Y khoa, y dược - Chapter 16: Pharmacology & drug calculations

Chapter 16Pharmacology & Drug Calculations IntroductionA large percentage of a qualified nurse’s time is spent in dispensing medication and related information and advice. A sound knowledge base is required if potentially catastrophic errors are to be avoided, and medication is to have the desired effect.An understanding of pharmacology requires consideration of the concepts of pharmacokinetics and pharmacodynamics. Errors in drug calculations are the cause of many Adverse Drug EventsMethods of calculation, along with practical examples, will be provided to ensure the reader can effectively and safely calculate medication doses in practice.DefinitionsPharmacology comes from the Greek Pharmakos (drug) and Logos (science). It is concerned with the study of how a substance interacts with an organism to produce a change. We may give drugs for a number of reasons, either for diagnostic purposes, prophylaxis, or for a therapeutic effect. The central concepts in pharmacology are Pharmacokinetics Pharmacodynamics. The Central Concepts in PharmacologyPharmacokinetics is concerned with the study of the effects of the body on the drug, including Absorption, Distribution, Metabolism and Excretion. Pharmacodynamics is concerned with the study of the effects of the drug on the body, including effective doses and response.AbsorptionAbsorption begins once the drug accesses the body. The rate and method of absorption affects the bioavailability of the drug. Absorption is affected by the physical properties of the drug itself (capsule, tablet or liquid), as well as by the physiology of the person taking the drug.If the drug is released too swiftly, an excessive response may occur due to high blood levels. If it is released too slowly, it may be eliminated in the faeces prior to being absorbed. Understanding Methods of AdministrationCrushing medications should only be undertaken where it can be confirmed that it is acceptable. Crushing can fundamentally affect the absorption of the drug, particularly in the case of modified or slow release medications.A large toxic dose may be provided if you crush such a tablet. If you are considering crushing a tablet, you should establish if a liquid formulation is available – this will be preferable. Distribution Once absorption has begun to occur, and the drug enters the bloodstream, distribution of the drug can take place. The rate at which the drug is delivered is determined by the rate and quantity of blood perfusing the part of the anatomy targeted. There are a number of other factors which impact on drug distribution.Water-soluble drugs remain within the blood and interstitial spaces, whereas fat-soluble drugs (lipophilic) cross cell membranes easily. Drugs may bind to plasma proteins within the bloodstream, being released when the plasma concentration (of ‘free’ drug) falls. MetabolismMetabolism of a drug involves chemical alteration of the drug by the body into a metabolite. The body will attempt to metabolise what it sees as a potential toxin as fast as it can. This generally occurs in the liver, and can either activate or deactivate the drug – some drugs (termed pro-drugs) are given as inactive substances, which are then metabolised into an active form. A group of enzymes termed the Cytochrome P-450 enzymes are primarily responsible for drug metabolism in the liver. This can lead to problems where several drugs compete for levels of the enzyme. Where one drug inhibits the enzyme-mediated metabolism of another drug the second drug may accumulate to a toxic level. EliminationElimination is the removal of the drug from the body. Drugs are excreted in sweat, saliva, breast milk, and in the case of inhaled anaesthetic agents, exhaled air.Some drugs are excreted unchanged in bile, which then enters the gastrointestinal tract. The drug is then excreted in faeces, or reabsorbed and ‘recycled’. This explains why such a low dose of Oestrogen may be used for the oral contraceptive pill, as it is constantly recycled. Any diarrhoea may reduce this enterohepatic cycling.ExcretionThe primary route for excretion is the kidney.Elimination via the kidneys is affected by urine flow, the rate of blood flow through the kidney, and the health of the kidneys. Patients with hypertension, diabetes, or recurrent kidney infections may have impaired ability to eliminate drugs, and will require a dose reduction. Kidney function decreases with age – a person aged 70 will have only 50% of the renal function they had at 30. Drugs and the Older Patient Elderly patients have a higher level of chronic disease than other patients; by their seventies, 75% of patients will have at least one chronic disease. They may be subject to polypharmacy and are often given drugs to counteract the side effects of other medications. The ability of elderly patients to eliminate drug metabolites may be significantly reduced due to poor hepatic or renal function, or in acute illness. Age related changes affect all 4 pharmacokinetic stages:Absorption DistributionMetabolism Elimination Limiting Adverse Effects27% of all reported adverse drug events happen in elderly patients, and 10-12% of emergency hospital admissions in the over 70s are caused by prescribed drugs (McGavock 2005).Nurses are perhaps best placed to ensure this understanding prior to the patient’s discharge from hospital, or while caring for the patient in the community.Ensuring that a patient is educated may help to reduce ADEs. While drug interactions may cause an ADE we can minimize the risks to the patient by taking some elementary precautions. Ensuring that we know all the drugs that a patient is taking will help us to reduce potential ADEs. Medication ErrorsApart from drug interactions a major source of ADE’s is ‘medication error’.Perhaps the best available definition of a medication error comes from the National Coordinating Council for Medication Error Reporting and Prevention in the USA:“A medication error is any preventable event that may cause or lead to inappropriate medication use or patient harm while the medication is in the control of the health care professional, patient, or consumer. Such events may be related to professional practice, health care products, procedures, and systems, including prescribing; order communication; product labeling, packaging, and nomenclature; compounding; dispensing; distribution; administration; education; monitoring; and use.” The ‘5 rights’ of Medication Administration The right patient Should receive the right drug In the right dose At the right time Via the right route Checking a patient’s wristband before giving them any medication is essential. Many errors of prescription are picked up by nurses and subsequently corrected, this means ensuring your drug knowledge is current and comprehensive. Noting each unfamiliar drug you are asked to give, and looking it up in the BNF, will soon ensure you have the necessary knowledge to provide safe care. Calculating the DoseProviding the right dose is fundamental to safe provision of medication. Moving a decimal point in error one place may involve giving 10 times the recommended doseMistaking micrograms for milligrams will involve an error in the magnitude of 1000 times the dose! If you find yourself drawing up several vials of a drug, or getting more that 2 tablets for a dose, it is worth checking to ensure you have the right dose. Always check the strength of the medication you are about to give – something with which you feel you are familiar may have a new supplier, with a different strength. Applying Theory to Practice: Exercise Medicines, which are prescribed as a weight related, dose (e.g. mg/kg) entail accurate weighing of the patient. Estimating weights is very difficult, especially in the prone patient – some medicines have a very narrow effective dose related to the patient’s weight, and under or over-estimating the weight may reduce their effectiveness. Estimate the weight of a variety of your colleagues. Weigh them and check your accuracy. Try the same exercise with patients. Providing medication at the right time is necessary to provide the consistent therapeutic level designed by the drug’s manufacturer. Missing doses or providing them early or late may entail unwanted effects or reduced efficacy. The Routes of AdministrationOral - Buccal -Nasal - Rectal Intravenous – Subcutaneous - IntramuscularAural Sublingual Sublingual Transdermal InhaledTopicalVaginalUrethralIntrathecalMedicines are licensed for specific routes – giving an intramuscular injection intravenously, or vice-versa, may be painful and ineffective or deadly. Drug Administration: ExerciseYou need to gain information on each route of administration; advice on the intravenous route is provided, research the other routes and add information to your expanding portfolio.Intravenous injections are given directly into a vein – normally through an intravenous cannula, but also through ‘Central’ lines, where the cannula is placed in a large vein. Infection control is extremely important for any procedure involving direct access to the bloodstream Medicines should be prepared and administered using a ‘no touch’ technique – this means avoiding touching critical areas, such as needles, syringe tops, and infusion ports. Medications Antibiotics can be further subdivided into;bacteriostatic medicinesbacteriocidal medicinesBacteriostatic medicines do not destroy the invading organism. They work by inhibiting reproduction of the bacteria, allowing the host’s natural defences to eventually overcome them. Many organisms are becoming resistant to antibacterial drugs, it is vital to only use antibiotics where there is indication for them. Prior to giving antibiotics it is very important to ask about previous drug reactionsAnalgesics Used to relieve pain. They work at various sites Opioid drugs work at the level of the brain or spinal cord to reduce the perception of pain. They have powerful Central Nervous System and respiratory depressant effects, potentially producing significant drowsiness. These effects, if severe, may be reversed with the opioid antagonist Naloxone, which competes for the opioid binding sites and has a rapid effect.Morphine is often given intravenously for severe pain and is titrated to the patient’s pain Morphine can precipitate vomiting, and an anti-emetic is often given concurrently. Cardiac Drugs Diuretics work by decreasing re-absorption of water and electrolytes by the renal tubulesDiuretics are often discontinued by patients because of the need to go more frequently to the toilet, which may be more difficult for patients with infirmities. Digoxin has 2 effects that make it a useful cardiac drug it has a positive inotropic effect - it strengthens the heartit slows the heart through its effect on the parasympathetic nervous systemIn order to obtain a rapid effect, Digoxin is often given with a loading dose, which serves to obtain an effective level of the circulating drug as quickly as possible. Anti-Hypertensives There are a number of classes of anti-hypertensive drugs Vasodilators act on the smooth muscle cells of blood vessels to decrease peripheral resistance to blood flow.Sympathetic blocking drugs also decrease peripheral resistance by blocking vasoconstrictor nerve supply to small arteries and arterioles.Angiotensin-Converting Enzyme (ACE) inhibitors work by inhibiting the conversion of Angiotensin 1 to Angiotensin 2, which inhibits the release of aldosterone, further reducing Sodium retention. Since water follows Sodium, the effect is to reduce the circulating volume, and consequently the blood pressure falls.Exercise Find the section on Hypertension in the BNF. Find an example of each of the classes of drug Write a short description on each class Focus on the last few letter of each class as this will provide you with future clues to enable quick identification of a drug. I.e. Atenolol (lol = beta blockers) In addition to providing medication, patients should receive advice on diet, lifestyle and smoking cessation. This advice can halt the progression of future disease.Drug Calculations If you divide the dose that has been prescribed (what you want) by the strength of the tablet that you have (what you’ve got), this will provide you with the number of tablets you need. Ensure that the units you are using are the same, so dividing 500(mg) by 500(mg) is fine, but dividing 500(mg) by 0.5(g) will give you a quite different answer! An ExampleFor example – you have 25mg tablets of Diclofenac, and you have been asked to give 75mg. Dividing what you want (the prescribed dose of 75mg) by what you’ve got (25mg tablets) gives you a number of 3 tablets to administer: (What you want) 75mg / (What you’ve got) 25mg = 3 Calculating Solutions/Mixtures/Injection Volumes This is slightly more complex. A tablet contains a specific amount of medication in each unit e.g. Paracetamol has 500mg in each tablet. Solutions are the same, but the ratio varies e.g. Amoxicillin comes as 125mg in 5mL and 250mg in 5mL strengths. This adds an additional step to your calculations.You start by dividing your prescribed dose (What you want) by the stock strength (let’s use Amoxicillin 125mg/5mL, so our stock strength in this case is 125mg). You then need to multiply the resulting figure by the stock volume (in this case 5mL) to give the amount of solution in mL. In this example, if we wanted to give 250mg, using the 125mg/5mL solution, we have the calculation: (What you want) 250mg / (Stock strength) 125mg = (2) x (Stock volume) 5mL = 10ml Weight Dependent DosesA number of drugs are prescribed based on the patient’s body weight. This simply involves an additional step prior to the drug calculation – e.g. the patient is prescribed a drug at 30 micrograms per kilogram body weight. To derive the dose required, a simple multiplication of the patient’s weight in kg (e.g. 75kg) by the amount per kg (in this case 30 micrograms) gives a dose of 30 x 75 = 2250 micrograms. The dose calculation can then be processed in the normal fashion. IV InfusionsIn addition to dosage calculations, you may be required to calculate the rate of an infusion. Infusion pumps provide a continuous infusion at a given rate, so it is important to be able to program them to ensure an accurate hourly dose is delivered. To calculate the flow rate, the prescribed volume (in mL) is divided by the duration of the infusion (in hours) – e.g. 1000 mL over 4 hours requires a flow rate of 1000/4 = 250 mL per hour. IV Infusions cont.Calculating the rate of an infusion which contains a medication requires a further step First, the stock concentration must be calculated – this is the medication strength (generally in mg) divided by the volume of infusion fluid in which it is contained – e.g. if we add 900mg of Amiodarone to 500mL of Dextrose, we have a stock concentration of 900/500 = 1.8mg/mL. Working out the required infusion rate (in mL/hour) then involves dividing the dose required (in mg/hour) by the stock concentration – e.g. if we want a rate of 60mg hour, the calculation will be 60/1.8, which gives an infusion rate of 33.3 mL/hour. ConclusionAccurate and competent drug calculations are part of every qualified nurse’s clinical armament. You should ensure that you feel competent and confident to undertake this role before venturing to prepare infusions or undertake a drug round. By demonstrating to your mentors or preceptors in clinical practice that you have some insight into medications and how to calculate the dose you are far more likely to receive beneficial input from them. If at the start of your placements they identify you know little about commonly used medications they potentially will not trust you to administer drugs. When asked by mentors about drugs, never lie be honest, no-one expects you to know every drug in the BNF! Both pharmacology and drug calculations play a major role within your future chosen career.