Chapter 35: Respiratory Flashcards
Histamine is a substance that performs many functions
It is involved in nerve impulse transmission in the central nervous system (CNS), dilation of capillaries, contraction of smooth muscle, stimulation of gastric secretion, and acceleration of the heart rate.
Histamine 1 (H1) receptors
mediate smooth muscle contraction
and dilation of capillaries
histamine 2 (H2) receptors
mediate acceleration of the heart rate and gastric acid secretion.
The release of excessive amounts of histamine can lead to anaphylaxis
and severe allergic symptoms and may result in any or
all of the following physiologic changes
• Constriction of smooth muscle, especially in the stomach
and lungs
• Increase in body secretions
• Vasodilatation and increased capillary permeability, which results in the movement of fluid out of the blood vessels and into the tissues and thus causes a drop in blood pressure an edema
Histamine is a major inflammatory
mediator in many allergic disorders
such as allergic rhinitis
(e.g., hay fever and mold, dust allergies), anaphylaxis, angioedema,
drug fevers, insect bite reactions, and urticaria (itching).
H1 antagonists include drugs such
as diphenhydramine (Benadryl), chlorpheniramine (generic), fexofenadine (Allegra), loratadine (Claritin), and cetirizine (Zyrtec). They are of greatest value in the treatment of nasal allergies, particularly seasonal hay fever. They are also given to relieve the symptoms of the common cold, such as sneezing and runny nose. In this regard they are palliative, not curative; that is, they can help alleviate the symptoms of a cold but can do nothing to destroy the virus causing it.
primary anticholinergic actions of antihistamines
They also affect the secretions
of the lacrimal, salivary, and respiratory mucosal glands, which
are the primary anticholinergic actions of antihistamines
The binding of H1 blockers to these receptors prevents
the adverse consequences of histamine binding:
vasodilation;
increased GI, respiratory, salivary, and lacrimal secretions; and
increased capillary permeability with resultant edema
The primary causes of nasal congestion
are allergies and URIs, especially the common cold. There
are three separate groups of nasal decongestants:
adrenergics (sympathomimetics), which are the largest group; anticholinergics (parasympatholytics), which are somewhat less commonly used; and selected topical corticosteroids (intranasal steroids
Contraindications to the use of decongestants include drug
allergy. Adrenergic drugs are contraindicated
in narrow-angle glaucoma, uncontrolled cardiovascular disease, hypertension, diabetes, hyperthyroidism, and prostatitis. They are also contraindicated
in situations in which the patient is unable to close
his or her eyes (such as after a cerebrovascular accident), as well as in patients with a history of cerebrovascular accident or transient ischemic attacks, cerebral arteriosclerosis, long-standing
asthma, benign prostatic hyperplasia, or diabetes.
Systemic sympathomimetic drugs and sympathomimetic
nasal decongestants are likely to cause drug toxicity when
given together.
Monoamine oxidase inhibitors (MAOIs) may
result in additive pressor effects (e.g., raising of the blood pressure)
when given with sympathomimetic nasal decongestants.
Other interacting drugs include methyldopa and urinary acidifiers
and alkalinizers.
During a cold, the blood vessels that surround
the nasal sinus are dilated and engorged with plasma,
white blood cells, mast cells, histamines, and many other blood
components that are involved in fighting infections of the respiratory
tract.
This swelling, or dilation, blocks the nasal passages,
which results in nasal congestion.
When these drugs (decongestion) are administered
intranasally, they cause dilated arterioles to constrict,
which reduces nasal blood flow and congestion
Dextromethorphan
is the most widely used of the nonopioid
antitussive drugs and is a derivative of the synthetic opioid levorphanol.
Benzonatate is another nonopioid antitussive.
Benzonatate suppresses the cough reflex by anesthetizing
(numbing) the stretch receptor cells in the respiratory tract,
which prevents reflex stimulation of the medullary cough center.
antitussive
Additional contraindications and cautions include the
following:
• Benzonatate: no known contraindications but cautious use in those with productive cough
• Dextromethorphan: contraindications of hyperthyroidism, advanced cardiac and vessel disease, hypertension, glaucoma,
and use of MAOIs within the past 14 days
• Diphenhydramine: see antihistamines
• Codeine and hydrocodone: contraindicated with alcohol use; cautious use required with CNS depression, anoxia, high
serum levels of carbon dioxide (hypercapnia), and respiratory depression; increased intracranial pressure, impaired renal function, liver diseases, benign prostatic hyperplasia, Addison’s disease, and chronic obstructive pulmonary disease
