Enhanced slow caudad fluid shifts in orthostatic intolerance after 24-h bed-rest

Pathophysiological mechanisms and therapeutic approaches in obstructive sleep apnea syndrome

Obstructive sleep apnea syndrome (OSAS) is a common breathing disorder in sleep in which the airways narrow or collapse during sleep, causing obstructive sleep apnea. The prevalence of OSAS continues to rise worldwide, particularly in middle-aged and elderly individuals. The mechanism of upper airway collapse is incompletely understood but is associated with several factors, including obesity, craniofacial changes, altered muscle function in the upper airway, pharyngeal neuropathy, and fluid shifts to the neck. The main characteristics of OSAS are recurrent pauses in respiration, which lead to intermittent hypoxia (IH) and hypercapnia, accompanied by blood oxygen desaturation and arousal during sleep, which sharply increases the risk of several diseases. This paper first briefly describes the epidemiology, incidence, and pathophysiological mechanisms of OSAS. Next, the alterations in relevant signaling pathways induced by IH are systematically reviewed and discussed. For example, IH can induce gut microbiota (GM) dysbiosis, impair the intestinal barrier, and alter intestinal metabolites. These mechanisms ultimately lead to secondary oxidative stress, systemic inflammation, and sympathetic activation. We then summarize the effects of IH on disease pathogenesis, including cardiocerebrovascular disorders, neurological disorders, metabolic diseases, cancer, reproductive disorders, and COVID-19. Finally, different therapeutic strategies for OSAS caused by different causes are proposed. Multidisciplinary approaches and shared decision-making are necessary for the successful treatment of OSAS in the future, but more randomized controlled trials are needed for further evaluation to define what treatments are best for specific OSAS patients.

Investigating the Dynamics of Supine Fluid Redistribution Within Multiple Body Segments Between Men and Women

While supine, fluid moves from the legs and accumulates in the chest and neck. However, patterns of rostral fluid shift are not clear. Furthermore, real-time measurement of neck fluid volume has not been investigated. The objective of this study was to investigate the dynamics of rostral fluid shift in men and women. We developed a bioelectrical impedance system to measure leg, abdominal, thoracic and neck fluid volumes (LFV, AFV, TFV, NFV) continuously. Forty healthy non-obese adults (20 men) lay supine for 90 min while fluid volumes were measured. After 90 min, a similar volume of fluid shifted out of the legs in both sexes (p = 0.079), but men accumulated more fluid in their thorax (63 ± 6 vs. 44 ± 11 ml, p = 0.016) and neck (17 ± 2 vs. 14 ± 1 ml, p = 0.029) than women. In both sexes, the increase in NFV caused a significant increase in neck circumference, which was greater in men (p = 0.009). Furthermore, 80% of rostral fluid shift would occur in the first 2 h of lying supine. These results suggest that greater fluid shift into the thorax and neck may contribute to the higher prevalence of sleep apnea in men than in women.

Space physiology IV: mathematical modeling of the cardiovascular system in space exploration

Mathematical modeling represents an important tool for analyzing cardiovascular function during spaceflight. This review describes how modeling of the cardiovascular system can contribute to space life science research and illustrates this process via modeling efforts to study postflight orthostatic intolerance (POI), a key issue for spaceflight. Examining this application also provides a context for considering broader applications of modeling techniques to the challenges of bioastronautics. POI, which affects a large fraction of astronauts in stand tests upon return to Earth, presents as dizziness, fainting and other symptoms, which can diminish crew performance and cause safety hazards. POI on the Moon or Mars could be more critical. In the field of bioastronautics, POI has been the dominant application of cardiovascular modeling for more than a decade, and a number of mechanisms for POI have been investigated. Modeling approaches include computational models with a range of incorporated factors and hemodynamic sophistication, and also physical models tested in parabolic and orbital flight. Mathematical methods such as parameter sensitivity analysis can help identify key system mechanisms. In the case of POI, this could lead to more effective countermeasures. Validation is a persistent issue in modeling efforts, and key considerations and needs for experimental data to synergistically improve understanding of cardiovascular responses are outlined. Future directions in cardiovascular modeling include subject-specific assessment of system status, as well as research on integrated physiological responses, leading, for instance, to assessment of subject-specific susceptibility to POI or effects of cardiovascular alterations on muscular, vision and cognitive function.

Role of nocturnal rostral fluid shift in the pathogenesis of obstructive and central sleep apnoea

Obstructive sleep apnoea (OSA) is common in the general population and increases the risk of motor vehicle accidents due to hypersomnolence from sleep disruption, and risk of cardiovascular diseases owing to repetitive hypoxia, sympathetic nervous system activation, and systemic inflammation. In contrast, central sleep apnoea (CSA) is rare in the general population. Although their pathogenesis is multifactorial, the prevalence of both OSA and CSA is increased in patients with fluid retaining states, especially heart failure, where they are associated with increased mortality risk. This observation suggests that fluid retention may contribute to the pathogenesis of both OSA and CSA. According to this hypothesis, during the day fluid accumulates in the intravascular and interstitial spaces of the legs due to gravity, and upon lying down at night redistributes rostrally, again owing to gravity. Some of this fluid may accumulate in the neck, increasing tissue pressure and causing the upper airway to narrow, thereby increasing its collapsibility and predisposing to OSA. In heart failure patients, with increased rostral fluid shift, fluid may additionally accumulate in the lungs, provoking hyperventilation and hypocapnia, driving below the apnoea threshold, leading to CSA. This review article will explore mechanisms by which overnight rostral fluid shift, and its prevention, can contribute to the pathogenesis and therapy of sleep apnoea.

Recovery of muscle atrophy and bone loss from 90 days bed rest: results from a one-year follow-up

Earlier studies found the recovery of bone loss after clinical immobilization to be incomplete. It has been argued that this is due to the human skeleton's inability to accrue bone mass once peak bone mass has been attained. However, recent studies suggest that bone losses can fully recover when complete functional rehabilitation is achieved. Accordingly, we hypothesized that bone losses by experimental bed rest would recover within one-year of follow-up. Twenty-five men (mean age 32 years, SD 4.2) were randomly assigned to either bed rest only (Ctrl), resistive flywheel exercise (FW), or to a group receiving 60 mg. i.v pamidronate prior to bed rest (Pam). Calf muscle cross sectional area and bone mineral content of the tibia was measured by peripheral quantitative computed tomography. Calcium, PTH and alkaline phosphatase blood levels were assessed along with urinary desoxypyridinoline excretion. Physical activity was assessed by the Freiburg questionnaire. In Pam and FW, diaphyseal bone losses were completely recovered at a 180-day follow-up, and there was even a small surplus after 1 year (p=0.016). Epiphyseal bone losses were largely, although not completely recovered after 1 year, when they still amounted to -0.6% (SD 1.3%, p=0.034, averaged over all groups). Bone formation and resorption markers had returned to baseline values at this time. However, epiphyseal recovery may still have been on-going, and fitting an exponential model yielded full recovery of the epiphysis within 2 years. Importantly, recovery of calf muscle cross-section and resumption of impact sport activities seemed to precede bone recovery, and bone accrual was closely matching the prior losses on an individual basis. No relationship was found between the epiphyseal BMC deficit at one-year follow-up and the participants' age. Results demonstrate recovery of bed rest induced bone losses in healthy adults. The initial re-accrual rate was remarkably high and is comparable to the accrual of bone mass during the pubertal growth spurt. This and the fact that the recovery of bone appeared to be tightly regulated, and generally followed neuromuscular recovery underline the adult skeleton's capability to adapt to mechanical stimuli.

Steep fall in cardiac output is main determinant of hypotension during drug-free and nitroglycerine-induced orthostatic vasovagal syncope

BACKGROUND

How much of the hypotension occurring during postural syncope is cardiac output-mediated and how much can be ascribed to a fall in systemic vascular resistance are unknown. The contribution of both determinants may be influenced by the use of vasoactive drugs.

OBJECTIVE

The purpose of this study was to assess the determinants of hypotension during drug-free and nitroglycerine (NTG)-induced vasovagal presyncope in routine tilt table testing.

METHODS

In this retrospective study, a total of 56 patients (37 female; age 36 +/- 19 years) with suspected vasovagal syncope and a positive tilt test at two clinical centers were selected. In 29 patients, presyncope was provoked by 0.4 mg sublingual NTG, administered in the 60 degrees head-up tilt position. In the other 27 patients, presyncope was provoked by passive tilt alone. Finger arterial pressure was monitored continuously, and left ventricular stroke volume was computed from pressure pulsations.

RESULTS

After NTG administration, heart rate rose, and peak heart rate was similar in all patients. Use of NTG did not affect circulatory patterns precipitating a vasovagal response. On average in all patients, marked hypotension was mediated by an approximately 50% fall in cardiac output, whereas systemic vascular resistance was well maintained until presyncope.

CONCLUSION

Hypotension during routine tilt testing is cardiac output-mediated, and the mechanism appears independent of the use of 0.4 mg sublingual NTG. The study data challenge the conventional idea of systemic vasodilation as the overriding cause of hypotension during postural syncope.

Influence of a radiofrequency electromagnetic field on cardiovascular and hormonal parameters of the autonomic nervous system in healthy individuals

The potential health risks of radiofrequency electromagnetic fields (EMFs) emitted by mobile phones are of considerable public interest. The present study investigated the hypothesis, based on the results of our previous study, that exposure to EMFs can increase sympathetic vasoconstrictor activity. Forty healthy young males and females underwent a single-blind, placebo-controlled protocol once on each of two different days. Each investigation included successive periods of placebo and EMF exposure, given in a randomized order. The exposure was implemented by a GSM-like signal (900 MHz, pulsed with 217 Hz, 2 W) using a mobile phone mounted on the right-hand side of the head in a typical telephoning position. Each period of placebo exposure and of EMF exposure consisted of 20 min of supine rest, 10 min of 70 degrees upright tilt on a tilt table, and another 20 min of supine rest. Blood pressure, heart rate and cutaneous capillary perfusion were measured continuously. In addition, serum levels of norepinephrine, epinephrine, cortisol and endothelin were analyzed in venous blood samples taken every 10 min. Similar to the previous study, systolic and diastolic blood pressure each showed slow, continuous, statistically significant increases of about 5 mmHg during the course of the protocol. All other parameters either decreased in parallel or remained constant. However, analysis of variance showed that the changes in blood pressure and in all other parameters were independent of the EMF exposure. These findings do not support the assumption of a nonthermal influence of EMFs emitted by mobile phones on the cardiovascular autonomic nervous system in healthy humans.

Assessment of capillary fluid shifts during orthostatic stress in normal subjects and subjects with orthostatic intolerance

Orthostatic stress causes, in addition to venous pooling, a loss of plasma fluid from capillaries to the dependent tissues. The rate of this loss may be one of the factors determining orthostatic tolerance. In this study we assessed the use of a multichannel impedance plethysmograph for determining changes in volume in the calf, thigh, and abdominal segments, in asymptomatic volunteers and in patients shown to have poor tolerance to orthostatic stress. Impedance plethysmography showed, for leg segments, that following head-up tilt there was an initial rapid change in volume followed after 2 to 4 minutes by an almost linear change. Results from the abdominal segment were more variable. The rate of change of leg (thigh + calf) volume was significantly correlated with the estimated loss of plasma volume derived from the changes in the concentration of plasma protein, using Evans Blue dye as the marker. Comparison of results of leg filtration rates between patients and volunteers indicated that some of the patients had abnormally high filtration rates and suggests that impedance plethysmography may have a role in assessing the possible reasons for orthostatic intolerance.