Pulmonary Function Tests (PFTs)

About pulmonary function tests

Pulmonary function tests (PFTs) are non-invasive tests that evaluate the respiratory function, essentially how well the lungs are working.

During PFTs, there are several parameters and volumes measured, some of which can also be estimated from patient data such as age, gender, height or ethnicity, which is what this calculator sets out to do, allowing to determine the possible values, before or after actual spirometry or plethysmography tests.

Spirometry tests are performed with the use of a spirometer, which is a device with a mouthpiece hooked up to a small electronic machine. Plethysmography tests are performed with a plethysmograph, which is a box similar to a short telephone booth, in which the patient is placed when performing the inhalation and exhalation tests.

Both methods are non-invasive, safe and easy to use but some of the risks of the procedure you may wish to know about include dizziness during the tests, shortness of breath during tests, coughing, asthma attack brought on by deep inhalation.

Pulmonary disorders diagnosed following pulmonary function tests can be classified as:

• Obstructive conditions – a decreased flow of air due to increased airway resistance;
• Restrictive conditions – when lung tissue or chest muscles don’t expand sufficiently, thus leading to low lung volumes.

Conditions diagnosed following PFTs include:

• Allergies;
• Respiratory infections;
• Chronic lung conditions (asthma, bronchiectasis, emphysema, or chronic bronchitis);
• Sarcoidosis;
• Scleroderma;
• Restrictive airway problems from malignancy or inflammation or scarring of the lungs.

Some surgery procedures may require a set of PFTs to be performed before. PFT results may be less accurate if the patient doesn’t fully cooperate, if bronchodilators are used, if pain medicines are used, during pregnancy, if patient suffers from stomach bloating during tests or other conditions that may affect patient’s ability to follow the instructions.

Tidal Volume (TV)

Tidal volume is the measure of the amount of air inhaled during a normal breath. Safe tidal volumes can be determined based on patient’s height and gender and the rule of thumb of 6 to 8mL per kg of ideal body weight (IBW) where:

• IBW male = 50kg + 2.3 x (Height in inches – 60)
• IBW female = 45.5kg + 2.3 x (Height in inches – 60)

Normal range for measured tidal volume is 500 – 780 mL.

Minute Volume (VE)

Minute ventilation or minute volume, is defined as the total volume of gas entering (or leaving) the lung per minute and is calculated as product of tidal volume and respiratory rate.

• VE in mL/min = Tidal volume (Vt) in mL x Respiratory rate (RR) in resp/min

Normal range for minute volume is 70 – 110 mL/kg/min or in average adults (4.0 – 8.0 L/min).

Peak Expiratory Flow Rate (PEFR)

The peak expiratory flow rate predicts the maximum speed of expiration and offers information about the airflow through the bronchi, thus can quantify how severe the degree of airway obstruction is.

Peak expiratory flow is estimated via one of the three formulas, depending on the patient characteristics:

• Children PEFR = ((Height in cm - 100) x 5) + 100
• Adult Men = (((Height in m x 5.48) + 1.58) - (Age x 0.041)) x 60
• Adult Women = (((Height in m x 3.72) + 2.24) - (Age x 0.03)) x 60

Normal range in adults is between 390 and 740 L/min.

Forced Vital Capacity (FVC)

Forced vital capacity (FVC) is the total amount of air exhaled during the FEV test.

• FVC = Race x 1.15 x [(0.0443 x Height) - (0.026 x Age) - 2.89]

Where Race variables are: 0.93 for Asian, 0.87 for Black or African American and 1 for White Caucasian.

Normal range FEV1 is between 3.0 and 5.0 L

Forced Expiratory Volume (FEV1)

Forced expiratory volume (FEV1) measures how much air a person can exhale during the first second of a forced breath.

• FEV1 = Race x 1.08 x [(0.0395 x Height) - (0.025 x Age) - 2.6]

Where Race variables are: 0.93 for Asian, 0.87 for Black or African American and 1 for White Caucasian.

Normal range FEV1 is between 2.4 and 4.0 L. Lower FEV1 values are indicative of obstructive lung disease, such as asthma or COPD.

Estimated Vital Capacity

Vital capacity can be indirectly estimated based on gender, age and height in centimetres.

• VC for men = [(27.63 – 0.112 x Age in years) x Height in cm] / 1000
• VC for women = [(21.78 – 0.101 x Age in years) x Height in cm] / 1000

Subject age is taken into account as a factor because vital capacity increases during the 20s and 30s and then follows a steady decrease towards the 50s. The VC estimation is gender specific (men tend to have a higher volume than women) but the general normal range is between 3.0 and 5.0 L.

Pulmonary Capacities

During spirometry, the four respiratory volumes are measured: inspiratory reserve volume (IRV), tidal volume (TV), expiratory reserve volume (ERV) and residual volume (RV). These are then used to determine the lung capacities, defined as:

References

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