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Teaching undergraduate students appropriate dose calculations in relation to intravenous infusion Correspondence Address:
1. Following the same steps of dose calculation for different drugs 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 (133167 drops/min) is not practically feasible. The correct calculation for the rate of aminophylline infusion is given below. Ø 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. Ø 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 = 1620 drops in normal i.v. set So, the rate of aminophylline administration will be 1620 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 = 1620 drops in normal i.v. set So, the rate of aminophylline administration will be 1620 drops/min. 2. Dose calculation by different methods 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 (6070 kg) basis: Formula: [Individual dose = Body weight (kg)/70 × average adult dose] i.e. 37/70 × 70 = 37 mg/hr C. BSAbased 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 drugMore 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 weightbased rules, but there is a definite difference when both are compared with the BSAbased rule. The dose calculated by the Salisbury rule is closer to the BSAbased 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 halflife is about 3.5 h in young children as compared to 89 h in adults).[3],[4] Considering these pharmacokinetic parameters as well as the narrow safety margin of aminophylline, the more accurate BSAbased rule may be preferred for dose calculation, keeping in mind its limited application in a clinical setup. 3. Dose calculation in relation to time duration 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


