15‐Days of Therapeutic Mild Intermittent Hypoxia Improves Cardiovascular, Upper Airway and Motor Limb Function in Participants with Motor Incomplete Spinal Cord Injury and Severe Obstructive Sleep Apnea

Abstract

IntroductionRepeated exposure to mild intermittent hypoxia (MIH) with sustained hypercapnia improves cardiovascular control in able‐bodied individuals with obstructive sleep apnea. Thus, we aimed to investigate if this treatment can improve cardiovascular function in individuals with motor incomplete spinal cord injuries (SCI). In addition, treatment with MIH improves cortical excitability, as well as, upper airway muscle and limb motor function. Consequently, we also explored if MIH improves upper airway motor function and walking endurance in individuals with motor incomplete SCI with concurrent sleep apnea.MethodsTwo individuals (age 51.0 ± 8.5) with traumatic motor incomplete SCI (T8, C6) and untreated severe obstructive sleep apnea (apnea hypopnea index 53.7 ± 34.0 events/hour) were treated with MIH for 15 days while receiving in‐home continuous positive airway pressure (CPAP) treatment. The MIH protocol consisted of twelve 2‐minute hypoxic exposures (PETO2 ≍ 50 mmHg) with PETCO2 maintained 2 mmHg above baseline. Therapeutic CPAP and the active critical closing pressure were measured before and after the 15‐day exposure period. In addition, blood pressure was measured over 24‐hours before and after the MIH protocol. Beat‐to‐beat blood pressure was measured during baseline (i.e. normoxia) prior to exposure to MIH on day 1 and day 15. Walking endurance was measured on the first and last day of the protocol prior to MIH exposure. Due to the small sample, Cohen’s D effect sizes are presented.Results24‐hour systolic (SBP), diastolic (DBP) and mean arterial (MAP) blood pressure (unit – mmHg) was reduced following 15 days of MIH (SBP: 131.4 ± 8.5 vs 113.4 ± 9.3, d= 2.0, DBP: 82.3 ± 1.4 vs 75.5 ± 3.9, d= 2.6, MAP: 98.5 ± 1.84 vs 88.0 ± 0.5, d= 9.04). Beat‐to‐beat blood pressure was also reduced following MIH (SBP: 135.3 ± 10.0 vs 116.9 ± 21.5, d= 1.2, DBP: 87.3 ± 0.46 vs 75.6 ± 3.1, d= 6.6, MAP 107.1 ± 0.4 vs 91.3 ± 7.0, d= 4.4). Additionally, an increase in indirect measures of cardiac output at rest (77.6 ± 39.9 vs 84.9 ± 30.2 L/min, d = 0.9) with concurrent reductions in total peripheral resistance (1.3 ± .7 vs 0.6 ± 0.1 mmHg*s/mL, d = 1.8) were found on the day 15. A reduction in therapeutic CPAP (11.0 ± 1.4 vs 9.0 ± 1.4 cmH2O, d= 1.4) and the critical closing pressure (4.5 ± 0.7 vs ‐0.6 ± 1.9 cmH2O, d = 3.9) was also evident following MIH. Lastly, walking endurance (155.8 ± 151.1 vs 240.5 ± 208.0 meters, d = 0.33) and walking velocity (0.4 ± 0.4 vs 0.7 ± 0.6 meters/second, d = 0.33) increased on day 15 compared to day 1.ConclusionFollowing 15 days of therapeutic MIH (i) blood pressure was reduced (ii) upper airway patency was improved and (iii) walking endurance and velocity increased in two individuals with motor incomplete SCI, hypertension and concurrent obstructive sleep apnea.