The duration of two carbon dioxide absorbents in a closed-circuit rebreather diving system
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Harvey D, Pollock NW, Gant N, Hart J, Mesley P, Mitchell SJ. The duration of two carbon dioxide absorbents in a closed-circuit rebreather diving system. Diving and Hyperbaric Medicine. 2016 June;46(2):92-97.) Introduction: Diving rebreathers use canisters containing sodalime preparations to remove carbon dioxide (CO 2) from the expired gas. These preparations have a limited absorptive capacity and therefore may limit dive duration. The Inspiration™ rebreather is designed for use with Sofnolime 797™ but some divers use Spherasorb™ as an alternative. There are no published data comparing the CO 2-absorbing efficacy of these sodalime preparations in an Inspiration rebreather. Methods: An Inspiration rebreather was operated in a benchtop circuit under conditions simulating work at 6 metabolic equivalents (MET). Ventilation was maintained at 45 L·min-1 (tidal volume 1.5 L; respiratory rate 30 min-1) with CO 2 introduced to the expiratory limb at 2 L • min-1. The P I CO 2 was continuously monitored in the inspiratory limb. The rebreather canister was packed to full volume with either Sofnolime or Spherasorb and 10 trials were conducted (five using each absorbent), in which the circuit was continuously run until the P I CO 2 reached 1 kPa ('breakthrough'). Peak inspiratory and expiratory pressures during tidal ventilation of the circuit were also recorded. Results: The mean operating duration to CO 2 breakthrough was 138 ± 4 (SD) minutes for 2.38 kg Spherasorb and 202 ± minutes for 2.64 kg Sofnolime (P < 0.0001). The difference between peak inspiratory and expiratory pressures was 10% less during use of Spherasorb, suggesting lower work of breathing. Conclusions: Under conditions simulating work at 6 MET during use of an Inspiration rebreather a canister packed with Spherasorb reached CO 2 breakthrough 32% earlier with 10% less mass than Sofnolime packed to similar volume. Divers cannot alternate between these two preparations and expect the same endurance.
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Although considered relatively safe with modern technology, diving and underwater exploration went through perilous milestones from its practices in the early days. Today there are still a number of risks and health consequences associated with underwater exposure. Diving enforces a number of physiological alterations to the human body. Water clearly has different physical properties compared to the atmospheric conditions the human body is used to, and there are significant physiologic effects of both, pressure and immersion. In order to understand and subsequently investigate any physiological changes due to immersion, a fundamental knowledge of underwater physics is required. The physical laws governing the behaviour of gas under pressure are introduced. Nearly all physiological systems are subjected to changes due to the prevailing underwater physics. In this chapter, we discuss the physiology of immersion, high ambient pressure exposure, and exposure to individual gases. The physiology of immersion focuses on the effects of the diving reflex, respiratory implications and thermal considerations for a diver. While the physiology of high pressure exposure includes the effects of pressure on the volume of gas and air spaces in the human body, the physiology of exposure to individual gases has physiological and neurological consequences due to the narcotic and toxic effects of individual gases.
Combined effects on respiratory minute ventilation (VE)--and thus, on end-tidal carbon dioxide partial pressure (P(ET)CO2)--of breathing resistance and elevated inspired carbon dioxide (CO2) had not been determined during heavy exercise. In this Institutional Review Board-approved, dry, sea-level study, 12 subjects in each of three phases exercised to exhaustion at 85% peak oxygen uptake while V(E) and P(ET)CO2 were measured. Participants inhaled 0%, 1%, 2% or 3% CO2 in air, or 0% or 2% CO2 in oxygen, with or without breathing resistance, mimicking the U.S. Navy's MK 16 rebreather underwater breathing apparatus (UBA). Compared to air baseline (0% inspired CO2 in air without resistance): (1) Oxygen decreased baseline V(E) (p < 0.01); (2) Inspired CO2 increased V(E) and P(ET)CO2 (p < 0.01); (3) Resistance decreased V(E) (p < 0.01); (4) Inspired CO2 with resistance elevated P(ET)CO2 (p < 0.01). In air, V(E) did not change from that with resistance alone. In oxygen, V(E) returned to oxygen baseline. End-exercise P(ET)CO2 exceeded 60 Torr (8.0 kPa) in three tests. Subjects identified hypercapnia poorly. Results support dual optimization of arterial carbon dioxide partial pressure and respiratory effort. Because elevated CO2 may not increase V(E) if breathing resistance and VE are high, rebreather UBA safety requires very low inspired CO2.
Cardiac events are responsible for a significant proportion of recreational diving fatalities. It seems inescapable that our current systems for selecting suitable recreational diver candidates and for longitudinal monitoring of diver health are failing to exclude some divers at high risk of cardiac events. Based on review of practice in parallel sporting disciplines and of the relevant literature, a series of recommendations for screening questions, identification of disqualifying conditions and risk factors, and investigation of candidates with risk factors was drafted. Recommendations for ongoing health monitoring in established divers were also generated. These recommendations were promulgated and debated among experts at a dedicated session of the Divers Alert Network Fatality Workshop. As a result, we propose a modified list of screening questions for cardiovascular disease that can be incorporated into health questionnaires administered prior to diver training. This list is confluent with the American Heart Association (AHA) preparticipation screen for athletes. The exercise stress test unmasks inducible cardiac ischemia and quantifies exercise capacity, and remains the tool of choice for evaluating diver candidates or divers with risk factors for coronary disease. An exercise capacity that allows for sustained exercise at a 6-MET intensity (possibly representing a peak capacity of 11-12 METS) is an appropriate goal for recreational divers.
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. © 2011 American Physiological Society. Compr Physiol 1:163-201, 2011.
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Intersorb 8 to 12 mesh indicating and non-indicating comparison with Sofnolime 797
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