Insulin Sensitivity During Hyperbaric Oxygen Compared to Hyperbaric Air

The 40 participants will be randomly allocated to two groups, matched for BMI. All participants will attend the Hyperbaric Medicine Unit on 2 occasions and metabolic testing will be undertaken at the same time of the day. Day 1 will be for baseline assessment sitting outside the hyperbaric chamber. Day 2 will be two days after Day 1 in the same week during which participants will undergo a 2-hour session in the hyperbaric chamber with a "10:90:30" compression profile (compression of the chamber in air to 2 atmospheres absolute, then donning a "hood" supplying high flow gas for 90-minutes followed by a linear decompression back to 1 atmosphere absolute over 30-minutes). Half of the participants will breathe oxygen during this compression profile while the other half will breathe medical air.

An overnight fast (10-hours) will be required prior to each day with modification of their diabetic medication. This regimen will be monitored in consultation with a diabetes specialist (IC):

• If prescribed metformin, withhold the evening dose before each study day but give other usual evening oral drugs
• If taking evening insulin, reduce dose to 2/3 of usual dose the night before each study day
• Hold diabetes drugs (oral or injectable) on morning of study until conclusion of study and then administer with food. Take usual morning oral medication at this time and modified insulin dose (if prescribed insulin)
• Check BSL on arrival and departure each study day

Day 1. Baseline assessment. The participant will attend at 0900 for a hyperinsulinaemic euglycaemic clamp, which will be performed with the same protocol as used in our three previous clamp studies. Two intravenous cannulae are inserted into veins in contralateral forearms using local anaesthetic (lignocaine 1%). Baseline blood is taken for fasting glucose and insulin and a primed insulin infusion is started (80mU/m2/min) for 3½ hours. Blood samples (<2mls) are obtained at 5-10 minute intervals so that blood glucose can be maintained at 5.5 mmol/L with a variable infusion of 25% Dextrose and a trained individual will be present for the duration of the clamping procedure. Insulin sensitivity will be assessed by the steady state glucose infusion rate calculated over a stable 30-minute period of the clamp. Assessments will be made at two points during the 3½ hour clamp; at 2½-3 hours and 3-3½ hours. Immediately post-clamp, volunteers are given orange juice and high carbohydrate lunch, and the glucose infusion is maintained on halving scale for 5 minutes each for at least 20 minutes. Blood sugar levels are monitored every 10-15 minutes for 60 minutes. Total blood taken during the clamping procedure will be less than 100mls including baseline samples. The researchers have performed several hundred clamps.

Day 2. The participant will attend at 0900 for a second 3½-hour hyperinsulinaemic euglycaemic clamp, similar to Day1. The procedure for the two groups (HBO and hyperbaric air) will be identical, the only difference will be the breathing gas used during the 10:90:30 hyperbaric exposures. The breathing gas delivered to the participant (oxygen or air) will be supplied from masked "research" gas outlets, so the participant will be blinded as to which group they are in. The participant will enter the chamber after a 1-hour clamp stabilising period and assessment of the steady state glucose infusion rate will take place at similar times to Day1. This means that during the 2-hour hyperbaric exposure, the assessment periods will correspond to the 30-minute decompression at the end of the hyperbaric session and the first 30-minutes after exit from the hyperbaric chamber. Post clamp will be managed as during Day1.

Blood samples (20 ml) for inflammatory markers will be taken before and after procedures on both days. With blood taken during 2 clamps (less than 100mls each) and four 20 ml samples, total blood taken will be approximately 280 ml.