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Spirometry – Terminology & Definitions

Spirometry is the classic pulmonary function test that measures the volume of air inspired and expired as a function of time. It can monitor quiet breathing and thereby measure tidal volume, and also trace deep inspiration and expirations to give information about vital capacity. Spirometry can also be used to measure forced expiration rates and volumes and to compute FEV1/FVC ratios.

  • VT – Tidal Volume – This is the amount of air that a person takes in during normal relaxed breathing, also known and tidal breathing.
  • VC – Vital Capacity – This is the amount of air that can be taken in after the patient had exhaled completely.
  • SVC - Slow Vital Capacity - This is the amount of air that patient can take in. It can either be measured from a maximal inspiration to a maximal expiration or from a maximal expiration to a maximal inspiration. This value may differ significantly from the FVC due to airway collapse during the forced maneuver.
  • FVC - Forced Vital Capacity - after the patient has taken in the deepest possible breath, this is the volume of air that can be forcibly and maximally exhaled out of the lungs until no more can be expired. FVC is usually expressed in units called liters. This PFT value is critically important in the diagnosis of obstructive and restrictive diseases.
  • FEV1 - Forced Expiratory Volume in One Second - this is the volume of air that can be forcibly exhaled from the lungs in the first second of a forced expiratory maneuver. It is expressed as liters. This PFT value is critically important in the diagnosis of obstructive and restrictive diseases.
  • FEV1/FVC - FEV1 Percent (FEV1%) - This number is the ratio of FEV1 to FVC - it indicates what percentage of the total FVC was expelled from the lungs during the first second of forced exhalation - this number is called FEV1%, %FEV1 or FEV1/FVC ratio. This PFT value is critically important in the diagnosis of obstructive and restrictive diseases.
  • FEV3 - Forced Expiratory Volume in Three Seconds - This is the volume of air that can be forcibly exhaled in three seconds - measured in Liters - this volume usually is fairly close to the FVC since, in the normal individual, most of the air in the lungs can be forcibly exhaled in three seconds.
  • FEV3/FVC - FEV3% - This number is the ratio of FEV3 to the FVC - it indicates what percentage of the total FVC was expelled during the first three seconds of forced exhalation. This is called %FEV3 or FEV3%.
  • PEFR or FEF Max- Peak Expiratory Flow Rate - This is maximum flow rate achieved by the patient during the forced vital capacity maneuver beginning after full inspiration and starting and ending with maximal expiration - it can either be measured in L/sec or L/min - this is a useful measure to see if the treatment is improving obstructive diseases like bronchoconstriction secondary to asthma.
  • FEF - Forced Expiratory Flow - Forced expiratory Flow is a measure of how much air can be expired from the lungs. It is a flow rate measurement. It is measured as liters/second or liters/ minute. The FVC expiratory curve is divided into quartiles and therefore there is a FEF that exists for each quartile. The quartiles are expressed as FEF25%, FEF50%, and FEF75% of FVC.
  • FEF25% - This measurement describes the amount of air that was forcibly expelled in the first 25% of the total forced vital capacity test.
  • FEF50% - This measurement describes the amount of air expelled from the lungs during the first half (50%) of the forced vital capacity test. This test is useful when looking for obstructive disease. The amount of air that will have been expired in an obstructed patient is smaller than that measured in a normal patient.
  • FEF75% - This measurement describes the amount of air that was forcibly expelled in the last 25% of the total forced vital capacity test.
  • FEF25%-75% - This measurement describes the amount of air expelled from the lungs during the middle half of the forced vital capacity test. Many physicians like to look at this value because it is an indicator of obstructive disease.
  • MVV - Maximal Voluntary Ventilation - this value is determined by having the patient breathe in and out as rapidly and fully as possible for 12 -15 seconds - the total volume of air moved during the test can be expressed as L/sec or L/min - this test parameter reflects the status of the respiratory muscles, compliance of the thorax-lung complex, and airway resistance. Surgeons like this test value because it is a quick and easy way to assess the strength of the patient’s pulmonary musculature prior to surgery - a poor performance on this test suggests that the patient may have pulmonary problems postoperatively due to muscle weakness. MVV can therefore be viewed as a measure of respiratory muscle strength. One major cautionary note is that this test is effort dependant and therefore can be a poor predictor of true pulmonary strength and compliance.
  • MIPs / MEPs or Peak Pressures - this is the maximum inspiratory and expiratory pressures that a patient can exert. This is determined by having the patient breath through a device that can be occluded. The patient takes in a maximal breath, the device is occluded and the patient blows as forcefully as they can for couple of seconds. The occlusion is then stopped. The process is repeated by having the patient exhale completely, to RV, the device is again occluded and the patient attempts to inhale forcefully. This test is used to access pulmonary and thoracic muscle strength. This test can be used to track a patient with neuromuscular disorders. This test is also very effort dependent and therefore can be a poor indicator of true pulmonary and muscular strength. (MIP is used as a determining factor when accessing a patient for extubation.) This value is measured in cm H2O.

Volume and Capacity Spirogram

 

 

 

These values are measured with a Body Box or Gas Dilution

  • IRV – Inspiratory Reserve Volume – This is the maximal amount of air that can be taken in after a normal inspiration. Measured in liters.
  • IC – Inspiratory Capacity – The amount of air that can be maximally taken in after a normal exhalation. Measured in liters. VT + IC = IRV
  • ERV – Expiratory Reserve Volume – This is the maximum amount of air that can be exhaled after normal exhalation. Measured in liters.
  • RV – Residual Volume – This is the amount of air that is left in the lungs after a patient exhales completely. It cannot be directly measured. Measured in liters. In restrictive lung and chest wall disorders, RV decreases less than do the FRC and TLC
  • TLC – Total Lung Capacity – This is the amount of air that the lungs can hold. This capacity cannot be directly measured. It is the total amount of air that can be taken in after the patient has exhaled completely (VC) plus the residual volume (RV). Measured in liters. VC + RV = TLC
  • RV/TLC - This number is the ratio of RV to TLC- it indicates what percentage of the total TLC is left in the lungs after a complete exhalation - This PFT value is critically important in the diagnosis of obstructive disease. It can indicate the amount of trapped gas is in the lung due to collapsed airways.
  • FRC – Functional Residual Capacity – This is the total amount of air that is in the lung after a normal exhalation.
  • TGV – Thoracic Gas Volume – This is the lung volume at which the shutter was closed. The value usually corresponds to the FRC.
  • RAW – Airway Resistance – The resistance to airflow in the airways is due to friction, which depends on the radius of the opening the air or gas flows through. Consequently, patients with smaller airway radii caused by bronchoconstriction or airway obstructions will have an increased airway resistance.

These values are measured using a mixture of gas containing Carbon Monoxide. The reason that CO is used is that the Hemoglobin in the blood has an affinity 210 X more that it does for O2 or CO2.

  • Dsb – Diffusion, single breath – This value indicates the ability of gas to cross the A-C (alveolar – capillary) membrane. This value is very important in the diagnosis of different disease states. As the lung gets more diseased, the A-C membrane may thicken making it harder for gas exchange to take place. When this happens it can affect the entire body.
  • DLCO  - Diffusing Capacity of the Lung for Carbon Monoxide
  • DsbHb – Diffusion Capacity of the Lung for Carbon Monoxide Hemoglobin Corrected


     
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