Static Metabolic Bubbles as Precursors of Vascular Gas Emboli During Divers’ Decompression: A Hypothesis Explaining Bubbling Variability

Abstract

Introduction The risk for decompression sickness (DCS) after hyperbaric exposures (such as SCUBA diving) has been linked to the presence and quantity of vascular gas emboli (VGE) after surfacing from the dive. These VGE can be semi-quantified by ultrasound Doppler and quantified via precordial echocardiography. However, for an identical dive, VGE monitoring of divers shows variations related to individual susceptibility, and, for a same diver, dive-to-dive variations which may be influenced by pre-dive pre-conditioning. These variations are not explained by currently used algorithms. In this paper, we present a new hypothesis: individual metabolic processes, through the oxygen window (OW) or Inherent Unsaturation of tissues, modulate the presence and volume of static metabolic bubbles (SMB) that in turn act as precursors of circulating VGE after a dive. Methods We derive a coherent system of assumptions to describe static gas bubbles, located on the vessel endothelium at hydrophobic sites, that would be activated during decompression and become the source of VGE. We first refer to the OW and show that it creates a local tissue unsaturation that can generate and stabilize static gas phases in the diver at the surface. We then use Non-extensive thermodynamics to derive an equilibrium equation that avoids any geometrical description. The final equation links the SMB volume directly to the metabolism. Results and Discussion Our model introduces a stable population of small gas pockets of an intermediate size between the nanobubbles nucleating on the active sites and the VGE detected in the venous blood. The resulting equation, when checked against our own previously published data and the relevant scientific literature, supports both individual variation and the induced differences observed in pre-conditioning experiments. It also explains the variability in VGE counts based on age, fitness, type and frequency of physical activities. Finally, it fits into the general scheme of the arterial bubble assumption for the description of the DCS risk. Conclusion Metabolism characterization of the pre-dive SMB population opens new possibilities for decompression algorithms by considering the diver’s individual susceptibility and recent history (life style, exercise) to predict the level of VGE during and after decompression.

Vascular dysfunction following breath-hold diving

“…With supersaturated N 2 in tissues, even VGE formation is possible due to very rapid ascents from deep breath-hold dives (Foster et al 2016). Recently, preexisting populations of gas bubbles populated and stabilized by tissue metabolism have been linked to the formation of nucleation sites that could relate to the VGE detected post-dive (Imbert et al 2019).…”

Physiology of repeated mixed gas 100-m wreck dives using a closed-circuit rebreather: a field bubble study

“…Even more, many modifications to the decompression schedule, implemented by technical divers, such as gradient factors or deep stops, have not been validated by any human dive trials (Doolette and Mitchell 2013 ). Although several thousand dives are reported to have been conducted safely in the field, one cannot assume a similar degree of confidence without proper data gathering (Imbert et al 2019 ; Buzzacott et al 2014 ), It has indeed already been demonstrated that technical diving in open circuit up to 70 m is associated with a high decompression stress (Ljubkovic et al 2010 ).…”

“…Indeed, regardless of the decompression algorithm used, detectable VGE may be measured after recreational, technical and (to a lesser degree) professional diving (Dunford et al 2002 ). The amount of VGE is considered to be statistically related to the risk of DCS after a dive (Imbert et al 2019 ).…”

Effect of Water Amount Intake before Scuba Diving on the Risk of Decompression Sickness

“…It has been reported that low plasma surface tension can be attributed to bubble formation [ 32 ]. On the other hand, the ingestion of normal saline solution prior to hypobaric exposure can temporarily raise surface tension, contributing to the protection against altitude DCS [ 33 , 34 ]. Another study suggested that pre-hydration can attenuate dehydration resulting from higher fluid retention and prevent hypovolemia during diving [ 9 ].…”

“…Moreover, in this study, the circulating vascular bubbles on the right and left subclavian veins and precordial region were measured only immediately after the dive and once again 30 min later, although it is recommended that bubble measurements should be conducted during the first 120 min following decompression with intervals being no longer than 20 min [ 31 ]. Secondly, although previous studies suggest that various factors such as age, diet or nutrition, fitness, and physical activities can affect the variability in vascular gas emboli or small metabolic bubble populations [ 32 , 33 , 34 , 35 ], we controlled only diet and physical activities during the experiment. Thirdly, water was used for pre-hydration in this study, but some studies have used other solutions, such as a saline-glucose solution.…”