[Cerebral Autoregulation]
Human cerebral autoregulation during long-duration spaceflight

RESEARCH OVERVIEW

• The human brain, as the most important organ in the body, requires a high blood flow supply. The brain blood flow regulation system must rapidly adjust the brain’s blood vessels to maintain blood flow to the brain, when the cardiovascular system cannot perfectly maintain arterial blood pressure. The changes in the regulation of brain blood flow may be related to “fainting after spaceflight” frequently seen in astronauts.
• In the Human Cerebral Autoregulation during Long-duration Spaceflight (Cerebral Autoregulation) investigation, the changes in the capacity of maintaining brain blood flow during short- and long-term spaceflight are estimated.
• Improved regulation for brain blood flow may be the compensatory mechanism for the brief loss of consciousness and lightheadedness that many astronauts experience in the upright posture after spaceflight. The findings from this investigation could have broad application to medicine, both for astronauts and syncope patients on Earth.

DESCRIPTION

From previous experiments from the Neurolab mission aboard the Space Shuttle (STS-90), it was hypothesized that human cerebral autoregulation is preserved, and possibly even improved by two weeks in space. Moreover, prolonged spaceflight may progressively improve human cerebral autoregulation more. The purpose of the Human Cerebral Autoregulation during Long-duration Spaceflight (Cerebral Autoregulation) investigation is to determine the effects of short and long-duration spaceflight on cerebral blood flow autoregulation in human, testing the above hypothesis.

Waveforms of arterial blood pressure in the finger artery, and blood flow velocity in middle cerebral artery, are measured before, during, and after long-term spaceflight using the Portable Doppler (PDOP). The PDOP is European Space Agency (ESA) hardware that is used to obtain waveforms of blood flow velocity in the middle cerebral artery, and records this data by non-invasive measurement. Cardiopres is another piece of ESA hardware, used to obtain waveforms of continuous arterial blood pressure by non-invasive measurement. The linear dependence of middle cerebral artery flow velocity on arterial blood pressure is to be estimated, as well as the magnitude of transfer from arterial blood pressure to middle cerebral artery flow velocity.

The estimated result is that long-duration spaceflight decreases the linear dependence of middle cerebral artery flow velocity on arterial blood pressure, and the magnitude of transfer from arterial blood pressure to middle cerebral artery flow velocity. If so, the results imply an improvement of dynamic cerebral autoregulation by long-duration spaceflight. This adaptation would play an important role to counteract decreases in cerebral perfusion pressure when the cardiovascular system cannot perfectly maintain arterial blood pressure after space flight. The determination of the effects of spaceflight on the autoregulation of cerebral blood flow is scientifically interesting, and important to human spaceflight.

SPACE APPLICATIONS

After returning to Earth, many astronauts faint or experience lightheadedness, which may be related to changes in blood flow in the brain. Understanding how these blood flows change improves efforts to treat this space-related lightheadedness, including possible countermeasures.

EARTH APPLICATIONS

When the brain cannot compensate for a sudden drop in blood pressure, a person becomes lightheaded or briefly loses consciousness in a fainting episode. This is known as syncope, and it affects millions of people worldwide. Understanding how to improve the brain’s self-regulation of blood flow would benefit these patients, as well as future space travelers.

OPERATIONAL REQUIREMENTS AND PROTOCOLS

Hardware Checkout
Equipment and JAXA unique configuration is checked out in ISS prior to the human experiment without human subject.
Number of subjects: 6 crew members.

In flight
The data collected is downlinked to the ground for analysis. No required samples are needed. Constraints for subjects are: Activity 1 is conducted at L+14 days (-7/+0day), Activity 2 is conducted at 4 weeks (-1/+1 week), Activity 3 is conducted at 16 weeks (-1/+1 week). No exercise including VO2max should be performed within 12 hours before each activity. No EVA should be performed for 12 hours prior to the session. More than 1 hour should be elapsed after nitroglycerin, catecholamines, etc. No caffeine or nicotine for 12 hours prior to testing. No heavy meal within the last 4 hours before performing experiment. A light snack of complex carbohydrates is OK within 2 hours of testing.

The hardware checkout and Man-in Loop Testing, which includes the measurement of cerebral blood flow velocity, blood pressure, and ECG is conducted in advance of the first session.

Pre and Post-BDC
Constraints for subjects are: crews whose Arterial Blood Pressure (ABP) and Cerebral Blood Flow Velocity (CBFV) wave form can be measured (check at precheck session). No maximal exercise including VO₂max, and NBL within 12 hours before each BDC session. More than 1 hour should be elapsed after nitroglycerin, catecholamines, etc. No caffeine, nicotine, or alcohol for 12 hours prior to testing. No heavy meal within the last 4 hours before performing experiment. A light snack of complex carbohydrates is OK within 2 hours of testing. Other medicine and exercise logs of subjects for 6 hours prior to testing are required, if acceptable. In addition, as secondary evaluation, intracranial pressure will be estimated using pre/postflight data of high resolution waveforms with increased high frequency.

The following data sharing are requested
Mandatory--Diagnosis of optic disc edema (MEDB 1.10) pre-/in-/postflight ( e.g. Fundoscopy, Vision Testing, Ultrasound, and MRI reports, including clinical specialist diagnosis)

If Available--

• Ophthalmology/Optometry (MEDB 1.10) reports pre-/in-/postflight (Vision Testing (pre/post), Visual Acuity (inflight), which measure amsler grid, visual fields, visual acuity changes, and refractive micropsia).
• Laboratory Testing (MEDB2.1) (e.g. Hemoglobin (Hb), Hematocrit (Ht), Red Blood Cell (RBC), etc.).
• On-Orbit Strength & Conditioning Monitoring (MEDB 5.2) (e.g. Number of training sessions/week, etc.).
• Medication history at pre-/in-/postflight (e.g. logs of 6 hours prior to each BDC and in-flight, etc.).
• Orthostatic tolerance (MEDB4.2).

1. Prepare Personal Computer (PC).
2. Measure blood flow velocity in middle cerebral artery, and continuous arterial blood pressure, 3 times. (L+2 weeks, L+1 month, L+4 months).
3. PC Close Out.
4. Data down link