Pulmonary Function, Arterial Blood Gases (ABGs), and Electrolyte Studies

Pulmonary Function, Arterial Blood Gases (ABGs), and Electrolyte Studies


Pulmonary Physiology

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, 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 airway resistance and limited chest wall excursion, the defect is termed a combined or mixed 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

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.

TABLE 14.1 Conditions That Affect Ventilation



Restrictive Ventilatory Impairments*

Chest wall disease

Injury, kyphoscoliosis, spondylitis, muscular dystrophy, other neuromuscular diseases

Extrathoracic conditions

Obesity, peritonitis, ascites, pregnancy

Interstitial lung disease

Interstitial pneumonitis, fibrosis, pneumoconioses (e.g., asbestosis, silicosis), granulomatosis, edema, sarcoidosis

Pleural disease

Pneumothorax, hemothorax, pleural effusion, fibrothorax

Space-occupying lesions

Tumors, cysts, abscesses

Obstructive Ventilatory Impairments

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 Impairments

Pulmonary congestion

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)

* Characterized by interference with chest wall or lung movement, “stiff lung,” and an actual reduction in the volume of air that can be inspired.

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.

Combined or mixed; exhibits components of both obstructive and restrictive ventilatory impairments.

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.

Symbols and Abbreviations

Pulmonary function studies and blood gas analyses measure quantities of gas mixtures and their components, blood and its constituents, and various factors affecting these quantities. The symbols and abbreviations given here are based on standards developed by American physiologists. Familiarity with the major and secondary symbols facilitates interpretation of any combination of these symbols (Charts 14.1, 14.2, 14.3, 14.4).


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. 14.1]). 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.

The forced expiratory volumes exhaled within 1, 2, or 3 seconds are referred to as timed vital capacities (FEV1, FEV2, 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 prebronchodilator 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.

FIGURE 14.1. Orbit™ portable spirometer. (Courtesy of QRS Diagnostic, Maple Grove, MN.)

Reference Values


FVC: >80% (>0.80) of the predicted value

FEVt: FEV1, FEV2, FEV3, >80% (>0.80) of the predicted value


FEV1, 80%-85% (0.80-0.85) of FVC

FEV2, 90%-94% (0.90-0.94) of FVC

FEV3, 95%-97% (0.95-0.97) of FVC FEF25-75: >60% (>0.60) of the predicted value

Predicted values are based on the patient’s age, height, ethnicity, and gender.

Interfering Factors

  • 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.

Reference Values


Approximately 300 L/min or 5 L/sec

Predicted values are based on age, sex, and height.

Interfering Factors

  • Poor patient effort compromises the test.

  • Inability to maintain an airtight seal around the mouthpiece

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.

Reference Values


Approximately 450 L/min or 7.5 L/sec

Predicted values are based on age, sex, and height.

Interfering Factors

  • Poor patient effort compromises the test.

  • Inability to maintain an airtight seal around the mouthpiece


Lung volumes can be considered as basic subdivisions of the lung (not actual anatomic subdivisions). They may be subdivided as follows:

  • Total lung capacity (TLC)

  • 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 14.2. Measurement of these values can provide information about the degree of air trapping or hyperinflation.

Functional Residual Capacity (FRC)

Functional residual capacity (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. 14.2).

Reference Values


Approximately 2.50-3.50 L

Predicted values are based on age, height, weight, ethnicity, and gender.

The observed value should be 75%-125% (0.75-1.25) of the predicted value.

FIGURE 14.2. 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)

Residual volume (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 expiratory reserve volume (ERV) from the measured FRC (see Fig. 14.2).

Reference Values


Approximately 1200-1500 mL

Predicted values are based on age, gender, and height.

Interfering Factors

Residual volume normally increases with age.

Expiratory Reserve Volume (ERV)

Expiratory reserve volume (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 inspiratory capacity (IC) from the vital capacity (VC). The ERV accounts for approximately 25% of the VC and can vary greatly in patients of comparable age and height (see Fig. 14.2).

Reference Values


Approximately 1200-1500 mL (1.20-1.50 L)

Predicted values are based on age, height, and gender.

Inspiratory Capacity (IC)

Inspiratory capacity (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 tidal volume (VT) and the inspiratory reserve volume (IRV) (see Fig. 14.2).

Reference Values


Approximately 3000-3300 mL (3.00-3.30 L) Predicted values are based on age, height, and gender.

Vital Capacity (VC)

Measurement of the vital capacity (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. 14.2).

Reference Values


Approximately 4.50-5.00 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. 14.2). This value is calculated indirectly from other tests.

Reference Values


Approximately 5.70-6.20 L

Predicted values are based on age, height, gender, and ethnicity.

All pulmonary volumes and capacities are about 20%-25% less in women than in men.


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.

FIGURE 14.3. 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.)

Reference Values


Approximately 25 mL/min/mm Hg (8.4 mmol/min/kPa) Predicted values are based on the patient’s height, age, and gender.

Interfering Factors

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).


Maximum Voluntary Ventilation (MVV)

Maximum voluntary ventilation (MVV) measures several physiologic phenomena occurring at the same time, including thoracic cage compliance, lung compliance, airway resistance, and available muscle force. It is the number of liters of air that the patient can breathe per minute with maximal voluntary effort.

Reference Values


Approximately 160-180 L/min

Predicted values are based on the patient’s age, height, and gender. A healthy person may vary by as much as 25%-35% from mean group values.

Interfering Factors

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 total lung capacity after a maximal inspiration, whereas the maximal inspiratory pressure (MIP) is measured at or near the residual volume after a maximal expiration.

Reference Values


Maximal expiratory pressure (MEP): approximately 100-250 cm H2O

Maximal inspiratory pressure (MIP): approximately 40-125 cm H2O

Predicted values (i.e., reference values) are based on the patient’s age and gender.

Interfering Factors

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.

Reference Values


Average is 10% to 20% (0.10 to 0.20) of the patient’s vital capacity (VC).

Predicted values are derived from mathematical regression equations and are based on the patient’s age and gender.

Interfering Factors

  • The CV increases with age.

  • Patients in congestive heart failure may show an increased CV.

Volume of Isoflow (VISO V)

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.

Reference Values


Average is 10%-25% of VC.

Predicted values are based on age.

Body Plethysmography: Thoracic Gas Volume (VTG), 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 airway resistance (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). Airway resistance (Raw) increases with decreased lung volumes and decreases with higher lung volumes in a nonlinear, hyperbolic fashion. Compliance (CL) increases in obstructive diseases (e.g., emphysema) and decreases in restrictive processes (e.g., interstitial lung disease).

Reference Values


Thoracic gas volume (VTG): approximately 2.50-3.50 L

Compliance (CL): 0.2 L/cm H2O (2.04 L/kPa)

Airway resistance (Raw): 0.6-2.4 L/s/cm H2O

Airway conductance (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. Airway resistance (Raw) tests are also sensitive monitors of response to bronchoconstrictive agents.

Reference Values


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

Jun 11, 2016 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on Pulmonary Function, Arterial Blood Gases (ABGs), and Electrolyte Studies

Full access? Get Clinical Tree

Get Clinical Tree app for offline access