Procedures
Spirometry
Spirometry is a physiological test that measures the ability to inhale and exhale air relative to time. Spirometry is a diagnostic test of several common respiratory disperses such as asthma and chronic obstructive pulmonary disease (COPD). It is also instrumental in monitoring the progression of various respiratory disorders. The main results of spirometry are forced vital capacity (FVC), forced expiratory volume exhaled in the first second (FEV1), and the FEV1/FVC ratio. The procedure of spirometry has 3 phases: 1) maximal inspiration; 2) a “blast” of exhalation; 3) continued complete exhalation to the end of the test. There are within-maneuver acceptability and between-maneuver reproducibility criteria for spirometry (Table 2).
The spirometry procedure is usually performed in a standard sitting position. However, spirometry measurement in the supine position may be indicated in certain neuromuscular disorders. For example, in individuals with spinal cord injury, at the level of T6 and above, the FVC and FEV1 are decreased and become lower in the supine position compared to the upright position, suggesting an increased risk of hypoventilation during sleep.[3]
Lung Volume
The measurement of lung volumes includes several important variables, such as functional reserve capacity (FRC), vital capacity (VC), slow vital capacity (SVC), expiratory reserve volume (ERV), and residual volume (RV). The lung volume measurement is very important to detect changes in lung volume independent of effort, especially when FVC is reduced on spirometry.
FRC is the volume of the amount of gas in the lungs at the end of expiration during tidal breathing. FRC is also the sum of ERV and RV. Once FRC has been measured, all other volumes can be calculated.
ERV is the volume of gas maximally exhaled after end-inspiratory tidal breathing. RV is the volume of gas in the airways after a maximal exhalation. VC is the volume of gas expelled from full inspiration to residual volume. The FVC is similar, but the patient exhales at maximal speed and effort.
In addition to RV and ERV, there is a tidal volume (TV) and inspiratory reserve volume (IRV). IRV is the volume of gas that can be maximally inhaled from the end-inspiratory tidal breathing. TV is the volume of gas inhaled or exhaled with each breath at rest. Slow vital capacity (SVC) can be measured as the maximal amount of air exhaled in a relaxed expiration from full inspiration to residual volume; exhalation should be terminated after 15 seconds.[6] The SVC may be a useful measurement when the FVC is reduced and airway obstruction is present.
Total lung capacity (TLC) is the volume of air in the lungs at the end of maximal inspiration. The sum of RV and VC or FRC and inspiratory capacity (IC) equals TLC, which is the gold standard for diagnosing restrictive lung disease. TLC less than 80% predicted is diagnostic of a restrictive ventilatory defect.[4][5]
There are two methods to measure lung volumes: body plethysmography and gas dilution methods (nitrogen washout and inert gas dilution). The gas dilution method uses an inert gas (poorly soluble in alveolar blood and lung tissues), either nitrogen or helium. The subject breathes a gas mixture until equilibrium is achieved. The volume and mixture of gas exhaled after the equilibrium has been achieved permit the calculation of FRC. In body plethysmography, the subject sits inside a body box and breathes against a shutter valve. FRC is calculated using Boyle's law (at a given temperature, the product of gas volume and pressure is constant). FRC calculated by body plethysmography is usually larger in subjects with obstructive lung disease and air trapping than FRC calculated using gas dilution methods. Body plethysmography is considered the gold standard for lung volume measurement, especially in heterogeneous airflow obstruction, such as in COPD or asthma, where plethysmography is more accurate than helium dilution.[6]
Diffusion Capacity
Diffusion studies the diffusion of gases across the alveolar-capillary membranes. Its measurement uses carbon monoxide (CO) to calculate the pulmonary diffusion capacity. The most common method is the standard single-breath D. It is measured in milliliters per minute per mm Hg.
The factors affecting the D are volume and distribution of ventilation, mixing and diffusion, the composition of the gas, characteristics of the alveolar membrane and lung parenchyma, the volume of alveolar capillary plasma, concentration and binding properties of hemoglobin, and gas tensions in blood entering the alveolar capillaries. A detailed list of factors affecting DLCO is listed in Table 3.
The diffusion capacity depends on multiple factors, and its value should be adjusted. For correct interpretation, specific adjustments should be made for hemoglobin, carboxyhemoglobin, and FiO2. Adjustment for lung volumes is controversial, and further studies are needed.
The DLCO is interpreted in conjunction with spirometry and lung volumes. Table 3 shows the severity classification for DLCO. For example, high DLCO is associated with asthma, obesity, and intrapulmonary hemorrhage. Normal spirometry and lung volumes with low DLCO can be present in pulmonary vascular diseases, early ILD, or emphysema. An obstructive ventilatory defect with low DLCO suggests emphysema or lymphangiomyomatosis.[7][8]
Respiratory Muscle Pressures
The respiratory muscle strength is assessed with maximal inspiratory pressure (MIP) and maximal expiratory pressure (MEP). The MIP reveals the strength of the diaphragm and other inspiratory muscles, whereas the MEP indicates the strength of the abdominal and other expiratory muscles. MIP and MEP are measured three times, and maximal value is reported. For adults 18 to 65 years old, MIP should be lower than -90 cmHO in men and -70 cmHO in women. In adults older than 65, MIP should be less than -65 cmH2O in men and -45 cmH2O in women. MEP should be higher than 140 cmH2O in men and 90 cmH2O in women. MEP less than 60 cmH2O predicts a weak cough and difficulty clearing secretions.
Bronchoprovocation testing
There are several types of bronchoprovocation testing available to assess airway responsiveness. The pharmacologic challenge or exercise challenge are the most common types for bronchoprovocation testing and the accurate diagnosis of asthma. Before initiating the bronchoprovocation testing, several precautions should be taken: (1) The person performing the test should be able to identify the occurrence of severe bronchospasm; (2) a rapid-acting beta-agonist (such as albuterol) should be immediately available; (3) FEV should be insured that has returned to or exceeded to baseline before the patient leaves the pulmonary function laboratory; (4) equipment for resuscitation should also be available and accessible with qualified personnel.
The most commonly used agent for bronchoprovocation is methacholine (acetylcholine derivative) at incremental doses ranging from approximately 0.03 mg/mL to 16 mg/mL. Bronchoprovocation testing is usually performed after obtaining baseline spirometry, using a diluent that is aerosolized via nebulizer for at least one minute, then spirometry is repeated twice. The dose of methacholine is then sequentially increased until a decrease in FEV > 20%. The dose of methacholine is associated with a greater than 20% decrease in FEV1 and is referred to as the provocative dose or PD20. A methacholine PD20 of 8 mg/mL or less is considered a positive bronchoprovocation test. However, a PD20 greater than 16 mg/mL is regarded as a negative test. [9]
Six-minute walk test (6MWT)
Six-minute walk test s a standard test to assess exercise capacity objectively and determine prognosis in many respiratory (such as COPD, idiopathic pulmonary fibrosis, and pulmonary hypertension) and non-respiratory conditions (such as heart failure).[10][11] The minimal clinically important difference for change in the 6MWT distance of adults is approximately 30 meters.[12] The 6MWT is not designed to be used for home oxygen titration and assessment, and a separate study is recommended to assess the need and dose of supplemental oxygen. Performance of the test requires the presence of a flat, straight corridor 30 m (100 feet) in length, the ability to monitor heart rate and pulse oximetry throughout the test, and if the patient uses supplemental oxygen, to record the flow rate and type of Oxygen device. [10]
