Medical Pharmacology Chapter 12: Anxiolytics and Sedative-Hypnotics
Preoperative Medication: Sedative Hypnotics and Other Agents and Issues
Evaluated on a case-by-case basis, anticholinergics may be included in some preanesthetic medication protocols
Factors which influence the likelihood that in anticholinergic would be included are
(1) the need for antisialagogue effects;
(2) the facilitation of sedative/amnestic effects
(3) the the need to reduce/eliminate reflex bradycardia (Reflex bradycardia in children may occur subsequent to laryngeal stimulation, laryngospasm, or hypoxia. Prophylactic use of atropine or glycopyrrolate (Robinul) [oral or intravenous] may prevent this reflex action)
Compared to other drugs, anticholinergics should not be considered effective in decreasing gastric fluid volume or increasing gastric fluid pH (patients had increased risk for aspiration pneumonitis can be managed using other drugs, e.g. prokinetic agents (metoclopramide (Reglan)), antacids, H2 receptor antagonists.
Most currently used inhaled/intravenous anesthetics do not cause significant salivation. Therefore, routine use of anticholinergic (antimuscarinic) drugs which cause "dry mouth", i.e. antisialagogue effects, would not be needed.
Ketamine (Ketalar) is an exception in that its use may provoke excessive salivation.
However, under some circumstances, antimuscarinic agents are appropriate for antisialagogue purposes.
For instance, preoperative anticholinergics may be helpful when tracheal tube is in place to decrease oral secretions during general anesthesia
Antisialagogue effects are helpful in procedures involving bronchoscopies or intra-oral surgeries.
When local anesthetics are used, antisialagogue effects limit the dilution of the anesthetic by excessive secretion.
Vagolytic effects of anticholinergics:
Acetylcholine exhibits normally a negative chronotropic effect on the heart (reduces heart rate in part by increasing K+ channel conductance at the SA node; increased K+ conductance will tend to hyperpolarize the membrane, i.e. more negative, requiring increased time to threshold and therefore reduced rate)
Generally, heart rate is controlled by the autonomic nervous system with the parasympathetic (cholinergic) component dominant. Therefore, there is a "tonic" level of autonomic inhibition of heart rate. Usually, the vagolytic activity of anticholinergic drugs account for an increase in heart rate or a block of effects that would otherwise decrease heart rate causing bradycardia.
Intraoperative factors that can promote bradycardia:
Traction on extraocular muscles or abdominal viscera
Carotid sinus stimulation
Following multiple doses of succinylcholine (Anectine)
Atropine or glycopyrrolate (Robinul), by IV administration appear equally effective in blunting bradycardic responses to multiple succinylcholine (Anectine) doses.
Route of Administration:
Intramuscular: relatively unreliable
Probably better choice: by IV administration just prior to surgery and/or anticipated stimulus which would cause the bradycardic response
Specific anticholinergic (antimuscarinic) drugs:
Three common anticholinergic drugs: scopolamine, atropine, and glycopyrrolate (Robinul).
Comparisons between these agents:
Scopolamine is a more potent (about threefold) a antisialagogue compared to atropine and scopolamine is noted to exhibit significantly enhanced CNS actions such as sedation.
Scopolamine is less likely to increase heart rate, compared atropine, along with providing relatively greater sedation and amnesia.
Scopolamine is more effective than atropine but probably less effective than commonly used benzodiazepines, e.g. midazolam (Versed), lorazepam (Ativan), or diazepam (Valium)
Scopolamine's amnestic effect is additive to that obtained with benzodiazepines
From the patients point of view, the combination scopolamine + morphine was "better" as compared to morphine + atropine.
Scopolamine may be the drug choice when both antisialagogue effects and sedation are desired
By contrast, glycopyrrolate (Robinul) is about twice as effective as an antisialagogue compared to atropine, does not exhibit CNS actions, and has an extended duration of action compared atropine. When sedation is not required but antisialagogue effects are, glycopyrrolate (Robinul) may be a good choice
Side effects of anticholinergic agents:
"Central anticholinergic syndrome": more likely observe following scopolamine or "high-dose" atropine
Older patients more susceptible
Anticholinergic CNS toxicity may be potentiated by inhalational agents
Treatment: try 1-2 mg physostigmine (Antilirium), by IV administration (rationale: physostigmine (Antilirium), a tertiary-amine anticholinesterase would be expected to gain ready access to the CNS where inhibition of acetylcholinesterase should increase free acetylcholine that could overcome competitive muscarinic receptor blockade caused by the anticholinergic medications.)
Anticholinergic drugs relax the lower esophageal sphincter, in theory promoting reflux and increasing aspiration risk.
However, this effect has not been clinically demonstrated.
Nevertheless, in patients with known reduced lower esophageal sphincter tone, such as patients with hiatal hernia, already present an aspiration risk that may be further increased by the use of anticholinergics.
In theory, mydriasis and cycloplegia which would be induced by anticholinergic agents would appear undesirable in glaucoma patients.
However, the low anticholinergic doses used preoperatively would be unlikely to produce adverse ocular effects.
Atropine and glycopyrrolate (Robinul) would probably be less likely to increase intraocular pressure compared to scopolamine
Usually, patients being treated for glaucoma, take their medication (eyedrops) as usual before surgery with low-dose (anesthesia-dose) anticholinergics used intraoperatively as needed
Pulmonary effects, secondary to reduced vagal activity:
Increase in respiratory dead space. Following anticholinergic agents, and dependent on pre-existing cholinergic tone, dead space may increase by 25%-33%.
Bronchial smooth muscle relaxation occurs with anticholinergic drugs.
This effect is used to advantage in asthma management by administration of ipratropium (Atrovent).
Normally, but variably, there is some bronchial smooth muscle tone maintained by the parasympathetic component of the sympathetic nervous system. Reduction of bronchial smooth muscle tone following anticholinergics is thus expected.
Effect of anticholinergic agents on bronchial secretions:
Secretions would tend to thicken with drying.
As a result of secretion thickening, airway resistance may increase, a concern of particular consequence of patients with cystic fibrosis for example
Anticholinergic drug effects on control of body temperature:
Anticholinergic drugs cause an increase in body temperature due to reduced sweating.
Sweat glands are normally innervated by sympathetic cholinergic fibers (an unusual innervation, given the most sympathetic fibers release norepinephrine not acetylcholine).
Therefore, blockade of muscarinic receptors, by inhibiting sweating, increases body temperature, which may be of clinical concern if the patient is a child with a fever.
1Preoperative Medication in Basis of Anesthesia, 4th Edition, Stoelting, R.K. and Miller, R., p 119- 130, 2000)
Hobbs, W.R, Rall, T.W., and Verdoorn, T.A., Hypnotics and Sedatives; Ethanol 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) TheMcGraw-Hill Companies, Inc., 1996, pp. 364-367.
3Sno E. White The Preoperative Visit and Premedication in Clinical Anesthesia Practice pp. 576-583 (Robert Kirby and Nikolaus Gravenstein, eds) W.B. Saunders Co., Philadelphia, 1994
4John R. Moyers and Carla M. Vincent Preoperative Medication in Clinical Anethesia, 4th edition (Paul G. Barash, Bruce. F. Cullen, Robert K. Stoelting, eds) Lippincott Williams and Wilkins, Philadelphia, PA, 2001
5Gertler, R., Brown, H. C, Mitchell, D.H and Silvius, E.N Dexmedetomidine (Precedex): a novel sedative-analgesic agent, BUMC Proceedings 2001; 14:13-21.