Medical Pharmacology Chapter 6:  Autonomic Pharmacology: Cholinergic Drugs

• Preoperative Medication

  • Primary current therapeutic rationale:

    • Sedation

    • Antisialagogue

  • Historical: IM atropine was used prior to anesthesia induction to limit excessive salivary secretion and reduce vagal reflex influence on the heart

    • Current inhaled/injected anesthetic agents are not consistently associated with these effects

    • Anticholinergic drugs are not mandatory as part of preoperative medication

  • Gastrointestinal effects:

    • Usual anticholinergic drug doses for preoperative medication (and adults) does not affect either gastric volume or pH

  •  Special considerations:

    • Patients with glaucoma:

      • Mydriatic effects of scopolamine > atropine; caution with scopolamine use in patients with glaucoma

      • Atropine + anticholinesterase causes little/no change in pupil size, therefore appearing apparently safe

      • Glycopyrrolate (Robinul): least effect on pupil size of all anticholinergic drugs used in preoperative medication

    • Parturients:

      • Atropine and scopolamine both cross the placenta, although the fetal heart rate is not significantly changed following atropine or glycopyrrolate IV administration.

• Sedation

  • Scopolamine is preferred when sedation is the objective for including anticholinergic drug in preoperative medication

  • Atropine:

    • Less sedation compared to scopolamine

    • Associated with increased incidence of memory deficit associated with anesthesia, compared to glycopyrrolate

    • Delayed arousal in the first 30 minutes after anesthesia if atropine-neostigmine mixtures were used to antagonize nondepolarizing neuromuscular-blocking drug effects {compared to glycopyrrolate-neostigmine mixtures}

  • Scopolamine enhances significantly sedative effects of other drugs administered at the same time, i.e. benzodiazepines and opioids

  • Scopolamine plus morphine: reliable preanesthetic sedation

  • Glycopyrrolate (Robinul): does not readily cross the blood-brain barrier; no sedative effects

  • CNS effects of anticholinergic drugs -- particularly scopolamine:

    •  Restlessness or somnolence

      •  Such symptoms more likely to occur in the elderly

    •  Inhaled anesthetics may potentiate anticholinergic CNS drug effects®increased postoperative restlessness/somnolence incidence 

• Treatment of Reflex-Mediated Bradycardia

  • Anticholinergic drugs: drugs of choice for management of intraoperative bradycardia --especially if due to increased vagal tone

   

First Degree Heart Block

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• Third Degree (Complete) Block

  • Atropine increases heart rate (by blocking acetylcholine effects on the SA node)

  • Young adults are most sensitive to atropine effects on heart rate {higher vagal tone} compared to infants or elderly patients

    •  In the elderly: atropine may have no effect on heart rate

  • During anesthesia with a volatile agent, atropine dosage required to raise the heart rate may be less than that required in an awake patient -- suggesting vagal center depression during anesthesia

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• Halothane (Fluothane) (like opioids) probably increases central vagal tone, explaining why atropine administration is more likely to result in an increase in heart rate compared to patients anesthetize with enflurane (Ethrane)

Taylor, P. Agents Acting at the Neuromuscular Junction and Autonomic Ganglia In, Goodman and Gillman's The Pharmacologial Basis of Therapeutics,(Hardman, J.G, Limbird, L.E, Molinoff, P.B., Ruddon, R.W, and Gilman, A.G.,eds) The McGraw-Hill Companies, Inc.,1996, pp.193-195. Adapted from Table 9-3.