Rodent Model for High Altitude and Ebullism Exposure Studies
INTRODUCTION: Ebullism is the pathophysiological process that occurs as a result of exposures to extremely low ambient pressures, traditionally below 47 mmHg (<0.9 psi, ∼63,000 ft/19,202 m equivalent). However, this field of research has made minimal progress since the 1940s–1960s, when the physiology of high altitude/space operations was being explored. This lack of progress is in part because it is thought of as invariably lethal and also because it requires unique facilities to simulate these environments. As a result, no standardized time/pressure profiles or animal models have been established. METHODS: A rodent animal model (N = 20) was exposed to rapid depressurization as low as 0.3 mmHg for up to 2 min; controls were placed in the chamber but not exposed to a pressure change. Autopsies were performed to characterize the pathophysiology of ebullism at extreme altitudes. RESULTS: A three-tiered pressure approach was developed that allows for varying degrees of exposure (pressure and time). Although previous studies focused on exposures above or below the Armstrong Line (∼63 kft), we noted significant thermal impacts due to exceeding the water triple point (∼120 kft). DISCUSSION: This initial study highlights the different pathophysiological regimes that exist beyond Armstrong’s line and subdivides ebullism exposures into two different classes, which can be operationally associated with cabin vs. suit depressurization events. These are now presented as Type A Complex Ebullism and Type B Simple Ebullism. The former is characterized by a combination of barotrauma, hypoxia, ebullism, and decompression sickness, while the latter presents as only ebullism. Garbino A, Nusbaum D, Goughnour S, Dalal S, Carminati M-VG, Clark J. Rodent model for high altitude and ebullism exposure studies. Aerosp Med Hum Perform. 2025; 96(3):198–205.
Schematic and control system of Midland Altitude Chamber Complex.
Planned exposure periods at each of the three targeted altitudes. The research protocol provided for exposures of 30 s, 60 s, and 90 s at 26 mmHg/75 kft and 2 mmHg/135 kft, but only 60 s and 90 s at 0.3 mmHg/190 kft.
Top: Actual chamber flights vs. target profiles. Due to the accumulator performance, arresting climb at 26 mmHg/75 kft resulted in some fluctuations before stabilizing the altitude. Bottom: Example flight profile: planned profile in orange, actual profile in blue, climb rates in red. Accumulator equalization achieved an initial climb rate > 1000,000 ft/min. After approximately 8 s the chamber would equalize with the accumulators and would be isolated from them while the vacuum pump continued to depressurize the chambers to the target altitude. Left axis: equivalent altitude (ft); right axis: climb rate (ft/min).
Animal mortality for different exposure profiles. Red X: died; Green circle: required euthanasia (N = 18, each symbol represents an animal; 2 controls not shown). Note that one of the animals in the 0.3 mmHg/130 kft/90 s group survived the chamber flight, but died within 2 min after exposure, prior to euthanasia. This animal was counted as deceased.
Pericardial bubbles observed postexposure.
Vacuum exposures can result in a mix of various pathophysiological processes from multiple factors such as rapid vs. slow pressure changes, initial and final pressure, oxygen percentage, previous denitrogenation, etc. Notional differences between Type A Complex and Type B Simple Ebullism are depicted.
Phase diagram of water, with progression from body temperature/pressure to vacuum through triple point. Modified from Wikimedia Commons (CC BY-SA 3.0).
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