Terminology

Terminology

Overview

Spirometry is a diagnostic test used to aid in the diagnosis of lung diseases and monitoring lung health. This physiological test measures how an individual inhales or exhales volumes of air as a function of time. The primary signal measured in spirometry may be volume or flow.

 

In order to perform the test properly, it is essential to know which parameters are measured during the test and how this relates to lung physiology. This module will provide definition and illustration of the different parameters measured during spirometry. The procedure of measurement will be discussed in Module 4: How to perform the procedure including reading and reporting of results.

Aims

  1. To provide the basic definition of the terms used in spirometry.
  2. To illustrate the different parameters used in spirometry.
  3. To describe the characteristics of a flow-volume curve.
  4. To describe the characteristics of a volume-time curve.

Assessment

Upon successful completion of this module, the participant should be able to:

  • Define the most commonly used spirometry indices.
  • Describe how a parameter is measured or calculated.
  • Identify a flow-volume curve and a volume-time curve in relation to the physiological process involved in performing the test.
  • Define the relationship between the flow-volume curve and the volume-time curve.

Definition of terms

Spirometry indices:

ATPS: ambient temperature and pressure, saturated; denoting a volume of gas saturated with water vapor at ambient temperature and barometric pressure.

 

BTPS: body temperature and pressure, saturated; denoting a volume of gas saturated with water vapor at 37°C / 99°F and ambient barometric pressure.

 

Spirometry devices measure lung function in ATPS conditions. Since ambient temperature differs from body temperature, the measured results need to be converted to BTPS so that the measured value reflects the conditions within the patient's lung.

 

Correction factor: In spirometry, the term correction factor refers to the correction factor which is needed to convert values measured from one set of conditions to another set of conditions.

 

Vital Capacity (VC): The volume change measured at the mouth between a full inspiration and full expiration. The manoeuvre may be performed in different ways. This parameter is expressed in litres at BTPS. (Illustration 1)

Illustration 1: Vital Capacity (VC)

 

Also known as Slow VC or Relaxed VC. Various ways to achieve the VC:

Inspiratory Vital Capacity (IVC): The vital capacity measured during an inspiratory manoeuvre. Starting from end-tidal volume, the subject exhales maximally until a plateau is reached then inhales fully. This parameter is expressed in litres at BTPS.

Illustration 2: Inspiratory Vital Capacity (IVC)

Expiratory Vital Capacity (EVC):
The vital capacity measured during an expiratory manoeuvre. Starting from end-tidal volume, the subject inhales fully and subsequently exhales maximally until plateau is reached. This parameter is expressed in litres at BTPS. (Illustration 2)


Forced Vital Capacity (FVC) or (FEVC): The volume of air that is forcefully exhaled after a maximum inhalation. This parameter is expressed in litres at BTPS. (Illustration 3)

 

Illustration 3: Volume time curve


Forced Inspiratory Vital Capacity (FIVC):
The volume of air that is forcefully inhaled after the FVC manoeuvre. This parameter is expressed in litres at BTPS.

 

The vital capacity is subdivided into the tidal volume (TV, VT), inspiratory reserve volume (IRV) and expiratory reserve volume (ERV). (Illustration 1)

Tidal Volume (TV, VT): The volume of air which is inhaled and exhaled during a respiratory cycle (normal breathing). This is a dynamic lung volume that can vary depending on the level of physical activity. This parameter is expressed in litres at BTPS. (Illustration 1)

 

Inspiratory Reserve Volume (IRV): The maximum volume of air that can be inhaled from the mean end inspiratory level. This parameter is expressed in litres at BTPS. (Illustration 1)

Expiratory Reserve Volume (ERV):
The maximum volume of air that can be exhaled from the level of the functional residual capacity (FRC). This parameter is expressed in litres at BTPS. (Illustration 1)

Forced Expiratory Volume in 1 second (FEV1):
The FEV1 is the volume of air that is forcefully exhaled in the first second during the FVC manoeuvre. FEV1 is an index used for assessing airway obstruction, bronchoconstriction or bronchodilation. This parameter is expressed in litres at BTPS. (Illustration 3)

FEV1/(F)VC ratio:
The FEV1 expressed as a percentage of the (F)VC is the standard index for assessing and quantifying airflow limitation. This parameter is reported with a decimal fraction to two decimal places.

As IVC > EVC > FVC in patients with obstructive lung disease, the VC should be specified when using the FEV1/VC ratio, hence FEV1%FVC (forced expiratory ratio, FER) or FEV1%IVC. The Tiffeneau-index is FEV1%IVC.

 

Forced inspiratory volume in 1 second (FIV1): The FIV1 is the volume of air that can be forcefully inhaled in the first second during a forced inspiratory manoeuvre following the FVC manoeuvre.

Most people find the inspiratory manoeuvre difficult to perform. Hence, this can also be measured following an EVC manoeuvre during an FVC measurement.

Peak Expiratory Flow (PEF):
The highest expiratory flow achieved during the FVC manoeuvre which is started without hesitation. This parameter can be expressed in litres per second or litres per minute. (Illustration 4)

Illustration 4: Flow-volume curve


Peak Inspiratory Flow (PIF):
The highest inspiratory flow achieved during the FIVC manoeuvre which is started without hesitation. This parameter can be expressed in litres per second or litres per minute. (Illustration 4)

 

Inspiratory Capacity (IC): The volume of air that is inhaled from FRC, to a position of maximum inspiration, expressed in litres at BTPS. (Illustration # 1)

 

Forced Expiratory Flow 25-75% (FEF 25-75%): The mean forced expiratory flow between 25% and 75% of the FVC. Expressed in litres at BTPS. This index is taken from the largest sum of FEV1 and FVC. It should be noted that it is highly dependent on the validity of the FVC measurement and the level of expiratory effort. (Illustration 4)

 

The following parameters fall under static lung volumes and are not measured during spirometry. These parameters were included in this module to provide the participant a complete parameter list of lung volumes and capacities.

Residual Volume (RV):
The volume of gas remaining in the lungs after a full exhalation. (Illustration 2)

Functional Residual Capacity (FRC):
The volume of gas present in the lungs after a normal expiration, is mainly determined by the interaction between elastic recoil of the chest and lungs. (Illustration 2)

 

Total lung capacity (TLC): The volume of gas contained in the lungs after a full inhalation. (Illustration 2)

 

Diagnostics:

Lung volume: refers to the volume of gas within the lungs

 

Lung capacity: refers to the volume of air which comprises two or more lung volumes.

 

Reversibility testing: Reversibility testing is usually performed for the diagnosis of asthma. Spirometry is performed, after which a bronchodilator is given that can either be a short-acting β-agonist or other agents, such as anticholinergics.

 

Bronchodilator responsiveness testing is a determination of the degree of improvement of airflow in response to bronchodilator administration as measured by changes in FEV1 and FVC.

 

Obstruction: Obstruction is characterised by airflow limitation; there is a decreased airway calibre through either smooth muscle contraction, inflammation, mucus plugging or airway collapse in emphysema.

 

Restriction: Restrictive disorders are characterised by a loss in lung volume and are much rarer. This occurs in pulmonary fibrosis, pleural disease, chest wall disorders (kyphoscoliosis), neuromuscular disorders, pneumonectomy, pulmonary oedema and obesity, to name a few. Many so-called restrictive spirometry traces are due to failure to reach the end of expiration, falsely reducing FVC.

 

End of forced expiration (EOFE): Previous standards used the term "end of test" and the abbreviation "EOT" to denote end of forced expiration (EOFE). These standards stress the importance of a maximal inspiration after the forced expiration. As such, the end of forced expiration is not the end of the manoeuvre, and hence the term EOFE is used.

 

Back-extrapolation (BEV): The volume of air measured before the start of the FVC manoeuvre. (Illustration 5)

Illustration 5: Back-extrapolation

 

If a patient is slow to start the expiratory manoeuvre, the measure can be corrected by back extrapolation. Time zero can be assessed by visual analysis, and should be reported.

  • Most computerised spirometers correct automatically.
  • If the machine does not have a printout, it can be difficult to assess quality of manoeuvre.