Aerosol Therapy
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Only about a tenth of every metered-dose inhaler puff reaches the lungs, and only droplets between 1 and 5 micrometres of mass median aerodynamic diameter settle in the lower airway — the physics that shapes all aerosol therapy. Nebulisers (jet, ultrasonic, vibrating mesh), pressurised MDIs and dry powder inhalers are simply different engines for placing the same drugs at that target, and the choice between them is decided less by device superiority than by the patient's flow, age and technique. In acute severe asthma, a jet nebuliser or an MDI with a spacer (four to ten puffs) deliver equivalent bronchodilation when used correctly; in croup, nebulised adrenaline 1:1000 at 0.5 mL/kg is the emergency dose every therapist must produce from memory.
What you must remember
- Particle size: 1-5 micrometres reaches the lower airways; above 5 impacts in the oropharynx, below 1 remains suspended and is exhaled.
- Jet nebuliser: driving gas 6-8 L/min, 8-10 minute treatment, residual volume of 0.5-2 mL means a substantial fraction of the dose never leaves the cup.
- Ultrasonic nebulisers are silent and fast but heat the solution — unsuitable for suspensions (budesonide) and labile proteins; vibrating mesh devices are the most efficient, with minimal residual volume.
- MDI technique: shake, exhale to functional residual capacity, actuate at the start of a slow deep inhalation, hold the breath 10 seconds, wait 30-60 seconds between actuations.
- A spacer roughly doubles MDI lung deposition and slashes oropharyngeal impaction; children under four use a mask, over four a mouthpiece, taking four to six tidal breaths per actuation.
- Dry powder inhalers need a forceful inspiratory flow (roughly 30-60 L/min): never shake them, never attach a spacer, and avoid them below about six years and in acute severe attacks, where flows are too low.
- Doses: nebulised salbutamol 2.5-5 mg (repeated or continuous in severe asthma), ipratropium 0.5 mg added in severe exacerbations, budesonide nebulisation 1-2 mg in croup, MDI salbutamol 100 micrograms per puff with 4-10 puffs via spacer equivalent to one nebulisation.
- Nebulised L-adrenaline 1:1000, 0.5 mL/kg (maximum 5 mL), is the classic Indian paediatric emergency dose for severe croup; observe 2-4 hours for rebound oedema.
- After inhaled corticosteroids, rinse and spit — oropharyngeal candidiasis and dysphonia are otherwise routine; watch for tremor, tachycardia and hypokalaemia with high-dose beta-agonists.
Acute severe asthma, worked through
A 22-year-old arrives with PEF 33% of predicted, respiratory rate 30, SpO2 91% on air, speaking in words. Sit her upright, attach monitors, and drive the first nebulisation — salbutamol 5 mg with ipratropium 0.5 mg — on 6-8 L/min of oxygen. Repeat every 20 minutes for the first hour, or run salbutamol continuously if severity demands. Give systemic corticosteroids early (oral prednisolone 40-50 mg). Nebulisers driven by oxygen are acceptable here because she is not hypercapnic; the moment an arterial gas shows rising CO2 or drowsiness, switch to air-driven nebulisation or MDI-plus-spacer so the driving gas stops adding dead-space oxygen.
Meanwhile the toddler in the next bay with stridor at rest follows a different protocol: dexamethasone 0.15-0.6 mg/kg plus nebulised adrenaline 1:1000 at 0.5 mL/kg up to 5 mL, on the parent's lap, with intubation equipment in the room and hours of observation after. Same device class, entirely different drug logic — vasoconstriction of oedematous subglottic mucosa rather than bronchodilation.
Where the technique fails
The MDI is failed by its users, not its engineering. The commonest errors: actuating after inspiration has already begun, skipping the breath-hold, firing two puffs into one breath, and never shaking the canister — each alone can halve delivery. Nebuliser errors are more institutional: driving with oxygen in a known CO2 retainer, filling beyond the maximum line so the baffle floods, and a facemask held centimetres from the face; for a crying child, quiet tidal breathing delivers more drug than screaming against a mask. And the examinable myth to retire: nebulisers are not "stronger" than MDI plus spacer — trials show equivalence when technique is good, and nebulisers earn their place only where flow, age or consciousness make the MDI impractical.
Frequently asked questions
What particle size reaches the lower airway?
One to five micrometres mass median aerodynamic diameter; larger droplets impact in the mouth and throat, smaller ones are exhaled.
What is the nebulised adrenaline regimen for severe croup?
L-adrenaline 1:1000 at 0.5 mL/kg, maximum 5 mL, with observation for rebound oedema over the next few hours.
Why rinse the mouth after inhaled corticosteroids?
To prevent oropharyngeal candidiasis and dysphonia from deposited drug.
Why is a dry powder inhaler unsuitable in acute severe asthma?
It depends on a high inspiratory flow the exhausted patient cannot generate; a nebuliser or MDI with spacer is required.
How does a small child use a spacer?
Mask for under-fours, mouthpiece above four, with four to six tidal breaths for each single actuation placed in the chamber.
Which nebuliser type should not be used for suspensions such as budesonide?
Ultrasonic nebulisers — their heating poorly nebulises suspensions and degrades labile agents.