An automated pressure-swing absorption system to administer low oxygen therapy for persons with spinal cord injury

Abstract

Mild episodes of breathing low oxygen (O2) (i.e., acute intermittent hypoxia, AIH) elicits rapid mechanisms of neural plasticity that enhance respiratory and non-respiratory motor function after spinal cord injury (SCI). Despite promising outcomes in humans and rodents with SCI, the translational potential of AIH as a clinical therapy remains dependent on a safer and more reliable air delivery system. The purpose of this study is to investigate the performance of a novel AIH delivery system to overcome inconsistencies in human AIH protocols using a hand-operated (manual) delivery system. Specifically, we characterized system performance of AIH delivery in terms of flow rate, O2 concentration, dose timing, and air temperature. Our data show that a novel 'automated' delivery system: i) produces reliable AIH with a goodness-of-fit at 98.1% of 'ideal'; ii) eliminates dose timing errors via programmable solenoid switches; iii) reduces fluctuations in O2 to less than 0.01%; and iv) delivers 62.7% more air flow than the 'manual' delivery method. Automated physiological recordings, threshold detection, and visual feedback of the participant's blood O2 saturation, heart rate, and blood pressure ensures real-time user safety. In summary, the 'automated' system outperformed the 'manual' delivery method in terms of accuracy, reliability, and safety. The 'automated' system offers several design features that move the technology closer to a medically approved treatment for clinical and home use.

Keywords: Breathing; Hypoxia; Motor function; Oxygen; Pressure swing adsorption; Rehabilitation; Spinal cord injury.

Conflict of interest statement

Declaration of Competing Interest The authors declare that they do not have competing interests.

Copyright © 2020 Elsevier Inc. All rights reserved.

Figures

Figure 1.

A block diagram depicts the ‘manual’ acute intermittent hypoxia (AIH) delivery system for humans (A). The system includes a single pressure-swing absorber (yellow box) that generates and distributes low oxygen air through a human breathing circuit (solid red line) manually connected/disconnected to the end-user’s face mask during AIH treatment. The dashed line denotes room air not supplied to humans during low O2 breathing. White open triangles point along the direction of air flow. An O2 sensor near the face mask records the percentage of O2 within the air mixture. A stand-alone patient monitor displays heart rate (HR), blood O2 saturation (SpO2), and blood pressure (BP) for safety. In B, a block diagram depicts the ‘automated’ AIH delivery system. The system includes 1) double pressure-swing absorbers (yellow boxes) that generate and distribute low O2 air (solid, red line) and 2) blower that distributes room air (solid, green line) through the breathing circuit and 3) a gas mixing chamber that reduces fluctuations ins steady state O2 concentration. A microcontroller board controls two pairs of one-way solenoid valves that route air from either blower or absorbers to face mask. As shown, dashed lines denote air not supplied to human. Patient monitoring unit acquires HR, SpO2, and BP for real-time feedback to a microcontroller that maintains AIH protocols within safe limits.

Figure 2.

Quantifying temporal accuracy during an acute intermittent hypoxia (AIH) delivery protocol of 90s breathing bouts of low O2 with 60s intervals of breathing ambient room air. A) Time-dependent changes in relative O2 (solid, black line) from the ‘manual’ AIH delivery system as compared to the ‘ideal’ AIH protocol (solid, gray line). B) Time-dependent changes in relative O2 from the ‘automated’ AIH delivery system (dashed, black line) as compared to the ‘ideal’ AIH protocol (solid, gray line).

Figure 3.

Cumulative temporal errors from S1 who administered a single sequence of AIH with the ‘manual’ delivery system. The plot with white-filled circles indicate a cumulative positive temporal error that corresponds to overall delay in the trained administrator (S1) who disconnected the tube from the face mask of participant S3. The plot with black-filled circles indicate the cumulative absolute error (secs) over time. There was a significant absolute error in switching times (p

Figure 4.

A) Relation between delivered O2 concentrations and pressure-swing absorption (PSA) flow rate. B) Effects of delivery system on flow rate at low O2 (10.0 ± 2.0%) and room air (20.9 ± 2.0%). Bars correspond to mean ± 1 standard error. Black bars correspond to the flow rate for ‘automated’ system with double PSA and white bars indicate the flow rate for ‘manual’ system with single PSA. Asterisk (*) corresponds to statistical significance at p < 0.01.

Figure 5.

Effects of a mixing chamber on magnitude of fluctuations in steady-state O2 concentration. In A, plots show the air delivery system without the mixing chamber that resulted in ~1% peak-to-peak O2 fluctuations (black trace) as compared to the delivery system with a 6L mixing chamber that resulted in ~0.06% peak-to-peak O2 fluctuation (gray trace). B) Bars represent mean ± 1 standard error in mean absolute deviation of O2 concentration within the breathing circuit. The mixing chamber (black bars) significantly reduced O2 deviations during room air and low O2 as compared to no mixing chamber (white bars). Greater O2 deviations occurred during low O2 as compared to room air. Asterisks indicate significant difference (p<0.05).

Figure 6.

Temporal changes in blood oxygen saturation (SpO2) of a research participant (S3) during a singles sequence (N=15 episodes) of acute intermittent hypoxia (AIH). Gray trace corresponds to the change in SpO2 levels during repetitive breathing bouts at 10.0% and 20.9% O2 (Black trace). Broken black horizontal trace indicates an 80% SpO2 safety threshold. The ‘automated’ system delivers room air when SpO2 dips below 80% during low O2 and extends room air intervals when SpO2 values remain below 80%.