Pressurized MDIs
The pressurized MDI is a pressurized aerosol canister that contains medication either in solution or suspension in a liquefied gas propellant, usually hydrofluoroalkane.29 The primary advantages of this device are that it is compact and portable, offers consistent dosing and rapid delivery, can be used independently and inconspicuously, and sometimes includes a dose counter. On the other hand, the pressurized MDI has been called the most complex dosage form in medicine.29
Difficulties using pressurized MDIs are widely documented. A meta-analysis of 24 studies of pressurized MDI use found that 77% of patients made at least one error during its use.12 Inadequate hand-breath coordination, poor fine motor control, and hand or finger muscle weakness from comorbidity, such as arthritis, stroke, or neuromuscular disease, all interfere with proper use. In one study of errors associated with pressurized MDIs, the proper use rate decreased with increasing age, with only 14% of patients aged ≥75 years using their pressurized MDI correctly (Figure 2).26 Todd et al studied a group of 30 independently living, cognitively normal volunteers aged 73–90 years and found that 29 of 30 (97%) were able to learn to use a pressurized MDI correctly, but when retested 24 hours later, the rate of correct use dropped to 36%.30 It is important to note that subjects with upper limb weakness were excluded from this study. Breath-actuated MDIs may mitigate problems with digital manipulation but require sufficient inspiratory flow (usually at least 30 L/sec) to trigger the device.31 Different inhaler devices require different techniques, so instruction for use by providers may not be correct. Spacers are used to minimize problems of poor inhalation associated with pressurized MDIs, but they present problems of their own. The walls of the spacers retain some of the drug, making it impossible to know how much is actually delivered into the airways. Many patients find spacers technically challenging and may prefer not to use them.13 The time clinicians spend teaching and assessing how patients use these devices is time well spent. A scheme for assessment of correct pressurized MDI use has been described.32