| 1) Original Aims
The management of major illness and/or trauma in space, will likely result in the need for delivery of oxygen. On orbit delivery of oxygen relies on oxygen stores which also provide environmental oxygen. A major concern of oxygen delivery is the elevation of ambient oxygen concentration and increased risk of fire. Our previous work has found that in previously healthy individuals (similar to NASA astronauts) oxygen requirements can be met by oxygen at 3-4 liters per minute. This finding suggests that oxygen on-orbit could be provided by a oxygen concentrator. This device has the advantage of operating solely from electric power and since it concentrates oxygen from ambient air, does not result increases in environmental oxygen concentration reducing fire risk. The waste gas from a concentrator is nitrogen, resulting in a net environmental change of 0.
Oxygen concentrators are used in home applications for patients with chronic lung disease and other respiratory disorders. We propose to evaluate the SeQual Eclipse and SeQual Integra oxygen concentrators at in an altitude chamber up to 30,000 feet. The investigators all have certifications for altitude chamber use and we will use the chamber at Brooks Air Force Base in San Antonio, TX.
2) Key Findings
We evaluated currently on the shelf portable concentrators
Results of the testing demonstrated the following important findings
o The volume of oxygen produced per minute varied from 0.5 to 3.0 liters
o As respiratory rate increases concentrator performance follows one of three characteristics
o Oxygen concentration remains constant
o Oxygen concentration remains constant, but only every other breath is rewarded with oxygen (the net effect being less oxygen to the patient)
o Oxygen concentration falls precipitously with increasing breath rate (60-70%)
o The SeQual Eclipse produces nearly three times the oxygen of second best performing device (Respironics EverGo)
The delivered FIO2 from the concentrators was highest at sea level. FIO2 diminished as barometric pressure decreased. The Eclipse II failed to operate above 22000 ft. Power consumption was reduced at higher altitudes. At the highest flow settings, power consumption diminished by 30% during continuous flow and 31% during pulse dose in the Eclipse II and 19% in the Integra comparing sea level to 8,000ft. Battery duration on the Eclipse II at 8000 ft and 3 lpm was 1 hr 48 mins compared to 1 hr and 22 mins at sea level. Conclusion: Oxygen is a finite commodity, which is cumbersome and hazardous to transport. The relatively high FIO2 delivered by the POCs makes this method of O2 delivery a viable alternative to compressed O2 in select situations. However, POCs cannot deliver an FIO2 of 1.0, necessitating complementary compressed gas for these scenarios. At operational PB, POC function remains equivalent to operation at sea level. The PIO2 available to the patient however, remains constrained by lower PB as altitude increases. At sea level (PB of 750 mmHg) an FIO2 of 0.90 produces an alveolar O2 of 582 mmHg, at 8,000 ft (PB of 564 mmHg) an FIO2 of 0.90 produces an alveolar O2 of 415 mm Hg whereas, at an altitude of 32,000 ft (PB of 206 mmHg) an FIO2 of 0.90 produces an alveolar O2 of 93 mmHg.
The findings suggest that even at extremes of altitude, oxygen concentrators are a safe alternative to compressed or liquid oxygen. At the extremes of mission altitudes, the device continues to operate with only a slight tail off in delivered oxygen concentration. An unsuspected finding was that at lower density the devices became more energy efficient owing to reduced resistance through the sieve beds, resulting in a longer battery life. Oxygen concentrators provide oxygen as long as there is sufficient electricity. In space, an oxygen concentrator is attractive as additional stores of oxygen are not required and use does not raise the oxygen concentration of the ambient environment. In austere environments (aircraft at altitude, remote areas, far forward military operations) oxygen concentrators provide a safe alternative to traditional oxygen sources.