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Summary |   |
Respiratory pharmacology consists of medications used to treat the pathological triad of bronchospasm, airway inflammation, and retained secretions. Drugs used for these are bronchodilators, decongestants, corticosteroids, and mucokinetic/mucolytic agents. There are several routes of administration with the aerosol route being the most often used by the RCP. Somc of the advantages of aerosol therapy include:
- rapid therapeutic effect
- a small total dose may be given
- topical administration minimizes systemic side-effects
The disadvantages of aerosol therapy consist of:
- underdosage or overdosage
- very little medication actually deposited in the lung
- airway irritation
- systemic absorption through oropharyngeal deposition
Excluding mainstream nebulization of large volumes of H20, there are 4 methods of aerosol medication delivery. They are:
- metered-dose inhaler (MDI)
- dry powder inhaler (DPI)
- small volume nebulizer (SVN)
- IPPB
The patient must be able to take a deep coordinated breath for the first three. IPPB is reserved for the patient who cannot spontaneously hyperinflate their lungs. Many conditions require modification of the recommended dose. Conditions such as liver dysfunction, kidney dysfunction, mechanical ventilation, emaciation, obesity, very young or very old patients require appropriate adjustment of dosage.
Bronchodilation is most often achieved through SNS stimulation. SNS stimulation results in conversion of ATP to cAMP through activation of the enzyme adenylate cyclase. This leads to airway smooth muscle relaxation. Beta adrenergic agonists increase the production of cAMP through the above mechanism. The enzyme phosphodiesterase inactivates cAMP thereby removing its bronchodilating influence.
Theophylline may work by inhibiting phosphodiesterase but this is unproven. Another mechanism to achieve bronchodilation is to block PNS stimulation of the airway. PNS stimulation leads to bronchoconstriction. Antimuscarinics are used to block PNS stimulation thereby leading to bronchodilation.
Complications of the Beta adrenergic agonists include tachycardia, skeletal muscle tremor and tachyphylaxis. Tachycardia is a result of excessive Beta1 stimulation. A worsening of V/Q relationships is also possible with these drugs. Beta2 stimulation can result in both bronchodilation and vasodilation. If the circulatory response exceeds the airway response, hypoxia may ensue. Complications of theophylline include tachycardia and tremors. It also can cause nausea and headache. Therapeutic levels should be maintained between 10-20 mcg/ml.
Decongestants and corticosteroids are not "bronchodilators" but their effects can lead to an increase in lumen size. Decongestants, through stimulation of receptors, cause vasoconstriction and decrease fluid in the airway. The resulting decrease in airway wall thickness and fluid in the lumen have the same effect as a bronchodilator.
Corticosteroids can have the same result through their anti-inflammatory properties. They also help prevent episodes of bronchospasm through inhibition of the allergic response to irritants. Chronic or excessive steroid use can result in adrenal insufficiency, cushinoid effects, decreased resistance to infection and osteoporosis. Acute complications include fluid and electrolyte imbalances. If steroids prove successful, cromolyn sodium may be of benefit to the patient. It is a prophylactic drug only, preventing bronchospasm through inhibition of mast cell degranulation.
Mucokinetic/mucolytic drugs aid secretion removal by one or more of several mechanisms. They dilute mucus, replenish the sol layer, stimulate bronchial glands, stimulate cilia, or chemically destroy components found in mucus. The safest and most common mucokinctic in water. Normal saline solutions also are very common, particularly for lavage purposes. A common mucolytic is N-acetylcysteine. Its action is to rupture disulfide bonds of mucus making it less tenacious and viscous. Bronchospasm is a possible complication of its use. Complications of the mucokinetic/mucolytic agents are overhydration, bronchospasm and tissue irritation.
Antibiotics are rarely aerosolized. Several (carbenicillin, streptomycin, gentamicin) have been aerosolized with mixed results. Antibiotic lavage solutions are administered during a bronchoscopy for severe infections. Cystic fibrosis is a condition that may benefit from this. The penicillins and aminoglycosides are common antibiotics. Complications of various antibiotics include: hypersensitivity reactions, nausea, vomiting, diarrhea, inner ear problems, renal problems, and rarely, a blockade of the phrenic nerve.