Monitoring Tissue Oxygen Saturation in Microgravity on Parabolic Flights

Near-infrared spectroscopy provides a noninvasive means of measuring tissue oxygenation, exploiting the characteristic light absorption properties of oxygenated and deoxygenated hemoglobin in the near-infrared wavelength range to determine tissue oxygen saturation (StO₂). Monitors typically measure oxygen saturation across a muscle tissue field containing both arterial and venous blood, such as the thenar eminence of the hand. In clinical practice, StO₂ has mainly been developed as an indicator of peripheral tissue perfusion in critically ill patients, as StO₂ is lowered when perfusion is reduced in association with, for example, major hemorrhage or sepsis (Duret et al., 2015; Neto et al., 2014). A fall in StO₂ can also be caused by a reduction in oxygen delivery to the tissues when blood oxygen content is reduced, such as at high altitude or when gas exchange is impaired in the lungs (Martin et al., 2009; Sandfeld et al., 2013).

Due to the shape of the hemoglobin-oxygen dissociation curve, a moderate fall in arterial partial pressure of oxygen from normal levels would be expected to cause a greater reduction in StO₂ than in arterial oxygen saturation measured by pulse oximetry (SpO₂), and may therefore provide a means of detecting more subtle changes in oxygenation. A handheld StO₂ monitor has recently been developed for portable use, introducing the potential to measure StO₂ noninvasively in challenging environments such as space. StO₂ monitoring could be investigated for its utility in various clinical scenarios in space, and could also be explored as a means of monitoring potential effects of the spaceflight environment on oxygenation.

In the future this could include the effects of moderately reduced cabin pressure with consequently reduced oxygen levels, which NASA and others have proposed for the design of future spacecraft and crew habitats (Bacal et al., 2008; Norcross et al., 2013; Scheuring et al., 2008). Such an atmosphere would expose astronauts to a mildly hypoxic environment comparable to a commercial aircraft cabin, which is known to stimulate classic physiological responses, such as ventilatory acclimatisation (Fatemian et al., 2001), erythropoietin secretion (Gunga et al., 1996), and hypoxic pulmonary vasoconstriction (Smith et al., 2013; Smith et al., 2012; Turner et al., 2015). This design approach would have the benefit of making extravehicular activity safer and easier with respect to the risk of decompression sickness and associated denitrogenation protocols, as well as other potential advantages including reduced risk of fire, decreased structural strength and weight requirements, reduced requirement for stored or generated oxygen, and even a speculative effect as a countermeasure against spaceflight anemia (Bacal et al., 2008; Buckey, 2006; Scheuring et al., 2008). However, the wider health and performance implications of this environment require careful consideration, including the scope for physiological interaction between hypoxia, hypobaria, and hypogravity, which has yet to be investigated (Norcross et al., 2013). Measurements of StO₂ could be a useful adjunct in this setting, and we aimed to evaluate portable StO₂ monitoring technology with a view to future use in space using parabolic aircraft flights to assess its function in microgravity.

The StO₂ monitor technology functioned normally in all gravity conditions, including powering on/off and performing its self-test function. The monitor was generally easy to use in microgravity, and this was aided by tethering it to the body. The 1-meter cable length was appropriate for use in weightless conditions and the visual display was easy to read. The monitor provided thenar StO₂ measurements appropriately during parabolas, which are shown in Figure 1. For each person, StO₂ is reported from an average of 17 parabolas, and there was considerable variability between and within individuals (Figure 1).

Figure 1

Baseline arterial oxygen saturation measured pre-flight was normal (SpO₂ 97±0.5%). Baseline StO₂ (80±2%) was also within the wide normal range of approximately 70–85% that has been reported in the literature. Due to the mildly hypoxic cabin pressure altitude, arterial oxygenation was lowered in-flight (SpO₂ 93±0.7%; unchanged with microgravity [P=0.3]), and thenar StO₂ was likewise affected by cabin pressure in-flight (74±3%) and is depicted in Figure 2. Transition to microgravity was associated with a small decrease in StO₂ of 1.1±0.3% (P=0.017), as shown in Figure 2. The StO₂ monitor is designed to be used on the hand and is not intended for use on the foot, and we found that StO₂ values at the foot were well below the physiological range both in 1 g (55±5% pre-flight) and in flight (50±3% during both ~1.8 g and 0 g). Heart rate did not change significantly during the parabolas (63±7 bpm; P=0.14).

Figure 2

This evaluation has established that the InSpectra StO₂ monitoring technology operates normally in microgravity. However we observed considerable inter-individual and intra-individual variability in StO₂ values, which is consistent with previous terrestrial studies and is one reason why StO₂ monitoring remains largely a research tool and has yet to be taken up widely in mainstream clinical practice (Lipcsey et al., 2012; Van Haren et al., 2013). It has been suggested that, in addition to possible limitations of the technology itself, various physiological factors could contribute to this variability, including tissue thickness and composition, fluid status, blood viscosity, and temperature-related changes in perfusion. Age did not vary greatly and is unlikely to have influenced our findings, although we note that long-duration astronauts are typically at least a decade older than this study's participants. Our findings do support using the device at the intended thenar site rather than the foot, where measurements were unreliable.

In the course of assessing this technology we obtained data on StO₂ during changing gravity conditions. This data is preliminary in nature and the usual limitations of parabolic flights apply, including a small sample size, short exposure times, and the potential for physiological effects of one phase (microgravity or hypergravity) to carry over into the subsequent alternate phase. Notwithstanding these limitations and the background variability, across multiple repeat measurements a small fall in StO₂ was observed in each individual on transition to microgravity. Such a small decrement has no physiological significance in itself, and an artifactual explanation cannot be excluded, but it is interesting to speculate whether this fall might represent the beginning of a larger effect of microgravity on tissue oxygenation that would have continued to develop had the weightless state been maintained beyond 17 seconds.

Although microgravity does not cause major impairment of gas exchange in the lungs (Prisk, 2014), limited evidence from a series of measurements on the Mir space station suggests that tissue oxygenation may nevertheless be somewhat decreased during spaceflight, perhaps due to slightly altered ventilation/perfusion matching in the lungs (Baranov, 2011; West, 2001). It is unfortunate that standard arterial blood gas analyses have not yet been performed in space, as these would be very helpful in interpreting isolated findings such as these, particularly in the light of the many ‘hypoxia-mediated physiologic concerns’ NASA has identified that could conceivably arise from or be worsened by a moderately hypoxic atmosphere in space (Norcross et al., 2013). These concerns include: visual impairment/intracranial pressure syndrome, sensorimotor performance deficits, neurocognitive impairment, sleep disturbance, acute mountain sickness, diminished exercise capacity and cardiovascular performance, immunosuppression, oxidative stress and damage, nutritional deficiencies, and increased bone resorption with acceleration of bone loss and higher risk of renal stones (Norcross et al., 2013). The true potential for any of these adverse factors to impact on future space missions is currently unknown, but would presumably be greater if the effects of a hypoxic cabin environment were superimposed on any underlying impairment of tissue oxygenation inherently associated with microgravity.

Although the use of a hypobaric exploration-class atmosphere of this kind is many years away, the acute combination of microgravity and mild hypoxia is likely to be experienced much sooner with the anticipated commencement of commercial suborbital spaceflights. For suborbital operators utilizing an airline-style cabin pressure, several minutes of microgravity will be superimposed on mild background hypoxia, and for spaceflight participants these factors will together interact with any influence of age, smoking, or pre-existing disease, and contribute to in-flight hypoxemia (as has been observed on a parabolic flight (Mackenzie et al., 2007)). In addition to the period of weightlessness, suborbital spaceflights will also include significant high-g exposures which are in fact more likely to cause hypoxemia, and arterial oxygen desaturation has been demonstrated during normoxic centrifuge studies simulating anticipated suborbital acceleration profiles (Blue et al., 2014; Blue et al., 2012). Of course the true acceleration-weightlessness-acceleration profile of suborbital spaceflight cannot be simulated, and in-flight studies will be required to characterize the associated physiology when suborbital operations commence.

In summary, we have established that StO₂ monitoring is possible in microgravity and demonstrated the potential for exploring the use of this technology in space. In theory, astronauts could use this technology to detect a degree of tissue hypoxia that may not otherwise be apparent during future spaceflight operations, although the noise from background variability would need to be overcome, possibly with multiple repeat measurements. This evaluation has highlighted our incomplete understanding of the physiological interaction between microgravity and hypoxia, and further research into the effects of changing gravity conditions on oxygenation is warranted.