摘要:

Times of heightened susceptibility are expressed by nonrandom patterns in the incidence of various diseases, not only along the scale of a day but also of a week and a year. Whereas these rhythms can be synchronized by the environment, their endogenicity is revealed by their persistence in the absence of time cues with a period slightly but statistically significantly different from the environmental match. In the case of some circadians, the gene involved has been identified and heritability in humans determined by studies on twins. Vital signs are now amenable to automatic monitoring around the clock. When analyzed by chronobiologic software, the data provide information regarding the given individual's time structure (chronome). Such physiologic monitoring serves the multiple purposes of deriving time‐specified reference norms on the basis of which rhythm alteration can detect an elevated risk early, thus prompting timely preventive action and timed treatment whenever warranted. For long journeys in space, the design of a multi‐disease prophylactic pill poses a chronopharmacologic challenge. Drugs such as aspirin and carnitine, used for the prevention of strokes, myocardial infarctions and depression, all show striking chronome‐dependent effects which can determine not only the presence or the absence of a desired effect, but even yield effects in opposite direc

ISSN:0091-2700    

DOI:10.1002/j.1552-4604.1994.tb02005.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

A remote computerized health care (telehygiene) system for space travel includes monitoring devices and pharmaceuticals aimed at the optimization of health and the environment. Early risk indicators are provided by dynamic characteristics of rhythms of several frequencies describing variation inside the physiologic range. These rhythm characteristics, assessed as one goes, can be updated and compacted as data accumulate by the use of chronobiologic software that resolves anticipated components of lower and lower frequency, thus providing summaries of data at intervals of differing length. At any time, harbingers of risk, including characteristics of circannual rhythms, can be retrieved in an instant. On their basis, early preventive action can be instituted for risk lowering and for delivery of timed treatment when needed. Preventive or curative health care in space and terrestrial spin‐offs are the more effective the more of the chronome (time‐structure) is resol

ISSN:0091-2700    

DOI:10.1002/j.1552-4604.1994.tb02006.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Exposure to microgravity with space flight, or to earth‐based analogs such as head‐down tilt (HDT) bedrest, results in cardiovascular deconditioning. With reapplication of gravity, deconditioning is most often observed as an elevated heart rate, a narrowed pulse pressure, and possibly even a failure to maintain blood pressure with symptoms of presyncope or syncope. Noninvasive measurements of heart rate variability (HRV) have been used to study cardiovascular control mechanisms during orthostatic stress (head‐up tilt and lower body negative pressure [LBNP]). Recently, the authors developed a new approach to study heart rate and blood pressure control mechanisms. Coarse graining spectral analysis (CGSA) allows simultaneous extraction of not only parasympathetic (PNS) and sympathetic (SNS) indicators from the HRV signal, but also determines the fractal dimension (DF), calculated from the slope (β) of the log spectral power‐log frequency relationship. The fractal dimension is an index of the complexity of the cardiovascular control system. Our investigations have indicated patterns of HRV consistent with reduced PNS and increased SNS activity and reduced DFwith the HRV signal during orthostatic stress. These findings also indicate that the cardiovascular system may be less stable when operating at a reduced level of co

ISSN:0091-2700    

DOI:10.1002/j.1552-4604.1994.tb02007.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

This study examined the heart rate response to lower body negative pressure (LBNP) during 17 weeks of horizontal bed rest to estimate the development and duration of orthostatic instability elicited by this model for space flight. Based on data from Skylab, the authors hypothesized that orthostatic (LBNP) instability would appear during the first 3 to 4 weeks, and would then remain constant for the duration of bed rest. Heart rates of four healthy adult male subjects were monitored at rest and during LBNP for 1 week of ambulatory control, 17 weeks of horizontal bed rest, and 5 weeks of recovery. The LBNP protocol consisted of 10 minutes of control (atmospheric pressure) and 5 minutes each at 5, 10, 20, 30, 40, and 50 mm Hg decompression, followed by a 10‐minute recovery period; this protocol was repeated weekly to document the progressive changes in heart rate response to LBNP. Lower body negative pressure was terminated early if symptoms compatible with the onset of syncope occurred. Throughout the study, heart rate was unchanged at 5, 10, and 20 mm Hg, but it increased at 30, 40, and 50 mm Hg LBNP. During the pre‐bed rest period, peak heart rate was 97 ± 10 beats/min (mean ± SE), occurring at 50 mm Hg for all four subjects. After 3 days of bed rest, all monitored heart rate responses, including values after release of LBNP, were only slightly elevated (NS) above pre‐bed rest level. Peak heart rate was 118 ± 21 beats/min at 50 mm Hg decompression (NS; N = 3). After 3 weeks of bed rest, peak heart rate was 110 ± 16 beats/min at 50 mm Hg decompression (NS; N = 4). The slight increase (NS) in heart rate persisted throughout the 17 weeks of bed rest. In three subjects, the duration of tolerance to LBNP decreased; in two subjects, decompression was ended before completion of the full 5‐minute period at 50 mm Hg; and in one subject, decompression was ended before completion of the full 5‐minute period at 40 mm Hg. This indicates a decrease in orthostatic tolerance in three subjects. The heart rate responses after 1 day of reambulation appeared no different than those for weeks 1 through 17 of bed rest. After 3 days of reambulation, the heart rate response during LBNP was similar to that recorded before bed rest (NS). After 32 days of reambulation, the heart rate was slightly lower (NS) than before bed rest at all levels of LBNP and recovery. The data are similar to those obtained during Skylab, including the increase in heart rate response after 1 week of space flight, which was maintained throughout the period of weightlessness. The data indicate that the heart rate changes associated with orthostatic instability develop early (after as few as 3 days of bed rest) and persist throughout the entire 17 weeks of horizo

ISSN:0091-2700    

DOI:10.1002/j.1552-4604.1994.tb02008.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

This paper summarizes the lower body negative pressure experiments performed in space, beginning with the experiments conducted on Skylab, because this program provided the most cardiovascular physiology data for United States space flight. Data obtained during studies of lower body negative pressure for use as a countermeasure after months of Russian space flight are also presented. Lower body negative pressure experiments conducted aboard Space Shuttle flights provide data about the deadaptation response of the cardiovascular system to orthostatic stress occurring during periods of zero gravity, and about protection against postflight orthostatic intolerance. Data obtained using Russian and American lower body negative pressure devices indicate that, when a crew member stands, as opposed to being supported by a seat or saddle as in the American device, there may be a slight detrimental effect in terms of the cardiovascular response to this orthostatic stress. Comparison of heart rate and blood pressure response to entry and landing of the Shuttle indicate that, although lower body negative pressure is a different stress and is applied in a different manner, the maximum heart rates during lower body negative pressure are reached at approximately the same point that the maximum heart rates are reached during entry and landing. Thus, the use of a lower body negative pressure stress in flight is a fairly good predictor of the cardiovascular response to the actual entry and landing of the Shuttle.

ISSN:0091-2700    

DOI:10.1002/j.1552-4604.1994.tb02009.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Microgravity induces fluid shifts which can alter the cardiovascular responses of astronauts both during space flight and on return to Earth. The decrease in orthostatic tolerance in astronauts returning from a weightless environment can be modelled in ground‐based studies using lower body negative pressure (LBNP). This study examined the physiological changes induced by LBNP and determined a reliable method of predicting the onset of presyncope to enable evaluation of countermeasures for loss of orthostatic tolerance, such as glycerol‐induced hyperhydration. Six healthy male subjects, aged 18 to 45 years, were each subjected to two LBNP tests, with or without glycerol ingestion. Continuous, non‐invasive measurements of middle cerebral artery blood flow velocities (CBF) by transcranial Doppler, arterial blood pressure (Finapres ABP), ECG and LBNP box pressures were recorded during each test. Negative pressure was increased in three minute intervals until symptoms of presyncope were observed. An increase in heart rate (HR), a relatively constant mean ABP and a steady decline in mean CBF were consistently observed as the box pressure was decreased. The continuous on‐line measurements clearly showed consistent dynamic changes in both CBF and ABP waveforms in response to changes in LBNP. At the onset of presyncope, sudden drops in mean ABP, HR and mean CBF were typically noted, the latter providing the earliest indication of presyncope. The time required to re‐establish original baseline values of CBF and ABP after release of box pressure varied widely from six to over ten minutes. The onset of presyncope during LBNP can be reliably predicted by continuous simultaneous analysis of CBF and ABP. Concomitant analysis of the dynamic changes may provide additional means of assessing an individual's degree of orthostatic tolerance by estimating and correlating the level and rate of reigonal cerebral autor

ISSN:0091-2700    

DOI:10.1002/j.1552-4604.1994.tb02010.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

The effects of regular aerobic exercise on orthostatic tolerance have been the subject of a long‐standing controversy that will influence the use of exercise during space flight. To examine these effects, astronauts performed continuous (CE) aerobic exercise (n = 8), interval (IE) aerobic exercise (n = 4), or no (NE) exercise (n = 5) during flights of 7 to 11 days. Heart rate (HR) responses to an orthostatic challenge (stand test) were measured 10 days before flight and on landing day. V̇O2peak(graded treadmill exercise) was measured 7 to 21 days before and 2 days after flight. No significant differences across the groups were observed in standing HRs before or after flight. However, the within‐group mean HRs significantly increased in the NE (71–89 beats/min) and CE (60–85 beats/min) groups after space flight. The HRs for the IE group did not significantly increase (75–86 beats/min) after space flight. V̇O2peakdecreased (P<.05) in the NE (‐9.5%) group, but did not change in the CE (‐2.4%) and IE (1%) groups. The relationship (r = 0.237) between the ΔHR andΔV̇O2peakwas not significant. These preliminary results indicate that: (1) continuous exercise does not affect the orthostatic HR response after space flight; (2) interval exercise may minimize an increase in the postflight orthostatic HR; and (3) both exercise protocols can

ISSN:0091-2700    

DOI:10.1002/j.1552-4604.1994.tb02011.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

A two‐period, crossover trial was conducted in the hypobaric chamber on human subjects to compare the influence of inflight exercise (experimental) and restricted activity (control) on altitude decompression sickness (DCS) during simulated extravehicular activities. Out of 39 pairs (total of 78 exposures), 4 cases of DCS occurred under control and 5 occurred under experimental conditions. Analysis of the crossover results showed that the P values for differences in DCS occurrence was 0.56. Under these circumstances, it was necessary to decide whether additional information would be obtained by accruing more subjects. This problem was examined by using a skew sequential design in which the “stopping rule” was based on an alpha of 0.05 (one‐sided) and power of 80%. The result of this analysis was in favor of the null hypothesis, and the trial was terminated. The authors recommend the use of similar stopping rules in aerospace trials to optimize sample size without compromising statistical v

ISSN:0091-2700    

DOI:10.1002/j.1552-4604.1994.tb02012.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Postflight orthostatic intolerance has been identified as a serious biomedical problem associated with long‐duration exposure to microgravity in space. High priority has been given to the development of countermeasures for this disorder that are both effective and practical. A considerable body of clinical research has demonstrated that people can be taught to increase their own blood pressure voluntarily, and that this is an effective treatment for chronic orthostatic intolerance in paralyzed patients. The current pilot study was designed to examine the feasibility of adding training in control of blood pressure to an existing preflight training program designed to facilitate astronaut adaptation to microgravity. Using an operant conditioning procedure, autogenic‐feedback training (AFT), three men and two women participated in four to nine training (15–30‐minute) sessions. At the end of training, the average increase in systolic and diastolic pressure, as well as mean arterial pressures, that the subjects made ranged between 20 and 50 mm Hg under both supine and 45° head‐up tilt conditions. These findings indicate that AFT may be a useful alternative treatment or supplement to existing approaches for preventing postflight orthostatic intolerance. Furthermore, the use of operant conditioning methods for training cardiovascular responses may contribute to the general understanding of the mechanisms of orthostatic i

ISSN:0091-2700    

DOI:10.1002/j.1552-4604.1994.tb02013.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Space flight represents a form of sensory stimulus rearrangement requiring modification of established terrestrial response patterns through central reinterpretation. Evidence of sensory reinterpretation is manifested as postflight modifications of eye/head coordination, locomotor patterns, postural control strategies, and illusory perceptions of self or surround motion in conjunction with head movements. Under normal preflight conditions, the head is stabilized during locomotion, but immediately postflight reduced head stability, coupled with inappropriate eye/head coordination, results in modifications of gait. Postflight postural control exhibits increased dependence on vision which compensates for inappropriate interpretation of otolith and proprioceptive inputs. Eye movements compensatory for perceived self motion, rather than actual head movements have been observed postflight. Overall, the in‐flight adaptive modification of head stabilization strategies, changes in head/eye coordination, illusionary motion, and postural control are maladaptive for a return to the terrestrial environmen

ISSN:0091-2700    

DOI:10.1002/j.1552-4604.1994.tb02014.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY