Dry powder inhalant devices offer several advantages over nebulization systems, including rapid drug administration, minimal risk of environmental contamination with drug, and no requirement for electricity. The dry powder inhalants comprise numerous capsules containing a single dose of drug and a rotor. The rotor of the dry powder inhalant device is breath-actuated, and the device punctures gelatin capsules containing powdered drug and releases it into a chamber for inhalation by the patient. This system eliminates the need for the operator to synchronize administration with inhalation. The entire dose from an individual dry powder capsule is delivered during a single inhalation; prolonged duration of inspiration and multiple inhalations do not improve pulmonary drug delivery.
dry powder inhalant devices are designed for use by human patients, but have been adapted for drug administration to horses using a specialized facemask (EquiPoudre, Agritronics Int) or a unique adaptor to the Equine AeroMask. The efficiency of drug delivery can be influenced by relative air humidity, airflow, and position. The masks used with dry powder inhalants must fit snugly around the muzzle to create adequate inspiratory pressure and flow rates by the horse to ensure sufficient inhalant emptying rates. The minimum flow rate necessary to trigger the device (60 L/min) is generated easily by healthy and heaves-affected horses. The dry powder inhalant device and mask must be aligned with the longitudinal axis of the nasal cavities to avoid affecting the powder within the mask or nasal passages.
High relative humidity increases retention of drug within the device because of aggregation of powder. If the relative air humidity exceeds 95%, water actually penetrates the dry powder inhalant and significantly limits drug delivery. Manufacturers recommend administration of dry powder inhalants under conditions of low relative humidity to minimize the loss of powder within the device. Ipratropium bromide is the most extensively investigated dry powder inhalant preparation for administration to horses and has demonstrated effective bronchodilation in heaves-affected horses.
Numerous devices may be used to deliver aerosolized antiinflammatory and bronchodilator drugs into the equine lung. The equine clinician should be familiar with the technical aspects of aerosolized drug administration because the appropriate drug dosage and frequency of administration for inhalation therapy varies depending on the efficacy of the drug, drug formulation, severity of disease, and efficiency of the delivery device. The quality and quantity of pulmonary drug deposition vary most among the commercially available mechanical nebulizers. The clinician must select a high-quality ultrasonic or jet nebulizer to ensure pulmonary drug delivery. The metered-dose inhalant systems produce the most consistent drug delivery given appropriately fitted equipment. Each system has advantages and disadvantages that must be taken into consideration relative to the size, cooperativity, and preferences of the horse and owner.