INTRODUCTION
Feline asthma has been recognized in the veterinary literature since at least 1911, when J. W. Hill described cats with increased airway mucus, airway inflammation, and the clinical signs of labored breathing and wheezing. Dr Hill's observations form the basis for the criteria we use today to make the diagnosis of asthma in cats:
DIAGNOSIS OF ASTHMA
1. A history of an acute onset of labored breathing. This is usually quickly relieved with some combination of oxygen, bronchodilators and steroids. In some cases however, the only clinical problem is chronic cough.
2. Radiographic evidence of bronchial wall thickening and air trapping. These changes are usually described as “doughnuts” and “tramlines.”
3. Clinicopathologic evidence of airway inflammation including airway eosinophilia.
Almost ninety years after Dr Hill’s description of feline asthma, we are just beginning to increase our recognition of the clinical disease and our understanding of the mechanism(s) involved in the pathogenesis of asthma in cats.
PATHOPHYSIOLOGY
Although there are many potential causes of asthma, the airways can respond to any irritant (real or imagined) in only three basic ways:
1. Airway smooth muscle contracts to prevent the irritant from moving deeper toward the lung.
2. Airway goblet cells and submucosal glands secrete mucus to trap the irritant within the airways before it can migrate toward the lung.
3. A cough is initiated to expel the trapped irritant.
These protective reflexes are all the result of the inflammatory response and directly lead to the clinical signs, including the cough, the wheeze and increased respiratory rate from a narrowed airway. Importantly, the effects of a small degree of airway narrowing on clinical signs can be dramatic. For example, a 50% reduction in the diameter of an airway results in a 16-fold reduction in the volume of air flowing through that airway. The important take-home message is that small changes in airway size result in dramatic changes in airflow through that airway. The clinical implications of this finding are twofold. First, relatively small amounts of mucus, bronchoconstriction, etc., can partially occlude airways and cause a dramatic fall in airflow. On the other hand, therapy that results in relatively small increases in airway size may cause a dramatic improvement in clinical signs.
A very important advance in our understanding of this relationship between airway structure and function is the recent discovery that human asthmatic airways are chronically inflamed whether or not the patient is symptomatic. (It is likely true in cats also, but this has not been proven.) Therefore, the cornerstone of treatment is the use of anti-inflammatory medication.
TREATMENT
Inhaled Medications
Corticosteroids and bronchodilators can now be given effectively by inhalation to cats with asthma. Both classes of drugs are available as metered dose inhalers (MDIs) for humans with asthma. Proper use of an MDI requires the patient to coordinate inhaling with the actuation of the device, and this has proven to be surprising difficult for most patients. This is also not realistic for infants and young children. An alternative was developed to allow these individuals to use the MDIs without the need to coordinate their breathing. Thus, the MDI is used in conjunction with a “spacer” designed for used with infants and small children, and a facemask specifically made for cats. The spacer is a plastic chamber the size of a cardboard inner role of toilet paper. The MDI fits into one end of the spacer; the other end of the spacer has an attachment for the facemask. The end of the spacer that connects with the facemask has an inner rubber gasket that acts as a one-way valve so that the medication within the spacer can only leave the spacer during an inhalation.
The client first attaches the MDI and the facemask to the spacer and then actuates (presses) the MDI twice to fill the spacer with medication. The client then places the facemask gently over the cats mouth and nose. The cat is allowed to breathe in and out 7–10 times with the mask in place, and the treatment is completed.
THE “SPACER”
Company: Respironics
Phone: 800-962-1266, or 973-857-3414
Address: 908 Pompton Avenue
Cedar Grove New Jersey 07009-1292
Why Use a Spacer?
It acts as a temporary storage area for the medication to sit in until the animal breathes in.
How to Use:
1. One end of the spacer is made to fit the metered dose drug device (MDI, the inhaler). The other end (flange) of the spacer is where we attach the facemask.
2. The end (flange) of our facemask is designed to fit with an endotracheal tube, but it has too small a diameter for the flange on the end of the spacer.
3. Take about two feet of ¼ inch cloth tape, and wrap it around the outside of the flange of the face mask until it is the right diameter to just fit snugly inside the flange of the spacer. (This is easier to do than to explain.) Once done, keep the mask attached to the spacer just like that and only take it off when you want to clean the clear dishwater-safe plastic.
Fluticasone Proprionate (FLOVENT)
The inhaled steroid I use is fluticasone. It comes in three doses (44, 110, and 220 mcg per actuation). I only use the 220 dose. This drug has virtually no side effects! Reported problems in people included growth retardation and oral candidiasis, but this has not occurred to my knowledge in more than 60 cats treated with flovent over the past two years. It has the clinical effect of oral prednisone 1 mg/kg q12h.
There are a few potential problems/limitations. First, the drug takes about seven to ten days to reach full effect. Second, the facemask needs to be properly fit for the animal to insure that the drug is being inhaled; it should snugly fit around the muzzle including the corners of the mouth. In the U.S., it is expensive. The highest dose (220) is about $120 per month. Finally, the owner may not be aware when the canister is empty.
How to Use:
1. Prescribe the 220 mcg dose as two puffs into spacer q12h.
2. Cat should breath through mask and spacer for 7-10 second.
3. If the animal is currently symptomatic and can tolerate short-term prednisone, begin 1mg/kg q12h oral prednisone for five days concurrent with flovent, then discontinue the prednisone.
4. Demonstrate how to use the mask/spacer/drug in front of the owner, with their pet.
5. Have them demonstrate the same technique to you, in the office, with their pet, by themselves.
6. This is a one-person job.
7. If the pet is doing well after two months on the 220 mcg dose, decrease to 110 mcg for another month.
8. The canister should last for one month if two puffs twice daily is followed.
Albuterol (Proventil or Ventolin)
The bronchodilator I use is albuterol. It only comes in one dose. I use this drug because it is more rapidly acting than the oral, sq or i.m. forms of terbutaline. It is also more effective than the theophylline compounds (theodur). It may be used daily or as needed for the asthmatic cat already on daily steroids if there is increased cough, wheeze, or increased respiratory rate and effort at rest. I usually prescribe this drug as needed for cats with intermittent signs of asthma (not daily signs). Potential side effects include musculoskeletal twitchiness, excitability, insomnia, and anorexia. These side effects in cats are very uncommon! In fact, this drug is very safe. It has been reported that profound overdose (cat chews canister) can cause problems with potassium regulation; I have never seen this.
How to Use:
1. Prescribe the drug as the generic (albuterol MDI)—two puffs into spacer q12h.
2. Cat should breathe the drug through the mask and spacer for 7–10 seconds.
3. Positive clinical effect should be seen within 5–10 minutes.
4. Can be used every half hour for two to four hours as needed in crisis.
SUMMARY
Inhaled steroids and bronchodilators are the standard of care in the treatment of humans with asthma. Over the past two years, we have treated more than 60 steroid-dependent asthmatic cats with twice-daily flovent and proventil on an as-needed basis. Approximately 2/3 of these patients no longer use oral prednisone. The methods described above are effective, practical, very safe, and avoid the complications associated with chronic oral corticosteroid use.