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Pharmacology |
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Introduction The term "pharmacology" refers to the study of drugs and their effects on body systems. A drug is defined as any chemical that effects living processes (the administration of drugs in the pre-hospital setting is what separates the basic emergency provider from the advanced provider). Due to the dynamic nature of emergency medicine, keeping current on your knowledge and skills in relation to drug administration is not going to be an easy task. New drugs are constantly being developed and old drugs are constantly being re-evaluated as new information on their actions are being discovered. The use of drugs to treat patients with various complaints is an ancient practice dating back to primitive times. There is written evidence that ancient Egyptians had knowledge of pharmacology. The Ebers Papyrus writings refer to such remedies as oil, wine, opium, resins, turpentine, yeast, lead, and soda. These types of remedies were used mostly by trial and error to determine the dosage and effects on the living. One example of a primitive drug that was widely used is "purple foxglove". It was first used in as early as 1250 A.D. as a diuretic to treat edema. This condition was known as "dropsy" in early history. It's effects on the heart were not discovered until around 1800. Digitalis is the active ingredient in purple foxglove. Digitalis is widely prescribed for heart conditions today. Drug Sources
Drug Names Chemical Name Usually the chemical name is the first of as many as four names assigned to a drug. The chemical name is an exact description of the chemical makeup of the drug. It describes the molecular structure. This is generally only used by chemists and lab personnel involved in the development of the particular drug. Generic Name The generic name is the official name given the drug by the United States Adopted Name Council. This name is completely independent of the manufacturer and often relates to the chemical name in some fashion. Trade Name This is also known as the brand name or proprietary name and is the name given to the drug by the manufacturer. For this reason a drug may have several trade names because it is produced by more than one company, it may only have one generic name. Official Name The official name of a drug is followed by USP (United States Pharmacopeia) or NF (National Formulary) which denotes its listing in official publications. Many times the official name and the generic name are the same minus the USP or NF. Let's look at an example of a drug and its various names: Chemical name - ethyl 1-methyl-4-phenylisonipecotate hydrochloride Generic name - meperidine hydrochloride Trade name - Demerol Hydrochloride Official name - meperidine hydrochloride, USP Many drugs are commonly referred to by their trade names and easily recognized as such. Drugs such as Narcan, Lasix, and Valium are well known and easily recognized by health care professionals by the trade names. It can be confusing, though, when using trade names and it is usually a good idea to refer to drugs by their generic name. Drug Forms Liquids Drug preparations for oral use are often found in liquid form. The following are various liquid preparations.
Solids
Parenteral The parenteral route refers to any route other than through the digestive tract. Parenteral drugs are usually in liquid form and are stored in various types of containers such as:
Pharmacokinetics Definition The term 'pharmacokinetics' refers to the study of drugs and how they enter the body, how they reach their intended target, how they are transformed by the body, and how they are eventually eliminated by the body. Factors that Influence Pharmacokinetics Every drug will have its own specific characteristics which determine its rate of absorption, distribution, metabolism (biotransformation), and excretion. Absorption The action that a drug will have on the body is dependent upon the rate and degree of absorption. Molecules pass through single layers of tissue easier than through multiple layers. This is why absorption by the gastrointestinal tract is faster than transdermally. The rate and degree of absorption is dependent upon the site of administration, dosage, the form of the administered drug, solubility of the drug, pH, blood flow to the site of administration, and the mechanisms involved in absorption which are diffusion, osmosis, and filtration. Distribution This term refers to the process by which the drug finds its way to its intended site of action. Some factors that affect distribution are cardiac function, physiologic barriers such as the blood-brain barrier or the placental barrier, and drug storage reservoirs. Drug storage reservoirs are found in two forms: plasma proteins and tissue binding. The determining factor in the drugs binding capacity are the chemical-physical properties of the drug. Biotransformation and Elimination Drugs can be eliminated from the body either in their original form or as a metabolite. The kidneys, liver, digestive tract, and the lungs play a role in elimination as do the mammary glands, sweat glands, and salivary glands to a small degree. Rates of elimination depend on several factors such as the type of medication and the state of a person's health. The process of eliminating a drug by metabolizing it is also known as 'biotransformation'. Biotransformation is when a drug is converted into a product that is easily eliminated by the body. The aforementioned organs and body systems aid in the elimination of drugs through biotransformation. Infants and older adults have a tougher time eliminating drugs from their systems. In the infant, the organs are not fully developed, and in the older patient they are not working at peak efficiency any longer. Pharmacodynamics Definition Pharmacodynamics is defined as the study of the effects of drugs on living tissues. Terminology
Drug Actions
Routes of Administration Enteral This route of drug administration refers to the introduction of the drug along any part of the gastrointestinal tract. These areas include the following:
Typically the oral route is the easiest and perhaps the safest enteral route. It is certainly the most common route in the hospital setting but not in the pre-hospital setting. There is a slow rate of absorption when using this route making it an unpopular route in the emergency setting. Another reason for its lack of use in the field is that the oral form of drug administration often requires water or some other liquid to help swallow the drug, this is not recommended for patients with an altered level of consciousness or problems such as airway compromise. Examples of pre-hospital medications given by the enteral route are nitroglycerin, nifedipine, activated charcoal, and oral glucose. Parenteral This refers to administration of a drug by any route other than the GI tract. It includes the following routes:
Essential Drug Safety Information Safety The business of pre-hospital care inherently provides us with many distractions and stressful situations. There is often intense pressure to perform quickly. There is often a crowd of responders as well as non-responders adding to the confusion. These components make it necessary for you to develop safe habits. Avoid Distractions Once you have decided that your patient is in need of a drug, it is your responsibility to deliver it safely. Concentrate on the five rights of drug administration and don't allow the confusion and noise to become a distraction. Any time you take your focus away from the drug that you have decided to administer, recheck the drug label, dosage and expiration date, especially if you have let go of it and set it down somewhere. The Five Rights
Some areas also include the Right Location, such as an SQ dose of Epinephrine not being administered via a finger or toe, but only deltoid area or quadriceps. Read the Label & Compare to Orders Typically, orders for giving medications include information such as the name of the drug, the dose, the administration route, and possibly the rate of administration. It would be ideal to be able to write the orders down so you could double check the orders prior to administration, and again after preparing the drug. Written protocols should be consulted if there is any question as to the dosage, route, rate, indications or contraindications. If the orders are given to you verbally you should always repeat the orders to verify them and avoid confusion. You have the responsibility to question any orders which may seem inappropriate. Always check the label of the drug prior to administration for expiration date, concentration, clarity, discoloration, and signs of tampering. Anytime your focus has been shifted away from the drug you are about to administer it would be a very good idea to recheck the label to reconfirm that it is the drug you were intending to give before you were distracted. Needles and Safety It goes without saying that safety with needles is very important to you, your crew, the patient, and the public. It would be ideal to immediately discard the needle after use into an appropriate container without having the needle ever change hands. You should personally discard the needle immediately after use into a sharps container without handing it over to someone else. We have all seen inappropriate handling of needles in the field such as sticking them into the ground or some other object, or otherwise discarded in an unwise fashion. The bottom line is that we have a major responsibility to the people around us to treat needles in the proper manner. The various bloodborne diseases we encounter on a daily basis do not need any help from medics to continue to spread. You need to do your part to maintain a high degree of safety when dealing with "sharps". Patient Monitoring It is important to monitor the patient to whom we have administered drugs, not only for the expected effects of the particular drug but also any unwanted side effects or reactions. Keep in mind the different problems that are associated with various age groups. The pediatric patients and the geriatric patients will need special considerations when administering drugs. Monitor the patients for patency of the IV, check the IV site for problems often. Verify initial placement of endotracheal tubes, and recheck placement of ET tubes at regular intervals. Monitor injection sites for adverse reactions. Documentation Documentation of drug administration serves two purposes, both clinical and legal. Recording the drug name, route of administration, dosage, site of administration, and any other pertinent information is very important. Documentation of reasons why you decided not to administer a drug following orders to do so would be very important. It may be a good practice to give reasons that you felt the need to administer the drug, the effects the drug seem to have had on the patient (expected and unexpected), or the reasons that you felt the drug was not indicated in this particular case. You never know which case may end up in court and how long it may take to get there. You need to document in such a way that you will be able to recall the events as clearly as possible. Metric System An understanding of the metric system is required to properly calculate drug dosages. The basic unit of measurement in the metric system of mass (a solid) is the gram (g or gm). As the gram is divided into smaller units, a prefix is given to the word "gram" to signify the unit of measurement and its relation to a gram. The gram is broken down into thousandths of a gram in this manner. The basic unit of volume (liquid and gas) measurement in the metric system is the liter (L). As with the gram, liters are commonly divided into smaller amounts and given a prefix to compare the amount to a liter. Liters are also commonly divided by thousandths. Metric Conversions The following are common prefixes associated with the metric system and paramedic drug calculations for solids. Kilo- A kilogram is equal to 1000 grams Milli- A milligram (mg) is equal to .001 of a gram (a gram divided by 1000) Micro- A microgram (mcg or µg)is equal to .001 of a milligram (milligram divided by 1000) or .000001 of a gram (a gram divided by 1,000,000 The following are common prefixes found when dealing with liquid measurement in the metric system. Milli- A milliliter is equal to .001 of a liter, so it is a liter divided by 1000 Micro- A microliter is equal to .001 of a milliliter, a milliliter divided by 1000 or a liter divided by 1,000,000 (not commonly used in drug calculations) It is important to note that a cubic centimeter (cc) is the same as a milliliter (ml). There are units of measurement for quantities larger than kilograms and liters but the need for these units are not common. Moving the Decimal Point The easiest way to convert grams and liters into smaller units is to move the decimal point either right or left depending on whether you wish to increase or decrease the unit of measurement. The following is an example: To convert 1.0 g to milligrams it is necessary to move the decimal place of 1.0 g to the left, three places. Moving the decimal point to the left makes the number smaller by 10 times for each time you move it.
1.0 g divided by 10 would look like this: 0.1 g 1.0 g divided by 100 would look like this: 0.01 g 1.0 g divided by 1000 would look like this: 0.001 g A gram divided by 1000 is equal to 1 mg. A milligram divided by 1000 is a microgram. To convert back to larger units, it is only necessary to move the decimal place back to the left again, to convert 1.0 mcg to milligrams simply move the decimal place to the right three times, and move it to the right six times to convert 1.0 mcg to grams Like this: 1.0 mcg = .001 mg 1.0 mcg = .000001 g Drug Calculations There are many instances in which a paramedic will be required to perform drug dosage calculations in the pre-hospital setting. There are many different ways to perform these mathematical calculations. They all have certain components in common and certain types of information that are required in order to correctly calculate the required dose. The information required in most methods of calculation is as follows: Desired Dose - It is important that you know the quantity of the medication or fluid that the medical orders call for. The orders for a drug will normally be expressed in either grams, milligrams, or micrograms. Sometimes the apothecary method of measurement is used and the order may be expressed in grains (not in common use). Concentration of Drug on Hand - It is equally important to know what concentration the desired drug on hand is found in. The concentration should be stated on the label and is usually expressed in grams, milligrams, micrograms, or grains. It may be expressed a as 5 mg in 10 ml of saline meaning that there is 5 milligrams of the drug in 10 ml of fluid. The concentration may then be expressed as 0.5 mg/ml. With these two pieces of information, you can accurately calculate the volume of a drug to be given to a patient. The standard math formula that utilizes these three pieces of information is as follows: 'Amount to give' = 'Desired dose' divided by 'Concentration' To use this formula, plug in the known values and work the formula as in this example: The orders you were given by a physician were to administer 60 mg of lidocaine to a patient by IV bolus. You find the drug supplied in a pre-filled syringe that contains 100 mg of lidocaine in 5 ml of saline. Plug in the known values like this: Desired dose = 60 mg Concentration on hand = 100 mg per 5 ml Amount to give = (60 mg) divided by (100 mg/5 ml) Amount to give = (60 mg) times (5 ml/100 mg) Amount to give = (60 mg) times (5 ml/100 mg) = (60 x 5)/100 = 300/100 = 3 This works out to 3 ml of the drug to be delivered to the patient to supply the correct amount of the drug. This type of formula only works when the units of measurement are the same. You can't mix grams and milligrams (or liters and milliliters) in the same formula, for example. You would have to convert everything to either grams or milligrams (liters or milliliters). It doesn't matter which you use, they only need to be the same. Variations The same type of formula works if you need to figure an infusion rate. 'Infusion rate' = [ 'Desired dose' x 'Administration set' ] / 'Concentration' Let's say the doctor orders an infusion of drug X at 2 mg/minute. The doctor orders you to prepare a mixture of drug X at a concentration of 4 g in 1 liter of saline. The doctor wants you to use a 60 gtts/ml administration set to deliver the drug. Work the formula as follows: Desired dose = 2 mg/min Concentration = 4 g (4000 mg) in 1000 ml or 4 mg/ml Infusion rate = [ Desired dose x Dosage ] / Concentration ? gtts/min = [ (2 mg/min) x (60 gtts/ml) ] / (4 mg/ml) = (2 x 60) / 4 = 120/4 = 30 gtts/min Many drugs are dosed according to weight and most pediatric dosages are according to weight. You will need to convert the patient's weight from pounds to kilograms. Simply divide the patient's weight in pounds by 2.2 to convert to kilograms. For example, for a patient that weighs 165 pounds: 165/2.2 = 75. So your patient that weighs 165 pounds weighs 75 kilograms. Lidocaine is a common pre-hospital drug that is dosed according to weight. The normal dosage of lidocaine is 1 - 1.5 mg/kg. For a patient that weighs 220 pounds, first you convert to kilograms: 220/2.2 = 100 kg. Then you plug the weight in the formula: 1 X 100 = 100 and 1.5 X 100 = 150. For this patient you would administer 100 - 150 mg of lidocaine. As mentioned earlier there are many different ways to calculate doses and you should find a way that you understand and are able to use at three o'clock in the morning, under pressure. That may be a bit tough to do but with a lot of practice you can do it!
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