AEROSOL DEVICES AND TECHNIQUES

Aerosol therapy has revolutionized the management of asthma. Aerosol is a suspension of the drug in gaseous medium, which is directly deposited in lower airways on inhalation via special devices.

Although only 10-15% of inhaled drug reaches the lower airway by these devices, major benefits accrue from the fact that the drug is directly delivered at the site of action with: (a) rapid onset of action, and (b) minimum side effects due to miniscule doses.

Various types of aerosol devices include: (a) metered dose inhaler (MDI) with/without spacers, (b) dry powder inhalers (DPI), and (c) nebulizers, each with own advantages and disadvantages (Table 32.3). The choice of device depends on the patient's age, cooperation and feasibility.

Generally, nebulizers are preferred in acutely sick hospitalized children. For domiciliary use, direct MDI is preferable in children gt;6 years, DPI for children between 3-6 years and MDI with spacer/mask in those lt;3 years of age.

Currently, almost all #946;-agonists and steroids pre­parations are available as aerosols (Ch 16.8). Some commonly used aerosol devices are discussed as follows: Metered dose inhaler (MDI) is a pressure-actuated aerosol device that delivers pre-fixed dose of aerosol particles (3-6 #956;) during each actuation, forced out by a liquid propellant at the velocity of ~100 km/hour. Released drug may be inhaled directly or via a spacer with/ without facemask. With MDI, ~10% of inhaled drug reaches the target airways.

Device: MDI consists of two separable parts: (a) pressurized aluminum canister with a valve and atomizing needle, which fits into, (b) outer plastic actuator sleeve with mouth piece (Fig. 32.15). The valve allows only a measured quantity of drug to be released after each accentuation.

*Combination MDI available. **Combination DPI not available.

Fig. 32.15: Metered dose inhaler.

Pressurised canister contains: (a) active drug pre­paration, (b) propellant, e.g. CFC, (c) surfactants, e.g. Sorbitan trioleate, oleic acid or lecithin, to prevent clumping of drug particles, and (d) preservatives, e.g. Benzalkonium chloride or metabisulfite.

Procedure: Adequate drug delivery via MDI requires proper coordination between drug release and inspi­ration, which can be acquired only by proper training and practice. Important steps include: shake the inhaler thoroughly gt; uncap mouth piece gt; hold the inhaler upright gt; exhale after tidal breathing gt; place the mouth piece in mouth and seal it with lips gt; press (actuate) the canister with simultaneous intake of a slow-deep breath gt; hold the breath for 5-10 seconds gt; breath out after removing inhaler.

After inhalation of steroid-aerosols, mouth must be rinsed with water to avoid local drug deposition and secondary oral candidiasis.

Cleaning: Outer plastic casing should be cleaned at least once a week by-remove the inner canister gt; immerse plastic body in warm water gt; rinse it under running water gt; dry gt; replace canister. Drug canister should not be immersed or rinsed.

Advantages: (a) self administration, (b) small and por­table, (c) independent of inspiratory airflow.

Disadvantages: (a) need for synchronized actuation and inhalation*, (b) cold-freon effect, (c) high drug deposition in the oropharynx (80% of inhaled drug), (d) adverse environmental effect with ozone depletion by CFC- containing canisters. However, most of the currently available MDIs are CFC-free.

*Consequent to the need for synchronized actuation-inhalation- breath holding, direct MDI is unsuitable for children lt;6 years or during acute severe asthmatic attack.

Spacers are devices, which hold the medication for a few seconds after it is released from the MDI, thus obviating the need for actuation-inhalation-breath holding synchronization. With MDI-spacer combination ~15% drug can reach target airways.

Fig. 32.16A and B: Spacer: (A) with face mask; (B) without face mask.

Device: Spacer devices have a two-piece compartment with combined volume of 250-820 ml, made up of anti­static polycarbonate/ polyamide material, with one inlet for MDI canister and another outlet as mouth-piece (Fig. 32.16). After actuation of MDI device, adequate quantity of aerosol remains in the spacer for ~ 10-40 seconds, during which it may be inhaled by multiple inspiratory efforts either directly through mouth-piece or via a face-mask (in infants). Some spacers also have a one-way valve that opens only during inspiration.

Procedure: Assemble the spacer gt; fix the MDI canister on inlet gt; close the lips around mouth-piece to create a good seal gt; actuate the canister gt; inhale deeply through mouth and hold the breath as long as possible gt; young children may have to breathe 5-6 times gt; repeat the second dose after at least 1 minutes, if necessary. A proper-size face­mask may be used to facilitate administration in children lt;3 years.

Cleaning: Spacer should be cleaned at least twice a week by - separate two halves gt; rinse in warm water gt; wipe dry with soft-cloth.

Advantage: MDI spacer with face-mask is ideal choice for day-to day aerosol therapy in children lt;3 years due to:

(a) no need for actuation-inhalation synchronization, (b) least drug deposition into oropharynx (~10% of inhaled drug, reducing local steroid side-effects, (c) increased drug delivery to the lungs, (d) no cold-Freon effect, (e) multiple dose administration at one sitting.

Dry powder inhalers (DPI) are breath-actuated devices, which use the patient's own inspiratory force to deliver the drug powder into lungs, using a special device- Rotahalers, Dischalers, Turbohaler, Spinhaler, etc.

Device: Commonly used Rotahaler is a two-part transparent chamber with upper half having a mouth­piece and a square hole to insert the medicine capsule,

i. e. Rotacap. Lower half (base) has a fin to break the

Fig. 32.17: Dry powder inhaler.

Fig. 32.18: Hudson chamber for nebulization (Schematic representation).

capsule inchamber, when it is rotated against the upper half (Fig. 32.17). Drugs for use with rotahalers are available as dry powders, tagged with an inert carrier, e.g. lactose and packed in a gelatin capsule.

Procedure: Assemble the rotahaler gt; insert a rotacap in square hole (transparent end first) gt; rotate the base to break the capsule and release drug powder into the chamber gt; breathe-out fully gt; seal the mouthpiece with lips gt; inhale as rapidly* as possible gt; remove the rotahaler and hold the breath as long as possible. Sometimes 2-3 additional inhalations are necessary to finish the powder.

* Note that very rapid inhalation is must with DPI use, while MDI should be inhaled deeply and slowly.

Cleaning: Rotahalers should be cleaned at least twice a week as follows-separate two halves gt; remove empty capsule gt; rinse with running water gt; dry gt; reassemble. Advantages: DPI is the ideal choice for day to day aerosol therapy in children between 3-6 years, due to (a) no need for actuation-inhalation coordination, (b) easy to assemble, use and carry, (c) economical than MDI, and (d) environment friendly (CFC free), and (e) multiple aerosol drugs can be administered with same rotahaler.

Disadvantages: Major limitation of DPI is the need for good inspiratory effort as a driving force, which may not be possible during severe acute attack*. Despite best efforts with DPI, only ~5% of inhaled drug reaches the target airways and lungs.

*Children unable to use DPI during acute attack, may be switched on MDI with spacer.

Nebulizer is an electric-device, used to convert a liquid drug into fine mist that can than be inhaled easily. Based on the methods of aerosolization, nebulizers may be divided in—(a) jet nebulizer, and (b) ultrasonic nebulizer.

Device: Commonly used Jet nebulizers use a compressed air/oxygen flow (4-6 L/minute) to convert the liquid drug into mist with a particle size of 2-5 #956;. Drugs, available as special nebulizing solutions, are filled into a special Hudson's chamber that has a baffle against the air/oxygen inlet to create necessary turbulence for mist formation (Fig. 32.18). This mist can be inhaled via a face-mask.

Procedure: Fill the desired dose of nebulizing solution in Hudson chamber gt; add normal saline* to make-up a volume of 2-4 ml gt; fix a face-mask over chamber-outlet

gt; start the machine to check adequate mist formation

gt; fix the face mask over patient's face covering mouth as well as nose properly gt; allow tidal inhalation till the chamber is empty.

Note that only normal saline, and not the distilled water, should be used as diluent in Hudson chamber, as hypo-osmolar distilled water mixture may provoke reflex bronchospasm. However, presently readymade liquid nebulizing formulations are available for use..

Cleaning: Hudson chamber, tubings, mouth-piece and mask should be cleaned with running water after each use. After each day, immerse all these components in warm water/ mild detergent for 2-3 minutes gt; scrub well gt; immerse in a disinfectant, e.g. acetic acid for 10 minutes gt; rinse under running water gt; dry in air.

Advantage: Nebulization is the preferred choice for aerosol therapy during acute severe asthmatic attack, due to: (a) no need for deep or rapid inhalations, (b) multiple drugs, e.g.

Fig. 32.19: Peak flow meter.

Disadvantage: (a) Expensive and needs regular electric supply, (b) bulky equipment, difficult to carry, (c) risk of air-borne infections if not cleaned properly or Hudson chamber is reused for many patients.

Peak expiratory flow meter is a simple and portable equipment to record PEFR-a reliable indicator of the severity of airway obstruction in asthma.

Components: Commonly used Mini-Wright's peak flow meter is a 20-25 cm long plastic tube with three parts: (a) detachable mouthpiece, (b) main body with a graduated scale (Lt/min) to read PEFR, and (c) diaphragm inside the tube attached to an external pointer that moves on main body's scale (Fig. 32.19).

Uses: PEFR measurements are used for: (a) assessment of the severity of acute attack and response to therapy, and (b) long-term monitoring of the course of disease.

Procedure: Move the pointer to zero gt; hold the peak flow meter horizontally gt; ask the patient to breath-in as deep as possible gt; place the mouthpiece in patient's mouth, sealing it with lips gt; ask him/her to blow-out as hard and fast as possible gt; record the forward displacement of pointer on the scale that gives direct value of PEFR in liters/minutes.

As it is an effort-dependent parameter, minimum three reading should be recorded before taking the best record as patient's PEFR.

32.6