Pulmonary function tests (PFTs) allows physicians to evaluate the respiratory function of their patients. They are reproducible and accurate. Ultimately, the results of the PFTs are affected by the effort of the patient. PFTs do not provide a specific diagnosis, but together with the history, physical exam, and laboratory data help clinicians reach a diagnosis. PFTs also allow physicians to quantify the severity of the pulmonary disease, follow it up over time, and assess its response to treatment.[1]

Spirometry

Spirometry is a physiological test which measures the ability to inhale and exhale air in relation to time. Spirometry is a screening test of general respiratory health. The main results of spirometry are forced vital capacity (FVC) and forced expiratory volume (FEV). 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).

Vital capacity (VC) is the volume of gas expelled from full inspiration to residual volume. The FVC is similar, but the patient is exhaling at maximal speed and effort.

The FEV is the forced expiratory volume in t seconds from a position of full inspiration. Forced expiratory volume in the first second (FEV)) is used to classify the severity of obstructive lung diseases. The reversibility testing is administered using a bronchodilator (short-acting beta 2-agonist or anticholinergic agent). An increase in either FEV or FVC of 3^12% and 3^200 mL is considered positive bronchodilator response. A lack of a response does not predict lack of response to bronchodilators. The patient should hold their bronchodilators before the reversibility testing.

The maximal flow-volume curves are a great asset to detect mild airflow obstruction (Figure 2). There is an inspiratory and expiratory loop. [2]

Lung Volume

The key measurement of lung volumes is functional reserve capacity (FRC). Once FRC has been measured, all other volumes can be calculated. FRC is the volume of the amount of gas in the lungs at the end of expiration during tidal breathing. FRC is the sum of expiratory reserve volume (ERV) and residual volume (RV). 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.

In addition to RV and ERV, there is 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.

There are 2 methods to measure lung volumes: body plethysmography and gas dilution methods (nitrogen washout and inert gas dilution). 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 have 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 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 volumes measurement.

Capacities are the sum of 2 or more volumes (Figure 1). TLC is the gold standard for diagnosis of restrictive lung disease. TLC less than 5 percentile of predicted or less than 80% predicted are diagnostic of a restrictive ventilatory defect.[3][4]

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. Specific adjustments should be made for hemoglobin, carboxyhemoglobin, and FiO for a correct interpretation. 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.[5][6]

Respiratory Muscle Pressures

The respiratory muscle strength is assessed with the maximal inspiratory pressure (MIP); also called negative inspiratory force (NIF) 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, 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 years old, 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.

Central and Upper Airway Obstruction

Central or upper airway obstruction (UAO) could occur in the extrathoracic (pharynx, larynx, and the extrathoracic part of the trachea) and intrathoracic airways (intrathoracic trachea and main bronchi). The FEV and/or FVC are not affected by this type of obstructions, but the peak expiratory flow (PEF) can be severely decreased. An FEV/PEF ratio of greater than 8 suggests central or UAO.

Three maximal and repeatable forced inspiratory and expiratory flow-volume curves are necessary. It is key that efforts, both inspiratory and expiratory are maximal.

The maximal inspiratory flow is largely decreased with an extrathoracic airway obstruction because there is a negative pressure inside the airways during inspiration. Inspiratory flows are not affected by intrathoracic lesions; the negative pleural pressure maintains the intrathoracic airways open. The maximal expiratory flow (peak flow) is decreased with intrathoracic and extrathoracic lesions.

The obstructions can be intrathoracic or extrathoracic and variable or fixed. In summary, fixed obstructions will have decreased inspiratory and expiratory flows; variable obstruction will depend on the location (intrathoracic or extrathoracic). The flow-curve is not a sensitive test; the absence of the classic pattern does not rule out a central or UAO.[1][7][8][9]

There are multiple indications to obtain PFTs. Table 1 summarizes the most commons indications.

PFTs can be physically demanding for patients, and it is recommended to wait one month after an acute coronary syndrome or myocardial infarction. Other relative contraindications are thoracic/abdominal surgery, brain/eye/ear/otolaryngological surgery, pneumothorax, ascending aortic aneurysm, hemoptysis, pulmonary embolism, severe hypertension (SBP greater than 200 mm Hg, DBP greater than 120 mm Hg).[7]