Study Objective: Rapid access to the vascular system for fluid resuscitation and medication administration via an intraosseous (IO) line has become a common practice in emergency medicine. However, if vascular dynamics can be determined from the intramedullary space, more invasive types of monitoring may be able to be avoided. The objective of this proof of concept pilot study is to describe IO pressure measures and their relationship to blood pressure obtained via external blood pressure cuff in intensive care unit patients. Methods: This is a prospective, convenience sample, proof of concept pilot study conducted in the medical and intensive care units at an urban, Level I trauma center. Patients were identified in the emergency department and enrolled under a waiver of informed consent granted by the HealthPartners Institute for Education and Research Institutional Review Board. Inclusion criteria included age 18 years old, presence of an IO placed by EMS or in the emergency department as standard of care, and planned admission to the medical or surgical intensive care unit. Patients were excluded if they had anticipated surgery within 12 hours of IO placement, current infection at the placement site, or prisoner of the state. A pressure transducer was attached to the IO catheter as soon as the line was no longer required for clinical care. External cuff pressure readings were recorded every 15 minutes, and IO pressure data were recorded continuously for up to 12 hours. IO systolic, diastolic, and mean pressure (IO SBP, IO DBP, IO Mean) readings were summarized for the minute before and minute following an external cuff pressure reading. The ratio of IO pressure to external cuff pressure (IO Systolic Blood Pressure / Cuff SBP; IO DBP / Cuff DBP; IO Mean / Cuff Mean) were calculated. Results: Ten patients were enrolled between January 2014 and April 2015. Average patient age was 60 (range ¼ 45-81), and 80% were male. Primary diagnoses were as follows: acute respiratory failure (4), meningitis (1), ingestion (1), hemorrhagic shock (1), congestive heart failure (1), cardiac arrest (1), and altered mental status (1). The average IO SBP, IO DBP, and IO mean were 39.47 12.73 mm Hg, 31.51 7.59 mm Hg, and 34.96 8.83 mm Hg respectively. The ratio of IO SBP to cuff SBP, IO DBP to cuff DBP, and IO mean to cuff mean are 34.5 13.4%, 40.5 22.3%, and 40.1 17.1% respectively. There were no adverse events reported during the monitoring period. Conclusions: In this convenience sample of severely ill and injured patients, IO pressure was reliably obtained and appears to be 35-40% of blood pressure readings obtained via external blood pressure cuff. This method of pressure monitoring may be an appropriate alternative invasive monitoring option in the future.