DRUG NOMENCLATURE

The nurse should be aware that each drug has various names — its chemical, generic and trade names. The chemical name provides an exact description of the drug’s chemical composition. This is usually only used by chemists and pharmacologists. Occasionally a drug’s generic name may describe its chemical compositions, as with lithium carbonate or potassium chloride.

The generic name is given by the manufacturer who first develops the drug. Even generic names are not standardised and nurses should be careful when using textbooks published in countries other than the one where they are practising nursing. Examples of different names used in different countries include adrenaline (Australia) and epinephrine (US); pethidine (Australia) and meperidine (US). Generic names are written with a lower-case first letter.

The trade (or proprietary) name is the name under which a manufacturer markets a drug. An example is the diuretic, frusemide, marketed as Urex, Lasix, Frusid and Uremide. Trade names are written with an initial capital letter.

DRUG FORMULATIONS AND ADMINISTRATION ROUTES

A drug may be presented in a variety of forms (Table 28.1) and administered via several different routes.

TABLE 28.1 DRUG FORMULATIONS

Routes of administration of drugs include:

ABSORPTION

Absorption is the movement of a drug from the site of administration into the bloodstream. The rate and amount of drug absorbed depends on three factors:

FIRST-PASS METABOLISM

Medication is absorbed from the gastrointestinal tract and enters the hepatic portal system, where it reaches the liver and is metabolised. Some drugs are almost entirely metabolised during this hepatic first pass, resulting in only very small amounts being available for therapeutic use. These drugs are better given via another route. An example of this is glyceryl trinitrate (Anginine), which is used to treat angina pectoris. If given orally, almost 96% is destroyed in this hepatic first-pass metabolism. Given sublingually, the drug bypasses the liver and is able to reach the target organs to have therapeutic effect. First-pass metabolism is also the reason why oral doses and parenteral doses are not equal — giving drugs via the parenteral route bypasses the first-pass metabolism and means that smaller doses can be given to achieve the same therapeutic effect.

DISTRIBUTION

After the drug has been absorbed into the general circulation it will then be distributed to different tissues for drug action to occur. To be transported in the blood, the drug molecules become protein bound (i.e. bind loosely to blood proteins); however, there is always some unbound (or free) drug. Equilibrium exists between the bound and unbound drug but only the unbound drug molecules are able to bind with the tissue (Figure 28.2).

Figure 28.2 Drug–plasma protein binding

The blood plasma contains a variety of plasma proteins (e.g. albumin, corticosteroid-binding globulin (CBG) and glycoproteins), which are able to bind to drugs to produce a drug–protein complex. If plasma proteins are deficient, for example, if a client has a condition such as liver disease, malnutrition or extensive burns, a greater portion of the drug remains unbound and is available to bind to the tissue, increasing the effects of the drug and requiring a decrease in the dose. Conversely, if there is an increased level of plasma proteins, in a client who has multiple myeloma, for example, there is an increase in the amount of bound drug, reducing the drug’s effectiveness, requiring an increase in dose.

Drugs may also compete for the same binding site on the plasma protein when more than one drug is administered concurrently. The drug with the higher affinity (or greater attraction) will be bound and displace the other(s) from the protein-binding site, resulting in the plasma concentration of the now unbound drug increasing. For example, aspirin will displace phenytoin from the plasma protein, resulting in an increased level of unbound phenytoin and, potentially, phenytoin toxicity. The effect is the same as giving an increased dose of the drug (Figure 28.3).

Figure 28.3 Displacement of phenytoin from its protein-binding site by aspirin

The blood–brain barrier of the central nervous system is generally very selective and allows very few drugs to pass through it. However, some conditions alter the permeability of the blood–brain barrier. For example, meningitis renders it permeable to penicillin, which is otherwise unable to pass through. The placental barrier is not as effective, and many drugs are able to cross the placenta to the fetus. Drugs that can reach the fetus in this way have the potential to cause significant damage, including congenital malformations. Unless absolutely necessary, drugs should not be taken during pregnancy and especially not during the first trimester, when organ development in the fetus occurs.

METABOLISM

Metabolism is the process whereby a substance is chemically altered, making it hydrophilic (water-loving) so that it can be readily excreted. The metabolism of drugs into inactive forms for excretion involves enzymes that are mainly found in the smooth endoplasmic reticulum in the liver. However, some enzymes are also found in the plasma, intestine and other organs. The main enzymes involved are those of the cytochrome P-450 family (several different types numbered from a to d). There are two types of enzymes involved in metabolism. During Phase I metabolism, enzymes modify the drugs by a series of chemical processes such as oxidation, reduction, hydrolysis and addition or removal of an active group from the drug molecule. During Phase II metabolism, the metabolite from Phase I, or a drug, may be directly conjugated (joined to another substance) by the enzymes, making the end product soluble for excretion.

Normally, these enzymes are present in small amounts. However, in some circumstances the amount of enzymes can alter, thereby also altering the rate of metabolism. For example, alcohol stimulates production of hepatic enzymes in habitual drinkers, resulting in the alcohol being more rapidly metabolised than in a non-drinker. This process of causing the amount of enzymes to be increased is called enzyme induction. Alcohol can also cause the levels of other enzymes involved in drug metabolism to increase, leading to the drug being more rapidly metabolised and therefore having a reduced therapeutic effect. Enzyme induction can be due to environmental pollutants, including benzopyretics in cigarette smoke, pesticides and some drugs, such as warfarin and phenytoin. Enzyme inhibition is the opposite effect and is the decreased synthesis of enzymes, resulting in the slowing down of the metabolic steps and an increased therapeutic effect. Some drugs are known enzyme inhibitors and include cimetidine, erythromycin, diltiazem, verapamil and ketaconazole. Figure 28.4 illustrates the process of drug metabolism simplified.

Figure 28.4 Drug metabolism simplified

DRUG EXCRETION

After metabolism, drug excretion may be through the kidneys, lungs, exocrine glands, liver and/or intestine. The chemical composition of the drug determines the organ(s) of excretion, for example, 100% of frusemide, 80% of digoxin and 50% of salbutamol are excreted in the urine. The kidneys are the main organs for drug excretion and, if a client’s renal function is impaired, the client is at risk of drug toxicity. Drugs that depend on renal function for excretion are eliminated more slowly in the very young and in older people. Gaseous compounds such as general anaesthetic agents are eliminated via the lungs. Alcohol is also partially excreted via the lungs and this is the basis of the police random breath testing to detect drink-drivers.

Some drugs, penicillin for example, exit unchanged in the urine, while others must undergo transformation in the liver before being excreted by the kidneys. Many drugs enter the hepatic circulation to be broken down by the liver and excreted into the bile and then into the bowel. The liver has a large metabolic capacity so, in people with liver disease, drug elimination is generally not affected until a large portion of the liver’s functional capacity is lost.

IMPORTANCE OF THERAPEUTIC DRUG MONITORING

For drugs to be therapeutic, a certain blood level needs to be reached and maintained. It is therefore a priority that medications are administered on time. If they are administered late, the level of the medication in the blood may drop below a therapeutic level; for example, if the level of an antibiotic is too low it provides an opportunity for microorganisms to multiply. If the blood levels are too high, toxicity and serious consequences might occur. The main aim of therapeutic drug monitoring is to optimise drug therapy by achieving adequate drug levels, while minimising toxicity. This is especially important in clients at the extremes of age. Clinical Interest Box 28.1 outlines some signs and symptoms of toxicity commonly seen in older clients. Clinical Interest Box 28.2 identifies changes related to ageing that influence pharmacokinetics.

Drug levels are measured to:

DRUG ACTION

The extent of the response to a drug depends on its concentration at the site of action, which in turn depends on dosage, absorption, metabolism and elimination. More than any other factor, the route of administration determines the onset of drug effect. Drugs that are administered directly into the bloodstream provoke a rapid response, whereas drugs administered orally or topically must be absorbed into the bloodstream before they can take effect. Drugs act by affecting or controlling changes in biochemical or physiological processes in the body. Drugs produce their actions in one of three ways: altering body fluids, altering cell membranes, or interacting with receptor sites.

Most drugs act at specific cell receptor sites. A specific drug forms a complex with only one type of receptor but may produce multiple effects because of the location of those receptors in cells of different tissues or organs. For example, the anticholinergic drug atropine sulphate not only reduces the production of saliva, bronchial, nasal and gastric secretions, but it also increases heart rate, stimulates ventilation, and raises intraocular pressure. A ‘selective’ drug acts at a receptor in a particular type of body tissue and produces little effect on similar receptors in other organs. For example, the bronchodilator salbutamol has specific selectivity for receptors in bronchial smooth muscle but produces little or no stimulation of similar receptors in cardiac muscle.

A drug may produce more than one effect:

LEGAL ASPECTS OF DRUG ADMINISTRATION

Before a drug can be administered safely, the nurse needs to be aware of the legal aspects of drug administrations. This includes knowledge of the laws governing the possession, use and dispensation of drugs and of the directives of the nurse’s registering body on the administration of medications to clients. It also means observing the employing health care facility’s occupational health and safety (OHS) regulations that are designed to promote safe storage, handling and use of drugs (see Chapter 26). Currently, the states and territories of Australia differ on the regulations governing administration of medication by an Enrolled Nurse (EN). In Tasmania an EN whose qualifications have been determined as appropriate for the purpose of administration of medications and whose practising certificate has been endorsed may administer medications listed in schedule 2, 3 and/or 4 of the Poisons List Order 2001 (Nurses Board of Tasmania 2007). In Queensland, the Health (Drug & Poisons) Regulation 1996 (section 58A) was amended in 2006 to enable the EN with medication endorsement to administer controlled Schedule 8 drugs (Controlled Drugs) under the supervision of a Registered Nurse (RN) or doctor (Queensland Nursing Council 2006). Other states and territories are currently moving towards, or have established, specific guidelines regarding limited medication administration by the EN. Each EN will need to ascertain the regulations specific to the state or territory of Australia in which nursing is practised.

Legal Acts concerning poisons and the poisons regulatory bodies in New Zealand and each state and territory in Australia (Table 28.2) deal with the control of all drugs, from prescription medication through to agricultural poisons and research drugs. The laws and regulations apply to sale, supply, storage, dispensing and labelling. The drugs and poisons schedules divide drugs into groups according to their mode of action, therapeutic use, potency, potential for abuse and addiction, and safety. While there is currently no national drugs and poisons schedule in Australia, and each state and territory has its own version, the recommendations of the National Drugs and Poisons Schedule Committee in the form of the Standard for the Uniform Scheduling of Drugs and Poisons (SUSDP, Table 28.3) are usually incorporated into the legislation and regulations of each state and territory. An agreement between Australia and New Zealand has led to harmonisation of trans-Tasman scheduling, and the two countries now have compatible schedules, labelling and packaging requirements (Bryant & Knights 2006).

TABLE 28.2 AUSTRALIAN LEGISLATION INVOLVED IN THE REGULATION OF DRUGS

TABLE 28.3 STANDARD FOR THE UNIFORM SCHEDULING OF DRUGS AND POISONS (SUSDP)

ADMINISTRATION OF DRUGS

Before administering medications in any form or by any route the nurse must check that the client does not have any drug allergies. The nurse also observes the client who is starting a new medication for any signs of allergy or adverse reaction, and reports and records such responses promptly to the medical officer and the nurse in charge. If the situation arises that a client chooses not to take a prescribed medication, this must be recorded on the client’s medication chart and also reported to the client’s medical officer and the nurse in charge. The principles of asepsis (see Chapter 25) are employed during the preparation and administration of all medications.

To ensure that medications are administered correctly and safely, the nurse must observe the seven rights of drug administration (Reiss, Evans & Broyles 2002):

Before any medication is administered the client medication chart (Figure 28.5) must be checked thoroughly and systematically to determine the name of the drug, the route, the dosage, and the date and time for administration of the medication prescribed.

Figure 28.5 Example of a client medication chart (Paterson 1996)

The nurse compares the label of the drug container with the client medication chart three times:

To identify a client correctly, before a drug is administered, the nurse:

Special care should be taken when identity bracelets are not worn (e.g. in residential care settings) or where the client is unable to state his or her own name, because of dementia or mental disturbance for example. One safety measure that has been implemented in some aged-care residential settings is to have a current photograph of each resident that can help with identification.

Most medication errors occur when a nurse fails to follow the recommended safety guidelines, or when the nurse is distracted while preparing or administering medications. To prevent drug administration errors the nurse should:

ERRORS IN ADMINISTRATION

Errors can arise for many reasons. Information on ways to avoid errors in administration is provided throughout this chapter, and also in Clinical Interest Box 28.3. Each health care facility has its own protocol for dealing with medication errors, and the nurse must understand and adhere to that protocol.

If the nurse does make a medication error (e.g. administering the wrong drug, wrong dose or via the wrong route) or if a nurse identifies an error made by another nurse, the incident must be reported immediately to the nurse in charge. The nurse has a professional and ethical responsibility for reporting any error, no matter how minor or trivial it may seem at the time. Measures to counteract the effects of the error may be necessary, such as administering an antidote (with a medical officer’s order) or monitoring the drug’s effects over time. The nurse is also responsible for completing an incident report form describing the nature of the incident. The incident report provides an objective analysis of why the incident occurred and is a means for the facility’s safety personnel to monitor such events and to implement measures to prevent recurrence. The incident, measures taken and outcomes should also be recorded in the client’s medical notes (see Chapter 20 for information on nursing documentation).

SI UNITS

The International System of Units (SI units) for mass and volume are kilograms (kg) and litres (L). Smaller measures of mass commonly used are:

Smaller units of volume commonly used are millilitres (mL: 1000 mL = 1 L) and microlitres (µL). See Appendix 2 for information concerning other units of measurement.

The strength of a pharmaceutical preparation used in electrolyte replacement therapy is normally expressed in millimoles (mmol) per tablet or millimoles per given volume of solution (e.g. mmol/L). A millimole is one thousandth of a mole, which is the molecular weight of a substance expressed in grams. For example, sodium has a molecular weight of 23, so a mole of sodium weighs 23 g, a millimole of sodium weighs 23 mg, and a 1 mM aqueous solution of sodium chloride, for example, contains 1 mmol of sodium chloride (containing 23 mg of sodium) in 1 L of water. Millimoles are also used to express the concentration of substances other than electrolytes and are used widely in laboratories, for example, in haematology reports.