CHAPTER 6 Calculating parenteral doses
large volume
Introduction
Water makes up between 50% and 70% body weight. Most body processes are dependent on water. Movement of a wide range of substances round the body depends on water (e.g. nutrients, hormones, electrolytes, and waste products).
Fluid losses, mainly through the kidney, must be replaced, in normal circumstances, by drinking (about 2 litres per day).
When the fluid balance of the body is compromised by, say, diarrhoea, vomiting or trauma (burns) and the oral route cannot be used, fluids and electrolytes are administered parenterally (Sexton and Rahman 2008).
Many body processes depend on electrical activity at the cellular level, e.g. the passage of a nerve impulse. This electrical activity is made possible by the presence of electrolytes, mainly sodium, potassium and calcium in cells and plasma. An electrolyte can be defined as a chemical substance that dissociates in water to yield electrically charged particles known as ions. Sodium chloride dissociates in water to yield Na+ (cation) and Cl− (anion)
Intravenous infusions
Special points
Practice aspects
Products
The more common solutions used for intravenous infusion (large volumes) are listed in Table 6.1. The information on a typical infusion solution bag is shown in Figs. 6.1–6.3. Nurses have to be ready to interpret common abbreviations and chemical symbols (see Abbreviations). Normal volumes in containers are 100 mL, 500 mL and 1000 mL.
Table 6.1 Common infusion solutions
| Active ingredient of solution | Content and common strengths w/v | Notes |
|---|---|---|
| Glucose (dextrose anhydrous) | 5% | Commonly used solution to replace water deficit |
| Energy source | ||
| Potassium chloride | Potassium chloride 0.3% or 0.15% with 5% glucose | Used to correct hypokalaemia. In view of dangers of overdose, use ready made up solution rather than add a concentrated solution to an infusion solution. |
| Sodium bicarbonate | Used to control severe metabolic acidosis | |
| Sodium chloride | 0.9% (normal saline) often in combination with glucose and other electrolytes | Used for electrolyte/water imbalance and as a vehicle for drug administration |
| Sodium lactate compound infusion | Sodium chloride, sodium lactate, potassium chloride, calcium chloride (see BNF for details) (BMA and RPSGB 2009) | Used for electrolyte and water deficiency |
* These solutions are hypertonic.
Administration
Infusions are administered using an administration set which conveys the fluid from the bag either to a cannula already sited in a peripheral vein or by central venous access. Administration sets vary depending on whether the infusion is to be administered by gravity or by pump and whether the infusion is of a clear fluid or of blood. It is essential to select the appropriate set as the rate of flow has been predetermined in the different types available.
For adults, the set to use for:
If there is any doubt about which set to use, reference should be made to the administration set packaging.
The safest way to administer an intravenous infusion is to deliver it via an infusion pump, as this method provides very precise control over the rate of the fluid. Where large volumes are to be given or where the fluid and/or its contents are highly potent, the standard expected would involve the use of an infusion pump. However, infusion pumps are costly, and it may not be possible to provide one in every case. It is obvious therefore that nurses are required to be competent in managing infusions with or without an infusion pump.
Gravity-assisted infusion
An infusion may be set up which simply allows the fluid to travel from the bag through the administration set to the cannula or by central venous access under the influence of gravity. With this type of infusion, the rate of fluid can be controlled in three ways. These are the:
To calculate how to set the infusion at the prescribed rate, you need to know the amount to be infused and the length of time the infusion is to take. The next step is to work out how much will be infused in 1 hour. This is done by dividing the total volume by the time the infusion is to take:
For example, 500 mL of a clear fluid is to be transfused over 4 hours:
The prescribed rate is 125 mL/h.
In order that the infusion can be regulated, however, the nurse needs to know how many drops per minute at which to set the infusion. In this instance, a clear solution is in use and therefore the appropriate administration set would be one set at 20 drops/mL.
To calculate the rate of flow in drops per minute, which the nurse will be able to count using a watch:
The infusion can be controlled at 42 drops per minute.
Some prescription sheets provide a ready reckoner to assist the nurse in calculating the rate of flow and should be used where provided.
Since other variables come into play, such as the positioning of the patient which may increase the flow rate, this method is not wholly reliable, and so to limit the risks associated with the infusion of large volumes of fluid, the maximum size of bag used is often limited to 500 mL.
Pump-assisted infusion
When using an infusion pump (Fig. 6.4), the appropriate administration set (specifically for use with a pump) must be used.
The infusion pump has been designed to provide very precise control of the rate at which the fluid is infused. It does not serve the purpose of a calculator and so it is essential to know how to calculate what the patient is to receive, i.e. the flow rate. This is expressed in millilitres per hour (mL/h).
Once the calculation has been made, and only then, the rate and the total volume to be infused can be keyed in to the pump.
To calculate the flow rate, the prescribed volume (always in mL) is divided by the duration of the infusion (always per hour):
If a volume of 1000 mL is to be infused over 8 hours, it can be represented as:
The rate is therefore 125 mL per hour.
These figures are then carefully entered using the appropriate keys. The pump will not correct any errors made; it will only perform on the basis of the information that has been entered into it. Over-reliance on it is to be avoided and periodic checks (e.g. at least hourly) must be made to ensure that the amount infused and the amount left, when added together, are the same as the original volume to be infused.
Reconstitution and dilution of parenteral medicines
Some parenteral medicines may need to be reconstituted, by adding a suitable volume of a diluent and then being further diluted for administration. Often dilution is required because of the irritancy of the drug to the tissues. Whenever possible, dilution to the required strength should be undertaken in a controlled environment (in pharmacy) in order to minimise the risk of microbial contamination. The BNF gives details of any necessary dilutions. The dilution will depend on the method of administration (Table 6.2), i.e.:
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