The Objectives of a Spacer Device

The primary objective of a spacer is to minimize the need for coordination between actuation of the pMDI and inhalation. In addition, the spacer should ensure that the aerosol particles trail the inspiratory flow of the patient, reduce the proportion of the dose contained in large particles, and increase the proportion contained in small particles (12). The need for coordination between actuation and inhalation is reduced by presenting the aerosol to the patient as a standing cloud of particles, which decreases oropharyngeal deposition. Oropharyngeal deposition with the actuator alone ranges from 30 to 70%, compared with 5-10% with spacer devices (13,14). 

The aerosol cloud should be available over a prolonged period to minimize still further the need for coordination between actuation and inhalation. This is particularly important in young children with poor compliance, and shallow and irregular breathing patterns (Fig. 1). If inspiratory flow and tidal volume are low, several breaths may be required to obtain the whole volume of aerosol retained in the spacer. These factors may also be important in older children and adults with faulty coordination between inhalation and actuation, in whom emptying of a large volume spacer may be delayed. 

When a drug is inhaled from a spacer, the aerosol particles will trail the inspiratory flow of the patient. This improves drug targeting to the lungs compared with an aerosol with an inherent flow, such as that generated by a nebulizer or pMDI alone, where the flow of aerosol may be misdirected and hence depositing on the skin or in the oropharynx. 

Large particles tend to deposit in the upper airways and, depending on the drug, this may add to the risk of both local side effects and systemic absorption. Fine particles add to the desired clinical effect and, by increasing the ratio between fine and large particles, the therapeutic index of the treatment is improved 

The spacer improves the particle profile by two mechanisms. First, the spacer acts as a settling bag, allowing large particles to impact or sediment. Second, the micronized dmg particles from a pMDI are wet particles enveloped in propellant, which evaporates within the spacer and thus reduces the particle size only a minor proportion of the propellant flashes inunediately into vapor as the propellants leave the pMDl. Farther evaporation of the liquid droplets occurs during passage throngh the air. Thus, the size of the dmg-containing droplets depends on the time available for evaporation of the propellants and the distance from the actuator orifice. 

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