Chapter 12
Quality Systems in the Pulmonary Function Laboratory
1. Describe the twelve quality system essentials and path of workflow for pulmonary function testing.
2. Describe three types of mechanical quality control devices.
3. Perform and evaluate spirometry linearity testing on a flow-based system.
4. Determine whether spirometers, single-breath diffusing equipment, a plethysmograph, or a blood gas analyzer is “in control” using a control chart.
5. Compose technologist’s comments to describe acceptable and unacceptable spirometry, Dlco, and lung volumes.
1. Evaluate results from a customer satisfaction survey and apply to process improvement.
2. Apply results obtained from biologic control subjects to troubleshoot pulmonary function equipment.
3. Describe two methods for performing quality control of a Dlco system.
4. Define and describe the three components of a quality manual—policy, process, procedure—as applied to each QSE.
5. Describe key components of an information management QSE.
6. Identify key components of a safety and infection control QSE for arterial blood gas collection and prevention of cross-contamination of pulmonary function equipment.
The chapter discusses issues related to a quality system, as introduced in Chapter 1. Although an in-depth review of the quality system is beyond the scope of this chapter, an introduction to the basics of each quality system essential (QSE) and the path of workflow is included to move the laboratory toward total quality management. The referenced Clinical and Laboratory Standards Institute (CLSI) documents will broaden the reader’s understanding of each of the concepts introduced in the chapter and may be essential for meeting accreditation and regulatory standards. The concept of developing a quality manual that addresses policies, processes, and procedures is introduced. Each of the twelve QSEs is discussed with examples for application to the pulmonary function laboratory. The personnel QSE addresses personnel standards, training, and competence assessment. Personnel who perform pulmonary function tests must make decisions during the path of workflow (POW) (pre-testing, testing, and post-testing) that determine the quality of data obtained. Special attention is given to methods by which the pulmonary function technologist can assess data quality through discussion of each component of the path of workflow. Documentation of pulmonary function data quality (e.g., acceptability, repeatability, and reproducibility) is discussed.
Quality manual
The initial step in building the quality system is developing a quality manual. The quality manual addresses policies for each of the twelve QSEs. Policies are written to answer the question: What do we do in our organization? Each policy describes the organization’s intent and provides direction for the specific QSE. Policies for the personnel QSE often include the intent and direction for job descriptions and qualifications, orientation, training, competence assessment, and continuing education. Processes are described to transform the policy into action and answer the question: How does this happen in our pulmonary laboratory? Processes are generally a group of activities or procedures. There is a process to obtain reliable spirometry data that may result in a need for several procedures such as quality control, test performance, test result selection and reporting, selection of reference values, and interpretation of results (Table 12-1). Procedures answer the question: How do I do this activity? Each QSE incorporates policies, processes, and procedures to build a platform for the path of workflow to be followed for each test procedure.
Table 12-1
| Operating Process: Spirometry Test Result Selection and Reporting | |
| Purpose | To describe the process for spirometry test result selection and reporting. |
| Process | This process is supported by the steps and documents in the table that follows: |
| What Happens | Who’s Responsible | Results: Documents |
Support staff
Expected Results: Accurate and reliable test results.
*NOTE: The “best curve” is selected from the largest sum of FVC + FEV1.
(From Blonshine S, Mottram CD, Berte LM, et al. Application of a quality management system model for respiratory services: Approved guidelines. 2nd ed. CLSI document HS4-A2. Wayne, PA: Clinical and Laboratory Standards Institute; 2006.)
Quality system essentials
Organization
Customer Focus
The pulmonary laboratory should identify their customers and expectations. Customers include both external and internal groups. Examples of external customers include accreditation and governmental agencies, patients, physicians, nurses, clinical support services, asthma educators, home care companies, and payers. All staff involved in the path of workflow are considered internal customers. An initial step is to evaluate the laboratory’s capability to meet the identified expectations. Physicians ordering tests likely will have an expectation of how long it should take to schedule a patient for testing and the length of time to receive an interpreted copy of the report. Surveys will identify expectations met and potential for continual improvement (Box 12-1). Complaints are recorded and managed according to the nonconforming event management QSE.
Box 12-1 Pulmonary Function Laboratory Customer Service Summary
How close to your scheduled appointment time were you called in for your test?
 |
Before appointment time |
 |
On-time |
 |
30 minutes late |
 |
1 hour late |
 |
Longer than 1 hour |
How well did the technologist explain your test?
 |
Excellent |
 |
Very good |
 |
Good |
 |
Fair |
 |
Poor |
How knowledgeable and technically skilled was the technologist performing your procedure?
 |
Excellent |
 |
Very good |
 |
Good |
 |
Fair |
 |
Poor |
How courteous and professional was your technologist?
 |
Excellent |
 |
Very good |
 |
Good |
 |
Fair |
 |
Poor |
Test performed: PFT ABG CPET O2 titration
Comment:________________________________________________
Courtesy of Mayo Clinic PF Laboratory.
Facilities and Safety
Each laboratory should have written guidelines defining safety and infection control practices. The guidelines should be part of a policy and procedure manual (Box 12-2). Procedures should include, but not be limited to, handwashing techniques, use of protective equipment such as laboratory coats and gloves, and guidelines for equipment cleaning. The handling of contaminated materials (e.g., waste blood) should be clearly described. Policies and procedures should include education of technologists regarding proper handling of biologic hazards. Department policies and procedures should be consistent with those mandated by individual hospitals or institutions. Most accrediting agencies require written plans for safety, waste management, and chemical hygiene. In the United States, the Occupational Safety and Health Administration (OSHA) has published strict guidelines regarding handling of blood and other medical waste (see Evolve). Safety training should be documented and the training records maintained per institutional requirements, accreditation, and regulatory requirements in the records management system. Generally, some areas will require retraining on an annual basis.
Pulmonary Function Tests
1. Disposable mouthpieces and noseclips should be used by the patient during spirometry. Reusable mouthpieces should be disinfected or sterilized after each use. Proper handwashing should be done immediately after direct contact with mouthpieces or valves. Gloves should be worn when handling potentially contaminated equipment. Hands should always be washed between patients and after removing gloves.
2. Tubing or valves through which subjects rebreathe should be changed after each test. Any equipment that shows visual condensation from expired gas should be disinfected before reuse. This is particularly important for maneuvers such as the FVC, where there is a potential for mucus, saliva, or droplet nuclei to contaminate the device. Breathing circuit components should be stored in sealed containers (e.g., plastic bags) after disinfection.
3. Spirometers should be cleaned according to the manufacturer’s recommendations. The frequency of cleaning should be appropriate for the number of tests performed. For open-circuit systems, only that part of the circuit through which air is rebreathed needs to be decontaminated between patients. Some flow-based systems offer pneumotachometers that can be changed between patients. Pneumotachometers not located proximal to the patient are less likely to be contaminated by mucus, saliva, or droplet nuclei.Disposable flow sensors should not be reused. Volume-displacement spirometers should be flushed using their full volume at least five times between patients. Flushing with room air helps clear droplet nuclei or similar airborne particulates. Water-sealed spirometers should be drained at least weekly and allowed to dry completely. They should be refilled with distilled water only. Bellows and rolling-seal spirometers should be disinfected on a routine basis. The spirometer may require recalibration after disinfecting.
4. Systems used for spirometry, lung volumes, and diffusing capacity tests often use breathing manifolds that are susceptible to contamination. Bacteria filters may be used to prevent contamination of these devices. Filters may impose increased resistance, affecting measurement of maximal flows as well as airway resistance or conductance. Some types of filters show increased resistance after continued use in expired gas. Spirometers with bacteria filters should be calibrated with the filter in line; the spirometer should meet the minimal recommendations in Table 12-2 with the filter in place. If filters are used for procedures such as lung volume determinations, their volumes must be included in the calculations.
Table 12-2
Minimal Recommendations for Spirometers
A 3-L calibration syringe is recommended for testing VC and FVC. Twenty-four standardized waveforms are available for validating FVC, FEV1, and FEF25%–75%. Twenty-six standard flow waveforms are available for validating PEF. Other flows require manufacturer’s proof of performance. A sine-wave pump is recommended for MVV validation.
| Test | Range/Accuracy (BTPS) | Flow Range (L/sec) | Time (sec) | Resistance/Back-Pressure |
| VC | 0.5–8 L/ ±3% of reading or ± 0.05 L, whichever is greater | 0–14 | 30 | N/A |
| FVC | 0.5–8 L/ ±3% of reading or ± 0.05 L, whichever is greater | 0–14 | 15 | <1.5 cm H2O/L/sec (0.15 kPa/L/sec) |
| FEV1 | 0.5–8 L/ ±3% of reading or ± 0.05 L, whichever is greater | 0–14 | 1 | <1.5 cm H2O/L/sec (0.15 kPa/L/sec) |
| Time zero | Time point for calculating all FEVT values | N/A | N/A | Back extrapolation |
| PEF | Accuracy: ±10% of reading or ±0.3 L/sec (20 L/min), whichever is greater; repeatability: ±5% of reading or ±0.15 L/sec (10 L/min), whichever is greater | 0–14 | N/A | Mean resistance at 200, 400, 600 L/min (3.3, 6.7, 10 L/sec) <2.5 cm H2O/L/sec (0.25 kPa/L/sec) |
 (except PEF) |
±5% of reading or ±0.2 L/sec, whichever is greater | 0–14 | N/A | <1.5 cm H2O/L/sec (0.15 kPa/L/sec) |
| FEF25%–75% | 7.0 L/sec /±5% of reading or ±0.2 L/sec, whichever is greater | ±14 | 15 | <1.5 cm H2O/L/sec (0.15 kPa/L/sec) |
| MVV | 250 L/min at VT of 2 L/ ±10% of reading or ±15 L/min, whichever is greater | ±14 (±3%) | 12–15 | <1.5 cm H2O/L/sec (0.15 kPa/L/sec) |
Adapted from Miller MR, Hankinson J, Brusasco V, et al. Standardisation of spirometry. Eur Resp J. 2005; 26:319-338.
5. Small-volume nebulizers, such as those used for bronchodilator administration or bronchial challenge, offer the greatest potential for cross-contamination. These devices, if reused, should be sterilized to destroy vegetative microorganisms, fungal spores, tubercle bacilli, and some viruses. Disposable single-use nebulizers are preferable but may not be practical for routines such as inhalation challenges. Metered-dose devices may be used for bronchodilator studies by using disposable mouthpieces or “spacers” to prevent colonization of the device. Common canister protocols for metered-dose inhaler usage have been evaluated in the laboratory.
6. Gloves or other barrier devices minimize the risk of infection for the technologist who must handle mouthpieces, tubing, or valves. Special precautions should be taken whenever there is evidence of blood on mouthpieces or tubing. There is a risk of acquiring infections such as tuberculosis or pneumonia caused by Pneumocystis carinii from infected patients. The technologist should wear a mask when testing subjects who have active tuberculosis or other diseases that can be transmitted by coughing.
7. Patients with respiratory diseases such as tuberculosis may warrant specially ventilated rooms, particularly if many individuals need testing. Risk of cross-contamination or infection can be greatly reduced by filtering and increasing the exchange rate of air in the testing room. Equipment can be reserved for testing infected patients only. An example may be using a spirometer for cepacia positive patients in a clinic patient room. Special patient organizations such as the Cystic Fibrosis Foundation may have additional requirements for infection control. Patients with known pathogens can also be tested in their own rooms or at the end of the day (to facilitate equipment decontamination).
8. Surveillance may include cultures of reusable components, such as mouthpieces, tubing, and valves, after disinfection.
Blood Gases
These standard precautions should be applied in the pulmonary function and/or blood gas laboratory:
1. Treat all blood and body fluid specimens as potentially contaminated.
2. Exercise care to prevent injuries from needles, scalpels, or other sharp instruments. Do not resheath used needles by hand. If a needle must be resheathed, use a one-handed technique or a device that holds the sheath. Do not remove used, unprotected needles from disposable syringes by hand. Do not bend, break, or otherwise manipulate used needles by hand. Use a rubber block or cork to obstruct used needles after arterial punctures. Use needle safety devices (now used in almost all blood gas kits) as described by the manufacturer. Place used syringes and needles, scalpel blades, and other sharp items in puncture-resistant containers. Locate the containers as close as possible to the area of use.
3. Use protective barriers to prevent exposure to blood, body fluids containing visible blood, and other fluids to which standard precautions apply. Examples of protective barriers include gloves, gowns, laboratory coats, masks, and protective eyewear. Gloves should be worn when drawing blood samples, whether from a needle puncture or an indwelling catheter. Gloves cannot prevent penetrating injuries caused by needles or sharp objects. Gloves are also indicated if the technologist has cuts, scratches, or other breaks in the skin. Protective barriers should be used in situations where contamination with blood may occur. These situations include obtaining blood samples from an uncooperative patient, performing finger-heel sticks on infants, and receiving training in blood drawing. Examination gloves should be worn for procedures involving contact with mucous membranes. Masks, gowns, and protective goggles may be indicated for procedures that present a possibility of blood splashing. Blood splashing may occur during arterial line placement or when drawing samples from arterial catheters.
4. Wear gloves while performing blood gas analysis. Laboratory coats or aprons that are resistant to liquids should also be worn. Protective eyewear may be necessary if there is risk of blood splashing during specimen handling. Maintenance of blood gas analyzers, such as repair of electrodes and emptying of waste containers, should be performed wearing similar protective gear. Laboratory coats or aprons should be left in the specimen handling area. Blood waste products (e.g., blood gas syringes) should be discarded in clearly marked biohazard containers.
5. Immediately and thoroughly wash hands and other skin surfaces that are contaminated with blood or other fluids to which the standard precautions apply. Hands should be washed after removing gloves. Blood spills should be cleaned up using a solution of 1 part 5% sodium hypochlorite (bleach) in 9 parts of water. Bleach should also be used to rinse sinks used for blood disposal.
Personnel
An orientation/training plan and process is required for all new employees. An orientation manual and training guide is developed and updated as required. The training manual may include, but is not limited to, quality management, safety, computer systems, ethics, work processes, and procedures. Records documenting the orientation and training processes are maintained according to the document and records QSE (Table 12-3). Training occurs and documentation is maintained when processes or procedures are updated or added. Performance of quality control procedures has been identified as a primary training need in pulmonary function laboratories globally, regardless of professional credentials. Orientation also includes review of the quality manual and application to work duties in the laboratory.
Table 12-3
• Main organization’s system (e.g., hospital information system [HIS])
• Department’s system (e.g., laboratory information system [LIS])
• PC applications (e.g., e-mail, scheduling, word processing, spreadsheets, database)
• Other computer applications used in the job (e.g., documentation of training, continuing education, competence assessment)
(From Blonshine S, Mottram CD, Berte LM, et al. Application of a quality management system model for respiratory services: Approved guidelines. 2nd ed. CLSI document HS4-A2. Wayne, PA: Clinical and Laboratory Standards Institute; 2006.)
Equipment
Equipment acquisition includes meeting the minimum equipment standards as outlined by the ATS-ERS standards as represented in Tables 12-2 and 12-4 for spirometers. Additional guidance may be found in the ATS-ERS standards for other types of equipment. The selection process may include a list of acceptable vendors, development of a product-evaluation matrix, equipment evaluation (on-site is preferred), written acceptable limits of accuracy and precision, information management options, computer standards, warranty and service agreements, and training options (Box 12-3). The initial selection may include an on-site evaluation with a comparison of old and new equipment.
Table 12-4
Minimum Recommended Scale Factors for Recorders and Displays
| Instrument Display | Printed Graphs | |||
| Resolution | Scale Factor* | Resolution | Scale Factor* | |
| Volume | 0.05 L | 5 mm/L | 0.025 L | 10 mm/L |
| Flow | 0.20 L/sec | 2.5 mm/L/sec | 0.10 L/sec | 5 mm/L/sec |
| Time | 0.2 sec | 10 mm/sec | 0.2 sec | 20 mm/sec |
*Scale factors for flow and volume produce an aspect ratio of 2:1.
Adapted from Miller MR, Hankinson J, Brusasco V, et al. Standardisation of spirometry. Eur Resp J. 2005; 26:319-338.
Installation, Validation, and Verification
Selection for equipment location in the pulmonary laboratory includes consideration of environmental conditions that impact acceptable function. For example, equipment specifications outline temperatures appropriate for spirometry equipment. A plethysmograph is sensitive to pressure changes in the room, vibrations close to the device, or other significant changes such as a fan blowing on the device. The installation process should include a correlation study between old and new equipment. Correlation studies are required for blood gas equipment by CLIA’88 and are also prudent to complete with pulmonary function equipment. Studies have shown bias between vendors and equipment within a specific vendor. Equipment function is validated initially to determine compliance with manufacturer’s specifications, expected accuracy and precision, and quality control standards. The question to answer is: Have the requirements for the intended use or application been fulfilled? Verification means that the specified requirements are fulfilled such as calibration verification. All institutional, regulatory, or governmental requirements should be considered during the installation process (Box 12-4).
Equipment Maintenance
An equipment maintenance plan should be developed and approved. If maintenance is provided by an outside contractor, it should be in compliance with the institutional plan. The type and complexity of instrumentation for a specific test determines the long-term and short-term maintenance that will be required. Daily maintenance includes replacing disposable items such as filters and gas conditioning devices. Preventive maintenance is scheduled in anticipation of equipment malfunction to reduce the possibility of equipment failure. Corrective maintenance or repair is unscheduled service that is required to correct equipment failure. These types of failures are often detected by QC procedures or unusual test results. Familiarity with the operating characteristics of spirometers, gas analyzers, Dlco systems, plethysmographs, metabolic systems, and application software requires manufacturer support and thorough documentation. Accurate records are essential to a comprehensive maintenance program. Documentation of procedures and repairs is required by most accrediting organizations. Upgrades to application software should be considered an essential component of equipment maintenance in the pulmonary function laboratory. Software upgrades generally require a re-verification process. Re-verification is often required after preventive maintenance and repairs (Table 12-5).
Table 12-5
| Date | Time | Maintenance or Problem | Resolution | Technologist |
(Courtesy Mayo Clinic, Rochester, MN)
Control Methods: Mechanical and Biologic
Quality Control Tools and Materials
Syringes
A large-volume syringe is the most common and frequently used mechanical QC tool used in pulmonary function testing. Syringes used for calibration should be accurate to within ±15 mL or ±0.5% of the stated volume (i.e., 15 mL for a 3-L syringe). Accuracy of calibration syringes should be verified annually. Several companies provide the service globally with the device in Figure 12-1, A and B. Syringes can be checked for leaks simply by occluding the port and trying to empty the syringe. Some laboratories use two syringes: one to calibrate and another to verify volume accuracy. This is highly recommended to exclude the syringe as a source of error. An alternative to have a 3-L and 7-L syringe is depicted in Figure 12-1.
A, Standard 3-L syringe is used for calibration of spirometers for testing of adults and adolescents. Smaller calibration syringes may be used for calibration and verification of pulmonary function equipment used for small children and infants. B, A 7-L syringe. (Courtesy Hans Rudolph, Inc., Shawnee, KS.)
A syringe of at least 3-L volume (see Figure 12-1) should be used to generate a control signal for checking spirometers. A 3-L syringe can be used to verify volume-displacement spirometers and associated deflection of mechanical recorders. A large-volume syringe may also be used to check the volume accuracy of flow-based spirometers. Computerized systems often have the user inject (or withdraw) a 3-L volume to calibrate the spirometer, and then immediately perform additional injections to verify the calibration. Some portable flow-based spirometers (i.e., those using disposable flow sensors) do not provide for calibration but do allow checking or verification of a stored calibration. The calibration or verification should minimally be completed each day testing is completed.
Sine-Wave Rotary Pumps
Sine-wave rotary pumps produce a biphasic volume signal. A biphasic or sine-wave signal may be useful for checking volume and flow accuracy for both inspiration and expiration. A rotary-drive syringe may be useful for checking the frequency response of a spirometer or to evaluate a spirometer’s ability to adequately record tests such as the MVV. Sine-wave pumps are also commonly used in the calibration of body plethysmographs(Figure 12-2).
Dlco Simulator
A Dlco simulator is a commercially available device, as depicted in Figure 12-3. This simulator uses precision gas mixtures to allow repeatable Dlco measurements at different levels (e.g., high Dlco, low Dlco) to simulate Dlco results over the range of expected results for patient testing. Two large-volume syringes are included; an adjustable 5-L syringe provides measured inspiratory volumes. A smaller second syringe is loaded with one of the precision gases; this gas is “exhaled” at the end of the breath-hold interval and sampled by the gas analyzers. Application software calculates the expected Dlco with the known gas concentrations (inspired and expired) along with the inspired volume, breath-hold time, and environmental conditions. The measured Dlco is then compared with the expected value and the percent error reported. By using different precision gases and varying the inspired volume, a range of expected Dlco values can be generated. This type of simulator is useful for all laboratories to identify the source of error in Dlco measurements and may also be used for performance qualification when installing new equipment. Use of the device is standard practice by manufacturers developing Dlco systems and evaluation of systems before shipment. It is also very useful for large laboratories with multiple Dlco systems, multicenter research applications in which accurate Dlco measurements are critical, and accreditation or regulatory programs. Studies have shown that the Dlco simulator data correlates with biologic data but may be more sensitive to identify the source of the problem or error.
A Dlco simulator that incorporates two large volume syringes and multiple precision gas mixtures provides quality control for Dlco systems. The simulator attaches to the patient port of the Dlco system. (Courtesy Hans Rudolph, Inc., Kansas City, MO.)
Plethysmograph volumes can be verified with an isothermal bottle. This device is available commercially (Figure 12-4) or can be constructed. An isothermal volume analog can be constructed from a glass bottle or jar of 4–5 L (for adult-size plethysmographs). The jar is filled with metal wool, usually copper or steel. The metal wool acts as a heat sink (see Figure 12-4) so that small pressure changes within the jar can be measured with minimal temperature change. The mouth of the bottle is fitted with two connectors. One connector attaches to the mouth shutter (or the patient connection). The other is attached to a rubber bulb with a volume of 50–100 mL (e.g., the bulb from a blood pressure cuff). The actual volume of the lung analog can be determined by subtracting the volume of the metal wool from the volume of the bottle. The volume occupied by metal wool is calculated from its weight times its density. The gas volume of the empty bottle may be measured by filling it with water from a volumetric source. The volume of the connectors and rubber bulb should be added to the total volume.
Quality Control Concepts
Precision may be defined as the extent to which repeated measurements of the same quantity can be reproduced. If a control is measured repeatedly and the results are similar, the instrument may be considered precise. Precision is often defined in terms of variability based on the standard deviation (SD) of a series of measurements (Box 12-5).
Box 12-5
The Mean and the Standard Deviation
Calculation of the mean and the standard deviation
The mean (
) may be calculated as:
∑ = a symbol meaning “the sum of”
The SD is calculated as follows:
X2 = deviations from the mean (X–
) squared
If the SD is computed from a sample of 30 data points or less, N-1 is usually substituted for N.
Example calculation of the mean and SD for a series of Pco2 values:
| Sample | Pco2 (mm Hg) | Deviation from the Mean (X) | Deviation Squared (X2) |
| 1 | 39 | − 0.9 | 0.81 |
| 2 | 40 | 0.1 | 0.01 |
| 3 | 43 | 3.1 | 9.61 |
| 4 | 42 | 2.1 | 4.41 |
| 5 | 39 | − 0.9 | 0.81 |
| 6 | 38 | − 1.9 | 3.61 |
| 7 | 40 | 0.1 | 0.01 |
| 8 | 41 | 1.1 | 1.21 |
| 9 | 38 | − 1.9 | 3.61 |
| 10 | 39 | − 0.9 | 0.81 |
| Total | 399 | 24.90 | |
| Mean | 39.9 |
Accuracy and precision are desirable but may not always be present together in the same instrument. For example, a spirometer that consistently measures a 3-L test volume as 2.5 L is precise but not very accurate. A spirometer that evaluates a 3-L test volume as 2.5, 3.0, and 3.5 L on repeated maneuvers produces an accurate mean of 3.0 L, but the measurements are not precise. Determining both the accuracy and precision of instruments such as spirometers is critical because many pulmonary function variables are effort-dependent. The largest observed value, rather than the mean, is often reported as the best test (see Chapter 2). Reporting the largest result observed is based on the rationale that the subject cannot overshoot on a test that is effort-dependent. Other pulmonary function tests, such as the Dlco, are reported as an average of two or more acceptable maneuvers; in these instances, precision of the measuring devices (e.g., gas analyzers) needs to exceed the normal physiologic variability of the parameter being measured.
Spirometry Calibration and Mechanical Quality Control
Spirometers that produce a voltage signal by means of a potentiometer (see Chapter 2) normally allow some form of “gain” adjustment so that the analog output can be matched to a known input of either volume or flow. For example, a 10-L volume-displacement spirometer may be equipped with a 10-V potentiometer. The potentiometer amplifier would be adjusted so that 0 V equals 0 L (zero), and 10 V equals 10 L (gain). The calibration could be verified by setting the spirometer at a specific volume and noting the analog signal (e.g., 5 L should equal 5 V).
