Toward safer skies - the role of NIRS and capnography in G-tolerance monitoring: A narrative review

Abstract

G-force-induced loss of consciousness (G-LOC) is a significant concern in high-performance aviation, where rapid gravitational shifts impair cerebral perfusion and oxygenation. The resulting physiological decline can originate from both peripheral (e.g., systemic hypoperfusion, ventilation-perfusion mismatch) and central (e.g., cerebral hypoxia, impaired autoregulation) mechanisms. Peripheral impairments involve cardiovascular redistribution and reduced end-tidal CO₂, while central disruptions reflect compromised oxygen delivery to brain tissue. Recent advancements in physiological Monitoring, particularly Near-Infrared Spectroscopy (NIRS) and capnography, offer real-time, non-invasive assessment of these critical variables. NIRS enables continuous monitoring of regional cerebral oxygen saturation (rSO₂), while capnography measures end-tidal CO₂ (EtCO₂) as a proxy for ventilation and perfusion efficiency. When used in combination, these modalities offer a synergistic view of pilot physiology under G-stress, supporting early detection of hypoxic decline and informing targeted countermeasures. As aerospace operations expand in intensity and complexity, wearable and integrative monitoring solutions become increasingly relevant. This narrative review explores the physiological mechanisms underlying G-tolerance, the technological advancements in NIRS and capnography, and the potential of these tools to mitigate G-LOC risk through individualized diagnostics, predictive modeling, and enhanced anti-G training protocols.

Keywords

Aerospace physiology aviation G-induced loss of consciousness hypoxia near-infrared spectroscopy ventilation-perfusion mismatch

Introduction

In modern aviation and spaceflight, maintaining cognitive and physiological stability under extreme conditions is critical. One of the main threats in these environments is exposure to high gravitational forces (G-forces), particularly along the head-to-foot axis (+Gz).¹ These forces, common in combat maneuvers and during launch or re-entry, reduce cerebral blood flow and oxygenation by redirecting blood to the lower extremities.² This can result in visual disturbances, motor dysfunction, and ultimately G-force-induced loss of consciousness (G-LOC),³ a serious risk for both mission success and pilot survival.⁴ Even with countermeasures like anti-G suits, positive pressure breathing, and physical conditioning, G-LOC remains a persistent issue,⁵ highlighting the need for improved detection and prevention strategies.⁶

Current physiological monitoring in high-G scenarios typically includes cardiovascular metrics such as heart rate and blood pressure. While useful for general systemic assessment, these tools lack the sensitivity and real-time responsiveness to detect the rapid cerebral and respiratory changes preceding G-LOC.⁷ Cognitive assessments, though valuable in controlled environments, are subjective and impractical during high-speed operations. This leaves a critical gap in monitoring cerebral and respiratory function in a continuous, actionable way during G-stress.

To address this, attention has turned to Near-Infrared Spectroscopy (NIRS) and Capnography. NIRS is a non-invasive method that tracks cerebral oxygenation by measuring light absorption by oxygenated and deoxygenated hemoglobin - particularly useful in monitoring the prefrontal cortex, a region vulnerable to G-induced hypoperfusion.⁸ Capnography, by measuring end-tidal carbon dioxide (EtCO₂), reflects ventilatory status and cardiac output, offering insights into systemic oxygen delivery and perfusion.⁹ Independently, each provides critical physiological information; together, they offer a synergistic view of the body's response to G-loads.

The integration of NIRS and capnography has been proposed as a promising approach for improving physiological monitoring, with potential applications in performance optimization and predictive safety. Combining cerebral oxygen saturation (rSO₂) with EtCO₂ trends may allow for early detection of G-LOC, even before symptoms manifest,¹⁰ which may support future development of personalized thresholds, tailored training strategies, and real-time physiological feedback. These technologies also provide objective data to improve anti-G countermeasures.^11,12 The development of wearable solutions, such as helmet-based NIRS and in-mask capnographs, enhances operational feasibility.¹³

This narrative review aims primarily to examine the physiological mechanisms underlying G-force intolerance and to evaluate the potential role of NIRS and capnography as complementary monitoring tools for assessing cerebral oxygenation and respiratory dynamics during high-G exposure. A secondary objective is to discuss current technological developments, practical limitations, and future research directions for integrating these monitoring approaches into aerospace training and operational environments.

Methods

Search strategy and scope

Given the narrative nature of this review, the evidence was synthesized descriptively and thematically rather than through a formal systematic or quantitative approach. This narrative review focuses primarily on literature published over approximately the last 10–15 years (2010–2024), reflecting recent developments in aerospace physiology, wearable biosensors, and neurophysiological monitoring technologies. Earlier foundational studies were included where necessary to contextualize the physiological mechanisms underlying G-tolerance and G-induced loss of consciousness (G-LOC). Relevant literature was identified through searches of major biomedical databases, including PubMed and Scopus, using combinations of keywords related to G-force exposure, cerebral oxygenation, NIRS, capnography, and aerospace physiology. Both original experimental studies and relevant review articles were considered when they provided important conceptual or technological insights.

Physiological mechanisms of G-force intolerance and the relevance of monitoring technologies

High gravitational forces (G-forces), frequently experienced by fighter pilots and astronauts, impose significant physiological stress, especially on cerebral perfusion. If left unaddressed, this can lead to G-induced loss of consciousness (G-LOC). Understanding these mechanisms is crucial for developing monitoring strategies and countermeasures to preserve cognitive function and mission safety. G-force intolerance stems mainly from blood pooling in the lower extremities during +Gz exposure, reducing cerebral blood pressure and oxygen delivery.^14,15 Compensatory responses, such as increased heart rate and systemic vascular resistance, may not suffice during prolonged or high G-loads.^1,5,16 Positive Gz forces (head-to-foot) decrease cerebral perfusion, while negative Gz (foot-to-head) can cause vascular congestion.^17,18 When cerebral oxygenation drops below critical levels, symptoms progress from visual disturbances to G-LOC.¹⁹ Cerebral hypoxia occurs when blood flow is inadequate to meet metabolic demands,²⁰ commonly under high-G conditions.^15,21,22 Although the brain's autoregulatory systems attempt to maintain perfusion, they can be overwhelmed during intense G-loads.^23,24 Impaired CO₂ clearance can worsen hypoxia. Hypocapnia (low CO₂) leads to vasoconstriction, further reducing blood flow. Observations from real-flight conditions further highlight the relevance of cerebral monitoring. For example, NIRS measurements obtained during aerobatic training have demonstrated dynamic fluctuations in cerebral oxygenation and heart rate associated with repeated positive and negative load factors, illustrating how in-flight monitoring can provide valuable insights into pilot physiological responses and potential safety risks during high-performance maneuvers.²⁵ Monitoring both oxygenation and gas exchange is therefore essential to fully understand G-force effects.

Future directions for monitoring improvements

Traditional tools like heart rate and blood pressure are often too delayed or indirect to detect early G-related physiological changes. Real-time, non-invasive technologies are becoming critical for monitoring cerebral and respiratory functions under G-load. Functional near-infrared spectroscopy (fNIRS) tracks cerebral oxygen saturation (rSO₂) in real time. Devices like the NIRSense Aerie have shown the ability to detect deoxygenation trends prior to G-LOC in simulated flight.¹⁶ Other tools, such as transcranial Doppler ultrasound and rheoencephalography, offer additional cerebral hemodynamic data. Capnography, which measures end-tidal CO₂ (EtCO₂), provides valuable information on ventilation and circulatory efficiency. Changes in EtCO₂ can indicate ventilation-perfusion mismatch or reduced cardiac output, key risks in G environments.

Monitoring systems must be sensitive, wearable, and compatible with flight gear.⁵ The combined use of fNIRS and capnography offers a full view of oxygen transport, ventilation and perfusion via EtCO₂, and cerebral uptake via rSO₂.¹⁶ These systems may support future real-time monitoring strategies capable of identifying early signs of physiological decompensation.¹⁹ They offer valuable tools for improving anti-G maneuvers, cockpit design, and even automated safety systems. Ultimately, understanding cardiovascular strain and cerebral hypoxia reinforces the need for real-time monitoring. Technologies like fNIRS and capnography improve physiological awareness and operational safety,^17,23 and will be increasingly important as flight demands intensify.¹

Current monitoring techniques and their limitations

Monitoring physiological responses to high-G environments is essential for pilot safety. While tools like G-suits, Electrocardiography (ECG), Blood Pressure (BP) monitoring, and cognitive tests offer basic insights, they lack the specificity and real-time responsiveness needed to assess cerebral oxygenation and respiratory function - highlighting the need for more advanced, integrated monitoring solutions in aerospace operations. G-suits help maintain cerebral perfusion during +Gz exposure but provide no physiological or real-time data.²⁶ Tilt table testing, used clinically to assess orthostatic tolerance, simulates gravitational stress via posture change but lacks the dynamic realism of actual flight. Centrifuge simulations closely mimic in-flight G-forces and are the gold standard for training and research. However, they are expensive, location-limited, and only offer episodic monitoring, not continuous in-flight physiological tracking.²⁶ These limitations highlight the need for more accessible, real-time monitoring tools in operational aerospace settings. ECG monitors cardiac rhythm and arrhythmias, but motion artifacts and signal issues reduce its reliability during real-time G-exposure.²⁷ Blood pressure monitoring is typically intermittent, and while wearable BP tech shows promise for continuous use, most remain unvalidated prototypes for aerospace settings.²⁸ Cognitive tests assess operational readiness and neurological function but rely on user input and post-event analysis, offering limited real-time value. Their inability to detect acute cognitive decline during G-stress further restricts their effectiveness in high-performance flight environments where immediate, actionable data is essential. Current monitoring systems lack real-time, specific data on cerebral and respiratory function, relying instead on indirect metrics like heart rate or blood pressure. Even when data are available, real-time analysis remains limited, reducing countermeasure effectiveness.^29,30 Motion artifacts under G-loads degrade signal quality across devices,³¹ and many systems are unsuitable for integration into aviation environments.^32,33 Additionally, poor interoperability hinders comprehensive physiological assessment. Integrating data from multiple biosensors faces challenges including software incompatibility, ergonomic constraints, and regulatory hurdles, limiting unified response strategies.^34,35 To overcome current limitations, future efforts should focus on non-invasive, wearable technologies for continuous, high-fidelity monitoring.³⁶ NIRS and capnography provide real-time data on cerebral oxygenation and ventilation-perfusion status, key but underused metrics. Enhancing data interpretation through machine learning and real-time algorithms is vital. Additionally, standardizing protocols, fostering collaboration, and adopting pilot-centric design are crucial for successful implementation in aviation and space environments.^37,38

The role and potential of near-infrared spectroscopy (NIRS) in G-force tolerance monitoring

NIRS is a non-invasive technique that measures changes in cerebral tissue oxygenation using near-infrared light. By detecting oxygenated (O₂Hb) and deoxygenated hemoglobin (HHb) levels, it provides continuous, real-time data on regional cerebral oxygen saturation (rSO₂).^39–42 This is especially valuable in aerospace settings, where G-forces can severely reduce cerebral perfusion. Its portability, motion artifact resistance, and trend-monitoring capability make NIRS a promising tool for enhancing G-tolerance monitoring and safety. NIRS has shown strong potential in aerospace physiology, particularly in tracking rSO₂ during high-G exposure.^8,43,44 Centrifuge-based studies suggest that reductions in cerebral oxygen saturation (rSO₂) frequently occur prior to the onset of G-LOC symptoms, although the magnitude and timing of these changes vary depending on individual physiological responses, G-load intensity, and experimental conditions.¹¹ Reported reductions in rSO₂ typically occur progressively during sustained or rapidly increasing +Gz exposure, but the precise thresholds associated with loss of consciousness remain variable across studies and populations. These observations suggest that NIRS monitoring may provide an early physiological indicator of impending cerebral hypoperfusion, potentially allowing earlier implementation of countermeasures. During pilot training, NIRS helps evaluate AGSM effectiveness by tracking oxygenation levels under stress.^45–47 When AGSMs are properly executed, many trainees maintain or improve cerebral oxygenation, which supports personalized training and performance optimization.¹¹ However, most available evidence derives from centrifuge-based experiments or small pilot cohorts, highlighting the need for further validation in operational flight environments. Beyond training, NIRS facilitates individualized monitoring for predictive modeling of G-tolerance.^48,49 Machine learning approaches have been proposed as potential tools to analyze physiological signals and may support future predictive monitoring systems. Integration into pilot helmets or lightweight headgear has been proposed as a potential approach for real-time monitoring without interfering with cockpit function, which may, after further validation, support future algorithm-based or AI-assisted monitoring systems. Despite these strengths, NIRS has limitations. One major issue is extracranial contamination, where scalp blood flow can distort cerebral-specific readings.^50,51 Other challenges include variability in device protocols, sensitivity to task-related brain activity, and limited penetration depth, which restricts assessment to superficial cortical regions.^50–52 Standardized calibration methods and sensor placement protocols are needed to improve reliability and ensure that measurements reflect true cerebral oxygenation. In conclusion, NIRS offers a powerful complement to traditional monitoring tools by providing real-time cerebral oxygenation data. Its application spans early detection of hypoperfusion, training feedback, and algorithm-based prediction. As a next-generation technology in aerospace medicine, future research should prioritize enhancing its accuracy, integrating it with other modalities, and validating its use in real flight conditions.

Capnography: Physiological role and operational applications

Capnography, the measurement of end-tidal carbon dioxide (EtCO₂) in exhaled air, is a non-invasive, real-time indicator of respiratory and metabolic status.⁵³ Widely used in clinical settings - such as anesthesia, emergency care, and intensive care - capnography provides critical insights into ventilation efficiency, perfusion adequacy, and systemic metabolic function. In high-G environments, it is particularly relevant for monitoring fighter pilots’ physiological status. EtCO₂ serves as a key proxy for ventilation-perfusion balance and cardiac output. Normally ranging from 30–45 mmHg, deviations may signal hypoventilation, hyperventilation, or cardiovascular compromise. In G-stress situations, changes in EtCO₂ can reveal mismatches in gas exchange or impaired cerebral perfusion before clinical symptoms emerge. Importantly, drops in EtCO₂ can precede G-induced Loss of Consciousness (G-LOC), offering a vital early warning. These changes can prompt countermeasures such as anti-G maneuvers or automated flight adjustments. Unlike pulse oximetry, which may remain normal despite hypoxia, capnography captures dynamic physiological shifts more responsively.⁵⁴ In aviation, capnography can significantly enhance situational awareness by enabling continuous monitoring of respiratory and circulatory responses to G-loads.⁵⁵ Its use in emergency and prehospital settings supports its potential in high-performance aircraft. Under G-load, altered blood distribution and increased intrathoracic pressure can affect respiratory mechanics; monitoring EtCO₂ helps assess the adequacy of ventilation and perfusion in real time. By detecting early physiological decline, capnography offers pilots and flight crews actionable data to prevent decompensation. This suggests that capnography could become a valuable tool for supporting in-flight physiological monitoring and decision-making systems. To be viable in aviation, capnography devices must be compact, lightweight, and robust. Current systems include waveform and numerical displays for rapid interpretation. Advances in miniaturization have enabled cockpit integration, but key challenges remain: maintaining calibration across pressure changes, resisting motion artifacts, and integrating with onboard oxygen and communication systems. Comprehensive pilot training is also essential to ensure correct interpretation under stress. Despite these barriers, capnography presents a strong case for broader aerospace adoption. It offers timely, non-invasive insight into respiratory and cardiovascular function, supporting early intervention, improved safety, and enhanced pilot performance in extreme environments.⁵⁵

Synergistic use of NIRS and capnography

Using NIRS and capnography together provides a comprehensive view of oxygen supply and demand in high-G environments.^56,57 NIRS monitors cerebral oxygenation, while capnography assesses ventilation and perfusion. Combined, they can detect early signs of compromised brain oxygen delivery - such as simultaneous drops in rSO₂ and EtCO₂ - potentially predicting G-LOC and which may support the future development of automated safety interventions in integrated cockpit monitoring systems.^46,58 These tools also enhance training by tailoring anti-G strategies to individual responses, and may inform future machine learning models for predictive and personalized physiological monitoring.⁵⁹ A summary comparison of the main characteristics, advantages, and limitations of NIRS and capnography for high-G monitoring is presented in Table 1. In addition, Figure 1 illustrates a conceptual model of how these technologies may be integrated to support real-time physiological monitoring and early detection of G-LOC risk in aviation environments.

Figure 1.

Conceptual model of integrated physiological monitoring in high-G environments.

Table 1.

Comparison of near-infrared spectroscopy (NIRS) and capnography for monitoring physiological responses to high-G exposure.

Feature	Near-infrared spectroscopy (NIRS)	Capnography
Physiological variable measured	Regional cerebral oxygen saturation (rSO₂)	End-tidal CO₂ (EtCO₂)
Physiological domain	Cerebral oxygen delivery and perfusion	Ventilation–perfusion balance and respiratory dynamics
Primary monitoring target	Brain oxygenation	Respiratory and circulatory efficiency
Response to high-G exposure	Detects decreases in cerebral oxygenation caused by reduced cerebral perfusion	Detects ventilation–perfusion mismatch and potential reductions in cardiac output
Advantages	Non-invasive, continuous monitoring of cerebral oxygenation; early indication of cerebral hypoxia	Real-time respiratory monitoring; widely used clinically; sensitive to ventilation changes
Limitations	Extracranial contamination, shallow penetration depth, variability between individuals	Sensitive to respiratory mechanics changes, motion artifacts, and pressure variations
Operational integration	Helmet-mounted sensors or lightweight headgear	Integrated into pilot oxygen masks or breathing circuits
Potential operational applications	Early detection of cerebral hypoperfusion, monitoring AGSM effectiveness, individualized G-tolerance assessment	Monitoring ventilation adequacy and circulatory changes during high-G maneuvers
Role in integrated monitoring	Provides direct information on cerebral oxygenation	Complements NIRS by monitoring respiratory and systemic physiological responses

Limitations and challenges of NIRS and capnography in G-force tolerance monitoring

While NIRS and capnography hold promise for physiological monitoring in high-G environments, several limitations must be addressed before their reliable integration into operational aerospace systems. These challenges include technical constraints, environmental influences, and broader issues related to usability, data interpretation, and regulatory compliance.

Technical and operational limitations

A major challenge for NIRS is extracranial contamination. Signals detected through the scalp and skull can distort cerebral oxygen saturation (rSO₂) readings, particularly under high-G conditions, where superficial blood flow increases and reduces measurement accuracy.¹³ Additionally, NIRS has shallow tissue penetration, primarily capturing data from the superficial frontal cortex. This limits its ability to detect ischemia in deeper or posterior brain regions during G-stress.^13,29,42 Variability in scalp vascularity between individuals may further distort readings, producing misleading results such as elevated rSO₂ during actual hypoperfusion.^4,7 External conditions - such as hypotension, rapid hemodynamic shifts, and ambient light - can further compromise signal integrity, especially during high-motion flight scenarios.¹³ These issues highlight the need for improved algorithms and robust, motion-resistant sensors. Capnography also presents limitations in high-G contexts. EtCO₂ readings may be affected by changes in respiratory mechanics, ventilation-perfusion mismatch, and motion artifacts during flight.⁵⁵ Additionally, capnographs must be adapted to perform reliably under rapid acceleration and varying pressures, while remaining lightweight and compatible with cockpit equipment. Interpreting EtCO₂ values under G-load is complex. A drop in EtCO₂ could indicate multiple physiological conditions, such as hyperventilation or reduced cardiac output with distinct implications. Without integration with cerebral data from NIRS, capnography alone may lack diagnostic specificity during flight.

Validation and implementation challenges

For cockpit integration, monitoring systems must be compact, durable, and non-intrusive. Current designs often fail to maintain signal quality in high-vibration environments and require better attachment mechanisms for consistent contact. Moreover, real-time data interpretation depends on sophisticated algorithms that distinguish between critical deterioration and benign variation - otherwise risking false alarms and desensitization. Most validation studies occur in controlled settings; few assess these technologies in real-world flight. High-G conditions such as multi-axis acceleration and pressure variation introduce confounding variables. Large-scale, in-flight testing is needed to confirm reliability and define operational protocols. Successful implementation also depends on certification, data privacy, and user training. Wearable systems must be comfortable and non-distracting. Pilot acceptance is critical, and continuous monitoring raises ethical concerns, especially in mixed civil-military settings. In addition to the technological and operational challenges discussed above, this review has limitations inherent to its narrative design. As a narrative review, the manuscript does not follow a fully systematic study selection process and is therefore subject to potential selection bias.⁶⁰ Although the literature search was guided by defined databases, time-frame, and topic-related keywords, study inclusion and interpretation were based on thematic relevance rather than formal systematic review procedures. Furthermore, no quantitative synthesis was performed. These factors should be considered when interpreting the conclusions of the present review.

Future research directions for NIRS and capnography in G-force tolerance monitoring

As aerospace operations evolve, G-induced loss of consciousness (G-LOC) remains a critical safety concern. Despite countermeasures like G-suits and anti-G maneuvers, G-LOC continues to affect pilots and astronauts. There is a growing need for real-time physiological monitoring to predict and prevent such events. NIRS and capnography offer complementary, non-invasive insights into cerebral oxygenation (rSO₂) and ventilation-perfusion status (EtCO₂), making them promising tools for addressing G-tolerance. Future research should focus on validating, optimizing, and operationalizing these technologies - especially in pilot populations. Several proposed applications remain conceptual and require validation in operational aviation environments.

Enhancing NIRS technology: Accuracy, integration, and wearability

Improving NIRS accuracy is essential. Extracranial contamination remains a key issue, requiring algorithmic corrections and refined optode designs to isolate cerebral signals.¹³ Motion artifacts also limit reliability during flight; thus, motion-resistant, skin-adaptive headgear is needed.⁶¹ Integrating NIRS with other metrics, like EEG or cardiac force index (CFI), could improve G-tolerance prediction by enabling multimodal physiological profiles.⁴⁶

Capnography: Advancing respiratory monitoring in flight

Capnography remains underutilized in aerospace despite its capacity to track ventilation, perfusion, and cardiac output. Research should examine how EtCO₂ changes under G-loads and correlates with cerebral oxygenation and G-LOC risk. Devices must be engineered to perform in pressurized, vibration-heavy cockpits. Predictive modeling using capnographic data and machine learning could identify individual baselines and early decompensation patterns.⁶²

Controlled human centrifuge trials and algorithm development

Centrifuge trials are crucial to validate sensor performance and establish physiological thresholds across varied G-loads and durations.^63,64 These studies may help inform the future development of algorithm-based alert systems for real-time data synthesis and early warning. Adaptive machine learning approaches have been proposed as a possible means of accounting for individual variability and complex G-profiles, but these applications still require substantial validation before operational use can be considered.^65–70

Pilot screening and individualized training protocols

NIRS and capnography can support individualized screening by identifying low G-tolerance or atypical responses early.^11,71,72 This data can shape customized training and countermeasure plans, improving long-term resilience.^70,73 Incorporating metrics like aerobic capacity and cardiovascular health may further reduce risk and optimize pilot readiness.⁷⁴

Operational challenges and collaborative development

Physiological responses vary significantly, making fixed thresholds inadequate.¹⁷ Personalized analytics and robust engineering solutions are needed to address device bulk, interface design, and system integration. Cross-sector collaboration between aerospace agencies, manufacturers, and researchers is vital to create standardized, scalable monitoring systems suitable for operational flight.

Conclusion

Integrating NIRS and capnography into aerospace monitoring marks a key advancement in enhancing human performance and safety. These tools provide complementary insights into cerebral and respiratory function, essential for preventing G-LOC. Realizing their full potential requires improved device accuracy, smart data interpretation, and real-world validation. With continued technological development, validation in operational environments, and interdisciplinary collaboration, these technologies may eventually support advanced physiological monitoring strategies in future flight systems.

Footnotes

Abbreviations

Acknowledgments

The authors would like to thank the Portuguese Air Force for its support and collaboration in the conceptualization and development of this manuscript.

ORCID iDs

João Bruno

Rohit Kumar Thapa

Hugo Sarmento

Ethical approval

Not applicable. This article is a narrative review and does not involve research with human or animal participants.

Author contributions

João Bruno conceptualized the manuscript, conducted the literature synthesis, and led the drafting and revision of the text. Rohit Kumar Thapa contributed to the review of respiratory physiology literature and co-drafted sections related to capnography and its applications. Raynier Montoro-Bombú supported the literature review on aerospace physiology and helped structure the theoretical framework of G-tolerance. Hugo Sarmento provided methodological oversight, critical revisions, and final approval of the manuscript. All authors have read and approved the final version of the manuscript.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

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