Download Med Math Unit 5 Drug Dosages

Chapter 5
Drug Dosages and
Intravenous Calculations
Objectives




Interpret drug orders and labels
Calculate drug orders
Identify and calculate medication volumes
Calculate the volume of reconstituted
medication in powdered form
 Perform intravenous and intramuscular
calculations
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Objectives (cont’d.)
 Perform titration calculations
 Calculate drug dosages based on body
weight
 Calculate drug dosages based on BSA using
formulas and the West Nomogram chart
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Interpreting Drug Orders
 Physicians use
short-hand
notations to
prescribe drugs
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Interpreting Drug Labels
 Labels give information such as:
•
•
•
•
Dosage strength
Brand name (or generic name)
Manufacturer
How to administer (orally, sublingually, injection,
or other)
• Drug form (e.g., tablet, capsule, liquid)
• Expiration date
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Interpreting Drug Labels (cont’d.)
 What is the drug?
 What is the total volume?
 What is the strength?
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Dosage Calculation Methods
 D = desired dosage
 V = vehicle
 Dosage formula:
H = on-hand dose
A = amount to give
• .
 Proportions:
• .
• Dimensional analysis
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Dosage Calculation Methods (cont’d.)
 An order for 75 mg of phenytoin is given
• Only 100 mg are available
• How many tablets should be given?
 Dosage formula:
• .
 Proportions:
• .
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Dosage Calculation Methods (cont’d.)
 Dimensional analysis:
• .
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Parenteral Dosage Calculations
 Administered through a non-gastrointestinal
route (e.g. syringe)
Figure 5.1 A 3-mL syringe
Figure 5.2 A 1-mL syringe
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Parenteral Dosage Calculations
(cont’d.)
 0.25 mg of a medication by IM injection is
ordered
• In stock is a vial labeled 0.5 mg in 2 mL
• What volume should be administered?
 Dosage formula:
• .
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Parenteral Dosage Calculations
(cont’d.)
 Proportions:
• .
 Dimensional Analysis:
• .
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Reconstitution of Solutions
 300 mg Zithromax IV is ordered
• In stock is a vial of powdered Zithromax that
states:
– “Constitute to 100 mg/mL with 4.8 mL of Sterile Water
for Injection”
– Single-dose vial contains 500 mg
 Dosage formula:
• .
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Reconstitution of Solutions (cont’d.)
 Proportions:
• .
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Reconstitution of Solutions (cont’d.)
 Dimensional analysis:
• Express:
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Intravenous Flow Rates
 Rate of flow: number of drops (gtt) that flow
into patient per minute
• .
• Drop factor: number of drops it takes to produce
1 cc
• Amount of solution: total amount of solution
administered
• Time: time it takes for solution administration
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Intravenous Flow Rates (cont’d.)
 An IV bag contains 1,000 mL solution
• Drop factor is 90 gtt/mL
• Flow rate is 125 gtt/min
• Find infusion time:
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Intravenous Flow Rates (cont’d.)
 Dimensional analysis:
• .
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Titration of Intravenous Medications
 Four-step process:
•
•
•
•
If order has different units than strength of
solution, convert the strength
Determine lower limit by using lower range
given together with patient’s weight
Use step 1 result to convert lower limit in step 2
into units of milliliters per hour
Use step 3 result together with ratio of upper
titration range to lower titration range, to find
upper limit
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Titration of Intravenous Medications
(cont’d.)
 IV line was ordered to titrate between 2 and
4 mcg/kg/min
• Patient weighs 60 kg
• IV solution contains 60 mg of drug in 300 mL
solution
• Determine rate of flow in milliliters per hour
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Titration of Intravenous Medications
(cont’d.)
.
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Titration of Intravenous Medications
(cont’d.)
 Using dimensional analysis:
• .
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Dosages Based on Weight
 Convert patient’s weight to kilograms
 Find dose (range) for body weight
• Example: 400 mg/kg/day for 99 lbs
– How many milligrams a day?
– How many every 6 hours?
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Dosages Based on BSA
 Formulas:
• .
• West Nomogram: common chart used to
estimate BSA
– See Figure 5.3
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Summary
 Physicians use short-hand notations
 Drug labels contain a lot of information
 Dosages can be calculated using formulas,
proportions and dimensional analysis
 Many parenteral dosages and reconstitution
of solutions can be determined by using:
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