With the usual jet velocity from MDI, aerosol particle impaction theory predicts that aerosol particles 10 |xm or greater should impact on a surface 10 cm away from the actuator orifice, the distance from the MDI orifice to the posterior pharyngeal wall. Therefore, if droplets completely vaporize to their final size before reaching the oropharyngeal surface, the impaction loss should be minimal. However, a number of studies have shown that a large fraction (50 to 65 percent) of MDI aerosols delivered from the MDI is lost in the mouth, suggesting that the droplets do not have a sufficiently long residence time for complete vaporization within the oropharyngeal space.
Kim et al, using an oropharyngeal model (Table 2), showed that major loss of large particles took place in the oropharynx for Bronkometer aerosol, whereas only minimal loss occurred for both Alupent and Duo-Medihaler aerosols. Together with predictions from impaction theory, this suggests that a large fraction of Bronkometer aerosol might be greater than 10 |xm diameter on approaching the oropharyngeal surface, whereas only a small fraction of both Alupent and Duo-Medihaler aerosols contain such large particles. This difference is probably due to the formulation of Bronkometer aerosol. It contains a large amount of slowly vaporizing alcohol (30 percent weight) as a solvent, whereas no alcohol is present in the suspension-type formulations. In feet, Pengilly et al found that the MMAD of spray droplets containing 15 percent alcohol by weight was about 30 |xm even at a distance of 28 cm from the actuator. MDI spray, which consists only of a mixture of propellants (P11:P12:P114=1:2:1), has a 14-pim MMAD at a distance 10 cm from the actuator orifice. The MMAD of MDI therapeutic aerosols penetrating the oropharynx, larynx and subsequently the trachea is 2.5 to 2.8 |xm. Since 60 percent of such sized particles deposit within the lung during resting tidal breathing, maximum effective deposition of MDI aerosols in the lung would be expected to be about 20 percent of the initially delivered dose. However, actual aerosol deposition in the lung may be much lower than this theoretical expectation, because loss of MDI aerosols in the oropharynx in vivo may be higher than those obtained from in vitro models or mouth-washing techniques. In fact, with controlled administration of radionuclide-tagged aerosol to patients with obstructive airways disease, only 9 percent of the dose is deposited in the lungs. Under such circumstances, 80 percent is deposited in the mouth, 1 percent is expired, and 10 percent is deposited in the aerosol actuator. To become acquainted with the latest news you may on Canadian health&care mall news website.
Deposition of MDI Aerosol in Oropharynx
The impaction theory described above does not take into account the influence of inspiratory flow rate developed by the subject synchronous with inhalation of MDI aerosol. In patients who have been trained to breathe “slowly” while inhaling the MDI aerosol, monitoring of their breathing pattern with respiratory inductive plethysmography, left to their own efforts, revealed a mean inspiratory flow rate of 0.54 L/sec (SD±0.16). The highest value observed in one of a group of ten patients was 1.3 L/sec.
To ascertain the influence of inspiratory flow rate on deposition of MDI aerosol in the oropharynx, Kim et al“ delivered MDI aerosol to an oropharyngeal model while air was continuously drawn through the model at 0.3 and 0.8 L/sec. Table 3 lists the loss of MDI aerosols in the oropharyngeal model as a function of the inspiratory flow rates. These data indicate that there is wide variation in deposition among the various MDI aerosols delivered directly to the model with minimal increases in aerosol deposition with the higher flow rates. Beclovent showed the least and Alupent the greatest deposition losses, a difference among MDI aerosols which appears to be related to the mass output (Table 4).
Deposition of MDI Aerosol in Larynx
Loss of aerosol occurs in the larynx because of a sudden narrowing of flow passage. Aerosol particles impact on the laryngeal surface and the aerosol loss directly correlates with an inertial factor. The latter is a function of particle diameter and flow velocity. The initial jet velocity of the MDI aerosols has minimal effect on the flow rate through the larynx. With an inspiratory flow rate of 0.5 L/sec, the loss of MDI aerosols of 4 to 5 \im MMAD in the larynx is less than 10 to 15 percent.
Table 2—Aerodynamic Size Distribution of Selected MDI Aerosols FtnetraHng the Oropharyngeal Model?
|Trade Name||Without Larynx||With Larynx|
|MMAD ± SD||GSD±SD||MMAD±SD||GSD±SD|
|Alupent||4.1 ±.4||2.3 ±.1||2.7±.l||2.1 ±. 1|
|Duo-Medihaler||4.0±.l||2.1 ±. 1||2.8 ±.2||1.9±.0|
|Bronkometer||2.9±.l||2.1 ±. 1||2.5±.l||2.2 ±.1|
Table 3—Effecta of Inspiratory Flow Rato of 0.3 and 0.8 L/sec on Aerosol Deposition as Fercent qfDose in Oropharyngeal Model Delivered by MDl and Auxiliary MDl Delivery Systems
|Beclovent||Aerobid||Trade b Duo-Medihaler||fameAlupent||Bronkometer||Azmacort|
Table 4—Mass Output (l^g/puff) of MDI Aerosols Delivered Directly and from Auxiliary Delivery Systems
|Direct||78 ±5||627 ±21||923 ±56||1510 ±51||541 ±51||176 ±13|
|Spacer||60 ±2(23)||366 ±16(42)||613 ± 38(34)||769 ± 58(49)||243 ±11(55)||84 ±5(52)|
|Aerochamber*||51 ±4(35)||314 ±17(50)||468 ± 24(49)||423 ±21(72)||124 ± 9(72)||77 ±6(56)|
|InspirEase*||51 ±2(35)||351 ±24(44)||562 ±16(39)||568 ±25(62)|