Expected outcome of a space-equivalent decompression has improved dramatically in the past 40 years, from an a priori assumption of non-survivability to the possibility of survival and rehabilitation. This paper outlines the history of Mans struggle with altitude, examines the known pathophysiology of Ebullism, explores the measures taken to improve survival in the Shuttle era, and investigates the state-of-the-art in treatment of rapid/explosive decompression.
Since the earliest days of Americas Manned Spaceflight program, one of the foremost concerns has been that of decompressions beyond Earths atmosphere. Early experiments in the 1800s (1) on laboratory animals revealed the catastrophic consequences of decompression to near-vacuum: hypoxia, decompression sickness (DCS), arterial gas embolism (AGE) and ebullism, a boiling away of water vapor from the body, generally considered (at the time) to be almost immediately fatal. While the conditions necessary for ebullism are present at an altitude of roughly 63,000 feet (referred to as the Armstrong Line), variations in the bodys temperature and pressure can allow this to occur as low as 55,000 feet (2); thus, the line is perhaps better thought of as a band (3).
For as long as weve known the consequences of altitude, weve made plans to improve our survival. Croce-Spinelli and Sivel first used supplemental oxygen in their 1874 balloon flight (4). Paul Bert developed the first altitude chamber (with supplemental oxygen) in 1878, and in 1934 Wiley Post developed and demonstrated the first pressure suit for high-altitude flights (4). As human flight neared the 40,000 foot level (at which the partial pressure of oxygen in air equals the combined pressures of carbon dioxide and water in the body), positive-pressure breathing (PPB) was instituted to provide sufficient oxygen despite falling pressure. Counterpressure garments, designed to facilitate high levels of PPB, evolved into the more extensive partial-pressure suit (5), capable of protecting the wearer from ebullism down to 15 mm Hg (6), but ineffective against the concomitant DCS and AGE.
Full-pressure suits, like the ILC Dover used in the Apollo program, incorporated a helmet, surrounding the body with a pressurized gas envelope. Since a full-pressure suit for extravehicular activity (EVA) presents an enormous pressure differential with the surrounding environment, they are inflated to only about one-third of normal (sea level) pressure, about 4.3 psi, with 100% oxygen. Since this level of oxygen imposes a high risk of fire, the shirt-sleeve environment of the Space Shuttle utilizes a 20% oxygen content at 14.7 psi.. This Earth-normal atmosphere will also be used on the International Space Station (ISS), necessitating extensive prebeathing with 100% oxygen before EVA and increasing the risk and morbidity of an unplanned loss of pressurization (7).
If and when such a loss in pressure occurs, what can we expect?
What does happen to a human exposed to vacuum? Can we successfully plan for a rapid decompression at 1,000,000 feet?
Hypobaria (low pressure) has life-threatening effects primarily on 3 systems: the Lungs, the Heart and the Brain. We examine each of these in turn.
Pulmonary Damage to the lungs in rapid or explosive decompression occurs primarily due to pulmonary overpressure, the tremendous pressure differential inside versus outside the lungs. 80 mm Hg is enough to cause pulmonary tears and alveolar rupture (8); pulmonary hemorrhaging, ranging from petechiae to free blood (depending on the magnitude and rate of decompression) is also seen (9). Emphysematous changes are seen especially in the upper lungs, while atelectasis and edema predominate in the lower lungs (10). When we get to the patient, the lungs will be a bloody, ruptured mess.
Cardiovascular Myocardial damage associated with ebullism is caused by stretching of the myocardium and anoxia (11). Heart rate rises the first 20 seconds (12), then drops to 40% of baseline at sixty seconds (12). By 2 minutes the arterial pressure wave is lost (13), but the cardiac contractility is maintained at least 5-7 minutes (14). Apneic animals resumed spontaneous respirations within 30 seconds of recompression as long as the heart continued to beat, but could not be resuscitated once asystole occurred (15). If our patient has a pulse, we might get him back.
Central Nervous System In rapid or explosive decompression above 60,000 feet, CNS damage is due to decreased cerebral blood flow and global cerebral anoxia (16). Little evidence of herniation is noted (16), though some damage to cerebral white matter and myelinated spinal cord is seen (13). Cases of accidental rapid and explosive decompression to date have not shown any lasting neurological damage (17, 18), though this includes only 2 cases. Our astronaut-patient may be rehabilitatable.
Though these are the most life-threatening changes seen in ebullism, subcutaneous swelling is also seen, due to creation of water vapor under the skin (19). This can rapidly distend the body to twice its normal volume (20). Our patient will look no better than he feels, though this means little in terms of survival.
This, then, is what we can expect to face. What measures have been taken to date to avoid ebullism in the space environment?
Shuttle Design The outcome of rapid decompression depends on multiple factors: rate of pressure change, absolute change in pressure, absolute pressure before decompression, ratio of initial pressure to final pressure, ratio of lung volume at the time of decompression to maximal lung capacity, and ratio of the cabin wall orifice (hull rupture) over the total cabin volume compared to the ratio of airway orifice over lung volume (21). The Shuttle, for example, is designed to sustain a pressure of 414 mm Hg for 165 minutes (long enough for an emergency return to Earth) in the presence of an 11 mm (0.45 inch) diameter hole (22). Data from the Long-Duration Exposure Facility indicates a flux in low-Earth orbit of 0.00001 meteoroids (and 0.0001 pieces of orbital debris) this size or larger per square meter per year (23). Thus, a good measure of protection from explosive decompression is provided by the ship itself.
Crew Altitude Protection Suit (CAPS) During launch and reentry, crewmembers are required to wear a fitted elastic garment, capable of preventing ebullism at pressures as low as 15 mm Hg (an altitude-equivalent of 70,000 feet) (5). In fact, these would be likely to reduce the risk of ebullism; unfortunately, this partial-pressure garment provides no protection from DCS or AGE.
Personal Rescue Sphere (Space Ball) Should the shuttle become compromised and unable to return to Earth, a unique emergency system is in place. Each crew member has a Personal Rescue Sphere (PRS), a 34-inch diameter fabric garment into which they can be zipped (Plate 1) (22). The sphere is then inflated with oxygen, and can be carried through space to a rescue vehicle. Each PRS has its own supply of oxygen, a window, and a small telephone. In addition to emergency decompression, the Space Ball might also be used if the cabin air became contaminated, allowing a suited crewmember to vent the entire atmosphere and replace it. Simple and rapidly donned, the Personal Rescue Sphere is the final refuge in the event of a decompressive emergency.
These are our current countermeasures for an emergency decompression. What technology is currently available to improve survival, which could be implemented in the near future?
There is currently no treatment protocol for ebullism; until recently, exposure to vacuum was generally accepted as nonsurvivable, based in part on 1960s animal research (24). But much research has been done in the intervening 30 years, and viable treatment options are beginning to emerge. Loosely characterized and in order of occurrence, these treatment phases are Return to Pressure (with hyperbaric oxygen), Basic Life Support (with high frequency ventilation), and Drug Therapy.
Return to Pressure Foremost in treatment considerations is rapid return to a pressure consistent with adequate oxygenation. Recompression reverses the massive bubbling and tissue swelling of ebullism (25), and allows for further treatment of the patient. On EVA or during planetary exploration, only on-site patching of the suit and repressurization would likely repressurize the patient adequately before asystole ensued.
On board the ISS, a depressurized crewmember would be brought to the combination airlock/compression chamber for recompression. Hyperbaric oxygen therapy would likely be used, due to its proven efficacy in treatment of DCS and AGE (both conditions associated with ebullism), although not all reports show it improves survival (26). Previous ISS design calls for a multiplace chamber rated to 3 ATA, sufficient for a Table 6 protocol (which does not increase body nitrogen; a Table 6A recompression to 6 ATA would, as 100% oxygen could not be used due to toxicity) (27). I am unable to ascertain at this time the proposed rating for the current design crew lock.
Basic Life Support Ebullism necessitates rigorous attention to basic life support (BLS) principles. Pulmonary hemorrhage and respiratory embarrassment will make vigorous endotracheal suctioning and intubation necessary. High Frequency Ventilation (HFV), a small-bore catheter ventilating at or above the resonance frequency of the lungs (600-2000 breaths per minute!), will avoid the pulmonary barotrauma and cardiac compromise associated with positive end-expiratory pressure (28).
Internal bleeding and plasma loss immediately following exposure (29) will require placement of 2 large-bore IVs and use of fluid expanders (e.g. Dextran).
Due to water phase change and evaporative cooling, body temperature drops and facial and extremity tissues can freeze. However, numerous investigators report improved survival from decompression with hypothermia (30, 31), and thus it is felt this decreased temperature should be maintained for at least 2 hours before rewarming (32).
In addition, although asystole ensues after about 2 minutes, cardiac contractility continues for at least 5 minutes; advanced cardiac life support may be called for (current ISS design includes a defibrillator with data telemetry to the Flight Surgeon).
Drug Therapy A number of drugs have been tested for the prevention and treatment of Ebullism and the attendant conditions of DCS and AGE, with varying degrees of success.
Pentoxifylline (Trental) This drug, commonly prescribed for peripheral vascular insufficiency, increases red cell deformability and decreases blood viscosity. Cerebral blood flow, decreased by slow experimental decompression, has been shown to increase significantly with pre-exposure treatment (33). Its mild side effect profile and potential for increasing cerebral oxygenation following recompression recommend it as a possible EVA pretreatment.
NMDA Antagonists Hypoxia causes influx of calcium to the hippocampus and dorsal thalamic nucleus, causing hyperexcitability and seizure activity, mediated by the N-methyl-D-aspartate (NMDA) receptor (34). Blockade of this receptor protects the CNS (35). Two agents worthy of further research are MK-801, which protects when given as late as 75 minutes after exposure (35), and HWA 285 (Propentofylline) (36).
Calcium Channel Blockers This hypoxia-calcium-hyperexcitability cascade can also be interrupted at the calcium level, with calcium channel blockers that penetrate the CNS and prevent calcium loading without causing hypotension. Several that have been investigated are, in order of potency, vinpocetine, l-eburnamonine, vinconate and vincamine (37). All have been shown to increase survival following hypoxic injury.
Prostaglandins Several prostaglandins present a dose-dependent protection against cerebral hypoxia, including PGE, PGI2 and PGD2. All 3 inhibit platelet aggregation and vasodilation, which occur during ebullism (38).
EVA Modification Currently-used Extravehicular Mobility Units (EMUs) operate at 4.3 psi; Russian Orlan-M suits are pressurized to 5.7 psi. As mentioned earlier, one determinant of the severity of ebullism is absolute change in pressure. A space suit pressurized to a lower pressure would impose a lower pressure differential on its occupant, and thus a lower severity of decompression. Research is currently underway to produce a suit pressurized to 3.5 psi, using ARGOX (62% argon 38% oxygen) gas (39).
Attitudes towards survivability of humans in vacuum began to change as humans underwent accidentally decompressions and survived. In 1960, Joe Kittinger was ascending to 102,800 feet when he lost pressurization to his right hand. Instead of descending, he decided to continue the ascent, and his hand became painful and useless in the near-vacuum. On descending (by way of his record-breaking parachute jump), however, the hand returned to normal (40).
In a videotaped case in 1966, a technician in Houston was altitude-testing a space suit when he lost suit pressure and was instantaneously exposed to an altitude of 120,000 feet (18). He recalled the saliva boiling off his tongue as he passed out, and regained consciousness as the chamber monitor called 14,000 feet. He suffered no neurological sequelae and was not hospitalized.
In 1982 a technician was decompressed over 3 minutes to an altitude greater than 74,000 feet, and held at maximum altitude for another 60 seconds A manager had to kick in a glass ionization gauge atop the chamber to allow air to leak in (due to cycle stopped in mid-process), and by the time the chamber was opened the victim had been above 63 millibar for 1 to 3 minutes. The patient was cyanotic, frothing at the lips, bleeding from his lungs and had grade 4 barotrauma of both eardrums. He was given IV Decadron and recompressed to 6 ATA using NITROX (50% nitrogen 50% oxygen) 5 _ hours after exposure. By 24 hours after exposure he was awake and alert; he was extubated at day 5, and at 1 year follow-up had neurological performance superior to testing before the accident (14).
Could an astronaut ever suffer direct contact with space, bleed out into the vacuum, and survive? In fact, one already has. Posting to sci.space, Gregory Bennett wrote:
"Incidentally, we have had one experience with a suit puncture on the Shuttle flights. On STS-37, during one of my flight experiments, the palm restraint in one of the astronauts gloves came loose and migrated until it punched a hole in the pressure bladder between his thumb and forefinger. It was not an explosive decompression, just a little 1/8 inch hole, but it was exciting down here in the swamp because it was the first injury weve ever had from a suit incident. Amazingly, the astronaut in question didnt even know the puncture had occurred; he was so hopped on adrenaline it wasnt until after he got back in that he even noticed there was a painful red mark on his hand. He figured his glove was chafing and didnt worry about it . What happened: when the metal bar punctured the glove, the skin of the astronauts hand partially sealed the opening. He bled into space, and at the same time his coagulating blood sealed the opening enough that the bar was retained inside the hole." (41)
Despite the losses of gravity, oxygen, warmth, pressure and every other life-sustaining characteristic of Earth, we have learned to adapt our surroundings to carry us higher and farther than the mass of humanity once thought possible. From Croce-Spinellis first use of oxygen to the union of the first 2 pieces of the next milleniums space station, we have not allowed any environment to hold us out; nor, if we can judge by history, should we.
Advances in medicine have given us the tools to start treating this latest problem, the boiling away of our own flesh, and accidents at altitude have allowed us to begin to test them, with some success. We have only begun to crawl out of the protective envelope of our birth planet, and we will not stop here.
"The Earth is the Cradle of mankind, but one does not live in the cradle forever."
-- Konstantin Tsiolkovsky, 1895