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Background: We compared microvascular and macrovascular blood flows of the tibia and anterior tibial artery during graded whole-body tilt. We hypothesized equal responses for bone microvascular and macrovascular blood flows during varying angles of tilt., Methods: There were 18 volunteers who were randomly positioned in the following postures: supine, 15° head-up tilt, 6° head-up tilt, 6° head-down tilt, and 15° head-down tilt using an inversion table with reference to seated posture (baseline control). Ultrasonography quantified anterior tibial arterial diameter and peak systolic velocity, enabling calculation of macrovascular blood flow to the tibia. Tibial bone microvascular blood flow was measured noninvasively using photoplethysmography in the same leg., Results: Transitioning from a seated position to a supine position, macrovascular blood flow did not change significantly (1.81 ± 1.18 to 2.80 ± 1.74cm 3 · s-1). However, bone microvascular flow increased significantly (0.36 ± 0.23 to 1.11 ± 0.79 V) from the seated to the supine position. Transitioning from a seated posture to 15° head-down tilt, both arterial macrovascular and bone microvascular flows increased significantly (1.81 ± 1.18 to 3.32 ± 2.08 cm3 · s-1 and 0.36 ± 0.23 V to 2.99 ± 2.71 V, respectively). The normalized flow for microvascular blood flow as a function of body tilt was significantly greater than that for macrovascular blood flow at 6° and 15° head-down tilt., Discussion: These data do not support our hypothesis that bone microvascular flow and arterial macrovascular flow share equal responses to altered body tilt. Therefore, for a given decrease in local blood pressure in the leg with head-down body tilt, the magnitude of increase in blood flow is greater in the microcirculation as compared to the feeding artery.Becker RL, Siamwala JH, Macias BR, Hargens AR. Tibia bone microvascular flow dynamics as compared to anterior tibial artery flow during body tilt. Aerosp Med Hum Perform. 2018; 89(4):357-364.

Background: Seven astronauts after 6-mo missions to the International Space Station showed unexpected vision problems. Lumbar punctures performed in the four astronauts with optic disc edema showed moderate elevations of cerebral spinal fluid pressure after returning to Earth. We hypothesized that lower body negative pressure (LBNP) imposed during head-down tilt (HDT) would reduce intraocular pressure (IOP) and transcranial ultrasound pulse amplitude, a noninvasive intracranial pressure (ICP) surrogate., Methods: Participating in this study were 25 normal healthy nonsmoking volunteers (mean age: 36 yr). Subjects were positioned supine (5 min), sitting (5 min), 15° whole body HDT (5 min), and 10 min of HDT with LBNP (25 mmHg). The order of HDT and HDT+LBNP tests was balanced. Right and left IOP, transcranial ultrasound pulse amplitude, arm blood pressure, and heart rate were measured during the last minute (steady state) of each testing condition., Results: IOP significantly decreased from supine to sitting posture by 3.2 ± 1.4 mmHg (mean ± SD: N = 25), and increased by 0.9 ± 1.3 mmHg from supine to the HDT position. LBNP during HDT significantly lowered IOP to supine levels. In addition, LBNP significantly reduced transcranial ultrasound pulse amplitudes by 38% as compared to the HDT condition (N = 9). Sitting mean blood pressure (BP) was significantly higher (+5 mmHg) than BP values after 10 min of LBNP during HDT. However, heart rate was not significantly different across all conditions., Discussion: These data suggest that short duration exposures to LBNP attenuate HDT-induced increases in IOP and ICP. Macias BR, Liu JHK, Grande-Gutierrez N, Hargens AR. Intraocular and intracranial pressures during head-down tilt with lower body negative pressure.

Introduction: Resistive exercise helps prevent muscle atrophy in microgravity, but better exercise equipment is needed. Therefore, the purpose of this study was to determine if a pneumatic, gravity-independent leg-press device (LPD) provides sufficient force to leg musculature. We hypothesized that intramuscular pressure (IMP), a quantitative index of muscle force, is greater in the antigravity superficial posterior and deep posterior compartments than in the non-antigravity anterior compartment during bilateral leg-press exercise., Methods: Millar pressure transducers were inserted into the anterior, lateral, superficial posterior, and deep posterior muscle compartments of the left leg of eight healthy subjects (three women, five men). Subjects were supine on the Keiser SX-1, a pneumatic LPD. Then maximal voluntary contraction (MVC) was determined; each subject performed three consecutive voluntary contractions at approximately 18%, 50%, and 100% MVC while continuously measuring IMP. Repeated measures ANOVA were used to determine differences of IMPs between compartments and loads., Results: The magnitudes of IMP (mean +/- SEM) at 18 - 3% (abbreviated approximately 18%), 50%, and 100% MVC in the superficial and deep posterior compartments were significantly greater than that in the anterior compartment during exercise (P < 0.05). Additionally, IMPs in all four compartments significantly rose as resistance increased at approximately 18%, 50%, and 100% MVC (P < 0.05)., Conclusion: The LPD provides significantly increased resistance to all four compartments, but with greater loading of the antigravity compartments as compared to the non-antigravity compartment. Since antigravity muscles of the leg are contained primarily in the superficial and deep posterior compartments, the LPD may help prevent muscle atrophy associated with microgravity.

Introduction: Intracranial pressure (ICP) may be an important contributor to symptoms of space adaptation syndrome during the initial days of microgravity exposure. The temporary nature of these symptoms suggests that some physiologic adaptation or compensation occurs. Fluid shifts similar to those in microgravity can be simulated on Earth using head-down tilt (HDT) bed rest. This study was performed to calibrate a new noninvasive ICP instrument and to investigate ICP adaptation during 30 d of HDT bed rest., Methods: A noninvasive ultrasound technique that measures small skull expansions with fluctuations in ICP was used to measure cranial oscillations before and near the end of 30-d HDT bed rest in eight healthy, male volunteers. Pulse phase-locked loop (PPLL) output voltage and arterial BP were continuously monitored and correlated., Results: The amplitude of intracranial distance pulsation decreased during 30-d bed rest. Prior to bed rest, the PPLL amplitude was 25 +/- 9 mV and this amplitude was reduced by 60% to 9 +/- 4 mV (a value consistent with that of upright posture) at the end of HDT bed rest (p = 0.01)., Discussion: PPLL measurements of skull pulsations are acutely posture dependent, being significantly higher in supine and HDT as compared with upright posture. A cephalad fluid shift is probably the responsible mechanism. Our results indicate that there are adaptations to intracranial pooling of blood and tissue fluid during bed rest that reduce skull pulsation amplitudes to values similar to those obtained in normal upright posture. Detailed studies of the time course of cranial vessel and bone adaptations may provide insights into the potential adaptative mechanisms.

Introduction: Intracranial pressure (ICP) may play a significant role in physiological responses to microgravity by contributing to the nausea associated with microgravity exposure. However, effects of altered gravity on ICP in astronauts have not been investigated, primarily due to the invasiveness of currently available techniques. We have developed an ultrasonic device that monitors changes in cranial diameter pulsation non-invasively so that we can evaluate ICP dynamics in astronauts during spaceflight. This study was designed to demonstrate the feasibility of our ultrasound technique under the physiological condition in which ICP dynamics are changed due to altered gravitational force., Methods: Six healthy volunteers were placed at 60 degrees head-up, 30 degrees headup, supine, and 15 degrees head-down positions for 3 min at each angle. We measured arterial blood pressure (ABP) with a finger pressure cuff, and cranial diameter pulsation with a pulsed phase lock loop device (PPLL)., Results: Analysis of covariance demonstrated that amplitudes of cranial diameter pulsations were significantly altered with the angle of tilt (p < 0.001). The 95% confidence interval for linear regression coefficients of the cranial diameter pulsation amplitudes with tilt angle was 0.862 to 0.968. However, ABP amplitudes did not show this relationship., Discussion: Our noninvasive ultrasonic technique reveals that the amplitude of cranial diameter pulsation decreases as a function of tilt angle, suggesting that ICP pulsation follows the same relationship. It is demonstrated that the PPLL device has a sufficient sensitivity to detect changes non-invasively in ICP pulsation caused by altered gravity.