Redistribution of body fluids during postural manipulations

Abstract

Inter-compartmental body-fluid distribution is contingent upon posture, exercise state and environmental temperature. This investigation aimed at quantifying the distribution of intra- and extravascular fluid volumes during postural manipulations. Fluid shifts were measured in eight males utilizing a simultaneous, radionuclide dilution technique, in which radioiodinated serum fibrinogen, radiochromated erythrocytes, radiobromine and tritiated water were used to measure plasma, red cell, extracellular and total body water volumes. Subjects were exposed to three postural changes [seated (control), supine and standing] for 30 min at an air temperature of 22.0 degrees C, with each posture separated by 30 min seated rest. Total body water content remained stable throughout postural changes (P = 0.842). Relative to seated volumes, BV increased by 89 mL when supine, and decreased by 406 mL while standing (P = 0.003), with such shifts being primarily a result of plasma movement (P = 0.011). Red cell volume changes were not significant. Vascular fluid lost during standing was filtered into the interstitial compartment (P = 0.014), with the extracellular and intracellular volumes remaining unaffected. (P = 0.271 and P = 0.800, respectively). These observations confirmed the influence of posture on inter-compartmental body-fluid distribution. The intravascular fluid loss when standing was caused by the filtration of plasma into the interstitium, while, during supine rest, intravascular volume increased, reflecting fluid flux from the interstitium to the circulation.

Influence of posture on pharmacokinetics

“…Due to gravity, the standing position results in a high hydrostatic pressure in lower body parts, leading to an intravascular fluid loss caused by filtration of plasma into the interstitium [24]. This fluid loss accounts for approximately 9.5% of the blood volume and 16.2% of plasma volume, respectively (compared to the supine position; see Table 1) [25].…”

“…The effects of posture in this context have been studied extensively over the past decades, with the results being largely consistent [24][25][26][27]. A change in body position from recumbent to sitting and to erect results in a significant increase in haematological laboratory values, such as haemoglobin, haematocrit, erythrocytes, leucocytes and platelets, due to reversible changes in the concentration and redistribution of the blood [25,27].…”

“…A change in body position from recumbent to sitting and to erect results in a significant increase in haematological laboratory values, such as haemoglobin, haematocrit, erythrocytes, leucocytes and platelets, due to reversible changes in the concentration and redistribution of the blood [25,27]. Relative to the sitting position, blood volume increases by 89 ml (mean) after 30 min in supine posture and decreases by 406 ml while standing, with these differences being primarily the result of plasma shifts between the interstitium and circulation [24] (see Table 1). The haemo-concentration while standing is supposed to involve all blood compounds-and drugsthat cannot permeate the capillary membrane.…”

A Brief History of Fluid and Sleep

“…Current prevalence estimates of obstructive sleep apnea (OSA) in the general adult population between the ages of 30 and 70 years range from 17 to 33%, increasing with older age (6). This represents a greater than 5% rise in OSA prevalence since the landmark study of Young in 1993, most likely attributable to rising societal obesity (7).…”

“…Visceral adiposity, upper airway anatomy, age, and sex are recognized as important contributors, but the pathophysiology of sleep apnea is multifactorial and complex. Changes in posture can have immediate effects on respiratory mechanics (4) and neural respiratory drive (5) and slower effects on intra-and extravascular volumes as a result of plasma movement (6). However, the role of fluid, and in particular nocturnal fluid shifts, in OSA remains a relatively underexplored area.…”