The Pharmaceutical Journal 1 SEP 2008
Katharina Floss, MRPharmS, DipClinPharm, Mark Borthwick, MA, MRPharmS and Christine Clark, PhD, FRPharmS
Safe and effective administration of intravenous fluids requires knowledge of the physiology of fluid and electrolyte homeostasis, physiological responses to injuries and diseases, and the properties of intravenous fluids. Studies have shown that prescribing intravenous fluids is usually left to younger doctors whose knowledge may be limited1
Iatrogenic problems resulting from improper fluid therapy may increase morbidity and prolong hospital stay. Pharmacists should be prepared to advise on the prescription of intravenous fluids in addition to other medications.
Basic physiology of fluids
The levels of fluids and electrolytes in the body are kept relatively constant thanks to various homeostatic mechanisms. Usually the liquid comes from food intake and drink intake (including a small amount of carbohydrate metabolism). Loss of urine, sweat and faeces as well as subtle loss of lungs and skin.
In the body, water is distributed to the intracellular and extracellular compartments. The extracellular compartment includes both interstitial and plasma compartments. The water moves freely across the membranes that separate the compartments to maintain an osmotic balance.
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Sodium potassium pumps on cell membranes usually cause potassium to pump into cells and pump out sodium, so that the intracellular sodium concentration is lower than the concentration of extracellular sodium (in contrast, in the case of potassium) – see Table 1.
Panel 1: main constituents of body fluid
Plasma constitutive concentration (mmol / L) Intra-interstitial fluid concentration (mmol / L) Intracellular fluid concentration (mmol / L)
Sodium 142 145 12
Potassium 4 4.1 150
Chloride 103 113 4
Bicarbonate 25 27 12
In healthy people, volumetric homeostasis is largely regulated by an anti-diuretic agent (ADH). Osmoreceptors and baroceptors detect a small decrease in osmolality and blood pressure and trigger ADH. This causes a feeling of thirst and reduces the excretion of water through the kidneys.
Renal mechanisms also play a role in volumetric homeostasis – the renin-angiotensin mechanism is activated by reducing renal perfusion pressure.
It is important to remember that normal homeostatic mechanisms may not work properly after injury (due to injury or surgery) or during episodes of sepsis or other critical diseases.
Indications for liquid therapy IV
Intravenous fluid therapy is used to maintain homeostasis when intestinal consumption is insufficient (e.g., when the patient is „toothless” or has reduced absorption) and replaces additional losses. These losses may be caused by the gastrointestinal tract (due to vomiting, diarrhea or fistulas) or urinary tract (eg, Straight Disease) or by blood loss due to injury or surgery. In addition, insensitive losses in fever or burns may increase as the barrier function of the skin is impaired.
Liquids can accumulate in spaces that usually contain a minimum volume of fluid (eg, peritoneal or pleural cavity) during surgery, anesthesia or as a result of inflammation (eg sepsis). This is known as the „third distance” and is caused by the dilation of blood vessels and „leakage” of the vascular epithelial walls. This distribution of normal compartment integrity may result in loss of circulating intravascular volume.
Evaluate the requirements
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Example of a prescription for intravenous fluid (Mark Borthwick)
The patient’s history indicates the expected state of fluids. The causes of dehydration include pre-operative fasting, persistent gastrointestinal disorders and self-neglect after acute confusion. Knowledge of a detailed diagnosis is crucial to obtain information about the likely composition of the lost fluid. Physicians must also be aware of all the accompanying conditions that may alter fluid distribution or increase the patient’s compliance to the adverse effects of fluid therapy (eg heart failure in a history).
Detect dehydration
Signs of dehydration in a physical state