Pulmonary Function, Arterial Blood Gases (ABGs), and Electrolyte Studies
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OVERVIEW OF PULMONARY FUNCTION TESTS
There are three aspects of pulmonary function: perfusion, diffusion, and ventilation. Perfusion relates to blood flow through pulmonary vessels; diffusion refers to movement of oxygen and carbon dioxide across alveolar capillary membranes; and ventilation relates to air exchange between alveolar spaces and the atmosphere.
During breathing, the lung-thorax system acts as a bellows to provide air to the alveoli for adequate gas exchange to take place. Like a spring or rubber band, the lung tissue also possesses the property of elasticity. When the inspiratory muscles contract, the thorax and lungs expand; when the same muscles relax and the force is removed, the thorax and lungs return to their resting position. Also, when the thorax and lungs expand, the alveolar pressure is lowered below atmospheric pressure. This permits air to flow into the trachea, bronchi, bronchioles, and alveoli. Expiration is mainly passive. It occurs because the thorax and lungs recoil to their resting position. The alveolar pressure increases above atmospheric pressure, and air flows out through the respiratory tract. The major function of the lungs is to provide adequate ventilation to meet the metabolic demands of the body during rest and during exercise. The primary purpose of pulmonary blood flow is to conduct mixed venous blood through the capillaries of the alveoli so that oxygen (O2) can be taken up by the blood and carbon dioxide (CO2) can be removed from the blood.
Purpose of Tests
Pulmonary function tests determine the presence, nature, and extent of pulmonary dysfunction caused by obstruction, restriction, or both. When ventilation is disturbed by an increase in airway resistance (Raw
), the ventilatory defect is called an obstructive
ventilatory impairment. When ventilation is disturbed by a limitation in chest wall excursion, the defect is referred to as a restrictive
ventilatory impairment. When ventilation is altered by both increased Raw
and limited chest wall excursion, the defect is termed a combined
defect. Table 14.1
presents the conditions that affect ventilation.
Pulmonary function studies may reveal locations of abnormalities in the airways, alveoli, and pulmonary vascular bed early in the course of a disease, when the physical examination and radiographic studies still appear normal.
Indications for Tests
Early detection of pulmonary or cardiogenic pulmonary disease (see Table 14.1
Differential diagnosis of dyspnea
Presurgical assessment (e.g., ability to tolerate intraoperative anesthetics, especially during thoracic procedures)
Evaluation of risk factors for other diagnostic procedures
Detection of early respiratory failure
Monitoring progress of bronchopulmonary disease
Periodic evaluation of workers exposed to materials harmful to the respiratory system
Epidemiologic studies of selected populations to determine risks for or causes of pulmonary diseases
Workers’ compensation claims
Monitoring after pharmacologic or surgical intervention
TABLE 14.1 Conditions That Affect Ventilation
Restrictive Ventilatory Impairmentsa
Chest wall disease
Injury, kyphoscoliosis, spondylitis, muscular dystrophy, other neuromuscular diseases
Obesity, peritonitis, ascites, pregnancy
Interstitial lung disease
Interstitial pneumonitis, fibrosis, pneumoconioses (e.g., asbestosis, silicosis), granulomatosis, edema, sarcoidosis
Pneumothorax, hemothorax, pleural effusion, fibrothorax
Tumors, cysts, abscesses
Obstructive Ventilatory Impairmentsb
Peripheral airway disease
Bronchitis, bronchiectasis, bronchiolitis, bronchial asthma, cystic fibrosis
Pulmonary parenchymal disease
Upper airway disease
Pharyngeal, tracheal, or laryngeal tumors; edema; infections; foreign bodies; collapsed airway; stenosis
Mixed-Defect Ventilatory Impairmentsc
Both increased airway resistance and limited expansion of chest cavity and/or chest wall; obstruction caused by bronchial edema, compression of respiratory airway owing to increased interstitial (and intravenous fluid) pressure; restriction caused by impaired elasticity, anatomic deformity (e.g., kyphosis, lordosis, scoliosis)
a Characterized by interference with chest wall or lung movement, “stiff lung,” and an actual reduction in the volume of air that can be inspired.
b Characterized by the need for increased effort to produce airflow; respiratory muscles must work harder to overcome obstructive forces during breathing; prolonged and impaired airflow during expiration; airway resistance increases and lungs become very compliant.
c Combined or mixed; exhibits components of both obstructive and restrictive ventilatory impairments.
Classification of Tests
Pulmonary function tests evaluate the ventilatory system and alveoli in an indirect, overlapping way. The patient’s age, height, weight, ethnicity, and gender are recorded before testing because they are the basis for calculating predicted or normal reference values.
Pulmonary function tests are generally divided into three categories:
Airway flow rates typically include measurements of instantaneous or average airflow rates during a maximal forced exhalation to assess airway patency and resistance. These tests also assess responses to inhaled bronchodilators or bronchial provocations.
Lung volumes and capacities measure the various air-containing compartments of the lung to assess air trapping (hyperinflation, overdistention) or reduction in volume. These measurements also help to differentiate obstructive from restrictive ventilatory impairments.
Gas exchange (diffusion capacity or transfer factor) measures the rate of gas transfer across the alveolar capillary membranes to assess the diffusion process. It can also monitor for side effects of drugs, such as bleomycin (antineoplastic) or amiodarone (antiarrhythmic), which can cause interstitial pneumonitis or pulmonary fibrosis. Diffusion capacity in the absence of lung disease (e.g., anemia) can also be evaluated.
See Figure 14.1
for a sample Pulmonary Function Report.
FIGURE 14.1. Pulmonary function report of a 47-year-old woman whose chief complaint is shortness of breath. The report includes spirometry, lung volumes, diffusion capacity, maximal voluntary ventilation, and maximal respiratory pressures. Note: The shape or configuration of the flow-volume loop (lower left corner of report) is significant for airflow obstruction (i.e., obstructive ventilatory impairment). The current flow-volume loop is essentially normal in appearance. (Reprinted with permission from Froedtert Hospital, Milwaukee, WI.)
AIRWAY FLOW RATES
Airway flow rates provide information about the severity of airway obstruction and serve as an index of dynamic function. The lung volume at which the flow rates are measured is useful for identifying a central or peripheral location of airway obstruction.
• Spirometry, Forced Expiratory Maneuver Volume-Time Spirogram (V-T Tracing); Flow-Volume Spirogram (F-V Loop)
Lung capacities, volumes, and flow rates are clinically measured by a mechanical device called a spirometer
. The mechanical signal is converted to an electrical signal, which records the amounts of gas breathed in and out and produces a spirogram
. Spirometers can be grouped into two major categories: (1) the mechanical or volume-displacement types (water-filled, dry-rolling seal, wedge, or bellows) and (2) the electronic or flow-sensing types (pneumotachometer or hot-wire anemometer [Fig. 14.2
]). Spirometry determines the effectiveness of the various mechanical forces involved in lung and chest wall movement. The values obtained provide quantitative information about the degree of obstruction (obstructive ventilatory impairment) to expiratory airflow or the degree of restriction (restrictive ventilatory impairment) of inspired air. The forced expiratory maneuver (spirometry) is useful to quantify the extent and severity of airway obstruction. It measures the maximum amount of air that can be exhaled rapidly and forcibly after a maximal deep inspiration. The results are a measure of airway function and the patency of the airway.
Forced vital capacity (FVC) is the maximum amount of air that can be exhaled forcibly and completely after a maximal inspiration. The forced expiratory volumes exhaled within 1, 2, or 3 seconds are referred to as timed vital capacities
, and FEV3
, respectively), whereas the FEF25-75
is the flow of air during the middle 50% (0.50) of the forced volume. These measurements are useful for evaluating a patient’s response to bronchodilators. Generally, if the FEV1
is <80% (<0.80) of predicted (reference value) or the FEF25-75
is <60% (<0.60) of predicted, bronchodilators (e.g., albuterol) are
administered with a handheld nebulizer, and the spirometry is repeated. Studies have shown a better bronchodilator response with combined drugs (e.g., albuterol plus ipratropium) than either alone. An increase in these values of 20% or more (>0.20) above the pre-bronchodilator level suggests a significant response to the bronchodilator and is consistent with a diagnosis of reversible obstructive airway disease (e.g., asthma). Persons with emphysema typically do not demonstrate this type of response to bronchodilator. Measured (actual) spirometry values are compared with predicted values by means of regression equations using age, height, weight, ethnicity, and gender and are expressed as a percentage of the predicted value. Typically, a value >80% (>0.80) of predicted is considered within normal limits.
CHART 14.3 Lung Volume Symbols: Pulmonary Function Terminology
This list indicates terms used in measuring lung volumes and the units that express these measurements.
Forced vital capacity: maximum amount of air that can be exhaled forcibly and completely after a maximal inspiration (liters)
Forced expiratory volume at specific time intervals (e.g., 1, 2, and/or 3 seconds): volume of air expired during the first, second, third, etc., seconds of FVC maneuver (liters)
Ratio of a timed forced expiratory volume to the forced vital capacity (e.g., FEV1/FVC) (percent)
Forced expiratory flow between 25% and 75%: average flow of expired air measured between 25% and 75% of the FVC maneuver (liters/second)
Peak expiratory flow rate: maximum flow of expired air attained during an FVC maneuver (liters/second or liters/minute)
Peak inspiratory flow rate: maximum flow of inspired air achieved during a forced maximal inspiration (liters/second or liters/minute)
Forced instantaneous expiratory flow rate at 25% of lung volume achieved during an FVC maneuver (liters/second or liters/minute)
Forced instantaneous expiratory flow rate at 50% of lung volume achieved during an FVC maneuver (liters/second or liters/minute)
Forced instantaneous expiratory flow rate at 75% of lung volume achieved during an FVC maneuver (liters/second or liters/minute)
Functional residual capacity: volume of air remaining in the lung at the end of a normal expiration (i.e., end-tidal expiration) (liters)
Inspiratory capacity: maximum amount of air that can be inspired from end-tidal expiration (liters)
Inspiratory reserve volume: maximum amount of air that can be inspired from end-tidal inspiration (liters)
Expiratory reserve volume: maximum amount of air that can be expired from end-tidal expiration (liters)
Residual volume: volume of gas left in the lung after a maximal expiration (liters)
Vital capacity: maximum volume of air that can be expired after a maximal inspiration (liters)
Total lung capacity: volume of gas contained in the lungs after a maximal inspiration (liters)
Carbon monoxide diffusing capacity of the lung: rate of diffusion of carbon monoxide across the alveolar capillary membrane (i.e., rate of gas transfer across the alveolar capillary membrane) (milliliters/minute per millimeter of mercury)
Closing volume: volume at which the lower lung zones cease to ventilate, presumably as a result of airway closure (percent of vital capacity)
Maximum voluntary ventilation: maximum number of liters of air a patient can breathe per minute by a voluntary effort (liters/minute)
Volume of isoflow: volume for which flow is the same with air and with helium during an FVC maneuver (percent)
CHART 14.4 Miscellaneous Symbols
This list shows some of the other symbols found in this chapter.
Frequency (of breathing)
Compliance of the lung
Oxygen diffusing capacity of the lung
Alveolar-to-arterial oxygen gradient
Body surface area (square meters)
Thoracic gas volume (also expressed as VTG)
FIGURE 14.2. Orbit portable spirometer. (Courtesy of QRS Diagnostic, Maple Grove, MN.)
FVC: >80% (>0.80) of the predicted value
FEVt: FEV1, FEV2, FEV3, >80% (>0.80) of the predicted value
FEV1, 80% to 85% (0.80 to 0.85) of FVC
FEV2, 90% to 94% (0.90 to 0.94) of FVC
FEV3, 95% to 97% (0.95 to 0.97) of FVC
FEF25-75: >60% (>0.60) of the predicted value
Predicted values are based on the patient’s age, height, ethnicity, and gender.
Bronchodilators (e.g., albuterol) should be withheld for at least 4 hours if tolerated.
Respiratory infections may decrease airflow during the maneuver.
Patient noncompliance can adversely affect the results because this test is effort dependent.
• Peak Inspiratory Flow Rate (PIFR)
The peak inspiratory flow rate (PIFR) measures the function of the airways, identifies reduced breathing on inspiration, and is totally dependent on the effort the patient makes to inspire. The PIFR is the maximal flow of air achieved during a forced maximal inspiration.
Approximately 300 L/min or 5 L/sec
Predicted values are based on age, gender, and height.
• Peak Expiratory Flow Rate (PEFR)
The peak expiratory flow rate (PEFR) measurement is used as an index of large airway function. It is the maximum flow of expired air attained during a forced expiratory maneuver.
Approximately 450 L/min or 7.5 L/sec
Predicted values are based on age, gender, and height.
LUNG VOLUMES AND CAPACITIES
Lung volumes can be considered as basic subdivisions of the lung (not actual anatomic subdivisions). They may be subdivided as follows:
Tidal volume (VT)
Inspiratory capacity (IC)
Inspiratory reserve volume (IRV)
Residual volume (RV)
Functional residual capacity (FRC)
Expiratory reserve volume (ERV)
Vital capacity (VC)
Combinations of two or more volumes are termed capacities
. These volumes and capacities are shown graphically in Figure 14.3
. Measurement of these values can provide information about the degree of air trapping or hyperinflation.
• Functional Residual Capacity (FRC)
FRC is used to evaluate both restrictive and obstructive lung defects. Changes in the elastic properties of the lungs are reflected in the FRC. The FRC is the volume of gas contained in the lungs at the end of a normal quiet expiration (see Fig. 14.3
Approximately 2.50 to 3.50 L
Predicted values are based on age, height, weight, ethnicity, and gender.
The observed value should be 75% to 125% (0.75 to 1.25) of the predicted value.
FIGURE 14.3. Subdivisions of lung volume in the normal adult male. (From Geschickter CF: The Lung in Health and Disease. Philadelphia, JB Lippincott, 1973.)
• Residual Volume (RV)
RV can help to distinguish between restrictive and obstructive ventilatory defects. It is the volume of gas remaining in the lungs after a maximal exhalation. Because the lungs cannot be completely emptied (i.e., a maximal expiratory effort cannot expel all of the gas), RV is the only lung volume that cannot be measured directly from the spirometer. It is calculated mathematically by subtracting the measured ERV from the measured FRC (see Fig. 14.3
Approximately 1200 to 1500 mL
Predicted values are based on age, gender, and height.
Residual volume normally increases with age.
• Expiratory Reserve Volume (ERV)
ERV is the largest volume of gas that can be exhaled from end-tidal expiration. This measurement identifies lung or chest wall restriction. The ERV can be estimated mathematically by subtracting the IC from the VC. The ERV accounts for approximately 25% of the VC and can vary greatly in patients of comparable age and height (see Fig. 14.3
Approximately 1200 to 1500 mL (1.20 to 1.50 L)
Predicted values are based on age, height, and gender.
• Inspiratory Capacity (IC)
IC measures the largest volume of air that can be inhaled from the end-tidal expiratory level. This measurement is used to identify lung or chest wall restrictions. Mathematically, the IC is the sum of the VT and the IRV (see Fig. 14.3
Approximately 3000 to 3300 mL (3.00 to 3.30 L)
Predicted values are based on age, height, and gender.
• Vital Capacity (VC)
Measurement of the VC identifies defects of lung or chest wall restriction. The VC is the largest volume of gas that can be expelled from the lungs after the lungs are first filled to the maximum extent and then slowly emptied to the maximum extent. Mathematically, it is the sum of the IC and the ERV (see Fig. 14.3
Approximately 4.50 to 5.00 L
Predicted values are based on age, gender, height, and ethnicity.
• Total Lung Capacity (TLC)
Total lung capacity (TLC) is used mainly to evaluate obstructive defects and to differentiate restrictive from obstructive pulmonary disease. It measures the volume of gas contained in the lungs at the end of a maximal inspiration. Mathematically, it is the sum of the VC and the RV, or the sum of the primary lung volumes (see Fig. 14.3
). This value is calculated indirectly from other tests.
Approximately 5.70 to 6.20 L
Predicted values are based on age, height, gender, and ethnicity.
All pulmonary volumes and capacities are about 20% to 25% less in women than in men.
GAS EXCHANGE (DIFFUSING CAPACITY), TRANSFER FACTOR
Gas exchange in the lungs is referred to as respiration, whereas the movement of gas in and out of the lung is ventilation. Gas exchange involves the movement of oxygen (O2) from the alveolus (gas exchange units in the lung) to the blood (i.e., diffusion across the alveolar capillary membrane) and movement of carbon dioxide (CO2) from the blood into the alveolus for subsequent removal.
• Carbon Monoxide Diffusing Capacity (DLCO, DL), Diffusing Capacity, Transfer Factor
The diffusing capacity measurement determines the rate of gas transfer across the alveolar capillary membranes. Carbon monoxide (CO) combines with hemoglobin about 210 times more readily than does O2
. If there is a normal amount of hemoglobin in the blood, the only other significant limiting factor to CO uptake is the state of the alveolar capillary membranes. Normally, the amount of CO in the blood is insufficient to affect the test. Two categories of factors (i.e., physical and chemical) determine the rate of gas (CO) transfer across the lung. The physical determinants are CO driving pressure,
surface area, thickness of capillary walls, and diffusion coefficient for CO. The chemical determinants are red blood cell volume and reaction rate with hemoglobin.
This test is used to diagnose pulmonary vascular disease, emphysema, and pulmonary fibrosis and to evaluate the extent of functional pulmonary capillary bed in contact with functional alveoli. The alveolar volume (VA) can also be determined. The DLCO measures the diffusing capacity of the lungs for CO. The DLO2 is obtained by multiplying the DLCO by 1.23.
Approximately 25 mL/min/mm Hg (8.4 mmol/min/kPa)
Predicted values are based on the patient’s height, age, and gender.
Exercise (with an increased cardiac output) and polycythemia increase the value. Because increased levels of COHb (as seen in smokers) and anemia decrease the value, the DLCO is adjusted for COHb levels >10% (>0.10) and hemoglobin (Hb) values <8 g/dL (<80 g/L).
OTHER PULMONARY FUNCTION TESTS
• Maximum Voluntary Ventilation (MVV)
Maximum voluntary ventilation (MVV) measures several physiologic phenomena occurring at the same time, including thoracic cage compliance, lung compliance, Raw, and available muscle force. It is the number of liters of air that the patient can breathe per minute with maximal voluntary effort.
Approximately 160 to 180 L/min
Predicted values are based on the patient’s age, height, and gender. A healthy person may vary by as much as 25% to 35% from mean group values.
Poor patient effort can be ruled out by using the following formula to predict the MVV of the patient: Predicted MVV = 35 × FEV1. This is a useful check to determine whether the recorded MVV is indicative of adequate patient effort. Low values can be related to patient effort and not to pathophysiology.
• Maximal Respiratory Pressure (MRP), Maximal Expiratory Pressure (MEP), Maximal Inspiratory Pressure (MIP)
The maximal respiratory pressure (MRP) measurements assess ventilatory muscle strength in persons with neuromuscular disorders such as poliomyelitis, emphysema, and pulmonary fibroses. The maximal expiratory pressure (MEP) is the greatest pressure that can be generated at or near TLC after a maximal inspiration, whereas the maximal inspiratory pressure (MIP) is measured at or near the residual volume after a maximal expiration.
MEP: ˜ 100 to 250 cm H2O
MIP: ˜ 40 to 125 cm H2O
Predicted values (i.e., reference values) are based on the patient’s age and gender.
The MIP and MEP measurements depend on patient effort; low values may be caused by poor effort rather than loss of respiratory muscle strength. If the patient does not inspire or expire maximally before performing the pressure measurement, the value may be low. Also, sustained efforts longer than 3 seconds should be avoided because they can cause a decrease in cardiac output as a result of increased intrathoracic pressures.
• Closing Volume (CV)
In a healthy person, the concentration of alveolar nitrogen, after a single breath of 100% O2, rapidly increases near the end of expiration. This rise is caused by closure of the small airways in the bases of the lung. The point at which this closure occurs is called the closing volume (CV). CV is used as an index of pathologic changes occurring within the small airways (those <2 mm in diameter). The conventional pulmonary function tests are not sensitive enough to make this determination. This test relies on the fact that the upper lung zones contain a proportionately larger residual volume of gas than the lower lung zones; there is a gradient of intrapleural pressure from the top to the bottom of the lung. Additionally, the uniformity of gas distribution within the lungs can be measured.
Average is 10% to 20% (0.10 to 0.20) of the patient’s VC.
Predicted values are derived from mathematical regression equations and are based on the patient’s age and gender.
• Volume of Isoflow (VISO
This test is designed to detect pathologic changes occurring in the small airways and may be more sensitive than conventional pulmonary function tests. Helium has the unique property of lowering gas density. Therefore, after the patient breathes a helium-oxygen gas mixture, the effects of convective acceleration and turbulence are negated. Any abnormality observed in the F-V loop, then, results from an increase in resistance to laminar (nonturbulent) flow, which indicates small airway abnormalities or lung disease.
Average is 10% to 25% of VC.
Predicted values are based on age.
• Body Plethysmography: Thoracic Gas Volume (VTG), Lung Compliance (CL), Airway Resistance (Raw), Airway Conductance (Gaw)
This test measures several parameters. Thoracic gas volume (VTG) composes all the air contained within the thorax, whether or not it is in ventilatory communication with the rest of the lung. Compliance of the lung (CL) is an indication of its elasticity, and Raw is a measurement of the resistance to airflow in the tracheobronchial tree (which is a hyperbolic function). Airway conductance (Gaw) is the reciprocal of Raw, decreasing in a linear fashion as Raw increases.
The measurement of VTG through body plethysmography is an application of Boyle’s law, which states that for a gas at constant temperature, pressure and volume vary inversely (P1V1 = P2V2). Raw increases with decreased lung volumes and decreases with higher lung volumes in a nonlinear, hyperbolic fashion. CL increases in obstructive diseases (e.g., emphysema) and decreases in restrictive processes (e.g., interstitial lung disease).
VTG: ˜ 2.50 to 3.50 L
CL: 0.2 L/cm H2O (2.04 L/kPa)
Raw: 0.6 to 2.4 L/sec/cm H2O
Gaw: reciprocal of Raw
Predicted values are based on the patient’s age, height, weight, and gender.
• Bronchial Provocation: Methacholine Challenge, Histamine Challenge
Bronchial provocation challenge testing is performed in patients with normal pulmonary function tests who have suspected underlying bronchial hyperreactivity. Additionally, the asthmatic patient is more sensitive to the bronchoconstrictive effects of cholinergic agents (e.g., methacholine chloride) than is the healthy person as observed on a spirometry test. Raw tests are also sensitive monitors of response to bronchoconstrictive agents.
Positive response: >20% (or >0.20) decrease in FEV1 from baseline or >35% (>0.35) increase in Raw
Negative response: <20% (or <0.20) decrease in FEV1 from baseline or <35% (<0.35) increase in Raw
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