Effects of HBO on Glucose in Patients With DM

The purpose of this Research Study is to determine whether the Dexcom G6® continuous glucose monitoring (CGM) system accurately estimates blood glucose in diabetic patients undergoing hyperbaric oxygen (HBO2) exposure. The study-specific CGM system is approved by the FDA (U.S. Food and Drug Administration).

Subjects will have the Dexcom CGM inserted underneath the skin of their abdomen. This is done in the office. It is secured to the skin by use of an adhesive patch. The sensor measures glucose levels just underneath the skin. A transmitter is fastened on top of the sensor and sends this information wirelessly to a receiver or compatible smart device. Study participants will also have second sensor inserted in the subcutaneous tissue on the back of the arm as a backup in case of data loss. The subjects will wear the sensors for at least 48 hours and then return to the Hyperbaric Medicine Clinic.

At the Hyperbaric Medicine Clinic, study staff will check the CGM system to make sure it has been working properly. The first 2-hour session will be a control where participants will sit quietly (as they would in the hyperbaric chamber) and will not be allowed to consume carbohydrates unless directed to by the hyperbaric department hypoglycemia protocol. The following day for the HBO2 session, subjects will be fitted for an oxygen hood and given a standard HBO2 treatment. Investigators will obtain concurrent transcutaneous oximetry measurements (TCOM) attached no farther than 3 cm from the commercial CGM on the abdomen to measure tissue oxygenation in the region of the CGM only during HBO2 exposure.

The hyperbaric oxygen therapy chamber can accommodate 12 patients at a time. Study subjects could be in the chamber along with regularly scheduled patients. In the chamber the pressure will increase to approximately 2-1/2 times normal atmospheric pressure (2.4 atmospheres absolute \[ATA\]). Once they reach the treatment pressure, subjects will breathe oxygen by placing the hood over their head and securing it in place. They will breathe oxygen for a 30-minute period, and then take the hood off for 5 minutes for an "air break." They will have a total of three 30-minute oxygen periods and two 5-minute air breaks. A subject will be in the hyperbaric chamber for approximately 2 hours.

Blood sampling will be identical for both the control and HBO2 exposure days. Subjects will have a peripheral intravenous (IV) catheter placed and a total of 12 samples collected to measure venous blood glucose (12 timepoints), glucagon levels (3 timepoints) and HgA1c, (one time). A RN or MD will be inside the chamber to monitor the subjects and to obtain the IV blood samples. Because CGMs have not been established to be reliable under HBO2 conditions in patients with DM, the protocol will collect Point-of-Care (POC) glucose measurements using the hospital's standard glucometer (Nova Biomedical, Waltham, MA) and the AccuChek Inform II POC glucometer (Roche Diagnostics, Indianapolis, IN) at each testing point. This will be done using a lancet to pierce the skin of the fingertip to get a drop of blood.