A blinded trial to investigate whether 'pressure-familiar' individuals can determine chamber pressure.

Introduction: We conducted a prospective study to determine whether divers or hyperbaric chamber attendants could discern chamber pressure--important to protecting blinding if they were to participate in blinded hyperbaric oxygen (HBO2) trials.
Methods: Experienced divers and chamber attendants were randomized by appointment group to one of four pressure changes. These pressure changes caused volume changes corresponding to starting compression from sea level or altitude, with final pressures of 1.5 atm abs (breathing 100% oxygen) or 1.2 atm abs (breathing air). Following the pressure excursions, participants estimated their depth in feet of sea water (fsw) and breathing gas.
Results: 42 volunteers participated: 10 females, 32 males, age 46 +/- 10 years (mean +/- 1 SD), dive experience 366 +/- 562 dives. Four were chamber attendants. When estimating the depth exposure, 19/42 (45%) selected I do not know. 3/23 (13%) were within 2 fsw of the actual depth (correlation between actual depth and estimate -0.0067) (p = 0.98). Two underestimated their depth. We found no indication of cluster randomization effect (p = 0.16). Participants could not discern breathing air or oxygen.
Conclusions: The results suggest experienced divers cannot discriminate chamber pressures of 1.2 and 1.5 atm abs, and could be included in hyperbaric clinical trials using this dosing without compromising masking integrity.