Hypoxia Pathways for Early Recovery After Spinal Cord Injury

The specific focus of this pilot study is to obtain preliminary data to inform a larger, more definitive clinical trial. The long-term objective of this line of work is to test the hypothesis that an acute intermittent hypoxia protocol (3, 5 min episodes, 9-13% FiO2; augmented AIH (aAIH)) favoring adenosine mechanisms, enhances respiratory motor plasticity more than an AIH protocol targeting serotonin mechanisms (1 min 9% FiO2 + CO2) in people with sub-acute SCI. Since an individual's genetics can influence the response to rehabilitation, investigators are also collecting preliminary data regarding how certain genes are related to breathing changes after these treatments. Data acquired through this pilot study will be used to estimate the magnitude and direction of effects. This line of research will inform AIH protocol optimization to enhance the clinical effects of AIH treatments designed to improve rehabilitation outcomes. The following Aims will be addressed: Aim 1: To preliminarily test the hypothesis that augmented acute intermittent hypoxia (aAIH) is a more potent stimulus to induce respiratory motor plasticity than AIHH (low oxygen with hypercapnia) in people with sub-acute SCI. The investigators predict that 6 sessions of aAIH (3, 5-minute episodes of 9-13% O2; 3-minute intervals) will elicit greater respiratory motor plasticity than AIHH (15, 1-minute episodes of 9-13% O2 with 4-5% CO2; 1.5-minute intervals) or Sham (5, 3-minute episodes of 21% O2) in individuals with SCI, 2 weeks to 6 months post injury. The primary outcomes include maximum pressure generation and cough function 1-day and 1-week post-intervention. Secondary, exploratory outcomes will be biceps (elbow flexion) force output. Aim 2: To obtain preliminary data regarding the association between dysfunctional genetic variants, linked with molecules necessary for AIH-induced respiratory motor plasticity, and the magnitude of response (change in primary outcomes) in response to aAIH or AIHH in individuals with sub-acute SCI. The investigators predict that individuals with dysfunctional single nucleotide polymorphisms (SNPs) associated with pro-plasticity genes have blunted responses to aAIH and/or AIHH treatments. * Augmented Acute intermittent hypoxia (aAIH): This intervention will involve breathing low-oxygen air through a facemask that covers the nose and mouth. Participants will be asked to breathe the low-oxygen air for 3 periods (about 5 minutes each period). A mask will be secured and adjusted to the face and the participant will be able to breathe naturally and normally. Between the \~ 5 minute periods of lower oxygen, the participant will breathe higher oxygen levels that are 'normal' or consistent with room air for about 3 minutes. Heart rate, blood oxygen saturation, and blood pressure will be monitored. The gas mixture will be adjusted if a participant's blood oxygen saturation drops below 80%. * Acute intermittent hypercapnic hypoxia, lower oxygen with CO2 (AIHH): This intervention delivery using the facemask will be identical to the aAIH described above, except that lower oxygen will be combined with higher carbon dioxide air. For this intervention, the participant will be asked to breathe the low oxygen air with higher carbon dioxide air for 15 periods (\~1 minute each). Between periods of breathing this gas mixture through the facemask, the participant will breathe higher oxygen levels that are normal for \~1.5 minutes. Heart rate, blood oxygen saturation, and blood pressure will be monitored. The gas mixture will be adjusted if a participant's blood oxygen saturation drops below 80%. * Sham/Placebo: This intervention will be identical to the two procedures described above, except that normal-oxygen, normal-carbon dioxide air (same as room air) will be delivered through the facemask for 5 periods (\~3 minutes each). Heart rate, blood oxygen saturation, and blood pressure will be monitored.