Respiratory: Antitussives Flashcards
Types of Respiratory Drugs:
Bronchodilators: Beta2-Receptor Agonists:
Basic Understanding:
++++EXAMPLES AND ADVERSE REACTIONS SECTIONS ARE MISSING
Antitussives are widely used by patients to self-treat coughs.
It is essential for the health-care provider to educate the patient on the useful physiological mechanism a cough provides
by clearing the airway of secretions and foreign material.
Therefore, a cough should not be suppressed if it is protecting
the airway. There are times when an antitussive is necessary
to provide rest or sleep. The cough reflex is complicated, involving both the CNS and peripheral nervous system, as well
as the smooth muscle of the bronchial tree. The drugs that
can affect this complex mechanism are diverse, ranging from
bronchodilators to drugs that act centrally or peripherally to
suppress cough. This section discusses the nonprescription
antitussives dextromethorphan and benzonatate. Codeine,
which is also used as an antitussive, is covered in Chapter 16
with the other opioids. Dosing of codeine for antitussive use
is included here.
Types of Respiratory Drugs:
Bronchodilators: B2RAs:
Pharmacodynamics:
Cough results when sensory stimuli or irritation in the
bronchial tree stimulates cough receptors, probably located
in the bronchial smooth muscle. A message is sent via the afferent nervous system to the cough centers in the medulla.
Antitussives work either centrally or peripherally to affect the
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cough. The exact mechanism of action of antitussives is
poorly understood. Dextromethorphan, the d-isomer of the
codeine analogue levorphanol, acts centrally in the cough
center in the medulla to elevate the threshold for coughing.
Codeine works as an antitussive through direct action on receptors in the cough center of the medulla, at lower doses
than is required for analgesia. Benzonatate (Tessalon) is related to tetracaine and is thought to anesthetize the stretch
receptors in the respiratory passages, thereby decreasing their
activity and calming the cough peripherally at its source.
Types of Respiratory Drugs:
Bronchodilators: B2RAs:
Pharmacokinetics: Absorption and Distribution:
Dextromethorphan, codeine, and benzonatate are absorbed
well from the GI tract. The distribution of dextromethorphan
and benzonatate is unknown. Codeine is 7% protein bound
and widely distributed, including in the CNS. Codeine freely
crosses the placenta and is distributed into breast milk.
Types of Respiratory Drugs:
Bronchodilators: B2RAs:
Pharmacokinetics: Metabolism and Excretion:
Dextromethorphan is extensively metabolized by the liver and
excreted in the urine, mostly as metabolites. Codeine is metabolized in the liver by glucuronidation into morphine and
norcodeine. The metabolism of codeine into morphine is
mediated by CYP450 2D6. Codeine is eliminated in the urine
as unchanged drug, norcodeine, and free and conjugated
morphine. The metabolism and excretion of benzonatate
is unknown. See Table 17-16 for the pharmacokinetics of
selected preparations.
Types of Respiratory Drugs:
Bronchodilators: B2RAs:
Pharmacotherapeutics: Precautions and Contraindications:
Antitussives are not to be used for persistent or chronic cough
caused by smoking, asthma, or emphysema. In asthma, antitussives may impair expectoration and thus cause increased
airway resistance. Expectorants must not be used by patients
with excessive respiratory secretions for the same reason. Patients must be cautioned not to self-medicate their cough for
long periods (more than 7 days) without seeking the care of
their health-care provider. If high fever or rash accompanies
a cough, patients must be seen by their health-care provider.
Benzonatate is contraindicated for patients allergic to
tetracaine, procaine, or related compounds.
Dextromethorphan, codeine, and benzonatate can cause
drowsiness, dizziness, nausea, and GI upset. In addition,
patients taking benzonatate may experience headache, constipation, pruritus, skin eruptions, a sensation of burning
eyes, a vague “chilly” sensation, chest numbness, and hypersensitivity. Patients with hepatic function impairment should
be monitored if dextromethorphan is prescribed because
metabolism of the drug may be impaired. The metabolism of
codeine can be affected by deficiency of CYP450D or by medications that may inhibit CYP2D6.
Codeine may cause dependence and should be used with
caution in a patient with a history of substance abuse.
Although dextromethorphan is not addictive, there have been
reports of abuse of dextromethorphan-containing products,
especially among teenagers. The FDA issued a Talk Paper in
2005 to warn the public, providers, and law enforcement
of the potential for dextromethorphan abuse and has begun
exploring whether dextromethorphan should become a
scheduled drug.
Codeine causes decreased gastric motility and therefore
should be used cautiously by patients with GI obstruction,
ileus, or preexisting constipation. Patients with acute ulcerative
colitis may be more sensitive to the constipating effects of
codeine.
Dextromethorphan and codeine are Pregnancy Category C,
but no teratogenic effects have been demonstrated. Codeine
should be used with caution near term in pregnancy. Benzonatate is Pregnancy Category C and is to be given to pregnant women only if clearly needed. There are better-studied
choices for antitussives in pregnancy, such as dextromethorphan or short-term codeine.
Types of Respiratory Drugs:
Bronchodilators: B2RAs:
Pharmacotherapeutics: Drug Interactions:
Use of antitussives with any CNS depressant may cause increased CNS depression. Concurrent use of dextromethorphan and MAOIs is contraindicated.
Codeine should be used with caution concurrently with
medications that are metabolized by CYP2D6 isoenzymes.
Quinidine has been shown to interfere with the metabolism
of codeine. Other medications that inhibit CYP2D6 are amiodarone (Cordarone), tricyclic antidepressants, metoclopramide (Reglan), selective serotonin reuptake inhibitors
(SSRIs), cimetidine (Tagamet), thioridazine (Mellaril),
propafenone (Rythmol), and haloperidol (Haldol).
Drugs interactions are shown in Table 17-17.
Types of Respiratory Drugs:
Bronchodilators: B2RAs:
Pharmacotherapeutics: Rational Drug Selection:
Patients may self-medicate their cough with a nonprescription form of dextromethorphan, and the health-care provider
has little to do with the choice of the medication. (Advertising
has the largest impact.) The health-care provider becomes
involved when the patient asks for a recommended formula
or if nonprescription products are not effective.
Cost
Although nonprescription dextromethorphan is less expensive than benzonatate- or codeine-containing preparations,
patients with good prescriptive coverage may actually pay less
out of pocket for the prescription product. Cost must therefore be evaluated on an individual basis.
Effectiveness
Patients might feel that a prescription medication is more
effective than nonprescription, but dextromethorphan has
been found to be as effective as codeine in the treatment of
cough.
Types of Respiratory Drugs:
Bronchodilators: B2RAs:
Pharmacotherapeutics: Monitoring
There is no specific monitoring required when prescribing
antitussive medications.
Types of Respiratory Drugs:
Bronchodilators: B2RAs:
Pharmacotherapeutics:
Patient Education:
Patient education centers on proper administration, adverse
reactions, and drug interactions with the antitussive agents.
Administration
Patients should be aware of the proper dosing of antitussive
medication. When they are self-medicating, they are often
not following the recommended dosing schedule. The healthcare provider needs to determine if the patient is taking the
proper amount, measured with a calibrated measuring spoon
(not a flatware teaspoon or tablespoon), and spacing the
dosage appropriately. The medications may be taken without
regard to food but may be better tolerated if taken with food
or milk.
Adverse Reactions
CNS depression is the major concern. Some of the antitussives are in alcohol-containing syrup form, and others may
cause sedation. Driving or operating hazardous machinery
should be undertaken with caution, and not at all if the
patient is sensitive to the sedating effects of the antitussives.
Patients should also be aware that if they have long-term cough
(occurring for more than 7 days) or cough accompanied by
fever, they should be seen by their health-care provider.
Patients concurrently taking MAOIs should not take
antitussives. Antitussives should be taken with caution if
the patient is concurrently taking any other CNS-sedating
medications.
Lifestyle Management
The patient with a cough should be encouraged to increase
fluid intake to improve the viscosity of the respiratory secretions. The patient should refrain from smoking and, if possible, stop smoking. Avoidance of respiratory irritants and
people with respiratory infections will decrease the incidence
of cough.
Table 17-18 presents available dosage forms.
