Submarine rescue decompression procedure from hyperbaric exposures up to 6 bar of absolute pressure in man: effects on bubble formation and pulmonary function

Recent advances in submarine rescue systems have allowed a transfer under pressure of crew members being rescued from a disabled submarine. The choice of a safe decompression procedure for pressurised rescuees has been previously discussed, but no schedule has been validated when the internal submarine pressure is significantly increased i.e. exceeding 2.8 bar absolute pressure. This study tested a saturation decompression procedure from hyperbaric exposures up to 6 bar, the maximum operating pressure of the NATO submarine rescue system. The objective was to investigate the incidence of decompression sickness (DCS) and clinical and spirometric indices of pulmonary oxygen toxicity. Two groups were exposed to a Nitrogen-Oxygen atmosphere (pO2 = 0.5 bar) at either 5 bar (N = 14) or 6 bar (N = 12) for 12 h followed by 56 h 40 min resp. 60 h of decompression. When chamber pressure reached 2.5 bar, the subjects breathed oxygen intermittently, otherwise compressed air. Repeated clinical examinations, ultrasound monitoring of venous gas embolism and spirometry were performed during decompression. During exposures to 5 bar, 3 subjects had minor subjective symptoms i.e. sensation of joint discomfort, regressing spontaneously, and after surfacing 2 subjects also experienced joint discomfort disappearing without treatment. Only 3 subjects had detectable intravascular bubbles during decompression (low grades). No bubbles were detected after surfacing. About 40% of subjects felt chest tightness when inspiring deeply during the initial phase of decompression. Precordial burning sensations were reported during oxygen periods. During decompression, vital capacity decreased by about 8% and forced expiratory flow rates decreased significantly. After surfacing, changes in the peripheral airways were still noticed; Lung Diffusion for carbon monoxide was slightly reduced by 1% while vital capacity was normalized. The procedure did not result in serious symptoms of DCS or pulmonary oxygen toxicity and may be considered for use when the internal submarine pressure is significantly increased.

Oxygen breathing or recompression during decompression from nitrox dives with a rebreather: effects on intravascular bubble burden and ramifications for decompression profiles

Preventive measures to reduce the risk of decompression sickness can involve several procedures such as oxygen breathing during in-water decompression. Theoretical predictions also suggest that brief periods of recompression during the course of decompression could be a method for controlling bubble formation. The aim of this study was to get clearer information about the effects of different experimental ascent profiles (EAPs) on bubble reduction, using pure oxygen or recompression during decompression for nitrox diving. Four EAPs were evaluated using bubble monitoring in a group of six military divers using Nitrox 40% O(2) breathing with a rebreather. For EAP 1 and 2, 100% O(2) was used for the end stage of decompression, with a 30% reduction of decompression time in EAP 1 and 50% in EAP 2, compared to the French navy standard schedule. For EAP 3 and 4, nitrox 40% O(2) was maintained throughout the decompression stage. EAP 3 is based on an air standard decompression schedule, whereas EAP 4 involved a brief period of recompression at the end of the stop. We found that EAP 1 significantly reduced bubble formation, whereas high bubble grades occurred with other EAPs. No statistical differences were observed in bubbles scores between EAP 3 and 4. One diver developed mild neurological symptoms after EAP 3. These results tend to demonstrate that the "oxygen window" plays a key role in the reduction of bubble production and that breathing pure oxygen during decompression stops is an optimal strategy to prevent decompression sickness for nitrox diving.

Influence of repetitive open sea dives and physical exercises on right-to-left shunting in healthy divers

OBJECTIVE

Paradoxical gas embolism through right-to-left (R/L) shunts is considered as a potential cause of certain types of decompression sickness.

AIM

To assess whether 4 months of repetitive diving and strenuous exercises would lead to an increased prevalence of R/L shunting in a group of military divers.

METHODS

Using a standardised contrast-enhanced transcranial Doppler technique, 17 divers were re-examined for the presence of a R/L shunt 4 months after their initial examinations. R/L shunts were classified as type I if observed only after a straining manoeuvre, and type II if present at rest.

RESULTS

Initial prevalence of R/L shunt was 41%: six type I shunts and one type II. At the second examination, prevalence was 47%, with the appearance of one type I shunt that was not previously present. We found no significant increase in the prevalence and size of R/L shunts.

CONCLUSION

It is speculated that diving-related phenomena, such as variations in right atrial pressures during the end stages of or events immediately after a dive could generate an R/L shunt. However, extreme conditions of repetitive diving and strenuous exercises do not cause permanent modification in R/L permeability over a period of 4 months.

Thalidomide reduces MPTP-induced decrease in striatal dopamine levels in mice

The effects of thalidomide, a sedative, anti-inflammatory and immunosuppressive agent were studied in the MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) murine model of Parkinson's disease. The striatal levels of dopamine (DA) and of its main metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were measured both in the MPTP control group (3 x 15 mg/kg intraperitoneally) and in the thalidomide groups (repeated treatments at 25 mg/kg or 50 mg/kg postoperatively). For mice treated with thalidomide, a dose-dependent protection was observed against the MPTP-induced decrease in DA. The decrease in HVA levels was totally antagonized by thalidomide at both doses. That thalidomide has activity in this model suggests that an inflammatory process may be involved in the induction of lesions by MPTP in DAergic neurons.

Neuroprotective effects of RPR 104632, a novel antagonist at the glycine site of the NMDA receptor, in vitro

The NMDA antagonist and neuroprotective effects of RPR 104632 (2H-1,2,4-benzothiadiazine-1-dioxide-3-carboxylic acid), a new benzothiadiazine derivative, with affinity for the glycine site of the NMDA receptor-channel complex are described. RPR 104632 antagonized the binding of [3H]5,7-dichlorokynurenic acid to the rat cerebral cortex, with a Ki of 4.9 nM. This effect was stereospecific, since the (-)-isomer was 500-fold more potent than the (+)-isomer. The potent affinity of RPR 104632 for the glycine site was confirmed by the observation that RPR 104632 inhibited [3H]N-[1-(2-thienyl)cyclohexyl]-3,4-piperidine ([3H]TCP) binding in the presence of N-methyl-D-aspartate (NMDA) (IC50 = 55 nM), whereas it had no effect on the competitive NMDA site or on the dissociative anaesthetic site. RPR 104632 inhibited the NMDA-evoked increase in guanosine 3',5'-cyclic monophosphate (cGMP) levels of neonatal rat cerebellar slices (IC50 = 890 nM) in a non-competitive manner and markedly reduced NMDA-induced neurotoxicity in rat hippocampal slices and in cortical primary cell cultures. These results suggest that RPR 104632 is a high-affinity specific antagonist of the glycine site coupled to the NMDA receptor channel with potent neuroprotective properties in vitro.