Symposium on Therapeutic Problems
The Clinical Roles of Pulmonary Function Testing Joseph L. Andrews, Jr., M.D.'"
In addition to its well-accepted role of helping to diagnose the nature and extent of respiratory defects, pulmonary function testing is becoming increasingly useful in helping to select the most specific effective therapy for different respiratory conditions. Sequential pulmonary function tests are important in judging the effectiveness of pulmonary therapy. Other therapeutic roles of pulmonary function tests include assessing patients preoperatively, assessing disability, and planning specific types of therapy as a part of total rehabilitation programs, monitoring and treating patients in respiratory failure, and helping to educate patients and physicians about the nature of the lung problems and what can be done about them. Although specific diagnoses are usually established for patients with various cardiac or renal problems, many physicians still group all patients with pulmonary problems into the "chronic lunger" category. However, with the help of lung function tests a more specific diagnosis can and should be established (for example, asthma, bronchitis, emphysema, or asbestosis) to determine the patient's clinical course and prognosis and to select the most effective therapeutic program. After the diagnosis has been made, sequential pulmonary function tests will assess the degree of improvement or deterioration after therapy, so that the type and dose of drug(s) can be modified if necessary. The goals of this article are to explain the indications for and practical uses of pulmonary function tests, to give an overview of the most widely used contemporary techniques, to provide clinical guidelines for interpretation of the tests, and to untangle the ingredients of the indigestible alphabet soup of terms and parameters of pulmonary function testing.
NORMAL LUNG FUNCTION The main function of the respiratory system is to supply oxygen to and eliminate carbon dioxide from the tissues to maintain cell met abo"Section of Puhlionary Medicine, Lahey Clinic Foundation and New England Deaconess Hospital; Harvard Medical School.
Medical Clinics of North America - Vol. 63, No. 2, March 1979
355
356
JOSEPH
L. ANDREWS, JR.
lism. Normal pulmonary function can be divided into four linked phases: ventilation, distribution, diffusion, and perfusion4 • 9, 13. 16. 41 (Figs. 1 and 2) Ventilation involves transport of adequate volumes of air by inhalation from the atmosphere to the respiratory tract where it is distributed via the airways to the alveoli. After alveolar gas exchange has occurred, waste air, depleted of oxygen and rich in carbon dioxide, is exhaled. Ventilation can be controlled voluntarily or it can occur automatically through a feedback system that involves afferent stretch receptors in the alveoli; sensors for pH, Pco 2 , and P0 2 in the carotid and aortic bodies and medulla; a central ventilatory control integrator in the medulla; and efferent nerves innervating inspiratory muscles. The phrenic nerve controls contraction of the diaphragm, and intercostal nerves control intercostal muscles. During inspiration they both actively expand the volume of the chest bellows creating a negative intrathoracic pressure that causes air to enter the lungs. (Expiration occurs passively owing to the elastic recoil of the lung and chest wall.) Thus, normal ventilation is dependent on intact neurologic afferent,
Figure 1. The normalrespira, tory system. On the left, the normal airway delivery system from nasopharynx to bronchus to alveolus, as well as the mechanical respiratory structure of chest cage, intercostal muscles, pleura, and diaphragm. On the right, the control of ventilation is shown: the carotid and aortic bodies which sense hypoxemiaand signal the medulla, which controls the diaphragm and chest wall respiratory movements. Inset shows the structure of the respiratory bronchiole and alveolus.
357
THE CLINICAL ROLES OF PULMONARY FUNCTION TESTING
Figure 2. Factors in normal O 2 transport from inspired air to blood and CO 2 elimination from blood to air are ventilation of the lungs during inspiration and expiration, distribution of the air to and from small airways and alveoli, diffusion of O2 from alveolus to capillary and of CO 2 from capillary to alveolus; and perfusion ofthe alveolar capillary bed by an adequate blood supply. The straight line "tube" below depicts normal "shunted" venous blood (2 to 6 per cent of cardiac output) that receives no oxygen, since it is not exposed to inspired air. Tidal volume (V T ) is comprised of dead space (VD) and alveolar volume (V A)' Illustrative normal values are shown.
VENTI LATI ON DISTRIBUTION
150c] VT 500 cc VA
DIFFUSION VT=VA+VO VE =VA+VO=
VTx Freq
VA =VE-VO
Pulmonary arteriole
Pulmonary venule
P02=40mmHg PC02=46mm
P02=90mm Hg PC02=40mm
Ifs
integrative, and efferent mechanisms, normal musculoskeletal chest bellows structure, and normal lung parenchymal tissue. Distribution of air occurs from nasopharynx and trachea to the alveoli via large airways (bronchi) and via smaller peripheral airways (bronchioles less than 2 mm in diameter). Approximately 27 divisions of the airways are found between the trachea and the terminal bronchioles. The large bronchi have rings of cartilage in their walls and thus are fairly rigid. However, the diameter of smaller bronchioles, which are embedded within the lung tissue and lack cartilage, narrows on expiration as the surrounding intrathoracic pressure increases. The diameter of the bronchioles, and thus airflow through them, is also controlled by the tone of the bronchial smooth muscle that encircles them. In disease of the lung, large airways can be obstructed by tumors or secretions (as in bronchitis) and small airways by muscular bronchospasm, mucus plugs, and mucosal edema (as in asthma). Airways can also be obstructed by dynamic collapse resulting from loss of elastic recoil of the surrounding lung tissue (as in emphysema). Diffusion of oxygen from the inspired air to the arterial capillary and of carbon dioxide from the metabolizing cell to the venous capillary to the alveoli occurs along gas pressure gradients across the alveolar-capillary membrane, which is one cell thick. Diffusion is dependent on many factors, the most important of which are gas pressure gradients, thickness of the alveolar-capillary membrane, and affinity of the red cell hemoglobin and plasma for the diffusing gas. Perfusion of the lungs by the pulmonary arteries involves a vast capillary network surrounding each alveolus, which provides a huge
358
JOSEPH
L. ANDREWS, JR.
alveolar-capillary diffusion surface. Normal perfusion requires adequate cardiac output (about 5 liters a minute), normal hemoglobin, sufficient patent pulmonary arteries, and capillaries, all of which should produce a normal overall balance of ventilation to perfusion (V/Q balance) to produce normal arterial oxygenation. CLINICA.L INDICATIONS FOR PULMONARY FUNCTION TESTS Diagnosis of Dyspnea The key role of pulmonary function tests both in clarifying the many different pathophysiologic mechanisms that cause dyspnea and in selecting the most specific and effective therapy for each cbndition is shown in Table 1 and is described in detail later. 7.9. 13.21.23.29 Selection of Specific Therapy and Evaluation of the Effectiveness of Therapy Repeat pulmonary function tests after inhalation of bronchodilator aerosol medication show whether bronchospasm is present and whether bronchodilator therapy will be effective. Tests taken during treatment indicate the effectiveness of therapy (for example, of steroids in sarcoidosis). Serial pulmonary function tests with arterial blood gases will determine the need for oxygen therapy for patients with hypoxemia resulting from bronchitis or interstitial lung disease. Arterial blood gas measurements at varying oxygen flow rates at rest and during exercise confirm which flow rates will provide optimal therapy. 4. 6 Assessment of Prognosis The degree of severity of a patient's obstructive or restrictive pul-· monary defect can provide an objective guide to the clinician in helping to judge that patient's prognosis. For example, the degree of airway obstruction, hypoxemia, and hypercarbia in patients with obstructive pulmonary disease determine to some extent how active they will be, their long-term prognosis, and whether cor pulmonale will develop.31, 33 The extent of pulmonary restriction in patients with interstitial lung disease will help determine how active they can be. 6 Preoperative Assessment The postoperative pulmonary complication rate is up to 20 times higher for patients whose preoperative values (especially air flow rates) are poor. These poor-risk patients require intensive preoperative and postoperative respiratory therapy to prevent complications. Results of pulmonary function tests may help to indicate the best specific form of anesthesia and respiratory therapy. 16, 37, 38 For example, local
THE CLINICAL ROLES OF PULMONARY FUNCTION TESTING
359
anesthesia may have to be used when very poor lung function exists; postoperative bronchodilator aerosol therapy will be most effective for patients with reversible bronchospasm. Pulmonary function tests are essential for patients having lobectomy or pneumonectomy to determine whether they can tolerate further loss of lung function. The cost of pulmonary function tests may be covered by third party insurance as part of the hospitalization even though the tests are performed on an outpatient basis four days before the patient is admitted to the hospital.
Disability Assessment Since values may be normal at rest, exercise pulmonary function tests are often required to determine the degree of disability. Exercise testing may reveal the level of effort required and the cardiopulmonary mechanism producing dyspnea. 17, 20 Rehabilitation Graded exercise (sometimes with supplemental oxygen) is often effective in improving the endurance and general physical condition of patients with chronic bronchitis and emphysema. 2 , 20, 31 Periodic assessment of cardiac and pulmonary function will help to determine appropriate levels of exercise for rehabilitation and to assess improvement. Determination of maximal oxygen uptake is the best method for assessing the degree of physical fitness ("training effect") and is becoming useful as a test for weekend joggers, marathon runners, and patients who have had a myocardial infarction and want to participate in exercise training programs. 20, 28 Diagnosis of Respiratory Failure, Patient Monitoring, and Treatment Objective physiologic parameters, including arterial blood gas analysis for detecting respiratory failure and for monitoring medical and postoperative patients, are becoming increasingly important because early hypoxemia,hypercarbia, and acidosis are often difficult to diagnose clinically. 10, 32 By monitoring physiologic cardiopulmonary parameters, the clinician can best decide what type of therapy should be instituted and whether a mechanical ventilator should be used, its settings, and when its use can be discontinued. New Knowledge for Patient and Physician Unsuspected pulmonary diseases, such as emphysema and pulmonary fibrosis, and epidemiologic clues to help prevent exposure from industrial and environmental toxins can be detected by periodic screening pulmonary function tests in asymptomatic patients. Also, smokers may be convinced to stop smoking if they can be shown to have early lung damage. More basic knowledge about pulmonary disorders and long-term effects of specific therapy must be gained if further progress is to be made.
Parenchymal
Decreased total lung capacity 0 TLC); increased respiratory rate and minute ventilation (t \rE) (compensatory for decreased tidal volume); decreased compliance
Restriction of lung volumes
Extraparenchymal
Bronchospasm (allergic, irritative) Secretions (infection, irritants) Decreased elastic recoil and air trapping Endobronchial infiltration Upper airway obstruction
Decreased air flow rates: decreased forced expiratory volume in one second (FEV,); decreased peak flow rate (PFR); decreased maximum midexpiratory flow rate (MMFR); decreased maximum voluntary ventilation (MVV); (?reversible after bronchodilators); increased tidal volume (VT ); increased residual volume (RV)
MECHANISMS
TESTS
DEFECT
Airway obstruction
PA THOPHYSIOLOGIC
PULMONARY FUNCTION
Antibiotics Thoracentesis Reduce weight Remdve ascitic fluid Hyperventilate postoperatively
Pneumonia Pleural effusion Kyphoscoliosis Obesity Ascites Thoracic and abdominal surgery
Fibrosis Pneumoconiosis Pulmonary edema
Excise obstructing lesion
Bronchial adenoma
Oxygen Eliminate dust; steroids Digitalis; diuretics
Steroids
Antibiotics; physical therapy; expectorants; stop smoking Improved ventilation (breathing training)
Bronchodilators
TREATMENT
Sarcoidosis
Emphysema
Bronchitis
Asthma
EXAMPLES
DIAGNOSTIC
Dyspnea: Diagnosis and Treatment
FUNCTIONAL
Table 1.
?"
'-
-l
t-l
(")
Z
c::
>< "'I
~
;.-
Z
0
a::
t"'
c::
"t:I
~
'"0
~
0 t"' t
The Clinical Roles of Pulmonary Function Testing Joseph L. Andrews, Jr., M.D.'"
In addition to its well-accepted role of helping to diagnose the nature and extent of respiratory defects, pulmonary function testing is becoming increasingly useful in helping to select the most specific effective therapy for different respiratory conditions. Sequential pulmonary function tests are important in judging the effectiveness of pulmonary therapy. Other therapeutic roles of pulmonary function tests include assessing patients preoperatively, assessing disability, and planning specific types of therapy as a part of total rehabilitation programs, monitoring and treating patients in respiratory failure, and helping to educate patients and physicians about the nature of the lung problems and what can be done about them. Although specific diagnoses are usually established for patients with various cardiac or renal problems, many physicians still group all patients with pulmonary problems into the "chronic lunger" category. However, with the help of lung function tests a more specific diagnosis can and should be established (for example, asthma, bronchitis, emphysema, or asbestosis) to determine the patient's clinical course and prognosis and to select the most effective therapeutic program. After the diagnosis has been made, sequential pulmonary function tests will assess the degree of improvement or deterioration after therapy, so that the type and dose of drug(s) can be modified if necessary. The goals of this article are to explain the indications for and practical uses of pulmonary function tests, to give an overview of the most widely used contemporary techniques, to provide clinical guidelines for interpretation of the tests, and to untangle the ingredients of the indigestible alphabet soup of terms and parameters of pulmonary function testing.
NORMAL LUNG FUNCTION The main function of the respiratory system is to supply oxygen to and eliminate carbon dioxide from the tissues to maintain cell met abo"Section of Puhlionary Medicine, Lahey Clinic Foundation and New England Deaconess Hospital; Harvard Medical School.
Medical Clinics of North America - Vol. 63, No. 2, March 1979
355
356
JOSEPH
L. ANDREWS, JR.
lism. Normal pulmonary function can be divided into four linked phases: ventilation, distribution, diffusion, and perfusion4 • 9, 13. 16. 41 (Figs. 1 and 2) Ventilation involves transport of adequate volumes of air by inhalation from the atmosphere to the respiratory tract where it is distributed via the airways to the alveoli. After alveolar gas exchange has occurred, waste air, depleted of oxygen and rich in carbon dioxide, is exhaled. Ventilation can be controlled voluntarily or it can occur automatically through a feedback system that involves afferent stretch receptors in the alveoli; sensors for pH, Pco 2 , and P0 2 in the carotid and aortic bodies and medulla; a central ventilatory control integrator in the medulla; and efferent nerves innervating inspiratory muscles. The phrenic nerve controls contraction of the diaphragm, and intercostal nerves control intercostal muscles. During inspiration they both actively expand the volume of the chest bellows creating a negative intrathoracic pressure that causes air to enter the lungs. (Expiration occurs passively owing to the elastic recoil of the lung and chest wall.) Thus, normal ventilation is dependent on intact neurologic afferent,
Figure 1. The normalrespira, tory system. On the left, the normal airway delivery system from nasopharynx to bronchus to alveolus, as well as the mechanical respiratory structure of chest cage, intercostal muscles, pleura, and diaphragm. On the right, the control of ventilation is shown: the carotid and aortic bodies which sense hypoxemiaand signal the medulla, which controls the diaphragm and chest wall respiratory movements. Inset shows the structure of the respiratory bronchiole and alveolus.
357
THE CLINICAL ROLES OF PULMONARY FUNCTION TESTING
Figure 2. Factors in normal O 2 transport from inspired air to blood and CO 2 elimination from blood to air are ventilation of the lungs during inspiration and expiration, distribution of the air to and from small airways and alveoli, diffusion of O2 from alveolus to capillary and of CO 2 from capillary to alveolus; and perfusion ofthe alveolar capillary bed by an adequate blood supply. The straight line "tube" below depicts normal "shunted" venous blood (2 to 6 per cent of cardiac output) that receives no oxygen, since it is not exposed to inspired air. Tidal volume (V T ) is comprised of dead space (VD) and alveolar volume (V A)' Illustrative normal values are shown.
VENTI LATI ON DISTRIBUTION
150c] VT 500 cc VA
DIFFUSION VT=VA+VO VE =VA+VO=
VTx Freq
VA =VE-VO
Pulmonary arteriole
Pulmonary venule
P02=40mmHg PC02=46mm
P02=90mm Hg PC02=40mm
Ifs
integrative, and efferent mechanisms, normal musculoskeletal chest bellows structure, and normal lung parenchymal tissue. Distribution of air occurs from nasopharynx and trachea to the alveoli via large airways (bronchi) and via smaller peripheral airways (bronchioles less than 2 mm in diameter). Approximately 27 divisions of the airways are found between the trachea and the terminal bronchioles. The large bronchi have rings of cartilage in their walls and thus are fairly rigid. However, the diameter of smaller bronchioles, which are embedded within the lung tissue and lack cartilage, narrows on expiration as the surrounding intrathoracic pressure increases. The diameter of the bronchioles, and thus airflow through them, is also controlled by the tone of the bronchial smooth muscle that encircles them. In disease of the lung, large airways can be obstructed by tumors or secretions (as in bronchitis) and small airways by muscular bronchospasm, mucus plugs, and mucosal edema (as in asthma). Airways can also be obstructed by dynamic collapse resulting from loss of elastic recoil of the surrounding lung tissue (as in emphysema). Diffusion of oxygen from the inspired air to the arterial capillary and of carbon dioxide from the metabolizing cell to the venous capillary to the alveoli occurs along gas pressure gradients across the alveolar-capillary membrane, which is one cell thick. Diffusion is dependent on many factors, the most important of which are gas pressure gradients, thickness of the alveolar-capillary membrane, and affinity of the red cell hemoglobin and plasma for the diffusing gas. Perfusion of the lungs by the pulmonary arteries involves a vast capillary network surrounding each alveolus, which provides a huge
358
JOSEPH
L. ANDREWS, JR.
alveolar-capillary diffusion surface. Normal perfusion requires adequate cardiac output (about 5 liters a minute), normal hemoglobin, sufficient patent pulmonary arteries, and capillaries, all of which should produce a normal overall balance of ventilation to perfusion (V/Q balance) to produce normal arterial oxygenation. CLINICA.L INDICATIONS FOR PULMONARY FUNCTION TESTS Diagnosis of Dyspnea The key role of pulmonary function tests both in clarifying the many different pathophysiologic mechanisms that cause dyspnea and in selecting the most specific and effective therapy for each cbndition is shown in Table 1 and is described in detail later. 7.9. 13.21.23.29 Selection of Specific Therapy and Evaluation of the Effectiveness of Therapy Repeat pulmonary function tests after inhalation of bronchodilator aerosol medication show whether bronchospasm is present and whether bronchodilator therapy will be effective. Tests taken during treatment indicate the effectiveness of therapy (for example, of steroids in sarcoidosis). Serial pulmonary function tests with arterial blood gases will determine the need for oxygen therapy for patients with hypoxemia resulting from bronchitis or interstitial lung disease. Arterial blood gas measurements at varying oxygen flow rates at rest and during exercise confirm which flow rates will provide optimal therapy. 4. 6 Assessment of Prognosis The degree of severity of a patient's obstructive or restrictive pul-· monary defect can provide an objective guide to the clinician in helping to judge that patient's prognosis. For example, the degree of airway obstruction, hypoxemia, and hypercarbia in patients with obstructive pulmonary disease determine to some extent how active they will be, their long-term prognosis, and whether cor pulmonale will develop.31, 33 The extent of pulmonary restriction in patients with interstitial lung disease will help determine how active they can be. 6 Preoperative Assessment The postoperative pulmonary complication rate is up to 20 times higher for patients whose preoperative values (especially air flow rates) are poor. These poor-risk patients require intensive preoperative and postoperative respiratory therapy to prevent complications. Results of pulmonary function tests may help to indicate the best specific form of anesthesia and respiratory therapy. 16, 37, 38 For example, local
THE CLINICAL ROLES OF PULMONARY FUNCTION TESTING
359
anesthesia may have to be used when very poor lung function exists; postoperative bronchodilator aerosol therapy will be most effective for patients with reversible bronchospasm. Pulmonary function tests are essential for patients having lobectomy or pneumonectomy to determine whether they can tolerate further loss of lung function. The cost of pulmonary function tests may be covered by third party insurance as part of the hospitalization even though the tests are performed on an outpatient basis four days before the patient is admitted to the hospital.
Disability Assessment Since values may be normal at rest, exercise pulmonary function tests are often required to determine the degree of disability. Exercise testing may reveal the level of effort required and the cardiopulmonary mechanism producing dyspnea. 17, 20 Rehabilitation Graded exercise (sometimes with supplemental oxygen) is often effective in improving the endurance and general physical condition of patients with chronic bronchitis and emphysema. 2 , 20, 31 Periodic assessment of cardiac and pulmonary function will help to determine appropriate levels of exercise for rehabilitation and to assess improvement. Determination of maximal oxygen uptake is the best method for assessing the degree of physical fitness ("training effect") and is becoming useful as a test for weekend joggers, marathon runners, and patients who have had a myocardial infarction and want to participate in exercise training programs. 20, 28 Diagnosis of Respiratory Failure, Patient Monitoring, and Treatment Objective physiologic parameters, including arterial blood gas analysis for detecting respiratory failure and for monitoring medical and postoperative patients, are becoming increasingly important because early hypoxemia,hypercarbia, and acidosis are often difficult to diagnose clinically. 10, 32 By monitoring physiologic cardiopulmonary parameters, the clinician can best decide what type of therapy should be instituted and whether a mechanical ventilator should be used, its settings, and when its use can be discontinued. New Knowledge for Patient and Physician Unsuspected pulmonary diseases, such as emphysema and pulmonary fibrosis, and epidemiologic clues to help prevent exposure from industrial and environmental toxins can be detected by periodic screening pulmonary function tests in asymptomatic patients. Also, smokers may be convinced to stop smoking if they can be shown to have early lung damage. More basic knowledge about pulmonary disorders and long-term effects of specific therapy must be gained if further progress is to be made.
Parenchymal
Decreased total lung capacity 0 TLC); increased respiratory rate and minute ventilation (t \rE) (compensatory for decreased tidal volume); decreased compliance
Restriction of lung volumes
Extraparenchymal
Bronchospasm (allergic, irritative) Secretions (infection, irritants) Decreased elastic recoil and air trapping Endobronchial infiltration Upper airway obstruction
Decreased air flow rates: decreased forced expiratory volume in one second (FEV,); decreased peak flow rate (PFR); decreased maximum midexpiratory flow rate (MMFR); decreased maximum voluntary ventilation (MVV); (?reversible after bronchodilators); increased tidal volume (VT ); increased residual volume (RV)
MECHANISMS
TESTS
DEFECT
Airway obstruction
PA THOPHYSIOLOGIC
PULMONARY FUNCTION
Antibiotics Thoracentesis Reduce weight Remdve ascitic fluid Hyperventilate postoperatively
Pneumonia Pleural effusion Kyphoscoliosis Obesity Ascites Thoracic and abdominal surgery
Fibrosis Pneumoconiosis Pulmonary edema
Excise obstructing lesion
Bronchial adenoma
Oxygen Eliminate dust; steroids Digitalis; diuretics
Steroids
Antibiotics; physical therapy; expectorants; stop smoking Improved ventilation (breathing training)
Bronchodilators
TREATMENT
Sarcoidosis
Emphysema
Bronchitis
Asthma
EXAMPLES
DIAGNOSTIC
Dyspnea: Diagnosis and Treatment
FUNCTIONAL
Table 1.
?"
'-
-l
t-l
(")
Z
c::
>< "'I
~
;.-
Z
0
a::
t"'
c::
"t:I
~
'"0
~
0 t"' t
