APNEA DIVERS: HYPOXYPHILIACS, ATHLETES, OR VALUABLE MEDICAL RESEARCH SUBJECTS?
by Lynne Ridgway, PhD cand, and Ken McFarland, PhD
Neuroscience in the School of Psychology University of Queensland, Brisbane, Queensland, Australia
‘Hypoxic brain damage’ is a common diagnosis for patients referred for assessment and rehabilitation and is seen as the culprit for the cognitive difficulties experienced by patients with sleep apnea, chronic obstructive pulmonary disorder, and high altitude mountain climbers. Documented neuropsychological changes include poor memory, attention, judgment and decision making, visual and motor problems, headache and fatigue. However, animal models (3,4) and some clinical case studies (1,2,5,7) have found no cell death, or neurocognitive changes respectively following extended hypoxemia. Elite healthy apnea divers provide an excellent model of neuropsychological functioning following repetitive, long-duration pure hypoxemia in the absence of ischemia, illness or sleep deprivation confounds usually present in patients experiencing hypoxia.
Aims and hypotheses: To examine the long term or cumulative effects from repetitive long duration apneas with multiple negative neurological events (NNE = loss of consciousness or loss of motor control).
1) Elite apnea divers would demonstrate worse than expected performance on standardized neuropsychological tests compared to population normative data.
2) That there would be a cumulative effect such that those divers with the greatest number of years of apnea experience and/or reported over their apnea career would score lower on the neuropsychological tasks compared to their peers.
A literature review was conducted to examine the physiological and neurological adaptations involved in the ‘mammalian diving reflex’ and across similar hypoxic conditions in occupational, recreational, and medical conditions. A comprehensive neuropsychological evaluation of 21 English- speaking elite apnea divers participating in the 2002 Hawaii Pacific Cup of Freediving was conducted. A battery of tests with known sensitivity to minor alterations to brain functioning included basic neurological observations, estimates of premorbid intelligence, speed and accuracy of visuo- motor responding, speed of language comprehension, response inhibition, and visual and verbal attention and memory tasks. 10 Australian, 4 American and 7 English divers (12 males, 9 females) gave informed consent and were tested in conditions designed to simulate the competition environment to allow for repeat testing in later planned studies of the acute effects of apnea. None of the divers had engaged in apnea or in substance use for a period of at least 12 h prior to the testing. For a complete description of methods and procedures see Ridgway and McFarland (6).
Divers were 31.0± 7.2 (mean ± standard deviation) years of age, had 13.6±2.0 years of education, 6.3±6.4 years of apnea activity, 4.9±0.23 min recent competition static apnea time, 111±9 estimated intelligence quotient (IQ), and 5.1±6.2 total number of lifetime NNE. Neuropsychological testing revealed that as a group, the 21 elite apnea divers performed tasks within one standard deviation of published norms adjusted for age and education. Multivariate general linear modeling revealed no significant correlations or group differences on cognitive functioning between the divers according to the number of previous NNE (n= 8 with 0-2 NNE; n= 7 with 3-5 NNE; and n=6 with >6 NNE) (F(2,18) 0.47 p = 0.63).
These results suggest that 1-20 years of repeated exposure to hypoxemia and multiple negative neurological events including previous concussions, apnea related loss of consciousness and loss of motor control does not impact on cognitive functioning as measured by standardized sensitive neuropsychological tests. While no attempt was made to answer extraneous questions of hypoxyphilia we conclude that this group of elite athletes provides medicine with a valuable model to examine neuropsychological functioning in pure hypoxemia without the confounding effects of aging, ischemia, illness or sleep deprivation observed in other clinical models of hypoxia.
One explanation for this surprising result is that trained apnea divers may benefit from maximizing their adaptive response to low oxygen as predicted by the mammalian diving reflex and observed in physiological studies of mountaineers, clinical conditions and apnea divers. It was beyond the scope of this study to investigate if the mammalian dive reflex provides a mechanism for neuroprotection.
Alternate explanations for the null result are that the relative above average intelligence of this group of divers may have obscured any decline in cognitive functioning. Therefore, we would caution against assuming that an absence of statistically significant cognitive decline implies that multiple NNE does not cause persisting deficits in individuals. Further studies on the acute neurocognitive effects of engaging in long duration apneas with and without NNE are planned.
The authors would like to thank the organizers and participants of the 2002 Pacific Cup of Freediving for their immense support for this research.
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DR. MUTH: Thank you very much for your presentation. This is a very exciting and important area of research, because we really do not know anything about long-term effects on the brain. Being diving doctor on one side and an anesthesiologist on the other, I am very interested in drowning.
You have to be very careful to compare those two kinds of diving, apnea diving and drowning, because all those cases where we had protective effects were associated to drowning. And it was not the blood flow itself, it was the transportation of low temperature to the brain, the cooling down of the brain which was protective, which slows down the metabolism. In all those cases where people drowned because of a heart attack, we have no protective effects. And only when the heartbeat was going on and transported code temperature, code blood through the brain, we have this effect. So be cautious to take this two-part. This is like comparing apples with peas.
NOTE: To access the entire proceedings of the UHMS DAN 2006 Breath-hold Proceedings, visit Divers Alert Network.
In: Lindholm P, Pollock NW, Lundgren CEG, eds. Breath-hold diving. Proceedings of the Undersea and Hyperbaric Medical Society/Divers Alert Network 2006 June 20-21 Workshop. Durham, NC: Divers Alert Network; 2006.