6.2: Flow-Volume Loops

Flow-volume loops are briefly discussed in context of the relevant physiology. Figure 6.3 shows a normal flow-volume loop. Note that the volume axis seems to be the wrong way around; this is because expired volume and flow are generally more useful, so the plot has expiratory flow as positive and lung volume orientated for expiration. While breathing on a spirometer, the patient begins to breathe in from residual volume (bottom half of maroon line). As inspiration continues, lung volume increases (moves toward the y-axis) and airflow increases (moves downward). The patient continues inhaling until they are at total lung capacity (or TLC).

Figure 6.3: Typical and normal flow-volume loop. FVC: forved vital capacity.

They then exhale as hard and as fast as they can, forcefully emptying the lung as quickly as possible. During forced exhalation of the first liter or so, expiratory flow rapidly increases until it reaches peak expiratory flow; this is the first clinically pertinent measure. After this point expiratory flow begins an exponential decline; as lung volume continues to decrease, so does the flow rate until flow reaches zero when the lung is emptied (at residual volume).

The rate of this decline in flow rate is also an important clinical measure and brings together a couple of important physiological points:

1. Flow is declining during this phase of expiration due to the forceful expiration causing airway compression and thereby increasing airway resistance.
2. Airways will also become smaller as radial traction of the parenchyma also declines with lung volume.

Although there are a number of measurements that are calculated from this forced exhalation, two are most commonly reported. First, the total volume that is expelled from the lung is referred to as the forced vital capacity (FVC). The forced expiratory volume that is expelled from the lung in the first second of expiration is referred to as FEV₁. The ratio of these two values, known as FEV₁/FVC, describes the percentage of lung volume that can be emptied in one second and is a useful indicator of airway resistance. A normal FEV₁/FVC is 90 percent or higher, meaning over 90 percent of vital capacity can be emptied from the lung within a second. This value is dependent on age, gender, and body size, but commonly used predicted values take these variables into account when assessing for disease.

The loop produced by a patient with chronic obstructive lung disease, or COPD, looks very different (gray line in figure 6.4). With disease causing airway narrowing, the peak expiratory flow is significantly reduced, and the decay in expiratory flow as lung volume declines is much more pronounced as the narrowed airways can be easier to collapse due to a lower starting radius and/or loss of radial traction.

Figure 6.4: Normal (maroon) and obstructive disease (gray) flow-volume loops.

This means that FEV₁ is significantly reduced, but FVC may remain unchanged (i.e., the lung volume is the same, but it takes longer to empty). An FEV₁/FVC significantly less than 90 percent is indicative of obstructive disease. Notice that the inspiratory loop of the COPD patient appears normal, illustrating the effect of increasingly negative intrapleural pressure, increasing lung volume and radial traction on airway resistance.

Alternatively, diseases that restrict lung expansion (figure 6.5), such as pulmonary fibrosis, demonstrate a reduced lung volume, where FVC is substantially reduced, but FEV₁ may not be significantly affected; in fact it is not uncommon for FEV₁/FVC to increase to about normal in restricted diseases, but this is of course due to a decline in FVC rather than a rise in FEV₁. Notice also that the inspiratory loop is affected, with volumes being reduced here as well.

Figure 6.5: Normal (maroon) and restrictive (gray) flow-volume loops.

A flow-volume loop is a quick, cheap, and powerful diagnostic measure, but it is highly dependent on the patient performing a forced expiration to encourage dynamic compression and peak flows be obtained so that any airway abnormalities can be seen. This is why you may hear a pulmonary function technologist (PFT) shouting encouragement to a patient as you walk past the lab.

Summary

So we have dealt with a couple of relatively complex issues in this chapter, particularly the interaction between intrapleural pressure and airway pressure during forced or active expirations and how airways can become compressed.

We have also looked at the use of flow-volume loops to determine the degree of airway obstruction and to distinguish between obstructive and restrictive disorders.

References, Resources, and Further Reading

Text

Levitsky, Michael G. "Chapter 3: Alveolar Ventilation." In Pulmonary Physiology, 9th ed. New York: McGraw Hill Education, 2018.

West, John B. "Chapter 7: Mechanics of Breathing—How the Lung Is Supported and Moved." In Respiratory Physiology: The Essentials, 9th ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams and Wilkins, 2012.

Widdicombe, John G., and Andrew S. Davis. "Chapter 3." In Respiratory Physiology. Baltimore: University Park Press, 1983.

Figures

Figure 6.1: Intrapleural and airway pressures during normal/passive expiration. Grey, Kindred. 2022. CC BY 4.0. https://archive.org/details/6.1_20220125

Figure 6.2: Intrapleural and airway pressures during forced expiration. Grey, Kindred. 2022. CC BY 4.0. https://archive.org/details/6.2_20220125

Figure 6.3: Typical and normal flow-volume loop. Grey, Kindred. 2022. CC BY 4.0. https://archive.org/details/6.3_20220125

Figure 6.4: Normal (maroon) and obstructive disease (gray) flow-volume loops. Grey, Kindred. 2022. CC BY 4.0. https://archive.org/details/6.4_20220125

Figure 6.5: Normal (maroon) and restrictive (gray) flow-volume loops. Grey, Kindred. 2022. CC BY 4.0. https://archive.org/details/6.5_20220125