Spirometry is a test performed to objectively measure lung capacity/function (ventilation) on the patients with medical indications. The tool used in this test is called a spirometer.
- to statistically and dynamically measure the lung volume
- to assess the change or abnormality in pulmonary function
Basically, spirometry measures the speed of air volume change within the lung during forced respiration which is known as forced volume capacity (FVC). The most common procedure used is the one where the subject performs maximum inspiration and exhales as fast and as whole as possible. FVC value is compared to normal value and prediction valued based on age, body height and sex.
Prior to doing spirometry, anamnesis, as well as body height and weight measurement are performed with the patient. In spirometer, there is a prediction value for Asians based on age and body height. If the prediction value does not conform with Indonesian standard, adjustment will be done using the Indonesian standard. The resulting air volume will be calculated as a percentage of prediction value achievement.
Spirometry can be done in the form of social vital capacity (SVC) or forced vital capacity (FVC). In SVC, the patient will be asked to breathe normally for 3 times (mouthpiece is already put in mouth) before taking a deep breath and exhale to the fullest extent. In FVC, the patient is asked to take a deep breath before the mouthpiece is put into the mouth and the air is exhaled maximally.
Reported lung function measurement:
- Forced vital capacity (FVC) is the amount of air that can be forcefully exhaled after maximum inspiration. It is measured in liter.
- Forced expiratory volume in one second (FEV1) is the amount of air that can be exhaled in 1 second. It is measured in liter. Together with FVC, this is the main indicator of the lung function.
- FEV1/FVC is the ratio of FEV1/FVC. In healthy adults, the value is around 75% - 80%.
- Forced expiratory flow (FEF) 25-75%, optional
- Peak expiratory flow (PEF), is the speed of air movement when it comes out of the lung at the beginning of expiration, measured in liter/second.
- FEF 50% and FEF 75%, optional, is the average flow (speed) of air coming out of the lung in the middle of breathing process (often called as maximal mid-expiratory flow [MMEF]).
Classification of ventilatory defect (% predictive value):
Restrictive defect: Vital capacity (VC) < 80% predictive value FVC < 80% predictive value
Obstructive defect: FEV1 < 80% predictive value FEV1/FVC < 75% predictive value
Restrictive and obstructive defect: FVC < 80% predictive value FEV1/FVC < 75% predictive value
Spirogram is the result of spirometry. Several factors causing a spirogram to not qualify are:
- Using the wrong method in taking a breath, or exhaling too fast
- Early termination
- The closure of glottis
- Varied expiration
Each measurement should be done at least 3 times. The criteria for a reproducible spirogram (after 3 times of expiration) is the two values of FVC and FEV1 from 3 expiration performed having minimum variation/difference (the difference is less than 5% or 100 mL).