Lung Volumes and Capacities PDF

TLC = FRC + IC or TLC = (FRC-ERV) + Lung Volumes and VC. Capacities Values Measured During Spirometry The so-called lung compartments consist The VT is measured directly from a of four lung volumes and four lung...

TLC = FRC + IC or TLC = (FRC-ERV) + Lung Volumes and VC. Capacities Values Measured During Spirometry The so-called lung compartments consist The VT is measured directly from a of four lung volumes and four lung spirogram (see Fig. 20.8). For the capacities. A lung capacity consists of two purposes of ensuring test validity and or more lung volumes. The lung volumes standardization, the patient should be in a are tidal volume (VT), inspiratory reserve sitting or reclining position and wearing a volume (IRV), expiratory reserve volume nose clip. It sometimes takes the patient 1 (ERV), and residual volume (RV). The four to 2 minutes to be at rest and become lung capacities are (1) TLC, (2) IC, (3) accustomed to the nose clip and FRC, and (4) VC. These volumes and mouthpiece. The patient breathes through capacities are shown in Fig. 20.8. The a tight-fitting mouthpiece until a normal, lung volumes that can be measured rhythmic breathing pattern is established. directly with a spirometer or Because VT varies normally from breath pneumotachometer include VT, IC, IRV, to breath, an average VT is a more ERV, and VC. Because the RV cannot be reliable measurement. In the laboratory, exhaled, the RV, FRC, and TLC must be an average VT sometimes is measured measured using indirect methods. during 3 minutes of quiet breathing while the spirometer records volumes and Knowing TLC is necessary to identify graphs volume and time. At the bedside, patients with a restrictive pattern of an average VT usually is measured over 1 pulmonary impairment. Measuring FRC is minute; the patient breathes normally into necessary to quantify hyperinflation, which a spirometer that stores in memory each may be associated with obstructive volume exhaled for 1 minute and impairment such as with asthma and computes an average. An alternative emphysema. approach is to measure the total volume of air exhaled for 1 minute known as VE and Calculating RV is necessary to gauge any divide by the breathing frequency (f) air trapping present in these and other counted during the same period. The obstructive conditions. Measurements of following formula can be used to calculate VT, IC, ERV, IRV, and VC may be used in the VT: calculations of TLC or be useful to VT = (VE÷f) clinicians considering weaning parameters such as the rapid-shallow-breathing index The IC is also measured directly from a (f/VT) or inspiration goals of hyperinflation spirogram. The patient is asked to inhale therapy. Standards for measuring lung maximally from the resting FRC at the end volumes and capacities were initially of a normal effortless exhalation. To published in 2005; these standards focus ensure validity, a consistent resting primarily on the techniques to measure expiratory level should be obvious on the FRC. Following the measurement of FRC, spirogram before inhaling. To ensure measurements of ERV and VC enable reliability, the IC should be measured at calculation of TLC according to the least twice, and the two largest formulas: measurements should agree within 5%. Because the definition of IC is the maximal volume inhaled, the largest measurement is the patient’s IC.  Because the RV cannot be exhaled (i.e., The ERV is measured directly from the RV is by definition the air left in the lung spirogram (see Fig.20.8). The patient is after maximal exhalation), RV, FRC, and asked to breathe normally for a few TLC cannot be measured directly with a breaths and then exhale maximally. The spirometer or pneumotachometer. There ERV is the volume of air exhaled between are three indirect techniques to measure the resting expiratory level (FRC) and the these lung volumes: (1) helium dilution, (2) maximal exhalation level on the nitrogen (N2) washout, and (3) body spirogram. To ensure validity, a consistent plethysmography. The He dilution and N2 resting expiratory level should be obvious washout techniques measure whatever on the spirogram before exhaling gas is in the lungs at the beginning of the maximally. To ensure reliability, like many test, if the gas is in contact with other volumes and capacities, the ERV unobstructed airways. The body should be measured at least twice and the plethysmographic technique measures all two largest measurements should agree the gas in the thorax at the resting within 5%. Because the definition of ERV expiratory volume. Because the is the maximal volume exhaled, the largest plethysmographic technique measures all measurement is the patient’s ERV. gas in the thorax, including gas that is trapped distal to obstructed airways or gas The VC is the most commonly measured in the pleural space, the lung volume lung volume. There are several methods measured by this technique is called the of measuring the VC. The VC can be thoracic gas volume (TGV) (VTG, or measured during inspiration or during a FRCPleth). In healthy individuals, TGV is slow prolonged expiration when air identical to FRC measured by both the trapping is of concern. To measure the VC gas dilution and washout techniques. during inspiration, the patient exhales However, in patients with obstructive lung maximally and then inhales as deeply as disease with gas trapping, TGV is often possible. The volume of the maximal larger than FRC measured by other inspiration is called the inspiratory VC. To methods. TGV is also more complex and measure the VC during expiration, the cumbersome to perform. Therefore, patient inhales maximally and then unless specifically requested by a exhales maximally, taking all the time physician, He dilution or N2 washout necessary to exhale completely. When methods are routinely used. The latter two exhalation is slow, the exhaled volume is allow measurements of either FRC or called the slow VC. An alternative method TLC. Once one of them is measured, the is to measure the IC and the ERV and add other can be calculated using the previous these volumes together for a formulas. For simplicity, we will describe “combined”VC, but this method should be as follows only the maneuvers used to reserved only for patients who cannot measure FRC. otherwise execute the VC. The VC also is measured when air is exhaled forcefully Helium Dilution and as rapidly as possible. The VC The helium dilution technique for measured using this technique is called measuring lung volumes uses a closed, the FVC, as previously mentioned in this rebreathing circuit (Fig. 20.9). This chapter. technique is based on the assumptions that the patient has no He in his or her Values Not Measured During Spirometry lungs and that an equilibration of He can occur between the spirometer and the lungs. First, volume (V1) and Corrections for temperature and He concentration (C1) of He are measured at absorption are normally applied. All lung the beginning of the test. Next, the valve is volumes and capacities must be reported turned to connect the patient to the under BTPS conditions. Volumes breathing circuit, usually at the resting measured by spirometers are at ambient expiratory level of the FRC. The patient is temperature, pressure, and saturated connected to the He-air mixture, and the (ATPS) conditions and must be adjusted concentration of He is diluted slowly by the for the temperature difference between the patient’s lung volume. Wearing nose clips, spirometer and the patient’s body the patient breathes normally in the closed temperature. This ATPS to BTPS circuit. Exhaled CO2 is absorbed with adjustment can increase volumes 5% to soda lime, and O2 is added at a rate equal 10%, and the difference is large enough to to the patient’s O2 consumption. A invalidate the test results, unless the constant volume is maintained to ensure correction is made. Although He is an inert accurate He concentration measurements. gas with a negligible solubility in plasma, it The patient rebreathes the gas in the is assumed that a small amount of He system until equilibrium of He diffuses across the alveolar-capillary concentration is established. In healthy membrane. To account for the loss, 30 mL patients and patients with a small FRC, of BTPS-corrected volume is subtracted equilibration occurs in 2 to 5 minutes. for each minute of He breathing, up to 200 Patients with obstructive lung disease may mL for a 7-minute test. Once these require 20 minutes to equilibrate because corrections are made, TLC can be of slow gas mixing in the lungs. The He calculated using spirometry data. dilution time or the duration of the test gives a reasonable indication of the Nitrogen Washout distribution of ventilation. The nitrogen washout technique uses a nonrebreathing or open circuit (Fig. For FRC to be calculated using the He 20.10). The technique is based on the dilution technique, several observations assumptions that the N2 concentration in must be made:V1 and C1 (see earlier the lungs is 78% and in equilibrium with discussion), before the patient is the atmosphere, that the patient inhales connected to the breathing circuit; the final 100% O2, and that the O2 replaces all of He concentration (C2) after He equilibrium the N2 in the lungs. Similar to the He between the spirometer and patient is dilution technique, the patient is established, the spirometer temperature, connected to the system at FRC. The and the time necessary for He patient’s exhaled gas is monitored, and its equilibration to occur. If there was no volume and N2 percentage are measured. absorption of the He across the pulmonary capillaries, In general, two types of circuits are used to measure lung volumes with this V1xC1=V1xC2 technique. In one type of circuit, all of the exhaled gases are collected in a large where V2 is the volume of the He at the container, where the volume and end of the test. After measuring V1, C1, concentration of N2 are measured. In the and C2, V2 is calculated: second type of circuit, the volume and concentration of each exhaled breath are FRC = V2-V1 measured separately and stored in a memory; the sum of the volumes and the weighted average of the N2 concentration essentially all other mathematical are calculated by a computer. calculations used in PFTs. Wearing nose clips, the patient breathes Plethysmography 100% O2 until nearly all of the N2 has The plethysmography technique applies been washed out of the lungs, leaving less Boyle’s law and uses measurements of than 1.5% N2 in the lungs. When the peak volume and pressure changes to exhaled concentration of N2 is less than determine lung volume, assuming 1.5%, the patient exhales completely, and temperature is constant. The the fractional concentration of alveolar N2 plethysmography technique measures the (FAN2) is noted. Similar to the He volume of all compressible gas in the technique, the time it takes to wash out thorax, including gas trapped behind the N2 is approximately 2 to 5 minutes in airway obstructions or in the pleural healthy individuals and longer in patients space. Gas in the abdomen also may be with obstructive lung disease. The test included in the measurement. The must occur in a leak-proof circuit because whole-body plethysmograph consists of a the presence of any air increases the sealed chamber in which the patient sits measured N2 percentages and results in (Fig. 20.11). Pressure transducers grossly elevated measurements of lung (electronic manometers) measure volume. pressure at the mouth and in the chamber. An electronically controlled shutter near For FRC to be calculated by the N2 the mouthpiece allows the airway to be washout technique,several measurements occluded periodically, measuring airway must be made: the total volume of gas pressure changes under conditions of no exhaled during the test (VE), the fractional airflow. Without airflow, pressure changes concentration of exhaled N2 in the total measured at the mouth are pressure gas volume (FEN2), the fractional changes in the alveoli. According to concentration of N2 in the alveoli at the Boyle’s law (V × P = k), when temperature end of the test (FAN2), and the spirometer is constant, volume changes in the thorax temperature. FRC can be calculated with create volume changes in the chamber, the following equation: which are reflected by pressure changes in the chamber. When measurement of FRC = VExFEN2/0.78-FAN2 TGV is being done, the patient sits in the chamber and initially breathes normal tidal The calculated FRC must be adjusted for volumes through the mouthpiece. When the temperature difference between the the patient is near FRC, the shutter is spirometer and the patient’s body closed at end expiration for 2 to 3 temperature using the BTPS correction seconds. The patient holds his or her factor. During the test, some N2 from the cheeks and performs gentle panting at 1 plasma and body tissues is usually Hz (60 times per minute) or one pant per excreted and exhaled with lung N2. For second. During panting, changes in airway this reason, another correction is needed, pressure (ΔP) and changes in chamber using duration of the test and the weight of volume (ΔV) are measured. Because the the patient. Note that the computer panting maneuver occurs with small interfaces used today in pulmonary pressure changes around barometric function labs eliminate the need to pressure, the simplified equation used to manually calculate such corrections and calculate TGV is: TGV = PB x (∆V/∆P) obstruction may be associated with increased TLC and fibrosis with a where PB is the barometric pressure in cm decreasing TLC, causing offsetting H2O differences). Such a pattern may occur in an entity called “combined pulmonary A series of three to five panting fibrosis and emphysema.”When maneuvers should be performed. After obstruction and restriction occur together, panting, the patient should exhale the TLC may be a less sensitive measure completely to record ERV and then inhale of the restrictive impairment. Other maximally to record the inspiratory VC. volumes and capacities may remain Because the body plethysmographic normal with mild obstructive or restrictive method of measuring FRC actually disease. The pattern of lung volume measures TGV, the value obtained for changes and the proportion of FRC and some patients may be larger than values RV to TLC are also important. resulting from either the He dilution or N2 washout techniques. Such a difference Normal values for lung volumes. The occurs whenever there is gas in the thorax normal VT is approximately 500 to 700 mL that is not in communication with patent for an average healthy adult. In the normal airways, as might be the case in patients population, great variation of tidal volumes with pneumothorax, pneumomediastinum, and measurements beyond the normal or emphysema. Assuming that TGV range do not necessarily indicate a actually represents FRC, calculations can disease process. Conversely, normal VT is be made for TLC and RV, similar to He often observed in both restrictive and dilution or N2 washout methods. obstructive lung diseases. VT alone is not useful in characterizing whether there is RULE OF THUMB restrictive or obstructive disease. Plethysmography measures the entire chest volume, whereas helium dilution and The normal IC is approximately 3.6 L, with nitrogen washout methods measure the a significant variation in the normal volume of lung that is connected with population. IC may be normal or reduced outside air. For this reason, in restrictive and obstructive lung plethysmography may give more accurate diseases. A reduction of IC occurs in FRC measurement in patients with bullous restrictive lung diseases because the lung disease, such as emphysema. patient’s inhaled volume is reduced and there is a reduction in TLC. In mild Interpretation obstructive lung diseases, IC is usually Changes in lung volumes and capacities normal. In moderate and severe are generally consistent with the pattern of obstructive diseases, IC can be reduced impairment. TLC, FRC, and RV increase because the resting expiratory level of with obstructive lung diseases and FRC has increased owing to hyperinflation decrease with restrictive impairment. By of the lungs. An increase in IC may occur definition,restrictive disease is present when the patient inhales from below the only when the TLC is decreased. Some resting expiratory level when the lung volumes provide valuable diagnostic measurement is performed; athletes and information. For example, TLC is always musicians who play wind instruments may reduced in restrictive lung disease such as have increased inspiratory capacities. RTs pulmonary fibrosis, unless obstruction and use the measurement of IC in clinical restriction occur together (because protocols to decide between methods of because of a loss of lung volume; RV and lung expansion therapy (see Chapter 43). FRC are often reduced proportionately. Certain acute disorders, such as IRV is not commonly measured. Similar to pulmonary edema, atelectasis, and VT and IC, IRV can be normal in both consolidation, also cause a reduction of restrictive and obstructive diseases and is TLC and FRC. not a useful diagnostic measurement. The normal value for IRV for an adult is INTERPRETATION OF THE approximately 3.1 L. PULMONARY FUNCTION REPORT Interpretive strategies for pulmonary The normal adult ERV is approximately function testing abound. Most 1.2 L and represents approximately 20% computer-based pulmonary function to 25% of the VC. It can be either normal testing systems have algorithms in their or reduced in obstructive and restrictive software programs for computer-assisted lung diseases. ERV is subtracted from interpretations of the pulmonary function FRC to calculate RV. report. Table 20.4 summarizes pulmonary function changes that may occur in The normal value of the VC is 4.8 L and advanced obstructive and restrictive represents approximately 80% of TLC. patterns of lung diseases, and Fig. 20.14 Normal values for VC can vary presents a simple algorithm to assess PFT significantly depending on age, gender, results in clinical practice. height, and ethnicity. A reduction of VC occurs in restrictive lung diseases When considering a pulmonary function because the patient’s inhaled volume is report, the FEV1/FVC ratio is a good place reduced and there is a reduction in TLC. to start because it provides an initial focus In mild obstructive lung diseases, the slow as normal, restrictive, or obstructive VC is usually normal if the patient exhales impairment. When the FEV1/FVC is less leisurely and has had enough time to than the LLN, there is airway obstruction. exhale completely or if the VC is FEV1/FVC is normal in healthy individuals measured during inspiration. and patients with restriction. However, Measurements made from FVC provide some clinicians still use a somewhat valuable data for pulmonary mechanics arbitrary value of 0.70. Although reliable in and are often reduced in patients with younger patients, using the fixed ratio of obstructive disease. FEV1/FVC less than 0.70 versus the LLN value for FEV1/FVC simplifies the issue RV, FRC, and TLC are the most important and often over diagnosed obstruction in measurements of lung volumes. For elderly individuals. adults, the normal TLC is approximately 6.0 L; RV is approximately 1.2 L and The next to consider is TLC. If the TLC is represents approximately 20% of TLC; less than the LLN, the patient has a and FRC is approximately 2.4 L,which restrictive impairment according to this represents approximately 40% of the TLC. algorithm. Patients with obstruction will RV and FRC are usually enlarged in acute often have normal or elevated TLC. and chronic obstructive lung diseases Naturally, some patients may have a because of hyperinflation and air trapping mixed obstructive/restrictive pattern.In (Fig. 20.12). TLC also may be increased in those patients, TLC may be reduced, COPD. By definition, TLC is always normal, or elevated. reduced in restrictive lung diseases Once an obstructive or restrictive pattern is ascertained, DLCO will help to differentiate between the conditions that do and do not affect the gas transfer across the alveolar-capillary membrane. If the percent predicted normal DLCO is less than the LLN, the patient has a diffusion impairment. According to most recent ATS/ERS guidelines on PFT interpretation, the severity of obstructive and restrictive impairment is judged by the patient’s FEV1, and the severity of gas transfer is based on the DLCO. Severity categories, based on percent predicted values, are outlined in Box 20.2. Use of other indices, such as FRC or TLC, to quantify the severity is controversial and will not be discussed here. Although FEV1 is used to quantify the severity of illness across the disease categories, it is important to note its limitations. FEV1 is a poor measurement of upper airway obstruction; it may not be suitable comparing different pulmonary conditions, it may not be reliable in the extremes of severity assessment, and FEV1 does not correlate well with clinical symptoms or disease progression.