Hyperbaric exposure in rodents with non-invasive imaging assessment of decompression bubbles: a scoping review

Decompression sickness (DCS) is the primary hazard for undersea commercial and military operations, and a factor for astronauts during decompression for extra-vehicular activity in space, but a reliable biomarker for predicting DCS remains to be identified. This scoping review investigated imaging modalities previously used for estimating decompression stress and detecting bubbles in rodents after hyperbaric exposure. Search methods were established under the Joanna Brigg’s Institute Scoping Review Framework and Preferred Reporting Items for Systematic reviews and Meta Analyses. Two independent reviewers considered each study for inclusion, with a third available for settling disputes. Eligible publications met the following requirements (1): used a rodent test subject for (2) hyperbaric exposure to simulate a dive profile and (3) implemented an imaging modality for post-dive evaluation of decompression bubble presence. An exposure factor was calculated for each publication, that included sufficient procedure methodology, using an established Mixed-Gas Model or pressure-time based index and the relationship was compared with the post-dive bubble presence. Ultrasound was the most common modality utilized post-dive, except for 6 publications using either magnetic resonance imaging (MRI), positron emission tomography (PET), or computed tomography (CT). Venous gas emboli (VGE) detected using echocardiography was regularly used to assess the severity of the exposure. Massive bubble presence post-dive typically corresponded with fatal DCS. Several factors have been identified for relationship to VGE including body weight, temperature, and exercise. Recent publications using MRI and PET have presented novel approaches for detecting bubble formation and monitoring inert gas kinetics, respectively. A mixed-gas model previously proposed for calculating the risk of DCS for a given dive profile was found to underestimate the true experimental result, yet performed better than a pressure-time approach. This scoping review summarizes the imaging modalities that have been used in past rodent studies with DCS and discusses the relationship between VGE and post-dive mortality in small animals. Future reports should include sufficient information for repeatability including imaging settings, example views of the imaged area, and a complete list of VGE grades (if collected). Additional work is needed to confirm safe thresholds for ultrasound imaging of decompression bubbles, and for the development of other modalities in the decompression context including MRI, PET, and bioelectrical impedance.