MEDICAL CARE IN SPACE
For a few minutes during launch of a space vehicle, gravitational
force increases as the spacecraft accelerates to orbital speed (typically 3g for
the NASA Shuttle, 4g for the Russian Soyuz). During the period in space, the major
physiologic stress on astronauts is the absence of gravitational stress (microgravity),
which results in an increase in left ventricular end-diastolic volume, paradoxically
accompanied by a decrease in central venous pressure.[290]
The central redistribution of blood causes facial edema, initiation of diuresis,
and consequently, depletion of plasma volume (up to 20% or more), which persists
until after landing.[291]
Shortly after attaining
orbit, most astronauts experience self-limited dizziness, drowsiness, nausea, and
vomiting.[292]
During reentry, astronauts are re-exposed
to increased gravitational force, typically 1.5g for the Shuttle and 3g to 4g for
the Soyuz craft. After landing, astronauts generally experience some degree of orthostatic
intolerance[293]
and often a recurrence of nausea
and vomiting.[292]
Postural hypotension has been
attributed to hypovolemia, enhanced expression of endothelial NO synthase, and downregulation
of α-adrenergic receptors.[294]
Cabin pressure in the Shuttle and International Space Station
is 760 mm Hg, but during extravehicular activity (EVA), space suit pressure is 4.3
psi (222 mm Hg). Astronauts prevent hypoxia during EVA by breathing 100% O2
;
however, the reduced pressure engenders the potential risk of decompression sickness.
[295]
[296]
If
the
suit becomes disrupted during EVA, the astronaut's ambient pressure would quickly
drop to zero and result in hypoxia and generalized bubble formation, referred to
as ebullism (body fluids boiling because of ambient
pressure lower than saturated water vapor pressure).
Gravitational unloading causes loss of calcium from bones, which
during long space flights could produce significant osteoporosis. This problem,
along with the resulting hypercalciuria and increased probability of nephrolithiasis,
are major obstacles to human interplanetary travel.
Emergency care during space presents numerous challenges.[296]
[297]
[298]
[299]
[300]
[301]
Hypovolemia
from hemorrhage may not be adequately treatable because of limited quantities of
supplies, and hypotension could be exacerbated by preexisting volume loss and increased
gravitational forces during reentry. Air-fluid interfaces in intravenous fluids
generate bubbles ( Fig. 70-18
).
Thus, intravenous bags must be degassed before flight or bubbles must be removed
by in-line filtration. The physiologic effects of general or regional anesthesia
in microgravity are unknown. Conventional anesthesia vaporizers require gravity
to confine the liquid to the bottom of the reservoir; thus a different design would
be required for use in space. Additional practical limitations include the need
to prevent "pollution" of the enclosed environment by volatile anesthetics or exhaust
oxygen (fire hazard). Tracheal intubation may be difficult because of facial edema
and the need to restrain (tether) both the patient and the intubator. Gastroesophageal
reflux is more common in microgravity, so aspiration during
Figure 70-18
Air in the bag of intravenous fluid in microgravity with
an infusion pump. The lack of a gravitational field makes it difficult to remove
gas. (Courtesy of NASA.)
general anesthesia may be more likely. On-board physicians may not be skilled at
performing either surgical or anesthetic procedures.
Remote robotic surgery has been considered as a substitute for
an on-board surgeon. Indeed, robotic laparoscopic cholecystectomy was successfully
performed on a patient in Strasbourg, France, by surgeons operating from New York
City. The total delay time between movement initiated by the surgeon and detection
on the monitor was approximately 155 milliseconds.[302]
The limit of the acceptable time delay is believed to be around 330 milliseconds.
Whether training or technology will allow this limit to be increased to permit surgery
during interplanetary flights (with a much longer delay time) is unknown.
Anesthetic challenges may extend into the period after landing.
After 14 days in space, two rhesus monkeys were administered general anesthesia
(ketamine, 10 mg/kg intramuscularly and then 1% to 1.5% isoflurane for 3 ½
to 4 hours) within 24 hours after landing by a competent veterinary anesthesiologist
to obtain biopsy specimens. One animal aspirated during emergence and could not
be resuscitated. Three hours after recovery, lethargy and facial edema inexplicably
developed in the other.[303]
From this experience
it can be inferred that well-accepted standard-of-care techniques may not be safe
when anesthetizing astronauts shortly after space flight.
