Hypercapnia in diving: a review of CO₂ retention in submersed exercise at depth.
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Carbon dioxide (CO₂) retention, or hypercapnia, is a known risk of diving that can cause mental and physical impairments leading to life-threatening accidents. Often, such accidents occur due to elevated inspired carbon dioxide. For instance, in cases of CO₂ elimination system failures during rebreather dives, elevated inspired partial pressure of carbon dioxide (PCO₂) can rapidly lead to dangerous levels of hypercapnia. Elevations in PaCO₂ (arterial pressure of PCO₂) can also occur in divers without a change in inspired PCO₂. In such cases, hypercapnia occurs due to alveolar hypoventilation. Several factors of the dive environment contribute to this effect through changes in minute ventilation and dead space. Predominantly, minute ventilation is reduced in diving due to changes in respiratory load and associated changes in respiratory control. Minute ventilation is further reduced by hyperoxic attenuation of chemosensitivity. Physiologic dead space is also increased due to elevated breathing gas density and to hyperoxia. The Haldane effect, a reduction in CO₂ solubility in blood due to hyperoxia, may contribute indirectly to hypercapnia through an increase in mixed venous PCO₂. In some individuals, low ventilatory response to hypercapnia may also contribute to carbon dioxide retention. This review outlines what is currently known about hypercapnia in diving, including its measurement, cause, mental and physical effects, and areas for future study.
Dr Freiberger works on the translation of basic science research on reactive oxygen species signaling into clinical practice involving hyperbaric oxygen (HBO). He has performed animal experiments in the use of HBO for ischemic preconditioning and he is currently funded to conduct a randomized controlled trial of the use of HBO for the treatment of bisphosphonate-induced osteonecrosis of the jaw. The mechanisms of action for HBO in the treatment of: diabetic wounds, bony and soft tissue radionecrosis and decompression sickness are also areas of his inquiry. Dr Freiberger also does basic epidemiological research into accidents and injuries associated with diving, high altitude exposure and other adverse conditions associated with extreme environments. Dr. Freiberger directs the fellowship program at the Duke Center for Hyperbaric Medicine and Environmental Physiology.
Research interests include the study of cardiorespiratory function in humans during challenging clinical settings including the perioperative period, and exposure to environmental conditions such as diving and high altitude. Studies have included gas exchange during diving, the pathophysiology of high altitude and immersion pulmonary edema, the effect of anesthesia and postoperative analgesia on pulmonary function and monitoring of tissue oxygenation. Ongoing human studies include the effect of respiratory muscle training on chemosensitivity and blood gases during stressful breathing: underwater exercise.
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