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Aerosols

Methods of Aerosol Delivery

Aerosols are produced in respiratory therapy by utilizing devices known as nebulizers. There are a variety of nebulizers in use today, but the most common is one in which the Bernoulli principle is used through a Venturi apparatus.

As discussed earlier, the Bernoulli principle states that when gas flows through a tube, it exerts a lateral wall pressure within that tube due to its velocity. As the gas reaches a smaller diameter in the tube, the velocity is increased, which decreases lateral wall pressure. This decrease in diameter within the tube is at a structure called a jet. Just distal to the jet is a capillary tube that is immersed in a body of fluid. The decreased pressure is transmitted to the capillary tube and fluid is drawn up it. When the fluid reaches the jet, it is then atomized.

The absolute humidity that will be delivered from these devices can be increased by the use of a heater. A baffle is distal to this atomization process in the stream of gas/fluid flow. Nebulization takes place here as the liquid is impelled against the baffle. This baffle causes the larger particles to coalesce and collect in the reservoir. The smaller particles will be delivered to the patient in aerosol form. If the baffle is not used, the device is known as an atomizer. When the baffle is used, it is then called a nebulizer. In addition to the physically placed baffle, any 90° angle to gas flow can be considered a baffle. Large bore corrugated tubing should be used with baffles. This will enable the aerosol particles to be delivered to the patient.

There several ways to deliver aerosol therapy, and the modalities available today include:

1. Aerosol mask
2. Face tent
3. Mouthpiece
4. Aerosol tent (mist tent)
5. In conjunction with IPPB

Physician orders for aerosol therapy should contain identification of:

1. Type of aerosol
2. Source gas (FI02)
3. Fluid composition (NaCl, water, etc.)
4. Delivery modality
5. Duration of therapy
6. Frequency of therapy
7. Temperature of the aerosol

When a prescribed aerosol therapy has been completed, be sure to chart your actions and observations, making sure to include the following information:

• time of administration
• duration of therapy
• type or composition of the aerosol (NaCl)
• pulse
• respiratory rate and pattern
• breath sounds
• characteristics of sputum
• if sputum was or was not produced
• the ease of breathing
• any benefits observed
• and any other relevant observations.

The reasons for administering aerosol therapies include:

1. For bronchial hygiene
   1. Hydrate dried secretions
   2. Promote cough
   3. Restore mucous blanket
      
      
2. Humidify inspired gas
   
   
3. Deliver prescribed medications
   
   
4. Induce sputum lab culture

Aerosol delivery is accomplished in a variety of ways:

• nasal spray pump
• metered-dose inhaler (MDI)
• dry powder inhaler (DPI)
• jet nebulizer
• small volume nebulizer (SVN)
• large volume nebulizer
• small-particle aerosol generator (SPAG)
• mainstream nebulizers
• ultrasonic nebulizer (USN)
• intermittent positive pressure breathing (IPPB) devices

Spray pumps are the most common devices used for nasal aerosol administration of: antiallergics, sympathomimetics, antimuscarinics, and anti-inflammatory drugs. The spray pump generates low internal pressure, and produces large particles that are well-targeted for nasal deposition.

Metered dose inhalers (MDIs) consist of a pressurized cartridge and a mouthpiece assembly. The cartridge, which contains from 150-300 doses of medication, delivers a pre-measured amount of the drug through the mouthpiece when the MDI is inverted and depressed.

One controversial problem with MDIs involves their use of chlorofluorocarbons (CFCs) which have been identified by scientists as culprits in causing the growing hole in the earth's ozone layer, contributing to global warming and increased ultraviolet radiation. While the manufacture and importation of other sources of CFCs, like refrigerants, have been banned in the U.S. since 1996, the FDA exempted CFCs used in "medically essential" products like MDIs. Alternatives (such as hydrofluorocarbons--HFAs) to CFCs for use in MDIs have been discovered, and the FDA has already formulated plans for facilitating a transition from CFCs to alternatives like HFAs.

However, the FDA has stipulated that CFC-medications will not be phased out until:

• acceptable treatment alternatives exist for a particular MDI or other drug product so that the patient can find a product that meets his or her medical,
  
  
• the alternatives are marketed for at least one year and are acceptable by patients, and
  
  
• the supply of alternative products is sufficient to ensure that there will be no shortages of the drug.

Successful delivery of medications with an MDI depends on the patient's ability to coordinate the actuation of the MDI at the beginning of inspiration. Patients need to be alert, cooperative, and capable of taking a coordinated, deep breath. Patients should be instructed to:

1. Be sure to shake the MDI canister well before using.
   
   
2. Hold the MDI a few centimenters from the open mouth.
   
   
3. Holding the mouthpiece pointed downwards, actuate the MDI at the beginning of a slow, deep inspiration, with a 4-10 second breath hold. Late actuation, or at the end of the inspiration, or stopping inhaling when the cold blast of propellant hits the back of the throat will cause the medication to have only a negligible effect.
   
   
4. Exhale through pursed-lips, breathing at a normal rate for a few moments before repeating the previous steps.
   
   
5. Patients should also be instructed to rinse their mouths after taking the medication.

After instructing the patient, the RCP should ask the patient to act out the procedure, observing to see if the patient has really understood the instructions. Proper instruction and observation of the patient are crucial to the success of MDI of therapy.

The particle size of the drug released is controlled by two factors: the vapor pressure of the propellant blend, and the diameter of the actuator's opening. Particle size is reduced as vapor pressure increases, and as diameter size of the nozzle opening decreases. The majority of the active drug delivered by an MDI is contained in the larger particles, many of which are deposited in the pharynx and swallowed.

• The advantages of MDI aerosol devices include:
• They are compact and portable.
• Drug delivery is efficient.
• Treatment time is short.

On the other hand, the disadvantages of using MDIs to deliver aerosolize medications include:

• They require complex hand-breathing coordination.
• Drug concentrations are pre-set.
• Canister depletion is difficult to ascertain accurately.
• A small percentage of patients may experience adverse reactions to the propellants.
• There is high oropharyngeal impaction and loss if a spacer or reservoir device is not used.
• Aspiration of foreign objects from the mouthpiece can occur.
• Pollutant CFCs, which are still being used in MDIs, are released into the environment until they can be replaced by non-CFC propellant material.

Extension or reservoir devices can be used to modify the aerosol discharged from an MDI. The purposes of these spacers or extensions include:

• Allow additional time and space for more vaporization of the propellants and evaporation of initially large particles to smaller sizes.
  
  
• Slow the high velocity of particles before they reach the oropharynx.
  
  
• As holding chambers for the aerosol cloud released, reservoir devices separate the actuation of the canister from the inhalation, simplifying the coordination required for successful use.