Knowing a detailed diagnosis is vital to gain information on the likely composition of the fluid lost. Causes of dehydration include preoperative fasting, ongoing gastrointestinal illness and self-neglect following acute confusion. Patients’ medical histories give an indication of their expected fluid status. This breakdown of normal compartment integrity can result in loss of circulating intravascular volume. This is known as “third spacing” and is caused by vasodilation and “leakage” of vascular epithelial walls. In addition, insensible losses can increase during fever or after suffering from burns because the barrier function of the skin is impaired.įluids can accumulate into spaces that normally contain minimal fluid volumes (eg, the peritoneal or pleural cavities) during surgery, anaesthesia or as a result of inflammatory conditions (eg, sepsis). These losses may occur from the gastrointestinal tract (due to vomiting, diarrhoea or a fistula) or the urinary tract (eg, diabetes insipidus), or be caused by blood loss from trauma or surgery. IV fluid therapy is used to maintain homeostasis when enteral intake is insufficient (eg, when a patient is “nil by mouth” or has reduced absorption), and to replace any additional losses. It is important to remember that normal homeostatic mechanisms may not work well after injury (due to trauma or surgery), or during episodes of sepsis or other critical illness. Renal mechanisms also play a part in volume homeostasis - the renin-angiotensin mechanism is activated by falling renal perfusion pressure. This elicits a sensation of thirst and reduces renal excretion of water. Osmoreceptors and baroceptors detect small decreases in osmolality and blood pressure, triggering the release of ADH. In healthy individuals, volume homeostasis is regulated largely by antidiuretic hormone (ADH). Sodium-potassium pumps on cell membranes normally ensure that potassium is pumped into cells and sodium is pumped out, thus the intracellular sodium concentration is lower than the extracellular sodium concentration (the reverse applies to potassium) - see Panel 1. Water moves freely across the membranes that separate the compartments to maintain osmotic equilibrium. The extracellular compartment comprises both interstitial and plasma compartments. Within the body, water is distributed into intracellular and extracellular compartments. It is lost via the urine, sweat and faeces, as well as through insensible losses via the lungs and skin. Normally, fluid is gained from a person’s food and drink intake (including a small amount from carbohydrate metabolism). Basic fluid physiologyįluid and electrolyte levels in the body are kept relatively constant by several homeostatic mechanisms. Pharmacists should be prepared to advise on the prescribing of IV fluids alongside that of other medicines. 1 Iatrogenic problems arising from inappropriate fluid therapy can increase morbidity and prolong hospital stays. Research has shown that the prescribing of intravenous fluids is generally left to junior doctors - whose knowledge may be limited. Safe and effective prescribing of intravenous fluids requires understanding of the physiology of fluid and electrolyte homeostasis, physiological responses to injury and disease, as well as knowledge of the properties of intravenous fluids. We do not recommend that you take any clinical decisions based on this information without first ensuring you have checked the latest guidance. Journal of Pharmaceutical Health Services Research.International Journal of Pharmacy Practice. ![]() Antimicrobial resistance and stewardship.
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