THE MOVEMENT OF FLUID THROUGH THE HUMAN CAPILLARY WALL IN RELATION TO VENOUS PRESSURE AND TO THE COLLOID OSMOTIC PRESSURE OF THE BLOOD

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.

The Microvascular-Lymphatic Interface and Tissue Homeostasis: Critical Questions That Challenge Current Understanding

Lymphatic and blood microvascular networks play critical roles in the clearance of excess fluid from local tissue spaces. Given the importance of these dynamics in inflammation, tumor metastasis, and lymphedema, understanding the coordinated function and remodeling between lymphatic and blood vessels in adult tissues is necessary. Knowledge gaps exist because the functions of these two systems are typically considered separately. The objective of this review was to highlight the coordinated functional relationships between blood and lymphatic vessels in adult microvascular networks. Structural, functional, temporal, and spatial relationships will be framed in the context of maintaining tissue homeostasis, vessel permeability, and system remodeling. The integration across systems will emphasize the influence of the local environment on cellular and molecular dynamics involved in fluid flow from blood capillaries to initial lymphatic vessels in microvascular networks.

Blink-sensing glasses: A flexible iontronic sensing wearable for continuous blink monitoring

Blink reflex has long been considered closely related to physiological states, from which abundant information on ocular health and activities can be revealed. In this study, a smart glasses wearable has been developed, incorporating a flexible and sensitive pressure sensor, to monitor blink patterns by continuously detecting ocular muscular movements, referred to as blink-sensing glasses. By applying the emerging flexible iontronic sensing (FITS) sensor with the sensitivity of 340 pF/mmHg, the skin pressure variations induced by movements of the orbicularis oculi muscles can be monitored in real time. The blink-sensing glasses can successfully capture blink patterns with a high accuracy of 96.3% and have been used to differentiate the blink features from both dry-eye subjects and healthy controls. This device can be potentially used as a new clinical and research monitoring tool for continuous eye blink analysis, while providing patients with high comfortableness in long-term ambulatory and home settings.

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Blink-sensing glasses can capture blink patterns with clinical-grade high accuracy

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A FITS sensor is applied to monitor the blink by detecting the muscle movement

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Blink-sensing glasses can be of potential use to prognose the dry eye

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The glasses are a continuous detection manner with immunity to ambient lights

Fluid Overload and Acute Kidney Injury

Commonest intervention in hospitalized patient is fluid therapy, and practically every critically ill patient receives fluid resuscitation. Commonest indication for fluid administration is to achieve hemodynamic stability and prevent or manage acute kidney injury (AKI). However, fluid administration is a two-edged sword, i.e., inadequate fluids give rise to hypoperfusion and organ injury and overzealous fluid therapy can give rise to fluid overload and related consequences. Though fluids are commonly given to prevent development of AKI, hypervolemia itself has the potential to cause AKI.

Leg fluid accumulation during prolonged sitting

The accumulation of fluid in the legs due to sedentariness can be a health risk in extreme cases. Negative health impacts associated with leg fluid accumulation include leg edema and risk of blood clots. Furthermore, fluid accumulating in the legs is accompanied by fluid shift into the upper body which is also associated with health risks such as: increased blood pressure when lying down, respiratory problems in people with heart failure, and increased sleep apnea. Understanding the pattern by which fluid accumulates in the legs can aid in the development of devices for reducing leg fluid accumulation. The purpose of this study was to characterize the time course of fluid accumulation over a two-and-half-hour seated period. Non-obese participants with sleep apnea and no other co-morbidities were included in the sample as part of a larger study. Leg fluid was measured continuously using a method of bioelectrical impedance. Participants were first asked to lie supine for 30 minutes as a washout, and then sat with their legs still for two and a half hours. The main finding of this study is that the pattern of leg fluid accumulation differed in the first 45 minutes compared to the latter 105 minutes. In the first 45 minutes, fluid accumulated according to first order exponential function. In the latter period, fluid accumulated according to a linear function. The initial exponential accumulation is likely due to the large increase in capillary pressure caused by rapid blood flow into the legs due to gravity, leading to substantial filtration of blood plasma into the tissue spaces. The latter linear portion likely represents continued slow filtration of fluid out of the vasculature and into the tissue spaces. This is the first study to show that fluid accumulation in the legs is a combination of an exponential and linear functions. The linear increase identifies that there is no foreseeable point in which leg fluid stops accumulating while sitting for prolonged periods.

Impact of Compression Stockings vs. Continuous Positive Airway Pressure on Overnight Fluid Shift and Obstructive Sleep Apnea among Patients on Hemodialysis

INTRODUCTION

Obstructive sleep apnea (OSA) is common in edematous states, notably in hemodialysis patients. In this population, overnight fluid shift can play an important role on the pathogenesis of OSA. The effect of compression stockings (CS) and continuous positive airway pressure (CPAP) on fluid shift is barely known. We compared the effects of CS and CPAP on fluid dynamics in a sample of patients with OSA in hemodialysis, through a randomized crossover study.

METHODS

Each participant performed polysomnography (PSG) at baseline, during CPAP titration, and after 1 week of wearing CS. Neck circumference (NC) and segmental bioelectrical impedance were done before and after PSG.

RESULTS

Fourteen patients were studied (53 ± 9 years; 57% men; body mass index 29.7 ± 6.8 kg/m²). Apnea-hypopnea index (AHI) decreased from 20.8 (14.2; 39.6) at baseline to 7.9 (2.8; 25.4) during CPAP titration and to 16.7 (3.5; 28.9) events/h after wearing CS (CPAP vs. baseline, p = 0.004; CS vs. baseline, p = 0.017; and CPAP vs. CS, p = 0.017). Nocturnal intracellular trunk water was higher after wearing CS in comparison to baseline and CPAP (p = 0.03). CS reduced the fluid accumulated in lower limbs during the day, although not significantly. Overnight fluid shift at baseline, CPAP, and CS was -183 ± 72, -343 ± 220, and -290 ± 213 ml, respectively (p = 0.006). Overnight NC increased at baseline (0.7 ± 0.4 cm), decreased after CPAP (-1.0 ± 0.4 cm), and while wearing CS (-0.4 ± 0.8 cm) (CPAP vs. baseline, p < 0.0001; CS vs. baseline, p = 0.001; CPAP vs. CS, p = 0.01).

CONCLUSION

CS reduced AHI by avoiding fluid retention in the legs, favoring accumulation of water in the intracellular component of the trunk, thus avoiding fluid shift to reach the neck. CPAP improved OSA by exerting local pressure on upper airway, with no impact on fluid redistribution. CPAP performed significantly better than CS for both reduction of AHI and overnight reduction of NC. Complementary studies are needed to elucidate the mechanisms by which CPAP and CS reduce NC.

Hypervolemia and Sleep Apnea in Kidney Disease

In end-stage renal disease (ESRD) and heart failure, conditions characterized by fluid overload, both obstructive sleep apnea (OSA) and central sleep apnea (CSA) are highly prevalent. This observation suggests that fluid overload may be a unifying mechanism in the pathogenesis of both OSA and CSA in these conditions. An overnight rostral fluid shift from the legs to the neck and lungs has been shown to contribute to the pathogenesis of OSA and CSA, respectively, in various different patient populations. This article reviews the evidence that supports a role for fluid overload and overnight fluid shift in the pathogenesis of sleep apnea in ESRD. The diagnosis, epidemiology, and clinical features of sleep apnea in patients with ESRD also are considered.

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.

Fluid resuscitation in acute medicine: what is the current situation?

The administration of intravenous fluids for resuscitation is the most common intervention in acute medicine. There is increasing evidence that the type of fluid may directly affect patient-centred outcomes. There is a lack of evidence that colloids confer clinical benefit over crystalloids and they may be associated with harm. Hydroxyethyl starch preparations are associated with increased mortality and use of renal replacement therapy in critically ill patients, particularly those with sepsis; albumin is associated with increased mortality in patients with severe traumatic brain injury. Crystalloids, such as saline or balanced salt solutions, are increasingly recommended as first-line resuscitation fluids for the majority of patients with hypovolaemia. There is emerging evidence that saline may be associated with adverse outcomes due to the development of hyperchloraemic metabolic acidosis, although the safety of balanced salt solutions has not been established. Fluid requirements vary over the course of critical illness. The excessive use of fluids during the resuscitative period is associated with increased cumulative fluid balance and adverse outcomes in critically ill patients. The selection of fluid depends on the clinical context in which it is administered and requires careful consideration of the dose and potential for toxicity. There is an urgent need to conduct further high-quality randomized controlled trials of currently available fluid therapy in patients with critical illness.

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.

Perspective: physiological role(s) of the vascular myogenic response

The vascular myogenic response is an inherent property of VSM in the walls of small arteries and arterioles, allowing these principal resistance segments of the microcirculation to respond to changes in transmural pressure. Elevated intraluminal pressure leads to myogenic constriction, whereas reduced pressure leads to myogenic dilation. This review focuses on the physiological significance of the myogenic response in microvascular networks. First, historical concepts related to the detection of stretch by the vessel wall are reviewed, including the wall tension hypothesis, and the implications of the proposal that the arteriolar network responds to Pp changes as a system of series-coupled myogenic effectors. Next, the role of the myogenic response in the local regulation of blood flow and/or Pc is examined. Finally, the interaction of myogenic constriction and dilation with other local control mechanisms, including metabolic, neural and shear-dependent mechanisms, is discussed. Throughout the review, an attempt is made to integrate historical and current literature with an emphasis on the physiological role, rather than the underlying signaling mechanisms, of this important component of vascular control.

Revised Starling equation and the glycocalyx model of transvascular fluid exchange: an improved paradigm for prescribing intravenous fluid therapy

I.V. fluid therapy does not result in the extracellular volume distribution expected from Starling's original model of semi-permeable capillaries subject to hydrostatic and oncotic pressure gradients within the extracellular fluid. Fluid therapy to support the circulation relies on applying a physiological paradigm that better explains clinical and research observations. The revised Starling equation based on recent research considers the contributions of the endothelial glycocalyx layer (EGL), the endothelial basement membrane, and the extracellular matrix. The characteristics of capillaries in various tissues are reviewed and some clinical corollaries considered. The oncotic pressure difference across the EGL opposes, but does not reverse, the filtration rate (the 'no absorption' rule) and is an important feature of the revised paradigm and highlights the limitations of attempting to prevent or treat oedema by transfusing colloids. Filtered fluid returns to the circulation as lymph. The EGL excludes larger molecules and occupies a substantial volume of the intravascular space and therefore requires a new interpretation of dilution studies of blood volume and the speculation that protection or restoration of the EGL might be an important therapeutic goal. An explanation for the phenomenon of context sensitivity of fluid volume kinetics is offered, and the proposal that crystalloid resuscitation from low capillary pressures is rational. Any potential advantage of plasma or plasma substitutes over crystalloids for volume expansion only manifests itself at higher capillary pressures.

Lymphatic dysfunction, not aplasia, underlies Milroy disease

OBJECTIVE

Milroy disease is an inherited autosomal dominant lymphoedema caused by mutations in the gene for vascular endothelial growth factor receptor-3 (VEGFR-3, also known as FLT4). The phenotype has to date been ascribed to lymphatic aplasia. We further investigated the structural and functional defects underlying the phenotype in humans.

METHODS

The skin of the swollen foot and the non-swollen forearm was examined by (i) fluorescence microlymphangiography, to quantify functional initial lymphatic density in vivo; and (ii) podoplanin and LYVE-1 immunohistochemistry of biopsies, to quantify structural lymphatic density. Leg vein function was assessed by colour Doppler duplex ultrasound.

RESULTS

Milroy patients exhibited profound (86-91%) functional failure of the initial lymphatics in the foot; the forearm was unimpaired. Dermal lymphatics were present in biopsies but density was reduced by 51-61% (foot) and 26-33% (forearm). Saphenous venous reflux was present in 9/10 individuals with VEGFR3 mutations, including two carriers.

CONCLUSION

We propose that VEGFR3 mutations in humans cause lymphoedema through a failure of tissue protein and fluid absorption. This is due to a profound functional failure of initial lymphatics and is not explained by microlymphatic hypoplasia alone. The superficial venous valve reflux indicates the dual role of VEGFR-3 in lymphatic and venous development.

THE FLOW AND COMPOSITION OF LYMPH IN RELATION TO THE FORMATION OF EDEMA

1. The experimental observations have been summarized at the end of an earlier section. The more important facts only will be recapitulated here. The capacity of the lymphatics for removing fluid from the tissues greatly exceeds the rate at which freshly formed tissue fluid can be made available for removal. Edematous regions can be rendered non-edematous by the application of measures, such as massage, passive motion, or normal exercise, which activate the lymphatics. During continuous activity the rate of lymph flow is first variable and later relatively constant. Constant rates of flow must correspond to the production of fresh lymph. A study of the constant rates indicates that lymph formation in the edematous animal is certainly only slightly greater, and possibly not greater at all, than under conditions of normality. When the protein of plasma decreases, the protein of lymph is also lowered. The loss of protein from lymph takes place at a faster rate than from plasma, so that the ratio of serum protein to lymph protein is greater in the edematous than in the normal animal. In edematous animals the concentration of protein in lymph is of the same order of magnitude as the concentration in edema fluids. The two fluids are not, however, identical in composition. Minor fluctuations in the protein content of lymph always occur during a period of continuous collection. 2. The factors involved in the circulation and accumulation of tissue fluid are discussed. Reasons are given for offering the following suggestions. Significant differences in tissue pressure or tension exist between the states resulting from quiescence and activation of the lymphatics. The differences give rise to variations in the relative areas of capillary wall, functioning for filtration and reabsorption. When the lymphatics are activated it is possible that capillary reabsorption may be completely in abeyance. A decline in the proteins of plasma may be associated with a diminished permeability of the capillaries. Such a lowering of capillary permeability would account for two features, both of which have been demonstrated: (1) failure to observe an appreciably increased rate of lymph formation in the edematous animal, and (2) the extremely low concentration of protein in lymph from edematous animals. Although the difference between the protein concentrations of edema fluid and lymph from the same region is small, the conclusion is not yet justified that a similarly small difference exists between normal tissue fluid and normal lymph.

Effect of endurance training on muscle microvascular filtration capacity and vascular bed morphometry in the elderly

AIM

Exercise training is a strong stimulus for vascular remodelling and could restore age-induced vascular alterations. The purpose of the study was to test the hypothesis that an increase in vascular bed filtration capacity would corroborate microvascular adaptation with training.

METHODS

We quantified (1) microvascularization from vastus lateralis muscle biopsy to measure the capillary to fibre interface (LC/PF) and (2) the microvascular filtration capacity (K(f)) in lower limbs through a venous congestion plethysmography procedure. Twelve healthy older subjects (74 +/- 4 years) were submitted to a 14-week training programme during which lower-limbs were trained for endurance exercise.

RESULTS

The training programme induced a significant increase in the aerobic exercise capacity of lower limbs (+11% V(O2peak); P < 0.05; +28% Citrate Synthase Activity; P < 0.01). K(f) was largely increased (4.3 +/- 0.9 10(-3) mL min(-1) mmHg(-1) 100 mL(-1) post-training vs. 2.4 +/- 0.8 pre-training, mean +/- SD; P < 0.05) and microvascularization developed as shown by the rise in LC/PF (0.29 +/- 0.06 post- vs. 0.23 +/- 0.06 pre-training; P < 0.05). Furthermore, K(f) and LC/PF were correlated (r = 0.65, P < 0.05).

CONCLUSION

These results demonstrated the microvascular adaptation to endurance training in the elderly. The increase in K(f) with endurance training was probably related to a greater surface of exchange with an increased microvessel/fibre interface area. We conclude that measurement of the microvascular filtration rate reflects the change in the muscle exchange area and is influenced by exercise training.