PFTs will detect significant increased resistance to airflow (airway obstruction) and increased resistance to expansion (parenchymal disease, weakness of respiratory muscles or abnormalities of the chest wall or diaphragm). ABGs supplement PFTs by measuring the effect of pulmonary and other illnesses on oxygenation and ventilation (Figure 110-1).
Lung Volumes. (Reproduced, with permission, from Weinberger SE. Principles of Pulmonary Medicine, 4th ed. Philadelphia: Saunders; 2004.)
TLC = Total lung capacity or the total volume of gas within the lungs after a maximal inspiration
RV = Residual volume or the volume of gas remaining in the lungs after a maximal expiration
VC = Vital capacity or the volume of gas expired after a maximal inspiration followed by a maximal expiration
FRC = Functional residual capacity or the volume of gas within the lungs at the end of expiration during normal tidal breathing at rest
To quantitate VC, ask the patient to breathe into a spirometer and obtain a spirometric tracing. To quantitate RV, FRC, and TLC, other methods such as dilution tests or body plethysmography are needed to measure the amount of air left in the lungs. These measurements require significant expertise on the part of the respiratory therapist in the PFT laboratory and maximal patient cooperation and ability to follow instructions. Inert gas dilution may underestimate lung volumes when there is airflow obstruction in patients who have air spaces such as bullae within the lung that do not communicate with the bronchial tree. Body plethysmography may overestimate lung volumes in airflow obstruction but may provide a more accurate measurement of intrathroacic gas volume in patients with noncommunicating airspaces within the lung.
Most diffuse lung disease is associated with decreased lung volumes. Restrictive PFTs means limitation to full expansion of the lungs. Volumes are decreased but flow rates are normal. Interstitial lung disease has reduced lung compliance and a restrictive defect. PFTs will reveal a decreased TLC, FRC, and RV. Although FEV1 and FVC may be decreased secondary to decreased volumes, the FEV1/FVC ratio is normal or increased. When the TLC and VC are decreased, the differential diagnosis includes restrictive lung disease (pulmonary fibrosis) or loss of lung volume (surgery, diaphragmatic paralysis, or skeletal problems). Decreases in the TLC, RV, and FRC can be interpreted as mild (60%-80% reduction), moderate (40%-60% reduction) or severe (<40% reduction). Marked decreases in the VC may also occur in certain neuromuscular diseases, and serial testing may be used to monitor disease progression in Guillain-Barre Syndrome, myasthenia gravis, and muscular dystrophy.
Measurement of flow rates requires that the patient breathe into a spirometer as hard and as fast as possible from TLC down to RV. The information can be displayed as a flow versus volume graph or a volume versus time graph. The volume expired during this test is the forced vital capacity or FVC. The amount expired during the first second is the forced expiratory volume in 1 second, or FEV1. This maneuver also reports the forced expiratory flow between 25% and 75% of VC (FEF 25%-75%), also referred to as the maximum midexpiratory flow rate (MMEFE or MMFR), which is the rate of airflow during the middle one-half of the expiration capacity. It is not effort or technique dependent (Figure 110-2).
Volume versus time. (Reproduced, with permission, from Fauci AS, Braunwald E, Kasper DL, et al. Harrison's Principles of Internal Medicine. 17th ed. New York: McGraw-Hill; 2008. Figure 246-2, p. 1586.)
Obstructive lung disease such as chronic obstructive pulmonary disease (COPD) is a spectrum of disorders that have in common impairment in expiratory flow. Diagnosis is made by spirometry showing a permanent reduction in FEV1/FVC ratio below 75%. The slow vital capacity or SVC will be greater than the FVC when the FVC is decreased in the setting of obstruction. Decreases in both FEV1 and FVC and a normal FEV1/FVC would suggest restrictive disease. Spirometry can be used to determine the degree of reversibility of the airways to bronchodilators and adequacy of treatment of obstructive airway disease. An increase in FEV1 of 200 cc or 12% is considered a significant response to bronchodilators and inadequately treated obstructive lung disease.
In addition to spirometry, flow rates may be measured using peak flow meters. Peak flow rates usually occur in the very early stages of the FVC maneuver and may be useful in measuring obstructive airway disease. Although a very simple test easily performed without too much training on the part of the operator or the patient, measuring the peak flow is very effort dependent and is not accurate enough to replace spirometry. The predicted values are rather nonspecific and the previously described prediction of severity of disease does not apply to these values. However, in the hospital it allows for a very simple, objective way of monitoring a patient's obstructive airway disease and response to therapy. Measuring the peak flow is a simple way for patients with asthma to objectively measure the degree of obstruction, starting in the hospital in preparation for discharge home when patients should monitor their peak flow and understand when they need to return for medical evaluation before they are so ill that they need to be rehospitalized. A peak expiratory flow calculator using age, sex, and height may be found at the link http://www.dynamicmt.com/PEFform.html.
The flow volume loop graphically records the maximal inspiratory and maximal expiratory maneuvers with flow on the Y-axis and volume on the X-axis. The patterns of the flow volume loops can be diagnostic of various types of obstruction including intra and extrathoracic obstruction, or fixed or variable obstruction. Although the beginning of a forced expiratory maneuver depends on effort, the latter part of forced expiration primarily reflects the mechanical properties of the lung and the resistance to airflow. The evaluation of the flow volume loop may be a qualitative visual analysis of the shape and concavity of the expiratory portion of the curve or a quantitative analysis comparing observed flow rates at specified volumes with predicted values.
In a fixed obstruction, changes in pleural pressure do not affect the degree of obstruction, and the inspiratory and expiratory portions of the curve reveal a plateau, reflecting a limitation in peak airflow of both inspiration and expiration.
In a variable obstruction, the location of the lesion and the effect of alterations in pleural and airway pressure with inspiration and expiration determine the amount of obstruction. If the lesion is intrathoracic, the flow volume loop will demonstrate airflow limitation during expiration. If the lesion is extrathoracic, the flow volume loop will demonstrate airflow limitation during inspiration (Figure 110-3).
Normal flow volume loop, spirometry, and DLCO.
In obstructive disease, the flow volume loop demonstrates a decrease in the expiratory loop, especially at the later part of the expiration (Figure 110-4).
Flow volume loop patterns.
MAXIMUM VOLUNTARY VENTILATION
This is the measurement of the maximum amount of air a patient can move in 12 to 15 seconds, using a spirometer. The results are expressed in liters/min. Because MVV tests airflow through major airways and muscle strength, consider weakness of respiratory muscles, especially the diaphragm, if MVV is low and flow rates are normal. Major airway lesions and neuromuscular disease result in a decreased MVV. Obstructive disease may also have a low MVV (MVV = FEV1 × 33). In theory, isolated restrictive disease should have a normal MVV; however, it is a nonspecific test and reductions are seen in pulmonary diseases (restrictive and airway obstruction) and in neuromuscular disease (loss of coordination, diminished cognitive function, and overall deconditioning).
Between 10% and 30% of patients with Guillian-Barre Syndrome will require ventilatory support. Patients most likely requiring mechanical ventilation present within 7 days of onset of symptoms, FVC <60% predicted, maximal inspiratory pressure (MIP) <30 cm H2O, or an expiratory pressure <40 cm H2O. These patients are at great risk of requiring mechanical ventilation and should be monitored carefully with serial FVC and inspiratory pressure measurements.
MAXIMUM INSPIRATORY AND EXPIRATORY PRESSURE
This is a good estimate of muscle strength, which would be presumed to be normal with a normal maximum inspiratory and expiratory pressure. Patients must be able to cooperate with the test.
DIFFUSING CAPACITY (DLCO)
Usually measured as DLCO, diffusing capacity is the measurement of the rate of gas transfer from the alveolus to the capillary measured in relation to the driving pressure of CO across the alveolar-capillary membrane. It is reported as ml of CO/min/mm Hg. The CO behaves like O2 and is usually measured in a single breath. The diffusing capacity is most dependent on the surface area available for gas exchange and the volume of blood or hemoglobin in the pulmonary capillaries available to bind to CO. The observed value of the diffusing capacity is corrected for the patient's hemoglobin level. The diffusing capacity should be normal in diseases that only affect the airway such as asthma and chronic bronchitis and not pulmonary parenchymal disease, such as interstitial lung disease or emphysema.
CASE 110-1 INTRATHORACIC OBSTRUCTION
This case demonstrates the PFT findings of a patient with a tracheal tumor. At the bedside the patient with a presumptive diagnosis of COPD exacerbation was found to have disproportionate upper airway wheezing.
The DLCO does not correlate with disease severity due to dependence on the Hgb level, pulmonary capillary volume, and thickness of the alveolocapillary membrane. Conditions that decrease the DLCO include:
Anemia, in which a decrease of 1 gm of Hg will decrease the DLCO by 7%.
Anatomical emphysema, pneumonectomy, pulmonary hypertension, and recurrent pulmonary emboli due to decreased pulmonary capillary volume.
Restrictive lung diseases such as interstitial lung disease due to increased thickness of the alveolar capillary membrane.
In the setting of obstructive disease, the DLCO helps distinguish between emphysema (decreased DLCO) and other causes of chronic airway obstruction such as asthma (normal DLCO). In the setting of restrictive disease, the diffusing capacity helps distinguish between intrinsic (or interstitial) lung diseases, in which DLCO is usually reduced, from other causes of restriction, in which DLCO is usually normal. Conditions that increase the DLCO include:
Alveolar hemorrhage, polycythemia, left to right shunt, and in some asthmatics due to increased pulmonary capillary volume.
The supine posture, after exercise, high cardiac output states.
The DLCO can be useful in the evaluation of diffuse pulmonary infiltrates when there is a large differential diagnosis.
HYPOXIA ALTITUDE SIMULATION TEST (HAST)
The purpose of the HAST is to simulate the oxygen tension while traveling by commercial airline flight. Aircraft pressure is maintained at an altitude of 8000 ft. The pO2 at this level is approximately 55 mm Hg. If the pO2 falls below 50 mm Hg, supplemental oxygen is supplied and the test repeated. Predictive of the risk of hypoxemia seen in flight, the HAST test identifies patients in need of supplemental oxygen when flying. Through a tight fitting mask the patient inspires air with 15.1% oxygen content. An ABG is obtained prior to the test and at 20 minutes. Continuous oxygen saturation and ECG monitoring is conducted. Symptoms are monitored.
INPATIENT PULMONARY COMPLIANCE STUDY
For intubated, mechanically ventilated patients, inpatient pulmonary compliance studies assess pulmonary mechanics by esophageal balloon. Esophageal balloon manometry provides an estimate of pleural pressure and lung compliance. Transpulmonary pressure can be recorded with PEEP and tidal volume titration. This information can be effectively used to improve oxygenation and respiratory system compliance in ventilated ARDS patients by identifying patients who derive benefit from higher levels of PEEP than would ordinarily be used.
PATTERNS OF PFT ABNORMALITIES
Patients with asthma, chronic bronchitis, or emphysema typically have an obstructive pattern: decreased FEV1 with normal or decreased FVC.
The FEV1/FVC is normally decreased in obstructive lung disease, but may be near normal when the FVC is at very low volumes. The FEF 25% to 75% is usually quite low. The flow volume loop will be abnormal when flow rates measured by spirometry are reduced.
In addition to asthma syndromes and COPD, wheezing may be caused by tumors, laryngeal or vocal cord dysfunction due to GERD, postnasal drip or psychological states, allergic reactions, CHF, vasculitis, aspiration, and other pulmonary causes such as pulmonary embolism, bronchiectasis, sarcoidosis, and interstitial lung disease. Decreased FEV1 and FEV1/FVC are the hallmarks of obstructive disease. Flow volume loops can help identify if wheezing is originating from an intrathoracic or extrathoracic source and they can help identify if the obstructing lesion is fixed or variable. Measurement of the DLCO can be helpful in differentiating obstruction due to chronic bronchitis, emphysema, and asthma. The DLCO is lower in emphysema and is usually not affected in other forms of airway obstruction that cause wheezing. The presence of reversibility of airway obstruction suggests asthma or COPD requiring intensification of bronchodilator therapy. Measurements of ABGs can be obtained to evaluate severity of illness and need for home O2 therapy (Figures 110-5-110-8).
No response to bronchodilators despite the presence of severe obstructive disease (FEV1, FEF25-75, and FEV1/FVC).
An intrathoracic pattern by flow volume curve (a flattening of the expiratory portion of the curve with a preserved inspiratory portion of the curve).
A typical pattern of severe obstruction with a scooped terminal portion of the expiratory loop.
A markedly decreased FEV1, FEF25-75, and FEV1/FVC with a significant response to bronchodilators consistent with severe COPD. The lung volumes suggest air trapping and the decreased DLCO the presence of emphysema.
CASE 110-2 SEVERE OBSTRUCTIVE LUNG DISEASE
This case demonstrates the PFT findings of a patient with progressive and persistent obstructive symptoms; he would likely benefit from more intense therapy and pulmonary rehabilitation.
Patients with severe skeletal deformities, diaphragmatic paralysis, s/p surgical repair, or pulmonary fibrosis typically have a restrictive pattern. A low TLC, a decrease in FEV1 and FVC with a normal FEV1/FVC, a normal or super-normal FEF 25% to 75%, and a low DLCO would suggest restrictive lung disease. The flow volume loop usually is normal in configuration but has low lung volumes (Figures 110-9 and 110-10).
The flow volume loop with a normal configuration but low lung volumes.
A decreased FEV1 and FVC, an increased FEV1/FVC and FEF25%-75%, and a decreased DLCO consistent with parenchymal disease.
The combined obstructive and restrictive pattern
Patients with cystic fibrosis, bronchiolitis obliterans, COPD and congestive heart failure, COPD and pneumonia, or COPD with obesity typically have a combined obstructive and restrictive pattern (Table 110-1).
TABLE 110-1Summary of Pulmonary Function Findings in Certain Diseases |Favorite Table|Download (.pdf) TABLE 110-1 Summary of Pulmonary Function Findings in Certain Diseases
|Pattern ||FVC ||FEV1 ||FEV1/FVC ||TLC ||RV ||DLCO |
|Normal ||Normal ||Normal ||Normal ||Normal ||Normal ||Normal |
|Obstructive ||Normal ↑or ↓ ||↓ ||Normal or ↓ ||Normal or ↑ ||Normal or ↑ ||Normal or ↓ |
|Restrictive ||↓ ||Normal or ↑ ||Normal or ↑ ||↓ ||Normal or ↓ ||↓ |
|Neuromuscular ||↓ ||↓ ||Normal or ↓ ||↓ ||Normal or ↑ ||Normal |