AEROSOL THERAPY
Aerosols are small liquid droplets or particles of solids that
are suspended in the air and can be inhaled into the lungs. Distribution of aerosols
is dependent on intrinsic characteristics such as the mass and size of the particles.
Larger particles (>4 µm) tend to deposit in the large central airways,
whereas smaller particles (<0.5 µm) tend to deposit distally in the alveoli
and small airways. Extrinsic factors such as patient technique also significantly
influence the distribution of aerosolized medications.
Aerosol Delivery Systems
There are three types of systems by which aerosolized medications
can be delivered: nebulizers, metered-dose inhalers (MDIs) and dry powder inhalers
(DPIs). Small-volume nebulizers are the most frequently used nebulizers in clinical
practice. These rely on an external gas source at a flow of 5 to 10 L/minute to
aerosolize the solution. In general, a liquid volume of 4 to 6 mL is gradually aerosolized
over several minutes while being inhaled by the patient through a mouth piece or
facemask.
Figure 75-3
Metered-dose inhaler (MDI). The medication is stored
in the canister along with other ingredients (see text). The MDI is actuated by
pressing down on the canister. The metering chamber dispenses the appropriate amount
of medication, which is then propelled into the mouthpiece.
MDIs are also used to deliver aerosolized medications. These
devices are pressurized canisters that contain the drug (in suspension or solution),
a propellant (traditionally a chlorofluorocarbon [CFC]), a surfactant, preservatives,
flavoring agents and dispersal agents. As the MDI is actuated, the metering device
delivers the appropriate dose of aerosolized medication ( Fig.
75-3
). Proper patient technique is the most important determinant of adequate
drug delivery when using an MDI. Improper technique can result in inappropriate
delivery of the drug to the oropharynx. Accessory devices, such as spacers or holding
chambers, can improve aerosol delivery from an MDI to the lungs ( Fig.
75-4
). Proper technique for MDI use with and without a spacer is shown
in Table 75-3
.
An environmental ban on CFCs has led to the use of other propellants,
such as hydrofluoroalkanes (HFAs), and to the increased use of DPIs. There is a
variety of DPI
Figure 75-4
Spacer apparatus. The metered dose inhaler (MDI) is
actuated and the aerosolized medication flows from the MDI into the spacer. The
aerosol is then drawn into the lungs as the patient inspires from the spacer mouthpiece.
TABLE 75-3 -- Metered-dose inhaler (MDI) technique
|
Without Accessory Device |
With Spacer or Holding Chamber |
|
1. Warm MDI to body temperature. |
1. Warm MDI to body temperature. |
|
2. Open mouth piece and check for loose objects that may be aspirated. |
2. Assemble apparatus and check for loose objects that may be
aspirated. |
|
3. Shake canister vigorously. |
3. Shake canister vigorously. |
|
4. Hold canister upright 4 cm away and aimed at the mouth. |
4. Hold MDI upright. |
|
5. Breath out normally. |
5. Breath out normally. |
|
6. Breath in slowly while actuating the MDI at the beginning
of inspiration. |
6. Place accessory mouthpiece in mouth or place mask over face
and nose. |
|
7. Continue to breathe to total lung capacity, then hold breath
for 10 seconds. |
7. Breath in slowly while actuating the MDI at the beginning
of inspiration. |
|
8. Wait at least 15 seconds between actuations. |
8. Continue to breathe to total lung capacity, then hold breath
for 10 seconds. |
|
9. Wait at least 15 seconds between actuations. |
delivery devices, but basically these are simple, handheld devices in which a pre-specified
amount of dry powder medication is first released into the device and then delivered
to the lungs after breath actuation. Attractive features of DPIs include breath
actuation, which requires less timed coordination than do MDIs, the absence of CFCs,
and the higher fraction of particles in the respirable range. However, the breath
actuation feature requires high inspiratory flows and a certain degree of coordination
that may be difficult for some patients, because accidental exhalation into the device
after the powder has been released results in dispersal of the medication.
