Keywords

Definitions; See Figure 2.1

Total Lung Capacity (TLC)

  • Is the volume of air (in liters) that a subject’s lungs contain at the end of a maximal inspiration [1].

Figure 2.1
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A volume spirogram showing the different lung volumes (on the left) and capacities (on the right)

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Residual Volume (RV)

  • Is the volume of air that remains in the lungs at the end of a maximal exhalation [1]. An abnormal increase in RV is called air trapping. The techniques used to measure lung volumes are primarily designed to measure the Residual Volume, as this volume can’t be exhaled to be measured. The rest of the lung volumes can then be measured by simple spirometry, using the SVC maneuver rather than the FVC maneuver. The TLC can then be calculated by adding RV to VC or functional residual capacity (FRC) to inspiratory capacity (IC); (Figure 2.1).

  • So, spirometry is an essential part of any lung volume study.

Functional Residual Capacity (FRC)

  • Is the volume of air that remains in the lungs at the end of a tidal exhalation, i.e. when the respiratory muscles are at rest [1]. This means that at FRC, the resting negative intra-thoracic pressure produced by the chest wall (rib cage and diaphragm) wanting to expand is balanced by the elastic recoil force of the lungs which naturally want to contract. Therefore, when the elastic recoil of the lungs decreases, as in emphysema, the FRC increases (hyperinflation), while when the elastic recoil increases as in pulmonary fibrosis, the FRC decreases.

  • The FRC is the sum of the expiratory reserve volume (ERV) and the RV and is ~50% of TLC.

  • FRC measured using body plethysmography (discussed below) is sometimes referred to as the thoracic gas volume (TGV or V TG ) at FRC or V FRC [1]. Indeed, FRC is the volume measured by all the volume measuring techniques and RV is then determined by subtracting ERV.

  • FRC has important functions:

    • – It aids mixed venous blood oxygenation during expiration and before the next inspiration.

    • – Decreases the energy required to re-inflate the lungs during inspiration. If for example each time the patient exhaled, the lungs fully collapsed, a large effort would be needed to re-inflate them. Such effort would soon result in exhaustion and respiratory failure [2].

Expiratory Reserve Volume (ERV)

  • Is the maximum volume of air that can be exhaled at the end of a tidal exhalation and can be measured by simple spirometry [1].

Inspiratory Reserve Volume (IRV)

  • Is similarly defined as the maximum volume of air that can be inhaled following a tidal inhalation [1].

Inspiratory Capacity (IC)

  • Is the maximum volume of air that can be inhaled after a normal tidal exhalation [1]. Accordingly, IC equals the IRV + Tidal volume (V T).

Tidal Volume (VT)

  • Is the volume of air that we normally inhale or exhale while at rest, and equals roughly 0.5 liter in an average adult and increases with exercise.

SVC or VC

  • Was discussed in Chapter 1. See Figure 2.6.

The Terms: “Volume” and “Capacity”; (Figure 2.1) [3]

  • The term “volume” refers to the lung volumes that can’t be broken down into smaller components (RV, ERV, VT and IRV).

  • While, the term “capacity” refers to the lung volumes that can be broken down into other smaller components (IC, FVC, TLC and VC)

    • – IC = IRV + V T

    • – FRC = ERV + RV

    • – VC = IC + ERV

    • – TLC = VC + RV

Correlation with the FV Curve

  • FV curve can be used as a volume spirogram (seen in Figure 2.1), in addition to its other uses; (Figure 2.2). The only three lung volumes that spirometry can’t measure are RV, FRC and TLC.

Figure 2.2
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To aid in understanding lung volumes as they relate to the FV curve, the FV curve may be rotated 90° clockwise and placed beside the volume spirogram

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Methods for Measuring the Static Lung Volumes

  • There are different ways of measuring the lung volumes, the most accurate and widely used of which is the body box or body Plethysmography. The other, less widely used, methods are the nitrogen washout method, the inert gas dilution technique and the radiographic method.

  • This section will discuss the principles, the advantages and the disadvantages of each method.

Body Plethysmography (Body Box)

  • This is an ingenious way of measuring the lung volumes. The primary goal is to measure the FRC by the body box, in addition to allowing measures of the ERV and the SVC. The RV and TLC can then be calculated from these 3 variables, (RV = FRC − ERV; TLC = RV + SVC); see Figure 2.1.

  • The principle of body plethysmography depends on Boyle’s law which states that the product of pressure and volume of a gas is constant at a constant temperature [4, 5]. For the details of how this law is applied in the body box to get the FRC, see Table 2.1.

  • The plethysmograph is the most popular way of measuring the lung volumes, as it is the fastest and probably the most accurate, but it is the most expensive, too. A comparison between the methods for measuring the lung volumes is shown in Table 2.4 [5, 6].

Table 2.1 Principle of Body Plethesmography
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Figure 2.3
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Principle of body plethesmography (the body box)

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Nitrogen Washout Method [1, 7]

  • This is another way of determining FRC. This technique is less accurate and more time consuming (at least 7 minutes [8]). Its principle is related to the concentration of nitrogen in the lungs (which is the concentration of the atmospheric nitrogen, 80%), which then can be washed out to determine the FRC volume. See Table 2.2 for details.

Table 2.2 Nitrogen washout method [1, 7]
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Figure 2.4
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Principle of Nitrogen washout method

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Inert Gas Dilution Technique [1, 9, 10]

  • An inert gas is a gas that is not absorbable in the air-spaces. As in N2 washout method, the inert gas technique is less accurate (under-estimates lung volumes in airway obstruction) and is more time consuming. See Table 2.3 for details.

Table 2.3 Inert gas dilution technique [1, 9, 10]
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Figure 2.5
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Principle of Inert Gas (Helium) Dilution Technique

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Radiographic Method (Planimetry or Geometry)

  • The TLC and RV are estimated by doing PA and lateral chest radiographs during full inspiration (TLC) and full expiration (RV). It is a method that is not used routinely, due to the unnecessary exposure to radiation. This method may yield a lower TLC by >10% compared to plethysmography [11,12,13]. CT scan and MRI are more accurate than radiography in determining TLC but they are more costly [14,15,16].

  • In a normal subject, all the above mentioned methods should give similar values for the lung volumes, if done properly [1]. It is only in disease states, that the values will vary to any significant degree between the different methods; Table 2.4.

Table 2.4 Comparison between the common methods for measuring lung volumes [5, 6]
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Technique for Body Plethysmography

  • The plethysmograph should be calibrated daily to ensure accuracy [1, 17,18,19]. The temperature and barometric pressure should be entered every morning.

  • The patient sits comfortably inside the body box, with the door closed, a nose clip applied and the mouth tightly applied to a mouth-piece.

  • The patient should breathe normally until 3 or 4 stable tidal breaths are achieved; (Figure 2.6). Then, (Step 1) at the end of the last tidal exhalation (FRC), the patient is instructed to pant fast and shallowly [20] against a closed valve (shutter), where the plethysmograph measures the FRC, as explained earlier.

  • Step 2: the patient is then instructed to take a full inspiration (IC) then (step 3) deep, slow expiration (SVC or VC) for at least 6 seconds, which is spirometry but an unforced maneuver. The subsets of lung volume can then be calculated, as shown in Figure 2.6.Footnote 1

  • The test is then repeated for reproducibility as ATS criteria should also be met in the measurements. The difference between the two measurements of FRC and TLC should be within 10% and RV within 20% [1].

  • Physical and biological calibrations are also needed.

    • – The physical calibration is done every morning and includes calibrating the mouth pressure transducer and the volume signal of the plethysmograph. The volume calibration is carried out using a container with a known volume (a 3-liter syringe) where the container’s gas volume measurements should be within 50 ml or 3% of each other, whichever is larger [1, 5].

    • – Biological calibration should be done once a month on two reference subjects [1]. Measurements shouldn’t be significantly different from the previously acquired measurements in the same subjects (<10% for TLC and FRC and <20% for RV) [1].

Figure 2.6
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Technique for plethysmography. Notice that SVC is used instead of FVC

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Correlating the Flow Volume Curve with Lung Volumes

  • When the FV curve is done while the patient is inside the body box, at the same time as the lung volume study, the TLC and RV can be accurately plotted on the curve too. As discussed in Chapter 1, TLC is represented by the left-most point of the curve and RV by the right-most point of the curve. Comparing these points with their equivalents in the predicted curve, will indicate whether these lung volumes are decreased, normal or increased.

  • In restrictive disorders, the TLC and RV are low, which means that the curve will shift to the right compared to predicted (remember, right = restrictive). The opposite is true in obstructive disorders where lung recoil is reduced i.e. emphysema, see Figure 2.7.

Figure 2.7
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(a) Represents the ideal curve; (b) represents an obstructive disorder with increased TLC and RV and shift of FV curve to the left; (c) represents a restrictive disorder with decreased TLC and RV and shift of FV curve to the right

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Reference Values [1, 6, 18, 21,22,23,24]

  • As with older spirometry databases, lung volume reference values were derived from Caucasian studies [25], and there remains a paucity of lung volume for other ethnic groups and therefore corrections need to be made for ethnic variability. These reference values are related to body size, with the height being the most important factor. Values above the fifth percentile are considered normal.

Components of a Lung Volume Study

  • The simple rule for lung volumes is that they increase in obstructive disorders and decrease in restrictive disorders. TLC and RV are the most important for interpreting PTFs. The RV/TLC ratio is similarly useful in interpreting lung volume studies. Table 2.5 discusses the causes for abnormal lung volumes. IC and IRV are not discussed as they have little diagnostic role.

Table 2.5 Causes of abnormal lung volumes
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Clinical Significance of FRC

  • A high FRC (as in emphysema) means, the lungs contain more air than normal at rest. Breathing at that high lung volume helps prevent collapse of the airways and air trapping in emphysematous lungs, but at the same time, increases the effort of breathing. This can be very uncomfortable and lead to dyspnea. By way of example take a deep breath and try to talk and breathe at that lung volume and see for yourself. The increased effort noticed when breathing at high lung volumes is caused by two consequences of a high lung volume. Firstly, the breathing muscles are shortened and contract at a mechanical disadvantage. As a result, more muscular activity is required to produce the pressure gradient that leads to airflow and tidal volume. Secondly, the lungs are less compliant as lung volume increases above FRC (more elastic recoil) and so more force is required to produce airflow.

  • When patients with emphysema exercise, their respiratory rate increases and the expiratory time decreases. The reduced expiratory time impairs lung emptying and leads to air trapping. The air trapping results in a progressive increase in the FRC with each respiratory cycle. This new volume is called End-Expiratory Lung Volume. This process continues until this volume approaches a critical point, at which time, the patient can’t continue exercising. This phenomenon is called “dynamic hyperinflation” and is characteristic of patients with emphysema and is responsible for much of their exercise limitation; Figure 2.8.

  • Breathing at a low FRC, as in pulmonary fibrosis and obesity, can also increase the work of breathing. In restrictive lung disorders, the lung compliance is reduced which means more effort is needed to inflate the lungs.

Figure 2.8
figure 8

Dynamic hyperinflation in patients with emphysema during exercise. Note that V T increases with exercise. Note also that the expiratory phase decreases progressively with continued exercise indicating progressive air trapping

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Disease Patterns

The lung volumes are diagnostically useful in many ways. Table 2.6 summarizes their usefulness, which is discussed in more detail in this section:

Table 2.6 Additional information acquired by a lung volume study compared to spirometry
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Defining Air Trapping and Hyperinflation in Obstructive Disorders

  • Current guidelines do not specify a fixed cut off for TLC when defining hyperinflation however, a value greater than 120% predicted is generally considered to be indicative of hyperinflation

  • Similarly, air trapping is identified by taking the upper limit of normal for RV in combination with an elevated percent predicted RV/TLC ratio. For example, if the measured RV is above the upper limit of normal and the ratio of RV over TLC is greater than 1.2 or 120%, then we have identified air trapping.

  • Differentiate subtypes of obstructive disorders

    • – Generally, obstructive disorders (emphysema and asthma) result in increased RV (air trapping) due to airway narrowing while TLC is increased only in emphysema due to loss of elastic recoil. In asthma, however, the lung has normal elastic recoil and, therefore a normal TLC [21].

    • – The RV/TLC ratio may be increased in both emphysema and asthma. The RV/TLC ratio can be used also to differentiate an obstructive from a non-obstructive increase in TLC, like acromegaly (where the RV is increased but the RV/TLC ratio is normal) [2].

    • – If lung volumes are measured pre- and post- bronchodilator, much can be learned from looking at the behavior of TLC and RV before and after the bronchodilator. TLC and RV may be shown to decrease following bronchodilator, even in the absence of a significant response in FEV1 and FVC. Furthermore, IC may increase as FRC may decrease more than TLC in response to bronchodilators. In this case, an increase in IC gives patients with emphysema more room or time to breathe before they develop dynamic hyperinflation to the point of stopping exercise. These volume changes indicate that the bronchodilators are clinically useful to such patients even though there is no change in FEV1; (Figure 2.9) [17, 21, 29].

  • Confirm the diagnosis of a restrictive disorder and differentiate its subtypes

    • – A decreased TLC is essential to make the diagnosis of a restrictive disorder with confidence [21]. The RV and RV/TLC ratio, however, may be used to differentiate the subtypes of restriction:

      • (a) In a parenchymal restriction (lung fibrosis), where there is increased elastic recoil and loss of air space, the RV and TLC are reduced with a normal RV/TLC ratio (both RV and TLC decrease proportionately) [2].

      • (b) In chest wall restriction (NMD, musculoskeletal disease, paralyzed diaphragms and obesity), where the lung parenchyma is normal, the RV is usually normal (or increased) with an increased RV/TLC ratio (remember that TLC is low). In NMD, RV may be increased because the ERV can be very low due to weakness of the expiratory muscles.

      • (c) The diffusing capacity for Carbon monoxide (DLCO) is a more reliable way of differentiation between parenchymal and chest wall restriction, as will be discussed in next chapter. Maximal Voluntary Ventilation (MVV) and maximal respiratory pressures are measures to help differentiate the different types of chest wall restriction.

    • – Obesity and mild asthma can show a spirometric pattern consistent with mild restriction (decreased FVC and normal FEV1/FVC ratio), the so called pseudorestriction. The way to differentiate parenchymal restriction from this pseudorestriction (caused by obesity or mild asthma) is by the IC/ERV ratio. This ratio is normally 2–3:1. This ratio decreases in parenchymal restriction to <2:1 and increases in pseudorestriction to >6:1. The maximal FV curve, combined with a FV curve during quiet breathing, can be used to make that distinction as in the Figure 2.10 [31].

    • – Poor patient effort during spirometry may mimic a restrictive disorder, with low FVC and FEV1 and a normal FEV1/FVC ratio. In this case a normal TLC can exclude restrictive disorders, as body plethysmography doesn’t need much patient effort to perform. The shape of the FV curve, can also easily exclude a poor effort study (PEF is not sharp and rounded in a poor effort study). In addition, the study is unlikely to be reproducible with a poor effort. The technicians usually indicate in their comments if a poor effort is apparent.

  • Separating obstructive from restrictive disorders

    • – Obstructive and restrictive disorders are sometimes hard to separate based on spirometry alone. Lung volumes may provide additional clues as they are generally increased with obstructive and decreased with restrictive disorders.

    • – As an example, when the FEV1 and FVC are at the lower limit of the normal range, with a normal FEV1/FVC ratio, a lung volume study may be of value:

      • (a) If the TLC and RV are high, then an obstructive disorder is most likely (RV/TLC ratio is usually high).

      • (b) If the TLC is normal and RV is mildly increased, then mild asthma and air trapping could be responsible (RV/TLC ratio is high) [2]. In this case, the airway obstruction is not severe enough to cause a significant drop in FEV1 or their ratio. A bronchodilator study may show a significant response.

      • (c) If the TLC is low, then a restrictive defect is likely to be the cause, provided that FVC is below the fifth percentile (a normal FVC rules out restriction [32, 33]). Before you make such a conclusion, have a quick look at the FV curve and the rest of the PFT values. If all the values are decreased proportionately with a normal FV curve make sure a correction for ethnic background is not required.

      • (d) If the TLC and RV are normal, then the study is most likely normal.

  • Detection of combined disorders

    • – Combined disorders are hard to diagnose based on spirometry alone. Spirometry coupled with a lung volume study is very useful:

      • (a) An obstructive disorder should be clear in spirometry, with low FEV1/FVC ratio. If this airflow obstruction is seen with a reduced TLC, then the reduced TLC suggests an additional restrictive disorder [21, 28]. The RV could be low, normal or high as airway obstruction may result in air trapping and increased RV [1]. Combined defects can be seen in conditions like sarcoidosis or co-existing COPD and lung fibrosis.

    • – Keep in mind that an obstructive disorder (like emphysema) with pulmonary resection (lobectomy or pneumonectomy) can give a similar pattern.

  • Chapter 6 discusses the approach to such PFTs in detail.

Figure 2.9
figure 9

Post-BD curve is closer to predicted curve indicating significant reduction in TLC and RV compared to that in the pre-BD curve. The morphology of the curve has not changed indicating no improvement in FEV1 or FVC. Despite that BD can be of help to such patients because of the lung volume change. Note that the change in TLC in this diagram is exaggerated

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Figure 2.10
figure 10

IC/ERV ratio is used to differentiate parenchymal restriction from pseudorestriction [30]

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