Drugs used in respiratory system disorders
Expectorants:
Drugs that liquefy and dilute viscid secretions of respiratory tract either directly on the mucussecreting glands or indirectly by reducing adhesivness of mucus, thereby helping in evacuation
of those secretions. These drugs are given orally and few are given by inhalation or parenterally.
Guaifenesine: Indication :1- in productive cough in combination with ammonium chloride,
antihistamines or dextromethorphan. 2- in equine practice used to induce or mantaine general
anesthesia. Side effects are rare, mild drowsiness or nausea may occur.
Mucolytics:
Acetylcysteine: decrease the viscosity of respiratory secretions by altering the chemical
composition of the mucus through the breakdown of chemical disulfide bounds.
Administration: by nebulization for breakdown thick respiratory mucus. Orally acetylcysteine
used as antidote for paracetamol toxicity. Side effects are few with nebulization. Nausea or
vomiting occur with oral administration.
Antitussives:
Drugs that inhibit or suppress cough (nonproductive). They are classified into two groups:
1- Peripherally acting agents: supress cough receptors in the airways. These drugs are
prepared as drops or lozenges and not practical to use for animal
2- Centrally acting agents: supress cough by depressing the cough center in brain. These are
used in veterinary medicine include:
Butorphanol: is a synthetic opiate, partial ogonist with significant antitussive activity.
Clinical uses: 1- used for relief of chronic nonproductive cough in dogs.
2- used for analgesia and preanesthesia in dogs and cats.
Side effects: sedation and ataxia.
Hydrocodone and codeine: are also narcotic antitussive , opiate agonist used in treatment of
non productive cough. Side effects: sedation and constipation.
Dextromethorphane: chemically similar to codeine. It is non narcotic antitusive, has no
analgesic or addictive properties. Used in treatment of nonproductive cough. Side effects are
rare but drowsiness and GIT upset may occur.
Bronchodilators:
Drugs that cause relaxation of contractile smooth muscle fiber in the wall of bronchioles to
allow to their diameter to return to normal size. There are four types of bronchodilators:
1cholinergic blockers:
bronchospasm
reduce
Ach release or inhibition of Ach-estrase
Ach
respiratory secretion airflow
dyspnea
Cholinergic blockers combine Ach receptors on smooth muscle fibers preventing the
bronchoconstrictive effect of Ach. These drugs include atropine, aminopentamide,
glycopyrrolate and ipratropium which is used in treatment of equine pulmonary obstructive
disease.
2Antihistamines:
Allergic or inflammatory mechanism
release of histamine from mast cell combine H1
bronchospasm
Receptors on
inflammatory response
Smooth muscle
secretion & viscosity
Antihistamines (H-blockers) include H1 and H2 blockers. H1 blockers are competitive
pharmacologic antagonist at H1 receptors.These drugs are more effective when given before
histamine release because they have no effect on histamine release from storage site, so they are
more effctive when used prevnetively (prophylactic).
Clinical uses: H1 blockers are used in the treatment of allergic and respiratory conditions
include heaves in horses, pneumonia in cattle, feline asthma and insect bites.
3β2 adrenergic agonist:
β blockers
β2 receptors in bronchioles
bronchoconstriction
treatment by
β2 agonist
These drugs combine with β2 receptors on smooth muscle fibers of bronchiols result in
relaxation and bronchodilation. Side effects include tachycardia and hypertension because they
may act on β1receptors.
4Methylxanthines:
β2 receptors stimulation
cyclic AMP release
relaxation of smooth muscle
metabolism
fibers in bronchioles
phosphodiestrase
An increase in phosphodiestrase result in bronchoconstriction due to decrease in cyclic
AMP. Methylxanthine tend to promote bronchodilation by inhibiting the inhibitor
(phosphodiestrase) of c-AMP allow to accumulate. Other effects of methylxanthines include
mild stimulation of the heart and respiratory muscle and minor diuresis. Methylxanthine include
aminophylline, theophylline, theobromine and caffeine.
Clinical uses: 1- for bronchoconstriction in respiratory and cardiac conditions.
2- to induce mild heart stimulation
Side effects: GIT upset, CNS stimulation, tachycardia, arrythmia and ataxia.
Decongestants:
Drugs that reduce the congestion of nasal membrane and reducing associated swelling by
vasoconstriction of nasal blood vessels.These drugs include ephidrine, pseudoephdrine and
phenylpropanolamine systemically and oxymetazoline and phenylephrine topically.
Corticosteroids:
Prednisolon, hydrocortisone and beclomethasone are corticosteroids used in treatment of
allergic and respiratory conditions like equine chronic obstructive pulmonary disease and
chronic airway inflammation in dogs and cats. The action of these drugs on bronchi is not fully
understood, their effect are the result of the following actions:
1-Anti-inflammatory
2- Reduction of mucosal edema so increase airfow.
3-modification of immune response and stabilization of mast cells.
4-Increased β2 receptor responsiveness to agonist.
Respiratory stimulants:
Doxapram is general CNS stimulant that used as stimulant for respiration during and after
anesthesia and to speed awaking and restoration of reflexes after anesthesia. In neonatal animals
doxapram is used to stimulate respiration after dystocia or cesarean section.
Side effects: include hypertension, arrythmia, hyperventilation, CNS excitation and seizures
occur in high dose.
Naloxone: is pure competitive antagonist at all opiod receptors used to stimulate respiration in
opiod overdose. Yobine: used to stimulate respiration in xylazine overdose.