The science of nebulised drug delivery

Effective nebuliser therapy requires a device distinguished from a simple atomiser by the incorporation of baffles which selectively rethat repeatedly and quickly delivers sufficient drug to the site of action, with minimal wastage, move large droplets from the outgoing spray. Early models were essentially atomisers conat a low cost. Clinicians are bombarded with competing claims about different nebuliser sysstructed of glass and operated manually by compressing a hand bulb attached to the air tems. In many cases, however, insufficient details are available to make the most appropriate inlet tube. In 1946 pumps providing a continuous flow of air were advocated and the choice. The rapid increase in the number of nebulisers marketed and significant differences Collinson nebuliser, constructed of ebonite with a plate baffle to filter out particles larger in design may result in drug delivery to patients varying by a factor of two or more. than 5 lm, became the most popular nebuliser in this country. In 1958 Wright described a new nebuliser, considerably more compact than the Collinson, with a moulded perspex top. Drug delivery from nebulisers Most of the prescribed medication for nebuNow discontinued, it found widespread use in bronchial challenge testing. 14 lisers never reaches the lungs. Of the dose placed in the nebuliser chamber, perhaps two With the advent of portable, oil free compressors and injection moulding of plastics, a thirds remains there at the end of nebulisation. Two thirds of the dose released from the nebuwide variety of disposable nebulisers has become available. Recent advances in their liser may be released during expiration and passes into the surrounding air. Some of the design have considerably altered the amount of drug patients receive. inhaled drug will be in particles too large to reach the lung, and some in particles so small that they do not deposit but are simply exhaled again. With many nebulisers only 10% of the    In a jet nebuliser the driving gas passes through prescribed dose may reach the lung. For bronchodilators, where a small dose may a very narrow hole, known as a Venturi, from a high pressure system (fig 1). At the Venturi achieve an adequate result, this may not matter. It is more important for drugs with dose related the pressure falls and the gas velocity increases greatly producing a cone shaped front. This effects (and side effects) such as steroids, and for expensive medications such as rhDNase. passes at high velocity over the end of a narrow liquid feed tube or concentric feeding system creating a negative pressure at this point. As a result of this fall in pressure, liquid is sucked Nebuliser types: how they work Nebulisers used in aerosol drug delivery proup by the Bernoulli effect (see Appendix 1) and is drawn out into fine ligaments. The duce a polydisperse aerosol where most of the drug released is contained in particles 1–5 lm ligaments then collapse into droplets under the influence of surface tension. This primary in diameter. Most nebulisers use compressed air for atomisation (fig 1), but some use ultrageneration (atomisation) typically produces droplets 15–500 lm in diameter. Coarse sonic energy (fig 2). A nebuliser may be