DRUG CALCULATIONS FOR NURSES A STEPBY-STEP APPROACH 3RD Edition By ROBERT LAPHAM BOOK.

DRUG CALCULATIONS FOR NURSES A STEPBY-STEP APPROACH 3RD Edition By ROBERT LAPHAM BOOK. Poison is in everything, and no thing is without poison. The dosage makes it either a poison or a remedy. Paracelsus (1493–1541) Medieval physician and alchemist Contents Preface x How to use this book xi Pre-test xii Basics xii Per cent and percentages xiv Units and equivalences xiv Drug strengths or concentrations xiv Dosage calculations xv Moles and millimoles xvi Infusion rate calculations xvi Answers xvii 1 First principles 1 Objectives 1 Before we start 1 Sense of number and working from first principles 1 Estimation of answers 3 The ‘ONE unit’ rule 3 Checking your answer: does it seem reasonable? 5 Putting it alltogether 6 Minimizing errors 7 Part 1: Mathematics 9 2 Basics 9 Objectives 9 Introduction 11 Arithmetic symbols 11 Basic maths 12 Rules of arithmetic 22 Fractions and decimals 25 Roman numerals 35 Powers or exponentials 36 Using a calculator 38 Powers and calculators 42 Estimating answers 42 3 Per cent and percentages 48 Objectives 48 Introduction 49 Per cent and percentages 49 Converting fractions to percentages and vice versa 50 Converting decimals to percentages and vice versa 50 Calculations involving percentages 51 Drug calculations involving percentages 54 How to use the percentage key on your calculator 55 4 Units and equivalences 59 Introduction 60 SI units 60 Prefixes used in clinical medicine 61 Equivalences 62 Conversion from one unit to another 63 Guide to writing units 68 5 Drug strengths or concentrations 71 Introduction 72 Percentage concentration 73 mg/mL concentrations 75 ‘1 in …’ concentrations or ratio strengths 77 Parts per million (ppm) 79 Drugs expressed in units 79 Part I1: Performing calculations 81 6 Dosage calculations 81 Introduction 82 Calculating the number of tablets or capsules required 82 Dosages based on patient parameters 83 Ways of expressing doses 86 Calculating drug dosages 87 Displacement values or volumes 91 7 Moles and millimoles 94 Introduction 95 What are moles and millimoles? 96 Millimoles and micromoles 97 Calculations involving moles and millimoles 98 Molar solutions and molarity 101 Contents vii 8 Infusion rate calculations 106 Introduction 107 Drip rate calculations (drops/min) 107 Conversion of dosages to mL/hour 109 Conversion of mL/hour back to a dose 114 Calculating the length of time for IV infusions 117 Part III: Administering medicines 120 9 Action and administration medicines 120 Introduction 121 Pharmacokinetics and pharmacodynamics 122 Administration of medicines 130 Promoting the safer use of injectable medicines 137 10 Infusion devices 140 Introduction 141 Gravity devices 141 Pumped systems 141 Infusion device classification 145 11 Children and medicines 147 Introduction 148 Drug handling in children 148 Routes of administration of drugs 151 Practicalimplications 152 Usefulreference books 156 Approximate values useful in the calculation of doses in children157 Calculating dosages 158 12 The elderly and medicines 159 Introduction 160 Drug handling in the elderly 160 Specific problems in the elderly 162 Generalprinciples 163 13 Sources and interpretation of drug information 165 Introduction 165 Sources of drug information 166 Summary of product characteristics (SPC) 166 viii Contents Contents Revision test 175 Basics 175 Per cent and percentages 176 Units and equivalences 176 Drug strengths or concentrations 177 Dosage calculations 177 Moles and millimoles 178 Infusion rate calculations 179 Compare your scores 180 Answers to revision test 180 Answers to problems set in chapters 183 3. Percent and percentages 184 4. Units and equivalences 184 5. Drug strengths or concentrations 187 6. Dosage calculations 188 7. Moles and millimoles 195 8. Infusion rate calculations 201 9. Action and administration of medicines 215 Appendices 216 1 Body surface area (BSA) estimates 217 2 Weight conversion tables 221 3 Height conversion tables 223 4 Calculation of Body Mass Index (BMI) 225 5 Estimation of renalfunction 232 6 Abbreviations used in prescriptions 234 Index 237 ix Preface Drug treatments given to patients in hospital are becoming increasingly complex. Sometimes, these treatment regimes involve potent and, at times, new and novel drugs. Many of these drugs are toxic or possibly fatal if administered incorrectly or in overdose. It is therefore very important to be able to carry out drug calculations correctly so as not to put the patient at risk. In current nursing practice,the need to calculate drug dosages is not uncommon. These calculationshave to be performed competently and accurately, so as not to put not only the nurse but, more importantly, the patient at risk. This book aims to provide an aid to the basics of mathematics and drug calculations. It is intended to be of use to nurses of all grades and specialities, and to be a handy reference for use on the ward. The concept of this book arose from nurses themselves; a frequently asked question was: ‘Can you help me with drug calculations?’ Consequently, a small booklet was written to help nurses with their drug calculations, particularly those studying for their IV certificate. This was very well received, and copies were being produced from original copies, indicating the need for such help and a book like this. The content of the book was determined by means of a questionnaire, sent to nurses asking them what they would like to see featured in a drug calculations book. As a result, this book was written and, hopefully, covers the topics that nurses would like to see. Although this book was primarily written with nurses in mind, others who use drug calculations in their work will also find it useful. Some topics have been dealt with in greater detail for this reason, e.g. moles and millimoles. This book can be used by anyone who wishes to improve their skills in drug calculations or to use it as a refresher course. This book is designed to be used for self-study. Before you start, you should attempt the pre-test to assess your current ability in carrying out drug calculations. After completing the book, repeat the same test and compare the two scores to measure your improvement. To attain maximum benefit from the book, start at the beginning and work through one chapter at a time, as subsequent chapters increase in difficulty. For each chapter attempted, you should understand it a fully and be able to answer the problems confidently before moving on to the next chapter. Alternatively, if you wish to quickly skip through any chapter, you can refer to the ‘Key Points’ found at the start of each chapter. A note about drug names In the past, the British Approved Name (BAN) was used for drugs in the UK. European law now requires use of the Recommended International Non-proprietary Name (rINN) for medicinal substances.In most cases, the old BAN and the new rINN are identical. Where the two differ, the BAN has been modified to the new rINN; for example: amoxicillin instead of amoxycillin. Adrenaline and noradrenaline have two names (BAN and rINN). However, adrenaline and noradrenaline are the terms used in the titles of monographs in the European Pharmacopoeia and are thus the official names in the member states. The British Pharmacopoeia 2008 shows the European Pharmacopoeia names first followed by the rINN at the head of its monographs (adrenaline/epinephrine); the British National Formulary (BNF) has adopted a similar style. For a full list of all the name changes, see the current edition of the BNF. Affected drugs that appear in this book will be referred to by their new name (rINN) followed by their old name (BAN) in brackets; for adrenaline, this book will follow the convention used by the British Pharmacopoeia. Case reports The journal Pharmacy in Practice highlights real-life medication errors to act as learning points for practitioners. Some of these have been used as Case Reports in this book to illustrate important points to remember. How to use this book To obtain the maximum benefit from this book, it is a good idea to attempt the pre-test before you start working through the chapters. The aim of this pre-test is to assess your ability at various calculations. The pre-test is divided into several sections that correspond to each chapter in the book, and the questions try to reflect the topics covered by each chapter. You don’t have to attempt questions for every chapter, only the ones that you feel are relevant to you. Answering the questions will help you identify particular calculations you have difficulty with. You can use calculators or anything else you find helpful to answer the questions, but it is best to complete the pre-test on your own, as it is your ability that is being assessed and not someone else’s. Don’t worry if you can’t answer all of the questions. As stated before, the aim is to help you to identify areas of weakness. Once again, you don’t have to complete every section of the pre-test, just the ones you want to test your ability on. Once you have completed the pre-test and checked your answers, you can then start working through the chapters. Concentrate particularly on the areas you were weak on and miss out the chapters you were confident with if you wish. It is up to you as how you use this book, but hopefully the pre-test will help you to identify areas you need to concentrate on. The pre-test consists of 50 questions and covers all the topics and types of questions in the book. Mark your score out of 50, then double it to find your percentage result. BASICS The aim of this section is to test your ability on basic principles such as multiplication, division, fractions, decimals, powers and using calculators, before you start any drug calculations. Long multiplication Solve the following: 1 678 × 465 2 308 × 1.28 Long division Solve the following: 3 3143 ÷ 28 4 37.5 ÷ 1.25 PRE-TEST Fractions Solve the following, leaving your answer as a fraction: 5 6 7 8 Convert to a decimal (give answers to 2 decimal places): 9 10 Decimals Solve the following: 11 25 × 0.45 12 5 ÷ 0.2 13 1.38 × 100 14 25.64 ÷ 1,000 Convert the following to a fraction: 15 1.2 16 0.375 Roman numerals Write the following as ordinary numbers: 17 VII 18 IX Powers Convert the following to a proper number: 19 3 × 104 Convert the following number to a power of 10: 20 5,000,000 9 16 2 5 5 6 3 8 ÷ 3 4 9 16 ÷ 3 4 12 16 × 5 9 3 7 × Basicsxiii PER CENT AND PERCENTAGES This section is designed to see if you understand the concept of per cent and percentages. 21 How much is 28% of 250 g? 22 What percentage is 160 g of 400 g? UNITS AND EQUIVALENCES This section is designed to test your knowledge of units normally used in clinical medicine, and how to convert from one unit to another. It is important that you can convert between units easily, as this is the basis for most drug calculations. Convert the following. Units of weight 23 0.0625 milligrams (mg) to micrograms (mcg) 24 600 grams (g) to kilograms (kg) 25 50 nanograms (ng) to micrograms (mcg) Units of volume 26 0.15 litres (L) to millilitres (mL) Units of amount of substance Usually describes the amount of electrolytes, as in an infusion (see Chapter 7 ‘Moles and millimoles’ for a full explanation). 27 0.36 moles (mol) to millimoles (mmol) DRUG STRENGTHS OR CONCENTRATIONS This section is designed to see if you understand the various ways in which drug strengths can be expressed. Percentage concentration 28 How much sodium (in grams) is there in a 500 mL infusion of sodium chloride 0.9%? mg/mL concentrations 29 You have a 5 mL ampoule of dopexamine 1%. How many milligrams of dopexamine are there in th