Autonomic Nervous System
Sympathetic (catecholamines; norepinephrine, epinephrine) vs Parasympathetic (acetylcholine) Stimulation
- Eyes: pupillary dilation mydriasis vs constriction miosis
- Heart: tachycardia vs bradycardia
- Airways: bronchodilation vs bronchoconstriction
- GI: hypomotility vs hypermotility
- Bladder: relaxation (urinary retention) vs contraction (voiding)
Autonomic Nervous System Drugs
- Adrenergic agents, from the word adrenaline, refers to drugs that affect the sympathetic system which runs on adrenaline.
- Cholinergic agents, from the word acetylcholine, refers to drugs that affect the parasympathetic system which runs on acetylcholine.
Adrenergics
Adrenergic agents act by stimulating the four types of receptors found in the sympathetic nervous system. These vary in their region of effect, which include:
- Alpha-1 Receptors: affects all organs affected by the sympathetic nervous system except the heart. Its primary targets are blood vessels, producing vasoconstriction.
- Alpha-2 Receptors: a modulator; prevents overstimulation by exerting opposing effects. Overall, this reduces blood pressure and causes hypotension.
- Beta-1 Receptors: affects the heart and kidneys, producing tachycardia of the heart and fluid retention and increased blood pressure through the release of renin, part of the RAA system.
- Beta-2 Receptors: affects the smooth muscles, particularly of the respiratory tract, producing bronchodilation to improve airflow. It also affects other organs such as the stomach to reduce peristalsis.
- Beta-3 Receptors: lipolysis and relaxation of the bladder.
Adrenergic agonists are those that increase the sympathetic response either non-selectively or selectively (which receptors are activated). As discussed above, this either targets Alpha 1 (increases BP), Alpha 2 (decreases BP), Beta 1 (increases HR), and Beta 2 (causes bronchodilation).
- Selective Alpha Adrenergic Agonist: Phenylephrine: a selective alpha stimulant that increases the blood pressure of a hypotensive patient.
- This can also be used for nasal congestion through vasoconstriction.
- Non-selective Adrenergic Agonist: Epinephrine and Dopamine: non-selective stimulant that is used for shock.
- Selective Alpha-1 Adrenergic Agonist: Pseudoephedrine and Oxymetazoline: used for hypotension.
- Selective Alpha-2 Adrenergic Agonist: Clonidine (Catapres), Methyldopa, Midodrine: used to reduce blood pressure.
- Non-selective Beta Adrenergic Agonist: Isoproperinol: increases heart rate and causes bronchodilation used commonly for asthma and also dysrhythmia and heart failure.
- Selective Beta-1 Adrenergic Agonist: Dobutamine, used in heart failure to increase heart rate. It may also be used for dysrhythmias.
- Selective Beta-2 Adrenergic Agonist: Albuterol, Salmeterol, Formoterol, Metaproterenol, and Terbutaline used as bronchodilators.
- Selective Beta-3 Adrenergic Agonist: Droxidopa, Mirabegran are used for urinary frequency
Nursing Considerations
With the use of sympathetic drugs, the nurse must monitor vital signs, especially those affected by the medication.
- Urine output and cardiac output must also be assessed.
- Breathing patterns should be checked.
- Check for photosensitivity, as sympathetic agonists will produce mydriasis, which can result in light sensitivity.
- Monitor for rhinorrhea especially during the use of phenylephrine. Epistaxis may also occur.
Adrenergic antagonists block the sympathetic response from exerting its effects. Primarily, these are used for lowering blood pressure.
- Non-selective Alpha Blockers: Phentolamine: used for severe hypertension, and at times for benign prostate hyperplasia (BPH).
- Selective Alpha-1 Blockers: “-zosin”s; Prazosin, Alfuzosin, Doxazosin, Terazosin, Tamsulosin: used for hypertension.
- Non-selective Beta Blockers: “-olol”s; Nadolol, Carteolol, Propranolol, Timolol (also suppresses aqueous humor production for glaucoma), Sotalol: slows the heart rate and produces bronchoconstriction. This may be used for angina.
- Selective Beta-1 Blockers: “-olol”s; Acebutolol, Atenolol, Bisoprolol, Metoprolol, Esmolol: decreases heart rate, alleviating hypertension.
Nursing Considerations
With the use of sympathetic blockers, the nurse must monitor vital signs, especially those affected by the medication.
- Urine output must be assessed. Urinary hesitancy, incomplete bladder emptying, and weak urination may occur.
- Check for orientation and LOC. Decreased perfusion to the brain may express itself in dizziness or drowsiness.
- Check cardiac output for adequacy.
Cholinergics
Cholinergic agents act by stimulating two types of receptors:
- Nicotinic Receptors: receptors that have a high affinity for nicotine. These are found in skeletal muscles and are situated pre-ganglionic in neurons. Stimulation of nicotinic receptors is non-selective; it can produce both sympathetic and parasympathetic effects due to its location in the pre-ganglion.
- Muscarinic Receptors: the stronger of the two receptors, with high affinity for muscarine (often found in poisonous mushrooms). Contrastingly, these are post-ganglionic, limiting its effects to those of the parasympathetic system. This makes it more appealing as a target for pharmacologic intervention. Thus, this is the focus of this discussion. There are five further divisions of muscarinic receptors:
- M1: affects the CNS
- M2: affects the heart
- M3: affects smooth muscles; most areas besides the heart
- M4: affects the CNS
- M5: affects the CNS
Cholinergic Stimulants promote this response through two primary methods: direct-acting stimulants and indirect-acting stimulants. The difference lies in whether the drug stimulates the receptors for acetylcholine through mimicry (direct) or blocks the effect of acetylcholinesterase or reuptake to increase its concentration (indirect).
- Direct-acting:
- Urecholine (Bethanicol): increases the parasympathetic response to the digestive and urinary tract, improving excretion through hypermotility and contraction. This is used in constipation and neurogenic urinary retention.
- Indirect-acting:
- “-stigmines”; Physostigmine (Antlirium). These contract skeletal muscles and increases blood flow to the CNS. This is used as an antidote for cholinergic crises such as in overdosing on anticholinergics.
Nursing Considerations
With the use of cholinergic stimulants, the nurse must monitor for the possibility of a cholinergic crisis. This produces an excess of parasympathetic responses, which result in hypersalivation, miotic pupils, twitching/spasms of skeletal muscles, diaphoresis, difficulty of breathing (bronchoconstriction), hypermotility, and urinary frequency. These are all effects of parasympathetic stimulation in excess.
Additionally, monitor liver function. Cholinergics are often hepatotoxic. Liver enzymes are checked to identify early liver damage.
The antidote used for cholinergic crisis is atropine.
Cholinergic Blockers (Anticholinergics) inhibit the parasympathetic response. This allows sympathetic effects to take hold, such as mydriasis, increased heart rate, decreased gastrointestinal motility, bronchodilation, urinary retention, and decreases blood flow to the central nervous system (useful for Parkinson’s). Indications include: peptic ulcer disease, opthalmic examination, heart abnormalities (e.g., heart failure), asthma, and an overactive bladder.
- Bronchodilator Cholinergic Blockers: “-tropium”s; Ipratropium (Atrovent), Aclidinium, and Tiotropium. These focus their effects on the lungs, allowing it to dilate.
- Chronotropic Cholinergic Blockers: Atropine, Glycopyrrolate. Atropine is the more common form given, often as pre-operative medications to counteract the downer effects of anesthetics given during surgery. It also produces dry mouth and decreases GI secretions in order to prevent aspiration precautions.
- Urinary Retention Cholinergic Blockers: Oxybutinine, Telterodine, Fesoterodine, Darifenacin, Solifenacin, Trospium. These are used for patients with urinary frequency.
- Mydriatic Cholinergic Blockers: Cyclopentolate, Tropicamide
- Gastrointestinal Cholinergic Blockers: Scopalamine, Methscopalamine, Dicyclomine, Propantheline. These are used to reduce peristalsis such as in cases of PUD.
Nursing Considerations
With the use of cholinergic blockers, the nurse must monitor for the possibility of an anticholinergic crisis. This produces an excess of sympathetic responses, which result in dry mouth, hot dry skin, decreased GI motility, vision problems due to mydriasis, and decreased LOC in severe cases.
- The antidote used for anticholinergic crisis is physostigmine.
- Provide ice cubes or hard candy for dry mouth.
- Avoid strenuous activities and hot temperatures.
- Monitor urine output.
- Monitor heart rhythms
- Watch out for abdominal distention due to gastrointestinal hypomotility