Article Figures & Data
Figures
- Figure 1
Anatomy of the flow–volume loop. (A) The flow–volume loop (FVL) is pictured here with volume on the x-axis (increasing toward the origin) and flow on the y-axis. By convention, the expiratory phase is depicted on the positive side of the y-axis and the inspiratory phase on the negative. Total lung capacity (TLC) is shown as the point on the x-axis after full inspiration, and residual volume is shown at the point of full exhalation, with vital capacity (VC) represented by the distance between the two. (B) The flow–volume loop is depicted here with a smaller tidal breath (Vt) in its interior. The point on the x-axis at which a tidal breath is exhaled is the functional residual capacity (FRC), and the distance between the FRC and the TLC is known as the inspiratory capacity (IC). (C) Peak expiratory flow rate (PEFR) is the highest positive value along the y-axis. FEF25% is the point at which one-quarter of the VC has been exhaled, FEF50% is the volume at which half of the VC has been exhaled, and FEF75% is the point at which three-quarters of the VC have been exhaled. ERV, expiratory reserve volume.
- Figure 2
Abnormalities of the flow–volume loop (FVL). (A) The airflow-limiting pattern is distinguished by a ‘scooping’ of the FVL, in which the flow is lower at any given volume. In addition, provided lung volumes were obtained along with spirometry, the loop is often shifted to the left, reflecting an increase in TLC and RV. The restrictive pattern, in contrast, is distinguished by a shift of the loop to the right, reflecting a decrease in TLC and RV. Although lung volumes are decreased, at any given volume, flow is often increased. (B) Mead developed a method of FVL quantification that makes use of tangent lines drawn along the expiratory limb at discrete intervals as well as chord lines created by drawing a line between a tangent on the loop and the end of expiration to generate slope ratio curves. The method of Kapp involves the description of an angle β generated by two lines: one from the point of PEF to the forced expiratory flow at 50% of the FVC (FEF50%) that leads to an extrapolated point on the x-axis, and the other from FEF50% to RV. (C) Vermaak's method of FVL quantification involves the creation of a hypothetical triangle using the VC as the base, the PEF as the perpendicular, and a line between PEF and RV as the hypotenuse. A later attempt by Lee makes use of a similar triangle but uses graphical analysis software to describe the area of obstruction (defined as the ratio of the area under the diagonal to the area under the triangle). The closer the ratio to 1, the more severe the obstruction. The closer it is to 0, the less obstructed the loop.
- Figure 3
Upper airway obstruction. (A) Variable intrathoracic obstruction causes a plateauing of the expiratory limb of the flow–volume loop (FVL) due to worsening obstruction when tracheal pressure is exceeded by pleural pressure during expiration within the thorax. (B) In contrast, variable extrathoracic obstruction causes a plateauing of the inspiratory limb due to worsening obstruction when tracheal pressure is exceeded by atmospheric pressure during inspiration. (C) A fixed airway obstruction does not vary with the respiratory cycle and therefore causes plateauing of both limbs of the FVL. Figures adapted from Acres and Kryger.16
- Figure 4
The volume–time curve. (A) The volume–time curve is presented with time along the x-axis and volume along the y-axis. The FEV1 is the volume exhaled in 1 s, while the forced vital capacity is the volume exhaled during the entirety of the breath (the forced expiratory time, FET). The forced expiratory volumes at 2, 3, and 6 s are determined by the volumes exhaled at those respective times. (B) The volume–time curve has a characteristic shape depending on the underlying pathology. In airflow limitation, the curve rises more slowly and does not plateau appropriately (ie, additional volume continues to be exhaled even at the conclusion of the breath). In restrictive lung disease, the curve rises and plateaus prematurely. (C) The maximal mid-expiratory flow (FEF25%–75%) is determined by drawing a line between the points at which 25% and 75% of the FVC has been exhaled. The triangle described by that line (as the hypotenuse), the change in volume (as the perpendicular), and the change in time (as the base) allow for assessment of the angle α, which is the FEF25%–75%.
- Figure 5
RV/TLC, IC/TLC, VA/TLC, and DL/VA. (A) The importance of a rising RV (assuming a constant TLC) is visualized first with volume on the y-axis and then with flow–volume loops (FVLs). (B) The relationship between inspiratory (IC) and functional residual capacity (FRC) is depicted here first with volume on the y-axis and then with FVLs. Assuming a constant TLC, as FRC increases, IC decreases. (C) The difference between alveolar volume (VA) and TLC is depicted schematically. While TLC as assessed by plethysmography or multiple-breath dilution measures the total volume of air in the chest, VA as assessed by inspiration of a tracer gas such as helium measures only the volume of air that reaches areas of the lung with normal gas mixing. With volume on the y-axis, the difference in volume between TLC and VA reflects areas of poor gas mixing and ventilation heterogeneity. (D) In conditions causing a low diffusing capacity of lungs for carbon monoxide (DLCO), the DL/VA can be used to point toward the underlying pathology. A very low DLCO associated with a low VA and a low DL/VA suggests alveolar or microvascular destruction as is seen in idiopathic pulmonary fibrosis (IPF). In association with a low DLCO, a low VA and a normal DL/VA suggest an increase in pulmonary capillary blood volume relative to alveolar volume, as is seen post pneumonectomy. A low DLCO associated with a normal VA and a low DL/VA suggests microvascular destruction or remodeling, as is seen in pulmonary arterial hypertension (PAH).
Tables
- Table 1
Basic patterns of pulmonary function tests
Airflow-limiting pattern Restrictive pattern Vascular pattern FEV1/FVC<LLN TLC<LLN DLCO<LLN The three primary patterns of disease distinguishable by traditional pulmonary function tests are summarized as follows: airflow limitation is characterized by a decrease in the forced expiratory volume in 1 s to the forced vital capacity (FEV1/FVC) ratio, restriction is characterized by a decreased total lung capacity (TLC), and vascular disease is characterized by a decrease in the diffusing capacity of lungs for carbon monoxide (DLCO).
LLN, lower limit of normal.
