McKenna's Pharmacology for Nursing, 2e

456 P A R T 5  Drugs acting on the autonomic nervous system A n adrenergic agonist is also called a sympathomimetic drug because it mimics the effects of the sympathetic nervous system (SNS). The therapeutic and adverse effects associated with these drugs are related to their stimulation of adrenergic receptor sites. That stimula- tion can be either direct, by occupation of the adrenergic receptor, or indirect, by modulation of the release of neurotransmitters from the axon. Some drugs act in both ways. Adrenergic agonists also can affect both the a - and b -receptors, or they can act at specific receptor sites. The use of adrenergic agonists varies from ophth­ almic preparations for dilating pupils to systemic preparations used to support individuals experiencing shock. They are used in people of all ages (Box 30.1). α- AND β-ADRENERGIC AGONISTS Drugs that are generally sympathomimetic (Figure 30.2) are called a -agonists (stimulate a -receptors) and b -agonists (stimulate b -receptors). These agonists stimu­ late all of the adrenergic receptors, that is, they affect both a - and b -receptors (Table 30.1). Agents that affect both a - and b -receptor sites include adrenaline ( Anapen, EpiPen ), dobutamine ( Dobutrex ), dopamine (generic), ephedrine (generic), metaraminol (generic) and noradrenaline ( Levophed ). Several of these drugs are naturally occurring catecholamines. Children are at greater risk for complications associated with the use of adrenergic agonists, including tachycardia, hypertension, tachypnoea and gastrointestinal complications.The dosage for these agents needs to be calculated from the child’s body weight and age. It is good practice to have a second person check the dosage calculation before administering the drug to avoid potential toxic effects. Children should be carefully monitored and supported during the use of these drugs. Phenylephrine is often found in over-the-counter (OTC) allergy and cold preparations, and parents need to be instructed to be very careful with the use of these drugs— they should check the labels for ingredients, monitor the recommended dose and avoid combining drugs that contain similar ingredients. ADULTS Adults being treated with adrenergic agonists for shock or shock-like states require constant monitoring and dosage adjustments based on their response. People who may be at increased risk for cardiac complications should be monitored very closely and started on a lower dose. Adults using these agents for glaucoma or for seasonal rhinitis BOX 30.1 Adrenergic agonists CHILDREN

Safe medication administration

Subdivision of α- and β-receptors There is a further subdivision of adrenergic receptors into a 1 , a 2 , b 1 , b 2 and b 3 receptors. The a 1 -receptors are found in the peripheral blood vessels, the eye and the heart. Stimulation of a 1 -receptors causes vasocon- striction of peripheral blood vessels and contraction of the dilator muscle of the pupil resulting in dilation of the pupil. In the heart, stimulation of a 1 -receptors increases the contractility of the heart. b 1 -receptors are also found in the heart and their stimulation causes increased heart rate as well as increased contractility Administering ophthalmic medications Some of the adrenergic agonists are applied in the eye; it is important to review the administration technique. First, wash hands thoroughly. Do not touch the dropper to the eye or to any other surfaces. Have the person tilt their head back or lie down and stare upwards. Gently grasp the lower eyelid and pull the eyelid away from the eyeball. Instill the prescribed number of drops into the lower conjunctival sac and then release the lid slowly (Figure 30.1). Have the person close the eye and look downwards. Apply gentle pressure to the inside corner of the eye for 3 to 5 minutes. Do not rub the eyeball and do not rinse the dropper. If more than one type of eye drop is being used, wait 5 minutes before administering the next one.

Drug therapy across the lifespan

need to be cautioned about the use of OTC drugs and alternative therapies that might increase the drug effects

and cause serious adverse effects. PREGNANCY AND BREASTFEEDING

Many of these drugs are used in emergency situations and may be used during pregnancy and breastfeeding. In general, there are no adequate studies about their effects during pregnancy and breastfeeding, and in those situations, they should be used only if the benefit to the mother is greater than the risk to the fetus or neonate. OLDER ADULTS Older people are more likely to experience the adverse effects associated with these drugs—central nervous system, cardiovascular, gastro-intestinal and respiratory. Because older people often have renal or hepatic impairment, they are also more likely to have toxic levels of the drug related to changes in metabolism and excretion. Older people should be started on lower doses of the drugs and should be monitored very closely for potentially serious arrhythmias or blood pressure changes. They also should be cautioned about the use of OTC drugs and complementary therapies that could increase drug effects and cause serious adverse reactions.