/ml to 20 mg/ml showed the "polycrystalline" texture of birefringence typical for liquid crystal structure. At shear rates less than 1 sec-1, flow occurred by the relative movement of irregular, roughly ellipsoidal actin domains 40-140 microns long; the appearance was similar to moving ice floes. At shear rates greater than 1 sec-1, domains decreased in size, possibly by frictional interactions among domains. Eventually domains flow in a "river" of actin aligned by the flow. Our observations confirm our previous domain-friction model for actin rheology. The similarities between the unusual flow properties of actin and cytoplasm argue that cytoplasm also may flow as domains.