Realización del proceso de Devolución de equipos policiales

EXCHANGE

Bubbles reduce the rate at which nitrogen is eliminated from tissue because nitrogen in a bubble must diffuse back into tissue before it can be transported by the circulation to the lungs (see Fig. 4–12E). Thus, the elimination of nitrogen from a bubble is slower than the elimination of nitrogen dissolved in tissue.

This has been demonstrated in both animal^65–67and human studies.^68–70

Most decompression models assume that bubbles do not form, but when bubbles are present, diffusion between bubble and tissue cannot be ignored. Diffusion is a simple physical process but difficult to describe mathematically. Figure 4–16 is a schematic representation of diffusion from a bubble filled with either oxygen, nitrogen, or helium into an adjacent perfusion-limited tissue.⁷¹ Because oxygen is metabolized in tissue, its tension falls rapidly with increasing distance from the bubble. Helium and nitrogen, on the other hand, are metabolically inert and are removed only by perfusion, so their concen-tration gradients extend deeper into tissue.

Helium penetrates further into tissue than nitrogen does because its diffusivity is greater.

The diffusion process is often simplified to make it more manageable mathematically.

Figure 4–17 shows three representations of diffusion in decreasing order of complex-ity. Figure 4–17A illustrates the situation depicted in Figure 4–16 in which diffusion is a continuous process throughout tissue.

Figure 4–15. The oxygen window as a function of inspired oxygen partial pressure. The values from Momsen⁵³are predictions, while the values from Hills⁶² and Hills and LeMessurier⁵⁴are measurements. The oxygen window in tissue does not increase indefinitely but reaches a maximum value, which is determined by the arteriovenous oxygen extraction.

(Redrawn from Van Liew.⁶¹)

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Momsen⁵³

Hills⁶², Hills and Le Messurier⁵⁴ Van Liew et al⁶² 20

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Diffusion into and out of the bubble is repre-sented by curvilinear concentration gradi-ents indicating bubble growth or resolution.

The difference between the inert gas partial pressure in the bubble and the dissolved inert gas tension in tissue is a consequence of the oxygen window.

In Figure 4–17B, the entire tissue region around the bubble is considered to be well-stirred, and all diffusion resistance is con-centrated in a barrier around the bubble.

This is the basis of Gernhardt’s decom-pression algorithm and commercial diving decompression schedules.^72,73

A further mathematical simplification in Figure 4–17C omits the diffusion barrier around the bubble such that the inert gas partial pressure in the bubble and the dis-solved inert gas tension in tissue are equal.

Hills offered the first analysis of this problem,⁶² which was later refined by Hennessy and Hempleman.⁷⁴ In this circum-stance of diffusion equilibrium between bubble and tissue, when a nitrogen molecule enters tissue from the arterial blood, another molecule moves from the tissue to the bubble. The reverse is also true, and the bubble shrinks by one molecule when there is a net loss of one inert gas molecule from tissue to venous blood.

As illustrated in Figure 4–18A, suppose a bubble forms in a diffusion-equilibrium tissue (see Fig. 4–17C) upon decompression from 60 fsw (18 msw; 2.8 ata) to sea level (1 ata). Because the bubble and tissue are in diffusion equilibrium, all supersaturated nitrogen dissolved in tissue immediately diffuses into the bubble. Figure 4–18B shows how the nitrogen tension in tissue changes with time when a diffusion-equili-brium bubble is (or is not) present. If no bubble forms, nitrogen uptake and elimi-nation follow the exponential kinetics expected of a well-stirred tissue, but the presence of a bubble causes the tissue nitro-gen tension to fall to a level defined by the oxygen window (equation 4–3) and to remain constant as long as the bubble is present.

Although the nitrogen tension in tissue and the partial pressure in the bubble are equal and constant, perfusion removes nitrogen from tissue and the bubble volume resolves at a linear rate (Fig. 4–18C ). When the bubble is gone, nitrogen kinetics revert to an exponential function. Vann described the mathematics of inert gas exchange in a diffusion-equilibrium bubble.⁷⁵

Diffusion-equilibrium bubbles are the basis of Thalmann’s exponential-linear (E-L) decompression model.^76,77 The nitrogen

Oxygen Nitrogen Helium Tissue

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Figure 4–16. Tissue tension gradients around a gas cavity (redrawn from Hlastala⁷¹). The oxygen gradient is steepest because oxygen is removed both by the circulation and by tissue metabolism, whereas nitrogen and helium are removed only by the circulation. The helium gradient extends further into tissue than the nitrogen gradient because helium diffuses faster than nitrogen.

Figure 4–17. Representations of diffusion for mathematical modeling. (A) Bulk diffusion through tissue as in Figure 4-16. (B) All diffusion resistance at a barrier around the bubble. (C) No diffusion resistance and the tissue and bubble are in equilibrium.

Growth

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kinetics of the E-L model are illustrated in Figure 4–18D. The E-L model uses a conven-tional M-value matrix as in Table 4–1 (called V-value to indicate E-L kinetics) that allows critical levels of supersaturation to exist in the tissues. (This excess supersaturation might be interpreted as surface tension and tissue elasticity.) When a critical supersatu-ration is exceeded, however, the nitrogen exchange kinetics change from exponential to linear, which is equivalent to the supposition that a bubble has formed in that tissue. After the supersaturation has resolved, the kinetics return to exponential,

which is equivalent to the supposition that the bubble has dissolved. The effect of bubble formation is to reduce the rate at which nitrogen is eliminated from tissue as indicated earlier. The E-L model provided a biophysical explanation for the asymmetry between nitrogen uptake and elimination that others had observed experimentally and was the basis for the U.S. Navy 0.7 atm oxygen partial pressure nitrogen-oxygen and helium-nitrogen-oxygen decompression tables.^78,79 The E-L model has been imple-mented in the recently approved U.S. Navy dive computer.⁸⁰

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Figure 4–18. Nitrogen exchange in response to bubble formation in a diffusion equilibrium tissue after decompression from 60 fsw (18 msw; 2.82 ata) to sea level. A. Formation of a diffusion equilibrium bubble upon decompression. B. While a diffusion equilibrium bubble is present, the tissue nitrogen tension remains constant and equal to the nitrogen partial pressure in the bubble. C. The bubble volume decreases linearly until it dissolves.

D. The exponential-linear (“E-L”) model in which the bubble is replaced by an “equivalent” dissolved gas tension that washes out at a linear rate so long as a bubble is present.^76,77When the bubble dissolves, washout becomes exponential.

PHYSICS OF BUBBLE