Fluid and electrolyte balance in children

Intravenous fluid and electrolyte management in paediatrics requires an understanding of the indication for intravenous fluid therapy and the underlying pathophysiology in order to meet requirements and avoid adverse events. Careful consideration needs to be given to the rate as well as the composition of intravenous therapy, and children's ongoing need should be reviewed regularly. Fluid therapy can be separated into maintenance fluids, replacement fluids that address electrolyte deficits and ongoing losses, and resuscitation fluids. We review the critical considerations when prescribing intravenous fluid for children and consider the challenges of non-osmotic secretion of anti-diuretic hormone (ADH), hyponatraemic encephalopathy and other electrolyte abnormalities. Fluid management in children with diabetic ketoacidosis is also reviewed.     

Fluid and electrolyte balance

The human body is made up principally of water, two-thirds of which is intracellular and one-third extracellular. Dissolved in body water are a variety of mineral and organic salts and proteins. These components are quantified in millimoles and are electrochemically balanced in each compartment. The relative quantities of each cation and anion are different in each compartment. Fluid motion through the body is determined by hydrostatic, osmotic and oncotic forces. The rate of extravasation is determined by the Starling equation. In perioperative medicine and critical care, temporal fluid shifts occur based on the stress response. Fluid management, in terms of volume and tonicity, should follow this response. Tissue oedema, particularly in the lungs, results from excess intravascular fluid or high venous pressures (hydrostatic pulmonary oedema) or damage to the capillary endothelium (capillary leak).     

Fluid and electrolyte balance in children

Fluid therapy in children requires an understanding of certain basic principles to avoid adverse events. Careful consideration needs to be given to both the appropriate rate and composition of the fluids to be administered with frequent re-assessment. Parenteral fluid management is used to meet maintenance requirements, correct any deficit and replace ongoing losses. Non-osmotic secretion of antidiuretic hormone (ADH) may occur, particularly in critically ill children and those in the perioperative period, resulting in an inability to compensate for an inappropriate administration of free water. Excess free water administration may result in cerebral oedema, which is poorly tolerated in children due to the proportionally larger size of the brain within the skull, compared to adults. Hyponatraemic encephalopathy continues to occur in hospitalized children and is associated with severe morbidity and mortality. Early recognition and aggressive management of this condition is required with hypertonic sodium chloride and further care within a paediatric high-dependency/intensive care unit. In the perioperative period concerns over hypoglycaemia have resulted in routine use of dextrose-containing solutions. However for the majority of children the stress response coupled with dextrose supplementation is likely to result in hyperglycaemia. Current recommendations regarding perioperative dextrose management are reviewed.     

Intravenous fluid and electrolyte management in children and young people

NICE published guidance NG29 in December 2015 entitled Intravenous fluid therapy in children and young people in hospital and in September 2016 Quality Standard QS131. These patient safety documents were developed following avoidable deaths in children who were given inappropriate or excessive volumes of intravenous fluids or who were inappropriately monitored, leading to acute symptomatic hyponatraemic encephalopathy. It is not mandatory to apply the recommendations of NG29, but anyone prescribing intravenous fluids should be familiar with them. This article does not reiterate NG29, but aims to help the reader: (i) revise their relevant basic science knowledge, while challenging their understanding of osmosis; (ii) use correctly the terms tonicity and osmolarity in clinical practice; and (iii) manage some electrolyte derangements.     

Paediatric fluid and electrolyte therapy guidelines

The advice in this article is based on a multidisciplinary consensus opinion generated by the Association of Paediatric Anaesthetists and on a National Patient Safety Agency (NPSA) recommendation of March 2007 entitled ‘Reducing the risk of hyponatraemia when administering intravenous infusions to children’. To this has been added advice from our specialist hospital fluid policy.     

Paediatric fluid and electrolyte therapy guidelines

The advice in this article is based on a multidisciplinary consensus opinion generated by the Association of Paediatric Anaesthetists and on a National Patient Safety Agency (NPSA) recommendation of March 2007 entitled ‘Reducing the risk of hyponatraemia when administering intravenous infusions to children’. To this has been added advice from our specialist hospital fluid policy.     

Regulation of fluid and electrolyte balance

Adequate tissue perfusion and cellular function is dependent on the maintenance of effective circulatory volume and serum osmolality, respectively. As sodium is the principal extracellular cation with the inability to pass freely across the cellular membrane, it therefore has the greatest effect on extracellular fluid osmolality. The extracellular fluid (ECF) volume can increase or decrease independent of the surrounding osmolality, indicating that control of plasma osmolality and volume occur through distinct physiological processes. Disorders in sodium balance with consequent effect on osmolality come about mainly due to disturbances in water homeostasis rather than an abnormality of sodium intake or excretion.     

Regulation of fluid and electrolyte balance

Adequate tissue perfusion and cellular function is dependent on the maintenance of effective circulatory volume and serum osmolality, respectively. As sodium is the principal extracellular cation with the inability to pass freely across the cellular membrane, it therefore has the greatest effect on extracellular fluid osmolality. The extracellular fluid (ECF) volume can increase or decrease independent of the surrounding osmolality indicating that control of plasma osmolality and volume occur through distinct physiological processes. Disorders in sodium balance with consequent effect on osmolality come about mainly due to disturbances in water homeostasis rather than an abnormality of sodium intake or excretion.     

COMPARISON OF THE EFFECT OF TWO HEIGHTS OF GLYCINE IRRIGATION SOLUTION ON SERUM SODIUM AND OSMOLALITY DURING TRANSURETHRAL RESECTION OF THE PROSTATE

Background: The absorption of sodium-deficient, hypotonic irrigation solution is believed to contribute, in certain cases, to hyponatraemia and hypo-osmolality and, in severe cases, to the so-called transurethral resection of the prostate (TURP) syndrome. Methods : The effect of the height of 1.5% glycine irrigation solution during intermittent-flow TURP on serum sodium and osmolality was studied pen-operatively in 40 patients. The height of the glycine was set at 70 (n= 20) or 150 (n= 20) cm above the operating table. Results: We found no statistically significant difference in the measured serum sodium (P= 0.929) and osmolality (P= 0.260) values between the two groups during the 24 hr study period. Conclusions: The height of the irrigation solution is not important in the development of hyponatraemia and hypo-osmolality, and other factors are probably more important.