Inferior Vena Cava (IVC) Diameters Before & After Interscalene Block

In recent practice, point-of-care ultrasound has been used to assess the size and respiratory variation of the inferior vena cava as a marker for volume status and fluid responsiveness. Little investigation has occurred regarding the mechanism of observed respiratory variation. Anesthetic plans for surgery (e.g. shoulder replacement) frequently employ interscalene injections to target and block the brachial plexus, which (on the same side) results in phrenic nerve palsy and paralysis of the diaphragm. The goal of this study is to examine the relationship between diaphragmatic function and variation in IVC diameter. Specifically, the investigators will evaluate and compare the size and variability of the inferior vena cava by point-of-care ultrasound before and after brachial plexus block.