McKenna's Pharmacology for Nursing, 2e

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P A R T 8  Drugs acting on the cardiovascular system

Lopresor ), nadolol ( Apo-Nadol, Apo-Nadolol [not available in Australia]), nebivolol ( Nebilet [not available in New Zealand]), pindolol ( Barbloc, Visken ), propranolol ( Deralin, Inderal ) and timolol ( Timoptol ) . • Alpha- and beta-blockers are useful in conjunction with other agents and tend to be somewhat more powerful, blocking all of the receptors in the sympathetic system. People often complain of fatigue, loss of libido, inability to sleep, and GI and genitourinary disturbances. As a result, they may be unwilling to continue taking these drugs. Alpha- and beta-blockers used to treat hypertension include the following agents: carvedilol ( Dicarz, Dilasig ) and labetalol ( Presolol , Trandate ) . • Alpha-adrenergic blockers inhibit the postsynaptic alpha 1 -adrenergic receptors, decreasing sympathetic tone in the vasculature and causing vasodilation, -receptors, preventing the feedback control of noradrenaline release. The result is an increase in the reflex tachycardia that occurs when blood pressure decreases. These drugs are used to diagnose and manage episodes of phaeochromocytoma, but they have limited usefulness in essential hypertension because of the associated adverse effects. Alpha- adrenergic blockers include the following agents: phenoxybenzamine ( Dibenyline ) and phentolamine ( Regitine ) . • Alpha 1 -blockers are used to treat hypertension because of their ability to block the postsynaptic alpha 1 -receptor sites. This decreases vascular tone and promotes vasodilation, leading to a fall in blood pressure. These drugs do not block the presynaptic alpha 2 -receptor sites, and therefore the reflex tachycardia that accompanies a fall in blood pressure does not occur. Alpha 1 -blockers used to treat hypertension include the following agents: doxazosin ( Dosan [not available in Australia]), prazosin ( Minipress ) and terazosin ( Hytrin ). • Alpha 2 -agonists (see Chapter 30) stimulate the alpha 2 -receptors in the CNS and inhibit the cardiovascular centres, leading to a decrease in sympathetic outflow from the CNS and a resultant drop in blood pressure. These drugs are associated with many adverse CNS and GI effects, as well as cardiac dysrhythmias. Alpha 2 -blockers used to treat hypertension include the following agents: clonidine ( Catapres ) and methyldopa ( Aldomet , Hydopa ). Moxonidine ( Physiotens [not available in New Zealand]) binds to both I 1 -imidazoline receptors and alpha 2 -adrenoreceptors, reducing sympathetic nervous system activity, and thus lowering blood pressure. which leads to a lowering of blood pressure. However, these drugs also block presynaptic alpha 2

KEY POINTS

■■ Hypertension is a sustained state of higher-than- normal blood pressure that can lead to blood vessel damage, atherosclerosis and damage to small vessels in end organs. ■■ The cause of essential hypertension is unknown; treatment varies among individuals. ■■ Drug treatment of hypertension aims to change one or more of the normal reflexes that control blood pressure. ■■ Sodium levels and fluid volume are decreased by diuretic agents. ■■ ACE inhibitors prevent the conversion of angiotensin I to angiotensin II, leading to a fall in blood pressure. ■■ ARBs prevent the body from responding to angiotensin II, causing a loss of effectiveness of the renin–angiotensin system. ■■ Calcium channel blockers interfere with the ability of muscles to contract, which leads to vasodilation, which in turn reduces blood pressure. ■■ Other drugs used to treat hypertension include diuretics, which decrease the sodium content in the body, and various sympathetic blockers, which block the blood pressure–raising effects of the sympathetic system. ANTIHYPOTENSIVE AGENTS As mentioned earlier, if blood pressure becomes too low (hypotension), the vital centres in the brain and the rest of the tissues of the body may not receive sufficient oxygenated blood to continue functioning. Severe hypo- tension or shock puts the body in serious jeopardy; it is often an acute emergency situation, with treatment required to save the person’s life. The first-choice drug for treating shock is usually a sympathomimetic drug. See Figure 43.3 for sites of action of drugs used to treat hypotension. Antihypotensive agents are also discussed in Table 43.3. S ympathetic adrenergic agonists or vasopressors Sympathomimetic drugs are the first choice for treating severe hypotension or shock. The sympathomimetic drugs are discussed in detail in Chapter 30. Sym- pathomimetic drugs used to treat shock include the following agents: adrenaline ( Adrenaline , EpiPen ), dobutamine ( Dobutrex ), dopamine (generic), ephed- rine (generic), isoprenaline ( Isuprel ), metaraminol (generic), noradrenaline ( Levophed ) and phenylephrine ( Neo-Synephrine ).