Ulrich Limper

Upright cardiac output measurements in the transition to weightlessness during parabolic flights.

OBJECTIVE Aims of this study were: 1) to determine cardiac output by inert gas rebreathing (CO(reb)) during transition into 0 Gz in the standing position; and 2) to compare impedance cardiography (ICG) and pulse contour method (PCM) with CO(reb) as a reference method. METHODS We measured baseline CO(reb) and heart rate (HR) on the ground, and CO(reb), CO(pcm), CO(icg), and HR in standing and supine positions in the transition to weightlessness in six subjects. We conducted repeated measures ANOVA, Bland and Altman analysis, and analysis of percentage error of each data set. RESULTS CO(reb) rose from 5.03 +/- 0.7 upright ground control to 11.45 +/- 3.6 L x min(-1) in 0 Gz. HR and stroke volume (SV) rose from 83 +/- 14 to 113 +/- 19 bpm and from 61 +/- 6 to 99 +/- 18 ml, respectively. Mean CO(reb), CO(pcm), and CO(icg) across all conditions were 10.45 +/- 3.04, 7.42 +/- 1.71, and 6.57 +/- 2.46 L x min(-1), respectively. Overall Bland and Altman analysis showed poor agreement for CO(pcm) and CO(icg) compared to CO(reb). DISCUSSION Large bias for both comparisons indicated that both PCM and ICG underestimate the true CO value. Paired CO values of individual subjects showed a better correlation between methods and a broad bias range, indicating a preponderant role for large between-subjects variability. Repeated CO(reb) determinations in 1 Cz (i.e., when the cardiovascular system is in a steady state) should be used for calibration of the PCM and of ICG data. PCM and ICG can then be used to track CO dynamics during rapid changes of acceleration profiles.

Timing and source of the maximum of the transthoracic impedance cardiogram (dZ/dt) in relation to the H-I-J complex of the longitudinal ballistocardiogram under gravity and microgravity conditions

The transthoracic impedance cardiogram (ICG) and the acceleration ballistocardiogram (BCG) measured close to the center of mass of the human body are generated by changes of blood distribution. The transthoracic ICG is an integrated signal covering the whole thorax and spatial resolution is poor. Comparison between both signals can be used to elucidate the source of the ICG signal. We recorded the ECG, ICG, and BCG simultaneously in healthy subjects under resting and microgravity conditions during parabolic flights. The time interval between the R-peak of the ECG and the maximum of the ICG (R-dZ/dtmax) and the time interval between the R-peak of the ECG and the I-peak in the BCG (R-I) differed significantly (p<;0.0001). The I-peak in the BCG always occurred earlier during systole than dZ/dtmax. The delay of dZ/dtmax ranged between 23 and 28 ms at rest and was lowest under microgravity conditions (12±4 ms, p<;0.02). Our results suggest that both signals have different sources. Combination of modern imaging techniques with classical non invasive approaches to detect changes of blood distribution may provide new insights into the complex interaction between blood flow and mechanocardiographic signals like the BCG.

An international collaboration studying the physiological and anatomical cerebral effects of carbon dioxide during head-down tilt bed rest: the SPACECOT study

Exposure to the microgravity environment results in various adaptive and maladaptive physiological changes in the human body, with notable ophthalmic abnormalities developing during 6-mo missions on the International Space Station (ISS). These findings have led to the hypothesis that the loss of gravity induces a cephalad fluid shift, decreased cerebral venous outflow, and increased intracranial pressure, which may be further exacerbated by increased ambient carbon dioxide (CO2) levels on the ISS. Here we describe the SPACECOT study (studying the physiological and anatomical cerebral effects of CO2 during head-down tilt), a randomized, double-blind crossover design study with two conditions: 29 h of 12° head-down tilt (HDT) with ambient air and 29 h of 12° HDT with 0.5% CO2 The internationally collaborative SPACECOT study utilized an innovative approach to study the effects of headward fluid shifting induced by 12° HDT and increased ambient CO2 as well as their interaction with a focus on cerebral and ocular anatomy and physiology. Here we provide an in-depth overview of this new approach including the subjects, study design, and implementation, as well as the standardization plan for nutritional intake, environmental parameters, and bed rest procedures.NEW & NOTEWORTHY A new approach for investigating the combined effects of cephalad fluid shifting and increased ambient carbon dioxide (CO2) is presented. This may be useful for studying the neuroophthalmic and cerebral effects of spaceflight where cephalad fluid shifts occur in an elevated CO2 environment.

Pulse contour methods to estimate cardiovascular indices in micro- and hypergravity.

BACKGROUND The importance of noninvasive health monitoring in space increased as a result of the long-duration missions on the International Space Station (ISS). In order to monitor changes in cardiovascular indices such as cardiac output (CO) and total peripheral resistance (TPR), many methods have been developed using signal processing and mathematical modeling techniques. However, their performance in various gravitational conditions has not been known. METHODS The present study compared 10 methods to estimate CO and TPR by processing peripheral arterial blood pressure signals recorded from 8 subjects in multiple gravity levels (1 G, 0 G, and 1.8 G) during parabolic flights. For reference data sets, CO and TPR were simultaneously obtained by an inert gas rebreathing technique. Root normalized mean square errors and Bland-Altman plots were used to evaluate the estimation methods. RESULTS The corrected impedance method achieved the lowest estimation errors (20.0% CO error and 23.5% TPR error) over the three gravity levels. In microgravity, mean arterial pressure was also demonstrated to be an indicator of CO (24.5% error). DISCUSSION The corrected impedance method achieved low estimation errors for a wide range of the gravity levels. Gravity-dependent performance was observed in the mean arterial pressure method that achieved low errors in the short-term 0 G.

Petechiae: reproducible pattern of distribution and increased appearance after bed rest.

BACKGROUND Exposure to acceleration can cause petechial hemorrhages, called G measles. Petechiae usually start to develop between 5 and 9 G with a high interindividual variance. Centrifuge training delays the onset to higher G levels. One might expect onset at lower G levels after bed rest; however, there is no evidence in the literature. A case of petechiae formation after bed rest is presented here. CASE REPORT Orthostatic tolerance was tested using a tilt table and lower body negative pressure before and after bed rest in both campaigns of a 2 x 21-d bed rest study with 6 degrees head-down tilt. A 42-yr-old male Caucasian without any history of thrombosis, venous disease, hemorrhage, or petechiae, and with a negative thrombophilia screening, took part in the bed rest study as 1 out of 10 subjects. He was the only one to develop petechiae during the orthostatic tests after, but not before, bed rest in both campaigns. Petechiae were distributed throughout the lower legs and most pronounced at the shin in a stocking-like fashion, surprisingly reoccurring in an identical pattern of distribution. Petechiae appeared slowly over minutes during hyperemia. DISCUSSION This case indicates that prolonged bed rest decreases the threshold for petechiae formation. A reproducible distribution pattern suggests that factors predisposing to petechiae formation keep their local distribution over time (possibly due to local vessel structures). Mechanisms of adaptation and interindividual variance are unclear. Findings are of clinical relevance as such cases might occur after prolonged bed rest in patients without need of expensive testing.

Bed Rest, Exercise Countermeasure and Reconditioning Effects on the Human Resting Muscle Tone System

The human resting muscle tone (HRMT) system provides structural and functional support to skeletal muscle and associated myofascial structures (tendons, fascia) in normal life. Little information is available on changes to the HRMT in bed rest. A set of dynamic oscillation mechanosignals ([Hz], [N/m], log decrement, [ms]) collected and computed by a hand-held digital palpation device (MyotonPRO) were used to study changes in tone and in key biomechanical and viscoelastic properties in global and postural skeletal muscle tendons and fascia from a non-exercise control (CTR) and an exercise (JUMP) group performing reactive jumps on a customized sledge system during a 60 days head-down tilt bed rest (RSL Study 2015–2016). A set of baseline and differential natural oscillation signal patterns were identified as key determinants in resting muscle and myofascial structures from back, thigh, calf, patellar and Achilles tendon, and plantar fascia. The greatest changes were found in thigh and calf muscle and tendon, with little change in the shoulder muscles. Functional tests (one leg jumps, electromyography) showed only trends in relevant leg muscle groups. Increased anti-Collagen-I immunoreactivity found in CTR soleus biopsy cryosections was absent from JUMP. Results allow for a muscle health status definition after chronic disuse in bed rest without and with countermeasure, and following reconditioning. Findings improve our understanding of structural and functional responses of the HRMT to disuse and exercise, may help to guide treatment in various clinical settings (e.g., muscle tone disorders, neuro-rehabilitation), and promote monitoring of muscle health and training status in personalized sport and space medicine.