Influence of body fat and plasma lipid compounds on venous gas embolism resulting from a dry air-dive simulation

For decades, it is well known that decompression sickness (DCS) in divers,
caisson workers and aeronautic personnel is caused by intravascular and/or
extra-vascular bubbles of inert gas. In many cases of DCS vascular bubbles,
especially venous bubbles are the cause; venous gas embolism (VGE). For many
years, one of the intriguing questions was why the one diver is much more
vulnerable to obtain VGE then the other. When breathing air, the gas forming
bubbles is nitrogen. For decennia, dive research tries to elucidate which
physiological parameters determine VGE. Susceptibility to VGE varies enormous
among divers with seemingly similar physical characteristics.
It has been well established that VGE susceptibility increases with age and
decreases with VO2max. The influence of BF as an independent determent is
controversial. One of the reasons of the fact that the influence of fat% is
still unknown is that fat% is highly correlated with age and negatively
correlated with VO2max.
BF as predisposing factor
The body fat compartment can enhance VGE, since after ascent a substantial part
of the total body dissolved nitrogen is accumulated in BF, and the more the
higher BF%. Since in the blood bubbles and the liquid phase compete for the N2
release in the blood, BF may directly influence VGE vulnerability. To study
fat% as an independent predisposing factor, we restrict the influence of the
factors age and VO2max by limiting their range..
Surfactants as predisposing factors
VGE bubbles arise from small nuclei (about 0.1 *m and smaller). However, such
small nuclei will collapse soon after their genesis due to their very large
surface tension that is larger the smaller the nucleus or bubble. A nucleus can
only survive when it is stabilized by a monomolecular layer of surfactant
molecules counteracting the inward force of the surface tension. These
molecules should have an amphiphilic nature: a hydrophilic group pointing
outward and a hydrophobic tail pointing inward. FFAs are theonly candiates
(3D-structure, concentration, physical intermolecular forces, thermodynamics).
FFA is weakly ionogenic bound to albumin and should be released
(thermodynamics) to frorm mycelles. The more FFAs, the easier bubbles are
stabiilized. Due to the (chemo-)physical characteristics of the mutual
interactions between bubbles, FFA and albumin, the amount of FFA is the
limiting factor.
TC/HDL ratio as predisposing factor.
The TC/HDL ratio is an indicator for the condition of the arterial endothelium.
A high ratio will be related to high numbers of crevases. This will result in
more microbubble generation and consequently a higher VGE after a dive.

Main objectives: Establishing the effect of BF as predisposing factor for the
development of VGE and hence DCS by using subjects with a restricted range in
age and VO2max. Establishing the effect of TC/HDL ratio and FFA as predisposing
factors for the development of VGE.

The design can be described by distinguishing in the following parts or
subsequent steps of execution.
1. Selection of subjects on the basis of the data of the obligatory
dive-medical examination (navy divers) or the data of a questionnaire
(recrational and professional divers)
2. Division of the divers in two equal age&VO2max matched groups.
3. The day before the simulation the divers are not allowed to perform physical
exercise, or using stimulating liquids and drugs etc. No diving is allowed 3
days in advance of the dive-simulation.
4. After rising in the early morning, the divers take liquid (in amounts
dependend on corrected body weight). Blood is sampled in the early morning
(empty stomach) before the simulation. BF% is determined with the bio-impedance
method as control of the skinfolds method, known from the data of the obligate
medical dive-examination of the examination-physician (with approval of the
diver).
5. A standardized breakfast, either fat-poor (the one group) or fat-rich (the
other group) with liquid intake is used an hour before the simulation (both
dependend on corrected body weight).
6. Dry dive *simulation in the morning to 21 m equivalent depth for 40 min in
total.
7. Blood sample directly after simulation (FFA).
8. Doppler measurements during 2h20min after *surfacing*.

Risks assessment, group relatedness
The risks associated with the dive simulation can be considered as very low
(2.0%). Due to the small depth and other conditions of the simulation, DCS when
occurring will be mild. DMC is THE authority to treat DCS of offshore and
recreational divers and caisson workers, in addition to navy divers. With their
experience and on the basis of knowledge of other dive medical centres (navy
and commercial) the risk of remaining symptoms and permanent damage is expected
to be very small (ca. 2% of the treated DCS cases).
The burden of the simulation can be considered minimal, compared to a real, wet
dive.