Sl Blogg

Magnetic Resonance Imaging and Neuropathology Findings in the Goat Nervous System following Hyperbaric Exposures

Divers may be at risk of long-term CNS damage from non-symptomatic hyperbaric exposure. We investigated the effect of severe, controlled hyperbaric exposure on a group of healthy goats with similar histories. Thirty goats were exposed to various dive profiles over a period of 5 years, with 17 experiencing decompression sickness (DCS). Brains were scanned using magnetic resonance (MR) imaging techniques. The animals were then culled and grossly examined, with the brain and spinal cord sent for neuropathological examination. No significant correlation was found between age, years diving, DCS or exposure to pressure with MR-detectable lesions in the brain, or with neuropathological lesions in the brain or spinal cord. However, spinal scarring was noted in 3 animals that had suffered from spinal DCS.

Indices of Increased Decompression Stress Following Long-Term Bed Rest

Human extravehicular activity (EVA) is essential to space exploration and involves risk of decompression sickness (DCS). On Earth, the effect of microgravity on physiological systems is simulated in an experimental model where subjects are confined to a 6° head-down bed rest (HDBR). This model was used to investigate various resting and exercise regimen on the formation of venous gas emboli (VGE), an indicator of decompression stress, post-hyperbaric exposure. Eight healthy male subjects participating in a bed rest regimen also took part in this study, which incorporated five different hyperbaric exposure (HE) interventions made before, during and after the HDBR. Interventions i–iv were all made with the subjects lying in 6° HD position. They included (C1) resting control, (C2) knee-bend exercise immediately prior to HE, (T1) HE during the fifth week of the 35-day HDBR period, (C3) supine cycling exercise during the HE. In intervention (C4), subjects remained upright and ambulatory. The HE protocol followed the Royal Navy Table 11 with 100 min spent at 18 m (280 kPa), with decompression stops at 6 m for 5 min, and at 3 m for 15 min. Post-HE, regular precordial Doppler audio measurements were made to evaluate any VGE produced post-dive. VGE were graded according to the Kisman Masurel scale. The number of bubbles produced was low in comparison to previous studies using this profile [Kisman integrated severity score (KISS) ranging from 0–1], and may be because subjects were young, and lay supine during both the HE and the 2 h measurement period post-HE for interventions i–iv. However, the HE during the end of HDBR produced significantly higher maximum bubble grades and KISS score than the supine control conditions (p < 0.01). In contrast to the protective effect of pre-dive exercise on bubble production, a prolonged period of bed rest prior to a HE appears to promote the formation of post-decompression VGE. This is in contrast to the absence of DCS observed during EVA. Whether this is due to a difference between hypo- and hyperbaric decompression stress, or that the HDBR model is a not a good model for decompression sensitivity during microgravity conditions will have to be elucidated in future studies.

Venous gas emboli in goats after simulated submarine escape from 290 msw breathing air or hyperoxic gas.

INTRODUCTION Escape from a disabled submarine has many inherent risks including the development of venous gas emboli (VGE). Breathing hyperoxic gas during rapid ascent from 2500 kPa (240 msw) reduces VGE; we investigated whether it would also be beneficial during an escape from 3000 kPa (290 msw), thought to be at the limit of human escape from a sunk sub. METHODS Adult castrated male or female goats (45-85 kg; N=35) underwent dry chamber compression to 3000 kPa in 28 s, then decompressed at a rate of 2.75 m x s(-1) while breathing either air or hyperoxic gas (60/40% O2/N2). Postsurfacing, precordial Doppler measurements were made using the Kisman Masurel (KM) scoring system; the animals were observed for signs of decompression illness (DCI) and/or oxygen toxicity. RESULTS Six animals in the air group (N=19) and two in the hyperoxic group (N=10) suffered from severe pulmonary barotrauma on surfacing and were euthanized. No cases of DCI arose later than 5 min postsurfacing. Oxygen toxicity was not observed. Although initial bubble scores (median KM score 4) were the same in both groups, the time taken for the median KM score to reach 3 or less was significantly faster in the hyperoxic group (20 min vs. 120 min). Disappearance of VGE was faster in the hyperoxic group. CONCLUSION Breathing hyperoxic gas during escape from 3000 kPa reduces the overall time with circulating VGE and, despite exposure to a maximum inspired partial pressure of oxygen of 1.8 MPa, symptoms of oxygen toxicity were not observed.