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.

Is the probable spillage of the lung surfactant dipalmitoylphosphatidylcholine the ultimate source of diabetes type 1?

The lung surfactant dipalmitoylphosphatidylcholine (DPPC) most probably leaks into the blood, settling on the luminal aspect of blood vessels to create active hydrophobic spots (AHS). Nanobubbles are formed at these spots from dissolved gas. We hypothesized that when a large molecule in the blood comes into contact with a nanobubble at the AHS, its tertiary structure is disrupted. An epitope not previously having undergone thymus education may then prompt an autoimmune response. There are thus two independent processes which may share the blame for autoimmune disease: spillage of large molecules into the blood, and the creation of AHS. DPPC was measured in 10 diabetes type 1 patients and 10 control subjects. DPPC in the diabetic group was 4.63 ± 0.68 µg/mL, non-significantly higher than in the control group (4.23 ± 0.94 µg/mL). However, in the diabetic group, DPPC was high when the samples were taken within 1.5 years of disease onset. This is closer to the time of AHS production, which takes place ahead of the disease. Further investigation, with sampling for DPPC as soon as possible after onset of the disease, may provide additional support for our hypothesis. If proved true, this may open up considerable therapeutic potential.

Power Equation for Predicting the Risk of Central Nervous System Oxygen Toxicity at Rest.

Patients undergoing hyperbaric oxygen therapy and divers engaged in underwater activity are at risk of central nervous system oxygen toxicity. An algorithm for predicting CNS oxygen toxicity in active underwater diving has been published previously, but not for humans at rest. Using a procedure similar to that employed for the derivation of our active diving algorithm, we collected data for exposures at rest, in which subjects breathed hyperbaric oxygen while immersed in thermoneutral water at 33°C (n = 219) or in dry conditions (n = 507). The maximal likelihood method was employed to solve for the parameters of the power equation. For immersion, the CNS oxygen toxicity index is K I = t2 × PO2 10.93, where the calculated risk from the Standard Normal distribution is Z I = [ln(K I 0.5) - 8.99)]/0.81. For dry exposures this is K D = t2 × PO2 12.99, with risk Z D = [ln(K D 0.5) - 11.34)]/0.65. We propose a method for interpolating the parameters at metabolic rates between 1 and 4.4 MET. The risk of CNS oxygen toxicity at rest was found to be greater during immersion than in dry conditions. We discuss the prediction properties of the new algorithm in the clinical hyperbaric environment, and suggest it may be adopted for use in planning procedures for hyperbaric oxygen therapy and for rest periods during saturation diving.

In vitro evidence of decompression bubble dynamics and gas exchange on the luminal aspect of blood vessels: Implications for size distribution of venous bubbles.

We found that lung surfactant leaks into the bloodstream, settling on the luminal aspect of blood vessels to create active hydrophobic spots (AHS). Nanobubbles formed by dissolved gas at these AHS are most probably the precursors of gas micronuclei and decompression bubbles. Sheep blood vessels stretched on microscope slides, and exposed under saline to hyperbaric pressure, were photographed following decompression. Photographs of an AHS from a pulmonary vein, containing large numbers of bubbles, were selected in 1-min sequences over a period of 7 min, starting 18 min after decompression from 1,013 kPa. This showed bubble detachment, coalescence and expansion, as well as competition for dissolved gas between bubbles. There was greater expansion of peripheral than of central bubbles. We suggest that the dynamics of decompression bubbles on the surface of the blood vessel may be the closest approximation to true decompression physiology, and as such can be used to assess and calibrate models of decompression bubbles. We further discuss the implications for bubble size in the venous circulation.

Calculated risk of pulmonary and central nervous system oxygen toxicity: a toxicity index derived from the power equation.

BACKGROUND: The risk of oxygen toxicity has become a prominent issue due to the increasingly widespread administration of hyperbaric oxygen (HBO) therapy, as well as the expansion of diving techniques to include oxygen-enriched gas mixtures and technical diving. However, current methods used to calculate the cumulative risk of oxygen toxicity during an HBO exposure i.e., the unit pulmonary toxic dose concept, and the safe boundaries for central nervous system oxygen toxicity (CNS-OT), are based on a simple linear relationship with an inspired partial pressure of oxygen (PO2) and are not supported by recent data. METHODS: The power equation: Toxicity Index = t2 × PO2c, where t represents time and c represents the power term, was derived from the chemical reactions producing reactive oxygen species or reactive nitrogen species. RESULTS: The toxicity index was shown to have a good predictive capability using PO2 with a power c of 6.8 for CNS-OT and 4.57 for pulmonary oxygen toxicity. The pulmonary oxygen toxicity index (PO2 in atmospheres absolute, time in h) should not exceed 250. The CNS-OT index (PO2 in atmospheres absolute, time in min) should not exceed 26,108 for a 1% risk. CONCLUSION: The limited use of this toxicity index in the diving community, after more than a decade since its publication in the literature, establishes the need for a handy, user-friendly implementation of the power equation.

Pulmonary oxygen toxicity in saturation dives with PO2 close to the lower end of the toxic range - A quantitative approach.

Pulmonary oxygen toxicity (POT) has been extensively described at partial pressures of oxygen (PO2) ≥ 1 bar, but much less so at lower PO2. We proposed the POT index [K = t2 × (PO2)4.57] as a means of evaluating the severity of POT, expressed either as reduced lung function or the incidence of POT in a group of divers. In the exponential recovery process (e - [- 0.42 + 0.384 × (PO2)ex] × tr), the time constant increases linearly from 0.0024 to 0.54 h-1 for a PO2 of 1.1 to 2.5 bar. A linear relationship was demonstrated between the incidence of POT and the POT index, given by the equation: POT incidence % = 1.85 + 0.171 × K. In saturation diving, PO2 is kept close to the lower end of the toxic limits for POT, which is approximately 0.5 bar. We suggested that at this low range of PO2, the two processes of cumulative toxicity and recovery operate simultaneously. For one example of saturation diving, we show that a recovery time constant of 0.0135 h-1 yields the measured incidence of POT. We therefore propose the formula K = t2 × PO24.57 × e-0.0135 × t for calculation of the POT index in further analyses of POT in saturation diving.

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 ±â€¯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.

Taravana, vestibular decompression illness, and autochthonous distal arterial bubbles.

Decompression bubbles can develop only from pre-existing gas micronuclei. These are the nanobubbles which appear on active hydrophobic spots (AHS) found on the luminal aspect of all blood vessels. Following decompression, with the propagation of blood along the arterial tree, diffusion parameters cause increased transfer of nitrogen from the tissue into the artery, and more so if perfusion is low. Taravana is a neurological form of decompression illness (DCI) prevalent in repeated breath-hold diving. A nanobubble on an AHS in a distal artery of the brain may receive an influx of nitrogen after each dive until it occludes the arterial blood flow. The vestibular organ has very low perfusion compared with the brain and the cochlea of the inner ear. We suggest that a nanobbubble on an AHS in the distal artery of the vestibular organ will receive a high influx of nitrogen from the surrounding tissue after decompression due to the low nitrogen clearance, thus expanding to cause vestibular DCI.