Asthma: Normal peak flow meter readings

My 15 year old son is having a problem with exercise induced asthma. To try to help us evaluate whether or not his prescribed inhaler is effective, could you please give us information on what his peak flow should be prior to exercise. Our family practice doctor is not very familiar with the condition, and I suspect she may have given us an inappropriate peak flow meter for a healthy 15 year old. He is easily able to max out the peak flow meter she gave him both before and after exercise, even when he is experiencing severe shortness of breath. Since he is able to max out the meter both before and after exercise, we have no way of knowing what his decrease in lung capacity is, if any. He is 5 ft 9" tall, weighs 165 lbs and is in excellent health other than the EIA. He does not suffer from asthma other than EIA.

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ABOUT THE EXPERT

Robert Steele

Robert W. Steele, MD, is a board certified pediatrician at St. John's Regional Health Center in Springfield, MO. He graduated from medical... Read more

First of all, congratulations to both you and your son for doing such a good job in trying to monitor his asthma. The appropriate methods of treating asthma have come quite a long way in such a short time due to the advances in medications. However, just as importantly, the methods for monitoring asthma have become more sophisticated as well. One of those methods is by using a peak flow meter. The peak flow meter is a simple device that can measure the maximum volume rate of air someone can blow during the first second or so of expiration. By checking what an individual's "personal best" peak flow is during times of no symptoms, the decrease in peak flow during an asthma attack can be identified and monitored. When the peak flow drops significantly, concern about the asthma rises. Conversely, a peak flow which remains at a high level helps in reassuring that the asthma is under control and perhaps any chronic medications being used may be decreased.

There are published normal predicted average peak flows for children. They are based upon height rather than age, weight, or sex. Just like blood pressure, normal peak flows increase with increasing height. And while age and sex have some influence upon normal peak flow values, it is the height (in prepubertal children) which tends to have the most effect. Below is the table of these average peak flows, but as I will explain, these values often have no meaning for the individual child with asthma.

Normal Predicted Average Peak Expiratory Flow for Children
(in liters/minute)

Height (inches) Peak Flow Height (inches) Peak Flow Height (inches) Peak Flow
43 147 51 254 59 360
44 160 52 267 60 373
45 173 53 280 61 387
46 187 54 293 62 400
47 200 55 307 63 413
48 214 56 320 64 427
49 227 57 334 65 440
50 240 58 347 66 454

Normal Predicted Average Peak Expiratory Flow (Post-pubertal)

 

Males
60" 65" 70" 75" 80"
554 602 649 693 740
    
Females
55" 60" 65" 70" 75"
390 423 460 496 529

Jim, you can get an idea of what is an average peak flow for a person your son's height. Which table you use depends upon how far into puberty he is. At 5'9", he is probably closer to the end of puberty than the beginning and should thus use the second table. Ultimately, it doesn't matter because the important thing is not whether he conforms to the table but what his own personal best is when he is not having asthma symptoms. It is this personal best that all other readings should be judged. A drop of 20% in peak flow from personal best suggests the beginnings of significant asthma symptoms. A drop of 40% suggests more severe symptoms. Peak flow meters come in both a child and adult size. If he is easily hitting the maximum peak flow value on his meter, he most likely has the child size and needs an adult one.

I hope this helps.

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