Measuring the effects of hypercapnia on diver performance

The study will be conducted at the Exercise Metabolism Laboratory at the University of Auckland. Participants (n = 20) randomized to no exposure (normocapnia) will receive a letter revealing their allocation. The letter will also contain a description of the symptoms of hypercapnia presented in a manner that reflects treatment of the participant in commonly available diver education material. Participants (n = 20) randomized to hypercapnia exposure will participate in an open-label hypercapnia exposure session. Participants will be directly observed by the research staff at all times when present for measurements to ensure adherence to study protocol. This will entail rebreathing of CO2 on a rebreather with no CO2 absorbent in the dedicated canister while performing a card recognition task. A commercially-available rebreather with a 100% O2 and air diluent cylinder will be used. The oxygen controller will be set to maintain a partial pressure of inspired oxygen at 0.7 atmospheres (71 kilopascals). A disposable anaesthetic circuit antimicrobial filter will be incorporated in the rebreather mouthpiece to mask changes in the circuit breathing resistance associated with the absence of CO2 absorbent and for sanitation purposes. Participants will be seated and will be instrumented with a 3-lead electrocardiogram, finger pulse oximeter and non-invasive finger cuff. A gas sampling line for continuous monitoring of inspired PO2 and ETCO2 will be attached to the mouthpiece filter port, and a pneumotachometer will be incorporated in the exhale limb for tidal volume, respiration rate, and minute volume measurements. A portable EEG system will be applied. A functional near infrared spectroscopy (fNIRS) system will be combined with the EEG electrodes using an EEG-compatible fNIRS cap. The session will begin with the participant breathing air through the rebreather mouthpiece in an open-circuit configuration while performing a two-minute baseline card-recognition task. The rebreather circuit will then be switched to a closed-circuit configuration. Subsequently, the card recognition test will commence. The hypercapnia exposure termination criterion includes an ETCO2 of 8.5 kPa (65 mm Hg) or participant self-termination due to symptoms. Errors made on the card recognition test will be recorded, but committal of errors will not be a termination criterion because self-termination due to symptoms, or reaching an ETCO2 of 8.5 kPa provide very safe end points for the exposure. Five minutes after the exposure, participants will be asked to recall the total number of errors they made and to rate the severity of potential hypercapnia symptoms on a VAS. The "test event" will take place approximately five weeks after the initial hypercapnic exposure. Participants in both original groups (no exposure (normocapnia) versus hypercapnia exposure) will participate in a second single-blind hypercapnia exposure. During the hypercapnic test event, participants will breathe from a rebreather with no functioning CO2 scrubber. Monitoring will be as described for the hypercapnic training event described above. Additionally, a virtual reality headset with embedded eye tracking technology will be employed. Participants will view a video of an underwater swim and will be asked to perform a virtual fish survey, similar to a scientific diving objective, to identify a specific fish species and to count these fish in a realistic and immersive underwater environment. This is intentionally a different cognitive task to that undertaken by those experiencing the initial hypercapnia exposure in the first allocation to avoid confounding due to practice effect. Participants will be asked to perform the fish counting task described above while exercising on a cycle ergometer, at approximately 110 watts of work; roughly equivalent to a diver finning at 0.5 to 1 knot. We have recently validated the use of the virtual reality oculography technique during ergometer cycling, as head mounted eye-tracking during exercise is novel. Participants will be instructed to perform a self-rescue task if hypercapnic symptoms are perceived. Self-rescue will involve operation of the mouth-piece bailout valve; a task which involves both hands and a certain degree of manual dexterity. Participants will be videoed, and accuracy of the distractor task and time to self-rescue initiation will be collected. We will also correlate these variables with end tidal CO2 levels. The primary outcome will be based on comparing these outcomes in participants allocated to no exposure (normocapnia) versus hypercapnia in the first allocation. For safety, if a participant allocated to a hypercapnia event ceases recording fish counts or reaches an end tidal CO2 of 8.5 kPa without bailing out, the participant will receive a visual stimulus to bail-out, and if not followed the event will be terminated. Five minutes after the exposure, participants will be asked to rate the severity of potential hypercapnia symptoms on a VAS.