Autonomic Drugs

Your patient's heart rate, blood pressure, pupils, bladder, gut, and airways are all being quietly managed — second by second — by a nervous system nobody chose to switch on. Autonomic drugs work by pushing on that system: turning up the gas, stepping on the brake, or cutting a wire. Once you understand which receptor a drug lands on and what that receptor does to the organ underneath it, the adverse effects stop being a list to memorize and start being something you can predict from first principles. This is the chapter that makes the rest of pharmacology click, because so many drugs — for hypertension, asthma, glaucoma, overactive bladder, shock, and anesthesia — are simply autonomic drugs wearing a different name badge.

Section 1The autonomic nervous system

Two divisions, one body

The autonomic nervous system (ANS) runs everything you do not consciously decide: heart rate, vessel tone, breathing passages, digestion, sweating, pupil size, bladder emptying. It has two divisions that mostly oppose each other, and the classic shorthand is worth keeping because it is genuinely predictive. The sympathetic division is fight or flight — the body preparing to run from something. The parasympathetic division is rest and digest — the body settling down to housekeeping. Ask yourself what a person sprinting from danger needs, and sympathetic effects fall out on their own: faster heart, higher pressure, open airways, dilated pupils to see the threat, glucose released for fuel, blood shunted to muscle and away from gut and bladder. Ask what a person napping after a meal needs, and the parasympathetic list appears: slower heart, constricted pupils, active gut, a bladder that empties.

The two divisions differ in their chemistry, and that is where drugs get their leverage. Every autonomic pathway uses two neurons in series with a ganglion (a synapse) between them. At every ganglion, in both divisions, the transmitter is acetylcholine (ACh) acting on nicotinic receptors. What changes is the last step, at the organ itself. Parasympathetic nerves release acetylcholine onto muscarinic receptors — these are the cholinergic synapses. Most sympathetic nerves release norepinephrine onto adrenergic receptors. Two important sympathetic exceptions are worth memorizing now, because they explain odd drug effects later: the adrenal medulla is a modified ganglion that dumps epinephrine straight into the blood as a hormone, and the sweat glands are sympathetic but use acetylcholine on muscarinic receptors — which is exactly why anticholinergic drugs make patients hot and dry.

The receptor map — the single most useful table in pharmacology

Nearly everything in this chapter is an application of the table below. A drug does not have effects because of its name; it has effects because of which receptor it hits and where that receptor lives. Learn the receptor and you have learned the drug, the therapeutic use, and the adverse effect profile in one stroke.

Two memory hooks pay for themselves all semester. “One heart, two lungs” puts beta-1 in the heart and beta-2 in the lungs and never lets you mix them up — and it instantly explains why a nonselective beta blocker is dangerous in asthma. SLUDGE (Salivation, Lacrimation, Urination, Defecation, GI upset, Emesis) is the muscarinic picture; the anticholinergic picture is simply its mirror image, which students remember as dry as a bone, red as a beet, blind as a bat, mad as a hatter, hot as a hare.

Four things a drug can do

Every drug in this chapter falls into one of four buckets, and the vocabulary is worth pinning down now. A sympathomimetic (adrenergic agonist) mimics the sympathetic system. A sympatholytic (adrenergic antagonist, or blocker) blocks it. A parasympathomimetic (cholinergic agonist) mimics the parasympathetic system. A parasympatholytic (anticholinergic) blocks it. Notice the elegance hiding in there: blocking one division looks a great deal like stimulating the other. A beta blocker slows the heart, and so does a cholinergic drug — different receptor, same bradycardia on your monitor. Do not let the clinical picture alone tell you which drug your patient took.

Section 2Adrenergic agonists

Turning up the sympathetic gas

An adrenergic agonist, or sympathomimetic, produces the effects the sympathetic nervous system would produce. Some do it directly, by binding the receptor themselves (epinephrine, norepinephrine, phenylephrine, albuterol). Others do it indirectly, by causing the nerve terminal to dump its own stored norepinephrine or by blocking its reuptake (ephedrine, which is mixed direct and indirect; amphetamines; cocaine). Either way the receptor sees more signal, and the organ responds.

What makes this class confusing at first is that a single drug can hit several receptors at once, so its effect depends on which receptors it reaches and at what dose. Selectivity is the whole game. A drug that hits only beta-2 opens airways without racing the heart; one that hits everything opens airways and races the heart and clamps down the vessels. Selectivity is also dose-dependent — push the dose high enough and a “selective” drug starts spilling onto its neighbors. This is precisely why an asthmatic patient who overuses albuterol develops tachycardia and tremor: at high doses the beta-2 selectivity leaks onto beta-1.

Epinephrine: the drug that changes personality with dose

Epinephrine is the prototype and the one to know cold. It is a nonselective agonist — alpha-1, alpha-2, beta-1, and beta-2 — but its receptors have different sensitivities, so the drug behaves differently at different doses. At low doses, beta effects dominate: the heart speeds up, contractility rises, and airways open while skeletal-muscle vessels actually dilate, which can drop the diastolic pressure. At high doses, alpha-1 effects take over and vasoconstriction wins, driving blood pressure up hard. This dose-dependent split is why the same molecule is used for anaphylaxis, for cardiac arrest, for shock, and (in dental and local anesthetic mixtures) to keep lidocaine from washing away.

In anaphylaxis, epinephrine is doing three jobs simultaneously, and the nurse should be able to name all three: alpha-1 constricts vessels to reverse the catastrophic drop in blood pressure and reduce airway edema; beta-1 supports cardiac output; beta-2 dilates the bronchi and stabilizes mast cells against further mediator release. No other drug does all of that. Antihistamines and steroids are adjuncts and never the first move. The first-line route for anaphylaxis is intramuscular into the vault of the lateral thigh (vastus lateralis), because that muscle's rich blood supply absorbs the drug faster and more reliably than subcutaneous tissue or the deltoid. Know the numbers, because this is where the fatal errors happen: the adult dose is 0.3–0.5 mg IM drawn from the 1 mg/mL (1:1,000) concentration, repeated every 5–15 minutes if the response is inadequate; for a child it is 0.01 mg/kg IM, to a maximum of 0.3–0.5 mg. The 0.1 mg/mL (1:10,000) prefilled syringe is the intravenous cardiac-arrest concentration and must never be substituted for the IM anaphylaxis dose — that swap, in either direction, is the classic tenfold concentration error this drug is known for.

The workhorses, drug by drug

Look again at clonidine, because it trips up nearly everyone. It is an agonist, yet it lowers blood pressure. The reason is where its receptor sits: alpha-2 receptors are presynaptic brakes. Stimulating them tells the nerve terminal to release less norepinephrine. So an alpha-2 agonist functionally behaves like a sympathetic blocker — a good reminder that receptor location matters as much as receptor type.

Section 3Adrenergic antagonists

Alpha blockers: releasing the vessels

An alpha-1 blocker stops norepinephrine from constricting vascular smooth muscle. The vessels relax, resistance falls, and blood pressure comes down. The same receptor sits in the bladder neck and prostatic smooth muscle, so blocking it also relaxes the outflow tract — which is why the “-osin” drugs are workhorses in benign prostatic hyperplasia (BPH), letting men empty a bladder that a big prostate had been squeezing shut. Tamsulosin is relatively uroselective and drops pressure less; doxazosin, terazosin, and prazosin lower blood pressure more and are sometimes used for that purpose (prazosin is also used off-label for PTSD-related nightmares, though the trial evidence is mixed).

The signature hazard is first-dose orthostatic hypotension. The first dose — or the first dose after any increase — can drop a patient's pressure sharply enough to cause syncope, and a syncopal fall in an older adult is a hip fracture waiting to happen. The nursing response is concrete: give the first dose at bedtime, teach the patient to rise slowly and to sit on the edge of the bed before standing, warn them not to drive after the first dose, and check orthostatic vitals. Phentolamine is a nonselective alpha blocker used differently — as the antidote for vasopressor extravasation and in the management of catecholamine excess such as pheochromocytoma.

Beta blockers: the “-olol” family

A beta blocker occupies beta receptors so catecholamines cannot. Blocking beta-1 slows the heart rate, weakens contraction, slows AV conduction, and suppresses renin release — the combination that lowers blood pressure, quiets angina by cutting myocardial oxygen demand, controls rate in atrial fibrillation, and, most importantly, improves survival after myocardial infarction and — for three specific agents — in chronic heart failure. That last point deserves emphasis because it is counterintuitive: a drug that weakens contraction helps the failing heart, because it protects it from years of catecholamine flogging. It also deserves emphasis because the benefit does not belong to the class as a whole. Only metoprolol succinate (the extended-release salt), bisoprolol, and carvedilol have proven a mortality benefit in heart failure with reduced ejection fraction; atenolol has no heart failure indication, and plain metoprolol tartrate is not the heart failure agent either. Do not transfer the survival claim to a beta blocker that has not earned it. In heart failure the beta blocker is started at a low dose and titrated up slowly — and never started during an acute decompensation.

The critical distinction in this class is cardioselectivity. Cardioselective agents (metoprolol, atenolol, bisoprolol, esmolol — the ones that start with A through M) block mainly beta-1 and largely spare the lungs. Nonselective agents (propranolol, nadolol, timolol, sotalol — N through Z) block beta-2 as well, which means they can constrict bronchi. That is the mechanism behind the caution in asthma and COPD. The A–M letter trick is a study aid, not a rule — carvedilol and labetalol are the classic exceptions, since both begin in the first half of the alphabet yet block beta-1, beta-2, and alpha-1. Cardioselectivity is also relative, not absolute, and is lost at higher doses, so even a “cardioselective” blocker gets used cautiously in reactive airway disease.

The three cautions a nurse must own

First, never stop a beta blocker abruptly. Chronic blockade upregulates beta receptors, and yanking the drug away exposes a hypersensitized heart to circulating catecholamines — rebound tachycardia, severe hypertension, angina, and infarction have all been documented. Taper over one to two weeks, and teach the patient this explicitly, because a patient who feels fine and stops on their own is the one who ends up in the emergency department.

Second, beta blockers mask hypoglycemia. The trembling, racing heart, and anxiety that warn a diabetic patient their glucose is falling are beta-mediated sympathetic signals — block beta and the warning goes silent. Sweating is the one sign that survives, because sweat glands are cholinergic. Teach patients on beta blockers with diabetes to check their glucose rather than trust how they feel, and never assume a patient without tremor is safe.

Third, hold parameters are yours to enforce. Take an apical pulse for a full minute before every dose. If the heart rate is under 60 or the blood pressure is below the ordered threshold, hold the dose and notify the prescriber — do not simply give it and chart the vitals. Beta blockers also blunt the compensatory tachycardia of hypovolemia, so a bleeding patient on metoprolol may not mount the heart rate you would otherwise expect. In beta blocker overdose — profound bradycardia and hypotension unresponsive to atropine — the specific antidote is glucagon, which raises cardiac cAMP by a route that bypasses the blocked receptor entirely.

Section 4Cholinergic and anticholinergic drugs

Cholinergic agonists: turning up rest-and-digest

A cholinergic agonist (parasympathomimetic) reproduces what acetylcholine does at muscarinic receptors, and the resulting picture is SLUDGE: salivation, lacrimation, urination, defecation, GI cramping, emesis — plus bradycardia, bronchoconstriction, and miosis. Written out that way it sounds purely like a poisoning, and at high doses it is. Used deliberately at therapeutic doses, though, two of those effects are useful. Bethanechol selectively stimulates the bladder detrusor and gut, which makes it the drug for urinary retention that is not caused by an obstruction — a distinction that matters enormously, because squeezing a bladder against a blocked outlet risks rupture. Pilocarpine, given as eye drops, constricts the pupil and opens the drainage angle in glaucoma, and orally it stimulates saliva in xerostomia. Cholinergic agonists are contraindicated in asthma (bronchoconstriction), in bradycardia or heart block, in peptic ulcer disease, and in bowel or bladder obstruction.

AChE inhibitors: making the body's own acetylcholine last longer

Acetylcholinesterase (AChE) is the enzyme that chews up acetylcholine in the synapse within milliseconds. Block the enzyme and acetylcholine lingers, so the same signal hits harder — an indirect cholinergic agonist. This is the mechanism behind three quite different therapies. In myasthenia gravis, where autoantibodies have destroyed nicotinic receptors at the muscle end plate, pyridostigmine keeps acetylcholine around long enough for the surviving receptors to work, restoring strength; the timing of doses around meals matters so patients can chew and swallow safely. In Alzheimer disease, donepezil, rivastigmine, and galantamine raise central acetylcholine — they modestly improve or stabilize symptoms for a time, but they neither cure the disease nor halt the underlying neurodegeneration. And in anesthesia, neostigmine reverses non-depolarizing neuromuscular blockade, which Section 5 takes up.

Push cholinergic stimulation too far and you get cholinergic crisis: profound SLUDGE with bradycardia, bronchoconstriction, copious secretions, and — the part that kills — muscle weakness and respiratory failure from a depolarized, overwhelmed end plate. This is the mechanism of organophosphate poisoning (insecticides and nerve agents), which irreversibly inactivates AChE. The treatment is two drugs, not one: atropine to block the muscarinic effects and dry the secretions, and pralidoxime to pull the organophosphate off the enzyme before the bond “ages” and becomes permanent. Atropine alone will not fix the nicotinic muscle weakness — a fact worth understanding rather than memorizing.

The cruel clinical trap in myasthenia is that myasthenic crisis (too little drug) and cholinergic crisis (too much) both present as severe weakness and respiratory failure. The differentiator is the company the weakness keeps: cholinergic crisis brings SLUDGE — drooling, tearing, cramping, tiny pupils, bradycardia — while myasthenic crisis does not. Either way the airway comes first.

Anticholinergics: blocking rest-and-digest

An anticholinergic (antimuscarinic, parasympatholytic) blocks muscarinic receptors, so the picture is SLUDGE in reverse: dry mouth, no sweat, blurred near vision and dilated pupils, tachycardia, constipation, and urinary retention, with confusion when the drug reaches the brain. Atropine is the prototype — the drug you push for symptomatic bradycardia, use to dry secretions preoperatively, and reach for in organophosphate poisoning. Ipratropium and tiotropium are inhaled anticholinergics that bronchodilate in COPD by a route completely different from albuterol's, which is why the two are often combined. Oxybutynin, tolterodine, and solifenacin quiet an overactive bladder. Scopolamine patches prevent motion sickness. Benztropine and trihexyphenidyl rebalance the dopamine–acetylcholine seesaw in Parkinson disease and in drug-induced extrapyramidal symptoms.

Every one of those anticholinergic effects is predictable from the table in Section 1, and so is the danger. Anticholinergic burden is the additive load of every drug with antimuscarinic activity a patient is taking — and it is not just the drugs above. Many antihistamines, tricyclic antidepressants, and antipsychotics carry hidden antimuscarinic activity. Stack three or four in an 80-year-old and you get confusion, falls, retention, and constipation that get misread as “dementia” or “just getting older.” Counting up anticholinergic burden on a medication list is one of the highest-yield things a nurse can do for an older patient.

Section 5Neuromuscular blockers

A different synapse, a much higher stake

Neuromuscular blocking agents (NMBAs) are technically somatic, not autonomic, but they belong here because they work on the same molecule — acetylcholine — at a nicotinic receptor. They act at the neuromuscular junction, on the N_M receptor of the skeletal muscle end plate, to produce flaccid paralysis for intubation, surgery, and mechanical ventilation.

Three facts must be absolutely fixed before any of the pharmacology matters. These drugs do not sedate. They do not relieve pain. They do not cause amnesia. A paralyzed patient who has not been given adequate sedation and analgesia is fully awake, feeling everything, and unable to move a finger or make a sound to tell you — among the most terrifying experiences medicine can inflict. NMBAs are high-alert medications, stored separately with auxiliary warnings, and they are never given except where the airway can be secured and mechanical ventilation is immediately available. Paralysis always includes the diaphragm. The nurse's obligations are non-negotiable: confirm deep sedation and analgesia are running first, never allow paralysis without a secured airway, provide eye lubrication and taping (the blink is gone), reposition to prevent pressure injury, provide DVT prophylaxis, and keep talking to the patient because hearing is unaffected.

Depolarizing blockade: succinylcholine

Succinylcholine is the only depolarizing agent in common use, and it works by an odd trick: it is an agonist that paralyzes. It binds the nicotinic receptor and opens it — producing an initial wave of disorganized contractions called fasciculations — but unlike acetylcholine it is not broken down by acetylcholinesterase, so it sits on the receptor and holds the end plate depolarized. A depolarized membrane cannot fire again, and the muscle goes flaccid. Onset is under a minute and the effect wears off in five to ten minutes as plasma pseudocholinesterase metabolizes it, which is exactly the profile you want for rapid sequence intubation.

Three hazards define it. First, there is no reversal agent — you wait it out, ventilating the patient the whole time. Giving an AChE inhibitor would only prolong the block by adding more acetylcholine to an already depolarized plate. Second, the depolarization leaks potassium out of every muscle cell in the body, so succinylcholine is contraindicated in patients with existing hyperkalemia, and in burns, crush injury, denervation, spinal cord injury, or prolonged immobility beyond roughly 24–48 hours, where receptor upregulation can produce a potassium surge large enough to cause cardiac arrest. The same mechanism drives the drug's boxed warning: a child or adolescent with an undiagnosed skeletal muscle myopathy — Duchenne muscular dystrophy above all, often not yet apparent in a young boy — can develop hyperkalemic rhabdomyolysis, ventricular dysrhythmia, and cardiac arrest after a single dose. For that reason succinylcholine is not used for routine or elective intubation in children and adolescents; its pediatric use is reserved for emergency airway control. Third, succinylcholine is a leading trigger of malignant hyperthermia in genetically susceptible patients — a hypermetabolic crisis whose earliest reliable sign is a rising end-tidal CO₂, with masseter rigidity, tachycardia, and a late temperature spike. The antidote is dantrolene, which must be immediately available wherever the drug is used. A rare patient with atypical pseudocholinesterase stays paralyzed for hours; the management is simply continued ventilation and sedation until it clears.

Non-depolarizing blockade: the “-curium” and “-curonium” drugs

Rocuronium, vecuronium, cisatracurium, and atracurium take the more intuitive route: they are competitive antagonists that occupy the nicotinic receptor without activating it, so acetylcholine cannot get in. There are no fasciculations, onset is slower (though a large dose of rocuronium approaches succinylcholine's speed), and the block lasts much longer. Because the blockade is competitive, it is reversible — flood the junction with enough acetylcholine and it outcompetes the drug. That is what neostigmine does, and it is always given with glycopyrrolate or atropine, because the extra acetylcholine spilling onto muscarinic receptors would otherwise cause bradycardia and drowning secretions. Sugammadex is the newer and cleaner option: it physically encapsulates rocuronium or vecuronium molecules and drags them out of circulation, reversing even deep blockade in minutes without touching the cholinergic system at all. Cisatracurium earns a special mention because it degrades spontaneously in plasma (Hofmann elimination) rather than through liver or kidney, making it the agent of choice in organ failure.

Monitoring is where the nurse lives with these drugs. Train-of-four (TOF) peripheral nerve stimulation delivers four small shocks and counts the resulting twitches; the depth of blockade is titrated to a target, commonly one to two twitches out of four for a continuous infusion. Fewer twitches means deeper block and a longer, harder recovery. Before extubation, recovery is judged by objective criteria — sustained head lift, adequate tidal volume and grip, the ability to protect the airway — and never by appearance alone, because a patient can look awake while still too weak to breathe. Remember too that several common drugs deepen and prolong blockade: aminoglycoside antibiotics, magnesium sulfate, calcium channel blockers, inhaled anesthetics, hypothermia, and acidosis. A postpartum patient on a magnesium drip is a different pharmacologic problem than the same patient without it.

ReferenceWords worth knowing

A quick reference for the terms in this chapter — skim it now, or circle back whenever a word trips you up.

Acetylcholine (ACh)

The transmitter at all autonomic ganglia, at parasympathetic organ synapses, at sympathetic sweat glands, and at the skeletal muscle end plate.

Acetylcholinesterase (AChE)

The enzyme that breaks down acetylcholine in the synapse; blocking it amplifies and prolongs cholinergic signaling.

Alpha-1 receptor

Receptor on vascular smooth muscle, pupil, and bladder neck; stimulation causes vasoconstriction, mydriasis, and urinary retention.

Alpha-2 receptor

Presynaptic and central receptor whose stimulation reduces norepinephrine release, lowering blood pressure (clonidine).

Anticholinergic (antimuscarinic)

A drug that blocks muscarinic receptors, producing dry mouth, blurred vision, tachycardia, constipation, retention, and confusion.

Anticholinergic burden

The cumulative antimuscarinic load from all of a patient's drugs, a major cause of confusion and falls in older adults.

Autonomic nervous system

The involuntary nervous system controlling heart, vessels, airways, gut, bladder, glands, and pupils.

Beta-1 receptor

Cardiac (“one heart”) and renal receptor; stimulation raises rate, contractility, conduction, and renin release.

Beta-2 receptor

Pulmonary (“two lungs”), vascular, uterine, and hepatic receptor; stimulation causes bronchodilation and vasodilation.

Cardioselective

Describing a beta blocker that acts mainly on beta-1 and largely spares beta-2; selectivity is relative and lost at high doses.

Cholinergic crisis

Excess cholinergic stimulation causing SLUDGE plus muscle weakness and respiratory failure; the mirror trap of myasthenic crisis.

Depolarizing blocker

A neuromuscular blocker (succinylcholine) that activates and then holds the end plate depolarized; has no reversal agent.

Extravasation

Leakage of a vesicant such as a vasopressor into surrounding tissue, risking necrosis; reversed locally with phentolamine.

Malignant hyperthermia

A hypermetabolic crisis triggered by succinylcholine or volatile anesthetics; earliest sign is rising end-tidal CO₂; antidote is dantrolene.

Muscarinic receptor

The acetylcholine receptor on parasympathetic target organs; stimulation produces the SLUDGE picture.

Neuromuscular blocking agent (NMBA)

A high-alert drug that paralyzes skeletal muscle at the nicotinic end plate without sedating or relieving pain.

Nicotinic receptor

The acetylcholine receptor at autonomic ganglia (N_N) and at the skeletal muscle end plate (N_M).

Non-depolarizing blocker

A competitive nicotinic antagonist (rocuronium, vecuronium, cisatracurium) reversible with neostigmine or sugammadex.

Parasympathetic division

The rest-and-digest division; acetylcholine on muscarinic receptors slows the heart and activates gut and bladder.

SLUDGE

Salivation, Lacrimation, Urination, Defecation, GI upset, Emesis — the muscarinic (cholinergic excess) picture.

Sympathetic division

The fight-or-flight division; mostly norepinephrine on adrenergic receptors, plus adrenal epinephrine release.

Train-of-four (TOF)

Peripheral nerve stimulation delivering four impulses to gauge the depth of neuromuscular blockade by counting twitches.