AHS

I: General pharmacology

Introduction and routes of drug administration

Pharmacology is the branch of medical science that primarily includes the in-depth study of medications/pills.  

A drug is a chemical substance, molecule, or compound that relieves symptoms or prevents or cures a disease.

Pharmacokinetics is the complete journey of drugs through the body. (How drugs go inside the body, reaches at the site of action, get metabolised and then are excreted out of the body.)

Pharmacodynamics is the mechanism of action of a drug, how it exhibits biological activity, either by binding to the receptor or without a receptor.

Pharmacy: a branch where the art of dispensing and formulating medicine is learned.

Toxicology is an allied branch of pharmacology that deals with the poisonous effects of drugs, including their specific management.

Routes of drug administration:

Drugs are administered through the topical or systemic route or using special drug delivery systems.

Topical Route:

Topical routes are used for localized action, usually applied over the skin or mucous membrane and having no or minimal systemic side effects. It is convenient, and the patient can apply it on their own. Ointment, gel, lotion, liniment, and creams are applied to the skin surface. Ear and nasal drops, mouthwash, gargles, paint, and pessaries are applied over the mucous membranes.

Examples: turpentine liniment to reduce pain and muscle stiffness in rheumatoid arthritis, calamine lotion for a patient with pruritus, nimesulide gel for muscular pain, and potassium permanganate gargles for patients with gingivitis. 

The route is readily acceptable to the patients; it does not induce pain and has minimal systemic side effects.

Systemic route:

The route is further subclassified into enteral, parenteral, rectal, and dermal routes.

Oral Route:                                                                                                                 

Solid (tablet, capsule) or liquid (syrup, suspension) forms of medication are administered via this route.  It is the most commonly used route because of its cost-effectiveness, convenience, and ease of administration.  It is non-invasive, and no special assistance from a staff nurse is required (like in IM and IV routes).

Example: a paracetamol tablet orally for headache and body ache.  

However, certain individuals cannot tolerate this route due to the bitter or metallic taste of the drug, leading to nausea and vomiting. Also, it cannot be used in unconscious and uncooperative patients. The route is contraindicated in patients with gastroenteritis.

Sublingual route: 

The most preferred route in acute angina. The tablet or capsule is kept under the tongue, which bypasses the first-pass mechanism and gets absorbed directly into the systemic circulation. This route will be preferred for drugs having a high first-pass effect. In patients with angina, sublingual nitroglycerine exhibits its action in 3-5 minutes. In case an adverse reaction has been observed to a sufficient degree, the drug can be spit out.

Rectal route:

Drugs that are not palatable orally are kept in the rectum or may be used in the form of a suppository (a cone-shaped medicated solid substance inserted into the rectum that melts with body temperature). This route has an edge over the oral route, as it can be inserted in an unconscious patient or one who is having intermittent vomiting, but the route cannot be used for those having diarrhea. Its use is limited, especially in apprehensive and nervous patients, as it is embarrassing to insert a tablet through the rectum.

Example: diclofenac suppository in postoperative patients, diazepam suppository in children with febrile convulsions.

Parenteral route:

A drug that is given by the parallel route to the enteral route is called the parenteral route.

Intravenous route

The drug is injected through superficial veins as a bolus dose or continuous infusion. It has 100% bioavailability. It has fast action and is useful in all types of emergencies. (Severe painful condition, acute heart failure, status asthmaticus, status epilepticus, hypovolemic shock, cardiac arrest, acute gastroenteritis, etc.) It can be used in an unconscious or uncooperative patient.

Disadvantages: The procedure is painful, and special staff are required to administer the drug. The patient may develop infection or thrombophlebitis if aseptic precautions are not taken.

Examples: Several antibiotics are given intravenously. Other examples are IV adrenaline in a cardiac arrest and IV midazolam in status epilepticus.

Intramuscular route:

The drug is administered aseptically via injection deep into the gluteal, deltoid, or thigh muscles. The muscle is a highly vascular region, which ensures absorption with nearly 100% bioavailability. This route provides a choice of administering aqueous solutions and depot injections. It has fast action, hence it is beneficial in emergencies.  It can be preferred in unconscious and uncooperative patients also. The considerable disadvantage of the route would be painful injections and the risk of development of an abscess if proper aseptic precautions are not taken during administration. Self-administration with this route is not possible, and on rare occasions painful injection may cause vasovagal shock.

Example: injection of streptomycin in TB, injection of diclofenac for severe renal or biliary colic.

Subcutaneous route:

The drug is given below the skin, which is a less vascular region. The preferred area for SC injection is the anterior abdominal wall and forearm.  The route provides the advantage of slow and steady absorption for a prolonged period. A special form of drug (implants) can be placed subcutaneously by using this route that ensures drug release for more than a year. The injection is usually administered by trained staff, but self-administration is possible, as the drug is injected into loose tissues and not as deeply as in muscles, as given in the intramuscular route. Irritant drugs cannot be given through this route, and drugs with larger volumes (>2 ml) are restricted by this route.

Examples: Drugs like insulin and low molecular weight heparin are used by this route. A few vaccines are also injected by this route.

Intradermal route:

An injection is given superficially in a horizontal direction, usually over the anterior surface of the forearm, to raise a small bleb over the skin.

Example: BCG vaccine, monkeypox vaccine. The route is also used for a test dose of antibiotics.

Other not frequently used routes  are:                      

Dermal route:

A drug is applied to the skin and gradually absorbed into systemic circulation. Small patches called transdermal patches are usually applied over the chest or back.

Examples: A nitroglycerin patch in a patient with angina and a scopolamine patch in a person having motion sickness.

Special drug delivery system:

In this type of drug delivery system, the drug is delivered through a special device. It provides site-specific delivery of the drug, thereby reducing toxicity. The drug delivered through a novel delivery system has sustained and prolonged action.

Prodrug: It is an inactive form of a drug that becomes active after undergoing biotransformation within the body.

Example: Levodopa, an inactive compound, gets converted to dopamine by the enzyme DOPA decarboxylase.  (Dopamine, being a tertiary amine, cannot cross the BBB; hence, Levodopa is used instead of dopamine in the treatment of Parkinsonism.)

Liposomes: Drugs are encapsulated into phospholipid bilayer vesicles and targeted at the site of action.

Example: Due to the nephrotoxic nature of amphotericin B, it is given in a special form called a liposomal drug delivery form.

Nanoparticles: Use of nanotechnology in the field of medicine is spreading rapidly. Nanoparticles are of small size (1 to 100 nm) and are used as drug carriers for targeted drug delivery in cancer therapy.

Monoclonal antibodies: They are genetically engineered antibodies synthesized against a specific antigen by using “hybridoma technology.”

Examples: Monoclonal antibodies are primarily used for the treatment of cancer (Rituximab, Cetuximab, Trastuzumab), but they have also shown promising results in other disorders. (Omalizumab for asthma, palivizumab against respiratory syncytial virus infection).

Norplant: contraceptives are implanted intradermally. Their action remains for almost 5 years. Also, contraceptives such as progesterone are directly inserted into the uterine cavity at a controlled rate using an implant called Progestasert.

Insulin pumps: These are portable electronic devices that are specifically designed to deliver insulin with a basal dose or bolus dose by continuous subcutaneous infusion. The major advantage of the device is that the delivery of insulin is analogous to the physiological secretion of insulin. Hence, it provides good control over blood sugar levels. The poor patient cannot afford it due to the high cost, and occasionally, mechanical pump failure can occur.

Nursing responsibility:

Nurses play a significant role in drug administration. The nurses must ensure the right dose, with the right route, at the right time for the right patient to avoid medication errors. In the parenteral route, aseptic precautions must be taken to avoid sepsis. Any warning signs after drug administrations or side effects of given drugs should be observed and timely reported to the concerned duty doctor. If a venous catheter is inserted for continuous IV infusion, it must be removed after 72 hrs of administration to avoid thrombophlebitis and sepsis. Any early sign of sepsis or thrombophlebitis must be recognized and reported in time.