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CORRESPONDENCE
Year : 2006  |  Volume : 38  |  Issue : 6  |  Page : 435-437
 

Teaching undergraduate students appropriate dose calculations in relation to intravenous infusion


Department of Pharmacology, M.P. Shah Medical college, Jamnagar-361008, India

Correspondence Address:
D M Parmar
Department of Pharmacology, M.P. Shah Medical college, Jamnagar-361008
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0253-7613.28216

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How to cite this article:
Parmar D M, Jadav S P. Teaching undergraduate students appropriate dose calculations in relation to intravenous infusion. Indian J Pharmacol 2006;38:435-7

How to cite this URL:
Parmar D M, Jadav S P. Teaching undergraduate students appropriate dose calculations in relation to intravenous infusion. Indian J Pharmacol [serial online] 2006 [cited 2018 Dec 17];38:435-7. Available from: http://www.ijp-online.com/text.asp?2006/38/6/435/28216


Medical emergencies such as cerebral malaria, diabetic coma, cardiogenic shock and many more require immediate treatment with parenteral drug administration. Intravenous (i.v.) infusion is one of the commonly preferred routes for this purpose. As one of the goals of undergraduate training is to train students in managing patients,[1] it is mandatory that the students acquire a sound knowledge of certain basic steps of intravenous drug infusion. These include dose calculation ( viz., based on body weight and body surface area) and clinical skills ( viz. , proper collection of drugs from ampoule/vial, introduction of drug into suitable diluting fluid, i.v. drip setting, etc.). One of the ways to achieve such a goal is by including many exercises on calculation of infusion rates in the undergraduate pharmacology practicals curriculum.

As dose calculation is an important initial step in such clinical procedures, we have emphasized it in relation to different aspects such as the following:

Total dose: The infusion rate will not differ in case of total dose administration (e.g., infusion rate of quinine will remain the same for a 30, 50 or a 70 kg patient since it is given in the dose of 10 mg/kg in 500 ml of 10% dextrose solution which is to be administered over 4 h). In contrast to this, the infusion rate will differ when drugs are to be given at the specified rates (e.g., infusion rate of aminophylline will differ for a 30, 50 or a 70 kg patient since it is to be given in the dose of 1 mg/kg/hr in 500 ml of normal saline).

Weight (mg/kg): The weight-based dose calculation (mg/kg) is more popular despite certain disadvantages, viz., one needs to know the different doses according to body weight (mg/kg) as well as according to the age of the patient.[2]

Body surface area: Body surface area (BSA) is another method which is considered as a more accurate basis for dose calculation since total body water, extracellular fluid volume and metabolic activity are better paralleled by it. The BSA of an individual can be calculated from the formula of Dubois [BSA (m 2) = Body weight (kg) 0.425 X Height (cm) 0.725 X 0.007184] or obtained from chart-form/slide-rule nomograms based on body weight and height. Dose recommendations in terms of BSA are available only for a limited number of drugs (e.g., anticancer drugs) as this method is more cumbersome in clinical practice.[3]

Time: The dose of some drugs is calculated per minute and per hour (e.g., dopamine 5 mcg/kg/min and insulin 0.1 U/kg/h). The infusion rates differ for different time duration in case of anticancer drugs like paclitaxel (see below).

Average adult body weight: The dose can also be calculated by using the average adult body weight (60-70 kg) basis from the formula: [Individual dose = Body weight (kg)/70 × average adult dose].[3]

Diluting fluid: Selection of a suitable diluting fluid is necessary for i.v., infusion. For example, 10% dextrose solution is being used for quinine infusion, which prevents hypoglycemia due to hyperinsulinemia caused by the powerful stimulatory effect of quinine on pancreatic beta cells,[4] whereas dopamine infusion is given with 5% dextrose (acidic pH) solution for its compatibility and stability with the latter (i.e., 5% dextrose).[5]

Some problems that may be encountered during dose calculation are listed below.


 » 1. Following the same steps of dose calculation for different drugs Top


To calculate the rate of quinine infusion is simple since quinine is to be given in 500 ml of 10% dextrose solution, to be administered over 4 h (i.e., we have to administer 500 ml of 10% dextrose solution with the required dose of quinine over 4 h).

But students can make mistakes if they follow this step of rate calculation for calculating the infusion rate of other drugs that are to be administered by their specified infusion rates (e.g., aminophylline: 1 mg/kg/h). Therefore, the students should be taught these aspects as well since many drugs come under this category (e.g., dopamine, insulin and oxytocin). [Please refer comparison given on page 1]

Thus, it is obvious that such an infusion rate for aminophylline (133-167 drops/min) is not practically feasible.

The correct calculation for the rate of aminophylline infusion is given below.

Ų Exercise:

You have decided to give i.v. infusion of aminophylline for a child (10 years old, 30 kg) suffering from acute bronchial asthma (Dose: 1 mg/kg/hr). Set an i.v. drip for this patient.

Ų Answer:

A. Calculate the required dose of Inj. aminophylline:

i.e. 30 mg of Inj. aminophylline (1 mg/kg)

B. Selection of fluid:

i.e. Normal saline

C. Aminophylline is available as 2.5% W/V in 10 ml ampoule, i.e. 250 mg/10 ml.

Introduce all 10 ml of Inj. aminophylline into 500 ml of normal saline solution.

Now we have 250 mg in 500 ml of normal saline solution, i.e. 30 mg will be present in 30/250 × 500 = 60 ml, which is to be administered in 1 h.

So, the desired rate of administration will be:

60 ml to be given in 1h (60 min),

i.e. 60/60 = 1 ml/min

Thus, in 1 min, 1 ml is to be given

1 ml = 16-20 drops in normal i.v. set

So, the rate of aminophylline administration will be 16-20 drops/min.

OR

Introduce all 10 ml of Inj. aminophylline into 500 ml of normal saline solution.

i.e. 500 ml of normal saline solution contains 250 mg of aminophylline

So, 1 ml will have 1/500 × 200 = 0.5 mg of aminophylline.

For 0.5 mg of aminophylline, 1 ml is required

So, 30 mg will be present in 30/0.5 × 1 = 60 ml, which is to be administered in 1 h.

So, the desired rate of administration will be:

60 ml to be given in 1 h (60 min)

i.e. 60/60 = 1 ml/min

Thus, in 1 min, 1 ml is to be given

1 ml = 16-20 drops in normal i.v. set

So, the rate of aminophylline administration will be 16-20 drops/min.


 » 2. Dose calculation by different methods Top


Problem:

Calculate the required dose for aminophylline infusion for a child suffering from acute bronchial asthma (12 years old, 37 kg, BSA 1.25 m 2 Dose: 1 mg/kg/h).

Solution:

A. mg/kg basis:

i.e. 37mg/h

B. Average adult body weight (60-70 kg) basis:

Formula: [Individual dose = Body weight (kg)/70 × average adult dose]

i.e. 37/70 × 70 = 37 mg/hr

C. BSA-based rule:

Formula: [Individual dose = BSA (m 2)/1.7 × average adult dose]

i.e. 1.25/1.7 × 70 = 51 mg/hr

D. Salisbury rule (proposed for children):[6]

  • Less than 30 kg: (Weight × 2) percentage of the adult dose of a drug
  • More than 30 kg: (Weight + 30) percentage of the adult dose of a drug


According to this, (37 + 30) percentage of the adult dose of a drug

i.e. 67 % of 70 mg of aminophylline,

i.e. 47 mg/h

This shows that there is no difference in doses calculated by mg/kg and average adult body weight-based rules, but there is a definite difference when both are compared with the BSA-based rule. The dose calculated by the Salisbury rule is closer to the BSA-based value.

Metabolism of theophylline (aminophylline) is often faster in children, after the first year of life,than in adults; and there is a marked interindividual variation in the rate of its elimination (average half-life is about 3.5 h in young children as compared to 8-9 h in adults).[3],[4] Considering these pharmacokinetic parameters as well as the narrow safety margin of aminophylline, the more accurate BSA-based rule may be preferred for dose calculation, keeping in mind its limited application in a clinical set-up.


 » 3. Dose calculation in relation to time duration Top


Paclitaxel (an anticancer drug) is to be given in different schedules [ Schedule 1: 175 mg/m 2 over 3 h (infusion rate: 58 mg/m 2 per hour, plasma clearance: 212 ml/min per m 2) and Schedule 2: 175 mg/m 2 over 24 hours (infusion rate: 7 mg/m 2 per hour, plasma clearance: 393 ml/min per m 2)].[4] A practical problem may arise if one calculates the dose of paclitaxel for schedule 2 from schedule 1. The dose and infusion rate will become 1400 mg/m 2 and 58 mg/m 2 respectively, which appear very different from the actual schedule 2 for 24 h. As clearance of paclitaxel is saturable and decreases with an increasing dose or dose rate,[4] such probable mistakes will definitely affect the clinical response.

To conclude, during undergraduate pharmacology practicals, different aspects, such as total dose, weight (mg/kg and average adult body weight basis), BSA, time duration and diluting solution, as well as likely practical problems that may arise during dose calculation, should be highlighted by demonstrating different exercises on the calculation of infusion rates.

 
 » References Top

1.Pattnaik KP, Mohapatra S, Mohanty M, Mohpatra BN, Patel D, Mukherji D. Clinical orientation of undergraduate pharmacology practicals: An intervention study. Indian J Pharmacol 2006;38:200-2.  Back to cited text no. 1    
2.British National Formulary. London. British Medical Association, 1995; 11.  Back to cited text no. 2    
3.Tripathi KD. Essential of medical pharmacology. 5th ed. New Delhi, India: Jaypee Brothers Medical Publishers (P) Ltd.; 2003.  Back to cited text no. 3    
4.Undem BJ, Lichtenstein LM. Drugs used in the treatment of asthma. In: Hardman JG, Limbird LE, Gilman AG. editors. Goodman and Gilman's The pharmacological basis of therapeutics. 10th ed. New York: McGraw-Hill; 2001.  Back to cited text no. 4    
5.Dollery C. Therapeutic drugs. 2nd ed. Edinburgh: Churchill Livingstone; 1999.  Back to cited text no. 5    
6.Lack JA, Stuart-Taylor ME. Calculation of drug dosage and body surface area of children. Br J Anaesth 1997;78:601-5.  Back to cited text no. 6  [PUBMED]  [FULLTEXT]


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