Initial Investigation of a Grating Stimulus as a Visual Endpoint for Human Centrifuge Research.

INTRODUCTION: G tolerance has been widely assessed using Peripheral Light Loss (PLL), but this approach has several limitations and may lack sensitivity. The aim of this study was to investigate the use of a foveal visual endpoint for centrifuge research (Grating Loss; GL) and assess its repeatability, reliability, and usability with PLL as a reference.METHODS: A total of 11 subjects undertook centrifuge assessment. Gradual onset sessions (GOR; 0.1 G · s-1) measured both endpoints simultaneously and were performed twice, consisting of six determinations with anti-G suits activated (GOR-On) and six without (GOR-Off). Four determinations of each endpoint were also taken during rapid onset runs (ROR; 3 G · s-1). Usability was scored subjectively.RESULTS: The GL endpoint was reached 0.3-0.5 Gz lower than PLL with each endpoint correlating strongly in GOR-Off (r = 0.93), GOR-On (r = 0.95), and ROR (r = 0.86). The GL had excellent test-retest repeatability (intraclass correlation coefficient: GOR-Off/On = 0.99, ROR = 0.92) and low within-subject variability. Between-subject variance equaled PLL in all conditions. Subjective usability endpoint ratings were equal for all conditions.DISCUSSION: For the 11 individuals tested, the GL was a reliable, repeatable, and usable endpoint, with similar performance to PLL. GL may prove useful as a supplementary endpoint for human centrifuge research as a secondary data point or to reduce fatigue in repeated measurements. The foveal GL stimulus was lost before PLL, contrary to popular models of visual changes under +Gz.Britton J, Connolly DM, Hawarden DE, Stevenson AT, Harridge SDR, Green NDC, Pollock RD. Initial investigation of a grating stimulus as a visual endpoint for human centrifuge research. Aerosp Med Hum Perform. 2023; 94(12):894-901.

Physiological Effects of Centrifuge-Simulated Suborbital Spaceflight.

BACKGROUND: High-G acceleration experienced during launch and re-entry of suborbital spaceflights may present challenges for older or medically susceptible participants. A detailed understanding of the associated physiological responses would support the development of an evidence-based medical approach to commercial suborbital spaceflight.METHODS: There were 24 healthy subjects recruited into 'younger' (18-44 yr), 'intermediate' (45-64 yr) and 'older' (65-80 yr) age groups. Cardiovascular and respiratory variables were measured continuously during dynamic combinations of +Gx (chest-to-back) and +Gz (head-to-foot) acceleration that simulated suborbital G profiles for spaceplane and rocket/capsule platforms. Measurements were conducted breathing air and breathing 15% oxygen to simulate a cabin pressure altitude of 8000 ft.RESULTS: Suborbital G profiles generated highly dynamic changes in heart rate, blood pressure, and cardiac output. G-induced hypoxemia was observed, with minimum arterial oxygen saturation < 80% in a quarter of subjects. Increased age was associated with greater hypoxemia and reduced cardiac output responses but did not have detrimental cardiovascular effects. ECG changes included recurrent G-induced trigeminy in one individual. Respiratory and visual symptoms were common, with 88% of subjects reporting greyout and 29% reporting blackout. There was one episode of G-induced loss of consciousness (G-LOC).DISCUSSION: Suborbital acceleration profiles are generally well tolerated but are not physiologically inconsequential. Marked hemodynamic effects and transient respiratory compromise could interact with predisposing factors to precipitate adverse cardiopulmonary effects in a minority of participants. Medically susceptible individuals may benefit from expanded preflight centrifuge familiarization that includes targeted physiological evaluation in the form of a 'G challenge test'.Smith TG, Pollock RD, Britton JK, Green NDC, Hodkinson PD, Mitchell SJ, Stevenson AT. Physiological effects of centrifuge-simulated suborbital spaceflight. Aerosp Med Hum Perform. 2022; 93(12):830-839.

Indirect Measurements of Acceleration Atelectasis and the Role of Inspired Oxygen Concentrations.

BACKGROUND: A growing number of symptom reports suggestive of acceleration atelectasis in fast jet aircrew have raised the question as to whether traditional guidelines on inspired gas composition remain valid. The aim of this study was to assess the effects of inspired O2 concentration on the development of acceleration atelectasis when wearing modern anti-G garments. METHODS: There were 14 nonaircrew subjects who completed 5 centrifuge exposures to +5 Gz lasting 90 s. During exposures subjects breathed a gas mixture containing 21, 35, 45, 60, or 75% O2. To assess the extent of atelectasis post-Gz, forced inspiratory vital capacity (FIVC), regional FIVC (EITFIVC), shunt, respiratory resistance, reactance, and compliance and peripheral O2 saturation during a hypoxic exposure were measured. RESULTS: Compared with baseline, FIVC was not statistically significantly altered. EITFIVC was 14.4% lower after the 75% O2 exposure only with a greater symptom reporting with higher FIO2 in some individuals. A significantly greater shunt (3>6%) followed the 60 and 75% O2 exposures. O2 concentration during Gz had no effect on respiratory resistance, reactance, compliance, or hypoxemia. DISCUSSION: There is evidence of mild acceleration atelectasis present when breathing 60% O2, particularly in susceptible individuals, with 75% O2 causing more obvious physiological compromise. An inspired oxygen concentration of <60% will prevent the majority of individuals from developing acceleration atelectasis. Pollock RD, Gates SD, Radcliffe JJ, Stevenson AT. Indirect measurements of acceleration atelectasis and the role of inspired oxygen concentrations. Aerosp Med Hum Perform. 2021; 92(10):780785.

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

AbstractBACKGROUND: 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.

Dynamic lung behavior under high G acceleration monitored with electrical impedance tomography.

Objective. During launch and atmospheric re-entry in suborbital space flights, astronauts are exposed to high G-acceleration. These acceleration levels influence gas exchange inside the lung and can potentially lead to hypoxaemia. The distribution of air inside the lung can be monitored by electrical impedance tomography. This imaging technique might reveal how high gravitational forces affect the dynamic behavior of ventilation and impair gas exchange resulting in hypoxaemia.Approach. We performed a trial in a long-arm centrifuge with ten participants lying supine while being exposed to +2, +4 and +6 Gx(chest-to-back acceleration) to study the magnitude of accelerations experienced during suborbital spaceflight.Main results. First, the tomographic images revealed that the dorsal region of the lung emptied faster than the ventral region. Second, the ventilated area shifted from dorsal to ventral. Consequently, alveolar pressure in the dorsal area reached the pressure of the upper airways before the ventral area emptied completely. Finally, the upper airways collapsed and the end-expiratory volume increased. This resulted in ventral gas trapping with restricted gas exchange.Significance. At +4 Gx, changes in ventilation distribution varied considerably between subjects, potentially due to variation in individual physical conditions. However, at +6 Gxall participants were affected similarly and the influence of high gravitational conditions was pronounced.

Indices of acceleration atelectasis and the effect of hypergravity duration on its development.

NEW FINDINGS: What is the central question of the study? The aim was to determine the effects of duration of acceleration in the cranial-caudal direction (+Gz) on acceleration atelectasis and identify measurement techniques that can be used to assess it. What is the main finding and its importance? Non-invasive measurement of acceleration atelectasis using electrical impedance tomography and estimates of pulmonary shunt provide more detailed assessment of acceleration atelectasis than traditional forced vital capacity measures. Using these techniques, it was found that as little as 30 s of exposure to +Gz acceleration can cause acceleration atelectasis. The results of the present study will allow a more accurate and detailed assessment of acceleration atelectasis in the future. ABSTRACT: Recently, there have been reports of acceleration atelectasis during fast jet flight despite the use of systems designed to minimize this. Before further investigation of this, indices suitable for use in applied settings and identification of acceleration durations that elicit it are required. Fifteen non-aircrew subjects underwented five centrifuge exposures lasting 15, 30, 60 and 2 × 90 s with a plateau of +5 Gz (acceleration in the cranial-caudal direction) while breathing 94% O2 during all but one control exposure (21% O2 ). Lung volumes and gas exchange limitation were assessed after each exposure. Regional lung impedance and compliance were measured after Gz exposure using electrical impedance tomography and the forced oscillatory technique, respectively. The presence of acceleration atelectasis was confirmed by reductions of 10-17% in vital and inspiratory capacity after 60 and 90 s Gz exposures (P < 0.05) and resulted in reduced regional lung impedance and a gas exchange limitation of 8.1 and 12.5%, respectively (P < 0.05). There was also a small but significant decrease in regional lung impedance after 30 s exposures. Functional residual capacity and lung compliance were unchanged in atelectatic lungs (P > 0.05). In the majority of individuals, >60 s of Gz exposure while breathing 94% O2 causes acceleration atelectasis. Electrical impedance tomography and the measurement of gas exchange limitation provide useful indicators of acceleration atelectasis. Acceleration atelectasis exerts its effects primarily through basal lung closure and reflex inspiratory limitation, both of which can be reversed by performing three maximal inspiratory breathing manoeuvres.

Hemodynamic Responses and G Protection Afforded by Three Different Anti-G Systems.

BACKGROUND: UK Royal Air Force fast jet aircrew use three different anti-G systems, however, little objective comparison of the G protection they provide exists. The G-protection afforded by each system and associated hemodynamic responses were investigated.METHODS: Ten subjects performed centrifuge acceleration exposures using Mk-10 (S1) and Mk-4 (S2) five-bladder anti-G trousers (AGT) and full coverage AGT plus pressure breathing for G-protection (PBG; S3). Measurements of relaxed G tolerance (RGT), eye-level blood pressure (BPeye), lower body blood volume (LBV), stroke volume (SV) and total peripheral resistance (TPR) were made during gradual onset runs (GOR) and rapid onset runs (ROR). The subjective effort required to maintain clear vision at +7 and +8 Gz provided an indication of the protection provided by the system.RESULTS: All systems moderated decreases in SV and BPeye and increases in LBV under increased +Gz. S3 provided the greatest mean RGT during GOR (+6.2 Gz) and ROR (+6 Gz), reduced the effort required to maintain clear vision at up to +8 Gz, prevented venous pooling and afforded the greatest rise in TPR. The majority of indices revealed no difference between S1 and S2 although RGT during the ROR was greater with S2 (+0.25 Gz).DISCUSSION: S3 effectively prevented pooling of blood in the lower limbs under +Gz, despite the use of PBG, and offers an advantage over five-bladder AGT. Given the similarities of S1 and S2, it was unsurprising that the majority of indices measured were similar. The objective measurement of hemodynamic parameters provides useful information for comparing the G-protection provided by anti-G systems.Pollock RD, Firth RV, Storey JA, Phillips KE, Connolly DM, Green NDC, Stevenson AT. Hemodynamic responses and G protection afforded by three different anti-G systems. Aerosp Med Hum Perform. 2019; 90(11):925-933.

Aircrew Conditioning Programme Impact on +Gz Tolerance.

INTRODUCTION: Physical conditioning may improve aircrew performance during exposure to high +Gz acceleration, although few studies have directly assessed this. The present study investigated the effects of a 12-wk Aircrew Conditioning Programme (ACP) on markers of G tolerance. The ACP comprises aerobic and muscle strengthening exercise performed twice weekly and targets improved fitness and reduced injury risk.METHODS: There were 36 UK Royal Air Force and Royal Navy aircrew who volunteered; 17 performed the ACP (Ex) and 19 acted as a control group (Con). Centrifuge testing was performed before and after the intervention. Relaxed G tolerance (RGT) and straining G tolerance (SGT), which had the addition of muscle tensing, were assessed. G endurance was also determined via repeated simulated air combat maneuvers (SACMs). During these centrifuge runs a number of physiological variables were recorded.RESULTS: During the G profile to determine RGT, neither RGT, HR, nor blood pressure responses were affected by the ACP. During SGT profiles, a lower HR at a given +Gz (+5.5 Gz) level following the ACP was observed (Ex: pre 146.0 ± 4.4, post 136.9 ± 5.6 bpm; Con: pre 148.0 ± 3.2, post 153.1 ± 3.3 bpm). BP was maintained and there was a tendency toward an improved SGT. The ACP increased the proportion of individuals completing the number of SACM profiles, although no meaningful differences were found between groups in other variables.CONCLUSION: Overall the ACP has no negative effect on RGT, reduced the physiological strain associated with a given level of +Gz (during SGT), and tended to improve the ability to tolerate repeated Gz exposure.Slungaard E, Pollock RD, Stevenson AT, Green NDC, Newham DJ, Harridge SDR. Aircrew conditioning programme impact on +Gz tolerance. Aerosp Med Hum Perform. 2019; 90(9):764-773.

Blood pressure, vascular resistance, and +Gz tolerance during repeated +Gz exposures.

BACKGROUND: Cardiovascular reflexes that regulate blood pressure (BP) adapt during repeated exposure to +Gz acceleration separated by short (< 20 s) breaks, but whether this effect is preserved with longer intervals remains unknown. METHODS: There were 17 subjects who completed 5 repeated gradual onset (0.1 G x s(-1)) runs (GOR1-5) to 60 degrees peripheral light loss (PLL) on a human centrifuge, separated by 2 min. Heart rate (HR) and BP were measured before and during each GOR and noninvasive estimation of cardiac output (CO) was used to calculate total peripheral resistance (TPR). RESULTS: Mean resting (+/- SE) systolic BP (138 +/- 4 vs. 128 +/- 3 mmHg) and TPR (13.9 +/- 1.2 vs. 12.7 +/- 1.0 mmHg x L(-1) x min(-1)) were elevated after GOR1 and remained elevated thereafter. Compared with that before GOR1, resting HR was decreased (5-6 bpm) before GOR2-5. Resting CO decreased from 8.3 +/- 0.6 L x min(-1) before GOR1 to a nadir of 7.2 +/- 0.5 L x min(-1) before GOR4 and GOR5. The change in HR under increased +Gz decreased from +39 +/- 3 bpm during GOR1 to +31 +/- 3 bpm during GOR4 and GOR5, but the decrease in eye level BP under +Gz was unaffected. RGT did not change across the five GORs. CONCLUSIONS: Increased resting BP and TPR following a single +Gz exposure suggest alterations in the cardiovascular system expected to confer improved tolerance during subsequent exposures. However, these changes were insufficient to improve +Gz tolerance measured during repeated GORs separated by 2 min.

+Gz tolerance, with and without muscle tensing, following loss of anti-G trouser pressure.

BACKGROUND: Fast jet aircrew are heavily reliant on anti-G trousers (AGT) and failure of the garment or its pressure supply would expose them to high levels of +Gz acceleration without protection. A previous study demonstrated the severity of this event at high +Gz, but no data exists as to the maximum +Gz level which could be tolerated. METHODS: : While relaxed and with lower body muscle tensing, 10 experienced centrifuge subjects had their AGT deflated after 5 and 30 s of +Gz exposure. Discrete +Gz exposures of increasing intensity were performed until deflation resulted in central light loss (CLL). Visual symptoms, eye-level systolic BP (SBPeye), and mean blood flow velocity in the middle cerebral artery (MCAVmean) were recorded. RESULTS: The mean +Gz level at CLL following AGT deflation was comparable to that without AGT inflation (+4.07 Gz) and increased by muscle tensing (+0.53 G) independent of exposure duration. Initial visual symptoms occurred more rapidly in the shorter +Gz exposures, while progression to CLL was quicker with muscle tensing compared with relaxed, but never less than 6.7 s. At CLL, the nadir in SBPeye was higher (+17 mmHg) with muscle tensing compared with relaxed, while MCAVmean was decreased by about 50% in all conditions. DISCUSSION: +Gz tolerance following AGT deflation is comparable to that without inflation and only modestly increased by muscle tensing. Although vision is maintained for several seconds following AGT deflation, progression of light loss symptoms can be rapid, possibly resulting in insufficient time to respond before G-induced loss of conciousness (G-LOC) ensues.

Subjective and objective measures of relaxed +Gz tolerance following repeated +Gz exposure.

BACKGROUND: Repeated exposure to +Gz acceleration provokes cardiovascular adaptations of potential benefit to pilots' +Gz tolerance, but whether such changes actually improve human tolerance to +Gz acceleration is uncertain. This study assessed +Gz tolerance before and after repeated exposure to +Gz at two different intensities as the role of frequency of +Gz exposure in adaptation also remains unknown. METHODS: In a cross-over design, 10 experienced male centrifuge volunteers completed two experimental conditions separated by at least 3 wk. Subjects completed four simulated air combat maneuvers (SACM) on a human centrifuge, either twice or four times per week, for 3 consecutive weeks. Relaxed +Gz tolerance (RGT) during a gradual onset run (GOR, 0.1 G x s(-1)) and cardiovascular responses to rapid and incremental head-up tilt were assessed before and after each condition. RESULTS: Rapid and incremental head-up tilt increased both mean arterial and diastolic blood pressures following +Gz exposure. +Gz exposure attenuated the increase in heart rate (+9 +/- 3 vs. +11 +/- 3 mmHg/Gz) and the decrease in eye-level systolic blood pressure (-11 +/- 3 vs. -14 +/- 4 mmHg/Gz) during GOR, but had no effect on RGT (4/wk: +3.88 +/- 0.56 vs. +3.92 +/- 0.63 Gz; 2/wk: +3.89 +/- 0.69 vs. +3.92 +/- 0.69 Gz). DISCUSSION: Frequent +Gz acceleration, either as 2 (8 SACMs) or 4 sessions (16 SACMs) per week for 3 wk, enhances cardiovascular tolerance to orthostatic stress but does not improve RGT measured during a GOR.

Garment fit and protection from sustained +Gz acceleration with 'full-coverage' anti-G trousers.

INTRODUCTION: Garment fit may influence the effectiveness with which Full-Coverage Anti-G Trousers (FCAGT) transmit pressure to the skin surface, and hence provide protection from sustained +Gz acceleration. A search of the available literature, however, did not reveal any prior work quantifying this effect. METHODS: Three related studies were performed. In Study I, using a mannequin, garment-to-surface pressure transmission ratios were measured at various locations under normally fitted (NF) and loosely fitted (LF) FCAGT. In Study II, garment pressure-volume ratios and lung volumes were measured at +1 Gz in six men wearing FCAGT in three conditions: NF, LF, or NF with inflatable socks at 13 kPa differential pressure (NF+SOCKS). In Study III, relaxed +Gz tolerance (RGT) and ratings of perceived exertion (RPE) at +7, +8, and +9 Gz were measured in eight men wearing FCAGT in four experimental conditions: NF; LF; abdomen LF, legs NF; or abdomen NF, legs LF. RESULTS: LF did not affect pressure transmission over the lower limbs or lower abdomen, but transmission over the upper abdomen was significantly compromised. Lung volumes were reduced with FCAGT inflation, but LF was associated with greater expiratory reserve volumes and increased FCAGT volume. Under +Gz acceleration, LF over the abdomen (with or without lower limb LF) decreased RGT and increased RPE, but not with lower limb LF when abdominal fit was normal. DISCUSSION: Care should be taken to achieve and maintain a snug FCAGT fit, especially of the abdominal portion of the FCAGT, to ensure optimal anti-G protection during sustained acceleration.