|Year : 2014 | Volume
| Issue : 3 | Page : 255-258
Components of respiratory function tests
Bahaa El-Din Ewees Hassan, Mai M Abdel-Aziz
Department of Anesthesiology, Intensive Care, and Pain Management, Faculty of Medicine, Ain Shams University, Cairo, Egypt
|Date of Submission||19-Apr-2014|
|Date of Acceptance||29-May-2014|
|Date of Web Publication||27-Aug-2014|
Mai M Abdel-Aziz
Department of Anesthesiology, Intensive Care, and Pain Management, Faculty of Medicine, Ain Shams University, Cairo 11566
Source of Support: None, Conflict of Interest: None
The respiratory system is composed of the lungs, the conducting airways, the parts of the central nervous system concerned with the control of the muscles of respiration, and the chest wall . The main functions of the respiratory system are to obtain oxygen from the external environment and supply it to the cells and to remove from the body the carbon dioxide produced by cellular metabolism . Pulmonary function tests provide valuable clinical information. They are designed to identify and quantify defects in the respiratory system .
Keywords: lung volumes, pulmonary function, respiratory function, spirometry
|How to cite this article:|
Hassan BDE, Abdel-Aziz MM. Components of respiratory function tests. Ain-Shams J Anaesthesiol 2014;7:255-8
Components of pulmonary function tests
The volume of air entering or leaving the lung in a single breath (SB) is called the tidal volume. It is usually 500 ml. The maximum volume of air that can be inhaled beyond this value is called the inspiratory reserve volume and is about 3000 ml. After normal expiration, the lung still contains the functional residual capacity (FRC) and it averages 2500 ml. The maximal volume of air, beyond the tidal volume, that can be exhaled using maximal expiratory effort is the expiratory reserve volume. Even after maximal expiration, 1000 ml air remains in the lungs and is termed the residual volume (RV) . The maximal volume of air expired after a maximal inspiration is the vital capacity; the forced expiratory volume in 1 s (FEV 1 ) is a variant of this method  [Figure 1].
Spirometry is used to measure the rate at which the lung changes volume during forced breathing maneuvers. It is the simplest and the most common test; it provides most of the information obtained from performing pulmonary function tests .
Spirograms and flow-volume curves
There are two methods for recording the flow-vital capacity (FVC). The first, called the classic spirogram, is that the patient blows into a spirometer that records the volume exhaled, which is plotted as a function of time. The FVC can also be plotted as flow-volume (FV) curve, in which the patient exhales forcefully and rapidly through a flow meter that measures the flow rate (l/s) at which the patient exhales  [Figure 2]. The FVC is the total volume of air expired during forceful expiration after maximal inhalation. Its normal value varies with age, sex, and height. The causes of decreased FVC are due to problems in either of the following:
(1) The lung itself - for example, resection, pulmonary fibrosis, congestive heart failure.
(2) The pleural cavity - for example, effusion.
(3) The chest wall - for example, scleroderma, obesity, kyphoscoliosis.
(4) The respiratory muscles - for example, diaphragmatic paralysis and myasthenia gravis.
|Figure 2: The two ways to record spirogram: volume recorded as a function of time (a) and fl ow-volume curve (b) . FEF, maximal forced expiratory fl ow; FEV, forced expiratory volume|
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Forced expiratory value in 1 s
The FEV 1 is perhaps the most useful measurement obtained from spirometry. It is the volume of air exhaled in the first second of the FVC test. Similar to the FVC, its normal value varies with age, sex, and height . In an obstructive defect, the FEV 1 is decreased by an amount that reflects the severity of the disease. The FVC may be also decreased but to a lesser degree. In a restrictive defect, the FEV 1 is also decreased. The FVC is almost always decreased. The FEV 1 /FVC ratio is used to differentiate obstructive form restrictive patterns .
FEV 1 /FVC ratio
0It is generally expressed as a percentage. The FEV 1 is a constant fraction of the FVC irrespective of lung size in the normal adult. The ratio normally ranges from 75 to 85%, but it decreases somewhat with aging . The significance of this ratio is that it differentiates between obstructive and restrictive defects when the FVC is low. For example, in pulmonary restriction, without any obstruction, the FEV 1 and the FVC are decreased proportionally; hence, the ratio remains in the normal range. In severe obstructive disease, the flow may be very low at the end of a forced expiration. Continuation of a forced expiration can be very tiring, and the FEV 6 can be substituted for the FVC  [Figure 3].
|Figure 3: Typical spirograms and fl ow-volume curves during forced expiration. (a) Normal individuals of different sizes; (b) patient with severe airway obstruction; (c) values typical of a pulmonary restrictive process .|
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This is the average maximal forced expiratory flow (FEF) rate over the middle 50% of the FVC.
This is the flow rate after 50% of the FVC has been exhaled.
This is the flow rate after 75% of the FVC has been exhaled.
Maximal forced expiratory flow or peak expiratory flow
This occurs shortly after the onset of expiration and can be calculated using hand-held devices, making this measurement valuable for asthmatic patients at home to monitor their status.
Maximum voluntary ventilation
The patient is instructed to breathe as hard and fast as possible for 10-15 s. The results are extrapolated to 60 s and reported in liters per minute. It correlates well with a patient's exercise capacity and with the complaint of dyspnea .
Some spirometers are capable of recording both expiratory and inspiratory flows. The patient exhales maximally (the FVC test) and then immediately inhales as rapidly and completely as possible, producing an inspiratory curve. The combined expiratory and inspiratory FV curves form the FV loop. Increased airway resistance decreases both maximal expiratory flow and maximal inspiratory flow, detecting lesions of the major airway . Two major characteristics are used to identify the obstruction as well as its site:
(1) According to the behavior of the lesion during forced expiration and inspiration, the lesion (obstruction) can be classified into:
(a) Variable: When narrowing occurs and flow decreases, during one phase of respiration but not the other.
(b) Fixed: When narrowing occurs and flow decreases, equally during both expiration and inspiration.
(2) The location of the lesion:
(a) Extrathoracic: This is when the lesion lies outside the thoracic outlet.
(b) Intrathoracic: This is when the lesion lies within the thoracic portion of the trachea down to the carina but generally not beyond  [Figure 4].
Static (absolute) lung volumes
|Figure 4: Comparison of typical flow-volume loops in normal individuals: (a) normal; (b) obstructive defect; (c) restrictive defect; (d) variable extrathoracic obstruction; (e) variable intrathoracic obstruction; (f) fi xed airway obstruction .|
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Measurement of the static lung volume is often useful. The most important volumes are:
(1) The vital capacity or the slow vital capacity, which is measured by having the patient inhale maximally and then exhale slowly and completely.
(2) The RV with complete exhalation; the volume of air remaining in the lungs is termed the RV.
(3) The total lung capacity.
The three most commonly used methods of measuring the FRC (from which the RV is obtained) are nitrogen (N 2 ) washout, inert gas dilution, and plethysmography.
Nitrogen washout method
At the end of a normal expiration, the patient is connected to the system. The lung contains an unknown volume (Vx ) of air containing 80% N 2 . With inspiration of N 2 -free oxygen and exhalation into a separate bag, all N 2 can be washed out of the lung. The volume of the expired bag and its N 2 concentration are measured, and the unknown volume is obtained with the simple mass balance equation .
Inert gas dilution technique
Helium, argon, or neon can be used. In the helium method, the spirometer system contains a known volume of helium (V1 ) with a known concentration (C1 ). At FRC, the patient is connected to the system and rebreathes until the helium concentration reaches a plateau, indicating equal concentration of helium (C2 ) in the spirometer and lung. As essentially no helium is absorbed, equations (1) and (2) can be combined and solved for Vx , the FRC .
The theory is based on Boyle's law, which states that the product of the pressure (P) and volume (V) (PV) of a gas is constant under constant temperature (isothermal) conditions. The gas in the lungs is isothermal because of its intimate contact with capillary blood. The plethysmographic method measures essentially all the gas in the lung, including that in poorly ventilated areas .
Diffusing capacity of the lungs
As measuring the diffusing capacity of oxygen is technically extremely difficult, the diffusing capacity of carbon monoxide (DLCO) is much easier and provides a valid reflection of the diffusion of oxygen. The most widely used method to measure the DLCO is the SB method. The patient exhales to RV and then inhales a gas mixture containing a very low concentration of carbon monoxide and an inert gas, usually helium. After a maximal inhalation to total lung capacity, the patient holds his or her breath for 10 s and then exhales completely. A sample of exhaled alveolar gas is collected and analyzed. By measuring the concentration of the exhaled carbon monoxide and helium, the value of the DLCO can be computed.
Performing the spirometry test before and after the administration of a broncholdilator is usually carried out for patients undergoing spirometry for the first time. A β-2 receptor agonist is usually selected.
Tests for distribution of ventilation
There are many tests used to detect abnormal patterns of ventilation distribution. The simplest method is the SBN 2 test.
The SBN 2 test is performed as follows; the patient exhales to RV and then inhales a full breath of 100% oxygen from the bag on the left [Figure 5]. A slow, complete exhalation is directed by the one-way valve through the orifice past the N 2 meter into the spirometer. The orifice ensures that expiratory flow will be steady and slow (<0.5 l/s). N 2 meter continuously records the N 2 concentration of the expired gas as it enters the spirometer. With simultaneous plotting of the expired N 2 concentration against expired volume, the normal graph is shown in [Figure 4]. There are four portions of the normal graph: phases I-IV. The events during expiration in a normal individual are as follows; the initial gas using the N 2 meter comes from the trachea and upper airway and contains 100% oxygen. Thus, phase I shows 0% N 2 . As expiration continues during phase II, alveolar gas begins washing out the dead space oxygen and the N 2 concentration gradually increases . Phase III consists entirely of alveolar gas. During a slow expiration, initially gas comes predominantly from the dependent alveolar regions, where the N 2 concentration is the lowest. As expiration continues, increasing amounts of gas come from the more superior regions, where N 2 concentrations are higher. This produces a gradually increasing N 2 concentration during phase III. An abrupt increase in N 2 concentration occurs at the onset of phase IV. This reflects the decreased emptying of the dependent regions of the lung. Most of the final expiration comes from the apical regions, which have higher concentration of N 2 . The onset of phase IV is said to reflect the onset of airway closure in the dependent regions, and it is often called the closing volume .
|Figure 5: Equipment required to perform the single-breath nitrogen washout test. A plot of exhaled nitrogen concentration (N2 conc) against exhaled volume is shown at the lower right .|
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| Acknowledgements|| |
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]