Data Acquired to Demonstrate a Low-Cost, Microcontroller-Based Gas Delivery System for Respiratory Stimuli in MRI Studies

Two respiratory stimuli were implemented: a hypercapnic stimulus and a hyperoxic stimulus. The hypercapnic stimulus consisted of three 60 s blocks of hypercapnia (5% CO2, 21% O2, 74% N2) interleaved with four 60 s blocks of normocapnia (medical air: 21% O2, 79% N2) to give a total duration of 7 minutes. The hyperoxia stimulus had two repeats of the following block: 120 s normocapnia, 30 s hyperoxia (100% O2) and 90 s with both normoxic and hyperoxic gases flowing simultaneously (equivalent to 60.5% O2, 39.5% N2). This block was repeated twice and followed by a further 120 s normocapnic period to give a total duration of 10 minutes. The initial 30 s 100% oxygen block was implemented to produce a large oxygen gradient between the alveolar gas and the blood and results in a rapid increase in end tidal partial pressure of oxygen (PetO2). The switch to 60.5% oxygen then rapidly reaches a plateau for the remainder of the block. Each of the medical flow meters were set to 15 lpm which resulted in a total flow of 30 lpm during the plateau of the hyperoxia stimulus. Expired gases were measured (O2 and CO2) using a respiratory gas analyser (ML206, AD Instruments, Dunedin, New Zealand).