Presence of dipalmitoylphosphatidylcholine from the lungs at the active hydrophobic spots in the vasculature where bubbles are formed on decompression

Endothelial Injury in Diving: Atomic Force-, Electronic-, and Light-Microscopy Study of the Ovine Decompressed Blood Vessels

We suggested that the nanobubbles, which appear at the active hydrophobic spots (AHSs) at the luminal aspect of the blood vessels, are the gas micronuclei from which the decompression bubbles evolve and the endothelial injury during the decompression is due to the tearing off the cell membranes with the detaching bubbles. Ovine blood vessels were stretched over the polycarbonate plates or glass microscopic slides and were exposed under saline to the hyperbaric pressure (1,013 kPa, 19 h). Following decompression, the blood vessels were photographed for the identification (by bubble formation) of the AHS. Nanobubbles could not be demonstrated at the AHS by using the atomic force microscopy (AFM) because of the roughness of the surface, which disabled the close contact of the probe. In the electron microscopy, no endothelial cells were observed in the samples from the area near to the AHS, but the underlying elastin layer of the intima was observed adjacent to the media. Some intact endothelial cells were observed only in the locations far from an AHS. In the optical microscopy, no endothelial cells were observed in the blood vessels in close proximity to the AHS and in some sections, debris or a detached cluster of the endothelial cells were observed. Intact endothelial cells could be found at the sites distant from an AHS. This study supports the assumption, where the detached bubbles tear off the endothelial cells and cause the initial endothelial injury following the decompression.

Decompression sickness, fatness and active hydrophobic spots

Since decompression sickness (DCS) in humans was first described, mankind has embarked on an odyssey to prevent it. The demonstration that decompression releases bubbles, which mainly contain inert gas (nitrogen, helium), into the circulation and that the slower the decompression rate the lesser the incidence of DCS, resulted in 1908 in the publication of the first, reasonably safe diving tables. Besides the development of proper diving tables, the selection of divers is also of importance. A relationship between body composition and DCS was observed in dogs as long ago as the nineteenth century, an observation supported early in the twentieth century: "Really fat men should never be allowed to work in compressed air, and plump men should be excluded from high pressure caissons…or in diving to more than about 10 fathoms, and at this depth the time of their exposure should be curtailed. If deep diving is to be undertaken…. skinny men should be selected." Alas, nothing is that simple! From my own experience it was not always the fat diver who ended up in the treatment chamber with DCS. Therefore, other factors must be at play; gender, age, physical fitness, and the existence of a persistent foramen ovale (PFO) have all been studied as possible factors for the development of vascular gas bubbles and, therefore, for DCS. However, none of these factors, alone or in combination, explain why there are intra-individual or intra-cohort differences in bubble grades (BG). In other words, why does a dive I did today led to a high BG but the same dive next week lead to a low one? Or, why is there such a difference in BG amongst divers of more or less the same age, gender, body composition and physical fitness? In a letter in this issue, a novel hypothesis is postulated that may fill in these gaps; active hydrophobic spots (AHS). These AHS can be found at the luminal side of capillary, venous and arterial walls and have an oligolamellar lining. In an in vitro experiment, nanobubbles developed on AHS after a 'dive' to 1,000 kPa (90 msw). It appears that AHS consist of dipalmitoylphosphatidylcholine (DPPC), which is the main component of surfactant. It is proposed that DPPC may leak from the alveoli into the alveolar capillary and be transported to veins and arteries where it precipitates and forms AHS. Based on these ideas, it is hypothesized that AHS generate nanobubbles that can grow into microbubbles. When these microbubbles detach from the AHS they might also take along pieces of the AHS membrane making the AHS smaller or even disappear. This phenomenon could explain some of the earlier findings regarding the formation of microbubbles in divers. The fact that the presence of microbubbles differs between younger and older divers, after repetitive dives, and between experienced divers and novice divers can be explained by this model, and AHS may be the missing link we are looking for in our quest to understand and treat DCS. However, some reservations must be made. Firstly, these observations are derived from in vitro and animal experiments and whether or not they reflect a similar process in man remains unclear. Secondly, it appears that female divers have lower bubble grades after similar dives compared to male divers, suggesting lower decompression stress. If AHS is the main generator for microbubbles, there should be a difference in the presence of AHS between men and women. We do not know from these animal experiments whether there is a gender difference, neither does a literature search in PubMed provide us with an answer. Thirdly, as said before, DPPC is the main component of surfactant. All alveolar surfactant phospholipids, such as DPPC, are secreted to the alveolar space via exocystosis of the lamellar bodies (LB) from alveolar type II (ATII) cells. To form a functional air-blood barrier, alveolar type I and ATII cells are connected to each other by tight junctions. These tight junctions constitute the seal of the intercellular cleft and in that way form a true barrier between the alveolus and the capillary. Only small molecules like oxygen, carbon dioxide, etc. can penetrate through this barrier by themselves due to passive diffusion. All other (macro)molecules, including DPPC, need intermediate processes such as ion transport proteins, channels, metabolic pumps, etc. to gain access to the pulmonary capillary lumen. To my knowledge, no such mechanisms for DPPC or LB are known. A theoretical explanation might be the fact that the production of DPPC and the exocystosis of DPPC-containing LBs into the alveolar space can be stimulated by stretch. Stretch of the alveoli can switch on Ca2+ entry by either mechanosensitive channels, store-operated channels or second messenger-operated channels, which induces LB exocystosis. Furthermore, an ATP-release mechanism might also be responsible for the pulmonary alveolar mechanotransduction of LB. During diving, transpulmonary pressure changes occur which might induce additional alveolar stretch and thus, theoretically, an extra release of LB. However, whether or not such exocystosis of LB is vascularly orientated remains unclear. Besides which, the leakage of DPPC from the alveolus to the pulmonary capillary might also be as simple as a malfunction of the tight junction due to epithelial membrane damage as a result of diving. Finally, it is also possible that DPPC is produced in other non-ATII cells in our body of which we are currently unaware. To conclude, this is an interesting hypothesis regarding the origin of microbubbles. Whether or not DPPC and LB are the main reason for individual sensitivity to DCS remains unclear. Further research will hopefully identify if DPPC and LB are indeed the missing link or just another branch on the big tree of the genesis of decompression sickness.

Ovine plasma dipalmitoylphosphatidylcholine does not predict decompression bubbling

Decompression illness (DCI) is the main risk associated with scuba diving. Some divers ("bubblers") are more sensitive to DCI than others ("non-bubblers"). We found that there are active hydrophobic spots (AHS) on the luminal aspect of ovine blood vessels, which contain the surfactant dipalmitoylphosphatidylcholine (DPPC). DPPC leaks from the lung into the plasma, settling on the blood vessel to create AHS. These are the main source of gas micronuclei from which bubbles develop after decompression. A correlation between bubbling ovine blood vessels and the animal's plasma DPPC might lead to the development of a blood test for vulnerability to DCI. Samples from ovine blood vessels were stretched on microscope slides, placed anaerobically in saline at the bottom of a Pyrex bowl, and exposed to high pressure. Automated photography was used after decompression to reveal AHS by visualising their bubble production. Phospholipids were extracted from the AHS and plasma for determination of DPPC. Bubbling was unrelated to the concentration of DPPC in the plasma (2.15 ± 0.87 μg/ml). Bubble production from the AHS (n = 130) as a function of their DPPC content yielded two groups, one unrelated to DPPC and the other which demonstrated increased bubbling with elevation of DPPC. We suggest this may be related to alternate layering with hydrophobic and hydrophilic phospholipids. This study reinforces the connection between DPPC and DCI. However, a blood test for diver vulnerability to decompression stress is not recommended.

A biophysical vascular bubble model for devising decompression procedures

Vascular bubble models, which present a realistic biophysical approach, hold great promise for devising suitable diver decompression procedures. Nanobubbles were found to nucleate on a flat hydrophobic surface, expanding to form bubbles after decompression. Such active hydrophobic spots (AHS) were formed from lung surfactants on the luminal aspect of ovine blood vessels. Many of the phenomena observed in these bubbling vessels correlated with those known to occur in diving. On the basis of our previous studies, which proposed a new model for the formation of arterial bubbles, we now suggest the biophysical model presented herein. There are two phases of bubble expansion after decompression. The first is an extended initiation phase, during which nanobubbles are transformed into gas micronuclei and begin to expand. The second, shorter phase is one of simple diffusion‐driven growth, the inert gas tension in the blood remaining almost constant during bubble expansion. Detachment of the bubble occurs when its buoyancy exceeds the intermembrane force. Three mechanisms underlying the appearance of arterial bubbles should be considered: patent foramen ovale, intrapulmonary arteriovenous anastomoses, and the evolution of bubbles in the distal arteries with preference for the spinal cord. Other parameters that may be quantified include age, acclimation, distribution of bubble volume, AHS, individual sensitivity, and frequency of bubble formation. We believe that the vascular bubble model we propose adheres more closely to proven physiological processes. Its predictability may therefore be higher than other models, with appropriate adjustments for decompression illness (DCI) data.

Nanobubbles Form at Active Hydrophobic Spots on the Luminal Aspect of Blood Vessels: Consequences for Decompression Illness in Diving and Possible Implications for Autoimmune Disease-An Overview

Decompression illness (DCI) occurs following a reduction in ambient pressure. Decompression bubbles can expand and develop only from pre-existing gas micronuclei. The different hypotheses hitherto proposed regarding the nucleation and stabilization of gas micronuclei have never been validated. It is known that nanobubbles form spontaneously when a smooth hydrophobic surface is submerged in water containing dissolved gas. These nanobubbles may be the long sought-after gas micronuclei underlying decompression bubbles and DCI. We exposed hydrophobic and hydrophilic silicon wafers under water to hyperbaric pressure. After decompression, bubbles appeared on the hydrophobic but not the hydrophilic wafers. In a further series of experiments, we placed large ovine blood vessels in a cooled high pressure chamber at 1,000 kPa for about 20 h. Bubbles evolved at definite spots in all the types of blood vessels. These bubble-producing spots stained positive for lipids, and were henceforth termed “active hydrophobic spots” (AHS). The lung surfactant dipalmitoylphosphatidylcholine (DPPC), was found both in the plasma of the sheep and at the AHS. Bubbles detached from the blood vessel in pulsatile flow after reaching a mean diameter of ~1.0 mm. Bubble expansion was bi-phasic—a slow initiation phase which peaked 45 min after decompression, followed by fast diffusion-controlled growth. Many features of decompression from diving correlate with this finding of AHS on the blood vessels. (1) Variability between bubblers and non-bubblers. (2) An age-related effect and adaptation. (3) The increased risk of DCI on a second dive. (4) Symptoms of neurologic decompression sickness. (5) Preconditioning before a dive. (6) A bi-phasic mechanism of bubble expansion. (7) Increased bubble formation with depth. (8) Endothelial injury. (9) The presence of endothelial microparticles. Finally, constant contact between nanobubbles and plasma may result in distortion of proteins and their transformation into autoantigens.