Cholinergic Drugs Study Notes

Cholinergic Drugs Study Notes

Objectives

  • Upon chapter completion, students will be equipped to:

    • Briefly review the functions of the autonomic nervous system (ANS) and analyze the impact of the parasympathetic division.

    • Enumerate various drugs classified as cholinergic agonists, also known as parasympathomimetics.

    • Discuss various aspects of cholinergic agonists, including:

    • Mechanisms of action.

    • Therapeutic effects.

    • Indications.

    • Adverse and toxic effects.

    • Drug interactions.

    • Cautions.

    • Contraindications.

    • Dosages and routes of administration.

    • Antidotal management.

    • Formulate a nursing care plan incorporating all phases of the nursing process for patients receiving cholinergic agonists.

Key Terms

  • Acetylcholine: A neurotransmitter that transmits nerve impulses to effector cells in the parasympathetic nervous system (PNS).

  • Acetylcholinesterase: The enzyme responsible for breaking down acetylcholine.

  • Alzheimer's disease: A progressive brain disease leading to confusion, disorientation, and memory loss.

  • Atony: Lack of normal muscle tone.

  • Cholinergic crisis: Severe weakness and respiratory paralysis due to excessive acetylcholine, often seen in myasthenia gravis patients due to medication side effects.

  • Cholinergic receptor: Nerve receptor that responds to acetylcholine stimulation.

  • Miosis: Constriction of the pupil.

  • Muscarinic receptors: Cholinergic receptors found in effector organs (smooth muscle, cardiac muscle, glands) that are post-synaptically located.

  • Nicotinic receptors: Cholinergic receptors found in ganglia of both PNS and sympathetic nervous systems; stimulated by nicotine.

  • Parasympathomimetics: Drugs which mimic the actions of the parasympathetic nervous system.

Overview

  • Cholinergic drugs, agonists, and parasympathomimetics refer to medications activating the PNS.

Parasympathetic Nervous System

  • The PNS is a branch of the ANS counteractive to the sympathetic nervous system (SNS).

  • Acetylcholine functions as the neurotransmitter for the PNS, binding to cholinergic receptors for action mediation.

Cholinergic Receptors

  • Two main types based on location and action:

    • Nicotinic receptors: Located in ganglia and can be activated by nicotine.

    • Muscarinic receptors: Located in post-synaptic effector organs and stimulated by muscarine, found in mushrooms.

Mechanism of Action and Drug Effects

  • When acetylcholine binds to its receptor, it leads to increased cellular permeability to sodium and calcium, depolarizing the cell and stimulating the organ.

  • Effects of cholinergic stimulation can categorize as follows (from Table 20.1):

    • Muscarinic Effects:

    • Bronchi: Increased secretions, constriction.

    • Blood vessels: Dilation.

    • Heart: Decreased rate, blood pressure.

    • Eyes: Miosis, decreased accommodation.

    • GI tract: Increased tone, motility; sphincter relaxation.

    • Genitourinary tract: Increased tone, motility; sphincter relaxation.

    • Glandular secretions: Increased.

    • Nicotinic Effects (primarily at higher doses):

    • Increased contraction of skeletal muscle.

Indications

Direct-Acting Drugs

  • Carbachol, Pilocarpine, Echothiophate: Used topically to manage intraocular pressure in glaucoma patients. Poor oral absorption limits systemic use.

  • Bethanechol: Administered orally for urinary bladder and GI tract stimulation. Indicated postoperatively for urinary retention.

  • Cevimeline: Treats xerostomia (dry mouth) in Sjögren's syndrome.

  • Succinylcholine: A neuromuscular blocker used in general anesthesia.

Indirect-Acting Drugs

  • Donepezil, Galantamine, Rivastigmine: Used in Alzheimer’s disease for increasing acetylcholine concentration in the brain by cholinesterase inhibition.

  • Physostigmine: Antidote for anticholinergic poisoning; important for reversing neuromuscular blockade. Indicated in myasthenia gravis for skeletal muscle stimulation.

Contraindications

  • Known drug allergies, GI or GU obstruction, bradycardia, cardiac impulse defects, hyperthyroidism, epilepsy, hypotension, COPD.

  • In Parkinson’s disease, rivastigmine is anxiously used despite caution.

Adverse Effects

  • Consequences of PNS overstimulation, including:

    • Cardiovascular: Bradycardia/tachycardia, hypotension/hypertension, syncope, conduction abnormalities.

    • Central nervous: Headaches, dizziness, convulsions, ataxia.

    • GI: Cramping, increased secretions, nausea, diarrhea.

    • Respiratory: Increased bronchial secretions, bronchospasm.

    • Other: Increased lacrimation, sweating, salivation, miosis.

Toxicity and Management of Overdose

  • A cholinergic crisis results from overdose, marked by circulatory collapse, hypotension, shock, or cardiac arrest.

  • Initial signs include abdominal cramps, salivation, flushing, nausea, and vomiting. Immediate administration of atropine alleviates symptoms.

  • Other symptoms (falls, orthostatic hypotension, dyspnea) may arise, and epinephrine can manage severe cardiovascular events.

  • SLUDGE acronym: Salivation, Lacrimation, Urinary incontinence, Diarrhea, GI cramps, Emesis.

Drug Profiles

  • Bethanechol (Urecholine):

    • Category: Muscarinic (direct-acting)

    • Dosage: 10-50mg PO, tid-qid.

    • Use: Urinary retention.

  • Donepezil (Aricept):

    • Category: Anticholinesterase (indirect-acting)

    • Dosage: 5-23mg daily as single dose.

    • Use: Alzheimer’s dementia.

  • Memantine (Namenda):

    • Category: NMDA-receptor antagonist

    • Dosage: Titrated up to 28mg/day.

  • Physostigmine (Antilirium):

    • Category: Anticholinesterase (indirect-acting)

    • Dosage: IM/IV: 0.5-2mg q10-30min if necessary.

  • Pyridostigmine (Mestinon):

    • Category: Anticholinesterase (indirect-acting)

    • Dosage: 600mg/day or IV 0.1-0.25 mg/kg.

  • Rivastigmine (Exelon):

    • Treats Parkinson’s dementia.

Nursing Process

Assessment

  • Monitor vital signs and health history for allergies or contraindications before administering cholinergic drugs.

  • Conduct comprehensive assessments of neurological status and renal function, with special attention to any underlying cardiovascular conditions.

Implementation

  • Nursing interventions must maximize therapeutic effects:

    • Encourage patient ambulation post-operatively for GI peristalsis.

    • Administer oral meds timely to enhance effects (e.g., 30 minutes prior to meals for myasthenia gravis).

    • Be prepared with atropine for overdose situations.

    • Educate on non-pharmacological methods to manage symptoms.

Evaluation

  • Track therapeutic outcomes: improvements in myasthenia gravis symptoms, GI activity post-operatively, and effective bladder voiding with bethanechol administration. Monitor also for adverse effects and therapeutic responses over time, remembering typical response times of up to 6 weeks for Alzheimer’s medications.

Evidence-Based Practice

  • Studies suggest a link between exercise (walking) and enhanced cognitive function in Alzheimer’s patients, indicating regular physical activity may stabilize cognitive decline.

Key Points

  • Cholinergic drugs stimulate the PNS and counteract SNS; acetylcholine is the primary neurotransmitter.

  • Key officer in patient education includes ensuring compliance with medication and monitoring for dizziness and other potential adverse effects during treatment.

Recommendations for Patient Education

  • Always follow medication instructions strictly and report adverse effects immediately. Encourage routine education on medications and available community resources.