Lung Volumes

Clinical evaluation of lung function and the study of static lung mechanics begin with the measurement of lung volumes (Fig. 21-1) and the factors that determine these volumes. All lung volumes are subdivisions of total lung capacity (TLC), the total volume of air that can be contained in the lung. Lung volumes are reported in liters either as volumes or as capacities. A capacity is composed of two or more volumes. Many lung volumes are measured with a spirometer. The patient is asked to first breathe normally into the spirometer, and the volume of air (the tidal volume [V_T]) that is moved with each quiet breath is measured. The subject then inhales maximally and exhales forcefully and completely, and the volume of that exhaled air is measured. The total volume of exhaled air, from a maximal inspiration to a maximal exhalation, is the vital capacity (VC). Residual volume (RV) is the air remaining in the lung after a complete exhalation. TLC is the sum of VC and RV; it is the total volume of air contained in the lungs, and it includes the volume of air that can be moved (VC) and the volume of air that is always present (trapped) in the lung (RV). Functional residual capacity (FRC) is the volume of air in the lung at the end of exhalation during quiet breathing and is also called the resting volume of the lung. FRC is composed of RV and the expiratory reserve volume (ERV; the volume of air that can be exhaled from FRC to RV).

Figure 21-1 The various lung volumes and capacities. ERV, expiratory reserve volume; FRC, functional residual capacity; FVC, forced vital capacity; IC, inspiratory capacity; IRV, inspiratory reserve volume; RV, residual volume; TLC, total lung capacity; VC, vital capacity; V_T, tidal volume.

The ratio of RV to TLC (RV/TLC ratio) is used to distinguish different types of pulmonary disease. In normal individuals, this ratio is usually less than 0.25. Thus, in a healthy individual, approximately 25% of the total volume of air in the lung is trapped. An elevated RV/TLC ratio, secondary to an increase in RV out of proportion to any increase in TLC, is seen in diseases associated with airway obstruction, known as obstructive pulmonary diseases. An elevated RV/TLC ratio can also be caused by a decrease in TLC, which occurs in individuals with restrictive lung diseases. Measurement of these lung volumes is described later.

Determinants of Lung Volume

What determines the volume of air in the lung at TLC or at RV? The answer lies in the properties of the lung parenchyma and in the interaction between the lungs and the chest wall. The lungs and chest wall always move together in healthy individuals. The lung contains elastic fibers that stretch when stress is applied, thereby resulting in an increase in lung volume, and that recoil passively when this stress is released, thereby resulting in a decrease in lung volume. The elastic recoil of the lung parenchyma is very high. In the absence of external forces (such as the force generated by the chest wall), the lung will become almost airless (10% of TLC). Similarly, chest wall volume can increase when the respiratory muscles are stretched and decrease when respiratory muscle length is shortened. In the absence of the lung parenchyma, the volume of the chest wall is approximately 60% of TLC.

Lung volumes are determined by the balance between the lung’s elastic properties and the properties of the muscles of the chest wall. The maximum volume of air contained within the lung and the chest wall (i.e., TLC) is controlled by the muscles of inspiration. With increasing lung volume, the chest wall muscles lengthen progressively. As these muscles lengthen, their ability to generate force decreases. TLC occurs when the inspiratory chest wall muscles are unable to generate the additional force needed to further distend the lung and chest wall. Similarly, the minimal volume of air in the lung (i.e., RV) is controlled by the expiratory muscle force. Decreasing lung volume results in shortening of the expiratory muscles, which in turn results in a decrease in muscle force. The decrease in lung volume is also associated with an increase in the outward recoil pressure of the chest wall. RV occurs when expiratory muscle force is insufficient to further reduce chest wall volume.

FRC, or the volume of the lung at the end of a normal exhalation, is determined by the balance between the elastic recoil pressure generated by the lung parenchyma to become smaller (inward recoil) and the pressure generated by the chest wall to become larger (outward recoil). When the chest wall muscles are weak, FRC decreases (lung elastic recoil > chest wall muscle force). In the presence of airway obstruction, FRC increases because of premature airway closure, which traps air in the lung.

Measurement of Lung Volumes

RV and TLC can be measured in two ways: by helium dilution and by body plethysmography. Both are used clinically and provide valuable information about lung function and lung disease. The helium dilution technique is the older and simpler method, but it is often less accurate than body plethysmography, which requires sophisticated and expensive equipment.

In normal individuals, the FRC measured by helium dilution and the FRC measured by plethysmography are the same. This is not true in individuals with lung disease. The FRC measured by helium dilution measures the volume of gas in the lung that communicates with the airways, whereas the FRC measured by plethysmography measures the total volume of gas in the lung at the end of a normal exhalation. If a significant amount of gas is trapped in the lung (because of premature airway closure), the FRC determined by plethysmography will be considerably greater than that measured by helium dilution.

IN THE CLINIC

In the helium dilution technique, a known concentration of an inert gas (such as helium) is added to a box of known volume. The box is then connected to a volume that is unknown (the lung volume to be measured). After adequate time for distribution of the inert gas, the new concentration of the inert gas is measured. The change in concentration of the inert gas is then used to determine the new volume in which the inert gas has been distributed (Fig. 21-2). Specifically,

Figure 21-2 Measurement of lung volume by helium dilution.

Measurement of lung volumes with a body plethysmograph (body box) uses Boyle’s gas law, which states that pressure multiplied by volume is constant (at a constant temperature). The subject sits in an airtight box (Fig. 21-3) and breathes through a mouthpiece that is connected to a flow sensor (pneumotach). The subject then makes panting respiratory effort against a closed mouthpiece. During the expiratory phase of the maneuver, the gas in the lung becomes compressed, lung volume decreases, and the pressure inside the box falls because the gas volume in the box increases. By knowing the volume of the box and measuring the change in pressure of the box at the mouth, the change in volume of the lung can be calculated. Thus,

Figure 21-3 The body plethysmograph.

where P₁ and P₂ are mouth pressures and V is FRC.

Lung Compliance

Lung compliance (C_L) is a measure of the elastic properties of the lung. It is a measure of how easily the lung is distended. Lung compliance is defined as the change in lung volume resulting from a 1–cm H₂O change in the distending pressure of the lung. The units of compliance are mL (or L)/cm H₂O. High lung compliance refers to a lung that is readily distended. Low lung compliance, or a “stiff” lung, is a lung that is not easily distended. The compliance of the lung is thus

Equation 21-1

where ΔV is the change in volume and ΔP is the change in pressure. Graphically, lung compliance is the slope of the line between any two points on the deflation limb of the pressure-volume loop (Fig. 21-4). The compliance of a normal human lung is about 0.2 L/cm H₂O, but it varies with lung volume. Note that the lung is less distensible at high lung volumes. For this reason, compliance is corrected for the lung volume at which it is measured (specific compliance) (Fig. 21-5). Changes in lung compliance are associated with certain types of lung disease (e.g., restrictive lung diseases) and are of great clinical importance. Compliance measurements are not often performed for clinical purposes, however, because they require placement of an esophageal balloon. The esophageal balloon, which is connected to a pressure transducer, is an excellent surrogate marker for pleural pressure. The change in pleural pressure (P_pl) is measured as a function of the change in lung volume; that is, C_L = ΔV/ΔP_pl.

Figure 21-4 Deflation pressure-volume (PV) curve. Because of hysteresis caused by surfactant, the deflation PV curve is used for measurements. Compliance at any point along this curve is the change in volume per change in pressure. From the curve it can be seen that lung compliance varies with lung volume. Compare the compliance at 1 versus 2. By convention, lung compliance is the change in pressure in going from FRC to FRC +1 L.

Figure 21-5 Relationship between compliance and lung volume. Imagine a lung in which a 5–cm H₂O change in pressure results in a 1-L change in volume. If half of the lung is removed, the compliance will decrease, but when corrected for volume of the lung, there is no change (specific compliance). Even when the lung is reduced by 90%, the specific compliance is unchanged.

The compliance of the lung is affected by several respiratory disorders. In emphysema, an obstructive lung disease usually of smokers associated with destruction of the alveolar septa and pulmonary capillary bed, the lung is more compliant; that is, for every 1–cm H₂O increase in pressure, the increase in volume is greater than in a normal lung (Fig. 21-6). In contrast, in pulmonary fibrosis, a restrictive lung disease associated with increased collagen fiber deposition in the interstitial space, the lung is noncompliant; that is, for every 1–cm H₂O change in pressure, the change in volume is less.

Figure 21-6 Fibrosis/emphysema pressure-volume curve.

Lung–Chest Wall Interactions

The lung and chest wall move together as a unit in healthy people. Separating these structures is the pleural space, which is best thought of as a potential space. Because the lung and chest wall move together, changes in their respective volumes are equal. An understanding of the pressures that surround the lung and chest wall and result in changes in lung volume is essential to comprehend how the lungs work. The pressure changes across the lung and across the chest wall are defined as transmural pressure. For the lung, this transmural pressure is called the transpulmonary (or translung) pressure (P_L), and it is defined as the pressure difference between the air spaces (alveolar pressure [P_A]) and the pressure surrounding the lung (pleural pressure]P_pl]). That is,

Equation 21-2

The lung requires positive transpulmonary pressure to increase its volume, and lung volume increases with increasing transpulmonary pressure (Fig. 21-6). The lung assumes its smallest size when transpulmonary pressure is zero. The lung, however, is not totally devoid of air when transpulmonary pressure is zero because of the surface tension–lowering properties of surfactant (see Chapter 20).

The transmural pressure across the chest wall (P_w) is the difference between pleural pressure and the pressure surrounding the chest wall (P_b), which is the barometric pressure or body surface pressure. That is,

Equation 21-3

During inspiration, the chest wall expands to a larger volume. Because pleural pressure is negative relative to atmospheric pressure during quiet breathing, the transmural pressure across the chest wall is negative.

The pressure across the respiratory system (P_rs) is the sum of the pressure across the lung and the pressure across the chest wall. That is,

Equation 21-4

The pressure-volume relationships for the lung alone, for the chest wall alone, and for the intact respiratory system are shown in Figure 21-7. A number of important observations can be made by examining the pressure-volume curves of the lung, chest wall, and respiratory system. Note that the transmural pressure across the respiratory system at FRC is zero. At TLC, both lung pressure and chest wall pressure are positive, and they both require positive transmural distending pressure. The resting volume of the chest wall is the volume at which the transmural pressure for the chest wall is zero, and it is approximately 60% of TLC. At volumes greater than 60% of TLC, the chest wall is recoiling inward and positive transmural pressure is needed, whereas at volumes below 60% of TLC, the chest wall tends to recoil outward.

Figure 21-7 Relaxation pressure-volume curve of the lung, chest wall, and respiratory system. The curve for the respiratory system is the sum of the individual curves. The curve for the lung is the same as in Figure 21-6.

The transmural pressure for the lung alone flattens at pressures greater than 20 cm H₂O because the elastic limits of the lung have been reached. Thus, further increases in transmural pressure produce no change in volume and compliance is low. Further distention is limited by the connective tissue (collagen, elastin) of the lung. If further pressure is applied, the alveoli near the lung surface can rupture and air can escape into the pleural space. This is called pneumothorax.

The relationship between pleural, alveolar, and elastic recoil pressure is shown in Figure 21-8. Alveolar pressure is the sum of the pleural pressure and elastic recoil pressure of the lung.

Figure 21-8 Relationship between transpulmonary pressure (P_L) and the pleural (P_pl), alveolar (P_A), and elastic recoil (P_el) pressures of the lung. Alveolar pressure is the sum of pleural pressure and elastic recoil pressure. Transpulmonary pressure is the difference between pleural pressure and alveolar pressure.

Because P_L = P_A − P_pl,

Equation 21-5

Thus,

In general, P_L is the pressure distending the lung, whereas P_el is the pressure tending to collapse the lung.

Pressure-Volume Relationships

Air flows into and out of the airways from areas of higher pressure to areas of lower pressure. In the absence of a pressure gradient, there is no airflow. Minute ventilation is the volume of gas that is moved per unit of time. It is equal to the volume of gas moved with each breath times the number of breaths per minute:

Equation 21-6

IN THE CLINIC

To understand the relationship between changes in pressure and changes in volume, it is helpful to examine the pressure changes during inspiration and exhalation (Fig. 21-9). In normal individuals during tidal volume breathing, alveolar pressure decreases at the start of inspiration. This decrease in alveolar pressure is usually small (1 to 3 cm H₂O). It is much larger in individuals with airway obstruction because of the larger pressure drop that occurs across obstructed airways.

Figure 21-9 Changes in alveolar and pleural pressure during a tidal volume breath. Inspiration is to the left of the vertical dotted line and exhalation is to the right. Positive (relative to atmosphere) pressures are above the horizontal dotted line and negative pressures are below. See text for details. At points of no airflow (points A and C), alveolar pressure is zero.

Pressure within the pleural space (pleural pressure) also falls during inspiration. This decrease equals the lung elastic recoil, which increases as the lung inflates. Pressure drops along the airways as gas flows from atmospheric pressure (zero) to the pressure in the alveolus (negative relative to atmospheric pressure). Airflow stops when alveolar pressure and atmospheric pressure become equal.

On exhalation, the diaphragm moves higher into the chest, pleural pressure increases (i.e., becomes less negative), alveolar pressure becomes positive, the glottis opens, and gas again flows from a higher (alveolus) to a lower (atmospheric) pressure. In the alveolus, the driving force for exhalation is the sum of the elastic recoil of the lung and pleural pressure.