Preparation for adverse cardiac events is vital to ensure the safety of space explorers, thus potentiating the development of the most effective protocol for BLS in simulated 0.38Gz and μG.
This study was the first of its kind to investigate the administration of effective ECCs using the 2010 ECC guidelines in comparison to the previous 2005 ECC guidelines during simulated 0.38Gz and μG, while looking at the physiological impact on the rescuer.
Both ECC guidelines emphasise that effective ECCs have two key components—adequate compression depth and rate—to ensure sufficient haemodynamics from time of arrest to application of ALS. When assessing the DMax achieved during ECCs, results from the 1Gz and simulated 0.38Gz sessions showed that all volunteers were able to perform according to both the 2005 and 2010 ECC standards. In fact, the ability of volunteers to abide by the previous 2005 ECC guidelines at 1Gz and during simulated 0.38Gz is in agreement with previous studies [10, 14]. Although mean DMax did not meet the 2005 ECC guidelines during simulated μG, a negligible difference of 0.2 mm would probably be deemed effective during in vivo BLS. However, the considerable inter-volunteer variability observed questions the efficacy of the ER method, which concurred with the findings of Rehnberg et al. . Mean DMax failed to abide by the 2010 ECC guidelines, which corresponds with the findings of Kordi et al.  (Figure 2B).
Previous studies noted that volunteers were failing to consistently allow full chest recoil using the ER method during simulated μG (Figure 2) . This can be detrimental to the effectiveness of BLS, as incomplete decompression decreases the change in thoracic pressure and thereby reduces perfusion to vital organs. To address this issue and a first for space CPR studies, ECC DT was calculated by adjusting for DIRecoil (Figure 3 and Table 1).
When assessing ECC DT, all 30 volunteers failed to abide by both ECC guidelines during simulated μG using the ER method. This could be attributable to rescuers inadequately decompressing between individual ECCs or interruptions during ECCs when using the ER position in simulated μG.
The inadequate decompressions between ECCs may be due to rescuers focussing on achieving the 100 compressions·min−1 rate set by guidelines during simulated μG. This is supported by mean ECC rate in keeping with both sets of ECC guidelines, whilst true depth of individual ECC sets was not (Table 1). This is not in accordance with a previous parabolic flight study using the ER method that found ECC rate to be lower whilst ECC depth remained adequate for the used ECC guidelines at that time, which were the same as 2005. These findings, however, may represent a limitation of the BSD system. The parabolic study had a sample size of 3 and was able to adhere to ECC guidelines even with such a small window of freefall, approximately 20 s per parabola .
In addition, the high SD seen in Table 1, which represents the inter-volunteer variability for ECC rate, increases with time in simulated μG. This suggests degradation of ECC rate during the course of BLS.
Overall, the results of this investigation suggest that the ECCs administered were ineffective in simulated μG, as only mean ECC rate was adhered to, which would reduce the benefit to the casualty due to inadequate vital organ perfusion. This also indicates that the efficacy of the ER method is deficient at producing true depth of ECCs in a simulated μG environment, which contradicts previous studies only analysing maximum depth of ECCs and that did not account for DIRecoil of the mannequin .
The efficacy of ECCs is dependent on the physiological impact of CPR on the rescuer. Increased HR, VE and VO2peak were inversely correlated with the simulated gravitational conditions studied, which indicates greater physical effort during the performance of BLS (Table 2 and Figure 5). The HR results support those found by both Dalmarco et al.  and Rehnberg et al. . Furthermore, there was no difference in HR, VE and VO2peak between ECC guidelines, which may imply that current ECC guidelines do not impose additional physical effort, unlike the simulated gravitational environment.
In our study, it is important to note that VO2peak was measured and used as an estimation of VO2max, allowing comparisons with previous literature findings to be drawn .
There are limited studies that evaluate VO2max during or post-spaceflight, all of which are short-duration missions (<14 days) . It has been hypothesised that appropriate exercise countermeasures may maintain VO2max during long-duration explorer-class missions. Thus, the additive effects of cardiac deconditioning would have less influence on the rescuer's aerobic capabilities to perform ECCs in μG, making the physical difficulty of the ER method the key variable in performing effective ECCs.
Interestingly, decreases in VO2max arise following re-entry. Levine et al.  noted that after the SLS-1 and SLS-2 missions, six astronauts showed VO2max levels of 2.1–2.9 L·min−1. In addition, the extra-vehicular activity (EVA) suit required for planetary surface exploration may also determine the level of cardiovascular exercise capacity. Studies at NASA's Johnson Space Center in simulated 0.38Gz showed an increase in VO2 by an additional 20 mL·kg−1·min−1 (40% of the volunteer's VO2max) while wearing a Mark III prototype exploration EVA suit .
After a Martian landing, crewmembers will most likely be required to begin work immediately without a sufficient period for acclimatisation to 0.38Gz. This reduced aerobic capacity, in conjunction with orthostatic intolerance and impaired blood flow from long-term microgravity exposure, may significantly impact a crewmember's capability during emergencies or while assisting an incapacitated crewmate.
Although any attempt to administer CPR in an EVA suit is unlikely to be achievable, the physiological aspect would be interesting to consider. Therefore, the mean VO2peak of a rescuer in an EVA suit would be 41.8 mL·kg−1·min−1 in simulated 0.38Gz, which accounts for our 21.8 mL·kg−1·min−1 (Figure 5B) and the expected additional 20 mL·kg−1·min−1 from wearing an EVA suit . For the average male weight (78.2 kg) in our study, this would equate to 3.3 L·min−1 after four sets of ECCs. This exceeds the VO2max of 2.9 L·min−1 found by Levine et al. . This value may be an underestimation, as the casualty would also be wearing an EVA suit and the pressurisation of their suit would have to be overcome as well.
Furthermore, this study looked at the range of elbow flexion, while previous studies only took maximum elbow flexion into account [10, 14]. The greater range of elbow flexion seen during simulated 0.38Gz could be accredited to the recruitment of upper arm muscle groups to compensate for the reduction in upper body weight. The lack of difference in the range of elbow flexion for either ECC guidelines may indicate that the upper limb muscle groups are recruited in the same manner (Figure 4).
An increase in elbow flexion range was also noted between simulated μG and 1Gz. Using the 2005 ECC guidelines during simulated μG, the mean increase of 15.5° (±8.7°) in the volunteer's dominant arm was similar to the approximate 11° (±8.3°) and 15° (±9.1°) of the right and left arms, respectively, found by Rehnberg et al. . However, it is important to highlight that the change from the terrestrial to the ER BLS position might have contributed to the recruitment of different muscle groups. Like simulated 0.38Gz, the lack of difference in the range of elbow flexion using the ER method for both ECC guidelines may indicate that the upper limb muscle groups are recruited in the same manner. This further suggests that guidelines are equally difficult in simulated μG, as this correlates with the inability to achieve effective true ECC depth and the non-significant difference in physiological variables between guidelines (Figure 4).
Although the physiological variables measured were not different between guidelines, volunteers perceived current ECC guidelines to be more difficult (Figure 6). This might have been influenced by the fact that volunteers had a pre-conception that illuminating more LEDs for current ECC guidelines could have been less attainable.
This study is not without limitations, since it is based on the evaluation of healthy young males performing 1.5 min of BLS. The simulated gravitational environment, using a BSD, may not replicate all physiological effects secondary to reduced gravity exposure, apart from weight reduction, which is essential for successful BLS. This also applies to the mannequin when considering that chest wall expansion would occur upon reduced gravity exposure, affecting chest compression depth. Other psychological and physiological factors may differ in a simulated study compared to an actual cardiac arrest, such as stress. Furthermore, there are differences in chest wall compliance between humans and mannequins, which do not take into account variations in body anthropometrics, as well as EVA suits. In addition, the sample may not be representative of the commercial space passenger population in terms of demographics.