On Neurolab, crewmembers will carry
out a comprehensive set of tests designed to investigate every aspect of
autonomic circulatory control, with the overall goal of understanding how
it has changed after space flight. Several key measurements will be made.
In addition to measuring blood pressure and other basic cardiovascular
parameters, blood flow to the brain will be estimated using the non-invasive
transcranial Doppler technique.
Measurement Technique 1: Sound
With this device, high-frequency
sound waves can be used to show how blood flow to the brain is regulated.
This is important because not all people who have orthostatic intolerance
after space flight experience a drop in blood pressure. They may have a
problem with the control of brain blood flow, and the transcranial Doppler
measurements can reveal this.
Measurement Technique 2: Tiny
Another innovative technique to
be used on Neurolab is microneurography. With this procedure, a small needle
the size of an acupuncture needle is placed in an easily accessible nerve
just below the knee (see box). With this needle, the nerve signals traveling
from the brain to the blood vessels can be measured directly. This provides
a clear insight into how the autonomic nervous system is functioning.
The measurements are made while
the autonomic nervous system is challenged by a variety of tests. The controlled
frequency breathing test allows for the natural oscillations in the control
of blood pressure to be measured precisely. The Valsalva test stimulates
the pressure receptors in the neck and chest and measures the responses.
The lower body negative pressure test places a stress on the cardiovascular
system very similar to the one experienced with standing in Earth's gravity.
The cold pressor and hand grip tests also activate the blood pressure control
system and raise blood pressure, but not by stimulating pressure receptors
as the Valsalva test does. The cuff occlusion test and body impedance measurements
show the fluid distribution in the body and how this may contribute to
problems in blood pressure control. When the results of these tests are
evaluated together, the part of the autonomic system that may be functioning
improperly can be localized.
One suggested reason for why the
blood pressure control system has changed is that it is underactive in
space. The autonomic nervous system releases chemical messengers when it
operates (norepinephrine is one). There has been some disputed evidence
from previous space flightS that the levels of norepinephrine are lowered
in space and that this indicates a low level of activity in the blood pressure
control system. Looking only at the blood levels of norepinephrine, however,
can lead to false conclusionS, since the blood level depends not only on
how much is released, but also on how fast it is removed from the circulation.
On Neurolab, crewmembers will infuse labeled norepinephrine into the blood
stream and collect blood samples. Investigators will use these samples
to assess whether the blood pressure control system is really underactive
Taken together, this comprehensive
set of measurements should help to solve the puzzle of why astronauts have
problems with blood pressure control after space flight. The answer is
important not only for helping astronauts to re-enter gravity safely after
being in space, but also may help with patients who suffer with diseases
of their blood pressure control systems.