Pulmonary Function Testing Equipment

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Chapter 11

Pulmonary Function Testing Equipment

The chapter describes pulmonary function equipment used for common testing applications, including spirometers (volume and flow), body plethysmographs, and blood gas analyzers.

Hutchinson introduced the precursor of the modern spirometer around 1844. This spirometer was a water-sealed volume-displacement device. Some aspects of the original device are still evident in today’s spirometers. Flow-sensing spirometers have become much more common with the advent of sophisticated electronics and software that can integrate flow signals to measure volume by a variety of methods. Microprocessor-based spirometers are now small enough to be handheld.

Analysis of respiratory gases by volumetric methods was pioneered by Haldane in the early part of the 20th century. However, modern gas analyzers use indirect means (e.g., electrodes or sensors) to measure partial pressures of gases, or physical separation of gases to measure fractional concentrations (gas chromatography). Almost every instrument in the pulmonary function laboratory today combines signal transducers, analog-to-digital converters, and computer software to process and record physiologic data. Some devices, such as the pulse oximeter, are based almost entirely on electronic components. Computers eliminate many tedious calculations, allowing the technologist to concentrate on obtaining high-quality, repeatable data.

VOLUME-DISPLACEMENT spirometers

Water-Seal Spirometers

For more than 100 years, the water-seal spirometer was the basic tool used to measure lung volumes and flows. The spirometer consists of a large bell (7–10 L) suspended in a container of water with the open end of the bell below the surface of the water (Figure 11-1, A and B). A breathing circuit inserted into the interior of the bell allows for accurate measurement of gas volumes. The patient breathes into the spirometer, moving the bell up during expiration and down with inspiration. Each spirometer bell has a “bell factor” relating the vertical distance moved to a specific volume (milliliters or liters). For many years, movement of the bell was recorded using a pen to make a tracing on a rotating drum, called a kymograph. Volumes were measured from the kymograph tracing by using paper that incorporated the bell factor for the spirometer. Volumes measured in this way reflect the gas in the spirometer that is at keytermambient temperature, pressure, and saturation (ATPS). These volumes, such as vital capacity (VC), had to be corrected to body temperature, pressure and saturation (BPTS).

This type of spirometer bell is more commonly used to activate a potentiometer. The potentiometer is a device that produces an analog DC voltage signal proportional to its position or displacement. For example, an output of 10 volts might equal a volume of 10 L in the spirometer. This analog signal can be used to drive a mechanical recorder such as a strip-chart recorder. More commonly, however, the analog signal is digitized with an analog-to-digital (A/D) converter. The digitized signal from the spirometer can then be stored and processed by a computer. Some potentiometers also produce analog signals representing the speed of movement of a volume-displacement spirometer. This signal is proportional to flow. All volume-displacement spirometers use some type of potentiometer or position encoder to produce signals that can be digitized and stored by a computer.

For simple spirometry, a single large-bore tube can be used for both inspiration and expiration. For rebreathing studies, the breathing circuit incorporates a carbon dioxide (CO2) absorber/scrubber (usually soda lime). Inspiratory and expiratory circuits are separated with one-way valves to reduce dead space. Water-seal spirometers are typically used for spirometry. They may also be used to measure ventilation, including image, VT, and respiratory rate. Water-seal spirometers can be used to obtain flow-volume (F-V) curves, although their frequency response may be limited by their physical characteristics. By including the rebreathing apparatus described (see Figure 11-1), lung volumes by helium dilution can be obtained. In combination with an appropriate reservoir for the test gas, water-sealed spirometers can be used to perform diffusing capacity tests, both single-breath and rebreathing. The water-seal spirometer can be used as a reservoir for special gas mixtures such as those used for Dlco tests.

The Stead-Wells water-seal or dry-seal spirometer is still used, although not commonly. The Stead-Wells spirometer uses a lightweight plastic bell (Figure 11-1, B). The water-sealed bell “floats” in the water well, rising and falling with breathing excursions. In the dry-seal version, a rubberized seal connects the bell to the internal wall of the spirometer well. The rubber seal then “rolls” over itself, much the same as the dry rolling seal spirometer (see next section). The Stead-Wells bell is usually attached to a linear potentiometer that provides analog signals proportional to volume and flow. These signals are passed to a computer through an A/D converter. The Stead-Wells design is capable of meeting the minimum requirements for flow and volume accuracy recommended by the American Thoracic Society and European Respiratory Society (ATS-ERS) (see Chapter 12).

Problems with water-seal (and dry-seal) spirometers are usually caused by leaks in the bell or in the breathing circuit. Gravity causes the spirometer to lose volume in the presence of leaks. Leaks in the spirometer, tubing, or valves can be detected by raising the bell and plugging the patient connection. Any change in volume can be detected easily by recording the spirometer volume over several minutes. Weights can be added to the top of the bell to enhance detection of small leaks. During patient testing, improper positioning of the spirometer can cause inaccurate measurements. If positioned too high, the bell can rise out of the water or reach the top of its travel range. This causes the volume-time tracing to appear abruptly flattened. The pattern observed may be mistaken for a normal end-of-expiration. If a Stead-Wells spirometer is positioned too low, it may empty completely. This may result in water being drawn into the breathing circuit, gas analyzer, or other system components. Inadequate water in the device may also lead to erroneous readings that are sometimes difficult to detect. The size of the water-seal spirometer and its weight when filled with water make it somewhat difficult to transport. The waterless version of the spirometer eliminates the last consideration.

Because lung volumes and flows are corrected to BTPS conditions, careful attention to the ambient conditions of volume-displacement spirometers is required. Although the temperature of gas in the spirometer can be easily measured, the temperature may change significantly during maneuvers such as a forced vital capacity (FVC). These changes can be difficult to monitor, resulting in volumes and flows that are not representative of lung physiology.

Maintenance of water-seal spirometers includes routine draining of the water well. Both wet and dry versions of the Stead-Wells spirometer must be checked for cracks or leaks in the bell. Chemical absorbers for water vapor must be routinely checked. Water absorbers can be rapidly exhausted because the gas in the spirometer is almost completely saturated with water vapor.

Infection control of water-seal spirometers typically involves replacing breathing hoses and mouthpieces after each patient. Although the patient’s expired gas comes into direct contact with the water in the spirometer, cross-contamination is uncommon. Some systems allow use of low-resistance bacteria filters to protect from contamination those parts of the breathing circuit that are not changed after patient use. Such filters should be used with caution for flow-dependent maneuvers. Water condensation in the filter element may significantly alter its resistance. The volume of these filters may need to be considered when calculating system volume or system dead space, as is required for dilutional measurements of lung volumes.

Because of problems such as leaks and maintenance required for water-seal spirometers, these devices have become relatively rare in clinical practice. Water-seal or dry-seal spirometers may be used in longitudinal studies that were begun before other types of spirometers were available.

Dry Rolling Seal Spirometers

Another type of volume-displacement spirometer is the dry rolling seal spirometer. A typical unit consists of a lightweight piston mounted horizontally in a cylinder. A rod that rests on frictionless bearings supports the piston (Figure 11-2, A and B). The piston is coupled to the cylinder wall by a flexible plastic seal. The seal rolls on itself rather than sliding as the piston moves. A similar type of rolling seal may also be used with a vertically mounted, lightweight piston that rises and falls with breathing (as in the dry-seal Stead-Wells described in the preceding section). The maximum volume of the cylinder with the piston fully displaced is usually 10–12 L. The piston has a large diameter so that excursions of just a few inches are all that is necessary to record large volume changes. The piston is normally constructed of lightweight aluminum to reduce inertia. Mechanical resistance is kept to a minimum by the bearings supporting the piston rod and by the rolling seal itself.

Although they can be used with a mechanical recorder, most dry rolling seal spirometers userolling seal spirometers use linear or rotary potentiometers. The potentiometer responds to piston movement to produce DC voltage outputs for volume and flow. For example, a 10 volt (V) potentiometer attached to a 10-L spirometer may produce an output of 1 V/L. On a separate channel, a flow of 1 L/sec may produce an output of 1 V. Flow in this case is proportional to the speed of the moving piston. These analog outputs for volume and flow are digitized so that a computer can store and manipulate the data.

The piston of the standard dry rolling seal spirometer (Figure 11-2, B) travels horizontally, eliminating the need for counterbalancing. The vertically mounted version (see Figure 11-1, B) depends on a lightweight piston and the rolling seal to reduce resistance to breathing. Temperature corrections (from ATPS to BTPS) are made by applying a correction factor to the digital value stored in the computer. A one-way breathing circuit and CO2 scrubber may be added so that dry rolling seal spirometers can be used for rebreathing tests in much the same way as water-seal spirometers.

To perform studies such as the open-circuit nitrogen washout test, a “dumping” mechanism is attached to the spirometer. The dumping device empties the spirometer after each breath or after a predetermined volume has been reached. Addition of an automated valve and alveolar sampling device allows the dry rolling seal spirometer to be used for single-breath diffusion studies. Dry rolling seal spirometers are typically capable of meeting the minimum standards recommended by the ATS-ERS (see Chapter 12).

Common problems encountered with dry rolling seal spirometers are sticking of the rolling seal and increased mechanical resistance in the piston-cylinder assembly. These difficulties can usually be avoided by adequate maintenance of the spirometer. As for other types of volume-displacement spirometers, simple correction of volumes from ATPS to BTPS may not completely reflect physiologic flow or volume changes. Infection control of the dry rolling seal involves disassembling the piston-cylinder. The interior of the cylinder and the face of the piston are usually wiped with a mild antibacterial solution. The rolling seal itself is also wiped with disinfectant. Alcohol or similar drying agents may cause deterioration of the seal and should not be used. The seal should be routinely checked for leaks or tears. After reassembly, the piston should be positioned at the maximum volume position. When the rolling seal is extended completely, the material of the seal is less likely to develop creases that can result in uneven movement of the piston. With the previously described reservations, bacteria filters may be used to avoid contamination of the spirometer.

Bellows-Type Spirometers

A third type of volume-displacement spirometer is the bellows or wedge bellows spirometer. Both devices consist of a collapsible bellows that folds or unfolds in response to breathing excursions. The conventional bellows design is a flexible accordion-type container. One end is stationary and the other end is displaced in proportion to the volume inspired or expired. The wedge bellows operates similarly except that it expands and contracts like a fan (Figure 11-3, A and B) One side of the bellows remains stationary; the other side moves with a pivotal motion around an axis through the fixed side. Displacement of the bellows by a volume of gas is translated either to movement of a pen on chart paper or to a potentiometer. For mechanical recording, chart paper moves at a fixed speed under the pen while a spirogram is traced. For computerized testing, displacement of the bellows is transformed into a DC voltage by a linear or rotary potentiometer. The analog signal is routed to an A/D converter and then to a computer.

The conventional and wedge bellows may be mounted either horizontally or vertically. The horizontal bellows is mounted so that the primary direction of travel is on a horizontal plane. This design minimizes the effects of gravity on bellows movement. The horizontal bellows (either conventional or wedge) with a large surface area offers little mechanical resistance. This type is normally used in conjunction with a potentiometer to produce analog volume and flow signals. Small (approximately 7–8 L), vertically mounted bellows are available and may be used for portable spirometry and bedside testing. Most of these types offer simple mechanical recording, digital data reduction, or both by means of a small, dedicated microprocessor.

Both bellows-type spirometers (see Figure 11-3, A and B) can be used to measure vital capacity and its subdivisions, as well as FVC, FEV1, expiratory flows, and MVV. Some bellows-type spirometers, especially those that are mounted vertically, are designed to measure expiratory flows only. These types expand upward when gas is injected, then empty spontaneously under their own weight. Horizontally mounted bellows can usually be set in a mid-range to record both inspiratory and expiratory maneuvers. This allows F-V loops to be recorded. With appropriate gas analyzers and breathing circuitry, bellows systems may be used for gas dilution FRC determinations and Dlco measurements. Most bellows-type spirometers meet ATS recommendations for flow and volume accuracy.

One problem that may occur with bellows-type spirometers is inaccuracy resulting from sticking of the bellows. The folds of the bellows may adhere because of dirt, moisture, or aging of the bellows material. Some bellows-type spirometers require the bellows to be partially distended when not in use. This technique allows moisture from exhaled gas to evaporate and prevents deterioration of the bellows. Leaks may also develop in the bellows material or at the point where the bellows is mounted. Leaks can usually be detected by filling the bellows with air, plugging the breathing port, and attaching a weight or spring to pressurize the contained gas.

Infection control of bellows-type spirometers depends on the method of construction. In some instruments, the bellows can be entirely removed; in others, the interior of the bellows must be wiped clean. Many bellows are made from rubberized or plastic-based material that can be cleaned with a mild detergent and dried thoroughly before reassembly. Bacteria filters may be used to avoid contamination of the bellows, with the reservations described previously.

Volume-displacement spirometers were once the main devices used for pulmonary function testing. Many such devices are still in use, and some manufacturers continue to produce sophisticated spirometers based on the volume-displacement principle. However, use of flow-sensing spirometers has become increasingly common, both in the pulmonary function laboratory and in small, portable devices for bedside or clinic use.

Flow-sensing spirometers

In contrast to the volume-displacement spirometer is the flow-sensing spirometer, or pneumotachometer. The term pneumotachometer describes a device that measures gas flow. Flow-sensing spirometers use various physical principles to produce a signal proportional to gas flow. This signal is then integrated to measure volume in addition to flow. Integration is a process in which flow (volume per unit of time) is divided into a large number of small intervals (time). The volume from each interval is summed (Figure 11-4). Integration can be performed easily by an electronic circuit or by computer software. Accurate volume measurement by flow integration requires an accurate flow signal, accurate timing, and sensitive detection of low flow.

One type of device that responds to bulk flow of gas is the turbine or impeller. Integration may be unnecessary because the turbine directly measures gas volumes. Some turbine spirometers produce volume pulses in which each “pulse” equals a fixed volume. These spirometers count pulses very accurately. Most flow-sensing spirometers use tubes through which laminar airflow is possible (see Evolve website (http://evolve.elsevier.com/Mottram/Ruppel/)). Five basic types of flow sensors are commonly used:

Turbines

The simplest type of flow-sensing device is the turbine, or respirometer. This instrument consists of a vane connected to a series of precision gears. Gas flowing through the body of the instrument causes the vane to rotate, registering a volume (Figure 11-5). The respirometer can be used to measure slow vital capacity (VC). It can also be used for ventilation tests such as VT and image. One such device is the Wright respirometer. This respirometer can measure volumes accurately at flows between 3 and 300 L/min. At flows greater than 300 L/min (5 L/sec), the vane is subject to distortion. Because of this limitation, it should not be used to measure FVC when the patient is capable of flows greater than 300 L/min. At low flows (less than 3 L/min), inertia of the vane-gear system may underestimate volume.

A special advantage of this type of respirometer is its compact size and usefulness at the bedside. Most respirometers can register a wide range of volumes using multiple scales. The standard Wright respirometer measures 0.1–1 L on one scale and up to 100 L on another scale. Turbine devices are also widely used for measurements of bulk flow in various dry gas meters.

An adaptation of the turbine flow device includes a photo cell and light source that is interrupted by the movement of the vane or impeller (Figure 11-6, D). Rotation of the vane interrupts a light beam between its source and the photo cell. This produces a pulse, with each pulse equivalent to a fixed gas volume. The pulse count is summed to obtain the volume of gas flowing through the device. The signal produced may not be linear across a wide range of flows because of inertia or distortion of the rotating vane.

image
Figure 11-6 Common flow-sensing devices (pneumotachometers). Each flow-sensing device is mounted in a tube that promotes laminar flow (center). (A) Pressure-differential pneumotachometer in which a resistive element causes a pressure drop proportional to the flow of gas through the tube. A sensitive pressure transducer monitors the pressure drop across the resistive element and converts the differential to an analog signal. The resistive element may be a mesh screen or capillary tube; it is usually heated to 37°C or higher to prevent condensation of water from expired gas; (B) Heated-wire pneumotachometer contains heated elements of small mass that respond to gas flow by heat loss. An electric current heats the elements (thin wires). Gas flow past the elements causes cooling. In one element, current is increased to maintain a constant temperature; the other element acts as a reference (see also Figure 11-11). The current change is proportional to gas flow, and a continuous signal is supplied to an integrating circuit as for the pressure differential flow sensor; (C) Pitot tube flow sensor uses a series of small tubes that are placed at right angles to the direction of gas flow. Sensitive pressure transducers detect changes in gas velocity. The Pitot tubes are mounted in struts in the flow tube; separate devices face either way so that bidirectional flow can be measured (see also Figure 11-12); (D) Electronic rotating-vane flow sensor. A vane or impeller is mounted in the flow tube. A light-emitting diode (LED) is mounted on one side of the vane, and a photodetector on the other side. Each time the vane rotates, it interrupts the light from the LED reaching the detector. These pulses are counted and summed to calculate gas volume; (E) Ultrasonic flow sensor. High-frequency sound waves pass through membranes on either side of a flow tube at an angle to the stream of gas. The sound waves speed up or slow down depending on which direction the gas is flowing. By measuring the transit time of the sound waves, gas flow can be measured very accurately and integrated to compute volume. (See also Figure 11-10.)

The accuracy of turbine flow devices is usually limited by the factors described. For this reason, turbine devices often do not meet the ATS-ERS minimum recommendations for spirometers. These devices may be used for monitoring or screening. Because of their simplicity and small size, several such devices are marketed for home use. This type of spirometer allows FVC, FEV1, and peak expiratory flow (PEF) to be monitored outside of the usual clinical setting.

Infection control of turbine-type respirometers depends on their construction and intended use. Devices such as the Wright respirometer usually must be gas sterilized. Water condensation from exhaled gas can damage the vane-gear mechanisms. Some turbine spirometers use disposable impellers. This avoids cross-contamination, but accuracy may be limited by the quality of the disposable sensor.

Pressure Differential Flow Sensors

Pressure differential flow sensors are among the most common implementations of flow sensing. They consist of a tube containing a resistive element. The resistive element allows gas to flow through it but causes a pressure drop (see Figure 11-6, A). The pressure difference across the resistive element is measured by means of a sensitive pressure transducer. The transducer usually has pressure taps on either side of the element (Figure 11-7, A). The pressure differential across the resistive element is proportional to gas flow as long as flow is laminar. This flow signal is integrated to measure volume (see Figure 11-4). Turbulent gas flow upstream or downstream of the resistive element may interfere with the development of true laminar flow. Most pneumotachometers attempt to reduce turbulent flow by tapering the tubes in which the resistive elements are mounted.

Although there are many designs for resistive elements, two types are commonly used. The Fleisch-type pneumotachometer uses a bundle of capillary tubes (or similar material) as the resistive element. Laminar flow is ensured by size and arrangement of the capillary tubes. The cross-sectional area and length of the capillary tubes determines the actual resistance to flow through the Fleisch pneumotachometer. The dynamic range of the Fleisch device must be matched to the range of flows to be measured. Different sizes (i.e., resistances) of pneumotachometers may be used to accurately measure high or low flows.

The other common type of pressure differential flow sensor is the Silverman (or Lilly) type. The Silverman pneumotachometer uses one or more screens to act as a resistive element. A typical arrangement has three screens mounted parallel to one another. The middle screen acts as the resistive element with the pressure taps on either side, whereas the outer screens protect the middle screen and help ensure laminar flow. The Silverman pneumotachometer usually has a wider dynamic flow range than the Fleisch type. As a result, it is better suited for measuring widely varying flows.

Most Fleisch and Silverman pneumotachometers use a heating mechanism to warm the resistive element to 37°C or higher. Heating the resistive element prevents condensation of water vapor from exhaled gas on the element. Condensation or other debris lodging in the resistive element changes the resistance across it, thus changing its calibration. A change in the resistive element such as condensation or a hole causes a change in the pressure-flow relationship. Pneumotachometers need to be recalibrated after cleaning or similar maintenance.

Some flow-sensing spirometers use resistive elements such as porous paper, rendering the flow sensor disposable (see Figure 11-7, B). These devices usually have a single pressure tap upstream of the resistive element. Pressure measured in front of the resistive element is referenced against ambient pressure. This design requires that the flow sensor be carefully “zeroed” before making any flow measurements. These types of flow sensors may be more susceptible to moisture or debris on the resistive element causing volumes and flows to increase with each successive blow. The flow sensor will need to be replaced if the technologist notices this occurring. The accuracy of spirometers using this type of flow sensor often depends on how carefully the disposable resistive elements are manufactured. If the resistance varies widely from sensor to sensor, each unit may need to be calibrated before use to ensure accuracy. Some manufacturers calibrate their disposable sensors and then provide a calibration code with each sensor. This code is used to identify a particular sensor by the software in the spirometer. One method of identifying the correct calibration factor for individual flow sensors is to imprint the sensor with a bar code (see Figure 11-7, B). The spirometer then includes a simple bar code reader to identify the appropriate calibration factors. Some portable spirometer systems that use precalibrated flow sensors do not provide for user calibration. However, verification of accuracy (using a 3-L syringe) is usually possible, even if the manufacturer has not provided for this in the software accompanying the spirometer.

Systems that use permanent pressure-differential flow sensors usually meet or exceed the ATS-ERS minimal recommendations for spirometers. Spirometers that use disposable sensors can meet or exceed the minimal requirements, depending on the quality of the sensor and the application software responsible for signal processing. Gas composition affects the accuracy of flow measurements in pressure-differential pneumotachometers. Correction factors for gases other than air can be applied by software so that these types of flow sensors can be used for most types of pulmonary function tests.

Infection control of pressure-differential flow sensors depends on their placement in the spirometer. In open-circuit systems in which only exhaled gas is measured, only the mouthpiece needs to be changed between patients. If inspiratory and expiratory flows are measured, the flow sensor itself may need to be disinfected between patients. Disassembly and cleaning of flow sensors usually require that the spirometer be recalibrated. Disposable or single-use sensors avoid this problem. In-line bacteria filters may be used to isolate the pneumotachometer from potential contamination. The spirometer should meet all ATS-ERS requirements for range, accuracy, and flow resistance with the filter in place (see Chapter 12). (See Figure 11-8.) If a filter is used, calibration with the filter in-line is usually required. The effect of bacteria filters on spirometric measurements has been reported to cause small errors in measured flows and volumes (30-50 mLs). Use of in-line filters can be an efficient and effective component of the PF laboratory’s infection control program.

Heated-Wire Flow Sensors

Heated-wire flow sensors are a third type of flow-sensing spirometer. They are based on the cooling effect of gas flow. A heated element, usually a thin platinum wire, is situated in a laminar flow tube (see Figure 11-6, B). Gas flow past the wire causes a temperature decrease so that more current must be supplied to maintain a preset temperature. The current needed to maintain the temperature is proportional to gas flow. The heated element usually has a small mass so that slight changes in gas flow can be detected. The flow signal is integrated electronically or by software to obtain volume measurements. The heated wire is usually protected behind a screen to prevent impaction of debris on the element. Debris or moisture droplets on the element can change its thermal characteristics. Some systems use two wires (Figure 11-9). One measures gas flow, and the second serves as a reference. Most heated-wire flow sensors maintain a temperature higher than 37°C. Heating prevents condensation from expired air that might interfere with sensitivity of the element.

Most heated-wire flow sensors meet or exceed ATS-ERS recommendations for accuracy and precision. Gas composition may affect the accuracy of flow measurements. Correction factors for gases other than room air can be applied via software. This allows heated-wire devices to accurately measure gases for pulmonary function tests using helium, oxygen (O2), and other gases. Heated-wire sensors can be used for routine pulmonary function tests, exercise testing, and metabolic studies. Infection control for heated-wire sensors is similar to that for pressure- differential devices. Disposable or single-use devices avoid cross-contamination even when the sensor is located proximal to the patient’s airway.

Pitot Tube Flow Sensors

Pitot tube flow sensors are a fourth type of flow sensor. They use the Pitot tube principle. The pressure of gas flowing against a small tube is related to the gas’s density and velocity. Flow can be measured by placing a series of small tubes in a flow sensor and connecting them to a sensitive pressure transducer (see Figure 11-6, C). The pressure signal must be linearized and integrated as described for other flow-sensing devices. In practice, two sets of Pitot tubes are mounted in the same sensor so that bidirectional flow can be measured (Figure 11-10). A wide range of flows can be accommodated by using two or more pressure transducers with different sensitivities. Because this type of flow-sensing device is affected by gas density, software correction for different gas compositions is necessary. This is accomplished by sampling the gas, analyzing O2 and CO2, and applying the necessary correction factors. Software corrections for test gases used for various pulmonary function tests (e.g., Dlco) can be easily applied.

Pitot tube flow sensors meet or exceed ATS recommendations for accuracy and precision. Their practical applications include routine pulmonary function tests, metabolic measurements, and exercise testing. Infection control for this type of device includes single-use, or disposable, flowmeters. If Pitot tube flow sensors are cleaned, care must be taken that disinfectant solution or rinse water is completely removed from the small tubes used to sense flow.

Ultrasonic Flow Sensors

Gas flow can be detected and measured by passing high-frequency sound waves across the stream of gas. Ultrasonic transducers on either side of the flow tube transmit sound waves alternately across the tube. By passing the sound waves at an angle to the flow of gas in two different directions, bidirectional flow can be measured (see Figure 11-6, E). The sound waves are sped up by gas flowing in one direction and slowed down by gas flowing in the opposite direction. By measuring the “transit time” of the pulses with a very accurate digital clock, flow can be integrated to measure volume. Analyzing the change in frequency of the sound waves passing through the flowing gas has the advantage of not being affected by the gas composition, temperatures, or humidity. In addition, there are no moving parts or elements to become occluded when measuring exhaled gas.

A distinct advantage of measuring gas flow by means of ultrasonic pulses is that a disposable flow tube can be inserted between the transducers, thus eliminating problems with cross-contamination between subjects. A further advantage of this design is that the disposable flow tube does not require calibration because it simply acts as a transparent barrier separating exhaled gas from the sensing transducers (Figure 11-11).

Flow Sensor Summary

Flow-sensing spirometers have some advantages over volume-displacement systems. When combined with appropriate gas analyzers and breathing circuits, flow-sensing spirometers can be used to perform lung volume determinations by the open-circuit method. Diffusing capacity can be measured with flow-sensing spirometers as well. Pressure-differential, heated-wire, and Pitot tube pneumotachometers are used to measure flow and volume in body plethysmographs, exercise testing systems, and metabolic carts. Because flow sensors require electronic circuitry to integrate flow or sum volume pulses, flow-based spirometers are microprocessor controlled. Some flow-sensing spirometers provide their analog signal (flow, volume, or both) to a strip chart or X-Y recorder. However, most flow sensors are integrated into a spirometer system and use computer-generated graphics to produce volume-time or flow-volume tracings.

Most flow-based spirometers can be easily cleaned and disinfected. Some flow sensors can be immersed in a disinfectant without disassembly. As noted, many systems use inexpensive, disposable sensors that can be discarded after one use. The use of in-line bacteria filters to prevent contamination of flow-based spirometers may result in changes in the operating characteristics of the spirometer. Any resistance to airflow through the filter will be added to the resistance of the spirometer. For this reason, the spirometer may need to be calibrated with the filter in place. Although the resistance offered by most filters is low, it may change with use. This may occur if water vapor from expired gas condenses on the filter media. Use of barrier filters does not eliminate the need for routine decontamination of spirometers.

Most types of the flow sensors produce a signal that is not linear across a wide range of flows. Some systems use two separate flow sensors (or variable orifices) to accommodate both low and high flows. Better accuracy can be obtained for flow and volume by matching the flow range of the sensor to the physiologic signal. Most flow-based spirometers “linearize” the flow signal electronically or by means of software corrections. In many systems, a simple “look-up table” is stored in the computer. The flow signal is continuously checked against the table and corrected. By combining a calibration factor (see Chapter 12) with the look-up table corrections, very accurate flow and integrated volume measurements are possible. Flows and volumes are corrected before variables such as FEV1 are measured.

Turbine, pressure-differential, heated-wire, and Pitot tube flow-sensing spirometers are affected by the composition of the gas being measured. Changes in gas density or viscosity require correction of the transducer signal to obtain accurate flows and volumes. In most systems, these corrections are performed by computer software with a stored table. A flow-sensing spirometer may be calibrated with air but then used to measure mixtures containing helium, neon, oxygen, or other test gases. Some gases cause a linear shift in flow proportional to their concentrations. Corrections are usually made by applying a simple multiplier to the signal.

The accuracy of flow-based spirometers depends on the electronics, software, or both that process the flow signal. Pulmonary function variables measured on a time base (e.g., FEV1 or FEV6) require precise timing and accurate flow measurement. Detection of the start or end of the test is critical in flow-based spirometers. Timing is usually triggered by a minimum flow or pressure change. Signal integration begins when flow reaches a threshold limit, usually 0.1–0.2 L/sec. Spirometers that initiate timing in response to volume pulses usually have a similar threshold that must be achieved to begin recording. Contamination of resistive elements, thermistors, or Pitot tubes by moisture or other debris can alter the flow-sensing characteristics of the transducer and interfere with the spirometer’s ability to detect the start or end of test. Similar problems can occur when flow drops to very low levels near the end of a forced expiration, causing measurement of flow (and volume) to be terminated prematurely. As a result, the volume (e.g., FVC or VC) may be underestimated.

Problems related to electronic “drift” require flow sensors to be “zeroed” frequently. Zeroing is simply a one-point calibration in which the output of the transducer is set to zero under a condition of no flow. Many systems “zero” the flow signal immediately before a measurement. Zeroing corrects for much of the electronic drift that occurs. A true zero requires no flow through the flow sensor. Thus, the flow sensor must be held still or occluded during the zero maneuvers. Most flow-based systems use a 3-L syringe for calibration. By calibrating with a known volume signal, the accuracy of the flow sensor and the integrator can be checked with one input. Calibration and quality-control techniques for volume-displacement and flow-sensing spirometers are included in Chapter 12.

Portable (Office) Spirometers

The widespread availability of microprocessors has resulted in a large number of small, portable spirometers based on various flow-sensing principles. These spirometers can be separated into two general categories: (1) those that interface with a laptop or desktop computer and (2) stand-alone devices that incorporate a dedicated microprocessor. In both cases, these devices may be referred to as screening or office spirometers.

Many flow-sensing spirometers interface directly with personal computers (Figure 11-12, A-C), typically a laptop computer. Other spirometers use an interface card that plugs directly into a personal computer (PC). With the appropriate software installed on the PC, spirometry can be performed. Other spirometers place the necessary electronic components in the flow sensor head or in an external adapter. This implementation allows the flow sensor to be connected to a serial port or USB (universal serial bus) port, both of which are standard on most computers. The pressure transducer and electronics for flow-based spirometry can also be mounted on a removable card. These cards allow spirometry to be performed with handheld computers, laptop computers, or personal digital assistants (PDAs). Many flow-based spirometers use dedicated microprocessors (Figure 11-13). Some of these are very compact so that the entire device is not much larger than a calculator. These designs allow the units to be handheld and portable.

Many portable spirometers use disposable flow sensors. Disposable sensors can provide accurate measurements if they are manufactured according to rigid specifications. Disposable flow sensors are usually precalibrated at the time of manufacture. Some manufacturers include calibration codes (or bar codes) on the sensor to be used in conjunction with the spirometer software. Ideally, each spirometer should provide a means for calibration using a 3-L syringe. At a minimum, the software in portable spirometers should allow verification of volumes, even if precalibrated flow sensors are used.

Interfacing a flow sensor to a PC or laptop computer makes spirometry available in a variety of clinical settings. Handheld or PC-based systems provide a relatively inexpensive way to perform spirometry, before and after bronchodilator studies, and even bronchial challenges. Increasingly sophisticated software allows spirometric data to be stored, manipulated, and displayed graphically. In spite of the availability of small, accurate spirometers, spirometry is still not widely used in primary care. Because spirometry is effort dependent, poorly performed maneuvers can result in misclassification (i.e., obstruction versus restriction versus normal). The choice of inappropriate reference equations and incorrect interpretation of results further limits the usefulness of spirometry.

The National Lung Health Education Program (NLHEP) provides recommendations for office spirometers to be used in primary care settings (Box 11-1). The goal of these recommendations is to standardize spirometry to promote early detection of chronic obstructive pulmonary disease (see Figure 11-14). Office spirometers should be simple and designed to measure three important parameters: FEV1, FEV6, and the FEV1/FEV6 ratio. To provide accurate measurements, office spirometers should display automated messages describing the acceptability and repeatability of efforts. Automated interpretation of simple spirometry can be performed if test quality is acceptable and appropriate reference values are used. Display or printouts of spirograms are optional. Office spirometers should meet the accuracy recommendations of the ATS-ERS (see Chapter 12).

Peak flowmeters

PEF can be measured easily with most spirometers, particularly the flow-sensing types. Many devices are available that measure PEF exclusively. PEF has become a recognized means of monitoring patients who have asthma. By incorporating a simple measurement into an inexpensive package, portable peak flowmeters allow monitoring of airway status in a variety of settings.

Most peak flowmeters use similar designs. The patient expires forcefully through a resistor or flow tube that has a movable indicator attached (Figure 11-15). An orifice provides the resistance in most devices. The movable indicator is deflected in proportion to the velocity of air flowing through the device. PEF is then read directly from a calibrated scale. Because these devices are nonlinear, different flow ranges are usually available. High-range peak flowmeters typically measure flows as high as 850 L/min. Low-range meters measure up to 400 L/min (Table 11-1). Low-range peak flowmeters are useful for small children or for patients who have marked obstruction.

Table 11-1

Peak Flowmeter Recommended Ranges

Children Adults
National Asthma Education Program 100–400 L/min ± 10% 100–700 L/min ± 10%
American Thoracic Society (1994) 60–400 L/min ± 10% or 20 L/min, whichever is greater 100–850 L/min ± 10% or 20 L/min, whichever is greater

The absolute accuracy of portable peak flowmeters is less important than their precision (i.e., repeatability of measurements). Within-instrument variability should be less than 5% or 0.15 L/sec (10 L/min), whichever is greater. Between-instrument variability should be less than 10% or 0.3 L/sec (20 L/min), whichever is greater. These devices are intended to provide serial measurements of peak flow as a guide to treatment. Patients who are carefully instructed should be able to reproduce their peak flow measurements within 0.67 L/sec (40 L/min). However, asthmatics may have difficulty repeating their PEF, particularly during exacerbations. PEF meters must be easy to use and easy to read. Scale divisions of 5 L/min for low-range devices and 10 L/min for high-range devices allow small changes in PEF to be detected. The scale should be calibrated to read flow in BTPS units. Corrections for altitude should be included because PEF meters tend to underestimate flow as altitude increases (i.e., approximately 7% per 100 mm Hg change in barometric pressure).

Although the simple design of portable peak flowmeters allows them to be used repeatedly, moisture or other debris can cause sticking of the movable parts. This can be problematic because it may suggest that the patient’s asthma has worsened. Some instruments can be cleaned but may need to be replaced periodically. Because portable peak flowmeters may have a limited life span, variability between same-model instruments should be 10% or 20 L/min as noted. This allows the patient to continue monitoring with a new device. Clear instructions on how to use and maintain the peak flowmeter should come with each device. Most peak flowmeters comply with the National Asthma Education Program’s “color zone” scheme for identifying clinically significant changes (see Chapter 2).

Body plethysmographs

Body plethysmographs (Figure 11-16, A-C) are used in many pulmonary function laboratories. Body plethysmographs are also called body boxes. Two types of plethysmographs are available: the constant-volume, variable-pressure plethysmograph, and the flow or variable-volume plethysmograph. These are sometimes called the pressure plethysmograph and flow plethysmograph, respectively. Pressure-type plethysmographs are more commonly used than flow types. Both designs are used to measure thoracic gas volume (VTG) and airway resistance (Raw) and its derivatives (see Chapter 4). Both types of box use some type of pneumotachometer to measure flow, as well as a mouth pressure transducer with a shutter to measure alveolar pressure. They differ in the method used to measure volume change in the box, and therefore in the lungs.

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