The Differences Between a Peak Flow Meter and a Spirometer

Peak flow and spirometry seem to have similar functions. While they both measure the amount of air your lungs can force out, a peak flow meter is used to monitor symptom management at home while spirometry is used to measure lung function. One instrument -- the peak flow meter -- alerts you when your symptoms are about to worsen while a spirometer provides your doctor with valuable information about your lung condition.

Function of a Peak Flow Meter

• A peak flow meter is a small instrument that measures the level of air flowing out of the lungs as you blow quickly and forcefully into the mouthpiece. This device alerts you to the narrowing of your airways before an asthma attack happens. When used daily, the peak flow meter can help you measure the effectiveness of your asthma management and treatment plan, when to add more medication or stop a medication, what triggers an attack and when to get emergency medical care.

Using a Peak Flow Meter

• The peak flow meter should be used daily and when you are experiencing the onset of asthma-related problems. The device should be used before taking a short-acting bronchodilator or rescue medication. To use the device correctly, move the pointer to zero, hold it by the handle, make sure your mouth is empty and stand up straight. Inhale deeply, put the device in your mouth and seal your teeth and lips around the mouthpiece. Blow out as fast and hard as you can -- give a "fast blast." Note where the pointer stops and write this number down. Reset it to zero and repeat twice more. If you have used the device correctly, all three of your numbers should be similar. Without averaging the numbers, write down the highest of the three numbers on a graph. This is your "peak flow" for the day.

Different Peak Flow Zones

• The different peak flow zones are based on the colors red, yellow and green. Red means you are in trouble medically, yellow means be cautious and monitor your condition more closely while green means safe. Each zone varies from one asthma patient to another. The red zone means you are under 60 percent of the amount of air your lungs should be able to expel. Yellow means your lungs are functioning at 60 to 80 percent of normal. The green zone means your lungs are functioning between 80 to 100 percent of normal.

What Is Spirometry?

• Your doctor uses a spirometer to measure your lung function. This instrument measures how well your lungs take in, hold and use air; it monitors asthma and other lung conditions, the effectiveness of your treatment, the severity of the lung disease and whether the disease is restrictive or obstructive. Spirometry measures expiratory vital capacity or how much air can be blown out of the lungs. It also measures inspiratory vital capacity or how much air can be breathed into the lungs.

How Are Spirometry Readings Interpreted?

• Your doctor uses your spirometry results to figure out what your lung function is. He also uses the readings to place your lung function into one of four classifications: normal, obstructive, restrictive or combined. Asthma, chronic bronchitis and chronic obstructive pulmonary disease fall into the obstructive category; scarring, fibrosis, a physical deformity, previous partial lung removal and pneumoconiosis fit into the restrictive category. A disease, such as cystic fibrosis, fits into both the obstructive and restrictive categories.

Differences Between Spirometry and Peak Flow Readings

• Spirometry instruments are used to measure the entire forced vital capacity (FVC) of your lungs over a span of time. This allows the time and volume curve to be evaluated. Peak flow, which measures the peak expiratory flow, captures the largest flow that can be maintained for 10 milliseconds starting from the full inflation of the lungs.