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Aerosols |   |
Dry powder inhalers (DPIs) consist of a unit dose formulation of a drug in a powder form, dispensed in a small MDI-sized apparatus for administration during inspiration. Because these devices are breath-actuated, using turbulent air flow from the inspiratory effort to power the creation of an aerosol of microfine particles of drug, they don't require the hand-breath coordination needed with MDIs.
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Cromolyn sodium and albuterol are the two primary drugs available in powder form. Cromolyn sodium is dispensed in a device called the Spinhaler, which pokes holes in capsules containing the powdered drug. The albuterol formulation is dispensed in a device called the Rotohaler, which cuts the capsule in half, dropping the powdered drug into a chamber for inhalation. In both cases, a single-dose micronized powder preparation of the drug in a gelatin capsule is inserted into the device prior to inhalation. |
Powder flow properties in DPIs depend on particle size distribution, with very small particles not flowing as well as the larger ones. A third drug, budesonide, is available in a pre-loaded, multi-dose (up to 200 doses) DPI unit called a Tubohaler. The advantages of using DPI devices for drug administration include:
- They are small and portable.
- Brief preparation and administration time.
- Breath-actuation eliminates dependence on patient's hand-breath coordination, inspiratory hold, or head-tilt needed with MDI.
- CFC propellants are not used.
- There is not the cold effect from the freon used in MDIs, eliminating the likelihood of bronchoconstriction or inhibited inspiration.
- Calculation of remaining doses is easy.
The disadvantages encountered when relying on DPIs for drug administration include:
- Limited number of drugs available for DPI delivery at this time.
- Dose inhaled is not as obvious as it is with MDIs, causing patients to distrust that they've received a treatment.
- Potential adverse reaction to lactose or glucose carrier substance.
- Inspiratory flowrates of 60Lpm or higher are needed with the currently available cromolyn and albuterol formulations.
- Capsules must be loaded into the devices prior to use.
Small volume nebulizers (SVNs) are gas powered (pneumatic) and are a common method of aerosol delivery to inpatients, and there are a variety of different SVNs available. Each has specific characteristics, especially in regard to output. These nebulizers fall into two subcategories: mainstream and sidestream. The mainstream nebulizer is one in which the main flow of gas passes directly through the area of nebulization. The sidestream nebulizer is one in which the nebulized particles are injected into the main flow or stream of gas as with IPPB circuits. The main difference, based upon their construction, is that the larger particles tend to rain-out with a sidestream nebulizer.
The advantage of SVN therapy is that it requires very little patient coordination or breath holding, making it ideal for very young patients. It is also indicated for patients in acute distress, or in the presence of reduced inspiratory flows and volumes. Use of SVNs allows modification of drug concentration, and facilitates the aeorsolization of almost any liquid drug.
Another advantage of a SVN is that dose delivery occurs over sixty to ninety breaths, rather than in one or two inhalations. Therefore, a single ineffective breath won't ruin the efficacy of the treatment.
Disadvantages of SVNs include:
- The equipment required for use is expensive and cumbersome.
- Treatment times are lengthy compared to other aerosol devices and routes of administration.
- Contamination is possible with inadequate cleaning.
- A wet, cold spray occurs with mask delivery.
- There is a need for an external power source (electricity or compressed gas).
In a 1990 study comparing the effectiveness of MDIs, DPIs, and SVNs, it was found that approximately the same amount of drug is delivered to the lung, regardless of the type of device used, given that all three devices contain the same loading dose. The clinical response measured by the improvement in FEV1 is also similar among the three devices, although the change with the MDI was statistically significantly greater than with DPI or SVN.
The greater response with the MDI correlates with the greater amount of drug delivered by the the MDI in that study. However, the study concluded, "The amount of bronchodilation obtained is a reflection of the dose of drug given, and not the method of delivery."
Since mainstream nebulizers are normally used for continuous administration of a bland aerosol (H20, normal saline) for airway humidification or secretion mobilization, they are not usually considered as medication delivery systems.
Also in use today is a pneumatic nebulizer that operates on the "Babbington Principle". This is called the hydrosphere or Babbington nebulizer. In this nebulizer, a source of gas enters a hollow sphere which is covered by a thin film of water. The hollow sphere has small ports in it, where the gas escapes to the outside. These ports act as jets. When the gas moves through these ports (jets), a negative pressure is produced and the flow of water is then drawn into the flow of gas producing an aerosol. A baffle is usually used in this system also. The baffle is placed distal to the atomization process in the flow of gas/aerosol. Particle sizes in these units are usually between three and five micron.