Metered Dose Inhalers

Metered Dose Inhalers 7 marks High-Yield Points

Definition and principle: a pressurised metered dose inhaler (pMDI) is a hand-held device that delivers a fixed, reproducible dose of drug suspended or dissolved in a hydrofluoroalkane (HFA) propellant as an aerosol. Optimal lung deposition requires particles of 1–5 µm; larger particles impact in the oropharynx, smaller particles are exhaled.

Why inhaled delivery is preferred: drug is delivered directly to the airway, giving a rapid onset at a much lower dose with fewer systemic adverse effects — important in the elderly, where systemic beta-agonist effects (tremor, tachyarrhythmia, hypokalaemia) and anticholinergic effects are poorly tolerated. GOLD states plainly that inhaled agents are preferred over oral agents.

Correct technique — eight steps to write out: (1) remove cap and inspect; (2) shake well; (3) breathe out gently and fully, away from the device; (4) place the mouthpiece between the teeth and seal the lips; (5) begin a slow, deep inhalation and actuate the canister simultaneously; (6) continue inhaling slowly over 3–5 seconds; (7) hold breath for about 10 seconds; (8) wait 30–60 seconds before a second dose. Rinse the mouth after inhaled corticosteroids.

The critical step is hand–breath coordination — actuation must coincide with the start of a slow deep breath. This is precisely the step that fails in older patients.

Barriers in the elderly (Pathy's, high-yield): Most cognitively intact, physically able older patients can use an inhaler, but learning and coordination difficulties are present in about 20%. Cognitive impairment dramatically impairs inhaler use: an MMSE score below 24 is associated with a very high risk of failure. Remarkably, the abnormal pentagon-copying test alone (the constructional component of the MMSE) identifies patients unable to use inhaler devices — a simple bedside screen. Board and Allen's "simple drawing test" makes the same point. Poor manual dexterity — weak hand grip, tremor, arthritis, impaired coordination and poor vision. Impaired executive function limits the acquisition of inhaling skills, particularly in hospitalised patients. Inadequate inspiratory flow and respiratory muscle strength, and reduced awareness of bronchoconstriction.

The spacer (valved holding chamber) solves most of these problems. It removes the need for coordination, slows the aerosol, allows the propellant to evaporate so particle size falls into the respirable range, increases lung deposition and reduces oropharyngeal deposition. Pathy's states directly that if an MDI is prescribed, a spacer will reduce oropharyngeal candidiasis and dysphonia. Use tidal breathing (5 breaths per actuation) in the frail; actuate one dose at a time; wash the spacer in detergent and air-dry without rinsing or wiping to reduce electrostatic charge; replace every 6–12 months. Add a face mask for patients who cannot form a lip seal.

Choosing between devices — the guiding principle is that "the perfect device does not exist"; select on an individual basis. pMDI — cheap, portable, dose counter available, but needs coordination; add a spacer. Breath-actuated MDI (Autohaler, Easi-Breathe) — removes the coordination problem but still needs adequate inspiratory flow; cannot be used with a spacer. Dry powder inhaler (DPI) — no coordination needed, but requires a forceful, deep inspiration generating adequate peak inspiratory flow, which many frail or severely obstructed older patients cannot achieve; also needs manual dexterity to load. Soft mist inhaler (Respimat) — slow-moving fine mist, low inspiratory flow requirement, good lung deposition; useful in the elderly but the cartridge needs loading and priming. Nebuliser — needs only tidal breathing; reserve for very advanced disease or where inhaler technique is irretrievably poor. Pathy's notes nebulised bronchodilators "may help patients with very advanced COPD... or where the inhaler technique is so poor that no other relief is possible".

Adherence and device burden: the number of devices independently reduces adherence, so fixed-dose combination inhalers should always be considered to reduce discontinuation and misuse. Cognitive impairment reduces adherence to both drug and non-drug therapy; the MMSE, the Mental Deterioration Battery or a secondary memory test such as Rey's 15-word test identify COPD patients at risk of poor compliance.

Practical rules for the clinic: ask every patient to bring their inhalers to every visit and demonstrate their technique; retrain the patient and the caregiver at each visit; check the technique before escalating therapy (a GOLD 2026 emphasis); use adapters fitted to the inhaler end to help weak grip; and check the dose counter. Short-acting oral beta-2 agonists exist for the few who cannot use any inhaler, but their unfavourable therapeutic window severely limits their use.

Adverse effects of MDI therapy: oropharyngeal candidiasis and dysphonia with inhaled corticosteroids (prevented by a spacer and mouth rinsing); local irritation and cough from the propellant; paradoxical bronchospasm; systemic effects — tremor, palpitations and hypokalaemia with beta-2 agonists, dry mouth, urinary retention and glaucoma with anticholinergics, and skin thinning, cataract and osteoporosis with high-dose inhaled corticosteroids. Latest Updates and Current Practice (2023–Present) Low-global-warming-potential propellants: pMDIs currently use HFA-134a and HFA-227ea, which are potent greenhouse gases. Manufacturers are transitioning to HFA-152a and HFO-1234ze, which cut the carbon footprint by over 90%; the first reformulated salbutamol and combination inhalers began reaching markets in 2025–2026 with unchanged dosing and technique. Examiners increasingly expect a line on the environmental footprint of inhalers (a pMDI carries roughly 10–30 times the carbon footprint of a DPI), balanced against the caution that a DPI is useless if the patient cannot generate the inspiratory flow. Smart inhalers and digital adherence monitoring — electronic dose counters with Bluetooth-linked reminders improve adherence and technique; GOLD 2026's new chapter on emerging technology and AI covers digital adherence tools, and a 2024 network meta-analysis (Eur Respir Rev) evaluated adherence-enhancing interventions for both inhaled and oxygen therapy. Current Practice Update — three points: (1) Pathy's recommends a spacer chiefly to reduce candidiasis and dysphonia; current guidance positions the spacer as the default for every older pMDI user, because it also compensates for poor coordination. (2) Device selection is now formally guided by measured peak inspiratory flow, not by clinician preference — low inspiratory flow contraindicates a DPI. (3) GOLD 2026 requires that inhaler technique, adherence and device suitability be re-checked before any escalation of therapy, making the inhaler review a formal treatment step rather than an afterthought.