Pulmonary Effects of Sustained Periods of High-G Acceleration Relevant to Suborbital Spaceflight

Abstract

BACKGROUND: Members of the public will soon be taking commercial suborbital spaceflights with significant Gx (chest-to-back) acceleration potentially reaching up to 6 Gx. Pulmonary physiology is gravity-dependent and is likely to be affected, which may have clinical implications for medically susceptible individuals.METHODS: During 2-min centrifuge exposures ranging up to 6 Gx, 11 healthy subjects were studied using advanced respiratory techniques. These sustained exposures were intended to allow characterization of the underlying pulmonary response and did not replicate actual suborbital G profiles. Regional distribution of ventilation in the lungs was determined using electrical impedance tomography. Neural respiratory drive (from diaphragm electromyography) and work of breathing (from transdiaphragmatic pressures) were obtained via nasoesophageal catheters. Arterial blood gases were measured in a subset of subjects. Measurements were conducted while breathing air and breathing 15 oxygen to simulate anticipated cabin pressurization conditions.RESULTS: Acceleration caused hypoxemia that worsened with increasing magnitude and duration of Gx. Minimum arterial oxygen saturation at 6 Gx was 86 1 breathing air and 79 1 breathing 15 oxygen. With increasing Gx the alveolar-arterial (A-a) oxygen gradient widened progressively and the relative distribution of ventilation reversed from posterior to anterior lung regions with substantial gas-trapping anteriorly. Severe breathlessness accompanied large progressive increases in work of breathing and neural respiratory drive.DISCUSSION: Sustained high-G acceleration at magnitudes relevant to suborbital flight profoundly affects respiratory physiology. These effects may become clinically important in the most medically susceptible passengers, in whom the potential role of centrifuge-based preflight evaluation requires further investigation.Pollock RD, Jolley CJ, Abid N, Couper JH, Estrada-Petrocelli L, Hodkinson PD, Leonhardt S, Mago-Elliott S, Menden T, Rafferty G, Richmond G, Robbins PA, Ritchie GAD, Segal MJ, Stevenson AT, Tank HD, Smith TG. Pulmonary effects of sustained periods of high-G acceleration relevant to suborbital spaceflight. Aerosp Med Hum Perform. 2021; 92(7):633641.

Oh G: The x, y and z of human physiological responses to acceleration

“…Limited measurements during simulated suborbital profiles also indicated a degree of arterial oxygen desaturation (Blue et al, 2012(Blue et al, , 2014) that is consistent with the hypoxaemia demonstrated during previous and subsequent work studying various magnitudes and durations of +Gx (Alexander et al, 1966;Ax et al, 2013;Glaister, 1970;Nolan et al, 1963;Pollock, Jolley, et al, 2021;Prisk, 2011;Rohdin et al, 2003).…”

“…Limited measurements during simulated suborbital profiles also indicated a degree of arterial oxygen desaturation (Blue et al., 2012, 2014) that is consistent with the hypoxaemia demonstrated during previous and subsequent work studying various magnitudes and durations of +Gx (Alexander et al., 1966; Ax et al., 2013; Glaister, 1970; Nolan et al., 1963; Pollock, Jolley, et al., 2021; Prisk, 2011; Rohdin et al., 2003). Any in‐flight hypoxaemia is likely to be exacerbated by the use of mildly hypoxic airline‐style cabin pressurization, which is anticipated on some suborbital vehicles (Pollock, Jolley, et al., 2021). Other factors that could compound the effects of +Gx during suborbital flights include the intervening μG phase and the simultaneous exposure to +Gz that is anticipated during some flight profiles (Albery, 2004; Blue et al., 2014).…”

“…Alongside worsening impairment of gas exchange and hypoxaemia, recent centrifuge experiments ≤ 6 Gx, sustained for 2 min, have demonstrated very high work of breathing and neural respiratory drive, in addition to a progressive reversal in the normal relative distribution of regional lung ventilation (from posterior to anterior), accompanied by anterior gas trapping (Menden et al., 2021; Pollock, Jolley, et al., 2021). In these experiments, ventilatory responses were limited by impaired pulmonary mechanics, with dose‐dependent neuroventilatory uncoupling and pronounced breathlessness, and most participants reported musculoskeletal chest pain at higher G levels (Pollock, Jolley, et al., 2021). These effects were analogous to a very short‐lived form of respiratory failure and were well tolerated by young, healthy volunteers but might have greater impact in other populations.…”

Anaesthetists and aerospace medicine in a new era of human spaceflight

“…At 6 Gs, bodyweight is multiplied six‐fold, which is half a tonne for an 85‐kg person, and the experience has colloquially been likened to an ‘elephant sitting on the chest’. Recent anaesthetist‐led research in the UK has characterised the pulmonary response to these magnitudes of G on a centrifuge, demonstrating ventilation/perfusion mismatching and hypoxaemia; reversal of regional lung ventilation with associated gas trapping; impaired breathing mechanics; and breathlessness [11]. These effects were well tolerated by young healthy participants, but studies are required to investigate their potential to become clinically meaningful in medically susceptible passengers, such as those with lung pathology, cardiac conditions or higher BMI [11].…”

“…Anaesthetists are experts in evaluating and optimising a patient’s capacity to withstand a major physiological challenge, and similar principles of pre‐assessment and optimisation apply to private citizens facing the stressors of spaceflight, which could one day include all of us. In both cases, the aim is to improve outcomes by stratifying physiological risk and then intervening to modify this risk as required, whether through widespread pre‐operative tools like cardiopulmonary exercise testing and prehabilitation, or by targeting in‐flight support with a pre‐flight ‘G challenge test’ on a human centrifuge [11]. Alongside colleagues in aerospace medicine and other specialties, anaesthetists are well placed to apply their skills and experience in pre‐assessment and optimisation as ‘peri‐flight physicians’.…”