Cardiorespiratory Responses to Acute Intermittent Hypoxia in Humans With Chronic Spinal Cord Injury

Brief exposure to repeated episodes of low inspired oxygen, or acute intermittent hypoxia (AIH), is a promising therapeutic modality to improve motor function after chronic, incomplete spinal cord injury (SCI). Although therapeutic AIH is under extensive investigation in people with SCI, limited data are available concerning cardiorespiratory responses during and following AIH exposure despite implications for AIH safety and tolerability. Thus, we recorded immediate (during treatment) and enduring (up to 30 min post-treatment) cardiorespiratory responses to AIH in 19 participants with chronic SCI (>1 year post injury; injury levels C1 to T6; American Spinal Injury Association Impairment Scale A to D; mean age = 33.8 ± 14.1 years; 18 males). Participants completed a single AIH (15, 60-s episodes, inspired O2 ≈ 10%; 90-s intervals breathing room air) and Sham (inspired O2 ≈ 21%) treatment, in random order. During hypoxic episodes: 1) arterial oxyhemoglobin saturation decreased to 82.1 ± 2.9% (P < 0.001); 2) minute ventilation increased 3.83 ± 2.29 L/min (P = 0.008); and 3) heart rate increased 4.77 ± 6.82 bpm (P = 0.010). Considerable variability in cardiorespiratory responses were found among subjects; some individuals exhibited large hypoxic ventilatory responses (≥ 0.20 L/min/%, n = 11), whereas others responded minimally (< 0.20 L/min/%, n = 8). Apnoeas occurred frequently during AIH and/or Sham protocols in multiple participants. All participants completed AIH treatment without difficulty. No significant changes in ventilation, heart rate or arterial blood pressure were found 30 min post-AIH (P > 0.05). In conclusion, therapeutic AIH is well-tolerated, elicits variable chemoreflex activation, and does not cause persistent changes in cardiorespiratory control/function 30 min post-treatment in people with chronic SCI.