Why do my peak flows not decrease

The following question from medscapes.com was a great one, and one I though my readers would enjoy hearing the answer to.

Question: Why do my peak flow readings not decrease when I'm having an attack?

Yesterday morning I was out doing my rabbit chores and I was having shortness of breath upon exertion (lifting, etc). I went inside and checked my Peak Flow Reading and it was a little lower than usual, but not in my yellow zone; I used my rescue inhaler anyway. I went back outside and my shortness of breath was better. Later yesterday evening I started coughing and had chest tightness. My pfr was in my yellow zone. I woke up during the night once.

What I don't understand is how come I had shortness of breath that responded to albuterol, but had a pfr that was relatively ok? Was it a precursor to what happened later that evening and at night?

Thank you.

Answer #1 by Dr. Matthew M. Mintz: though peak flows do correlate with asthma symptoms, they are far from perfect. In children, for example, even some of the most severe asthmatics have normal lung functions. The purpose of doing peak flows was to identify an attack before it happened; however, this process is far from perfect. That said, current guidelines do recommend peak flow monitoring for patients with moderate to severe asthma.

Answer #2 by Stephen Gaudet, RRT and asthmatic: Also, Peak flow measurements represent the function of the larger airways. When your smaller airways are obstructed ( from inflammation, mucus or constriction), it's not always reflected in your peak flow numbers. You might also have some air-trapping, which could make you short of breath while still having near normal peak flows. In either case, albuterol usually helps.

Answer #3 by your humble RT: If it makes you feel any better, in February 1998 my asthma got so bad I required a breathing treatment every hour for about a week. However, my peak flow readings never wavered below my predicted normal. As a kid my peak flows were reliable predictors of oncoming asthma symptoms, yet as an adult this doesn't seem to be the case. Why?

My theory for this would coincide with what Steve said, that as a kid my airways were small, and the increased inflammation that occurs during an attack caused both my upper and lower airways to become obstructed. As an adult, my airways are bigger, and thus increased inflammation is more likely to effect my smaller airways. At least that's the theory we'll stick to for now.