An oximeter clipped to the tip of your finger gives the quickest, most accurate picture of your oxygen transport system.
This is for information, not an advert!
http://www.fact-canada.com/Sportstat/sportstat-pulse-oximeter.html
I would think that this would be useful for determining whether a "difficult to breathe" condition was an effect rather than a cause. Sometimes excitement causes hyperventilation which would be labored breathing accompanied probably by a rise in blood oxygen.
The advantage of the inspirometer is that it can quantify the constricted airways which are the definition of asthma and it can do that when the attack is not severe and when the subject is at rest and not so troubled by the airway constriction. This permits early accomodation to a potentially deadly episode.
A detectable decrease in blood oxygen might not occur until well into an attack. I don't know anything about how the oximeter readings might tend to vary normally and how they might vary during an asthma episode.