Hyperbaric Conditions

Abstract

Exposure to elevated ambient pressure (hyperbaric conditions) occurs most commonly in underwater diving, during which respired gas density and partial pressures, work of breathing, and physiological dead space are all increased. There is a tendency toward hypercapnia during diving, with several potential causes. Most importantly, there may be reduced responsiveness of the respiratory controller to rising arterial CO₂, leading to hypoventilation and CO₂ retention. Contributory factors may include elevated arterial PO₂, inert gas narcosis and an innate (but variable) tendency of the respiratory controller to sacrifice tight control of arterial CO₂ when work of breathing increases. Oxygen is usually breathed at elevated partial pressure under hyperbaric conditions. Oxygen breathing at modest hyperbaric pressure is used therapeutically in hyperbaric chambers to increase arterial carriage of oxygen and diffusion into tissues. However, to avoid cerebral and pulmonary oxygen toxicity during underwater diving, both the magnitude and duration of oxygen exposure must be managed. Therefore, most underwater diving is conducted breathing mixtures of oxygen and inert gases such as nitrogen or helium, often simply air. At hyperbaric pressure, tissues equilibrate over time with high inspired inert gas partial pressure. Subsequent decompression may reduce ambient pressure below the sum of tissue gas partial pressures (supersaturation) which can result in tissue gas bubble formation and potential injury (decompression sickness). Risk of decompression sickness is minimized by scheduling time at depth and decompression rate to limit tissue supersaturation or size and profusion of bubbles in accord with models of tissue gas kinetics and bubble formation and growth.

Hypercapnia in diving: a review of CO2 retention in submersed exercise at depth

“…Despite this increased work of breathing, divers are prone to total pulmonary hypoventilation with exercise at depth, leading to progressive accumulation of CO 2 . As discussed by Doolette and Mitchell [26], this hypoventilation is not entirely explained by an insurmountable increase in WOB at depth. Hypoventilation and hypercapnia have been shown to occur even during submaximal exercise under hyperbaric conditions [27,[61][62][63].…”

“…(2) A = A 0 (ρ/ρ 0 ) -k where A is either MVV or peak expiratory flow at gas density ρ (related to the ambient pressure of the diver), A 0 is MVV or peak expiratory flow at 1 ATA, ρ 0 is gas density at 1 ATA and k is a constant (valued at 0.4-0.5) [55]. The mechanisms by which diving reduces MVV have been previously explored in detail [26,41]. Briefly, MVV is reduced in diving due to reductions in both inspiratory and expiratory flow.…”

“…Briefly, MVV is reduced in diving due to reductions in both inspiratory and expiratory flow. During expiration of dense gas, a small increase in flow requires a greater increase in pleural pressure [56], partially due to dynamic airway compression at lower flow rates [26]. Inspiratory flow is not affected by dynamic airway compression, but is similarly reduced at high gas densities, contributing to an overall reduction in MVV [51].…”

“…At this time, hypercapnia remains an unpredictable and sometimes lethal risk of diving. Yet, we are aware of only one recent comprehensive review of hypercapnia in diving [26]. This review outlines what is currently known about hypercapnia in diving -its measurement, cause and effects -and further explores two relevant topics: the relationship between end-tidal and arterial PCO 2 under submersed exercise conditions and the role of hyperoxia in the development of divingrelated hypercapnia, including a secondary analysis of four previously published studies.…”

DECOMPRESSION FROM He-N2-O2 (TRIMIX) BOUNCE DIVES IS NOT MORE EFFICIENT THAN FROM He-O2 (HELIOX) BOUNCE DIVES

“…Differences in gas uptake and washout and bubble growth can manifest in a difference in decompression obligation for dives conducted breathing heliox and nitrox. For instance, nitrogen washes out more slowly than helium from body tissues with slow gas exchange, [3][4][5] and this probably underlies the slower required rate of decompression from nitrox saturation dives than from heliox saturation dives 6,7 (a saturation dive is one of sufficient duration for all the body tissues to completely equilibrate with inspired inert gas partial pressures). A related phenomenon manifests in decompression algorithms used to schedule bounce dives.…”