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Bronchodilators |   |
High-Dosage Bronchodilators
Standard recommended dosages for bronchodilators have been established for patients with respiratory problems, but are not necessarily designed for those with severe bronchospasm or in status asthmaticus. Treatment of more severe bronchial problems require increasing both the dosage and frequency of aerosol medications. The greater the degree of the patient's bronchial constriction:
- the greater the amount of bronchodilator needed
- the faster the bronchodilator is degraded so more frequent administration also is necessary
Many of the newer compounds have been developed to allow for stronger and faster acting bronchodilation. Response to a drug is proportional to the drug concentration. As the drug concentration increases, the number of receptors occupied continues to increase until the drug has occupied all available receptors. The rate of response to the drug usually begins to diminish as dosages increase, until a ceiling of maximal effect is reached. At this point, delivering more drug elicits no further therapeutic effect.
The term potency refers to the concentration or dose a drug producing 50% of that drug's maximal response, and maximal response refers to the greatest response that can be produced by the drug, a dose above which no further response can be elicited. When acceptable and maximal dosages are being established for individual drugs, the ratio of the dose which provides relief to 50% of the test subjects, to the dose which is toxic or lethal to 50% of the subjects, is called the therapeutic index (TI).
This TI ratio represents a safety margin for the drug, meaning the smaller the TI, the greater the possibility of crossing from a therapeutic effect to a toxic effect. For example, theophylline is an example of a drug with a fairly narrow therapeutic margin. Toxic side effects from theophylline can be seen in some individuals at dose levels that are very close to dose levels that are therapeutic for other patients.
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Patients with more severe respiratory ailments benefit from the fact that many of the new bronchodilating compounds have a wider TI, allowing caregivers to safely administer maintenance doses that can be up to 8 or 10 times the conventionally recommended dose. In treating these more severe cases, caregivers test several combinations of dosage and frequency to arrive at what seems to be the optimal dosage regimen for each individual. Since patients with artificial airways actually receive far less of the drug being administered than nonintubated patients, they can also benefit from higher than normally recommended dosage regimens. |