Extracellular volume: its structure and measurement and the influence of age and disease

Quantitative in Vivo Molecular MRI

Molecular magnetic resonance imaging (MRI) combines chemistry, chemical biology, and imaging techniques to track molecular events non-invasively. Quantitative molecular MRI aims to provide meaningful, reproducible numerical measurements of molecular processes or biochemical targets within the body. In this review, the classifications of molecular MRI probes based on their signal-generating mechanism and functionality are first described. From there, the primary considerations for in vitro characterization and in vivo validation of molecular MRI probes, including how to avoid pitfalls and biases are discussed. Then, recommendations on imaging acquisition protocols and analysis methods to establish quantitative relationships between MRI signal change induced by the probes and the molecular processes of interest are provided. Finally, several representative case studies are highlighted that incorporate these features. Quantitative molecular MRI is a multidisciplinary research area incorporating expertise in chemical biology, inorganic chemistry, molecular probes, imaging physics, drug development, pathobiology, and medicine. The purpose of this review is to provide guidance to chemists developing MR imaging probes and methods in terms of in vitro and in vivo validation to accelerate the translation of these new quantitative tools for non-invasive imaging of biological processes.

Repeatability of quantitative sodium magnetic resonance imaging for estimating pseudo-intracellular sodium concentration and pseudo-extracellular volume fraction in brain at 3 T

The purpose of this study is to assess the repeatability of the quantification of pseudo-intracellular sodium concentration (C1) and pseudo-extracellular volume fraction (α) estimated in brain in vivo using sodium magnetic resonance (MRI) at 3 T. Eleven healthy subjects were scanned twice, with two sodium MRI acquisitions (with and without fluid suppression by inversion recovery), and two double inversion recovery (DIR) proton MRI. DIR MRIs were used to create masks of gray and white matter (GM, WM), that were subsequently applied to the C1 and α maps calculated from sodium MRI and a tissue three-compartment model, in order to measure the distributions of these two parameters in GM, WM or full brain (GM+WM) separately. The mean, median, mode, standard deviation (std), skewness and kurtosis of the C1 and α distributions in whole GM, WM and full brain were calculated for each subject, averaged over all data, and used as parameters for the repeatability assessment. The coefficient of variation (CV) was calculated as a measure of reliability for the detection of intra-subject changes in C1 and αfor each parameter, while intraclass correlation (ICC) was used as a measure of repeatability. It was found that the CV of most of the parameters was around 10-20% (except for C1 kurtosis which is about 40%) for C1 and α measurements, and that ICC was moderate to very good (0.4 to 0.9) for C1 parameters and for some of the α parameters (mainly skewness and kurtosis). In conclusion, the proposed method could allow to reliably detect changes of 50% and above of the different measurement parameters of C1 and αin neuropathologies (multiple sclerosis, tumor, stroke, Alzheimer's disease) compared to healthy subjects, and that skewness and kurtosis of the distributions of C1 and αseem to be the more sensitive parameters to these changes.

A method for estimating intracellular sodium concentration and extracellular volume fraction in brain in vivo using sodium magnetic resonance imaging

In this feasibility study we propose a method based on sodium magnetic resonance imaging (MRI) for estimating simultaneously the intracellular sodium concentration (C₁, in mM) and the extracellular volume fraction (α) in grey and white matters (GM, WM) in brain in vivo. Mean C₁ over five healthy volunteers was measured ~11 mM in both GM and WM, mean α was measured ~0.22 in GM and ~0.18 in WM, which are in close agreement with standard values for healthy brain tissue (C₁ ~ 10–15 mM, α ~ 0.2). Simulation of ‘fluid’ and ‘solid’ inclusions were accurately detected on both the C₁ and α 3D maps and in the C₁ and α distributions over whole GM and WM. This non-invasive and quantitative method could provide new biochemical information for assessing ion homeostasis and cell integrity in brain and help the diagnosis of early signs of neuropathologies such as multiple sclerosis, Alzheimer's disease, brain tumors or stroke.

Body fluid dynamics: back to the future

Pioneering investigations conducted over a half century ago on tonicity, transcapillary fluid exchange, and the distribution of water and solute serve as a foundation for understanding the physiology of body fluid spaces. With passage of time, however, some of these concepts have lost their connectivity to more contemporary information. Here we examine the physical forces determining the compartmentalization of body fluid and its movement across capillary and cell membrane barriers, drawing particular attention to the interstitium operating as a dynamic interface for water and solute distribution rather than as a static reservoir. Newer work now supports an evolving model of body fluid dynamics that integrates exchangeable Na(+) stores and transcapillary dynamics with advances in interstitial matrix biology.

Evaluation of between-methods agreement of extracellular water measurements in adults and children

BACKGROUND

Extracellular water (ECW), a relevant molecular level component for clinical assessment, is commonly obtained by 2 methods that rely on assumptions that may not be possible to test at the time the measurements are made.

OBJECTIVE

The aim of the current study was to evaluate the degree of agreement between ECW assessment by the sodium bromide dilution (ECW(NaBr)) and total body potassium (TBK; whole-body (40)K counting) to total body water (TBW; isotope dilution) methods (ECW(TBK-TBW)) in an ethnically mixed group of children and adults.

DESIGN

ECW was measured with the ECW(NaBr) and ECW(TBK-TBW) methods in 526 white and African American males and females (86 nonobese children, 193 nonobese adults, and 247 obese adults). Fat mass was assessed with dual-energy X-ray absorptiometry. Multiple regression analysis was used to examine the variables related to between-ECW method differences.

RESULTS

Significant but generally small group mean (+/-SD) differences in ECW were found in the obese adults (1.28 +/- 2.54 kg) and children (-0.71 +/- 1.78 kg). The magnitude of the differences was related to mean ECW in obese adults, children, and nonobese adults, and the relations between these variables were modified by sex for nonobese adults. ECW differences were also dependent on age, weight, sex, and race or on interactions between these variables.

CONCLUSIONS

Overall, although good between-method agreement was found across the 3 groups, the degree of agreement varied according to subject characteristics, particularly at the extremes of ECW and body weight. We advance a possible mechanism that may link subject characteristics with the degree of agreement between ECW measurement methods and their underlying assumptions.

Effects of sodium chloride and betaine on hydration status of lambs at slaughter

Under commercial conditions in Australia, lambs are often dehydrated at slaughter despite access to water at the abattoir. Dehydration results in reduced fluid content of meat, which causes liveweight and carcass weight loss. Two experiments evaluated hydration status and carcass characteristics in lambs fed increased levels of sodium chloride (salt) and/or betaine before slaughter. Both experiments were 2 × 2 factorial designs with two levels of salt added [0 and 50 g/kg dry matter (DM)] and two levels of betaine added (0 and 6.7 g/kg DM) to the diets. The treatment diets were introduced to sheep over a 7–8-day period and then fed at 1.5 kg/day for the following 7 days. In experiment 1, 96 Poll Dorset–Merino cross wether lambs (45.9 ± 0.17 kg) were fed in individual pens and in experiment 2, 204 Merino wether lambs (43.1 ± 0.35 kg) were fed in group pens. In the first experiment, water and feed were withheld from the end of the treatment feeding period until slaughter 48 h later. In the second experiment, feed and water were withheld for the first 24 h after treatment but water was available for the next 24 h immediately before slaughter. In both experiments, consumption of the high salt diet decreased urine specific gravity and osmolarity at the end of the feeding period and after 24 h without access to water. This indicates an improved hydration status at this time. However, the difference in hydration status related to feeding salt declined during the 48-h lairage period and there were no significant differences in hot carcass weight or muscle DM among groups at slaughter. It could be concluded from this result that the addition of salt has little commercial value; however, this may not be the case. The lambs in experiment 2 did not behave according to industry expectations. They all drank water in the last 24 h in lairage and none of the groups showed severe dehydration. Further research is justified to determine if the lambs fed the diets with no added salt drunk in lairage as a learned response from the lambs fed salt and, if so, whether this can be developed into a commercial strategy. Feeding betaine did not result in any changes in hydration status or carcass characteristics.

Body water compartment measurements: a comparison of bioelectrical impedance analysis with tritium and sodium bromide dilution techniques

BACKGROUND AND AIMS

This study was undertaken to assess the comparability of body water compartment estimates in healthy volunteers using single and dual frequency bioelectrical impedance analysis (BIA) with established reference methods of tritium and NaBr dilution.

METHODS

Total body water (TBW) was estimated in 10 healthy volunteers using single frequency (50 kHz) BIA (Bodystat 1500), dual frequency (5 and 200 kHz) BIA (Bodystat Dualscan 2005) and tritium dilution. Extracellular water (ECW) was measured with dual frequency BIA and NaBr dilution. BIA was performed using distal tetrapolar electrodes in the supine position. Venous blood was sampled for measurement of background concentrations of tritium and NaBr using a beta counter and high performance liquid chromatography respectively. 10 ml tritiated water (3.7 MBq) and 50 ml 5% NaBr solution were then injected intravenously and blood samples taken from the opposite arm every 45 min for 4.5 h for estimation of concentrations of tritium and NaBr.

RESULTS

There was good correlation (r(2)=0.76) between estimates of ECW using dual frequency BIA and NaBr dilution, with the former overestimating ECW by approximately 1 L. However, this difference varied systematically with body weight. Although TBW measurements obtained by single and dual frequency BIA correlated well with estimates using tritium dilution (r(2)=0.96 and 0.95 respectively), single frequency BIA underestimated TBW by approximately 1 L and dual frequency BIA by approximately 5 L compared to tritium dilution.

CONCLUSION

TBW measurements obtained using the single frequency BIA device were more accurate than those obtained using the dual frequency BIA device. Dual frequency BIA provided a reasonably accurate estimate of ECW.

[76Br]Bromodeoxyuridine PET in tumor-bearing animals

5-bromodeoxyuridine (BUdR) provides in vitro measures of tumor cell proliferation. We used positron emission tomography to study tissue and plasma kinetics of [76Br]BUdR in tumor-bearing animals. In order to account for the slow washout of the major plasma metabolite, [76Br]bromide, a mathematical correction for the distribution volume of [76Br]bromide was applied. However, following correction specific tumor tracer retention was low or even zero and did not correlate with independent measures of proliferation. The kinetic characteristics of [76Br]BUdR make this tracer unsuitable for proliferation imaging.

Body water content of extremely preterm infants at birth

BACKGROUND

Preterm birth is often associated with impaired growth. Small for gestational age status confers additional risk.

AIM

To determine the body water content of appropriately grown (AGA) and small for gestational age (SGA) preterm infants in order to provide a baseline for longitudinal studies of growth after preterm birth.

METHODS

All infants born at the Hammersmith and Queen Charlotte's Hospitals between 25 and 30 weeks gestational age were eligible for entry into the study. Informed parental consent was obtained as soon after delivery as possible, after which the extracellular fluid content was determined by bromide dilution and total body water by H(2)(18)O dilution.

RESULTS

Forty two preterm infants were studied. SGA infants had a significantly higher body water content than AGA infants (906 (833-954) and 844 (637-958) ml/kg respectively; median (range); p = 0.019). There were no differences in extracellular and intracellular fluid volumes, nor in the ratio of extracellular to intracellular fluid. Estimates of relative adiposity suggest a body fat content of about 7% in AGA infants, assuming negligible fat content in SGA infants and lean body tissue hydration to be equivalent in the two groups.

CONCLUSIONS

Novel values for the body water composition of the SGA preterm infant at 25-30 weeks gestation are presented. The data do not support the view that SGA infants have extracellular dehydration, nor is their regulation of body water impaired.

Comparison of total body chlorine, potassium, and water measurements in children with cystic fibrosis

BACKGROUND

Symptoms of cystic fibrosis (CF) may limit the utility of total body chlorine (TBCl) and total body potassium (TBK) measurements for assessing body fluid compartments of children.

OBJECTIVE

This study assessed relations among independent measurements of TBCl, TBK, and total body water (TBW) in children with CF.

DESIGN

We compared cross-sectional measurements of TBCl by in vivo neutron activation analysis, TBK by whole-body counting of (40)K, TBW by D(2)O dilution [TBW(D(2)O)], and TBW from TBCl and TBK [TBW(Cl + K)] in 19 prepubertal children (13 boys) aged 7.6-12.5 y who had mild symptoms of CF. Body-composition measurements were compared with data from previous studies of healthy children.

RESULTS

Subjects with CF had deficits in TBCl, TBK, TBW, and body weight compared with control reference data (P < 0.05). The ratios (TBCl + TBK)/TBW and TBCl/TBK were not significantly different from control reference values, and plasma chlorine and potassium concentrations were within control reference ranges. The sum of TBCl and TBK correlated with TBW(D(2)O) (r(2) = 0.79, P < 0.001), and TBW(Cl + K) correlated with TBW(D(2)O) (r(2) = 0.78, P < 0.001). TBW(Cl + K) was similar to TBW(D(2)O) (mean +/- SEM: 19.0 +/- 0.5 compared with 19.4 +/- 0.5 L; NS).

CONCLUSIONS

Prepubertal children with mild symptoms of CF can develop deficits in TBCl, TBK, and TBW that reflect chronic energy malnutrition. Mild symptoms of CF do not appear to affect normal relations among TBCl, TBK, and TBW. Measurements of TBCl and TBK may be used to assess body fluid compartments in these patients.

Extracellular water: sodium bromide dilution estimates compared with other markers in patients with acquired immunodeficiency syndrome

Extracellular fluid and closely related extracellular water (ECW) provide information on nutritional status in health and disease. Although various methods exist for ECW determination, little is known about their comparability in patients with wasting diseases such as acquired immunodeficiency syndrome (AIDS). One practical method, the dilution of sodium bromide (NaBr), is used widely in clinical research, although its relationship to other ECW markers has not been well characterized. The present study sought to compare ECW estimates as determined by NaBr and three other methods in 11 male patients with AIDS (mean +/- SD; age, 44+/-12 years; body weight, 64.5+/-8.8 kg; and height, 172+/-4 cm). ECW volumes were determined from NaBr dilution, total body chlorine (TBCl) by delayed gamma-neutron activation analysis, total body water (TBW) by tritium dilution combined with total body potassium (TBK) by whole body 40K counting, and radioactive sulfate dilution (35SO4). All correlations between the NaBr method and other methods were statistically significant (NaBr vs TBCl [r = .91; p < .001]; vs TBW/TBK [r = .76; p < .01]; and vs 35SO4 [r = .89; p < .001]). As expected from previous studies, ECW (L) derived by NaBr provided a group mean (15.1+/-2.2 L) similar to the TBCl method (15.4+/-1.7 L; p = .32), a significantly smaller ECW than by the TBW/TBK method (18.6+/-3.4 L; p = .0004), and a significantly larger ECW than by 35SO4 method (13.3+/-3.0 L; p = .002). Estimating ECW by NaBr dilution was comparable with other research-based ECW methods and, thus, offers a practical alternative for evaluating ECW in patients with AIDS.

Postnatal changes in water content and proton magnetic resonance relaxation times in newborn rabbit tissues

In the present study, using proton nuclear magnetic resonance relaxation (H1 NMR) measurements, an attempt was made to quantitate water fractions with different mobility in the skin, skeletal muscle, and liver tissues obtained from New Zealand white rabbit pups. Serial studies were carried out at the postnatal age of 0-1, 24, 48, and 72 h in pups nursed with their mothers and suckling ad libitum (group I) and in those pups separated from their mothers and completely withheld from suckling (group II). Tissue water content (desiccation method) and T1 and T2 relaxation times (H1 NMR method) were measured. Free, loosely bound, and tightly bound water fractions were calculated by applying multicomponent fits of the T2 relaxation curves. It was demonstrated that skin water content and T1 and T2 relaxation times decreased with age (p < 0.01), the decrease in T2 proved to be more pronounced in group II than in group I (p < 0.05). Muscle and liver water, and T1 and T2 relaxation times did not change with age in the suckling pups. In response to with-holding fluid intake muscle water remained constant, liver water increased paradoxically (p < 0.05). T1 relaxation time showed no consistent change in either tissues, whereas T2 relaxation time decreased significantly (muscle, p < 0.01) or tended to decrease (liver, p < 0.06). Using biexponential analysis fast and slow components of T2 relaxation curve could be distinguished that corresponded to the bound and free water fractions. Bound water accounted for 42-47%, 50-57%, and 34-40% of total tissue water in the skin, skeletal muscle, and liver, respectively, regardless of age and fluid intake. Triexponential fits of the T2 relaxation curve made possible the further partition of tissue water into tightly bound (fast component), loosely bound (middle component), and free (slow component) water fractions. In all tissues studied, loosely bound fraction predominated (skin, 48-64%; muscle, 54-65%; liver, 45-63%), followed by the free (skin, 26-45%; muscle, 23-32%; liver, 20-25%) and the tightly bound water fraction (skin, 6-14%; muscle, 10-16%; liver, 14-33%). Postnatal age and fluid intake had no apparent influence on this pattern of distribution. It is concluded that the majority of neonatal tissue water is motion-constrained. The free, the loosely bound, and the tightly bound water fractions appear to be interrelated and dependent on age, fluid intake, the tissues studied, and their hydration.

Bioelectrical impedance and body composition: present status and future directions

Single-frequency bioelectrical impedance analyzers used to assess body composition are being replaced by multiple-frequency analyzers. At low frequencies, the current flows primarily through extracellular fluids; at high frequencies, it completely penetrates all body tissues. Measures of bioelectrical impedance at multiple frequencies can differentiate total and extracellular fluid compartments in the body. This has considerable value for assessing clinical and nutritional status. Impedance measures at a single frequency contain only a small window of the available impedance spectrum information, which may explain the difficulty in discriminating among individuals. The impedance spectrum and its analysis may provide a much clearer picture of individual differences in body water and body composition. With increasing clinical uses of bioelectrical impedance in individuals and sample populations, the use of multiple-frequency impedance may help to elucidate differences that are not discernible with single-frequency impedance.

Extracellular volume in preterm infants: influence of gestational age and colloids

We assessed the effect of fresh frozen plasma (FFP) on extracellular volume (ECV) during the first few days of life in two groups of preterm infants, group 1 (gestational age less than 30 weeks) and group 2 (gestational age 30-34 weeks). The infants were randomly assigned to one of two treatment groups, one receiving FFP, the other not; group 1 infants receiving no treatment (n = 8) and group 1 infants receiving FFP (n = 11), and group 2 infants receiving no treatment (n = 9) and group 2 receiving FFP (n = 10). FFP was given at a dose of 10 ml/kg daily during a two-hour period for the first three days of life. ECV was measured on day 1 before FFP was given and on day 4 by the distribution of bromide. There was a significant correlation between birth weight and initial ECV (r = 0.85, P less than 0.001). In group 1 ECV was significantly higher than in group 2 (P less than 0.02). In group 1 receiving no treatment mean weight loss and mean decrease in ECV were equal (84 g/kg and 78 ml/kg, respectively), but no correlation between the two parameters could be found (r = 0.68, P = 0.06). In the other study groups, mean weight loss was higher than mean decrease in ECV, with no correlation between weight loss and change in ECV.(ABSTRACT TRUNCATED AT 250 WORDS)

Measuring body fat: calibrating the rulers. Intermethod comparisons in 389 normal Caucasian subjects

A systematic study of 389 normal Caucasians stratified for sex and age compared all of the traditional methods for measuring fat: body water, underwater weighing, body potassium, and anthropometrics and the newer methods of dual-photon absorptiometry, bioimpedance analysis, and total body electrical conductivity. Measurements by all methods are highly intercorrelated, but methods differences show the population means for fat percent to range from 26 to 35% of body weight across eight methods. All methods show increasing fat (as % body weight) with age in both sexes but vary in secular slope. The goal of this report is to provide direct translations between each of the eight methods. Intermethod comparison equations are given as simple linear regressions by using each method both as dependent and independent variable for each sex, permitting translation for results by any method to any other.

Postnatal weight loss in preterm neonates less than 1,500 g is due to isotonic dehydration of the extracellular volume

Weight, extracellular volume (ECV; distribution volume of sucrose) and renal function were studied in 13 preterm infants at birth (age 6 h (2-12); median, range) and again when postnatal weight loss exceeded 5% of birth weight (age 84 (64-97) h). Gestational age was 28 (26-32) weeks, and birthweight was 1,170 g (810-1,455). The infants were nursed in incubators and mechanically ventilated. Fluid therapy allowed a weight loss of up to 10% of birthweight. Body weight decreased significantly from 1,101 +/- 202 g at birth to 1,016 +/- 198 g at day 3 and ECV from 499 +/- 155 ml to 413 +/- 118 ml. Mean weight loss of 85 +/- 50 g was the same as mean ECV loss of 86 +/- 48 ml, suggesting that postnatal weight loss is water loss from the ECV. Weight loss was preceded by a marked increase in diuresis, exceeding fluid intake on day 2. Creatinine clearance did not change. The increased urine output led to a significant increase of sodium excretion without inducing hyponatremia but resulted in an isotonic reduction of ECV.

Sodium balance and extracellular volume regulation in very low birth weight infants

We assessed sodium balance and extracellular volume regulation in very low birth weight infants by examining the effect of differences in sodium intake on postnatal sodium homeostasis and body water composition. Twenty infants (mean birth weight 1103 +/- 216 gm, mean gestation 28.5 +/- 1.7 weeks) were randomly assigned to receive sodium in doses of either 1 or 3 mmol.kg-1.day-1 for the first 10 postnatal days. Extracellular volume (estimated by the bromide dilution method), sodium excretion, creatinine clearance, fractional sodium excretion, plasma atrial natriuretic factor level, urine aldosterone concentration, and vasopressin excretion were measured on postnatal days 1, 5, 10, 20, and 30. The corrected bromide space was large at birth and decreased in both groups during the first 5 days of observation, concomitant with a negative sodium balance. After 5 days of age, sodium excretion decreased in both groups so that sodium balance became positive and the corrected bromide space increased in proportion to increasing body weight. Differences in sodium intake were associated with differences in tubular sodium reabsorption; corrected bromide space and net sodium balance were similar in the two groups. Serum sodium concentration was significantly lower in the low-sodium intake group. Creatinine clearance, plasma atrial natriuretic factor level, and excretion of aldosterone and vasopressin were not significantly different between the two groups. We conclude that very low birth weight infants are able to regulate sodium balance by altering renal sodium excretion. However, the renal response to sodium intake may be insufficient to prevent changes in serum sodium concentration. The roles of specific renal and hormonal mechanisms regulating sodium excretion in very low birth weight infants remain incompletely defined.

The influence of age on lignocaine pharmacokinetics in young puppies

Eight mongrel puppies were studied at intervals of a few weeks. Lignocaine 4 mg/kg was infused over 60 seconds. Frequent samples were taken over 30 minutes for plasma lignocaine assay. Initially the concentrations were significantly higher in the oldest group (178-191 days) but after 10 minutes the youngest group (3-16 days) had significantly higher levels than the other groups. Pharmacokinetic data derived included the rate constants k21, k12 (drug movement between two compartments), k10 (elimination from the central compartment), and the volume of distribution (V beta). The elimination rate constant k10 was significantly lower in Group 1 (3-16 days) than all other groups and Group 2 (37-57 days) was lower than Group 4 (178-191 days). The calculated beta half-life was significantly longer in the youngest group than the others. There was no significant difference in the volume of distribution between these age groups--up to 6 months.

Rabbit tissue composition: developmental aspects of tissue fluid and fractional exclusion of albumin

The maturational changes in tissue water spaces and in the fractional exclusion of albumin from the interstitial gel were studied in the developing rabbit using multiple radioactive tracers. Total tissue water and extracellular water decrease with age in all organs studied. Intracellular water decreases in the heart and brain, transiently increases in the muscle, and is unchanged in the ileum and lung. However, the percentage of extravascular water located in the cell increases with age in all organs. Fractional exclusion of albumin is unchanged in the muscle and brain, fluctuates in the lung, increases in the ileum, and decreases in the heart. Possible mechanisms for regional differences in fluid and albumin changes with maturation are discussed. Until the role of the fractional exclusion of albumin in the interstitial matrix in the maintenance of tissue fluid balance is elucidated, the physiological impact of these developmental changes will not be evident.

Pharmacokinetics of oral and intravenous bromide in normal volunteers

The pharmacokinetics of oral and intravenous bromide was studied in 7 adult volunteers, who served as their own controls. They received 1 ml/kg of 3% sodium bromide, equivalent to 30 mg/kg bromide. Oral bioavailability ranged between 75-118% with a mean of 96 +/- 6%. Elimination T1/2 was 11.9 +/- 1.4 days after oral administration and 9.4 +/- 1.5 days after I.V. administration (P greater than 0.10). The employment of this ion for calculation of extracellular fluid and assessment of its potential significance in environmental toxicology necessitates accurate data on its disposition characteristics.

Muscle cell growth and the distribution of water and electrolyte in human pregnancy

Ten normal pregnant women had muscle composition analyses (rectus abdominis) carried out at 39-40 weeks of pregnancy. Water, chloride (Cl), chloride space (ECV), non-chloride space (ICW), potassium (K), sodium (Na), magnesium (Mg) and zinc (Zn) determinations were carried out. Analyses for DNA (cell number), protein: DNA ratio (cell size), RNA and collagen were also performed. Similar analyses were performed on uterine muscle and placentae before and after perfusion with Earle's solution. Data from pregnant patients were compared with similar estimations carried out on rectus abdominis samples from 13 non-pregnant subjects undergoing gynaecological procedures. Muscle tissue and predicted muscle mass (MM) (which constitutes 40% of body weight) demonstrated that the gain in body K was due to the products of conception, that ICW decreased per unit weight in muscle (8%), ECV increased (41%) without a radical change in muscle water content (2%). Overall a 6 l gain in ECV and a 2 l gain in ICW can be accounted for during pregnancy. The results of this study indicate that added hydration excluding the products of conception (placenta, infant, uterus) is mainly extracellular. Intracellular Na concentration decreases (50%) and it is speculated that the cation gap is made up by H+ in the presence of extracellular alkalosis. Muscle cells diminish in size but cell number per gram is constant. Zinc content (Zn/DNA) decreases. Previous experimental work suggests that MM increases by about 10% during pregnancy and this information has been included in considerations but it remains to be shown to what extent total muscle cell numbers increase and as to whether such increased muscle growth remains following pregnancy.

Composition of postnatal weight loss and subsequent weight gain in small for dates newborn infants

Using a sucrose and deuterium oxide dilution technique body water compartments and solids were serially determined in small for dates newborn infants at birth, at the moment of maximum postnatal weight loss and on recovery of birth weight. Compositions of weight loss and subsequent weight gain were calculated from the differences in body water compartments and solids between the first and the second and the second and the third study, respectively. Birth weight of the infants was 1.55 +/- 0.46 kg (mean +/- SD) (N = 7), gestational age was 35.7 +/- 3.1 weeks. Results show that despite changes in extra- and intracellular water volumes during weight loss, total body water volume and solids per unit of body weight remained remarkably constant throughout the study. Compositions of weight loss and subsequent weight gain were similar to body composition. This suggests that in small for dates newborn infants postnatal weight loss is the result of catabolism rather than dehydration and subsequent weight gain is the result of growth rather than rehydration.

The application of the extracellular base excess to children

The Base Excess of the Extracellular Fluid (BE-ecf) is the most complete model of acid-base physiology. It has gained considerable acceptance for use in adults as an index of the metabolic component of an acute acid-base disturbance. Several other commonly used indices were compared to the BE-ecf. The values of the Base Excess of Blood, Plasma Bicarbonate, Standard Bicarbonate, and T40-Bicarbonate differed significantly from the BE-ecf in 11.4, 6.3, 12.5, and 1.1% of samples, respectively. These differences are considered to be errors since the nonlinear relationship of the variables makes it difficult to clinically accommodate them. The standard (adult) form of the BE-ecf calculation overestimated the base excess by 1 meq/liter in 44.8% and by 2 meq/liter or more in 6.1% of samples from neonates and infants when compared to a form of the calculation which was individually adjusted based on the weight and hemoglobin concentration of each subject. Since it is no more difficult to make these corrections than to ignore them, if the BE-ecf is to be used in neonates and infants the correction should be applied.

Role of the renin-angiotensin-aldosterone system in hypertensive children with coarctation of the aorta

To investigate the role of the renin-angiotensin-aldosterone system as a cause of hypertension, 20 hypertensive patients with coarctation of the aorta were studied during normal and low sodium intake and after diuresis with flurosemide. Eight patients with essential hypertension and 13 control subjects were similarly studied. Plasma renin activity values in patients with coarctation were similar to those in patients with essential hypertension and in control patients during normal and low sodium diets. However, after the administration of furosemide, plasma renin activity values were significantly higher in the patients with coarctation than in the other two groups (P less than 0.005 and less than 0.01, respectively). The values for urinary aldosterone, plasma volume and extracell fluid volume (bromide space) were increased in patients with coarctation during both normal and low sodium intake. These renin and aldosterone responses and body fluid spaces in patients with coarctation suggest that their hypertension resembles a one-kidney Goldblatt model. The data help to better define the role of the renin-angiotensin-aldosterone system in the hypertension of coarctation and thus may help guide the clinician in therapeutic interventions.

Fluid and electrolyte requirements in the newborn infant

Most of the studies of water and electrolyte balance have been performed on full teren balancing the hydration and electrolyte status of all infants, it is most important to relate the treatment regimen to the infant's gestational age and weight, and to make appropriate adjustments for various environmental conditions which affect water and salt requirements, such as phototherapy, type of warmer, and humidity.

Paediatric pharmacology

Infants and children differ pharmacologically from adults because the progressive maturation of various systems during development leads to variations in drug action and dosage. These differences are reviewed. Pharmacogenetic factors which affect anaesthesia and the problems of drug overdosage, as seen in paediatric intensive care units, are briefly considered.

Weight minus extracellular fluid as metabolic reference standard in newborn baby

In 51 babies of differing size and gestational age, rates of oxygen consumption and corrected bromide space (as a measure of extracellular fluid) were measured. The results are used to examine the concept that weight minus extracellular fluid (ECF) is an appropriate metabolic reference standard in the newborn baby. When the whole group, which included large-for-dates and small-for-dates babies, is considered there is a systematic variation wherein the rate of oxygen consumption thus expressed varies with size. However, when appropriately grown babies only are considered, who varied in birthweight from 1210 g to 3820 g, rates of oxygen consumption thus expressed were constant. The implication is that when unusual rates of oxygen consumption per kilogram body weight are found, they should be interpreted bearing in mind the possibility of an unusual proportion of ECF in the baby. An incidental finding was that small-for-dates babies have a relatively large corrected bromide space.

Muscle biopsy measurement of body water and intracellular electrolytes in children

Body water and electrolytes have been measured in normal infants and children using a muscle biopsy technique. Extracellular water, sodium, and chloride contents per kilogram dry weight are high in infancy but fall to adult values by 18 months to 2 years. Intracellular water and potassium contents are initially high, but approximate to adult levels around 6 months of age. Large variations in water and electrolyte contents have been found in apparently normal children. There is good correlation between extracellular water and sodium content, and between intracellular water and potassium content. Therefore intracellular electrolyte concentrations are similar for children of all ages. This technique is presented as a useful method of obtaining information about fluid and electrolyte balance in children.