Page 1

Over the past 30 years, pulmonary function testing has come into widespread use. This has been facilitated by several developments:

Miniaturization and advances in computer technology, microprocessor devices have become portable and automated with fewer moving parts.

Testing equipment, patient maneuvers, and testing technique have become widely standardized throughout the world through the efforts of professional societies.

Widely accepted normative parameters have been established. 

DEFINITION

Pulmonary function testing is a valuable tool for the evaluation of the respiratory system, representing an important adjunct to the patient history, various lung imaging studies, and invasive testing such as bronchoscopy and open-lung biopsy. The overall approach is to compare the measured values for an individual patient at any particular point in time with normative values derived from population studies. Therefore, the percent predicted normal is used to define normal and abnormal as well as to grade the severity of the abnormality. Practicing clinicians must become familiar with pulmonary function testing because it is frequently used in clinical medicine for evaluation of respiratory symptoms such as dyspnea and cough, for preoperative risk stratification, and for diagnostic purposes for common diseases such as asthma and chronic obstructive pulmonary disease.

Pulmonary function tests (PFTs) is a generic term used to indicate a battery of studies or maneuvers that may be performed using standardized equipment to measure lung function. PFTs may include simple screening spirometry, formal lung volume measurement, diffusing capacity for carbon monoxide, and arterial blood gases. These studies may collectively be referred to as a complete pulmonary function survey.

Before a spirogram can be meaningfully interpreted, one needs to inspect the graphic data (i.e., volume-time curve and the flow-volume loop) to ascertain whether the study meets certain well-defined acceptability and reproducibility standards. The interpretative strategy usually involves establishing a pattern of abnormality (obstructive, restrictive, or mixed), grading the severity of the abnormality, and assessing trends over time. A variety of algorithms are available. Automated spirometry systems usually have built-in software that can generate a preliminary interpretation, especially for spirometry; however, algorithms for other pulmonary function studies are not as well established and necessitate appropriate clinical correlation and physician oversight.

 

PHYSIOLOGY

Basic concepts of normal pulmonary physiology that are involved in pulmonary function testing include mechanics (airflows and lung volumes), the ventilation-perfusion interrelationship, diffusion/gas exchange, and respiratory muscle or bellows strength. Ventilation is the process of generating the forces necessary to move the appropriate volumes of air from the atmosphere to the alveoli to meet the metabolic needs of the body under a variety of conditions. Simply, the thoracic muscles generate negative pressure in the chest and pleural space, favoring flow of air into the airways and lungs (inspiration). When the pressures equilibrate, the muscles relax and contract, increasing intrathoracic pressure and forcing air out of the lungs (expiration).

With exhalation, the early portion of the maneuver is characterized by high flows, mostly from large airways, and the latter portion is characterized by low flows with a larger contribution from the smaller airways. Inspiration is generally not flow limited and is a function of overall muscular effort. In contrast, a variety of factors affect expiratory flow, including the overall driving pressure (which is the pressure head at the alveolus, or PALV: the arithmetic sum of pleural pressure, or PPL; plus pressure from lung elastic recoil, or PELAST), airway diameter, overall dispensability of lung and chest wall, dynamic airway collapse (i.e., from a flow-limiting segment), and muscular effort. So:

PALV = PPL + PELAST

The mechanism for the maximal expiratory airflow limitation seen in normal airways is due to two factors: the gradual pressure drop along the airways as well as dynamic airway compression, with intrathoracic airways downstream becoming narrowed distally to a choke point or equal pressure point. At maximal airflow or during the early part of the expiratory maneuver, the main driving pressure is the intrinsic elastic recoil of the lungs.

BATTERY OF MANEUVERS

Pulmonary function studies use a variety of maneuvers to measure and record the properties of four lung components. These include the airways (both large and small), lung parenchyma (alveoli, interstitium), pulmonary vasculature, and the bellows-pump mechanism. Various diseases can affect each of these components.

Spirometry:

Spirometry is the most commonly used lung function screening study. It generally should be the clinician's first option, with other studies being reserved for specific indications. Most patients can easily perform spirometry when coached by an appropriately trained technician or other health care provider. The test can be administered in the ambulatory setting, physician's office, emergency department, or inpatient setting. The indications for spirometry are diverse. It can be used for diagnosing and monitoring respiratory symptoms and disease, for preoperative risk stratification, and as a tool in epidemiologic and other research studies.

Spirometry requires a voluntary maneuver in which a seated patient inhales maximally from tidal respiration to total lung capacity (TLC) and then rapidly exhales to the fullest extent until no further volume is exhaled at residual volume (RV) . The maneuver may be performed in a forceful manner to generate a forced vital capacity (FVC) or in a more relaxed manner to generate a slow vital capacity (SVC). In normal individuals, the inspiratory vital capacity, the expiratory SVC, and expiratory FVC are essentially equal. However, in patients with obstructive airways disease, the expiratory SVC is generally higher than the FVC.

A spirometer, including the waterless, rolling seal type, and Stead-Wells water seal type is an instrument that directly measures the volume of air displaced or measures airflow by a flow sensing device, such as a pneumotachometer or a tube containing a fixed resistance to flow (Table 2). Today, most clinical pulmonary function testing laboratories use a microprocessor-driven pneumotachometer to measure air flow directly and then to mathematically derive volume.

A spirogram is a graphic representation of bulk air movement depicted as a volume-time tracing or as a flow-volume tracing. Values generated from a simple spirogram provide important graphic and numeric data regarding the mechanical properties of the lungs, including airflow (forced expiratory volume in 1 second, or FEV1, along with other timed volumes) and exhaled lung volume (FVC or SVC). The measurement is typically expressed in liters for volumes or in liters per second for flows and is corrected for body temperature and pressure of gas that is saturated with water vapor. Data from a spirogram provides important clues to help distinguish obstructive pulmonary disorders that typically reduce airflow, such as asthma and emphysema, from restrictive disorders that typically reduce total lung volumes, including pulmonary fibrosis and neuromuscular disease.

A number of spirometry standards have been developed over the years. The American Thoracic Society standardization guidelines for acceptability and reproducibility criteria are shown in. A well-trained pulmonary function technician usually coaches the patient through the procedure so that the measurement represents the best possible measure of lung function.

 

 

 

Forced Expiratory Volume In 1 Second

The FEV1 is the most widely used parameter to measure the mechanical properties of the lungs. FEV1 accounts for the greatest part of the exhaled volume from a spirometric maneuver and reflects mechanical properties of both the large airways and medium-sized airways. In a normal flow-volume loop, the FEV1 occurs at about 75% of the FVC. This parameter is reduced in both obstructive and restrictive disorders. In obstructive diseases, FEV1 is reduced disproportionately to the FVC and is an indicator of flow limitation. In restrictive disorders, the FEV1, FVC, and total lung volume are all reduced, and in this setting FEV1 is a measure of volume rather than flow.

Forced Vital Capacity

FVC is a measure of lung volume and is usually reduced in diseases that cause the lungs to be "smaller." Such processes are generally termed restrictive and may include disorders of the lung parenchyma, such as pulmonary fibrosis, or of the bellows, including kyphoscoliosis, neuromuscular disease, and pleural effusion. However, a reduction in FVC is not always due to reduced total volumes and may occur in the setting of "large lungs" hyperinflated due to severe airflow obstruction and air trapping, as in emphysema. In this setting, the FVC is decreased due to reduced airflow, air trapping, and increased residual volume, a phenomenon referred to as pseudorestriction. Reduced FVC may occur despite a normal or increased total lung volume. Therefore, FVC is not a reliable indicator of TLC or restriction, especially in the setting of airflow obstruction. The overall accuracy of the FVC for restriction is about 60%.

Volume-time Tracing and Flow-volume Loop

The volume-time tracing and flow-volume loop ascertain the technical adequacy of a maneuver and therefore the quality of the data as well as identify the anatomic location of airflow obstruction. The volume-time tracing is most useful in assessing whether the end-of-test criteria have been met, whereas the flow-volume loop is most valuable in evaluating the start-of-test criteria. The zero time point on the volume-time tracing has been carefully defined and extrapolated to provide a uniform start point for measurement. It corrects for a possible delayed start that may not actually reflect airflow.

The shape of the flow-volume loop may indicate the location of airflow limitation, such as the large upper airways or smaller distal airways. With common obstructive airflow disorders, such as asthma or emphysema, the disease generally affects the expiratory limb and may reduce the effort-dependent peak expiratory flow as well as subsequent airflows that are effort-independent. The descending limb of the expiratory loop is typically concave. In contrast, several unusual anatomic disorders that narrow the large airways may produce a variety of patterns of truncation or flattening of either one limb of the loop (variable upper airway obstruction) or both limbs of the loop (fixed upper airway obstruction).

Additional Tests

A variety of parameters selectively reflect small airways. These include measures of flow from a spirogram, such as the maximal midexpiratory flow (MMEF) or forced expiratory flow at 25% to 75% vital capacity (FEF25-75). The FEF25-75 is the slope of the spirogram between the 25th and the 75th percentile of an FVC maneuver. The closing volume from a single-breath N2 test and frequency-dependent dynamic lung compliance also can be used to detect small airway disease. It is believed that small airways dysfunction may precede and exist separately in the setting of a normal FEV1 and FVC. The hypothesis is that smokers may have isolated small airways dysfunction and that there is an obligatory passage through a "silent period" during which only sensitive tests are impaired. However, there is a greater coefficient of variation for these tests of small airways function. In addition, because these measures are vitally influenced by lung volumes, they cannot be interpreted separately without volume correction. Therefore, in practice, these tests have not been particularly helpful to practicing clinicians, and the American Thoracic Society does not recommend their use for detecting small airways disease.6

Bronchoprovocation:

To define whether nonspecific airway hyperreactivity is a mechanism for atypical chest symptoms of unclear etiology, inhalational challenge tests are often used in the pulmonary function laboratory.7 Methacholine and histamine are the agents most often used with this procedure, although other agents may also be useful. Methacholine is considered safe, can be used in outpatient clinics, and has no systemic side effects.

When the baseline spirogram is relatively normal, inhalational challenge may be performed by aerosolizing progressive concentrations of methacholine by a dosimeter. This is typically performed as a five-stage procedure with five different concentrations. After each stage, the patient performs a spirometry. When there is a 20% reduction in the FEV1, the test is terminated and is considered positive for airway hyperreactivity. The provocative dosage level of the inhalational agent required to produce a 20% reduction in the FEV1 is labeled as PD20. If the drop in FEV1 is less than 20% after five stages of this procedure, the challenge test is considered negative for the presence of airway hyperreactivity.

Bronchial hyperreactivity, as assessed by this inhalational challenge procedure, is very sensitive for the presence of active or current asthma. A positive test is strongly suggestive of bronchial asthma. However, this test may be falsely positive by a variety of conditions, including chronic obstructive pulmonary disease, parenchymal respiratory disorders, congestive heart failure, recent upper respiratory tract infection, and allergic rhinitis. For practical purposes, a negative inhalational challenge with methacholine or histamine excludes active symptomatic asthma as a cause for the patient's chest symptoms.

Lung Volumes:

Because spirometry is an expiratory maneuver, it measures exhaled volume or vital capacity but does not measure residual volume, functional residual capacity (or the resting lung volume), or TLC. Vital capacity is a simple measure of lung volume that is usually reduced in restrictive disorders; however, vital capacity is only an indirect measure of other lung volumes. Because residual volume is not exhaled through the mouth, it is not measured by a spirometer.

Other pulmonary function methodology is required to formally measure TLC, which is derived from the addition of FRC to inspiratory capacity obtained from spirometry. FRC is usually measured by a gas dilution technique or body plethysmography. Gas dilution techniques are based on a simple principle, are widely used, and provide a good measurement of all air in the lungs that communicates with the airways. A limitation of this technique is that it does not measure air in "noncommunicating" bullae and, therefore, may underestimate TLC, especially in patients with severe emphysema.

Gas dilution techniques use either closed-circuit helium dilution or open-circuit nitrogen washout. They are based on the inhalation of a known concentration and volume of an inert tracer gas, such as helium, followed by equilibration of 7 to 10 minutes in the closed-circuit helium dilution technique. The final exhaled helium concentration is diluted in proportion to the unknown volume of air in the patient's chest (RV). Usually the patient is connected at the end-tidal position of the spirometer; therefore, the lung volume measured is FRC. In the nitrogen-washout technique, the patient breathes 100% oxygen and all the nitrogen in the lung is "washed out." The exhaled volume and the nitrogen concentration in that volume are measured. The difference in nitrogen volume at the initial concentration and at the final exhaled concentration allows a calculation of intrathoracic volume, usually FRC.

Body plethysmography is an alterative method of measuring lung volume that takes advantage of the principle of Boyle's law, which states that the volume of gas at a constant temperature varies inversely with the pressure applied to it. The primary advantage of body plethysmography is that it can measure the total volume of air in the chest, including gas trapped in bullae. Another advantage is that this test can be performed quickly. Drawbacks include the complexity of the equipment as well as the need for a patient to sit in a small enclosed space. A patient is placed in a sitting position in a closed "body box" with a known volume. From the FRC, the patient pants against a closed shutter to produce changes in the box pressure proportionate to the volume of air in the chest. The volume measured by this technique is referred to as thoracic gas volume (TGV) and represents the lung volume at which the shutter was closed, typically FRC.

 

Diffusing Capacity:

Understanding gas diffusion through the lungs requires recognition of the basics of the gas exchange interface and of the various forces at work by which oxygen and carbon dioxide move by molecular diffusion. Diffusion is limited by the surface area in which diffusion occurs, capillary blood volume, hemoglobin concentration, and the properties of the lung parenchyma that separate the alveolar gas from the red blood cell with the capillary.

Because all lung volume is not exchanged, most gas exchange occurs as a function of diffusion independent of bulk flow. The role of ventilation is to reset concentration of the bulk flow of gas with the ambient air and to provide a constant gradient for oxygen and carbon dioxide. As spirometry measures the components of this bulk flow exchange, diffusing capacity measures the forces at work in molecular movement with its concentration gradient from the alveolar surface through to the hemoglobin molecule. The clinical test diffusing capacity of the lung (DL) most commonly uses carbon monoxide (CO) as the tracer gas for measurement because of its high affinity for binding to the hemoglobin molecule. This property allows a better measurement of pure diffusion, such that the movement of the CO is in essence only dependent on the properties of the diffusion barrier and the amount of hemoglobin. The properties of oxygen and its relatively lower affinity for hemoglobin compared with CO also make it more perfusion dependent; thus, cardiac output may influence actual measurement of oxygen diffusion measurements.

Diffusing capacity for CO (DLCO) is the measure of CO transfer. In Europe, it is called the transfer factor of CO, which describes the process more accurately. DLCO is a measure of the interaction of alveolar surface area, alveolar capillary perfusion, the physical properties of the alveolar capillary interface, capillary volume, hemoglobin concentration, and the reaction rate of CO and hemoglobin. After a number of simplifications, the commonly used clinical tests to measure DLCO are based on a ratio between the uptake of CO in mL per minute divided by the average alveolar pressure of CO. Overall, DLCO is expressed as the uptake of CO in mL of gas STPD (standard temperature and pressure, dry) per minute and per mm Hg driving pressure of CO. In principle, the total diffusing capacity of the whole lung is the sum of the diffusing capacity of the pulmonary membrane component and the capacity of the pulmonary capillary blood volume.

All methods for measuring diffusing capacity in clinical practice rely on measuring the rate of CO uptake and estimating CO driving pressure. The most widely used and standardized technique is referred to as the single-breath breath-holding technique. This technique relies on a subject inhaling a known volume of test gas that usually contains 10% helium, 0.3% CO, 21% oxygen, and the remainder nitrogen. The patient inhales the test gas and holds his or her breath for 10 seconds. Exhalation is then performed to "wash out" mechanical and anatomic dead space. Subsequently, an alveolar sample is collected. DLCO is calculated from the total volume of the lung, breath-hold time, and the initial and final alveolar concentrations of CO. Alveolar volume is estimated by the helium dilution and the initial alveolar concentration of CO. The driving pressure is assumed to be the initial alveolar pressure of CO.

Hemoglobin concentration is a very important measurement in interpreting reductions in DLCO. Because the hemoglobin present in the alveolar capillaries serves as a CO sink such that oxygen and CO are removed from dissolved gases, the concentration gradient from alveolar to arterial blood remains relatively constant in favor of dissolved gas flow toward the arterial circulation. In this way, a DLCO may be decreased when the patient is anemic. Because the level of hemoglobin present in the blood and diffusing capacity are directly related, a correction for anemic patients (DLCOc.) is used to further delineate whether a DLCO is decreased due to anemia or due to parenchymal or interface limitation. If alveolar volume is low and the patient is anemic, both corrections may be performed and reported as the DLVAc.

 Diseases such as interstitial pulmonary fibrosis or any interstitial lung disease may make the DLCO abnormal long before spirometry or volume abnormalities are present. Low DLCO is not only an abnormality of restrictive interstitial lung disease but may also occur in the presence of emphysema. In emphysema, the lung volumes may be normal or hyperinflated; therefore, the DLVA is not useful. Additionally, the loss of alveolar surface area, the pathologic lesion of emphysema, is not proportionate to volume. As such, one can understand that other obstructive entities that predominantly affect the airways can have similar spirometry, but a low DLCO implies a loss of alveolar surface area consistent with emphysema. Unfortunately, it is not always this simple. Some forms of interstitial lung disease may have components of restrictive physiologies, such as low lung volume and clear evidence of decreased diffusion, but may also have airway flow limitation. Sarcoidosis and Wegener's granulomatosis may have an endobronchial component of airway webs or strictures, limiting flow before overt volume loss and still have enough interstitial granulomatous inflammation to reduce the DLCO.

On the other end of the spectrum, alveolar hemorrhage or congested capillary beds may actually increase the DLCO. The presence of trapped hemoglobin in proximity to alveolar gas will absorb CO despite the actual severe limitation of gas exchange and oxygen delivery.

As for spirometry, predicted formulas have been established for DLCO and DLVA. It is important to note, however, that differences in race have been observed in normal subjects, and a race correction of 7% is allowed for African-American patients.

EQUIPMENT

A detailed discussion of equipment is beyond the scope of this chapter. The American Thoracic Society has gone to great lengths to standardize and publish detailed recommendations regarding spirometry, lung volumes, and diffusing capacity. These guidelines include the selection of equipment, important technical considerations for variability, and standardization between laboratories for the maneuver. Table 5 lists the suggested performance standards for an office spirometer.

 

NORMALITY AND PREDICTED EQUATIONS

Studies from a healthy population indicate that parameters of lung function, such as FEV1 or FVC, are affected most significantly by standing height, age, gender, race, and to a lesser extent, by weight. If we assume that lung function has a normal Gaussian distribution, then a wide range of values may be considered normal. Because there is no absolute cut-off point for what is normal in biologic systems, an arbitrary statistical approach is widely used to define the lowest 5% of the population, or abnormal. Over the years, many regression equations have been generated by several investigators using different methodologies to study a variety of population cohorts. The recommendation is for clinical labs to choose a published reference standard that is most similar to the typical patient population at a given institution as well as the testing methods used. The most commonly used standards include Morris et al, Crapo et al, Knudson et al, and most recently, National Health and Nutrition Examination Survey (NHANES III). These reference standards are based on a cohort of normal people of similar age, height, and race, with normal being defined as individuals without a history of smoking or disease that may impact lung function.

Many approaches have been developed over the years to determine the normal range of spirometry. These approaches have included using a fixed percentage of predicted (i.e., 75%) and a fixed FEV1-to-FVC ratio, (i.e., >0.70), although both of these approaches have no statistical basis and are not recommended.

The American Thoracic Society recommends using the concept of lower limit of normal by identifying the lowest 5% of a population, or patients that fall outside the limits of 1.64 standard deviations from the mean. This value may be calculated by multiplying 1.65 times the standard error of estimate (1.65 x SEE). Weight is less important as a predictor of lung function. Obese patients may have abnormal spirometry (i.e., decrease in FVC) based on the diaphragm's ability to displace the intra-abdominal fat. Body weight has little impact on intrathoracic volume. Race plays an important role in determining normal lung function, as it has been recognized that individuals of different races for any given height and age have proportionately different lung volumes. Specifically, based on anthropometric differences, the lung function for African-Americans is systematically lower compared with Caucasians. The American Thoracic Society recommends a 12% correction for African-Americans for FEV1, FVC, and TLC. The FEV1-to-FVC ratio in African-Americans may be slightly higher compared with Caucasians. A 7% correction for lower values is recommended for FRC and residual volume. However, race-specific reference standards are preferred. Over time, the NHANES III reference equations will likely become the standard in most pulmonary function testing laboratories around the country. The methodologies and the sample size are most robust for this data set, as well as being representative of the American population.

CLINICAL INTERPRETATIVE STRATEGIES

Spirometry:

In 1991, the American Thoracic Society issued a position statement regarding interpretative strategies, which forms the basis for PFT interpretation in practice. As previously discussed, spirometry is the most widely used screening test of lung function or pulmonary function studies. It is usually the first test to be performed and interpreted. Supplemental studies may be conducted as needed, such as a formal lung volume measurement, diffusing capacity, methacholine provocation test, or cardiopulmonary exercise studies. Spirometry is usually adequate for preoperative risk assessment and stratification. It is also often adequate for rotated obstructive lung disease, such as emphysema or asthma. However, when a patient's symptoms or clinical history cannot be explained by findings on spirometry or when multiple coexisting processes (i.e., dyspnea with both heart and lung disease) are present, then further testing is usually warranted.

In a simplistic way, respiratory disease can be classified as obstructive or restrictive processes. Obstructive disorders, such as emphysema or asthma, are characterized by airflow limitation, have increased lung volumes with air trapping, and have normal or increased compliance (based on pressure volume profile). In contrast, restrictive disorders such as pulmonary fibrosis are characterized by reduced lung volumes and an increase in overall stiffness of the lungs (with reduced compliance). Once the technical adequacy of the spirogram has been established, the next step is to classify whether the study is normal, has an obstructive pattern, a restrictive pattern, or a mixed obstructive/restrictive pattern.  In general, the measured values are compared with the lower limits of normal predicted values from one of the published studies. Airflow obstruction exists, by definition, when the FEV1-to-FVC ratio is below the lower limits of normal. When this ratio is above the lower limits of normal, obstruction is usually excluded. However, occasionally, early termination or short expiratory time can artifactually reduce FVC and falsely normalize the FEV1-to-FVC ratio to mask obstruction. Once the presence of airflow obstruction is established, then a typical approach in the laboratory is to administer two puffs of inhaled albuterol and repeat the spirogram after 15 minutes to establish bronchodilator responsiveness. Lack of bronchodilator response certainly does not exclude asthma, and the result needs to be used in the context of a patient's clinical history.

Lung Volumes:

Because the FVC is not a reliable measure of TLC, spirometry can only be suggestive of a restrictive process and, in general, should be followed up by lung volume measurement.  When spirometry suggests a restrictive process or when the abnormalities seen on the spirogram do not adequately explain a patient's clinical history, then formal measurements of lung volume are helpful. TLC can be particularly helpful when a patient has severe airflow obstruction and has a reduction in FVC. In this case, a normal or increased TLC would exclude an associated restrictive process, and the reduction in FVC would actually be a pseudorestriction.

DLCO:

Diffusing capacity is a pulmonary function test that is commonly performed to help further characterize abnormalities in spirometry or lung volume measurements. The DLCO is a test that has greater degrees of variability between laboratories and that it is a study that requires some level of expertise to perform reliably. Several processes can affect diffusing capacity.  If diffusing capacity is reduced proportionately to airflow obstruction (a proportionate reduction in FEV1 and DLCO), then this is a pattern typical for emphysema. If the DLCO is reduced proportionately to a reduction in TLC in the context of restrictive abnormalities, then this would be suggestive of a parenchymal process such as pulmonary fibrosis. If there is an isolated or disproportionate reduction in diffusing capacity along with either normal or fairly well-preserved mechanics, then this would be suggestive of predominantly a pulmonary vascular process such as primary pulmonary hypertension or thromboembolic disease. Note that anemia or carboxyhemoglobinemia (from smoking) could affect the measured DLCO. The concept of a reduced DLCO that normalizes after correction for a lung volume measurement is often used to describe an extrathoracic or extraparenchymal disease process such as resection, obesity, or neuromuscular disease. However, this approach has many limitations.

Notes

Pulmonary Function Interpretation

Identify the indications for pulmonary function testing:

According to the AARC CPG, PFT need to be done to:

1.    diagnosis restrictive defects,

2.    to differentiate between restrictive and obstructive defects,

3.    assess the patient’s response to interventions

4.    pre-op assessment of patients at risk for pulmonary limitations

5.    evaluate pulmonary disability

6.    Quantify air trapping; is it getting worse, better

 

Identify the contraindications of pulmonary function testing:

According to the AARC CPG, the relative contraindications include

·         untreated pneumothorax

·         hemoptysis

·         unstable hemodynamics

·         aneurysms.

 

If persons have claustrophobia, upper body paralysis or cast that makes the ‘body box’ impossible, this single test may be deferred.

 

Identify the hazards of pulmonary function testing:

·         PFT that add supplementary 02 can cause 02-induced hypoventilation

·         cross-contamination

·         hypoxemia due to removal of supplementary 02 or increased WOB during certain maneuvers

·         hypercapnia

 

List the effects of gender and age and other issues on the patient normal pulmonary function values.

·         The most important factor for prediction of normal values is the height of the patient.

·         Sex: Adult men tend to have better values for all parameters than adult women

·         The age of a person has a inverse effect on values, the older the patient the lower the normal values. Peak values are found in the person in their mid-twenties and then start to drop.

·         BSA: body surface area [height and weight together] because some weight gain is in muscle mass, values will rise, but when weight gain is due to increased body fat, the values fall.

·         Race has a small effect on pulmonary function study results [Madama pp. 153]  normal values for volumes may be smaller for Asian and black person than European although the flows may not be that different. Studies are going on all over the world to find normal PFT values for different populations.

Quantifying PFT results:

Because normal values are based on gender, age, size and general conditions, absolute normal values vary so much that it is easier to look at these values as “percent of predicted” --- % predicted.

The normal parameters are as a rule considered normal if within +/- 20%, so a person with a value that is 79% of predicted has decreased values, and the person with 121% has increased values.

·         deranged measurements between 65-79% or 121-135% are mild

·         deranged measurements between 50-64% or 136-150% are moderate

·         deranged measurements between 35-49% or 151-165% are severe

·         deranged measurements between less than 35% predicted more than 165% predicted are very severe 

 

List & discuss those PFTs that measure and quantify lung volumes and capacities.

 

The 4 lung volumes are used to document the presence of restrictive defects. In the case of a restrictive defect, all or most of the volumes will be decreased.

We don’t worry about increased volumes unless it is the residual volume [RV], which is a problem because higher than normal RV imply there is air-trapping due to obstructive defect.

1.    VT:  tidal volume measured via spirometry; the normal VT is monitored at the bedside and during PFT.

·         The VT  is about 10% of the TLC & may or may not be decreased in restrictive & obstructive defects

·         For accuracy, at the bedside, we need collect the VE and RR then calculate the VT. An average is better than one or two single breaths

·         In the PFT lab we will collect 3 minutes for our average

·         The VT is a fairly unhelpful measurement in of itself & is frequently not even reported outside of discussions about weaning patients from or placing patients on mechanical ventilation.

2.    IRV: inspiratory reserve volume the amount of gas that is inhaled after a normal exhalation, the IRV is about 50% of the TLC.

·         In the case of restrictive defect, this figure will be less than 80% predicted.

·         In case of obstructive defect, this might be normal or decreased

3.    ERV: expiratory reserve volume the amount of gas that is exhaled at the end of a normal exhalation. The ERV is about 20% of the TLC

·         In the case of restrictive defect, this measurement will be less than 80% predicted

·         In the case of obstructive defect, the ERV is normal or decreased

4.    RV: residual volume: volume of gas that stays in the lung after a maximal exhalation. Because this gas cannot be exhales, it cannot be measured via spirometry, so indirect means such as the body box or He dilution are used to measure this gas. The normal RV is about 20% of the TLC.

·         In obstruction, the RV may be more than 120% predicted due to air-trapping

·         In restrictive defects the RV is lower than 80% of predicted

 

The 4 lung capacities:

A lung capacity is the sum of 2 or more lung volumes. Lung capacities that include RV will have to be measured indirectly.

1.    TLC = ERV + RV + VT + IRV. This is all the volume in the patient’s lungs including that air that cannot exit the lungs [RV]

·         Decreased TLC implies there is a restrictive defect

·         TLC more than 120% predicted implies that air trapping has become ‘emphysema.’

·         A variation of the indirect measurement of TLC by He dilution or N2 washout is the measurement of the thoracic gas volume [TGV] via the body plethysmograph.

·         When there is a large difference between the TLC measured by He dilution & N2 washout and the Thoraic Gas Volume by body box, this result implies that this patient might have areas of RV that are completely obstructed so that they don’t connect to the airways. In this case the TLC will be lower than the TGV.

·         In a healthy person the TLC and the TGV are identical [and both within 20% of predicted]

 

2.    VC = ERV + VT + IRV: The maximal amount of gas that is inhaled after a maximal exhalation. This is the most gas a person can move. It should be 80% of TLC.

·         The VC can be slow SVC or forced [FVC] in which the patient inhales and exhales as quickly as possible

·         The SVC will be decreased in restrictive defects

·         When there is a marked difference between the same person’s SVC and FVC, we know this patient has the decreased flow rates associated with obstructive defects.

·         When both the SVC & the FVC are decreased, we have restrictive defect

 

3.    IC = VT + IRV: The maximal amount of gas that is inhaled after a normal exhalation

·         is decreased in restrictive defects

·         if the patient has severe obstruction, this may be decreased because the FRC is raised so high above normal

 

4.    FRC = ERV + RV

·         The combination of the normal exhaled reserve volume, plus the RV that cannot be removed

·         Are decreased in restrictive defects

·         May be increased in obstructive defects associated with air-trapping

·         The “FRC point” at the end of a normal exhalation is considered the point at which most PFT tracings will start  

 

List & discuss those PFT that monitor flow characteristics & problems with RAW associated with obstructive defects

The volumes associated with obstruction include the FRC, the RV and the TLC. [see above lecture]

Assessment of patient’s flow rates at various points will track the presence, location and degree of airway obstruction.

Assessment of these flow before and after bronchodilator administration should give the RCP an idea about the reversibly of the obstructive defect.

Because FVC technique is so effort-dependent, a good PFT lab will collect several different types of flow measurement.

 

FVC measured with simple spirometry can yield the following information:

1.    FEV1  : the volume of gas a person can exhale in the first second during an FVC. This number is generally compared to the FVC.  Because we are measuring volume against time we are more or less measuring the flow rates

·         Decreased values of FEV1 imply there is an obstructive defect, but it could also be down in a person with restriction because the FRC is down.

·         We need to compare the FEV1 to the FVC

 

2.    FEV1/FVC the person with normal RAW will be able to exhale at least 70% of the FVC within the first second of exhalation.

·         Anything less than this implies there is an obstructive component to the lung disease.

 

·         A person with a decreased FVC due to a restrictive defect, may has a normal FEV1 because he has no obstruction… his FEV1/FVC maybe be abnormally large. So…oddly enough, high FEV1/FVC can, sometimes, imply restrictive defects.

 

3.    FEF 200-1200   the average flow is measured between the first 200 ml and the 1200 ml point. This is the average flow rate in the larger airways. Decreased flows imply obstructive disorders

 

4.    FEF25% the volume exhaled at the point of the first 25% of the FVC is a flow rate that can be decreased in obstructions in the larger airways. Decreased flows imply obstructive disorders 

 

5.    FEF 25-75% the average flow rate of the smaller airways. This is measured between the first 25% of the FVC and the last 25%. This is the average flow rate in the smaller airways Decreased flows imply obstructive disorders

 

6.    FEF 50% the volume exhaled at the mid-point the FVC is a flow rate that can be decreased in obstructions in the smaller airways

 

7.    FEF 75-85% the average flow rate of the smallest airways and will be decreased with obstruction of the smallest airways.

 

8.    FEF 75% the volume exhaled at the last 25% of the FVC& it is a flow rate that is decreased in obstruction in the smallest airways

 

The MVV – maximal voluntary ventilation in which the patient breaths as fast and as deeply as he can for at least [10-15]12 seconds with the results extrapolated to 1 minute. The units are LPM. This is a nonspecific test of the patient’s over-all ventilatory function.

·         Both obstructive defects and restrictive defects will have lower numbers

·         In the case of the obstructive defect, the actual shape of the tracing will change. The base line will move up from FRC point

·         The most important function of the MVV is its ability to predict the patient’s ability to withstand pulmonary rehabilitation. A person with a MVV less than 50% of predicted cannot handle the rigors of pulmonary rehabilitation

 

Flow/Volume loop the patient performs a FVC but the computer displays the values in a flow/volume graphic. Once the inspiratory flows are separated from the expiratory flows we can start measuring:

 

·         PIFR: peak inspiratory flow rates- most of the concern with PIFR is the patient’s ability to get inhaled medication via DPI

·         PEFR:  peak expiratory flow rates: easily measured gage of bronchodilation.  Most effort dependent- better to look at FEV1 for comparisons of various broncho-active agents

 

Shape of the flow/volume loop: in the normal flow-volume loop we see the inspiratory flow pattern as a sine wave while the expiratory flow rate is a descending curve.

Patients with restrictive disorders will have a normal flow but decreased volume-- tall narrow loop
Patients with small airway obstruction such as asthma or COPD will have slower expiratory flows with a scooped out expiratory tracing
Patients with large fixed obstructions [such as ET tubes] will have squared off inspiratory and expiratory flows [kinda box-looking]
Patients with intra-thoracic large airway obstruction will have a squared off expiratory flow with a slower sine wave on inspiration. This can be seen in persons with a tumor in the central airways or compression of the central airways.
Patients with an extra-thoracic large airway obstruction will show normal shape of the expiratory flow but decreased inspiratory flow. This would be seen in persons with tumors or other problems in the area of the larynx.

  

Measurement of the RV in order to calculate the TLC

Because we cannot exhale the residual volume, measurement of this data is a little tricky.

There are two basic ways to do this: [1] by dilution of a gas and [2] by changes in thoracic volume inside a body plethysmograph

 

He dilution test [also called closed circuit method]: Because He gas is rare in ambient air; we can use this gas to measure the RV indirectly.

  

·         The patient inhales a known percentage of He into a known volume of gas—usually quite large-- about 30 Liters.

·         The patient has an unknown volume of gas inside his RV, but a known He percentage of zero [because He is so rare]

·         Over several minutes, the patient inhales and exhales into the huge spirometer [holds almost 30 Liters] and after a few minutes the percentage of He is measured.

·         Knowing that the He will have diffused throughout container that includes the spirometer and the patient’s own lungs, we can calculate the actual RV.

·         The change in He percentage is directly proportional to the amount of the patient’s FRC volume added to the circuit volume. 

·         Once we figure out the FRC, we can subtract the ERV to get the RV

·         Note that this technique of He dilution works only if all the patient’s airways are patent. If there are significant areas of completely collapsed airways, this measurement will be lower than the actual FRC.

·         Because He dilution involves the patient rebreathing from the container, we must add a soda lime canister to absorb the exhaled C02 to prevent rebreathing C02.   

·         Extra 02 can be added before the test starts so the patient doesn’t become hypoxemia. 

·         Towards the end of the test, if the supplementary 02 runs low the patient might become hypoxemia

 

N2 washout study [or open circuit] Another way to indirectly measure RV is to washout the patient’s N2 with breathing 100% for several minutes [max 7 minutes].

·         The exhaled gases are collected and the N2 percentage measured.  The known volume of gas and known percentage of alveolar N2 [.75] at the start of the test is compared to the final N2 percentage so that the FRC can be calculated. Again once the FRC is measured, we subtract the ERV to get the RV

·         Needless to say, COPD patients with hypoxic drives require another means of RV, because they cannot handle the high Fi02.

·         Because the patient doesn’t rebreathe his Co2, the N2 washout test is a open-circuit method

 

Body plethysmography used to measure thoracic gas volume. The patient is placed inside the body boy and asked to pant for a second or two, then the shutter valve closes and the patient continues to pant. Because there is no flow, we can assume that the mouth pressure is equal to the alveolar pressure.

·         Because the patient is sealed inside an airtight box, the changes of pressure-- caused by the patient’s chest wall getting larger during inspiration--can be measured and with various formulae turned into thoracic gas volume.

·         Thoracic gas volume includes trapped gas that is not in contact with the airways.

·         If there is a large difference between the RV found during He dilution and that discovered during the TGV, we know that this patient has significant areas of airways that are completely collapsed.

·         RAW directly measured by body plethysmography

·         At the same time the patient is panting for the TGV measurement, we can also measure the airway resistance of the patient because while the shutter is closed we can measure the alveolar pressure.

·         Normal RAW determined this way is .6 to 2.8 cmH20/L/second

·         As we already know, increased RAW are signs of obstructive defect

 

List & discuss PFT that measure gas diffusion

We must measure the patient’s ability to diffuse gas for a complete picture of his lung disease.

Anyone who has a disease that causes hypoxemia most likely has some degree of diffusion defect, but we need to investigate further

While a person with a restrictive would have decreased alveolar ventilation which would limit diffuse of gases and a person with severe obstruction would also have limited alveolar ventilation thus diffusion- there is another class of patient.

 If a person had no flow or volume problems but still had decreased gas diffusion, we could diagnosis a pure diffusion problem such as pulmonary embolism [common] or pulmonary vascular edema, pulmonary vasculitis [both very rare.]

 

Normal diffusion of CO is 20-30 ml/minute/mmHg. This is called DLCO diffusion of CO -.

When we multiply the DLCO by 1.23 we get the DLO2

 

Increased diffusion

While we expect diffusion to be down in almost everyone who is significantly hypoxemic, occasionally we may find that a patient has increased gas diffusion due to polycythemia [more RBC with more Hb] or LV failure in which blood stays in the pulmonary bed too long.

Persons who live at high altitudes will have increased diffusion.

Diffusion increases during supine positioning and during exercise

To perform gas diffusion studies, the patient will inhale a small, known amount of carbon monoxide [CO].

Because the CO will bind with Hb, we know that there will be zero CO dissolved in the plasma [PcapillaryC0] so that we know that the PACo-PaC02 gradient is equal to the PACO

CO diffusion can be performed in a single breath study of .3%CO or during a steady-state inhalation of 0.1% CO over several minutes

 

Pulmonary function studies of gas distribution

We need to study the patient’s physiological VD to get an understanding of the patient’s actual alveolar ventilation.

While the anatomical VD stays the same unless the patient gets a tracheostomy, the alveolar VD varies with disease processes. So measurements of physiological VD gives us all the information we need.

Gas distribution: the basal areas fill first, then empty first

Single-breath N2 elimination

1.    N2 starts at zero because we are at the start of exhalation. This is pure VD

2.    mixture of central airways and alveolar gases as patient starts to exhale; so still exhaling VD gases

3.    pure alveolar gases

4.    Closing volume; small airways close so that there is no more gas out of the basal portion of the lung. Now we have mostly gas flow out of the apical areas which have more N2 because this is where air trapping occurs.

 

Interpretation of single-breath N2 washout

·         We can calculate the VD alveolar and the VD/VT ratio from these measurements as well as monitor the gas distribution.

·         Normal VD/VT ratio is .20 to .40 with VD anatomical and VD basically the same figure

 

 

Links

 

PFT Equipment

All about Spirometry

Spirometry - Introduction