Inhalant delivery offers alternatives to treating respiratory disease

Oct 01, 2006

Q Please review inhalant therapy in dogs and cats.

A Dr. Leah A. Cohn at the 2006 American College of Veterinary Internal Medicine Forum in Louisville, Kentucky gave a lecture on inhalant delivery of medications in small animals. Some relevant points in this lecture are provided below.

Inhalant delivery of aerosolized medication offers a number of benefits including an enormous absorptive surface area across a permeable membrane, a low enzyme environment that result 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 a high drug concentration directly at the site of disease with minimization of systemic toxicity and often at a fraction of the dose required if the same drug was administered through a systemic route.

Because of these advantages, inhalant delivery of medication has gained widespread use for the treatment of airway diseases in people. In veterinary medicine, the literature on inhalant therapy is sparse, and what does exist focuses on aerosol drug delivery to horses rather than to dogs and cats. Regardless, aerosol delivery of medication has become popular for the treatment of dogs and cats with respiratory tract disease.

Although there are advantages to inhalant drug delivery, there are difficulties in using this route as well. Respiratory defenses are efficient at preventing particulates from reaching the lower airways so only a small proportion of the administered medication reaches the lower airways — a significant amount of drug is lost in the delivery device or deposited in the oropharynx. Another difficulty is that most aerosol drug-delivery devices are designed for use in humans. Some require purposeful respiration and breath holding. Adaptations of some devices facilitate their uses in animals, and modified systems are now marketed for dogs and cats.

Drug delivery by the aerosol route depends in part of respiratory depth and rate, tidal volume and airflow rates, yet all of these may be negatively impacted by respiratory tract disease. Additionally, not all drugs are suitable for aerosol delivery, and drugs themselves or preservatives contained in the drug preparation may cause airway irritation and possible bronchoconstriction potentially worsening respiratory compromise.

Aerosol delivery systems

The basic types of aerosol delivery systems in common usage are nebulizers and metered-dose inhalers (MDI). In general, nebulizers deliver much smaller particles allowing deeper respiratory tract penetration and provide fluid along with drug. MDI devices deliver drug primarily to the larger airways. There are more than 30 drugs available as MDI, including anti-inflammatory drugs and bronchodilators.

Nebulizers use compressors to generate relatively high air pressures and flow rates. Generally, there is a source of compressed air or oxygen, a well into which fluid/drug can be placed, and a baffle which when hit by the drug causes the creation of small particles. The basic nebulizer types include jet nebulizers and ultrasonic nebulizers. Modifications exist (e.g., spinning-disc nebulizers and vibrating-mesh nebulizers) to improve delivery or modulate particle size. Nebulizers are available in portable sizes at a modest price, certainly suitable for use in veterinary hospitals and even practical for at-home use by owners (e.g., Nebulair Veterinary Portable Ultrasonic Nebulizer®, DVM Pharmaceuticals and many portable products for the human market). Nebulized liquid can be administered to dogs and cats by face mask, by tent, in a closed aquarium-like container into which the animal is placed, or into a tracheotomy tube. Any of these should be suitable for airway humidification via saline nebulization.

In general, the more removed the particle generator is from the respiratory tract, the more drug would be expected to be lost outside of the respiratory tract. For this reason, administration of drugs via nebulization would likely be more effective by mask than when simply administered into a tank containing the animal.