Study procedures: Arterial baroreflex gain measurements will be performed either at the patient's bedside or in the electrophysiology laboratory as described below. BP will be monitored using a non-invasive arterial tonometer (Colin, San Antonio, TX) if performed at bedside, or a preexisting arterial line if performed in the laboratory. Logistical considerations, such as timing, would be a reason for performing the assessment at the bedside. The majority of the studies will be performed in the laboratory. This procedure will take approximately 20 minutes.