Jochen D. Schipke

Breath-hold diving exposes humans to repeated episodes of profound hypoxia and hypercapnia, eliciting physiological adaptations that enable prolonged underwater performance. This article summarises current knowledge on chronic adaptations in elite breath-hold athletes and traditional diving populations, including the Bajau sea nomads of Southeast Asia and the Korean Haenyeo divers. Evidence indicates that repeated apnoea induces adaptations across multiple physiological systems. Haematological changes include increased spleen size and enhanced splenic contraction, augmenting circulating haemoglobin and oxygen stores during apnoea. In elite divers, structured training can increase resting spleen volume, whereas the Bajau exhibit genetically associated splenic enlargement linked to variants near the PDE10A gene. Cardiopulmonary adaptations include modified pulmonary vascular responses to hypoxia, improved oxygen conservation, and metabolic shifts favoring efficient mitochondrial energy production. Molecular adaptations involve enhanced antioxidant defenses and activation of hypoxia-responsive pathways that may mitigate oxidative stress associated with repeated hypoxia-reoxygenation cycles. Emerging evidence also suggests neural plasticity and possible structural brain adaptations, although the long-term neurological consequences of chronic intermittent hypoxia exposure remain uncertain. Studies of traditional diving populations indicate that both phenotypic plasticity and genetic selection contribute to diving capacity, highlighting interactions between training and evolution. Despite these benefits, breath-hold diving also carries risks, including hypoxic blackout, decompression sickness, and potential neurological injury. Understanding the mechanisms underlying human tolerance to extreme hypoxia may have implications beyond diving physiology, including applications in cardiovascular medicine, hypoxic diseases, and rehabilitation. Further longitudinal, genomic, and mechanistic studies are needed to clarify the limits, benefits, and clinical relevance of these adaptations.

Introduction: In diving medicine, critical flicker fusion frequency (cFFF) has been employed to assess alertness under varying ambient pressures and gas compositions, yet the effects of elevated oxygen partial pressure on cFFF performance underwater remain inconsistently characterised. This investigation aimed to assess the impact of breathing 140 kPa oxygen on underwater cFFF performance, a pressure level corresponding to established safety limits and common closed-circuit rebreather setpoints during operational diving. Methods: Nineteen experienced divers (10 females; mean age 23.8 [SD 3.0] years) completed cFFF assessments under three conditions: breathing air at poolside (21 kPa O2), breathing air at 4 m depth (29 kPa O2), and breathing 100% oxygen at 4 m depth (140 kPa O2). Concurrent measurements included heart rate, respiratory rate, and heart rate variability indices representing autonomic function; specifically the LF/HF ratio indicating sympathetic activity and RMSSD reflecting parasympathetic activity. Results: Mean cFFF breathing air at poolside was 37.9 (SD 2.9) Hz and breathing 100% oxygen at 4 m was 37.2 (SD 2.5) Hz, indicating that mild hyperoxia at 140 kPa does not impair cortical processing speed or visual temporal resolution. However, oxygen breathing produced notable cardiovascular and respiratory effects: heart rate decreased from 70.9 (10) to 67.8 (11.6) beats per minute, respiratory rate declined from 15.2 (2.7) to 13.4 (1.7) breaths per minute, the LF/HF ratio decreased from 5.9 (1.4) to 3.1 (2.0), and RMSSD increased from 73.7 (10.8) to 97.0 (6.6) milliseconds. These physiological changes indicate reduced sympathetic activity and enhanced parasympathetic tone during oxygen breathing. Conclusions: The findings suggest that breathing 140 kPa oxygen during shallow water immersion maintains neural integrity as measured by cFFF while beneficially modulating autonomic regulation toward parasympathetic dominance. This autonomic shift resembles known effects of both submersion and moderate hyperoxia, supporting the safety profile of current oxygen exposure protocols in recreational and technical diving.

The critical flicker fusion frequency (cFFF) is a non-invasive measure of central nervous system function and cortical arousal, increasingly used in diving and hyperbaric medicine to assess the effects of breathing gases under pressure. This feasibility study aimed to evaluate whether cFFF can be reliably measured through the porthole of a hyperbaric chamber. Forty-five experienced male divers underwent cFFF testing at various pressures (101.3 kPa outside chamber, then 101.3, 608, 132, 101.3 kPa inside [1.0 bar outside then 1.0, 6.0, 1.3, 1.0 bar inside]) using a manually operated LED flicker-device while standing at a fixed distance from the chamber window. Results showed that cFFF values were higher inside the chamber at 101.3 kPa (1.0 bar) compared to outside (45.6 Hz vs. 40.2 Hz), decreased under hyperbaric conditions (608 kPa [6 bar], 43.5 Hz), and declined further during decompression (132 kPa [1.3 bar], 42.1 Hz; 101.3 kPa [1.0 bar], 43.5 Hz). These findings support previous observations of gas-induced central nervous system effects and highlight the sensitivity of cFFF to pressure-related neural changes. The successful external measurement protocol addresses challenges associated with observer narcosis and movement artifacts in underwater settings. While limited by the homogenous participant group and lack of confirmatory measures, this approach may still be a valuable tool for future research into the temporal dynamics of gas narcosis and cortical excitation.

Introduction: Critical flicker fusion frequency (cFFF) measures visual perception, with higher values indicating better performance. It is widely used to assess the effects of breathing gases in diving and hyperbaric environments. Aim: Identify confounders explaining inconsistent cFFF study results. Methods: Four experiments using a manual flicker device in volunteers with healthy eyes: 1. cFFF was measured in 174 recreational divers to examine demographic effects. 2. One male diver was tested daily over 85 days. 3. In 28 divers, cFFF was measured before and after 5 min of normobaric oxygen. 4. In 24 divers, cFFF was tested with and without diving masks, plus a condition with diagonal flicker light on the mask. Results: 1. Mean cFFF: 39.2 ± 5.1 Hz; decreased with age ( p < 0.001), no gender/BMI effect. 2. Mean: 43.4 ± 2.1 Hz; stable across lighting. 3. Thresholds differed by frequency order; minimal O₂ effect. 4. Masks had no effect, but diagonal light reduced cFFF ( p < 0.01). Conclusion: cFFF varied by 13% across divers and 5% in a single diver. Age and observer-light geometry influenced results. Oxygen had minor impact. Findings support using age-specific norms and considering perceptual confounders in diving research.

TO THE AUTHORS This is a comprehensive study with many participants, over a long period, and with numerous variables. The problem with 'big data' is that statistics may provide appealing associations, but not causalities. And indeed, a lot of stati...

Background and Objectives: Scuba divers often experience persistent inert gas narcosis (IGN) even after surfacing. This study aimed to test the hypothesis that breathing oxygen (O₂) before surfacing can reduce postdive IGN. Materials and Methods: A group of 58 experienced divers underwent a 5 min dive at a depth of 50 m in a multi-place hyperbaric chamber. They were decompressed using air (air group). Another group of 28 divers (O₂ group) breathed 100% O₂ during the end of decompression. Prior to and after the dive, all participants performed the Sharpened Romberg test (SRT) and a modified tweezers test. Results: In the air group, the number of positive SRT results increased postdive (47% vs. 67%), indicating a greater impairment in the vestibular system (Cohen’s d = 0.41). In the O₂ group, the percentage of positive SRT results remained constant at 68% both before and after the dive. In terms of the modified tweezers test, the air group showed no significant change in the number of picked beads (40 ± 9 vs. 39 ± 7), while the O₂ group demonstrated an increase (36 ± 7 vs. 44 ± 10) (Cohen’s d = 0.34). Conclusion: The results reveal that the SRT revealed a negative effect of nitrogen (N₂) on the vestibular system in the air group. The increased number of beads picked in the O₂ group can be attributed to the learning effect, which was hindered in the air group. Consistent with our hypothesis, breathing O₂ during decompression appears to reduce postdive IGN.

First and foremost, we like to express our gratitude for the praise bestowed upon our narrative review

The critical flicker fusion frequency (cFFF) refers to the frequency at which a regularly recurring change of light stimuli is perceived as steady. The cFFF threshold is often assessed in clinics to evaluate the temporal characteristics of the visual system, making it a common test for eye diseases. Additionally, it serves as a helpful diagnostic tool for various neurological and internal diseases. In the field of diving/hyperbaric medicine, cFFF has been utilized to determine alertness and cognitive functions. Changes in the cFFF threshold have been linked to the influence of increased respiratory gas partial pressures, although there exist inconsistent results regarding this effect. Moreover, the use of flicker devices has produced mixed outcomes in previous studies. This narrative review aims to explore confounding factors that may affect the accuracy of cFFF threshold measurements, particularly in open-field studies. We identify five broad categories of such factors, including (1) participant characteristics, (2) optical factors, (3) smoking/drug use, (4) environmental aspects, and (5) breathing gases and partial pressures. We also discuss the application of cFFF measurements in the field of diving and hyperbaric medicine. In addition, we provide recommendations for interpreting changes in the cFFF threshold and how they are reported in research studies.