The extracellular fluid volume in infants and children

Fluid Homeostasis and Diuretic Therapy in the Neonate

Understanding physiologic water balance and homeostasis mechanisms in the neonate is critical for clinicians in the NICU as pathologic fluid accumulation increases the risk for morbidity and mortality. In addition, once this process occurs, treatment is limited. In this review, we will cover fluid homeostasis in the neonate, explain the implications of prematurity on this process, discuss the complexity of fluid accumulation and the development of fluid overload, identify mitigation strategies, and review treatment options.

Modelling intestinal glucose absorption in premature infants using continuous glucose monitoring data

BACKGROUND

Model-based glycaemic control protocols have shown promise in neonatal intensive care units (NICUs) for reducing both hyperglycaemia and insulin-therapy driven hypoglycaemia. However, current models for the appearance of glucose from enteral feeding are based on values from adult intensive care cohorts. This study aims to determine enteral glucose appearance model parameters more reflective of premature infant physiology.

METHODS

Peaks in CGM data associated with enteral milk feeds in preterm and term infants are used to fit a two compartment gut model. The first compartment describes glucose in the stomach, and the half life of gastric emptying is estimated as 20 min from literature. The second compartment describes glucose in the small intestine, and absorption of glucose into the blood is fit to CGM data. Two infant cohorts from two NICUs are used, and results are compared to appearances derived from data in highly controlled studies in literature.

RESULTS

The average half life across all infants for glucose absorption from the gut to the blood was 50 min. This result was slightly slower than, but of similar magnitude to, results derived from literature. No trends were found with gestational or postnatal age. Breast milk fed infants were found to have a higher absorption constant than formula fed infants, a result which may reflect known differences in gastric emptying for different feed types.

CONCLUSIONS

This paper presents a methodology for estimation of glucose appearance due to enteral feeding, and model parameters suitable for a NICU model-based glycaemic control context.

Size mismatch in liver transplantation

Size mismatch is an unique and inevitable but critical issue in live donor liver transplantation. Unmatched metabolic demand of recipient as well as physiologic mismatch aggravates the damage to liver graft, inevitably leading to graft failure on recipient. Also, an excessive resection of liver graft for better recipient outcome in live donor liver transplant may jeopardize the healthy donor well-being and even put donor life in danger. There is a fine balance between resected graft volume required to meet the recipient's metabolic demand and residual graft volume required for donor safety. The obvious clinical necessity of finding that balance has prompted a clinical need and promoted the improvement of knowledge and development of management strategies for size-mismatched transplants. The development of the size-matching methodology has significantly improved graft outcome and recipient survival in live donor liver transplants. On the other hand, the effect of size mismatch in cadaveric transplants has never been observed as being so pronounced. The importance of matching of the donor recipient size has been unrecognized in cadaveric liver transplant. In this review, we attempt to summarize the current most updated knowledge on the subject, particularly addressing the definition and complications of size-mismatched cadaveric liver transplant, as well as management strategies.

Body surface area index predicts outcome in orthotopic liver transplantation

BACKGROUND/PURPOSE

In living donor liver transplantation (LDLT), matching of liver volume between donor and recipient is critical to the success of the procedure; mismatch can result in 'small- or large-for-size syndrome'. In orthotopic liver transplantation (OLT), matching criteria are less stringent and non-uniform. We sought to determine whether a new parameter, the ratio of donor to recipient body surface area (BSAi), is predictive of size mismatch and/or post-transplant morbidity or mortality.

METHODS

We reviewed data on 1228 OLT recipients and stratified this data according to three categories: small-for-size (BSAi <0.6), control (BSAi = 0.6-1.4), and large-for-size (BSAi >1.4) donors.

RESULTS

We found that: (1) matching of grafts at the upper and lower extremes of BSAi had significantly reduced graft survival; (2) matches with lower BSAi sustained a less severe form of intraoperative post-reperfusion syndrome, and the incidence of hepatic artery thrombosis was high postoperatively in these grafts; (3) BSAi and donor age correlated well with the severity of intraoperative post-reperfusion hypotension; and (4) small-for-size (BSAi <0.6) and large-for-size (BSAi >1.4) grafts, as well as preoperative total bilirubin, were significant risk factors for decreased graft survival.

CONCLUSION

We conclude that the BSAi can predict clinically significant size mismatch and adverse outcomes in cadaveric whole OLT.

Tips and traps analyzing pediatric PK data

Pharmacokinetic (PK) and pharmacodynamic (PD) modeling has elucidated aspects of developmental pharmacology of value to the anesthetic community. The increasing sophistication of modeling techniques is associated with pitfalls that may not be readily apparent to readers or investigators. While size and age are considered primary covariates for PK models, the impact of birth on clearance maturation is poorly documented, dose in obese children is poorly investigated, pharmacologic implications of physiologic changes poorly portrayed, disease progression on drug response poorly depicted and the impact of metabolites on effect poorly illustrated. This review identifies some of these pitfalls and suggests ideas to circumvent or investigate these hazards.

Management of the surgical newborn: physiological foundations and practical considerations

The unique physiological changes that occur at birth and the adaptations that come in the first weeks of life profoundly affect the clinical management of the surgical newborn patient. The newborn has adaptive challenges that have to be met at birth: (a) establish a functional residual capacity and breathe; (b) change from a fetal to a newborn circulatory circuit; (c) change from fetal hemoglobin to hemoglobin A; (d) sustain a heart rate dependent cardiac output; (e) maintain normal body temperature; establish renal function and regulate postnatal body fluid composition; (f) establish hepatic function; and (g) maintain normal vital signs. Classical physiological studies of the last century describe the physiological basis of these adaptive tasks, and support practical considerations that we follow in clinical practice.

An allometric model to estimate fluid requirements in children following burn injury

OBJECTIVES

To evaluate the ability of an allometric 3/4 Power Model combined with the Galveston Formula (Galveston-3/4 PM Formula) to predict fluid resuscitation requirements in children suffering burn injuries in comparison with the frequently used Parkland Formula and Galveston Formula using the Du Bois formula for surface area estimation (Galveston-DB Formula).

AIM

To demonstrate that the Galveston-3/4 PM Formula is clinically equivalent to the Galveston-DB Formula for the estimation of fluid requirements in pediatric burn injury cases.

BACKGROUND

Fluid resuscitation requirements differ in children suffering burn injuries when compared to adults. The Parkland Formula works well for normal weight adults but underestimates fluid requirements when indiscriminately applied to pediatric burn patients. The Galveston-DB Formula accounts for the change in body composition with age by using a body surface area (BSA) model but requires the measurement of height. The allometric model, using an exponent of 3/4, accounts for the dependence of a physiological variable on body mass without requiring height measurement and can be applied to estimate fluid requirements after burn injury in children.

METHODS

Comparisons were performed between the hourly calculated fluid requirements for the first 8 h following 20%, 40%, and 60% BSA burns using the Parkland Formula, the Galveston-DB Formula and Galveston-3/4 PM Formula for children 2-23 kg.

RESULTS

In children less than 23 kg, the fluid requirements predicted by the Galveston-3/4 PM Formula are well correlated with those predicted by the Galveston-DB Formula (R2 = 0.997, P < 0.0001) and are much better than of the predictions made with the Parkland Formula, especially for children <10 kg.

CONCLUSIONS

For the purposes of clinical estimation of fluid requirements, the Galveston-3/4 PM Formula is indistinguishable from the Galveston-DB Formula in children 23 kg or less.

Mechanism-based concepts of size and maturity in pharmacokinetics

Growth and development can be investigated using readily observable demographic factors such as weight and age. Size is the primary covariate and can be referenced to a 70-kg person with allometry using a coefficient of 0.75 for clearance and 1 for volume. The use of these coefficients is supported by fractal geometric concepts and observations from diverse areas in biology. Fat free mass (FFM) might be expected to do better than total body weight when there are wide variations in fat affecting body composition. Clearance pathways develop in the fetus before birth. The use of postnatal age as a descriptor of maturation is unsatisfactory because birth may occur prematurely; therefore postmenstrual age is a superior predictor of elimination function. A sigmoid E(max) model (Hill equation) describes gradual maturation of clearance in early life leading to a mature adult clearance achieved at a later age.

Body surface area and body weight predict total liver volume in Western adults

Computed tomography (CT) is used increasingly to measure liver volume in patients undergoing evaluation for transplantation or resection. This study is designed to determine a formula predicting total liver volume (TLV) based on body surface area (BSA) or body weight in Western adults. TLV was measured in 292 patients from four Western centers. Liver volumes were calculated from helical computed tomographic scans obtained for conditions unrelated to the hepatobiliary system. BSA was calculated based on height and weight. Each center used a different established method of three-dimensional volume reconstruction. Using regression analysis, measurements were compared, and formulas correlating BSA or body weight to TLV were established. A linear regression formula to estimate TLV based on BSA was obtained: TLV = -794.41 + 1,267.28 x BSA (square meters; r(2) = 0.46; P <.0001). A formula based on patient weight also was derived: TLV = 191.80 + 18.51 x weight (kilograms; r(2) = 0.49; P <.0001). The newly derived TLV formula based on BSA was compared with previously reported formulas. The application of a formula obtained from healthy Japanese individuals underestimated TLV. Two formulas derived from autopsy data for Western populations were similar to the newly derived BSA formula, with a slight overestimation of TLV. In conclusion, hepatic three-dimensional volume reconstruction based on helical CT predicts TLV based on BSA or body weight. The new formulas derived from this correlation should contribute to the estimation of TLV before liver transplantation or major hepatic resection.

Calculation of child and adult standard liver volume for liver transplantation

Despite refinements in surgical techniques for liver transplantation, liver size disparity remains one of the most common problems in pediatric patients. Optimal liver graft size remains unknown and the volume of diseased liver in the recipient is not indicative of the volume (standard liver volume [LV]) optimal for the recipient's metabolic demands. To establish a formula for calculating the standard LV in the pediatric and adult populations for liver transplantation, whole LVs were measured using computed tomography (CT) in 96 patients (65 pediatric and 31 adolescent or adult subjects) with normal liver whose disease conditions did not seem to affect body weight (BW) or LV. In the 96 subjects, the ratio of estimated LV to BW decreased gradually as age increased until approximately 16 years, when it started to level off. On the other hand, there seemed to be a directly proportional relationship between the estimated LV in vivo and body surface area (BSA) (r = .981; r2 = .962; P < .0001) in the subjects as a whole, and the formula, LV (mL) = 706.2 x BSA (m2) + 2.4, was established from the measured data by simple regression analysis. Another predicting equation, LV (mL) = 2.223 x BW (kg)0.426 x body height (BH) (cm)0.682, was produced by multiple regression analysis (r2 = .969; P < .0001). Considering its simplicity of use, we adopted the first formula for predicting standard LV in an individual patient.

Advances in pediatric surgical critical care

The advances in pediatric intensive care outlined here point out the differences between children and adults that need to be considered when taking care of children with surgical diseases. In the past, advances in pediatric critical care have not kept pace with advances in adult care, but these and other new techniques have rapidly closed this gap in knowledge.

Pharmacokinetics of oral cefetamet pivoxil (Ro 15-8075) and intravenous cefetamet (Ro 15-8074) in humans: a review

Cefetamet pivoxil belongs to the class of orally absorbed pro-drug esters which are hydrolyzed to the active compound (cefetamet) on their first pass through the gut wall and/or the liver. The intravenously administered cefetamet is eliminated predominantly unchanged in the urine by glomerular filtration. Systemic and renal clearance values for cefetamet were 140 and 130 ml/min, respectively. The plasma protein binding is 22%, whereby the only binding protein is albumin. The steady state volume of distribution (0.29 l/kg) corresponds roughly to the extracellular water space which is consistent with other low protein-bound cephalosporins. In general, after intravenous doses, cefetamet follows the kinetic behaviour of a cephalosporin with low protein binding, limited non-renal clearance, and renal clearance that is predominantly due to glomerular filtration, e.g. ceftizoxime, ceftazidime. After oral administration, cefetamet pivoxil shows a significant food effect (F = 41% vs 51%). Hence, cefetamet pivoxil is recommended to be taken after food. The food effect, however, is not of such a magnitude that it will be of clinical consequence when this recommendation is not followed. The food effect is not related to a change in gastric pH because antacids and ranitidine do not affect the absorption of cefetamet pivoxil, although in approximately 20% of the subjects absorption of the drug is delayed. The elimination of cefetamet is directly proportional to renal function. In patients with varying degrees of renal insufficiencies, dosage should be decreased accordingly. Age has no effect on the bioavailability of cefetamet pivoxil. However, the clearance of cefetamet is higher in children and lower in the elderly.

Single-dose pharmacokinetics of ceftriaxone in infants and young children

The pharmacokinetics of ceftriaxone were studied in five infants (7 to 15 months old) and five young children (24 to 70 months old). Both groups received a single 50-mg/kg dose in an intravenous infusion over 5 min. No major pharmacokinetic differences were observed between the two populations. The total (bound plus unbound) plasma concentration-versus-time data could be described in each case by a biexponential equation. Changes in renal clearance indicated time- and dose- dependent pharmacokinetic behavior. The fraction excreted unchanged in the urine (fu) and the biological half-life (t 1/2 (beta)) were, however, dose independent. The average values were 47% for fu (0 to 12 h) and 6.5 for T 1/2 (beta). Weight-corrected total systemic clearance was C1TS = 0.71 ml/min per kg; volume of distribution was VD (beta) = 394 mg/kg. The data support intravenous administration of 50 mg of ceftriaxone per kg of body weight every 12 h in assessing its activity against Haemophilus influenzae, Streptococcus pneumoniae, and Neisseria meningitidis in postneonatal-stage pediatric patients.