Operational Medical Issues in Hypo- and Hyperbaric Conditions

The relation between the presence of a patent foramen ovale (PFO) and the risk of decompression illness (DCI) remains controversial. PFO is a common finding in the general population, with an autopsy prevalence of about 25%. Recent review articles in the diving literature have concluded the presence of a PFO may increase the relative risk for DCI but the absolute risk remains low. In space operations, DCI is a significant concern for extravehicular activities (EVA) because of the low suit pressure (the NASA EMU-suit pressure is 4.3 psia, equivalent to about 30,000 feet). The Canadian Space Agency (through contract to DCIEM) is participating in NASA-led Prebreathe Reduction Protocol (PRP) studies to assess the safety and efficacy of reducing EVA oxygen prebreathe time. Reduction in prebreathe time is accomplished by incorporating exercise protocols during a two hour oxygen prebreathe prior to decompression to EVA suit pressure. As for NASA astronauts, DCIEM PRP subjects are screened with a trans-thoracic echocardiogram (TTE). In 48 volunteers at DCIEM screened for the PRP studies with a TTE, 14 (29%) were found to have an echo-probable PFO. In 29 altitude-exposed subjects who had a TTE, there were 5 echo-probable PFOs. None of these 5 subjects experienced DCI. Two of these subjects had a high bubble load with grade IV bubbles on precordial Doppler monitoring. In total there were four cases of Type I DCI and no Type II DCI. None of these subjects had an echo-probable PFO. Introduction Decompression illness (DCI) represents a significant health risk for underwater divers, caisson workers, pilots of high performance aircraft and astronauts who perform EVAs, with symptoms ranging from local joint pain (Type I), to neurological symptoms (Type II), to circulatory collapse and death. Of particular concern are the serious neurological manifestations of DCI (Type II DCI) which untreated, may lead to system collapse and death. DCI occurs as a result of the evolution of nitrogen dissolved in the body fluids and tissues when ambient pressure is decreased to the point that gas phase separation occurs. Inert gas bubbles are generally believed to form extravasculary in tissue, or intravascularly in the venous side of the circulation. Intravascular bubbles in the venous system, or venous gas emboli (VGE), circulate to the right side of the heart and eventually travel to the pulmonary circulation where they are filtered out due to the very large concentration gradient between the pulmonary capillaries and the alveoli. Neurological complications in DCI are thought to occur as a result of the formation of gas bubbles in the tissues of the brain or spinal cord, or of arterial gas embolism (AGE). AGE may then flow to the brain and spinal cord, obstructing circulation in these tissues, causing mechanical damage, and altering biochemical and hematological balances. The presence of gas bubbles in the arterial side of the circulation is thought to occur by three possible mechanisms: 1) rupture of small airways in the lungs resulting in air embolism; 2) generation of inert gas bubbles in the arteries (de novo genesis or tribonucleation); or 3) cross-over of gas emboli from the venous to the arterial side of the circulation. Right-to-left cross-over may occur by: 1) anatomical shunts within the lung, or 2) intracardiac shunts including atrial septal defect (ASD) and patent foramen ovale (PFO). Recently, atrial septal defects amongst divers, particularly in the form of patent foramen ovale (PFO), have been associated with the right-to-left crossover of inert gas bubbles resulting in neurological DCI (Moon et. al., 1989 and Wilmhurst et.al., 1989). PFO is a common finding in the general population, with an autopsy prevalence of about 25-34% (Hagen et.al., 1984) and recent reports in the diving literature have concluded that the presence of a PFO increases the relative risk for DCI but the absolute risk remains low (Cross et. el., 1994 and Bove, 1998). Paper presented at the RTO HFM Symposium on “Operational Medical Issues in Hypoand Hyperbaric Conditions”, held in Toronto, Canada, 16-19 October 2000, and published in RTO MP-062.