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How Spirometry Works

Posted Aug 27 2008 8:33am

The world of cystic fibrosis is full of technical terminology and abbreviations. One such abbreviation is PFTs which stands for pulmonary function tests. PFTs are of critical importance in helping the CF team keep track of the progression of lung disease in CF patients. PFTs are typically administered by a respiratory therapist either in the clinical setting or during in-patient hospitalization.

This test checks the volumes and flow rates the patient is capable of producing by forcing (blowing) air out of their lungs into a mouthpiece. The test results can be compared at each visit to determine if lung function is staying the same, improving, or getting worse. A nose clip is used to keep air from coming out of the nose, so all the air goes into the mouthpiece. Three consistent forced expirations are required for a good test. A respiratory therapist in the pulmonary lab area will coach and encourage the patient during the test to get the best effort.

The Basics of Spirometry

The most common test used to evaluate how well the lungs are functioning is spirometry. Specifically, spirometry measures how quickly air moves in and out of the lungs. The patient breathes into an apparatus called a spirometer. The spirometer is connected to a computer, which uses a special program to measure the volume and speed of air.

A spirometer uses a disposable mouthpiece attached to a tube, which in turn, is connected to a variety of electronic sensors. In order to carry out the test, the patient must sit up straight, feet flat against the floor and head facing forward. The mouthpiece and tube is then positioned directly in front of the patient at a comfortable height. The patient is then given a pair of soft nose clips to ensure that all inhalation and exhalation occurs only via the mouth.

Spirometry is more than just breathing into a tube. In order to give the computer good data (i.e. repeatable and detectable) the patient must follow a specific breathing technique. This begins with what is referred to as “tidal” breathing. Basically this is just relaxed breathing that is not too shallow or too deep. The patient takes steady, evenly spaced breaths for a few moments. This gives the computer program time to recognize the individual’s breathing pattern.

When the respiratory technician gives the signal, the patient must breathe in quite deeply then exhale forcefully and quickly. The goal is to empty the lungs as much as possible in the first second of breathing out. This results in the measurements of Forces Vital Capacity (FVC) and Forced Expiratory Volume (FEV). Together these give a good indication as to the overall health of the lungs. The amount of air exhaled in the first second, is called FEV1. This is the value the respiratory technician and the doctor look at the most in cystic fibrosis.

Breathing Technique

Taking the deep breath in can be difficult, especially for someone who is experiencing respiratory distress, or other complications. The key to breathing deeply is to focus on filling the lungs completely. By using the lower abdominal muscles, the diaphragm is stretched and more air can enter the lungs.

It is important for the patient to resist the urge to raise his or her shoulders upon inhalation. This is especially difficult for asthmatic patients or those with severe congestion, as they are accustomed to feeling relief by holding their posture in such a way. However, this is not conducive to deep breathing.

What Do Spirometry Results Tell Us?

Spirometry results are displayed on a spirograph. An example of this is in the picture below. In addition to the graphical representation of the results, a table listing different numerical values or calculations is included. They are:

Force vital capacity (FVC): a measure of how much air the patient can exhale with force immediately following a deep breath in.

Forced expiratory volume (FEV): the measure of the amount of air that can be exhaled forcefully in one breath. FEV is measured at 1 (FEV1) second, 2 (FEV2) seconds, or 3 (FEV3) seconds. In the case of CF, FEV1 is most commonly used to diagnose the progression of the disease.

Forced expiratory flow (FEF )25% to 75%: these values give a measure of the amount of flow halfway through the exhaled breath.

Peak expiratory flow (PEF): In the past, pediatricians only used peak flow (determined by a simple hand held device called a peak-flow meter) to gauge breathlessness. Although this measurement gives a good indication of how quickly a patient can exhale, it is not very useful in determining whether the small airways or the larger airways are constricted.

Measures of capacity and volume are expressed as liters (L). All measurements of flow are expressed as liters per minute (L/min).

What Does the Doctor Do With the Results?

Everyone will experience a loss of lung function over time. People with CF lose it at a much quicker rate than their healthy peers. By keeping track of spirometry results, the respiratory therapist and CF specialist can determine the rate of progression of lung disease. By identifying problems early, severe damage can possibly be averted.

For example, if a patient who typically has an FEV1 of 2.7 Liters suddenly drops to 2.1Liters, this may indicate the presence of infection. Based on that assumption, the doctor may immediately prescribe anti-inflammatory medication, request a sputum sample to look for infection, or order a chest x-ray to get a better look at what’s going on in the patient’s lungs.

Some doctors are very proactive about preventive antibiotics. As such, patients who exhibit a dramatic decrease in FEV1 in a short period of time may find themselves hospitalized immediately. When a patient’s baseline pulmonary status decreases by 10%, the patient is said to be experiencing an exacerbation requiring treatment.

How often is spirometry done?

Patients admitted to the hospital perform PFT’s soon after admission and usually several more times before going home (depending on the length of stay in the hospital). This is one way to evaluate how well a patient is improving during the hospital stay and decide when they are well enough to go home.

Different clinics and hospital have their own guidelines for frequency of spirometry. Regular spirometry is an important component of aggressive treatment for cystic fibrosis. The American Thoracic Society has determined how often spirometry should be done. Unfortunately, people with CF who receive their care at facilities other than at an accredited CF Clinic, may not be having their spirometry measured frequently enough.

The Cystic Fibrosis Foundation recommends that adults should have spirometry done at each clinic visit. Pediatric patients need only be monitored every 3 to 6 months.

Can I Do Spirometry at Home?


Yes! Although you will not generate the same type of in-depth results as a formal spirometry study conducted at your CF Clinic, you can monitor your own peak flow and FEV1 levels in the privacy and comfort of your own home. There are a few different commercially available products, which are linked below.



Home-use Spirometers and Software to track FEV1

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