The use of inhaled respiratory medications in dogs and cats is becoming more common. Inhalant delivery of aerosolized medication
offers a number of theoretical benefits, including an enormous absorptive surface area across a permeable membrane, a low
enzyme environment that results in little drug degradation, avoidance of hepatic first-pass metabolism, and reproducible absorption
kinetics. When the target of inhaled medications is the respiratory tract itself, additional benefits include the potential
for attaining a high drug concentration directly at the disease site with minimal systemic absorption and toxicity. Often,
therapeutic effect can be achieved with only a fraction of the dose required for systemic delivery of the same drug.1
Because of these advantages, inhalant delivery of medication has gained widespread use for treating airway diseases in people.
More than 30 drugs licensed for people are available for inhalation, including anti-inflammatory drugs and bronchodilators.
An enormous body of evidence in the medical literature exists regarding the efficacy and toxicity of inhalational drug therapy
in people. In veterinary medicine, the literature on inhalant therapy to treat naturally occurring disease is sparse. Regardless,
aerosol delivery of medication has become popular for treating dogs and, especially, cats with respiratory disease.
CAUTIONS
Although inhalant drug delivery has many benefits, difficulties in using this route exist as well. Respiratory defenses are
efficient at preventing particulates from reaching the lower airways, so it should come as no surprise that only a small proportion
of the administered medication reaches the lower airways; a marked amount of drug is lost in the delivery device or deposited
in the oropharynx.1 In animals, another portion will be deposited on the fur, especially if an animal is placed in a tent or tank, and can be
ingested through grooming.
Another difficulty is that most aerosol drug delivery devices are designed to be used by people on a voluntary basis, and
some require purposeful respiration and breath-holding. Adaptations of some devices facilitate their use in animals, and modified
systems are now marketed for dogs and cats (e.g. Aerokat—Trudell Medical International; NebulAir Small Animal Chamber—DVM Pharmaceuticals; Breathe Easy—Jorgensen Laboratories).
Drug delivery by the aerosol route depends in part on respiratory depth and rate, tidal volume, and airflow rate, yet all
of these may be negatively affected by respiratory disease. Additionally, not all drugs are suitable for aerosol delivery,
and the drugs themselves (or preservatives contained in the drug preparation) may cause airway irritation and possible bronchoconstriction
that could worsen respiratory function.
AEROSOL DELIVERY SYSTEMS
Two basic types of aerosol delivery systems are used in veterinary practice: nebulizers and pressurized metered dose inhalers
(MDIs). The two are distinct devices with different uses. In general, nebulizers deliver much smaller particles, allowing
deeper respiratory penetration, and provide fluid along with the drug.1 MDI devices deliver drugs primarily to the larger airways.
Nebulizers
Basic nebulizer types include jet nebulizers and electronic nebulizers. Modifications exist (e.g. spinning disk nebulizers, vibrating mesh nebulizers) to improve delivery or modulate particle size.
How they work. Jet nebulizers have a compressed air or oxygen source, a well into which fluid or a drug can be placed, and a baffle that
when hit by the drug creates small particles. Jet nebulizers tend to be larger and sturdier than electronic nebulizers. Electronic
nebulizers use membrane vibration to produce an aerosol and are much smaller since no air compressor is required. However,
in my and other clinicians' experience, electronic nebulizers tend to malfunction more easily than jet nebulizers do.
1. Portable electronic nebulizers can be adapted for use in pets by attaching a face mask.
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Equipment needed. Both types of nebulizers are available in portable sizes at a modest price, suitable for use in veterinary hospitals and at
home (e.g. Nebulizer compressor—Jorgensen Laboratories; NebulAir Veterinary Portable Ultrasonic Nebulizer or Vet Neb Express Nebulizer—various
veterinary suppliers; nebulizers from human medical supply companies). The cost starts at less than $100 for a good nebulizer
unit, and small ultrasonic or used jet nebulizers can be purchased for under $50. Alternatively, with a prescription, many
respiratory supply pharmacies will rent nebulizers directly to clients and provide instructions for use.
2. Sturdy jet nebulizers are better-suited to use with tanks or tents (as seen here) than electronic nebulizers are. These
systems work well for delivery of saline nebulization but result in inefficient delivery of nebulized medications.
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Nebulized liquid can be administered to dogs and cats by face mask (Figure 1), by tent (Figure 2), in a closed aquarium-type container into which the animal is placed, or through an endotracheal or tracheotomy tube. Any
of these routes are suitable for airway humidification by saline nebulization. In general, the more removed the particle generator
is from the respiratory tract, the more the drug will be lost outside of the respiratory tract. For this reason, drug administration
by nebulization is more effective by mask than by simply administering the drug into a tank containing the pet. Tank or tent
drug delivery also results in marked drug wasting, and drug deposition on the fur allows oral ingestion after grooming.
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